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

Line-Focus Solar Power Plant 

Cost Reduction Plan 

Charles Kutscher, Mark Mehos, Craig Turchi, 

and Greg Glatzmaier

National Renewable Energy Laboratory

Timothy Moss

Sandia National Laboratories

Milestone Report

NREL/TP-5500-48175

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

Line-Focus Solar Power Plant 

Cost Reduction Plan 

Charles Kutscher, Mark Mehos, Craig Turchi, 

and Greg Glatzmaier

National Renewable Energy Laboratory

Timothy Moss

Sandia National Laboratories

Prepared under Task No. CP09.1001

Milestone Report

NREL/TP-5500-48175

December 2010
NOTICE

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i

Table of Contents

List of Figures................................................................................................................................ iii

Introduction......................................................................................................................................1

1 Cost Reduction Paths...............................................................................................................1

2 Examination of Potential Improvement Opportunities............................................................4

2.1 Capital Equipment............................................................................................................ 4

2.1.1 Cost ........................................................................................................................... 4

2.1.2 Performance .............................................................................................................. 8

2.2 Collector Field Component Improvement Opportunities................................................. 8

2.2.1 Collector Frames....................................................................................................... 8

2.2.2 Receivers................................................................................................................... 9

2.2.3 Reflectors................................................................................................................ 11

2.2.4 Optical Materials Testing........................................................................................ 12

2.2.5 Foundations............................................................................................................. 12

2.3 Storage Subsystem Improvement Opportunities............................................................ 13

2.4 Overall Collector Design Improvements........................................................................ 15

2.5 O&M Improvement Opportunities................................................................................. 15

2.6 Parasitic Power Reduction Opportunities ...................................................................... 17

2.7 Advanced Systems Concepts ......................................................................................... 18

2.7.1 Direct Steam Generation (DSG)............................................................................. 18

2.7.2 Hybrid Designs ....................................................................................................... 18

2.7.3 Supercritical Brayton Cycle.................................................................................... 19

2.8 Environmental Issues..................................................................................................... 19

3 Recommended Activities.......................................................................................................20

3.1 Items Affecting the Collector Field................................................................................ 21

3.1.1 Overall Collector Design ........................................................................................ 21

3.1.2 Collector Structure Improvement/Wind Load Investigation .................................. 21

3.1.3 Receiver Tubes........................................................................................................ 22

3.1.3.1 Advanced Receiver Coating Development ......................................................... 22

3.1.3.2 Hydrogen Mitigation........................................................................................... 22

3.1.3.3 Development of Lower-Cost Receivers.............................................................. 23

3.1.3.4 Receiver Heat Loss Measurement....................................................................... 23

3.1.4 Improved Mirrors.................................................................................................... 24

3.1.5 Optical Materials Testing........................................................................................ 24
ii

3.1.6 Foundations and Support Structures....................................................................... 25

3.2 Heat Transfer Fluid ........................................................................................................ 25

3.3 Storage Cost Reduction.................................................................................................. 26

3.3.1 Storage Fluid........................................................................................................... 26

3.3.2 Thermocline Storage............................................................................................... 26

3.3.3 PCM Storage........................................................................................................... 26

3.3.4 Thermochemical Storage ........................................................................................ 27

3.3.5 Other Advanced Storage Concepts......................................................................... 27

3.4 Parasitic Power Reduction ............................................................................................. 27

3.5 O&M Cost Reduction..................................................................................................... 27

3.6 CSP Environmental Impact............................................................................................ 28

3.7 Collector and System Performance Assessment ............................................................ 28

3.7.1 Mirror Measurement Methods................................................................................ 28

3.7.2 Collