NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Water Use in Parabolic Trough 

Power Plants: Summary Results 

from WorleyParsons' Analyses 

C.S. Turchi, M.J. Wagner, and C.F. Kutscher

Technical Report

NREL/TP-5500-49468

December 2010
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Water Use in Parabolic Trough 

Power Plants: Summary Results 

from WorleyParsons' Analyses 

C.S. Turchi, M.J. Wagner, and C.F. Kutscher

Prepared under Task No. SM10.1530

Technical Report

NREL/TP-5500-49468

December 2010
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iii

Table of Contents

Summary...................................................................................................................................................... 1

Background and Objectives....................................................................................................................... 1

Steam Rankine Power Cycle...................................................................................................................... 2

Cooling Options .......................................................................................................................................... 3

WorleyParsons Reports ............................................................................................................................. 7

Cost Assumptions................................................................................................................................ 7

Design Basis ......................................................................................................................................... 7

Selection of Air-Cooled Condenser (ACC) Size ................................................................................ 8

Description of Study Sites................................................................................................................. 11

Daggett, California Site ...................................................................................................................... 12

Las Vegas, Nevada Site ..................................................................................................................... 14

Alamosa, Colorado Site ..................................................................................................................... 16

Water Consumption ........................................................................................................................... 17

Analysis Using Solar Advisor Model (SAM)........................................................................................... 19

Impact of Time-of-Delivery (TOD) Rates on Dry-Cooled Plant Revenue ............................................. 23

Conclusions............................................................................................................................................... 25

References................................................................................................................................................. 26

Acknowledgements .................................................................................................................................. 27

Appendix A ................................................................................................................................................ 28

Appendix B ................................................................................................................................................ 58

Appendix C ................................................................................................................................................ 77
iv

List of Figures

Figure 1. Simple representation of a steam Rankine thermal power cycle. Cycle shows a cooling system 

that uses water evaporation to condense the low-pressure steam coming from the turbine................... 3

Figure 2. Representation of wet-bulb and dry-bulb temperature difference and the impact on condenser 

temperature for a warm day. Wet-cooled systems can always achieve equal or lower condenser 

temperatures than dry-cooled systems.................................................................................................... 4

Figure 3. Steam turbine power output as a function condenser temperature [3]. ......................................... 6

Figure 4. Hybrid cooling systems use an air-cooled condenser and a wet-cooled condenser in parallel. .... 7

Figure 5. Determining the optimum ITD for a plant with the Las Vegas climate file based on installed 

cost per kW............................................................................................................................................. 8

Figure 6. Determining the optimum ITD for a plant with the Alamosa climate file based on installed 

cost per kW............................................................................................................................................. 9

Figure 7. Impact of two different design assumptions on subsystem costs and plant output. .................... 10

Figure 8. Dry-bulb temperature duration curves for several Southwest locations...................................... 12

Figure 9. Estimated water consumption for the 13 cases............................................................................ 18

Figure 10. Water consumption by different land-intensive activities in the Southwest.............................. 19

Figure 11. SAM simulations of a summer day at design point (top) and a spring day at lower ambient 

temperatures (bottom) for wet- and dry-cooled plants. Constant-capacity design is assumed, i.e., the 

dry plant has a larger turbine and solar field to accommodate the lower cycle efficiency................... 22

Figure 12. Relative generation or revenue as a function of cooling type and TOD schedule for Alamosa 

(top) and Daggett (bottom) cases. The scale is relative to the wet-cooled, no TOD, values estimated 

by SAM. ............................................................................................................................................... 24

List of Tables

Table 1. Characteristics of the different cooling methods. ........................................................................... 4

Table 2. Summary of conditions evaluated by WorleyParsons. Consult the complete report for more 

information. .......................................................................................................................................... 11

Table 3. Comparison of wet- and dry-cooled parabolic trough plants in Daggett, California.................... 13

Table 4. Comparison of nominal 100-MW wet, hybrid, and dry cooling cases for the Las Vegas study 

with constant heat input design assumption. ........................................................................................ 14

Table 5. Comparison of 125-MW wet- and dry-cooling cases for Las Vegas, NV study with constantcapacity design assumption. ................................................................................................................. 16

Table 6. Comparison of wet-, hybrid-, and dry-cooling cases for the Alamosa, Colorado study............... 17

Table 7. Financial assumptions used in SAM analysis............................................................................... 19

Table 8. Summary of the WorleyParsons (WP) results and SAM estimates. SAM values were obtained 

using the solar field area, turbine size and efficiency, and design-point temperatures from the 

WorleyParsons analyses. ...................................................................................................................... 21

Table 9. Change in generation by TOD period and the resulting impact on annual revenue resulting from 

the switch from wet to dry cooling, assuming Daggett and Alamosa climates using the TOD schedule 

for SCE. The Daggett cases assume constant capacity while the Alamosa cases assume constant heat 

input design (See Table 1).................................................................................................................... 23
1 

Water Use in Parabolic Trough Power Plants: 

Summary Results from WorleyParsons’ Analyses

Summary

Water consumption for electric power generation is undergoing increasing scrutiny, with more 

emphasis being placed on low-water-use technologies. In 2009 and 2010, the National 

Renewable Energy Laboratory (NREL) contracted with WorleyParsons Group, Inc. (Golden, 

Colorado) to examine the efficiency and impact of dry and hybrid (both dry and wet operating in 

parallel) cooling systems on a nominal 100-MW parabolic trough concentrating solar power 

(CSP) plant. WorleyParsons analyzed 13 different cases spanning three different geographic 

locations (Daggett, California; Las Vegas, Nevada; and Alamosa, Colorado) and wet, dry, and 

hybrid cooling technologies to assess the performance, cost, and water use impacts of switching

from wet to dry or hybrid cooling systems. NREL developed cases in its Solar Advisor Model 

(SAM) for each scenario to provide a comparison to the WorleyParsons designs. Although 

absolute estimates are listed at times, the emphasis of this analysis is on the relative cost and 

performance of wet-, dry-, and hybrid-cooled plants. 

Our findings indicate that switching from 100% wet to 100% dry cooling will result in levelized 

cost of electricity (LCOE) increases of approximately 3% to 8% for parabolic trough plants 

throughout most of the southwestern United States. In cooler, high-altitude areas like Colorado’s 

San Luis Valley, WorleyParsons estimated the increase at only 2.5%, while SAM predicted a 

4.4% difference. For the single case that included a no-storage design, the addition of thermal 

energy storage reduced the negative impact of dry cooling on LCOE. In all cases, the transition 

to dry cooling will reduce water consumption by over 90%. The remaining water consumption is 

split between steam-cycle maintenance and mirror washing. Utility time-of-delivery (TOD)

schedules had similar impacts for wet- and dry-cooled plants, suggesting that TOD schedules 

have a relatively minor effect on the dry-cooling penalty. 

Hybrid cooling can reduce the LCOE increase—but at a higher capital cost and operational 

complexity. The economics of hybrid cooling depend on water cost and climate: for example, in 

the Alamosa climate, hybrid cooling had no benefit, while there was a clear LCOE advantage for 

hybrid cooling over dry cooling for the Las Vegas site with storage. A more detailed study of 

hybrid cooling is the subject of a pending NREL publication. 

Background and Objectives

Utility-scale solar power plants are currently being proposed at numerous sites throughout the 

southwestern United States. Large central-station plants in this region take advantage of both 

excellent solar resource quality and economy of scale in construction and operation to produce 

electricity at the lowest cost of any CSP installation. While there are obvious local, regional, and 

global environmental benefits of large-scale solar power (e.g., jobs, low pollution, low fossil-fuel 

consumption, domestic resource, etc.), local environmental impacts must also be considered. The 

greatest local impacts relate to the use of land and water. This report focuses on water 

consumption at parabolic trough CSP plants. 

Parabolic troughs represent the most mature of CSP technologies, with more than 500 MW of 

trough plants currently operating in the United States and Spain. In addition, approximately 1 
2 

gigawatt (GW) is under construction, with several GW of trough plants proposed for locations 

around the world [1]. Parabolic troughs offer a desirable blend of efficiency and proven 

performance that make them appealing to developers and financiers. In addition, thermal inertia 

and the ability to incorporate thermal energy storage allow trough (and power tower) plants to 

provide reliable, dispatchable generation that facilitates their integration into the electric grid. 

The huge generation potential of CSP in the Southwest has led to a focus on the possible impact 

of extensive CSP deployment on water resources in the region. Because steam-cycle cooling 

accounts for over 90% of water consumption in a typical wet-cooled CSP power plant, 

minimizing cooling water use is the most important step in water conservation. In 2009 and 

2010, NREL contracted with WorleyParsons Group (Golden, CO) to estimate the water use, 

performance, and cost impacts of switching from wet to dry cooling. Studies were performed for 

parabolic trough plants in three different locations: Daggett, California; Las Vegas, Nevada; and 

Alamosa, Colorado. Daggett and Alamosa were selected to represent two opposite climate 

extremes for the region, and the Las Vegas case was undertaken to assist with a specific U.S. 

Department of Energy (DOE) request. The WorleyParsons studies are included as appendices to 

this report. The three studies had differing assumptions that resulted in slightly different 

conclusions. 

The specific objectives of this report are as follows:

• Summarize and consolidate the WorleyParsons work and provide a single publication 

reference;

• Reconcile the conclusions of the WorleyParsons work on the basis of the differing 

study assumptions; 

• Compare the WorleyParsons results to estimates from SAM (available at 

https://www.nrel.gov/analysis/sam/) using the same plant size assumptions; and 

• Incorporate TOD price schedules into the economic evaluation of wet- and drycooled plants. 

The following section provides a brief overview of the importance of cooling for steam Rankine 

power plants; this review is targeted for those who are not experts in steam turbine design. This 

is followed by a review of the WorleyParsons results, and then the comparison analysis using 

SAM. 

Steam Rankine Power Cycle

Parabolic trough and power tower CSP technologies rely on steam Rankine power cycles that are 

essentially the same as those used in coal and nuclear power plants. These power cycles input 

high-quality thermal energy, produce electric power, and discharge low-quality heat (see Figure 

1). The heat rejection phase in a Rankine power block uses a cooling system (a heat sink) to 

condense steam back into water. The water can then be efficiently pumped back to high pressure 

and returned to the boiler to produce high-pressure steam. The overall conversion efficiency of 

thermal energy into electricity directly depends on the temperatures of the heat source and the 

heat sink. In its simplest form, the ideal thermal cycle efficiency (the Carnot efficiency) is 

proportional to 1 minus the ratio of heat sink temperature to the heat source temperature, where 

the temperatures are defined on an absolute scale: 
3 

Thus, cycle efficiency is maximized when the highest possible heat source temperature and the 

lowest possible heat sink temperature are obtained. Because of this relationship, power plant 

developers seek a cooling system that provides the lowest possible heat sink temperature. In 

general, the most convenient low-temperature heat sink is provided by water—either directly in 

the case of once-through cooling with a body of water or indirectly through evaporation. Where 

water is scarce or expensive, the water-cooled design may not be practical. Notably, the vast 

majority of power plants (whether solar, coal, gas, or nuclear) that utilize a steam Rankine power 

cycle use water cooling. 

Figure 1. Simple representation of a steam Rankine thermal power cycle. Cycle shows a cooling 

system that uses water evaporation to condense the low-pressure steam coming from the turbine. 

Cooling Options

While water is the preferred medium for power-cycle cooling, its availability may be limited by 

policy or cost in some locations, especially throughout much of the arid Southwest. Under these 

conditions, the plant designer can opt for an air-cooled system or, if some water is available, a 

hybrid design that uses both air and water for cooling. An air-cooled condenser (ACC) 

condenses the steam by forcing ambient air over a bundle of finned tubes containing the steam 

that exits the turbine. A typical hybrid system includes both an ACC and a wet-cooled tower 

operating in parallel, and the size of each can be adjusted depending on the design intent. The 

design of these systems has been discussed elsewhere [2-4]; the attributes of each are 

summarized in Table 1.

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ܶ௦௜௡௞

ܶ௦௢௨௥௖௘
4 

Table 1. Characteristics of the different cooling methods.

Cooling 

Type

Advantages Disadvantages

Wet 

(cooling tower)

Lowest installed cost

Low parasitic loads

Best cooling (i.e., lowest cooling temperature), 

especially in arid climates; gives highest power 

cycle efficiency

High water consumption

Water treatment and blowdown disposal 

required

Cooling tower plume in cold weather 

Dry (ACC)

No water consumption

No water treatment required

No cooling-tower or blowdown pond

Lower O&M costs 

More expensive equipment

Higher parasitic loads

Poorer cooling at high dry-bulb temps 

(cycle efficiency falls) 

Hybrid

Reduced water consumption

Potential for lower levelized energy cost 

compared to dry cooling

Maintains good performance during hot weather 

Complicated system involving wet and 

dry cooling; often highest capital cost

Same disadvantages of wet system, but to 

lesser degree 

A critical distinction between the effectiveness of wet- and dry-cooled systems relates to the 

minimum cooling temperature each technology can achieve. Dry-cooled processes rely on air 

cooling and are limited by the ambient dry-bulb temperature. In contrast, wet cooling processes 

use evaporation to reject heat and can achieve minimum temperatures that approach the ambient 

wet-bulb temperature. “Wet-bulb” refers to the temperature achieved by a moistened 

thermometer in flowing air and reflects the reduced temperature that is possible when 

evaporation from a surface is accounted for. The difference between wet-bulb and dry-bulb 

temperature depends on humidity: they are equal at 100% relative humidity, and wet-bulb is 

always lower than dry-bulb temperature in other conditions. Figure 2 depicts an example of how 

the wet-bulb and dry-bulb temperatures relate to the operating conditions of wet and dry 

condensers. 

Figure 2. Representation of wet-bulb and dry-bulb temperature difference and the impact on 

condenser temperature for a warm day. Wet-cooled systems can always achieve equal or lower 

condenser temperatures than dry-cooled systems. 
5 

A cooling tower can easily bring water to within 5°C of the wet-bulb temperature (known as the 

“approach temperature”). This cooling water is then used to condense the steam at a temperature 

typically a few degrees warmer, with the overall difference between the wet-bulb temperature 

and the condensate on the order of 10°C; that is, for a wet-bulb temperature of 24°C, a typical 

wet-cooled condenser may operate at 34°C (see Figure 2). In contrast, a dry-cooled condenser is 

designed to condense steam at a temperature above the dry-bulb temperature; this is denoted as 

the initial temperature difference (ITD). Because air has a low volumetric heat capacity, the heat 

transferred to the air results in a large temperature increase in the air as it passes through the 

ACC. (Higher air flow rates could reduce the temperature increase, but at a high cost in fan 

power. Similarly, larger ACC areas can be used, but also at a higher cost.) In the example shown 

in Figure 2, the dry-bulb temperature is 32°C, and even with an aggressive ITD temperature of 

16°C, a dry-cooled condenser can only achieve a condenser temperature of 48°C. Wet-cooled 

condensers are normally designed with a closer approach than ACCs because the marginal cost 

of an ACC climbs dramatically as one nears the dry-bulb temperature. As previously shown, the 

efficiency of the Rankine steam cycle depends on the condenser temperature, so as condenser 

temperature climbs, power-cycle efficiency falls. 

Figure 3 shows a typical response of cycle efficiency versus condenser temperature [3]. One can 

see that condenser temperature has a strong impact on plant output. Steam turbines are designed 

for optimum performance at the intended design conditions; two different turbine design curves 

are shown in Figure 3. A low-backpressure turbine, such as might be specified for a wet-cooled 

plant, has a higher efficiency—but only at low condenser temperatures. Performance falls rapidly 

as condenser temperature increases, but because wet-bulb temperature is normally relatively 

constant, this is not detrimental. In contrast, a high-backpressure turbine would be a better match 

for a dry-cooled plant running at conditions governed by the higher and more variable dry-bulb 

temperature. Such curves are commonly plotted versus condenser pressure, which is the 

“backpressure” seen by the turbine. Because the condenser holds saturated steam, either the 

temperature or pressure can be used to define the conditions. 
6 

Figure 3. Steam turbine power output as a function condenser temperature [3]. 

On hot summer afternoons dry cooling performance is at its least efficient. However these 

periods are often when electricity demand peaks and power generation is most valuable. Some 

utilities reward generation during these periods through TOD rates that provide cost incentives 

for on-peak generation. Hybrid cooling systems (Figure 4) have been proposed to allow for 

partial wet cooling during these periods. In hybrid systems, a wet-cooled condenser operates in 

parallel with a dry-cooled condenser. When the dry-bulb temperature is low, only the ACC is 

used. At high dry-bulb temperatures, the wet-cooled system is activated to reduce the cooling 

load on the ACC, thereby lowering the exit air temperature and allowing the steam to condense 

at a lower temperature closer to the air dry-bulb temperature. Such designs exchange water 

consumption for cycle efficiency. 
7 

Figure 4. Hybrid cooling systems use an air-cooled condenser and a wet-cooled condenser in 

parallel.

WorleyParsons Reports 

Cost Assumptions

All of the WorleyParsons analyses covered here assume an Engineering, Procurement, and 

Construction Management (EPCM) project contract. As such, several typical project costs are 

not included in the WorleyParsons estimates. The most notable exclusions are those items 

normally associated with owner’s costs, including permitting, land, legal fees, geotechnical and 

environmental surveys, taxes, interest during construction, and the owner’s engineering and 

project management activities; in addition, WorleyParsons did not include sales tax in their 

analysis. These categories are typically estimated by a factor applied to the project’s direct costs; 

for example, these indirect costs are often estimated as 20% to 30% of a project’s direct costs.

Assuming this percentage is unchanged between the wet- and dry-cooled cases, the relative 

change between wet- and dry-cooled costs in the WorleyParsons work are representative even if 

the absolute values are incomplete. The SAM program cost values in this report include project 

indirect costs of approximately 25%. 

Design Basis

There are two different design assumptions one can make when comparing wet- and dry-cooled 

designs: (1) constant design-point net capacity, or (2) constant design-point heat input. In the 

first case, the designer assumes that the plant owner wishes to maintain a particular design-point 

capacity for the facility regardless of the cycle design. That is, a 100-MW wet-cooled plant 

should produce 100 MW after switching to dry cooling. This design requires increasing the solar 

field area and turbine gross capacity in order to offset the efficiency penalty of dry cooling. Such 

an approach is likely when planning a project with a specific power output requirement. 

In contrast, if the goal is to highlight performance differences between wet and dry cooling, it is 

simpler to assume a constant heat input. In a constant heat input design, the turbine and solar 
8 

field sizes are unchanged, and any performance difference results purely from the change of 

cooling system. This is an oversimplification, because even with no change to the turbine 

capacity, one is likely to optimize the turbine for the higher condenser temperatures expected 

from a dry-cooled system. For example, WorleyParsons adjusted turbine design parameters to 

account for a higher backpressure when modeling the dry-cooled plants. 

Selection of Air-Cooled Condenser (ACC) Size

WorleyParsons examined different ACC ITD design points to determine the optimum balance 

between plant capital cost and efficiency for the Las Vegas and Alamosa sites. The ITD analysis 

assumes a dry-cooled plant with a fixed solar field size operating at the selected design-point 

dry-bulb temperature. To select an optimum ITD for the Las Vegas site, GateCycle performance 

models were run at the design conditions, varying the ITD from 5°F to 40°F (2.8°C to 22°C). 

The net plant power from each run took into account the varying ACC fan power loads and 

steam turbine outputs. The total installed plant cost was adjusted for the different sizes of ACCs 

as defined by the ITD. The total plant cost and the plant cost per net kW are plotted as a function 

of the ACC ITD in Figure 5 below. 

Figure 5. Determining the optimum ITD for a plant with the Las Vegas climate file based on 

installed cost per kW. 

As shown in Figure 5, total plant installed cost drops with ITD because a larger ITD leads to a 

smaller and less expensive ACC. However, ITD also affects plant efficiency and design-point 

capacity. The lowest cost per kW (net) at the design conditions was obtained using an ITD of 

25°F (13.9°C), and this ITD was used in the Las Vegas cooling study. A similar analysis was 

$9,000 

$9,500 

$10,000 

$10,500 

$11,000 

$11,500 

$12,000 

$900 

$950 

$1,000 

$1,050 

$1,100 

$1,150 

$1,200 

0 5 10 15 20 25 30 35 40 45

$/kw at design conditions

million $

Las Vegas site ITD (deg F)

Total Plant Installed Cost, Million $

Total Plant Installed Cost, $/kw
9 

performed for the Colorado site (see Figure 6). The cost per kW was level from 20°F to 30°F 

ITD. The average was subsequently selected, which conveniently aligned with the 25°F ITD 

chosen for the Las Vegas site. Note that these ITDs are considerably lower than what would 

typically be used for a fossil plant. The lower ITD and commensurately larger ACC can be 

justified because trough plants operate at a lower steam temperature and are thus more sensitive 

to the heat rejection temperature. In addition, for an equivalent power output, it is more cost 

effective to invest in a larger ACC than a larger solar field.

For a specified ITD, one can calculate the temperature and pressure that will be achieved in the 

condenser at design-point conditions. Due to the lower design-point dry-bulb temperature in 

Colorado, a 25°F ITD resulted in a 1.27 psia (0.088 bar) steam turbine backpressure compared to 

a design turbine exhaust pressure of 2.60 psia (0.180 bar) in the Las Vegas study. According to 

WorleyParsons, the lowest steam turbine backpressure that can be reasonably achieved with an 

ACC is about 1 psia (0.07 bar). Thus, the Colorado site’s 100%-dry design approaches this limit, 

and there is no advantage to selecting a smaller ITD. For comparison, WorleyParsons assumes 

that wet-cooled systems can achieve minimum pressures as low as 0.5 psia (0.035 bar). 

Figure 6. Determining the optimum ITD for a plant with the Alamosa climate file based on installed 

cost per kW. 

The ITDs selected above are optimized based on the design-point ambient temperature. Since 

this temperature is achieved only during a few of the hottest hours of the year, this method may 

not produce a system with an optimal LCOE. For this reason, WorleyParsons performed a 

parametric study based on the cost per MWh produced as opposed to cost per kW capacity. That 

analysis found a broad minima for ITDs from 25°F to 32°F (14°C to 18°C). 

Based on the above analysis for Las Vegas and Colorado, WorleyParsons concluded that a 25°F 

(14°C) ITD was close to optimal for both sites. It should be noted that the 14°C ITD optimum 

$8,000 

$8,500 

$9,000 

$9,500 

$10,000 

$10,500 

$11,000 

$11,500 

$800 

$850 

$900 

$950 

$1,000 

$1,050 

$1,100 

$1,150 

$1,200 

0 10 20 30 40 50

Installed Cost, $/kw

Installed Cost, Million $

Alamosa, CO site ITD (degF)

Total Installed Cost, million $

Total Installed Cost, $/kw
10 

selected by WorleyParsons was based on solar field costs that are higher than the solar industry 

predicts [6]. Optimum ITD is a function of solar field and ACC cost, and a separate NREL 

analysis selected higher values for ITD [11]. WorleyParsons also noted the following: 

• For a given duty and ITD, ACC manufacturers can reduce the fan parasitic load, at 

the expense of higher capital cost. Whether this is economically justified depends on 

the net/gross power metric, labor costs, hours of thermal energy storage (TES), etc. 

• The optimization was performed for a trough plant with significant TES. A plant that 

has little or no TES would likely not warrant as aggressive an ITD because the steam 

turbine would not be operating at full load as often. 

• Steam turbine exhaust selection, which is manufacturer-dependent, is as important as 

ITD selection and should be done simultaneously. Generally, a longer last-stage blade 

will improve turbine efficiency, but it will also limit the maximum backpressure the 

turbine can see. In this way, the turbine exhaust design can be optimized for the 

selected ITD WorleyParsons optimized the turbine design for each case using generic 

steam turbine guidelines. 

• Spraying the ACC inlet air may be a cost-effective alternative to a hybrid parallel 

cooling system, depending on hours of operation, ambient conditions, water costs, 

water analysis, and other factors.

Traditional ACC designs use an ITD in the range of 30°F to 50°F (16°C to 28°C) [5]. The 

smaller ITD selected here means a more expensive ACC, but it also minimizes the additional 

solar field and turbine capacity needed to overcome the reduced efficiency of the dry-cooled 

plant. In a constant-capacity design, this “oversized” ACC combined with the larger solar field 

and gross turbine size allows the dry-cooled plant to generate more energy on an annual basis. 

Figure 7 shows the relative impact of the design assumption on subsystem costs and plant output 

when switching from 100% wet to 100% dry cooling with a 25°F (14°C) ITD. The data come 

from the Daggett (constant capacity) and Las Vegas (constant heat input) cases described later in 

this report. 

Figure 7. Impact of two different design assumptions on subsystem costs and plant output.
11 

As shown in Figure 7, the turbine and solar field are unchanged in the constant heat input design, 

and design-point performance and annual generation drop due to lower plant efficiency. The 

power block cost increases in both cases primarily due to the greater cost of an ACC versus a 

cooling tower. Although the power block cost increase is significant, its impact on installed plant 

cost is dwarfed by the cost of the larger solar field in the constant-capacity case (note that the 

installed plant cost of the constant heat input case increases much less than that of the constantcapacity case). This fact drives the rationale to install a large ACC in order to minimize the 

required increase in solar field. 

Description of Study Sites

The cases evaluated by WorleyParsons are summarized in Table 2 and discussed individually in 

the following sections.

Table 2. Summary of conditions evaluated by WorleyParsons. 

Consult the complete report for more information.

Location 

(Climate File) Design Basis Storage Cooling Options

Complete

Report

Daggett, CA 

(TMY3) Constant capacity: 103 MW 6.3 h Wet, dry Ref [6] 

Las Vegas, NV 

(TMY2)

Constant heat input from 

562,440 m2 solar field None Wet, dry, hybrid Appendix A 

Constant heat input from 

931,950 m2 solar field 6 h Wet, dry, hybrid Appendix A 

Constant capacity: 125 MW 4 h Wet, dry Appendix B

Alamosa, CO 

(TMY3)

Constant heat input from 

905,790 m2 solar field 6 h Wet, dry, hybrid Appendix C 

The cost penalty for switching from wet to dry cooling is determined by the dry-bulb 

temperature profile for the site. Figure 8 shows duration curves for dry-bulb temperature taken 

from TMY climate files for five locations in the Southwest. All other factors being equal, we 

anticipate sites near Phoenix to have the greatest penalty associated with switching to dry 

cooling, with Daggett and Las Vegas following close behind. The colder, high-altitude climate of 

Alamosa suggests that the performance penalty for dry cooling will be modest. Accordingly, the 

Daggett, Las Vegas, and Alamosa studies performed by WorleyParsons most likely represent 

worst- and best-case examples, respectively, for dry cooling in the Southwest. 
12 

Figure 8. Dry-bulb temperature duration curves for several Southwest locations. 

Daggett, California Site

The most extensive WorleyParsons analysis was provided for a trough plant located near 

Daggett, California. Daggett’s excellent solar resource and proximity to the Solar Electricity 

Generating Systems (SEGS) plants have made this the traditional site for NREL’s reference 

power plants. The primary purpose of the Daggett plant analysis was to develop a line-item cost 

model for the Solar Advisor Model [6]: this analysis assumed a constant-capacity constraint 

when comparing wet- and dry-cooled configurations. The WorleyParsons contract reports on the 

wet- and dry-cooled plants are available in reference [6]; the major assumptions and results are 

summarized below in Table 3. 
13 

Table 3. Comparison of wet- and dry-cooled parabolic trough plants in Daggett, California.

Design Parameters Wet-Cooled 

Design

Dry-Cooled 

Design

% Change 

Power Block Net Capacity (MWe) 103 103 0

Thermal Energy Storage at Design Point (hours) 6.3 6.3 0

Design Conditions: DNI (W/m

2

) 1000 1000 0

Design Conditions: Wet-Bulb Temperature (°C) 21.8 - 0

Design Conditions: Dry-Bulb Temperature (°C) - 42.2 0

Size Parameters

Turbine Gross Capacity (MWe) 118 120.5 +2.1%

Plant Footprint (acres) 1018 1024 +0.6%

Solar Field Area (m

2

) 987,540 1,062,750 +7.6%

Thermal Storage Media (metric tonnes) 62,000 66,800 +7.8%

Thermal Storage Size (MWh-t) 1988 2144 +7.8%

Output Values Estimated by WorleyParsons

Annual Net Electricity Generation (MWh) 426,717 438,790 +2.8% 

Capacity Factor (Based on 103 MWe Net) 47% 48% +2.8%

Annual Water Consumption (m3) 1,530,000 

(1240 acre-ft)

114,000 

(90 acre-ft)

-93% 

Design Point Parasitic: HTF Circulation Pumps 

(MWe)

7.9 8.4 +6% 

Design Point Parasitic: Cooling System (MWe) 2.0 3.6 +80%

Design Point Parasitic: Total (MWe) 15.0 17.6 +17%

Annual O&M Costs $11.8M $11.7M -1%

Installed Cost ($M) 1,016 1,098 +8.0%

The Daggett analysis shows greater annual generation provided by the dry-cooled plant. The drycooled plant produces more energy because the solar field and power block are oversized to 

maintain design-point generation at high ambient temperatures. At the lower ambient 

temperatures that characterize much of the year, this combination generates more energy than 

does the slightly smaller wet-cooled plant. Switching from wet to dry cooling raised plant 

installed cost by about 8% while supplying 2.8% more energy over the course of the year. The 

greater generation indicates that the change in LCOE will be less than 8%, although this 

calculation was not part of WorleyParsons’ scope.

The primary advantage of the dry-cooled system is the dramatic decrease in water consumption. 

Despite the larger solar field, the plant footprint and O&M costs are virtually unchanged due to 

elimination of the evaporation ponds and maintenance associated with the wet-cooling tower.

Eliminating the evaporation ponds also produces a substantial savings in site improvement costs.
14 

The major disadvantage of the dry-cooled plant is the higher installed cost. This elevated cost is 

due to two factors: (1) the greater cost of an ACC and (2) the requirement of a slightly larger 

solar field, turbine, and thermal storage system to maintain the design-point capacity in the face 

of the lower thermal cycle efficiency and greater parasitic losses. Plant parasitics increase by 

roughly 17% at design point, mostly due to the energy consumption of the cooling fans. 

Las Vegas, Nevada Site

While the Daggett study was performed primarily as a cost analysis, the later Las Vegas and 

Alamosa studies were undertaken expressly to examine cooling impacts. At NREL’s request, 

WorleyParsons examined wet-, dry-, and hybrid-cooled plant configurations for the Las Vegas 

area; this report is provided as Appendix A. As documented above, an ITD of 25°F (14°C) was 

selected for the dry-cooled case. The same ACC size was used for the dry and hybrid cases, 

while the hybrid system's wet-cooling tower was sized at 90% of the full wet-cooled system size. 

The hybrid-cooled analysis arbitrarily assumed a goal to reduce overall water consumption by 

50%. The hybrid system was utilized during the periods of hottest dry-bulb temperatures, 

moving to lower dry-bulb temperatures until the specified 50% water usage limit was reached. 

The constant heat input assumption kept the solar field size fixed for all three cooling methods. 

WorleyParsons examined trough plants with and without thermal energy storage, which entailed 

two different solar field sizes. The steam turbine design was optimized for the anticipated 

backpressure from each of the different cooling approaches, and it exhibits slight differences 

between the cases. The results for the Las Vegas location are summarized in Table 4. 

Table 4. Comparison of nominal 100-MW wet, hybrid, and dry cooling cases for the Las Vegas 

study with constant heat input design assumption. 

Parameter (No Storage Case) 100% WetCooled Hybrid Cooling 100% Dry-Cooled 

Site Improvements ($) 23,979,000 19,365,000 -19% 16,723,000 -30%

Solar Field & HTF System ($) 312,952,000 312,952,000 0% 312,952,000 0%

Power Plant & Cooling System 

($) 120,949,000 160,680,000 +33% 150,147,000 +24%

Total Installed Cost ($) 528,140,000 564,831,000 +6.9% 550,419,000 +4.2%

Gross Turbine Efficiency at 

Design 0.3771 0.3745 -0.7% 0.3576 -5.2%

Net Plant Output at Design 

(MW) 100 98.71 -1.3% 93.11 -6.9% 

Annual Generation (MWh) 252,055 248,309 -1.5% 240,942 -4.4%

LCOE (¢/kWh) * 18.6 19.8 +6.4% 20.1 +8.1%
15 

Parameter (6 h Storage Case)

Site Improvements ($) 30,941,000 24,442,000 -21% 21,816,000 -30%

Solar Field & HTF System ($) 521,895,000 521,895,000 0% 521,895,000 0%

Power Plant & Cooling System 

($) 121,997,000 162,820,000 +33% 151,913,000 +24%

Total Installed Cost ($) 961,123,000 996,843,000 +3.7% 982,085,000 +2.2%

Gross Turbine Efficiency 0.3770 0.3744 -0.7% 0.3577 -5.1%

Net Plant Output at Design 

(MW) 100 98.92 -1.1% 92.82 -7.2%

Annual Generation (MWh) 396,062 395,070 -0.3% 377,000 -4.8%

LCOE (¢/kWh) * 18.4 18.9 +3.2% 19.5 +6.3%

* See Appendix A for financial assumptions 

Although the Daggett and 100-MW Las Vegas studies used different design assumptions, the 

overall conclusions of switching from 100% wet to 100% dry cooling are consistent. The 

greatest decrease in plant output occurs at design-point conditions; during the remainder of the 

year, the impact is less severe. The increase in LCOE for the Las Vegas plant with storage is 

6.3%, which is consistent with the Daggett location estimate of less than 8%. The presence of 

storage helps mitigate the penalty of switching to dry cooling, perhaps because some generation 

is shifted into periods of lower dry-bulb temperature [7]. 

The hybrid cooling option reduces water consumption by 50% (as was defined by the analysis) 

and allows for turbine efficiency and annual generation to remain relatively unchanged. 

However, this design has the highest installed cost and operational complexity. For plants with 

storage, the hybrid plant LCOE falls roughly halfway between the wet and dry plant LCOE, 

whereas there is little LCOE advantage in hybrid cooling for plants without storage. When TOD

factors influence revenue, the impact to LCOE is more complex. This issue will be considered in

a later section.

WorleyParsons also considered a plant with the Las Vegas TMY2 climate file that had a nominal 

125 MW capacity with 4 hours of storage (Table 5). This design was representative of a 

proposed project in Nevada. Unlike the 100-MW Las Vegas cases above, but consistent with the 

Daggett study, this design was considered under a constant-capacity assumption. The percent 

increase in installed cost and annual generation were very similar to those estimated for the 

Daggett case. The increase in LCOE was only 3.4%, which is much lower than that of the other 

dry-cooled Las Vegas cases (LCOE was not estimated for Daggett). 
16 

Table 5. Comparison of 125-MW wet- and dry-cooling cases for Las Vegas, NV study with 

constant-capacity design assumption.

Parameter (4 h Storage Case) 100% WetCooled 100% Dry-Cooled 

Total Installed Cost ($) 997,000,000 1,081,000,000 +8.4%

Gross Turbine Efficiency at Design 0.378 0.358 -5.3%

Net Plant Output at Design (MW) 125 125 - 

Annual Generation (MWh) 426,710 438,790 +2.8%

LCOE (¢/kWh) * 18.0 18.6 +3.4%

* See Appendix B for financial assumptions

Comparing the Las Vegas cases, WorleyParsons was asked to quantify why the LCOE increase 

for the dry-cooled plant was 3.4% for the 125-MW/4 h TES case (Table 5) and 6.3% for the ~93-

MW/6 h TES case (Table 4). Upon reviewing the data, WorleyParsons reported that several 

small effects combined to cause most of the difference: 

1. Estimated O&M cost savings for the switch from wet to dry cooling were higher for the 

125-MW plant. 

2. Based on turbine curves, the annual average cycle efficiency of the 125-MW dry-cooled 

plant was slightly greater than that of the smaller dry-cooled plant, while the converse 

was true for the two wet-cooled designs. This produced a greater annual penalty for the 

dry-cooled design for the smaller plants. 

3. Turbines and cooling systems scale with an exponent less than 1, while solar field scales 

linearly. This acts to reduce the influence of power block capital cost differential between 

larger wet- and dry-cooled plants. 

4. The plants with more storage ran at their design capacity for a slightly greater fraction of 

the year. Dry-cooled plants experience more pronounced parasitic loads at design and 

thus averaged a slightly lower annual efficiency. 

5. Lastly, the 125-MW plant analysis was carried out prior to the 100-MW cases. A more 

sophisticated algorithm was used in the latter cases, which is believed to have generated 

slightly better results. 

In summary, the differences between the wet- and dry-cooling cases are subtle enough that slight 

changes in turbine selection and analysis approach can yield what appear to be significant 

changes in the relative costs. Based on the other cases and the SAM analysis shown later, the 

6.3% differential is believed to be more representative of true costs. 

Alamosa, Colorado Site

The case for Alamosa, Colorado is significantly different from the other two sites because of its 

colder, high-altitude climate. The dry-bulb temperature rarely exceeds 30°C in Colorado’s San 

Luis Valley (see Figure 8), suggesting good potential for efficient dry cooling. The 

WorleyParsons results support this conclusion. Dry cooling is effective, and the hybrid-cooled 

system (when modeled) is rarely called into service. Under these conditions, it is unlikely that 

the benefits of the hybrid system justify its additional complexity. 
17 

Table 6. Comparison of wet-, hybrid-, and dry-cooling cases for the Alamosa, Colorado study.

Parameter (6 h Storage Case) 100% WetCooled Hybrid Cooling 100% Dry-Cooled 

Site Improvements ($) 34,501,000 19,616,000 -43% 18,483,000 -46%

Solar Field & HTF System ($) 408,587,000 408,587,000 0% 408,587,000 0%

Power Plant & Cooling System 

($) 106,559,000 141,601,000 +33% 136,348,000 +28%

Total Installed Cost ($) 816,524,000 838,653,000 +2.7% 831,798,000 +1.9%

Gross Turbine Efficiency at 

Design 0.3858 0.3837 -0.5% 0.3772 -2.2%

Net Plant Output atDesign 

(MW) 100 98.57 -1.4% 97.16 -2.8%

Annual Generation (MWh) 367,602 363,219 -1.2% 361,778 -1.6%

LCOE (¢/kWh) * 17.3 17.8 +2.9% 17.7 +2.3%

* See Appendix C for financial assumptions

Dry cooling in the Alamosa climate adds only 2% to 3% to the installed cost and LCOE for the 

trough plant. Despite a weaker solar resource, the estimated installed cost and LCOE for 

Alamosa are lower than those of Las Vegas due to the lower labor rates associated with the 

Colorado location. 

Water Consumption

Figure 9 highlights a comparison of water consumption by the different plant designs considered 

by WorleyParsons. Switching to dry cooling reduces water consumption by more than 90% as it 

eliminates the cooling tower. In very hot climates, dry-cooled systems may require a small 

amount of cooling water to supply low-temperature cooling for turbine components, but this 

represents less than 0.3% of the original cooling water consumption. Steam-cycle water 

consumption increases slightly in dry-cooled plants due to the lower plant efficiency, but these 

effects are overwhelmed by eliminating the cooling tower. 

The average water consumption per generation for the wet-cooled plants is 3.5 m3

/MWh; for the 

dry-cooled plants, it is 0.3 m3/MWh. These values can be compared to water consumption at 

other wet-cooled Rankine cycle plants: about 2.2 m3

/MWh for coal and 3.2 m3/MWh for nuclear, 

assuming closed-loop cooling towers with onsite evaporation ponds [8]. Compared to these 

plants, water consumption for the trough plants is higher due to lower cycle efficiency and more 

frequent startup and off-design operation. Lacking any specific site data, WorleyParsons’ 

assumed a fixed cooling tower cycles of concentration (COC) and water cost for all locations. 

COC depends on water quality and variations in COC will affect overall water consumption. 

Water consumption for hybrid-cooled plants is a direct function of the operating strategy of the 

plant. Greater use of the wet-cooling tower leads to higher plant efficiency but higher water 

consumption. In the Las Vegas case, the hybrid plant was arbitrarily designed to reduce water 

consumption by half, and this strategy resulted in virtually no change in annual generation—
18 

albeit at a higher capital cost. In the Alamosa case, the hybrid system was hardly used at all 

because of effective dry cooling. 

Figure 9. Estimated water consumption for the 13 cases. 

While water use by utility-scale CSP is an important consideration in plant design, it is 

interesting to compare CSP with other land-intensive activities. Figure 10 compares utility-scale 

solar plants to Southwestern agricultural products and golf courses in terms of water 

consumption [9, 10]. The wet-cooled CSP range includes parabolic trough and power tower 

technologies. Power towers are at the low end of the given range due to their higher thermal 

efficiency compared to trough plants. Dry-cooled CSP includes troughs, towers, and dish/engine 

systems. PV plants use water for panel washing only. In a relative sense, “growing megawatts” 

uses much less water than growing other commodities. Despite this favorable comparison, all 

water in the Southwest is precious, and all users should strive to minimize their consumption. 
19 

Figure 10. Water consumption by different land-intensive activities in the Southwest.

Analysis Using Solar Advisor Model (SAM)

NREL’s Solar Advisor Model (SAM) is designed to run hourly simulations covering a full year. 

SAM was updated in 2010 with a new parabolic trough simulation that incorporates a power 

block model based more on physical properties and less on empirical curve fits, thereby allowing 

for better simulation of dry-cooling than previous SAM models. In addition, utility TOD rates 

are incorporated into the latest release of SAM. These new features were employed to examine 

the impact of dry cooling on LCOE when TOD rates are in effect. 

LCOE is strongly dependent on the financial and incentive assumptions employed by the analyst, 

so it is critical to know these assumptions when comparing results from different studies. The 

assumptions used in SAM for the analyses in this report are summarized in Table 7. 

Table 7. Financial assumptions used in SAM analysis.

Analysis Period (years) 30 US Investment Tax Credit 30%

Inflation Rate 2.5% Federal Depreciation MACRS

Real Discount Rate 8% Contingency on Direct Costs 10%

Composite Income Tax

Rate 40.2% Indirect Costs incl. Sales Tax 24.7% 

Annual Insurance 0.5% Debt Fraction 42%

Loan Term (years) 20 Minimum Return on Equity 15%

Loan Rate 8% Minimum Debt Service Coverage 

Ratio 1.4 

Debt fraction was set at 42%, a value that minimizes LCOE based on the financial assumptions 

with the 30% ITC. Lastly, direct costs for SAM analyses follow those for the NREL reference 

plant as provided in reference [6], Table 3. 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

CSP 

(wet_cooled)

CSP 

(dry_cooled)

PV Alfalfa Cotton Fruit Trees Golf Courses

Acre-ft / acre per year
20 

NREL created SAM cases for each of WorleyParsons’ analyses; a summary comparison is 

presented in Table 8. The SAM case files used for this report were set up to match the 

WorleyParsons assumptions as closely as possible—for example, by matching solar field area, 

storage hours, gross turbine capacity and efficiency, and reference conditions. It is important to 

note that the turbine design-point efficiency is a function of design-point ambient temperature, 

approach (or ITD), and reference direct normal radiation. When running SAM simulations, these 

parameters must be considered a set and should not be independently changed. The SAM code 

was used to estimate generation, water consumption, and LCOE. 

On average, SAM underestimates generation by about 5% compared to WorleyParsons’ results. 

Agreement is best for the Daggett cases (within about 1%), while the Alamosa and no-storage 

Las Vegas cases disagree by about 9%. Variation between the two modeling approaches is not 

unexpected, and the SAM model appears to be slightly more conservative.

SAM’s water use estimates for the wet-cooled Daggett and Las Vegas plants are within 3% of 

WorleyParsons’ (normalized by estimated generation). SAM’s estimate for the wet-cooled 

Alamosa plant is about 12% higher than that of WorleyParsons. SAM consistently 

underestimates WorleyParsons’ water consumption for the dry-cooled plants by about 15% on 

average. Water estimates for the dry-cooled plants are dominated by mirror washing and steamcycle makeup. Mirror wash rates are subjective, so some variation is expected. The assumption 

used in this SAM analysis was 0.6 liters of wash water per square meter of solar field and an 

annual wash interval of five days (73 washes per year). WorleyParsons’ values for Daggett were 

0.81 L/m2

 and 63 washes per year. The steam-cycle blowdown fraction was set at 0.023. Drycooled plant water use would match WorleyParsons’ value more closely if the SAM defaults for 

mirror washing and steam-cycle blowdown were increased. Hybrid cooling cases were not 

modeled with SAM. 

The SAM cases predict that switching from wet to dry cooling would raise LCOE by 4.4% to 

5.7% depending on location and plant design. The range estimated by WorleyParsons was 2.5% 

to 7.5%. The low end of the range corresponds to the cooler Alamosa location. 
21 

Table 8. Summary of the WorleyParsons (WP) results and SAM estimates. SAM values were obtained using the solar field area, turbine 

size and efficiency, and design-point temperatures from the WorleyParsons analyses.

Notes:

• Daggett design turbine efficiencies are estimated by NREL and not provided by WorleyParsons. 

• SAM generation values use a 100% plant availability to be consistent with WorleyParsons’ assumptions. 

• SAM allows 105% turbine operation during TOD period 1 (although TOD periods vary by utility, TOD 1 normally 

corresponds to summer afternoons). 

• WorleyParsons’ LCOE values do not include owner’s project costs or incentives, but assume a higher solar field cost than 

used by NREL in SAM (see [6]). 

Location

Daggett 

TMY3

Daggett 

TMY3

Las Vegas 

TMY2

Las Vegas 

TMY2

Las Vegas 

TMY2

Las Vegas 

TMY2

Las Vegas 

TMY2

Las Vegas 

TMY2

Las Vegas 

TMY2

Las Vegas 

TMY2

Alamosa 

TMY3

Alamosa 

TMY3

Alamosa 

TMY3

Net Capacity (MW) 103 103 100 98.71 93.11 100 98.92 92.82 125 125 100 98.57 97.16

Turbine Gross Capacity (MW) 118 120.5 109.01 108.26 103.38 114.42 113.62 108.57 142.2 145.1 113.9 113.27 111.34

Cooling wet dry wet hybrid dry wet hybrid dry wet dry Wet Hybrid Dry

Storage (hrs) 6.3 6.3 0 0 0 6 6 6 4 4 6 6 6

Solar Field Area (m2) 987,540 1,062,750 562,440 562,440 562,440 931,950 931,950 931,950 1,013,700 1,088,910 905,790 905,790 905,790

Design Dry-bulb Temp (C) - 42.2 43.6 43.6 43.6 43.6 - 43.6 29.3 29.3

Design Wet-bulb Temp (C) 22 - 23 23 23 23 23.0 - 15.7 15.7

Design ITD or Approach (C) 5.6 14 5.6 14 5.6 14 5.6 14 5.6 14

Design Turbine Efficiency 0.3753 0.3552 0.3771 0.3745 0.3576 0.377 0.3744 0.3577 0.378 0.358 0.3858 0.3837 0.3772

WP design point parasitics (MW) 15 17.6 17.2 20.1

SAM design point parasitics (MW) 14 18 15 21

WP annual Generation (MWh) 426,717 438,790 252,055 248,309 240,942 396,062 395,070 377,000 426,710 438,790 367,602 363,219 361,778

SAM annual Generation (MWh) 421,000 433,000 228,000 - 219,000 377,000 - 360,000 419,000 429,000 339,000 - 329,000

WP annual water use (m3) 1,530,000 114,000 935,000 460,000 75,000 1,470,000 709,000 110,000 1,634,000 128,000 1,134,000 111,000 95,000

SAM annual water use (m3) 1,541,000 100,000 832,000 - 52,000 1,354,000 - 85,000 1,496,000 100,000 1,170,000 - 79,000

WP LCOE (cents/kWh) - - 18.62 19.81 20.12 18.36 18.94 19.52 17.96 18.57 17.27 17.78 17.72

SAM LCOE (Real, no TOD) 14.4 15.1 14.5 - 15.4 15.2 - 16.0 14.7 15.6 16.5 - 17.3

SAM LCOE (Bid Price, SCE TOD) 12.8 13.5 12.4 - 13.0 13.5 - 14.3 12.8 13.6 14.6 - 15.2

SAM LCOE (Bid Price, PG&E TOD) 13.5 14.2 13.6 - 14.4 14.3 - 15.0 13.5 14.3 15.5 - 16.2
22 

SAM and WorleyParsons agree that for a constant-capacity design, a dry-cooled plant will 

produce more annual energy than a wet-cooled plant. Figure 11 illustrates how a dry-cooled 

plant can generate more energy over the course of a year. The data are taken from SAM 

simulations of the constant-capacity Daggett cases. The top chart shows a summer day when the 

plant is running at design-point conditions on a hot afternoon. Note that the dry plant efficiency 

is more affected than the wet plant by the high temperature of the afternoon. As intended by the 

constant-capacity design, generation on this date is virtually identical: 1806 MWh for the wet 

plant and 1803 MWh for the dry plant. In contrast, on a March date when ambient temperatures 

do not approach the design points, dry plant efficiency is only slightly impacted by daily 

temperature. On this date, the dry plant output of 1573 MWh exceeds the wet plant output of 

1552 MWh. Because the design-point temperatures occur only a few days per year, the dry plant 

produces slightly more energy over the course of the year. 

Figure 11. SAM simulations of a summer day at design point (top) and a spring day at lower 

ambient temperatures (bottom) for wet- and dry-cooled plants. Constant-capacity design is 

assumed, i.e., the dry plant has a larger turbine and solar field to accommodate the lower cycle 

efficiency.

0.30

0.32

0.34

0.36

0.38

0.40

0.42

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Power Block Gross Cycle Efficiency

Plant Net Output (MW)

July Day - Design Point Conditions

Wet Plant Net Output (MW)

Dry Plant Net Output (MW)

Wet Plant Cycle Efficiency

Dry Plant Cycle Efficiency

0.30

0.32

0.34

0.36

0.38

0.40

0.42

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Power Block Gross Cycle Efficiency

Plant Net Output (MW)

March Day

Wet Plant Net Output (MW)

Dry Plant Net Output (MW)

Wet Plant Cycle Efficiency

Dry Plant Cycle Efficiency
23 

Impact of Time-of-Delivery (TOD) Rates on Dry-Cooled Plant Revenue

Many utilities offer varying rates for power based on when the power is produced. SAM allows 

users to input these TOD schedules and dispatch generation based on them. By tracking energy 

generation during each TOD period and applying TOD allocation factors, one can determine the 

revenue associated with each TOD period. Because several California utilities offer favorable 

TOD allocation factors for summer afternoons, solar plants in those locations receive a 

disproportionate amount of their revenue during the summer. 

While summer-weighted TOD rates are generally favorable for solar plants, the relative penalty 

for dry cooling may be exacerbated by heavily weighted peak generation rates. To examine this 

possibility, SAM was used to compare the change in generation and revenue for plants in 

Daggett and Alamosa when switched from wet to dry cooling. The 2009 TOD rate schedules for 

Southern Cal Edison (SCE) and Pacific Gas & Electric (PG&E) were applied. The results 

presented in Table 8 show the change in generation by TOD periods due to the switch to a drycooled plant for an SCE schedule, which represents the most extreme TOD rates. For Alamosa, 

annual revenue falls 0.15% more than annual generation, while for the more challenging climate 

of Daggett, annual revenue lags annual generation by 0.43%. The observed differences were 

lesser for the PG&E TOD schedule. In the SAM simulations, the steam turbine is allowed to run 

at up to 105% of its design rating during TOD period 1 (summer weekday afternoons). 

Overdesign operation during this period allows the plant to maximize production—and 

revenue—during periods of highest demand. When overdesign operation was not allowed, the 

TOD schedules had a greater impact on relative revenue, but the difference was still less than 

0.5%. 

Table 9. Change in generation by TOD period and the resulting impact on annual revenue 

resulting from the switch from wet to dry cooling, assuming Daggett and Alamosa climates using 

the TOD schedule for SCE. The Daggett cases assume constant capacity while the Alamosa cases 

assume constant heat input design (See Table 2).

Daggett, CA

Alamosa, 

CO

TOD 

Period

TOD 

Allocation 

Factor

Generation 

Change (%)

Generation 

Change (%)

1 3.13 1.04 -3.28

2 1.35 2.14 -3.06

3 0.75 2.15 -0.94

4 1.00 3.26 -3.20

5 0.83 4.26 -3.39

6 0.61 No gen. No gen.

Annual Generation 2.79% -2.87%

Annual Revenue 2.36% -3.02%

Figure 12 depicts the relative impact of changing cooling method and TOD schedule for the 

Alamosa and Daggett cases. On this figure, a value of 1.0 corresponds to the estimated annual 

generation or revenue for the wet-cooled plant with no TOD rates. For Alamosa, a dry-cooled 

plant of the same solar field size experiences a drop in annual generation due to its lower 
24 

efficiency. With no TOD rates, the drop in revenue exactly matches the drop in generation. Two 

points are significant: (1) in all cases, the TOD schedules result in an increase in revenue by 

favoring afternoon generation that coincides with solar availability, and (2) dry-cooling has 2.9% 

lower revenue versus wet cooling with or without TOD rates. 

The Daggett plant conclusions are similar, but a slight additional penalty is seen in the drycooled case with TOD rates for this climate. Whereas the revenue increase is 2.8% without TOD 

rates, it ranges from only 2.4% (SCE) to 2.6% (PG&E) with TOD rates. For both studies, the 

SAM runs allow 105% turbine operation during TOD 1; the dry-cooling penalty would be 

slightly greater without this assumption. 

Figure 12. Relative generation or revenue as a function of cooling type and TOD schedule for 

Alamosa (top) and Daggett (bottom) cases. The scale is relative to the wet-cooled, no TOD, values

estimated by SAM. 

1.000 1.000

1.289

1.224

0.971 0.971

1.251

1.189

0.800

0.900

1.000

1.100

1.200

1.300

1.400

Generation Revenue: No TOD Rates Revenue: SCE Rates Revenue: PG&E Rates

Relative Gen. or Revenue

Alamosa, CO location (constant heat input design assumption)

Wet Dry

1.000 1.000

1.267

1.209

1.028 1.028

1.297

1.240

0.800

0.900

1.000

1.100

1.200

1.300

1.400

Generation Revenue: No TOD Rates Revenue: SCE Rates Revenue: PG&E Rates

Relative Gen. or Revenue

Daggett, CA location (constant capacity design assumption)

Wet Dry
25 

Conclusions

Water consumption for electric power generation is undergoing increasing scrutiny, and more 

emphasis is being placed on low-water-use technologies. For Rankine-cycle power plants such as 

those used in parabolic trough and power tower CSP systems, dry cooling offers the potential to 

reduce water consumption by over 90%. Hybrid cooling (i.e., parallel wet/dry) uses water during 

hot periods when dry cooling is less efficient to trade some water use for increased generation 

efficiency. WorleyParsons analyzed 13 different cases covering three different geographic 

locations and wet, dry, and hybrid cooling technologies to assess the performance, cost, and 

water use impacts of the transition from a wet to a dry or hybrid cooling system. NREL 

developed SAM cases for each scenario to allow for hourly modeling and provide comparison of 

SAM and WorleyParsons model results. The analysis led to the following conclusions: 

• Two general design approaches can be employed when comparing alternative cooling 

systems: maintaining a constant plant capacity or maintaining a constant heat input 

(i.e., constant solar field size). The former is more representative of how developers 

and utilities would compare plant designs, while the latter better isolates the impact 

that the cooling system has on plant performance.

• For a constant-capacity plant assumption, 100% dry cooling increases capital cost by 

approximately 8%. The designer must balance the cost of the ACC versus the 

additional solar field, storage, and turbine capacities that are needed to maintain 

design-point power output. Due to the relatively high cost for additional solar field, a 

larger ACC (smaller ITD) than typical is recommended. WorleyParsons utilized an

ITD of 14°C in these analyses. While the general ACC sizing rule is valid, studies 

using different solar field and ACC costs are optimized at higher ITD values [11]. 

• Constant-capacity designs yield an increase in annual generation for the dry-cooled 

case. This increase results from the good performance of the larger solar field and 

turbine during periods of lower dry-bulb temperature. 

• The switch to dry cooling in the constant-capacity Daggett case caused no change in 

plant land usage. The larger solar field size was offset by elimination of the cooling 

tower evaporation ponds. Land area was not estimated for the other cases.

• The switch to dry cooling reduces annual O&M costs because maintenance costs for 

the ACC are less than the costs of water and water treatment for the wet cooling 

system. 

• For a constant heat input assumption, 100% dry cooling increases capital cost 

approximately 2% to 4%, primarily due to the more expensive cooling system. Under 

this assumption, annual generation of the dry-cooled plant is about 2% to 5% lower 

than for the wet-cooled plant. 

• For climates with dry-bulb temperatures exceeding 40°C, such as southern California 

and Nevada, WorleyParsons estimates the switch from wet to dry cooling increases 

the LCOE by approximately 3.5% to 7.5%. The increase for the relatively cool 

Colorado climate was only 2.5%. Thermal energy storage helped reduce the dry 

cooling penalty. 
26 

• SAM estimates the percent increase in LCOE from the switch from wet to dry cooling 

to be 4.5% to 5.7%, with the low end corresponding to the Alamosa location. 

• Hybrid cooling economics looked best for the Las Vegas case with storage and had 

no benefit for the Alamosa case. It is likely that application of TOD rates will 

improve hybrid cooling economics, although this was not evaluated in the current 

study (see [11]). 

• TOD schedules had only a minor impact on the economics of dry cooling. 

In summary, these findings indicate that switching from 100% wet to 100% dry cooling will 

result in LCOE increases of approximately 3% to 8% for parabolic trough plants throughout 

most of the Southwest. In all cases, the transition to dry cooling will reduce water consumption 

by over 90%. The remaining water consumption is split between steam-cycle blowdown and 

mirror washing. Hybrid cooling can reduce the LCOE increase, but at higher capital cost. A more 

detailed analysis of hybrid cooling is provided in [11].

References

[1] SolarPACES project lists available at http://www.nrel.gov/csp/solarpaces/ 

[2] Energy Demands on Water Resources, Report to Congress on the Interdependency of Energy 

and Water, U.S. Department of Energy, December 2006. 

[3] Concentrating Solar Power Commercial Application Study: Reducing Water Consumption of 

Concentrating Solar Power Electricity Generation, Report to Congress, U.S. Department of 

Energy, 2009. 

[4] Kelly, B., “Nexant Parabolic Trough Solar Power Plant Systems Analysis, Task 2:

Comparison of Wet and Dry Rankine Cycle Heat Rejection,” Subcontract Report, NREL/SR550-40163, July 2006. 

[5] Wet & Dry Cooling Options for a 250 MW Solar Thermal Plant, PG&E Concentrating Solar 

Power Engineering Services, Prepared by: WorleyParsons Group, Inc., 2006. 

[6] Turchi, C., “Parabolic Trough Reference Plant for Cost Modeling with the Solar Advisor 

Model (SAM),” Technical Report, NREL/TP-550-47605, July 2010. 

[7] Sioshansi, R., and P. Denholm, “The Value of Concentrating Solar Power and Thermal 

Energy Storage,” National Renewable Energy Laboratory, NREL-TP-6A2-45833, February 

2010. 

[8] Klara, J., “Cost and Performance Baseline for Fossil Energy Plants,” DOE/NETL-2007/1281, 

Volume 1: Bituminous Coal and Natural Gas to Electricity, Revision 1, August 2007.; Water 

Requirements for Existing and Emerging Thermoelectric Plant Technologies, DOE/NETL402/080108, April 2009. 

[9] Blaney, H.F., “Monthly Consumptive Use of Water by Irrigated Crops and Natural 

Vegetation,” General Assembly of International Union of Geodsy and Geophysics, Toronto, 

Canada, 1957. 
27 

[10] Consumptive Use of Water by Major Crops in the Southwestern United States, United 

States Department of Agriculture, ARS Conservation Research Report 29, May 1982. 

[11] Wagner, M.J., and C.F. Kutscher, “Assessing the Impact of Heat Rejection Technology on 

CSP Plant Revenue, SolarPACES 2010, Perpignan, France, September 21-24, 2010. 

Acknowledgements

The authors wish to thank the staff of WorleyParsons Group for their assistance with the original 

reports and subsequent questions. Engineers who assisted with the work included Ryan Bowers, 

Gary Aron, Dylan Ross, Robert Pieksma, and Rodney Gartner. 
28

Appendix A

Analysis of Wet, Dry, and Parallel Condensing

Parabolic Trough Power Plants with Fixed Solar Heat Input

WorleyParsons Group

NREL-2-ME-REP-0003

October 19, 2009
Analysis of Wet, Dry, and Parallel 

Condensing Parabolic Trough Power Plants 

with Fixed Solar Heat Input 

NREL Task 2, Subtask 1&2 Draft Report

NREL-2-ME-REP-0003 Rev 0 

 Prepared for:

 National Renewable Energy Lab

Prepared by:

WorleyParsons Group, Inc.

1687 Cole Blvd, Suite 300

Golden, Colorado 80401 USA

October 19, 2009

29
NOTICE

The information presented in this document was compiled and interpreted 

exclusively for the purposes stated in the document introduction. WorleyParsons 

provided this report for NREL solely for the purpose noted above.

WorleyParsons has exercised reasonable skill, care, and diligence to assess the 

information acquired during the preparation of this report, but makes no 

guarantees or warranties as to the accuracy or completeness of this information. 

The information contained in this report is based upon, and limited by, the 

circumstances and conditions acknowledged herein, and upon information 

available at the time of its preparation. The information provided by others is 

believed to be accurate but cannot be guaranteed.

WorleyParsons does not accept any responsibility for the use of this report for 

any purpose other than that stated in the document introduction and does not 

accept responsibility to any third party for the use in whole or in part of the 

contents of this report. Any alternative use, including that by a third party, or any 

reliance on, or decisions based on this document, is the responsibility of the 

alternative user or third party.

No part of this publication may be reproduced, stored in a retrieval system or 

transmitted, in any form or by any means, electronic, mechanical, photocopying, 

recording or otherwise, without the prior permission of WorleyParsons.

Any questions concerning the information or its interpretation should be directed 

to Rod Gartner, Project Manager. 

30
TABLE OF CONTENTS

1. INTRODUCTION ............................................................................................................................ 4

2. DESIGN ASSUMPTIONS AND METHOD ..................................................................................... 4

3. CAPITAL COSTS ......................................................................................................................... 13

4. PERFORMANCE.......................................................................................................................... 15

5. WATER CONSUMPTION............................................................................................................. 25

6 LEVELIZED COST OF ELECTRICITY ANALYSIS..................................................................... 28

7 CONCLUSIONS AND RECOMMENDATIONS............................................................................ 29

31
4 

1. INTRODUCTION

The United States Department of Energy’s National Renewable Energy Laboratory (NREL) has 

elected WorleyParsons to determine the relative economic differences between similar 

Concentrating Solar Power (CSP) parabolic trough plants with varying cooling and storage 

designs. The base case is a 100 MWe net with a 100% wet condensing system while two 

alternate cases, having the same solar field size, utilize a 100% dry condensing system and a 

parallel wet/dry condensing system. These three cases are evaluated with 6 hours of thermal 

energy storage (TES) and no storage. Two sets of solar data were evaluated for the selected 

Nevada Test Site near Las Vegas. 

The goal of this study is to assist CSP plant developers in selecting an appropriate condensing

system for their project based on water availability and levelized cost of electricity (LCOE). For 

each design case, WorleyParsons will provide overnight direct capital and reoccurring operations 

and maintenance (O&M) costs based on a preliminary engineering design effort which altogether 

yields a relative estimate accuracy of ±30%.

2. DESIGN ASSUMPTIONS AND METHOD 

2.1 Ambient Conditions

Heat balance modeling requires design and off-design ambient conditions. The design conditions 

(see Table 2.1 below) are used to physically size the Rankine cycle equipment whereas the offdesign conditions are used to model the performance of the plant while varying weather and 

turbine load. The design temperatures selected are the highest monthly 2% frequency dry and 

wet bulb temperatures for July and August. Historically these temperatures were exceeded 2% of 

the time. 

Table 2.1.1 Design conditions for Nellis Air Force Base, NV (2005 ASHRAE Handbook)

Parameter Units Value

Elevation Above Sea Level ft 1880

Standard Atmospheric Pressure psia 13.725

Design Dry Bulb Temperature °F 109.8

Design Wet Bulb Temperature °F 72.8

Off-design ambient temperatures are extracted from a Class A Typical Metrological Year (TMY) 2 

data file for Las Vegas, NV (WBAN No. 23169). This dataset provided hourly dry/wet bulb 

temperatures and solar radiation for a complete year which were ultimately used in performance 

modeling to arrive at the results. The application of the off-design temperatures is discussed later 

in the report. The 8760 hour dry bulb temperature data ranged from 24°F to 112°F. The 

temperature range across which SAM simulated operation of the steam turbine ranged from 38°F 

to 112°F. 

The figures 2.1.1 through 2.1.3 are scatter plots of 8760 hour dry bulb and coincident wet bulb 

temperatures. Figure 2.1.1 and 2.1.2 include overlays of plots of the hours during which the 

steam turbine is operating for the designs with and without Thermal Storage. Figure 2.1.3 

includes an overlay of a plot of when the solar field is exporting heat.

32
5 

Figure 2.1.1 Coincident power block operation with No TES depicted.

Dry Bulb & Coincident Wet Bulb Temperatures

Power Block Operation Shown For 100MW net CSP Trough Plant

Las Vegas, NV (TMY2 Data)

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Dry Bulb Temperature (F)

Coincident Wet Bulb Temperature (F)

Unfiltered

Pwr Blk Oper

33
6 

Figure 2.1.2 Coincident power block operation with 6 hours of TES depicted.

Dry Bulb & Coincident Wet Bulb Temperatures

Power Block Operation Shown For 100MW net CSP Trough Plant with 6 hrs TES

Las Vegas, NV (TMY2 Data)

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Dry Bulb Temperature (F)

Coincident Wet Bulb Temperature (F)

Unfiltered

Pwr Blk Oper

34
7 

Figure 2.1.3 Coincident solar field operation with 6 hours of TES depicted.

Dry Bulb & Coincident Wet Bulb Temperatures

Solar Field Operation Shown For 100MW net CSP Trough Plant 

Las Vegas, NV (TMY2 Data)

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Dry Bulb Temperature (F)

Coincident Wet Bulb Temperature (F)

Unfiltered

Solar Field Oper

35
8 

Figure 2.1.4 illustrates the number of hours and the percent occurrence of 5 degree interval dry 

bulb temperatures from the 8760 hourly data. 

Figure 2.1.4 

Annual Hours & Frequency of Dry Bulb Temperatures at 5 Degree Ranges

Las Vegas, Nevada, USA

0

100

200

300

400

500

600

700

800

900

1000

1100

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Hours of Annual Occurance (hours)

0%

1%

2%

3%

4%

5%

6%

7%

8%

9%

10%

11%

Frequency of Annual Occurance (%)

Hours

Frequency

36
9 

Figure 2.1.5 illustrates the number of hours of 5 degree interval dry bulb temperatures from the 

8760 hourly data and the average coincident wet bulb temperature associated with each dry bulb 

interval.

Figure 2.1.5 

Annual Hours of Dry Bulb and Average Coincident Wet Bulb Temperatures

in 5 Degree Ranges

Las Vegas, Nevada, USA

0

100

200

300

400

500

600

700

800

900

1000

1100

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Hours of Annual Occurance (hours)

0

10

20

30

40

50

60

70

Coincident Wet Bulb, °F

Hours

Coincident Wet Bulb

2.2 Wet Condensing 

The wet condensing system consists of a steam surface condenser, circulating water pumps, an 

induced draft counter-flow cooling tower, and an underground & aboveground interconnecting 

pipe network. This type of condensing system allows for the lowest steam turbine operating back 

pressure and efficiency, at the expense of increased water consumption. The wet condensing

system was modeled to utilize these advantages and was not configured with a higher 

backpressure similar to that of the dry systems to conserve water. The intent is to illustrate the 

impact of the increased performance of a wet condensing system on water consumption. The 

operation of the 100% wet condensing system was modeled to reduce steam turbine 

backpressure and maximize output as the ambient temperatures dropped rather than to minimize 

water consumption by operating the steam turbine at a higher back pressure. The cooling tower 

operation was modeled with 4 cycles of concentration. Without having site specific information, it 

is assumed that no pre or post water treatment is needed. 

37
10

2.3 Dry Condensing 

The dry condensing alternative utilizes an air cooled condenser (ACC) to cool the exhaust steam 

using a large array of fans that force air over finned tube heat exchangers. The heat is rejected 

directly to the atmosphere and no external water supply is needed for condensing the steam 

cycle exhaust steam. The initial temperature difference (ITD) is defined as the difference between 

the ambient air temperature at the design point and the steam condensation temperature within

an ACC. The smaller the ITD, the more aggressive the design resulting in better steam turbine 

generator (STG) backpressure but at a higher capital and fan power consumption cost. An ITD of 

25 °F was used in this study as a preliminary investigation suggests that this design is close to 

optimal in terms of cost vs. net plant generation for the proposed CSP plant. The operation of a 

100% dry condensing system was modeled to minimize condensing pressure and maximize 

steam turbine output as ambient temperatures decreased.

2.4 Parallel Condensing 

The parallel condensing system is a combination of wet and dry condensing systems. The steam 

turbine exhaust branches near the turbine exit and a duct runs to each condensing system. The 

steam flow naturally splits in proportion to the available condensing capacity of each system at 

the time. A parallel system is more expensive than a wet system and can be more expensive than 

a dry system depending on the design capacity of the parallel system. The goal of this study was 

to maximize the performance of the plant so the same size ACC was used for the parallel system 

as was used on the all dry system. The wet condensing portion is approximately 90% that of the 

all wet design. This large capacity wet portion allows the use of an efficient low back pressure 

steam turbine and the large ACC allows the wet system and corresponding water consumption to 

be curtailed as soon as possible with falling dry bulb temperatures. The operation of the parallel 

system was modeled to maximize performance at the expense of water consumption. The 

advantage of this system is that it can be operated with minimal water consumption at the 

expense of power production. 

2.5 Solar Field Designs

Two solar field sizes were fixed throughout this study. One solar field size collects approximately 

930 MWth of solar energy to output 100 MWe net to the grid with a 100% wet condensing 

Rankine power cycle with 6hrs of thermal energy storage. The other solar field collects 561 MWth 

for the same wet condensing design but without thermal energy storage. These same two solar 

field designs were used to evaluate the performance of the 100% dry and parallel condensing 

designs with and without thermal storage. Thus the heat input to steam cycles was the same for 

all the condensing designs utilizing thermal storage. Likewise the heat input to the steam cycles

without thermal storage was the same for the three condensing designs but different than that for 

the designs with thermal storage.

2.6 Sizing Criteria

The steam turbine exhaust sections for all three condensing designs were optimized to minimize 

the exhaust losses at full load conditions. The steam turbine in the wet and parallel condensing 

designs, which operates at a lower back pressure, utilized an exhaust design with a larger 33.5” 

last stage blade length and corresponding annulus area than the dry condensing design which 

had the least exhaust losses with a smaller 26” last stage blade. The steam turbine inlet 

temperature and pressure was the same for all 6 designs. This modeling was done based on 

38
11

WorleyParsons’ long term power plant experience and knowledge base, utilizing available steam 

turbine technology, but not tied to specific vendor design data.

The criteria in the following tables were used to size and estimate the costs of the three

condensing systems for both the 6 hour thermal storage designs and the non thermal storage 

designs. 

Table 2.6.1 Rankine cycle sizing criteria at design conditions for plants with TES. 

Parameters Units Wet Parallel Dry

Steam Turbine Exhaust Enthalpy Btu/lb 988.97 992.47 1018.8 

Steam Turbine Exhaust Flow lb/hr 705,841 707,905 722,293 

Steam Turbine Exhaust Back Pressure Inches HgA 1.28 1.40 2.53 

Air Cooled Condenser Duty MMBtu/hr n/a 65 662 

Cooling Tower Duty MMBtu/hr 646 584 n/a

Circulating Water Flow Rate gpm 64,500 58,200 n/a

Cooling Tower Approach °F 10 10 n/a

Cooling Tower Range °F 20 20 n/a

Condenser Terminal Temperature Difference 

(TTD) °F 7 10.5 n/a

Table 2.6.2 Rankine cycle sizing criteria at design conditions for plants with no TES. 

Parameters Units Wet Parallel Dry

Steam Turbine Exhaust Enthalpy Btu/lb 988.73 992.66 1018.82 

Steam Turbine Exhaust Flow lb/hr 672,420 674,480 688,092 

Steam Turbine Exhaust Back Pressure Inches HgA 1.28 1.40 2.53 

Air Cooled Condenser Duty MMBtu/hr n/a 62 631 

Cooling Tower Duty MMBtu/hr 616 556 n/a

Circulating Water Flow Rate gpm 61,400 55700 n/a

Cooling Tower Approach °F 10 10 n/a

Cooling Tower Range °F 20 20 n/a

Condenser Terminal Temperature Difference 

(TTD) °F 7 10.1 n/a

39
12

2.7 Method

Using the criteria given above, budgetary vendor cost and performance quotes for the cooling 

tower, ACC, surface condenser and steam turbine were obtained in order to determine impact on 

performance, capital cost, auxiliary loads, water consumption, and ultimately LCOE. 

The performance portion of this study is necessary to arrive at a LCOE. Net plant output and 

water consumption are the primary performance inputs to an LCOE model. These parameters 

were estimated using four different calculation tools which ultimately were driven by three inputs: 

ambient dry bulb, ambient wet bulb, and steam cycle heat input. 

Hourly ambient dry/wet bulb temperatures were obtained from the TMY2 weather dataset for Las 

Vegas NV, which also provided direct normal insolation (DNI) data used in NREL’s Solar Advisor 

Model (SAM). This SAM software provided the steam cycle heat input, also referred to as thermal 

energy to the power block (Q_PB), as well as thermal energy to storage (Q_to_ts) and thermal 

energy from storage (Q_from_ts). GateCycle was used to model the Rankine cycle behavior and 

initially determined the plant’s design point conditions. A unique GateCycle model was

developed for the three condensing types. Several off-design heat balance models were 

independently run varying dry-bulb temperature, wet-bulb temperature, and steam cycle heat 

input. Model results were compiled and numerically fit into a three-variable interpolative lookup 

functions using Microsoft Excel tools to arrive at the various plots and tables presented herein. 

Using this all-inclusive Excel spreadsheet, the inputs and equations were used to obtain cooling 

tower water makeup for every hour of a typical year. This same methodology was used to 

produce steam turbine electric gross output and Rankine cycle parasitic loads per hour. Solar 

system parasitic loads (i.e. HTF pumps, TES pumps, SCA drives, etc.) were calculated based on 

hourly heat input to the steam cycle, and heat input/output from the thermal energy storage (TES) 

system. The HTF pumps and TES pumps were assumed to be variable speed or variable 

frequency driven.

The results of the evaluation are presented in the following sections.

40
13

3. CAPITAL COSTS 

Capital costs have been determined using a combination of vendor budgetary proposals and

WorleyParsons’ equipment, commodity, and installation labor database. The capital costs are 

within a +/- 30% confidence range based on a conceptual engineering effort. 

The results illustrate the differences in capital cost between the wet, dry, and parallel condensing 

designs. The solar field effective mirror aperture area increased from 562,440 m2 without thermal 

storage to 931,950 m2 with thermal storage. The thermal energy storage, solar field civil-site 

work, balance of plant mechanical/electrical, HTF system, electrical, instrumentation/controls and 

all other cost items which makeup a complete CSP plant were adjusted as necessary in each 

design to accommodate the condensing system and thermal storage impacts.

The thermal energy storage equipment cost is based on a turnkey budgetary quote from the 

single commercially available salt storage vendor. An alternative cost savings approach would be 

to estimate the storage system from the ground up and compile vendor quotes for each subcomponent (tanks, pumps, HX, etc.). 

NREL has selected a 2.0 solar multiple. The solar multiple has a significant capital cost impact 

and is subject to the project developer's financial model.

Cost reflects NREL's selected 150-meter trough design. This trough is the most proven design 

with the most utility-scale installations; however, the associated materials and labor costs are 

higher than alternative emerging designs (i.e. 100-meter trough, or SkyFuel’s SkyTrough) 

Labor rates are union-based for Las Vegas, Nevada with a productivity factor of 1.2. 

Alternatively, merit-shop based labor rates can significantly reduce costs. 

3.1 Vender Quotes

WorleyParsons’ obtained budgetary quotes for the cooling tower, surface condenser, and aircooled condenser. All other equipment and materials included in the makeup of a complete CSP 

trough plant were priced based on WorleyParsons’ extensive archive of past vendor quotes and 

previously constructed projects.

The following tables are a summary of the complete cost analysis showing the line items that 

build up the overall total installed capital cost for the 3 condensing options different options with 

and without thermal storage. 

41
14

Table 3.1.1 Estimated capital cost summary for wet, parallel, and dry condensing plants with 6 hours of Thermal 

Storage. 

Case 100% Wet Parallel 100% Dry

Description 100% Wet-Cooled Parallel Case 100% Dry Cooled

Site Improvements $ 30,941,000 $ 24,442,000 $ 21,816,000 

Solar Field $ 434,392,000 $ 434,392,000 $ 434,392,000 

HTF System $ 87,503,000 $ 87,503,000 $ 87,503,000 

Thermal Energy Storage $ 187,100,000 $ 187,100,000 $ 187,100,000 

Fossil Backup $ - $ - $ - 

Power Plant $ 121,997,000 $ 162,820,000 $ 151,913,000 

EPCM Costs $ 29,001,000 $ 29,001,000 $ 29,001,000 

Project, Land, Misc. $ - $ - $ - 

%DC's Sales Tax Applies $ - $ - $ - 

Subtotal $ 890,934,000 $ 925,258,000 $ 911,725,000 

Contingency $ 70,189,000 $ 71,585,000 $ 70,360,000 

TOTAL INSTALLED COST $ 961,123,000 $ 996,843,000 $ 982,085,000

Table 3.1.2 Estimated capital cost summary for wet, parallel, and dry condensing plants with No Thermal 

Storage.

Case 100% Wet Parallel 100% Dry

Description 100% Wet-Cooled Parallel Case 100% Dry Cooled

Site Improvements $ 23,979,000 $ 19,365,000 $ 16,723,000 

Solar Field $ 264,585,000 $ 264,585,000 $ 264,585,000 

HTF System $ 48,367,000 $ 48,367,000 $ 48,367,000 

Thermal Energy Storage $ - $ - $ - 

Fossil Backup $ - $ - $ - 

Power Plant $ 120,949,000 $ 160,680,000 $ 150,147,000 

EPCM Costs $ 29,001,000 $ 29,001,000 $ 29,001,000 

Project, Land, Misc. $ - $ - $ - 

%DC's Sales Tax Applies $ - $ - $ - 

Subtotal $ 486,881,000 $ 521,998,000 $ 508,823,000 

Contingency $ 41,259,000 $ 42,833,000 $ 41,596,000 

TOTAL INSTALLED COST $ 528,140,000 $ 564,831,000 $ 550,419,000 

42
15

4. PERFORMANCE

This section provides data tables and plot illustrations of the performance results for all cases. 

Fixing the solar heat input to the steam cycle for the base and alternate cases allowed for a more 

direct comparison of the costs and performance. 

For the designs with thermal storage, SAM sends the solar energy to storage until there is 

enough energy to operate the steam turbine at or near full load, after which the energy is sent to 

the power block. If thermal storage is not present, SAM operates the steam turbines at lower load 

points to avoid wasting energy. This is illustrated in Figures 4.0.1 and 4.0.2 below which shows 

the operating hours of the steam turbines at various load points. The steam turbine minimum 

operating load without thermal storage is 20 MWe whereas with thermal storage it is 70 MWe

(Note:The parallel operation is very close to the wet operation and is not visible on these small 

charts). 

Figure 4.0.1

Annual Hours of Net Generation at 5MWe Ranges

100MWe Nominal Net CSP Trough Plant with No TES

Nevada, USA 

0

500

1000

1500

2000

2500

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105

Net Electric Generation (MWe)

Operation within Net Generation Range (hours)

Wet Condensing

Dry Condensing

Parallel Condensing

43
16

Figure 4.0.2

Annual Hours of Net Generation at 5MWe Ranges

100MWe Nominal Net CSP Trough Plant with 6 Hrs TES

Nevada, USA 

0

500

1000

1500

2000

2500

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115

Net Electric Generation (MWe)

Operation within Net Generation Range (hours)

Wet Condensing

Dry Condensing

Parallel Condensing

4.1 Performance Results

Tables 4.1.1 and 4.1.2 summarizes the performance results for the wet, dry, and parallel

condensing designs with and without thermal storage. 

Table 4.1.1 Nevada Site Performance Summary with 6 hrs of thermal storage.

Performance Results Wet Parallel Dry

Solar Input to Collector Field (MWth) 930.45 930.45 930.45

Design Steam Cycle Thermal Input (MWth) 303.49 303.49 303.49

Design Gross Steam Turbine Output (MWe) 114.42 113.62 108.57

Design Plant Parasitic Losses (MWe) 14.42 14.70 15.75

Plant Net Output (MWe) at Design Conditions 100 98.92 92.82

Design Gross Steam Turbine Efficiency (%) 37.70 37.44 35.77

STG Gross Annual Generation (MWe-hrs/yr) 429,320 427,190 411,927

Plant Net Annual Generation (MWe-hrs/yr) 396,062 395,070 377,000

Annual Backfeed Electricity (MWe-hrs/yr) 3,283 3,283 3,283

44
17

Table 4.1.2 Nevada Site Performance Summary with no thermal storage.

Performance Results Wet Parallel Dry

Solar Input to Collector Field (MWth) 561.25 561.25 561.25

Design Steam Cycle Thermal Input (MWth) 289.08 289.08 289.08

Design Gross Steam Turbine Output (MWe) 109.01 108.26 103.38

Design Plant Parasitic Losses (MWe) 9.01 9.55 10.27

Plant Net Output (MWe) at Design Conditions 100 98.71 93.11

Design Gross Steam Turbine Efficiency (%) 37.71 37.45 35.76

STG Gross Annual Generation (MWe-hrs/yr) 252,055 248,309 240,942

Plant Net Annual Generation (MWe-hrs/yr) 231,459 229,041 219,840

Annual Backfeed Electricity (MWe-hrs/yr) 2,052 2,052 2,052

Charts 4.1.1 through 4.1.4 illustrate annual net generation distribution as function of ambient dry 

bulb temperature. The plant net generation shown in these plots represents the electric energy 

produced for export to the grid and does not include plant auxiliary power that may be purchased 

from the grid when the plant is offline. 

Charts 4.1.5 and 4.1.6 illustrate the plant’s average net output as a function of the ambient dry 

bulb temperature. SAM’s default Southern California Edison (SCE) dispatch structure was used 

in all cases with TES. The trends recognized in the plots below are entirely driven by SAM’s use 

of the storage dispatch profile.

45
18

Figure 4.1.1 Net Energy production at different ambient temperatures for different cooling technologies without Thermal Storage

Annual Electric Energy Generation at 5 Degree Dry Bulb Temperature Ranges

100MWe Nominal Net Design CSP Trough Plant with No TES

Nevada, USA

0

5

10

15

20

25

30

35

40

45

50

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Generated Electric Power (MWe-hrs E+03)

Wet Net Gen

Dry Net Gen

Parallel Net Gen

46
19

Figure 4.1.2 Net Energy production at different ambient temperatures for different cooling technologies with 6 hrs of Thermal 

Storage

Annual Electric Energy Generation at 5 Degree Dry Bulb Temperature Ranges

100MWe Nominal Net Design CSP Trough Plant with 6 Hrs TES

Nevada, USA

0

5

10

15

20

25

30

35

40

45

50

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Generated Electric Power (MWe-hrs E+03)

Wet Net Gen

Dry Net Gen

Parallel Net Gen

47
20

Figure 4.1.3 Percentage of Net Energy production at different ambient temperatures for different cooling technologies without 

Thermal Storage

Fraction of Annual Net Generation Occuring at 5 Degree Dry Bulb Temperature Ranges

100We Net Nominal CSP Trough Plant with No TES

Nevada, USA

0%

2%

4%

6%

8%

10%

12%

14%

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Fraction of Annual Net Generation (MWe-hr/MWe-hr)

Wet Net Generation

Dry Net Generation

Parallel Net Generation

48
21

Figure 4.1.4 Percentage of Net Energy production at different ambient temperatures for different cooling technologies with 6 

hrs of Thermal Storage.

Fraction of Annual Net Generation Occuring at 5 Degree Dry Bulb Temperature Ranges

100We Net Nominal CSP Trough Plant with 6 Hrs TES

Nevada, USA

0%

2%

4%

6%

8%

10%

12%

14%

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Fraction of Annual Net Generation (MWe-hr/MWe-hr)

Wet Net Generation

Dry Net Generation

Parallel Net Generation

49
22

Figure 4.1.5 Average Net Plant output at different ambient temperatures for different cooling technologies with 6 hrs of 

Thermal Storage.

Average Electric Power Generation at 5 Degree Dry Bulb Temperature Ranges

100MWe Nominal Net CSP Trough Plant with 6 Hrs TES

Nevada, USA

0

20

40

60

80

100

120

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Average Electric Power Generation (MWe)

Wet Gross Pwr

Dry Gross Pwr

Parallel Gross Pwr

50
23

Figure 4.1.6 Average Net Plant output at different ambient temperatures for different cooling technologies with no Thermal 

Storage.

Average Electric Power Generation at 5 Degree Dry Bulb Temperature Ranges

100MWe Nominal Net CSP Trough Plant with No TES

Nevada, USA

0

20

40

60

80

100

120

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Average Electric Power Generation (MWe)

Wet Gross Pwr

Dry Gross Pwr

Parallel Gross Pwr

51
24

4.2 Performance Discussion

Plants with a 100% wet condensing design will typically have a more efficient steam cycle across 

all load points and ambient temperatures. Moreover, this is especially true in dry climates with 

high ambient temperatures such as Nevada. A cooling tower’s performance is governed by the 

wet bulb temperature, whereas a dry cooling system’s capabilities are dictated by the dry bulb 

temperature. The 2% maximum design dry bulb for this Nevada site is 109.8°F whereas the wet 

bulb is only 72.8°F, providing a large performance advantage to wet condensing. On these hot 

days, a wet system is able to condense the steam turbine exhaust at a lower temperature, 

pressure, and enthalpy, resulting in more power extracted from the steam flow. The wet system 

was able to achieve a steam cycle efficiency of 37.7% at the design conditions, whereas the dry 

system could only achieve 35.8% even with a very large ACC designed with an aggressive ITD of 

25.

For the same duty at fully capacity, an ACC will always have higher auxiliary loads than a wet 

system due to its larger number of fans. This further reduces the net plant output of the dry plant 

on hot days compared to a dry cooled plant. However, the dry bulb temperature drops faster than 

the wet bulb temperature with a reduction in overall ambient temperatures. As a result, the 

auxiliary loads of a dry condensing system drop quicker than those of wet system with the 

reduction in ambient temperatures.

These net output differences between the dry and wet cooled plants can be seen on figures 4.1.5 

and 4.1.6. As the ambient temperatures decrease this difference is reduced because the delta 

between the wet and dry bulb temperatures is also reduced. Due to the inherent exhaust losses 

in the ACC ducting system, the dry cooled system can never achieve as low a steam turbine 

exhaust pressure as the wet cooled system. 

Since a wet system is able to maintain a more consistent turbine exhaust pressure across the 

operating ambient temperature range, the exhaust velocities are also more constant. This allows 

a wet condensing system to operate closer to the peak efficiency point on the steam turbine 

exhaust loss curve more often than a dry condensing system.

The objective of a parallel system is to combine the hot day generating performance of the wet

system with the water conserving benefits of the dry system on cooler days. As shown by the 

results herein, the wet design generates 5% more annual MWe-hrs than the dry, but less than 1% 

more than that of the parallel system. In other words the parallel cooled plant has the ability to 

generate nearly as much annual power as a 100% wet cooled plant while only consuming half the 

amount of water. 

Figures 4.1.5 and 4.1.6 illustrates the different operating modes of a plant with and without 

thermal storage. With the thermal storage the steam turbine is not started until there is enough 

heat to operate at or near full load. Without thermal storage the turbine is operated whenever 

sufficient heat is available run at minimum load and on cooler days there is not enough to operate 

at full load. 

52
25

5. WATER CONSUMPTION

This analysis compares the water consumption between the wet and dry designs on an annual 

consumption basis. The three-variable equation discussed in Section 2.5 was used to arrive at an 

annual cooling tower makeup flow for the wet condensing system. Other water consumers 

considered in this study include mirror washing, steam cycle makeup, and WSAC makeup (dry 

condensing system only). 

Without having site specific information, raw water quality was assumed sufficient for cooling 

tower use with 4 cycles of concentration and therefore no pre or post water treatment is included. 

Cooling tower blowdown is discharged to onsite evaporation ponds. Electrostatic deionization and 

multimedia filtration equipment are included in the design to treat the raw water and produce 

demineralized (demin) water for steam cycle makeup and solar collector mirror washing. The 

water consumption table below identifies the water quality required by each consumer. Note that 

the demin water system rejects wastewater from the reverse osmosis process and backwash 

water from the multimedia filtration process. These reject quantities are included in the demin 

water consumption values below.

5.1 Cooling Tower

Cooling tower makeup is the largest user of water, primarily consuming it through evaporation 

and secondarily by blowdown rejection. Evaporation is a function of the cooling load and wet bulb 

temperatures. Blowdown is based on water quality (which assumes 4 cycles of concentration), 

evaporation, and cooling tower drift. The annual water consumption of the cooling towers was 

calculated as describe above. The circulating water system also provides the heat sink for the 

closed cooling water system that cools the auxiliary plant equipment. 

5.2 Air Cooled Condenser

Utilizing a dry condensing system consumes minimal water compared to a wet system. The ACC 

is a closed loop system not having evaporation or blowdown. Steam cycle makeup is assumed to 

be slightly higher for the dry condensing system than the wet because obtaining optimal cycle 

water chemistry is more difficult with ACCs and therefore more blowdown is needed during startup.. In the absence of a cool water source (i.e. cooling tower) a fin-fan cooler is needed to cool 

plant auxiliary equipment. In addition, a supplemental WSAC is used in parallel with the fin-fan 

cooler when ambient dry-bulb temperatures exceeds 85° F, as direct dry air cooling can not 

provide the 105° F auxiliary cooling temperature required by steam turbine lube oil, feedwater 

pumps, and the sample panel coolers. The WSAC, like a cooling tower, requires makeup water to 

replace blowdown, drift, and evaporation. The water consumption from the WASC has been 

included in the analysis. 

53
26

Table 5.2.1 Estimated Annual Water Consumption with Thermal Storage (Acre-Feet/Year)

Water Consumers (water quality) Wet 

Condensing

Parallel 

Condensing

Dry 

Condensing

Cooling Tower Makeup (Raw) 1,112 496 0

Steam Cycle Operating Makeup (Demin) 21 21 21

Steam Cycle Makeup at Startup (Demin) 13 13 19

Mirror Wash Water (Demin) 46 46 46

Wet Surface air Cooler Makeup (Raw) 0 0 3

Totals 1,192 575 89

Table 5.2.2 Estimated Annual Water Consumption without Thermal Storage (Acre-Feet/Year)

Water Consumers (water quality) Wet 

Condensing

Parallel 

Condensing

Dry 

Condensing

Cooling Tower Makeup (Raw) 705 320 0

Steam Cycle Operating Makeup (Demin) 13 13 13

Steam Cycle Makeup at Startup (Demin) 13 13 19

Mirror Wash Water (Demin) 27 27 27

Wet Surface air Cooler Makeup (Raw) 0 0 2

Totals 758 373 61

Figure5.2

Estimated Annual Water Consumption

Fixed Solar Field Size for 100MW (net) CSP Trough Plant with/without TES

Las Vegas, NV 

0

200

400

600

800

1,000

1,200

Wet, 6hrs TES Dry, 6hrs TES Parallel, 6hrs TES Wet, No TES Dry, No TES Parallel, No TES

Cooling Design Cases

Est. Annual Water Consumption (Ac-Ft/Yr)

Mirror Wash

Startup

BD Quench

CT Makeup

Note: WSAC makeup values are too small to display

54
27

5.3 Water Consumption Discussion

Approximately 93% percent of the wet condensing plant’s annual water consumption is used by 

the cooling tower. With steam cycle makeup being the roughly the same for all three condensing 

systems and the WSAC only contributing a small 3 Acre-Feet/Yr to the dry systems (not shown 

on the bar graph), the secondary contributor to the dry/wet difference is mirror wash consumption. 

Since the solar filed sizes were held constant for each of condensing systems, the mirror wash 

consumption only changes with the addition of thermal storage and its consequential larger solar 

field. Altogether the wet condensing plant consumes about 13 times that of the dry condensing 

plant under the design parameters of this study. Both the thermal storage and non thermal 

storage parallel designs were able to reduce the cooling tower water consumption by 

approximately 55% and reduce the overall plant water consumption by about 52%. This reduction 

was obtained by simulating the operation of the plant for maximum plant output. Further water 

consumption could be obtained by shifting more of the condensing load from the surface 

condenser to the ACC. This would result in a reduction of plant net output but nonetheless could 

be a valuable operating option. Annual water consumption contributes to the operations and 

maintenance cost as a plant consumable. This reoccurring cost will ultimately contribute to the 

final LCOE analysis discussed in the next section.

55
28

6 LEVELIZED COST OF ELECTRICITY ANALYSIS

The levelized cost of electricity was determined by calculating the net present value of the 

nominal capital and operations and maintenance costs per year and discounting them back to the 

present. The net present value was them divided by the annual net output per year to determine 

the cost per MW–hr. The annual (O&M) costs are escalated 2009 dollars. The levelized annual 

costs and LCOEs are nominal dollars as the effects of inflation are not included in this analysis. 

The capital costs are assumed to occur entirely in 2009 and the annual costs are assumed to 

start with the plant operation in year 2013. The LCOE is based on the plant operating for 30 years 

through 2042. Other items such as capital financing, construction financing, taxes, renewable 

credits, financial incentives, debt ratios, depreciation, loan periods, etc. were not included as they 

were considered to be project and company specific. The intent of this analysis was to provide a 

technical LCOE based on the data in the tables below that would allow a comparison of the 

performance and costs of the various designs. Further economic considerations can be added to 

this data to obtain an economic or financial analysis for a specific project within a specific 

company. 

Table 6.0 Levelized cost of electricity model inputs and output summary.

Discount rate: 5.25%

Water Cost $/acre-ft $450

Commercial Operation Date: 2013

Economic Operating Life: 30

Plant O&M cost nominal escalation: 1.50%

Designs with 6 hr TES Wet Parallel Dry

Plant Output (MW/yr) 396,062 395,070 377,000

Total Capex $961,123,000.00 $996,843,000.00 $982,085,000.00

Annual O&M Cost $11,333,902.00 $11,167,145.00 $10,889,096.00

NPV $1,086,559,740.50 $1,117,947,041.45 $1,099,671,773.95

Nominal Levelized Yearly 

Cost ($72,709,246) ($74,809,588) ($73,586,663)

Nominal LCOE ($/MW-hr) ($183.58) ($189.36) ($195.19)

Designs without TES Wet Parallel Dry

Plant Output (MW/yr) 231,459 229,041 219,840

Total Capex $528,140,000.00 $564,831,000.00 $550,419,000.00

Annual O&M Cost $9,292,383.00 $9,233,312.00 $9,035,758.00

NPV $643,944,622.77 $677,901,800.21 $661,186,634.55

Nominal Levelized Yearly 

Cost ($43,090,800) ($45,363,110) ($44,244,582)

Nominal LCOE ($/MW-hr) ($186.17) ($198.06) ($201.26)

56
29

The wet condensing designs had the lowest LCOEs as they had the lowest capital costs and the 

highest annual energy production. The parallel designs while having the highest capital costs, 

had LCOEs between that of the Wet and the Dry designs due to the fact that the energy 

production of the parallel plants was close to that of the wet condensing plants. The relatively low 

annual generation of the dry condensing designs resulted in them having the highest LCOEs of 

all the condensing designs. This is all based on a $450/acre-foot water cost. As the cost of water 

rises or the quality of water diminishes the dry cooled plant LCOE will approach the wet cooled 

plant and in some extreme cases it will be less.

7 CONCLUSIONS AND RECOMMENDATIONS

The LCOE’s for all the designs are very close and well within the +/- 30% tolerance of this study. 

In a hot dry climate such a Nevada, it is expected that wet condensing designs would have better 

economic results with the cost of water used. In a cooler more humid climate the differences 

would be smaller and the dry designs may even have the lowest LCOEs. A parallel condensing 

plant is more complicated to operate but with that comes operational flexibility. This study 

simulated the operation of the parallel plant to maximize net power output. It could be operated to 

conserve more water and in doing so the performance and costs would approach that of the all 

dry plant. The relative LCOEs of these designs are good guidelines of what to expect from the 

different condensing technologies for the site evaluated and other sites with similar climates. 

More in depth analysis based on specific project criteria could be expected to yield increased 

efficiencies and lower LCOEs for any of the designs.

57
Appendix B

Analysis of Wet and Dry Condensing

125 MW Parabolic Trough Power Plants

WorleyParsons Group

NREL-2-ME-REP-0002

September 29, 2009

58
Analysis of Wet and Dry Condensing 

125 MW Parabolic Trough Power Plants 

NREL-2-ME-REP-0002 Rev 0 

 Prepared for:

 National Renewable Energy Lab

Prepared by:

WorleyParsons Group, Inc.

1687 Cole Blvd, Suite 300

Golden, Colorado 80401 USA

September 29, 2009

59
NOTICE

The information presented in this document was compiled and interpreted 

exclusively for the purposes stated in the document introduction. 

WorleyParsons provided this report for NREL solely for the purpose noted 

above.

WorleyParsons has exercised reasonable skill, care, and diligence to assess 

the information acquired during the preparation of this report, but makes no 

guarantees or warranties as to the accuracy or completeness of this 

information. The information contained in this report is based upon, and 

limited by, the circumstances and conditions acknowledged herein, and upon 

information available at the time of its preparation. The information provided 

by others is believed to be accurate but cannot be guaranteed.

WorleyParsons does not accept any responsibility for the use of this report 

for any purpose other than that stated in the document introduction and does 

not accept responsibility to any third party for the use in whole or in part of 

the contents of this report. Any alternative use, including that by a third party, 

or any reliance on, or decisions based on this document, is the responsibility 

of the alternative user or third party.

No part of this publication may be reproduced, stored in a retrieval system or 

transmitted, in any form or by any means, electronic, mechanical, 

photocopying, recording or otherwise, without the prior permission of 

WorleyParsons.

Any questions concerning the information or its interpretation should be 

directed to Rod Gartner, Project Manager. 

60
TABLE OF CONTENTS

1. INTRODUCTION.............................................................................................................................4

2. DESIGN ASSUMPTIONS AND METHOD......................................................................................4

3. CAPITAL COSTS .........................................................................................................................10

4. PERFORMANCE ..........................................................................................................................11

5. WATER CONSUMPTION .............................................................................................................16

6 LEVELIZED COST OF ELECTRICTY ANALYSIS.......................................................................18

61
4

1. INTRODUCTION

The United States Department of Energy’s National Renewable Energy Laboratory (NREL) 

has elected WorleyParsons to determine the relative economic differences between a 125

MWe net Concentrated Solar Power (CSP) parabolic trough plant with 100% wet condensing 

and 100% dry condensing systems. The solar data is based on a Nevada site near Las 

Vegas and assumes 4 hours of thermal energy storage (TES) for both designs. This will allow 

future plant developers to determine the impact of site water availability and its associated 

condensing system options on plant levelized cost of electricity (LCOE). For each design 

case, WorleyParsons has provided relative overnight capital and fixed/variable operating & 

maintenance (O&M) costs based on a preliminary engineering design effort which altogether 

yields a relative estimate accuracy of ±30%. 

2. DESIGN ASSUMPTIONS AND METHOD 

2.1 Ambient Conditions

Heat balance modeling requires design and off-design ambient conditions. The design 

conditions (see Table 2.1 below) are used to physically size the Rankine cycle equipment 

whereas the off-design conditions are used to model the effects of the plant while varying 

weather and turbine load. The design temperatures selected for the 125MWe net plant are 

“Monthly Design” dry and wet bulb temperatures at a 2% frequency for July and August, 

respectively. Historically these temperatures were exceeded 2% of the time. 

Table 2.1 Design conditions for Nellis Air Force Base, NV (2005 ASHRAE Handbook)

Parameter Units Value

Elevation Above Sea Level ft 1880

Standard Atmospheric Pressure psia 13.725

Design Dry Bulb Temperature °F 109.8

Design Wet Bulb Temperature °F 72.8

Off-design ambient temperatures are extracted from a Class A Typical Metrological Year 

(TMY) 2 data file for Las Vegas, NV (WBAN No. 23169). This dataset provided hourly dry/wet 

bulb temperatures and solar radiation for a complete year which were ultimately used in 

performance modeling to arrive at the results. The application of off-design temperatures is 

discussed later in the report.

62
5

Figure 2.1.1 Coincident power block operation depicted.

Dry Bulb & Coincident Wet Bulb Temperatures

Las Vegas, NV (TMY2 Data)

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Dry Bulb Temperature (F)

Coincident Wet Bulb Temperature (F)

Unfiltered

Pwr Blk Oper

63
6

Figure 2.1.2 Coincident solar field operation depicted.

Dry Bulb & Coincident Wet Bulb Temperatures

Las Vegas, NV (TMY2 Data)

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Dry Bulb Temperature (F)

Coincident Wet Bulb Temperature (F)

Unfiltered

Solar Field Oper

64
7

Figure 2.1.3 

Annual Hours & Frequency of Dry Bulb Temperatures at 5 Degree Ranges

Las Vegas, Nevada, USA

0

100

200

300

400

500

600

700

800

900

1000

1100

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Hours of Annual Occurance (hours)

0%

1%

2%

3%

4%

5%

6%

7%

8%

9%

10%

11%

Frequency of Annual Occurance (%)

Hours

Frequency

Figure 2.1.4 

Annual Hours of Dry Bulb and Average Coincident Wet Bulb Temperatures

in 5 Degree Ranges

Las Vegas, Nevada, USA

0

100

200

300

400

500

600

700

800

900

1000

1100

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Hours of Annual Occurance (hours)

0

10

20

30

40

50

60

70

Coincident Wet Bulb, °F

Hours

Coincident Wet Bulb

65
8

2.2 Wet Condensing 

The wet condensing system consists of a steam surface condenser, circulating water pumps, 

an induced draft counter-flow cooling tower, and an underground & aboveground 

interconnecting pipe network. This type of condensing system allows for the lowest steam 

turbine operating back pressure and efficiency, at the expense of increased water 

consumption. The wet condensing system was modeled to utilize these advantages and was 

not configured with a higher backpressure similar to that of the dry systems to conserve 

water. The intent is to illustrate the impact of the increased performance of a wet condensing 

system on water consumption. The operation of the 100% wet condensing system was

modeled to reduce steam turbine backpressure and maximize output as the ambient 

temperatures dropped rather than to minimize water consumption by operating the steam 

turbine at a higher back pressure. The cooling tower operation was modeled with 4 cycles of 

concentration. Without having site specific information, it is assumed that no pre or post water 

treatment is needed. 

2.3 Dry Condensing 

The dry condensing alternative utilizes an air cooled condenser (ACC) to cool the exhaust 

steam using a large array of fans that force air over finned tube heat exchangers. The heat is 

rejected directly to the atmosphere and no external water supply is needed for condensing 

the steam cycle exhaust steam. The initial temperature difference (ITD) is defined as the 

difference between the ambient air temperature at the design point and the steam 

condensation temperature within an ACC. The smaller the ITD, the more aggressive the 

design resulting in better steam turbine generator (STG) backpressure but at a higher capital 

and fan power consumption cost. An ITD of 25 °F was used in this study as a preliminary 

investigation suggests that this design is close to optimal in terms of cost vs. net plant 

generation for the proposed CSP plant. The operation of a 100% dry condensing system was 

modeled to minimize condensing pressure and maximize steam turbine output as ambient 

temperatures decreased. 

2.4 Sizing Criteria

The criteria in the following table were used to size and estimate the costs of the two 

condensing systems. 

66
9

Table 2.4 Wet/Dry Design case sizing criteria.

Parameters Units Dry Wet

Steam Turbine Exhaust Enthalpy Btu/lb 1018.4 988.2 

Steam Turbine Exhaust Flow lb/hr 962946 874435 

Steam Turbine Exhaust Back Pressure Inches HgA 5.0 2.57 

Air Cooled Condenser Duty MMBtu/hr 882 n/a

Cooling Tower Duty MMBtu/hr n/a 801 

Circulating Water Flow Rate gpm n/a 80,000 

Cooling Tower Approach °F n/a 10 

Cooling Tower Range °F n/a 20 

Condenser Terminal Temperature Difference 

(TTD) °F n/a 7 

2.5 Method

Using the criteria given above, budgetary vendor cost and performance quotes for the cooling 

tower, ACC, surface condenser and steam turbine were obtained in order to determine 

impact on performance, capital cost, auxiliary loads, water consumption, and ultimately 

levelized costs of electricity. 

The performance portion of this study is necessary to arrive at a LCOE. Gross plant output, 

net plant output, and water consumption are the primary performance inputs to an LCOE 

model. These three parameters were estimated using four different calculation tools which 

ultimately were driven by three inputs: ambient dry bulb, ambient wet bulb, and steam cycle 

heat input. 

Hourly ambient dry/wet bulb temperatures were obtained from the TMY2 weather dataset for 

Las Vegas NV, which also provided direct normal insolation (DNI) data used in NREL’s Solar 

Advisor Model (SAM). This SAM software provided the steam cycle heat input, also referred 

to as thermal energy to the power block (Q_PB), as well as thermal energy to storage 

(Q_to_ts) and thermal energy from storage (Q_from_ts). GateCycle was used to model the 

Rankine cycle behavior and initially determined the plant’s design point conditions. A unique 

GateCycle model was developed for the two condensing types. Several off-design heat 

balance models were independently run varying dry-bulb temperature, wet-bulb temperature, 

and steam cycle heat input. Model results were compiled and numerically fit into a threevariable equation using MathCAD and Microsoft Excel Visual Basic tools to arrive at the 

various plots and tables presented herein. 

Using this all-inclusive Excel spreadsheet, the inputs and equations were used to obtain 

cooling tower water makeup for every hour of a typical year. This same methodology was 

used to produce steam turbine electric gross output and Rankine cycle parasitic loads per 

hour. Solar system parasitic loads (i.e. HTF pumps, TES pumps, SCA drives, etc.) were 

calculated based on hourly heat input to the steam cycle, and heat input/output from the 

thermal energy storage (TES) system.

The results of the evaluation are presented in the following sections.

67
10

3. CAPITAL COSTS 

Capital costs have been determined using a combination of vendor budgetary proposals and

WorleyParsons’ equipment, commodity, and installation labor database. The capital costs are 

within a +/- 30% confidence range based on a conceptual engineering effort. 

The results show a noticeable difference in capital cost between the wet and dry condensing 

designs. This difference is mainly attributed by the drop in cycle efficiency and increase in 

aux loads, altogether needing an increase in solar collectors to maintain a 125MW net output. 

The solar field effective mirror aperture area increased from 1,013,700 m2 (wet condensing) 

to 1,088,910 m2 (dry condensing). The thermal energy storage, solar field civil-site work, 

balance of plant mechanical/electrical, HTF system, electrical, instrumentation/controls and 

all other cost items which makeup a complete CSP plant were adjusted as necessary in each 

design to accommodate condensing system impacts. 

Although the increase in solar collectors contributes to the majority of cost increase, the 

larger HTF system and much larger cooling equipment are also major cost contributors. 

Differences in operating costs and plant performance are discussed later.

3.1 Vender Quotes

WorleyParsons’ obtained budgetary quotes for the cooling tower, surface condenser, and aircooled condenser. All other equipment and materials included in the makeup of a complete 

CSP trough plant were priced based on WorleyParsons’ extensive archive of past vendor 

quotes and previously constructed projects.

The following table is a summary of the complete cost analysis showing the line items that 

build up the overall total installed capital cost for the two different options. 

Table 3.1 Estimated capital and O&M cost summary for dry and wet condensing 125MWe net plants. 

COST ITEM Dry Condensing Wet Condensing

Total Installed Capital (EPCM Basis) $1.081 Billion $997.2 Million

Fixed & Variable O&M (% of Capital) $11.8 Million (1.09%) $12.2 Million (1.12%)

**labor rates based on union labor for Las Vegas, NV.

68
11

4. PERFORMANCE

Since both designs require 125 MWe net electricity to the grid at design conditions, the less 

efficient dry condensing system will require more solar collectors to achieve the same net 

output. The dry condensing system has a higher turbine back pressure driven by steam 

saturation temperature which is based on the condenser’s ITD and the site’s dry-bulb

temperature. This higher back pressure lowers the enthalpy delta that can be extracted from 

the low pressure (LP) steam turbine and therefore decreases the steam cycle efficiency. 

Increased gross power output is required to maintain net output; which is achieved by 

increasing steam flow and steam cycle heat input. Furthermore, because the ACC has 

significantly more fans than the cooling tower, the dry condensing system has higher auxiliary 

loads than the wet system; again requiring an increase in solar collectors. The dry 

condensing design also requires an air cooled heat exchanger (also known as fin-fan cooler) 

and a wet surface air cooler (WSAC) for auxiliary cooling, resulting in a further increase in 

auxiliary loads.

The steam turbine models were optimized in Gatecycle for the different operating conditions 

of each condensing design. SAM sends the solar energy to thermal storage until there is 

enough energy to operate the steam turbine at or near full load, after which the energy is sent 

to the power block. This is illustrated in Figure 4.0 below. Thus the steam turbine exhaust 

sections for both designs were optimized to minimize the exhaust losses at full load 

conditions. The steam turbine in the wet condensing design, which operates at a lower back 

pressure, utilized an exhaust design with a larger last stage blade length and annulus area 

than the design dry condensing design. This modeling was done based on WorleyParsons’ 

long term power plant experience and knowledge base, utilizing available steam turbine 

technology, but not tied to specific vendor design data.

69
12

Figure 4.0

Annual Hours of Net Generation at 5MWe Ranges

125MWe Net CSP Trough Plant with 4 Hrs TES

Nevada, USA 

0

200

400

600

800

1000

1200

1400

1600

1800

101520253035404550556065707580859095

100

105110

115120

125

130135140

145

0.0

Net Electric Generation (MWe)

Operation within Net Generation Range (hours)

Wet Cooled

Dry Cooled

4.1 Performance Results

Table 4.4 summarizes the performance analysis results for the wet and dry condensing 

designs. 

Table 4.1 Nevada Site Performance Summary

Performance Results Dry Condensing Wet Condensing

Solar Input to Collector Field (MWth) 1088 1012

Design Steam Cycle Thermal Input (MWth) 404.8 376.4

Design Gross Steam Turbine Output (MWe) 145.1 142.2

Design Plant Parasitic Losses (MWe) 20.1 17.2

Design Plant Net Output (MWe) 125 125

Design Gross Steam Turbine Efficiency (%) 35.8% 37.8%

Design Point Net Solar Use Efficiency (%) 11.49% 12.35%

Plant Net Annual Output (MWe-hrs/yr) 438,790 426,710

Plant Gross Annual Output (MWe-hrs/yr) 480,191 464,087

Annual Backfeed Electricity (MWe-hrs/yr) 3,792 3,757

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13

4.2 Performance Discussion

The wet condensing plant with the more efficient steam cycle produces less total net MWehrs on an annual basis than the dry condensing plant. As stated previously, the auxiliary 

loads of the dry design are larger than those of the wet design and therefore the gross steam 

turbine output must be larger to compensate for these loads. The auxiliary loads of dry 

cooling are a function of the dry bulb temperature as opposed to wet cooling loads which are 

a function of the wet bulb. The wet condensing designs can achieve a lower steam turbine 

back pressure because the wet bulb is lower than the dry bulb. However, the dry bulb 

temperature drops faster than the wet bulb temperature with a reduction in overall ambient 

temperatures. As a result, the auxiliary loads of a dry condensing system drop quicker than 

those of wet system with the reduction in ambient temperatures. The combination of a larger 

gross steam turbine output and auxiliary loads that are reduced quicker with falling ambient 

temperatures results in higher net output for the dry condensing designs on days with lower 

ambient temperatures. The higher output on cooler days translates into higher overall MWehrs on an annual basis.

The following charts illustrate the manner in which the SAM model dispatches a solar plant 

with Thermal Storage. The plant net generation in these charts is the energy produced for 

export to the grid and does not include plant auxiliary power that may be purchased from the 

grid when the steam turbine is not operating.

Figure 4.2.1

Fraction of Annual Net Generation Occuring at 5 Degree Dry Bulb Temperature Ranges

125MWe Net CSP Trough Plant with 4 Hrs TES

Nevada, USA

0%

2%

4%

6%

8%

10%

12%

14%

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Fraction of Annual Net Generation (MWe-hr/MWe-hr)

Wet Cooled

Dry Cooled

71
14

Figure 4.2.2

Annual Electric Energy Generation at 5 Degree Dry Bulb Temperature Ranges

125MWe Net Wet Cooled CSP Trough Plant with 4 Hrs TES

Nevada, USA

0

10

20

30

40

50

60

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Generated Electric Power (MWe-hrs E+03)

Gross STG Gen

Net Gen

72
15

Figure 4.2.3

Annual Electric Energy Generation at 5 Degree Dry Bulb Temperature Ranges

125MWe Net Dry Cooled CSP Trough Plant with 4 Hrs TES

Nevada, USA

0

10

20

30

40

50

60

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Dry Bulb Temperature (F)

Generated Electric Power (MWe-hrs E+03)

Gross STG Gen

Net Gen

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16

With thermal storage most of a solar plant’s operation is at or near full rated load. Because 

the plants are designed to output 125MWe at high ambient temperatures, At part load on the 

steam cycle side, when ambient temperatures are lower, the ACC performance is better than 

at high ambient (almost comparable to wet condensing performance). At part load (lower 

radiation but also lower dry bulb) the dry and wet plant cycle efficiencies (driven by back 

pressure) are much closer and the plant with the larger heat input produces more power. 

Altogether, dry plants generate more MWe-hrs per year because of their better part-load 

performance contributed by a larger mirror area and significantly better condensing

performance.

 

5. WATER CONSUMPTION

This analysis compares the water consumption between the wet and dry designs on an 

annual consumption basis. The three-variable equation discussed in Section 2.5 was used to 

arrive at an annual cooling tower makeup flow for the wet condensing system. Other water 

consumers considered in this study include mirror washing, steam cycle makeup, and WSAC 

makeup (dry condensing system only). 

Without having site specific information, raw water quality was assumed sufficient for cooling 

tower use with 4 cycles of concentration and therefore no pre or post water treatment is 

included. Cooling tower blowdown is discharged to onsite evaporation ponds. Electrostatic 

deionization and multimedia filtration equipment are included in the design to treat the raw 

water and produce demineralized (demin) water for steam cycle makeup and solar collector 

mirror washing. The water consumption table below identifies the water quality required by

each consumer. Note that the demin water system rejects wastewater from the reverse 

osmosis process and backwash water from the multimedia filtration process. These reject 

quantities are included in the demin water consumption values below.

5.1 Cooling Tower

Cooling tower makeup is the largest user of water, primarily consuming it through evaporation 

and secondarily by blowdown rejection. Evaporation is a function of the cooling load and wet 

bulb temperatures. Blowdown is based on water quality (which assumes 4 cycles of 

concentration), evaporation, and cooling tower drift. The annual water consumption of the 

cooling towers was calculated as describe above. The circulating water system also provides 

the heat sink for the closed cooling water system that cools the auxiliary plant equipment.

5.2 Air Cooled Condenser

Utilizing a dry condensing system consumes minimal water compared to a wet system. The 

ACC is a closed loop system not having evaporation or blowdown. Steam cycle makeup is 

assumed to be slightly higher for the dry condensing system than the wet because obtaining 

optimal cycle water chemistry is more difficult with ACCs and therefore more blowdown is 

needed during start-up.. In the absence of a cool water source (i.e. cooling tower) a fin-fan 

cooler is needed to cool plant auxiliary equipment. In addition, a supplemental WSAC is used 

in parallel with the fin-fan cooler when ambient dry-bulb temperatures exceeds 85° F, as 

direct dry air cooling can not provide the 105° F auxiliary cooling temperature required by 

steam turbine lube oil, feedwater pumps, and the sample panel coolers. The WSAC, like a 

74
17

cooling tower, requires makeup water to replace blowdown, drift, and evaporation. The water 

consumption from the WASC has been included in the analysis. 

Table 5.2 Estimated Annual Water Consumption (Acre-Feet/Year)

Water Consumers (water quality) Dry Condensing Wet Condensing

Cooling Tower Makeup (Raw) 0 1,242

Steam Cycle Operating Makeup (Demin) 18 18

Steam Cycle Makeup at Startup (Demin) 13 13

Mirror Wash Water (Demin) 64 52

Wet Surface air Cooler Makeup (Raw) 3 0

Totals 104 1,325

Figure 5.2

Water Consumption Comparison

125MW Net CSP Trough Plant with 4 Hrs Storage

Nevada, USA

CT Makeup

0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200 1,300

BD Quench

Startup

Mirror Wash

WSAC Makeup

Consumption Source

Water Consumption (AC-Ft/Yr)

Dry Cooled

Wet Cooled

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18

5.3 Water Consumption Discussion

Approximately 94% percent of the wet condensing plant’s annual water consumption is used 

by the cooling tower. With steam cycle makeup being the same for both condensing systems 

and the WSAC only contributing a small 3 Acre-Feet/Yr to the dry system, the secondary 

contributor to the dry/wet difference is mirror wash consumption. Mirror wash consumption is 

greater for the dry condensing plant due to its larger solar field. Altogether the wet 

condensing plant consumes more than 12 times that of the dry condensing plant under the 

design parameters of this study. Annual water consumption contributes to the operations and 

maintenance cost as a plant consumable. This reoccurring cost will ultimately contribute to 

the final LCOE analysis discussed in the next section.

6 LEVELIZED COST OF ELECTRICTY ANALYSIS

The levelized cost of electricity was determined by first determining the net present value of 

the capital and operations and maintenance cost per year and discounting them back to the 

present. The net present value was them divided by the annual output per year to determine 

the cost per MW –hr. The annual costs are in real 2009 dollars and escalated in real terms 

as shown in the spread sheet. The discount rate and inflation rate are inputs that can be 

easily modified. The capital cost was assumed to occur entirely in year 1. 

Table 6.0 Levelized cost of electricity model inputs and output summary.

Discount rate: 5.25%

Water Cost $/acre-ft $450

Fuel Escalation rate: 0.00%

Commercial Operation Date: 2013

Economic Life: 30

General Inflation: 1.00%

Plant O&M cost nominal escalation: 1.50%

Case: 4 Hr TES 100% wet 4 Hr TES 100% Dry

Plant Output (MW/yr) 426,710 438,790

Total capex $997,200,000.00 $1,081,063,000.00

NPV $1,161,122,832.77 $1,234,192,779.84

Nominal Levelized Yearly Cost ($76,648,858.75) ($81,472,403.59)

Levelized Cost of Electricity ($/MWhr) ($179.63) ($185.68)

76
Appendix C

Analysis of Wet, Dry, and Parallel Condensing

Parabolic Trough Power Plants with Fixed Solar Heat Input

San Luis Valley, Colorado Site

WorleyParsons Group

NREL-2-ME-REP-0004

May 10, 2010

77
Analysis of Wet, Dry, and Parallel 

Condensing Parabolic Trough Power 

Plants with Fixed Solar Heat Input 

San Luis Valley, Colorado Site 

NREL Task 2, MOD 2 Final Report - Colorado Site

NREL-2-ME-REP-0004 Rev 1 

 Prepared for:

 National Renewable Energy Lab

Prepared by:

WorleyParsons Group, Inc.

1687 Cole Blvd, Suite 300

Golden, Colorado 80401 USA

May 10, 2010

78
NOTICE

The information presented in this document was compiled and interpreted 

exclusively for the purposes stated in the document introduction. 

WorleyParsons provided this report for NREL solely for the purpose noted 

above.

WorleyParsons has exercised reasonable skill, care, and diligence to assess 

the information acquired during the preparation of this report, but makes no 

guarantees or warranties as to the accuracy or completeness of this 

information. The information contained in this report is based upon, and 

limited by, the circumstances and conditions acknowledged herein, and upon 

information available at the time of its preparation. The information provided 

by others is believed to be accurate but cannot be guaranteed.

WorleyParsons does not accept any responsibility for the use of this report 

for any purpose other than that stated in the document introduction and does 

not accept responsibility to any third party for the use in whole or in part of 

the contents of this report. Any alternative use, including that by a third party, 

or any reliance on, or decisions based on this document, is the responsibility 

of the alternative user or third party.

No part of this publication may be reproduced, stored in a retrieval system or 

transmitted, in any form or by any means, electronic, mechanical, 

photocopying, recording or otherwise, without the prior permission of 

WorleyParsons.

Any questions concerning the information or its interpretation should be 

directed to Ryan Bowers, Project Manager. 

79
TABLE OF CONTENTS

1. INTRODUCTION.............................................................................................................................4

2. DESIGN ASSUMPTIONS AND METHOD......................................................................................4

3. CAPITAL COSTS .........................................................................................................................11

4. PERFORMANCE ..........................................................................................................................12

5. WATER CONSUMPTION .............................................................................................................20

6 LEVELIZED COST OF ELECTRICITY ANALYSIS......................................................................23

7 CONCLUSIONS AND RECOMMENDATIONS ............................................................................24

80
4

1. INTRODUCTION

The United States Department of Energy’s National Renewable Energy Laboratory (NREL) 

has elected WorleyParsons to determine the relative economic differences between similar

Concentrating Solar Power (CSP) parabolic trough plants with varying cooling and storage 

designs. Similar to the previous cooling study for projects in Nevada, the purpose of this 

study is to understand the performance and cost impacts of cooling systems on parabolic 

trough solar power plants for projects in the San Luis Valley of Colorado. The base case is 

100 MWe net with a 100% wet condensing system while two alternate cases, having the 

same solar field size and thermal input to the steam cycle, utilize a 100% dry condensing 

system and a parallel wet/dry condensing system. These three cases are evaluated with 6 

hours of thermal energy storage (TES) where the storage system size is based on the 100% 

wet case and held constant across the other two cases. This methodology simplified the 

analysis, conversely the design net output could have been held constant and the solar field 

increased for the dry and parallel cases.

The goal of this study is to assist CSP plant developers in selecting an appropriate 

condensing system for their project based on water availability and levelized cost of electricity

(LCOE). For each design case, overnight direct capital and reoccurring operations and 

maintenance (O&M) costs will be estimated based on a preliminary engineering design effort 

which altogether yields an estimate accuracy of ±30%.

2. DESIGN ASSUMPTIONS AND METHOD 

2.1 Ambient Conditions

Heat balance modeling requires design and off-design ambient conditions. The design 

conditions (see Table 2.1.1 below) are used to physically size the Rankine cycle equipment 

whereas the off-design conditions are used to model the performance of the plant while 

varying weather and turbine load. The design temperatures selected are the highest monthly 

2% frequency dry and wet bulb temperatures for July and August. Historically these 

temperatures were exceeded 2% of the time. 

Table 2.1.1 Design conditions for Alamosa, CO (2005 ASHRAE Handbook)

Parameter Units Value

Elevation Above Sea Level ft 7,536

Standard Atmospheric Pressure psia 11.11

Design Dry Bulb Temperature °F 84.7

Design Wet Bulb Temperature °F 60.2

Off-design ambient temperatures are extracted from a Class II Typical Metrological Year 

(TMY) 3 data file for Alamosa, CO (WMO No. 724620), located in the San Luis Valley of 

Colorado. This dataset provided hourly dry/wet bulb temperatures and solar radiation for a 

complete year which were ultimately used in performance modeling to arrive at the results.

The application of the off-design temperatures is discussed later in the report. The 8760 hour 

dry bulb temperature data ranged from -26°F to 87°F. The temperature range across which 

NREL’s Solar Advisor Model (SAM) simulated operation of the steam turbine ranged from -

3°F to 87°F. 

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5

The figures 2.1.1 through 2.1.2 are scatter plots of 8760 hour dry bulb and coincident wet 

bulb temperatures with overlays of the hours during which the steam turbine operates and 

solar field exports heat.

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6

Figure 2.1.1 Dry bulb and coincident wet bulb temperatures during power block operation.

Dry Bulb & Coincident Wet Bulb Temperatures

Power Block Operation Shown For 100MW net CSP Trough Plant with 6 hrs TES

Alamosa, CO (TMY3 Data)

-30

-20

-10

0

10

20

30

40

50

60

70

-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Dry Bulb Temperature (F)

Coincident Wet Bulb Temperature (F)

8760 Hour Data

Power Block Oper

83
7

Figure 2.1.2 Dry bulb and coincident wet bulb temperatures during solar field operation.

Dry Bulb & Coincident Wet Bulb Temperatures

Solar Field Operation Shown For 100MW net CSP Trough Plant with 6 hrs TES 

Alamosa, CO (TMY3 Data)

-30

-20

-10

0

10

20

30

40

50

60

70

-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Dry Bulb Temperature (F)

Coincident Wet Bulb Temperature (F)

8760 Hour Data

Solar Field Oper

84
8

Figure 2.1.3 illustrates the number of hours of 5 degree interval dry bulb temperatures from 

the 8760 hourly data and the average coincident wet bulb temperature associated with each 

dry bulb interval.

Figure 2.1.3 Dry bulb and average coincident wet bulb frequency plot.

Annual Hours of Dry Bulb and Average Coincident Wet Bulb Temperatures

in 5 Degree Ranges

Alamosa, Colorado USA

0

100

200

300

400

500

600

700

800

900

1000

1100

-25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Dry Bulb Temperature (F)

Hours of Annual Occurance (hours)

-30

-20

-10

0

10

20

30

40

50

60

70

Coincident Wet Bulb, °F

Hours

Coincident Wet Bulb

2.2 Wet Condensing 

The wet condensing system consists of a steam surface condenser, circulating water pumps, 

an induced draft counter-flow cooling tower, and an underground & aboveground 

interconnecting pipe network. This type of condensing system allows for the lowest steam 

turbine operating back pressure and highest efficiency, at the expense of water consumption. 

The wet condensing system was modeled to utilize these advantages and was not configured 

to match the higher backpressure obtained with the dry condensing system. The intent is to 

illustrate the impact of the increased performance of a wet condensing system on water 

consumption. The operation of the 100% wet condensing system was modeled to reduce 

steam turbine backpressure and maximize output as the ambient temperature dropped rather 

than to minimize water consumption by operating the steam turbine at a higher back 

pressure. The cooling tower operation was modeled with 4 cycles of concentration. Without 

having site specific water quality information, it was assumed that the water available is

municipal quality and that no pre or post water treatment is needed. 

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9

2.3 Dry Condensing 

The dry condensing alternative utilizes an air cooled condenser (ACC) to condense the 

exhaust steam using a large array of fans that force air over finned tube heat exchangers. 

The heat is rejected directly to the atmosphere and no external water supply is needed for 

condensing the steam cycle exhaust steam. The initial temperature difference (ITD) is 

defined as the difference between the ambient dry bulb temperature at the design point and 

the steam condensation temperature within the ACC. The smaller the ITD, the more 

aggressive the design, resulting in lower steam turbine generator (STG) backpressure but at 

a higher capital and fan power consumption cost. A preliminary investigation suggests that an 

ITD of 25 °F is close to optimal in terms of cost vs. net plant generation for the proposed CSP 

plant at the design ambient conditions. The operation of a 100% dry condensing system was 

modeled to minimize condensing pressure and maximize steam turbine output as ambient 

temperature decreased.

2.4 Parallel Condensing 

The parallel condensing system is a combination of wet and dry condensing systems. The 

steam turbine exhaust branches near the turbine exit and a duct runs to each condensing 

system. The steam flow naturally splits in proportion to the available condensing capacity of 

each system. A parallel system is more expensive than a wet system and can be more 

expensive than a dry system depending on the design capacity of the parallel system. The 

same size ACC was used for the parallel system as the all dry system in order to increase 

annual generation. The duty of the parallel wet condensing portion is approximately 32% of 

the all wet design. The wet portion allows the steam turbine to operate at minimum back 

pressure at all dry bulb temperatures. The large ACC allows the wet system and 

corresponding water consumption to be curtailed as soon as possible with falling dry bulb 

temperature. The operation of the parallel system was modeled to maximize performance at 

the expense of water consumption. This design allows operation on hot summer days without 

an increase in steam turbine back pressure, compared to the all dry system. 

2.5 Solar Field Designs

The solar field size is selected based on 100MWe net plant output, with a 100% wet 

condensing Rankine power cycle, 6hrs of thermal energy storage and a solar multiple of 2.0

at 1,000W/m2 design irradiance. This same solar field design was used to evaluate the 

performance of the 100% dry and parallel condensing designs. Thus the heat input to steam 

cycle was the same for all the condensing configurations. 

2.6 Sizing Criteria

The steam turbine exhaust sections for all three condensing designs were optimized to 

minimize the exhaust losses at full load conditions. The steam turbines in the wet and parallel 

condensing designs, which operate at lower back pressures, utilized an exhaust design with 

a larger 37.5” last stage blade length and appropriate annulus area whereas the dry 

condensing design assumed a 33.5” last stage blade. The steam turbine inlet temperature 

and pressure was the same for all 3 designs. This exhaust section selection was based on 

generic steam turbine technology and not tied to specific vendor design data.

The criteria in the following tables were used to size and estimate the costs of the three

condensing systems. 

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10

Table 2.6 Rankine cycle sizing criteria at design conditions. 

Parameters Units Wet Parallel Dry

Steam Turbine Exhaust Enthalpy Btu/lb 974.49 978.05 988.24 

Steam Turbine Exhaust Flow lb/hr 678,343 680,159 684,741 

Steam Turbine Exhaust Back 

Pressure

Inches 

HgA 1.819 2.036 2.56 

Air Cooled Condenser Duty MMBtu/hr n/a 421 624 

Cooling Tower Duty MMBtu/hr 620 199 n/a

Circulating Water Flow Rate gpm 61,930 19,860 n/a

Cooling Tower Approach °F 10 10 n/a

Cooling Tower Range °F 20 20 n/a

Condenser Terminal Temperature 

Difference (TTD) °F 7 10.75 n/a

2.7 Method

Using the criteria given above, budgetary vendor cost and performance quotes for the cooling 

tower, ACC, surface condenser and steam turbine were obtained in order to determine 

impact on performance, capital cost, auxiliary loads, water consumption, and ultimately 

LCOE. 

The performance portion of this study is necessary to arrive at a LCOE. Net plant output and 

water consumption are the primary performance inputs to an LCOE model. These 

parameters were estimated using four different calculation tools which ultimately were driven 

by three inputs: ambient dry bulb, ambient wet bulb, and steam cycle heat input. 

Hourly ambient dry/wet bulb temperatures were obtained from the TMY3 weather dataset for 

Alamosa, CO, which also provided direct normal insolation (DNI) data used in NREL’s Solar 

Advisor Model (SAM) program. SAM provided the steam cycle heat input, also referred to as 

thermal energy to the power block (Q_PB), as well as thermal energy to storage (Q_to_ts) 

and thermal energy from storage (Q_from_ts). GateCycleTM was used to model the Rankine 

cycle behavior and initially determined the plant’s design point conditions. A unique 

GateCycleTM model was developed for the three condensing configurations. Several offdesign heat balance models were independently run varying dry-bulb temperature, wet-bulb 

temperature, and steam cycle heat input. Model results were compiled and numerically fit into 

a three-variable interpolative lookup function, using Microsoft Excel tools, to arrive at the 

various plots and tables presented herein. 

Using this all-inclusive Excel spreadsheet, the inputs and equations were used to obtain 

cooling tower water makeup for every hour of a typical year. This same methodology was 

used to produce steam turbine electric gross output and Rankine cycle parasitic loads per 

hour. Solar system parasitic loads (i.e. HTF pumps, TES pumps, SCA drives, etc.) were 

calculated based on hourly heat input to the steam cycle, and heat input/output from the TES 

system. The HTF pumps and TES pumps were assumed to be variable speed driven.

The results of the evaluation are presented in the following sections.

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11

3. CAPITAL COSTS 

Capital costs have been determined using a combination of vendor budgetary proposals and

WorleyParsons’ equipment, commodity, and installation labor database. The capital costs are 

within a +/- 30% confidence range based on a conceptual engineering effort. 

The results illustrate the differences in capital cost between the wet, dry, and parallel

condensing designs. The solar field effective mirror aperture area for all cases is 905,790 m2

. 

The thermal energy storage, solar field civil-site work, balance of plant mechanical/electrical, 

HTF system, electrical, instrumentation/controls and all other cost items which makeup a 

complete CSP plant were adjusted as necessary in each design to accommodate the 

condensing system and thermal storage impacts. Note, the higher Site Improvements cost 

associated with the 100% wet system is due to the significantly larger operational waste 

water evaporation pond.

The thermal energy storage equipment cost is based on a turnkey budgetary quote from the 

single commercially available salt storage vendor. An alternative cost savings approach 

would be to estimate the storage system from the ground up and compile vendor quotes for 

each sub-component (tanks, pumps, HX, etc.). 

NREL has selected a 2.0 solar multiple to minimize LCOE, for plants with 6 hours of thermal 

storage. The solar multiple has a significant capital cost impact and is subject to further 

optimization, based on actual plant costs and the project developer's financial model.

Cost reflects NREL's selected 150-meter trough design. This trough is the most proven 

design with the most utility-scale installations; however, the associated materials and labor 

costs are higher than alternative emerging designs (i.e. 100-meter trough, or SkyFuel’s 

SkyTrough) 

Labor rates are non-union merit-based for Pueblo, CO with a productivity factor of 1.2. This

project’s average labor rate is 35% lower than the average hourly rate (union) used in the 

study of the Nevada Test Site CSP plant. This difference in labor is the driving factor behind 

the large capital savings between the NV and CO plants. 

3.1 Vendor Quotes 

WorleyParsons’ obtained budgetary quotes for the cooling tower, surface condenser, and aircooled condenser. All other equipment and materials included in the makeup of a complete 

CSP trough plant were priced based on WorleyParsons’ extensive archive of past vendor 

quotes and previously constructed projects.

The following tables are a summary of the complete cost analysis showing the line items that 

build up the overall total installed capital cost for the three condensing options. 

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12

Table 3.1 Estimated capital cost summary for wet, parallel, and dry condensing plants. 

Case 100% Wet Parallel 100% Dry

Description 100% Wet-Cooled Parallel Case 100% Dry Cooled

Site Improvements $34,501,000 $19,615,958 $18,483,239

Solar Field $335,001,000 $335,001,000 $335,001,000

HTF System $73,585,928 $73,585,928 $73,585,928

Thermal Energy Storage $182,541,000 $182,541,000 $182,541,000

Fossil Backup - - - 

Power Plant $106,559,193 141,601,355 136,347,546

EPCM Costs $29,001,000 $29,001,000 $29,001,000

Project, Land, Misc. - - - 

%DC's Sales Tax - - -

Subtotal $761,189,121 $781,346,241 $774,959,713

Contingency $55,795,000 $57,307,153 $56,838,739

TOTAL INSTALLED 

COST $816,524,000 $838,653,394 $831,798,452

4. PERFORMANCE

This section provides data tables and graphs of the performance results for all cases. Fixing 

the solar heat input to the steam cycle for the base and alternate cases allowed for a more 

direct comparison of the costs and performance. 

For plants with thermal storage, SAM sends the solar energy to storage until there is enough 

energy to operate the steam turbine at or near full load, after which the energy is sent to the 

power block. If thermal storage is not present, SAM operates the steam turbines at lower load 

points to avoid wasting energy; however this study does not include plants without storage. 

This is illustrated in Figures 4.0 below which shows the operating hours of the steam turbines 

at various load points. The minimum steam turbine load used in this study is 20% of rated 

load. (Note: The parallel operation is very close to the wet operation and is not visible in

some of the figures below). 

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13

Figure 4.0

Annual Hours of Net Generation at 5MWe Ranges

100MWe Nominal Net CSP Trough Plant with 6 Hrs TES

San Luis Valley, CO

0

250

500

750

1000

1250

1500

1750

2000

2250

2500

2750

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115

Net Electric Generation (MWe)

Operation within Net Generation Range (hours)

Wet Cooling

Dry Cooling

Parallel Cooling

4.1 Performance Results

Table 4.1 summarizes the performance results for the wet, dry, and parallel condensing 

designs. 

Table 4.1 Colorado plant performance summary.

Performance Results Wet Parallel Dry

Solar Input to Collector Field (MWth) 905.69 905.69 905.69

Design Steam Cycle Thermal Input (MWth) 295.40 295.40 295.40

Design Gross Steam Turbine Output (MWe) 113.90 113.27 111.34

Design Plant Parasitic Losses (MWe) 13.90 14.70 14.18

Plant Net Output (MWe) at Design Conditions 100.00 98.57 97.16

Design Gross Steam Turbine Efficiency (%) 38.58 38.37 37.72

STG Gross Annual Generation (MWe-hrs/yr) 397,245 388,906 387,435

Plant Net Annual Generation (MWe-hrs/yr) 367,602 363,219 361,778

Annual Backfeed Electricity (MWe-hrs/yr) 3,354 3,354 3,354

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14

Figure 4.1.1 through 4.1.2 illustrates annual net generation distribution as function of ambient 

dry bulb temperature. The plant net generation shown in these plots represents the electric 

energy produced for export to the grid and does not include plant auxiliary power that may be 

purchased from the grid when the plant is offline. 

Figure 4.1.3 illustrates the plant’s average net output as a function of the ambient dry bulb 

temperature. SAM’s default Southern California Edison (SCE) thermal storage time of day 

dispatch structure was used in all cases. The trends recognized in the plots below are entirely 

driven by SAM’s use of the storage dispatch profile.

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15

Figure 4.1.1 Net energy production at different ambient temperatures for different condensing systems.

Annual Net Electric Energy Generation at 5 Degree Dry Bulb Temperature Ranges

100MWe Nominal Net Design CSP Trough Plant with 6 Hrs TES

San Luis Valley, CO

0

5

10

15

20

25

30

35

40

45

50

-25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Dry Bulb Temperature (F)

Net Generated Electric Power (MWe-hrs E+03)

Wet Cooling

Dry Cooling

Parallel Cooling

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16

Figure 4.1.2 Percentage of net energy production at different ambient temperatures for different condensing systems. 

Fraction of Annual Net Generation Occuring at 5 Degree Dry Bulb Temperature Ranges

100We Net Nominal CSP Trough Plant with 6 Hrs TES

San Luis Valley, CO

0%

1%

2%

3%

4%

5%

6%

7%

8%

9%

10%

11%

12%

13%

14%

-25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Dry Bulb Temperature (F)

Fraction of Annual Net Generation (MWe-hr/MWe-hr)

Wet Cooling

Dry Cooling

Parallel Cooling

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Figure 4.1.3 Average Net Plant output at different ambient temperatures for different condensing systems. 

Average Net Electric Power Generation at 5 Degree Dry Bulb Temperature Ranges

100MWe Nominal Net CSP Trough Plant with 6 Hrs TES

San Luis Valley, CO

0

10

20

30

40

50

60

70

80

90

100

110

120

-25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Dry Bulb Temperature (F)

Average Electric Power Generation (MWe)

Wet Cooling

Dry Cooling

Parallel Cooling

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4.2 Performance Discussion

Plants with a 100% wet condensing design will typically have a more efficient steam cycle 

across all load points and ambient temperatures. Moreover, this is especially true in dry 

climates with high ambient temperatures such as Nevada. A cooling tower’s performance is 

governed by the wet bulb temperature, whereas a dry cooling system’s capabilities are 

dictated by the dry bulb temperature. The 2% maximum design dry bulb for this Colorado site 

is 84.7°F whereas the wet bulb is only 60.2°F. Although this provides a performance 

advantage to wet condensing, the advantage pales in comparison to hotter climates like 

Nevada. On these hot days, a wet system is able to condense the steam turbine exhaust at a 

lower temperature, pressure, and enthalpy, resulting in more power extracted from the steam 

flow. The wet system was able to achieve a steam cycle efficiency of 38.58% at the design 

conditions, whereas the dry system could only achieve 37.72% even with a very large ACC 

designed with an aggressive ITD of 25°F. 

For the same duty at full capacity, an ACC with a 25°F ITD will typically have higher auxiliary 

loads than a wet system due to its larger number of fans. This further reduces the net plant 

output of the dry plant on hot days compared to the wet cooled plant. However, hotter drybulb temperatures generally have lower relative humidity and therefore the dry bulb 

temperature drops faster than the wet bulb temperature. As a result, the auxiliary loads of a 

dry condensing system drop quicker than those of wet system with the reduction in ambient 

temperature.

These net output differences between the dry and wet cooled plants can be seen on figure 

4.1.3. Due to the inherent exhaust losses in the ACC ducting system, the dry cooled system 

can never achieve as low a steam turbine exhaust pressure as the wet cooled system. The 

minimum turbine exhaust pressure with an ACC is typically 2.0 in HgA due to these duct 

losses. At ambient temperatures below freezing, this minimum pressure is raised to 2.5” HgA 

to prevent freezing in the bundle tubes. 

The cold Colorado climate also increases the heat loss in the HTF piping and receiver tubes. 

Higher heat loss will negatively impact plant start-up time, overall efficiency, freeze 

protection, and maintenance. Means to mitigate these impacts may include employing a gasfired start-up heater, increasing HTF pipe insulation thickness or material properties, and/or 

effectively equipping all critical equipment with adequate heat tracing and/or insulation. In this 

study, the auxiliary gas-fired boiler size and operation have been increased above the 

Nevada site to protect the HTF from freezing (using a steam condensing to HTF heat 

exchanger). No insulation or start-up system adjustments are included beyond what is 

calculated by SAM. 

Since a wet system is able to maintain a more consistent turbine exhaust pressure across the 

operating ambient temperature range, the exhaust velocities are also more constant. This

allows a wet condensing system to operate closer to the peak efficiency point on the steam 

turbine exhaust loss curve more often than a dry condensing system. The parallel system 

also has this benefit over the dry condensing system.

The objective of a parallel system is to combine the hot day generating performance of the 

wet system with the water conserving benefits of the dry system on cooler days. At the San 

Luis Valley site, the summer dry bulb temperatures are low enough that the performance of 

the all dry design performance is not reduced significantly compared to the wet. The 

improved steam turbine efficiency of the parallel design yields higher annual gross steam 

generation compared to the dry, but it is offset by the increased auxiliary loads of the wet 

system’s fan and circulating water pumps. 

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Figure 4.1.3 illustrates the manner in which the SAM model simulates the operation of a CSP 

plant with thermal storage. With thermal storage the steam turbine is not started until there is 

enough heat to operate at or near full load. If the plant were to have no thermal storage the 

turbine would operate whenever sufficient heat is available to run at minimum load. 

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5. WATER CONSUMPTION

This analysis compares the water consumption between the wet, dry, and parallel cooling

designs on an annual consumption basis. The three-variable equation discussed in Section 

2.7 was used to arrive at an annual cooling tower makeup quantity for the wet condensing 

system. Other water consumers considered in this study include mirror washing and steam 

cycle makeup. 

Without having site specific information, raw water quality was assumed sufficient for cooling 

tower use with 4 cycles of concentration and therefore no pre or post water treatment is 

included. Cooling tower blowdown is discharged to onsite evaporation ponds. Electrostatic 

deionization and multimedia filtration equipment are included in the design to treat the raw 

water and produce demineralized (demin) water for steam cycle makeup and solar collector 

mirror washing. The water consumption table below identifies the water quantity required by

each consumer. Note that the demin water system rejects wastewater from the reverse 

osmosis process and backwash water from the multimedia filtration process. These reject 

quantities are included in the demin water consumption values below.

5.1 Cooling Tower

Cooling tower makeup is the largest user of water, primarily consuming it through evaporation 

and secondarily by blowdown rejection. Evaporation is a function of the cooling load and wet 

bulb temperatures. Blowdown is based on water quality (which assumes 4 cycles of 

concentration), evaporation, and cooling tower drift. The annual water consumption of the 

cooling towers was calculated as describe above. The circulating water system also provides 

the heat sink for the closed cooling water system that cools the auxiliary plant equipment.

Due to the low ambient dry bulb and high precipitation frequency in the San Luis Valley

compared to the Nevada site, evaporation ponds tend to require more surface area for the 

same reject flow rates than in hotter climates.

5.2 Air Cooled Condenser

Utilizing a dry condensing system consumes minimal water compared to a wet system. The 

ACC is a closed loop system not having evaporation or blowdown. Steam cycle makeup is 

assumed to be slightly higher for the dry condensing system than the wet because obtaining 

optimal cycle water chemistry is more difficult with ACCs and therefore more blowdown is 

needed during start-up. In the absence of a cooling water sink (i.e. cooling tower) am aircooled heat exchanger (fin-fan) is needed to cool plant auxiliary equipment. In hot climates 

like Nevada where the ambient dry bulb temperature frequently (>2%) exceeds 85°F a 

supplemental wet surface air cooler (WSAC) is used in parallel with the fin-fan cooler to meet 

cold water temperature requirements that cannot be met by direct dry air cooling. This is not 

the case for cooler climates such as Alamosa CO, where dry bulb temperatures exceed 85°F 

less than 0.4% of the year. Therefore a fin-fan cooler can provide sufficient auxiliary cooling 

100% of the time. 

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Table 5.2.1 Estimated Annual Water Consumption (Acre-Feet/Year)

Water Consumers (water quality) Wet 

Condensing

Parallel 

Condensing

Dry 

Condensing

Cooling Tower Makeup (Raw) 848 13 0

Steam Cycle Operating Makeup (Demin) 13 13 13

Steam Cycle Makeup at Startup (Demin) 13 19 19

Mirror Wash Water (Demin) 44 44 44

Totals 919 90 77

Figure 5.2

Estimated Annual Water Consumption

100MW Nominal Net CSP Trough Plant with 6 hours TES

San Luis Valley, CO 

0

100

200

300

400

500

600

700

800

900

1,000

Wet, 6hrs TES Dry, 6hrs TES Parallel, 6hrs TES

Cooling Design Cases

Annual Water Consumption (Ac-Ft/Yr)

Mirror Wash

Startup

BD Quench

CT Makeup

5.3 Water Consumption Discussion

Approximately 92% percent of the wet condensing plant’s annual water consumption is used 

by the cooling tower. Mirror wash water consumption is fixed for all cases since all cases 

have the same solar field size. Regardless of the cooling system employed, steam cycle 

makeup is roughly the same for all cases; with the exception of dry cooled start-up where 

ACCs take longer to pull full vacuum and obtain optimal steam cycle chemistry and therefore 

consume more steam cycle and quench water in the process. Altogether the wet condensing 

plant consumes about 12 times that of the dry condensing plant under the design parameters 

of this study. The parallel design only consumed 17% more water than the all dry system, but 

it did not yield any performance benefits. In addition, due to the infrequency of wet cooling

operation in the parallel case, all auxiliary cooling loads are assumed to be cooled by a fin98
22

fan cooler while the cooling tower is presumably laid –up during the winter and shoulder 

months. This will save additional water and O&M costs for the parallel plant but the fin-fan 

cooler may consume more parasitic power compared to evaporative cooling, depending on 

configuration. 

Annual water consumption contributes to the operations and maintenance cost as a plant 

consumable. This reoccurring cost will ultimately contribute to the final LCOE analysis 

discussed in the next section.

99
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6 LEVELIZED COST OF ELECTRICITY ANALYSIS

The levelized cost of electricity was determined by calculating the net present value of the 

nominal capital and operations and maintenance costs per year and discounting them back to 

the present. The net present value was them divided by the annual net output per year to 

determine the cost per MW–hr. The annual (O&M) costs are escalated 2009 dollars. The 

levelized annual costs and LCOE are nominal dollars as the effects of inflation are not 

included in this analysis. The capital costs are assumed to occur entirely in 2009 and the 

annual costs are assumed to start with the plant operation in year 2013. The LCOE is based 

on the plant operating for 30 years through 2042. Other items such as capital financing, 

construction financing, taxes, renewable credits, financial incentives, debt ratios, 

depreciation, loan periods, etc. were not included as they were considered to be project and 

company specific. The intent of this analysis was to provide a technical LCOE based on the 

data in the tables below that would allow a relative comparison of the performance and costs 

of the various cooling system designs. Further economic considerations can be added to this 

data to obtain an economic or financial analysis for a specific project within a specific 

company. 

Table 6.0 Levelized cost of electricity model inputs and output summary.

Discount rate: 5.25%

Water Cost $/acre-ft $450

Commercial Operation Date: 2013

Economic Operating Life: 30

Plant O&M cost nominal escalation: 1.50%

Designs with 6 

hr TES Wet Parallel Dry

Plant Output 

(MW-hr/yr) 367,602 363,219 361,778

Total Capital Cost $816,524,000 $838,653,394 $831,798,452

Annual O&M Cost $11,296,625 $10,990,453 $10,972,639

NPV $948,603,278 $964,945,208 $958,159,692

Nominal 

Levelized Yearly 

Cost $63,477,622 $64,571,174 $64,117,108

Nominal LCOE

($/MW-hr) $172.68 $177.77 $177.23

100
24

The wet condensing design had the lowest LCOE as it had the lowest capital cost and the 

highest annual electricity production. The parallel design, while having more annual net 

generation than the dry design, did not produce enough generation to offset its higher capital 

costs. This resulted in the parallel case having the highest LCOE across all designs. As 

expected the dry design had a higher LCOE than the wet design, yet slightly lower than the 

parallel. This is all based on a $450/acre-foot water cost. As the cost of water rises or the 

quality of water diminishes the dry cooled plant LCOE will approach the wet cooled plant and 

in some extreme cases it will be less.

7 CONCLUSIONS AND RECOMMENDATIONS

The LCOE for all the designs are very close; the dry cooled plant increasing LCOE about 

2.6% above the 100% wet configuration and the parallel plant LCOE is slightly higher than 

the 100% dry configuration. In dry climates such as Colorado and Nevada, it is expected that 

wet condensing designs would have better economic results with the cost of water assumed 

in this study. This is due to the fact that wet cooling can achieve a lower steam turbine 

exhaust pressure and the cooling equipment is less expensive. In more humid climates, the

wet cooling is less efficient and thus the economic differences would be smaller. Cooler dry 

bulb temperatures reduce the negative impacts of dry cooling which is the reason that the 

LCOE difference between the wet and dry designs in Nevada is larger than that of the 

Colorado site. 

This study focused on the cooling technology differences and assumed that the water 

supplied to the plant was similar in quality to typical city tap water. The actual quality of the 

water available in remote sites where CSP plants would most likely be located could require 

very expensive water treatment systems that could drive the LCOE of the wet design higher 

than the dry design. Actual water supply and treatment costs need to be considered when 

performing a similar study for an actual project. 

A high level sensitivity analysis was performed to investigate the effect of varying the ACC 

ITD for the dry design on the total plant cost per MW-hr at full load in 5°F increments from 

30°F to 85°F dry bulb. The lower ITD designs resulted in more annual generation but at 

higher initial capital costs. The ACCs with 25°F, 30°F, and 35°F ITDs were within 0.1% of 

each other, in $/MW-hr, whereas the 40°F ITD ACC was 0.7% higher. The optimum ACC 

ITD for a specific site will depend on the actual steam turbine selected and the last stage 

blade and annulus area options available for that turbine.

In the San Luis Valley of Colorado, a parallel cooled CSP trough plant with an all dry ACC 

ITD of 25°F is not economically justified relative to an all dry design. The same large ACC 

used in the all dry design was used in the parallel design to minimize water consumption. 

However, the advantages of parallel cooling over dry cooling were not fully realized since 

there were few operating hours which benefited from wet cooling which did not offset the 

added expense and complexity of a parallel system. A high level sensitivity analysis was also 

performed on the parallel system which indicated a 0.8% improvement in $/MW-hr (vs. 

parallel configuration used in study) could be obtained by using an all dry ACC with a 40°F

ITD and a larger wet cooling system (compared to parallel cooling configuration used in

study). However, this improvement is accompanied by an approximate 130% increase in 

estimated annual water consumption, compared to the study’s parallel configuration 

101
25

This study simulated the operation of the parallel plant to maximize net power output. It could 

be operated to conserve more water and in doing so the performance and operating costs 

would approach that of the all dry plant with the same size ACC. Since a 40°F ITD parallel 

design (ITD with wet portion turned off) would have a smaller ACC, trying to reduce water 

consumption by running the wet portion less will result in lower net output than doing the 

same with a 25°F ITD parallel design.

This study does not consider market electricity price impacts. It is likely that power purchase 

agreements for some projects/locations may have a time-of-use payment structure for peak 

power which can make parallel cooling configurations more attractive than 100% dry cooled 

plants. 

The relative LCOE figures presented above are good guidelines of what to expect from the 

different condensing technologies for the site evaluated and other sites with similar climates. 

More in depth analysis based on specific project criteria is required in order to make a 

determination of the best cooling configuration for a San Luis Valley project. 

102
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1. REPORT DATE (DD-MM-YYYY)

December 2010

2. REPORT TYPE

Technical report

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Water Use in Parabolic Trough Power Plants: Summary Results 

from WorleyParsons' Analyses 

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

C.S. Turchi, M.J. Wagner, and C.F. Kutscher

5d. PROJECT NUMBER

NREL/TP-5500-49468

5e. TASK NUMBER

SM10.1530

5f. WORK UNIT NUMBER

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National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

8. PERFORMING ORGANIZATION

REPORT NUMBER

NREL/TP-5500-49468

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NREL

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14. ABSTRACT (Maximum 200 Words)

The National Renewable Energy Laboratory (NREL) contracted with WorleyParsons Group, Inc. to examine the 

effect of switching from evaporative cooling to alternative cooling systems on a nominal 100-MW parabolic trough 

concentrating solar power (CSP) plant. WorleyParsons analyzed 13 different cases spanning three different 

geographic locations (Daggett, California; Las Vegas, Nevada; and Alamosa, Colorado) to assess the performance, 

cost, and water use impacts of switching from wet to dry or hybrid cooling systems. NREL developed matching cases 

in its Solar Advisor Model (SAM) for each scenario to allow for hourly modeling and provide a comparison to the 

WorleyParsons results. Our findings indicate that switching from 100% wet to 100% dry cooling will result in levelized 

cost of electricity (LCOE) increases of approximately 3% to 8% for parabolic trough plants throughout most of the 

southwestern United States. In cooler, high-altitude areas like Colorado’s San Luis Valley, WorleyParsons estimated 

the increase at only 2.5%, while SAM predicted a 4.4% difference. In all cases, the transition to dry cooling will 

reduce water consumption by over 90%. Utility time-of-delivery (TOD) schedules had similar impacts for wet- and drycooled plants, suggesting that TOD schedules have a relatively minor effect on the dry-cooling penalty. 

15. SUBJECT TERMS

parabolic trough; water; dry cooling; CSP; solar

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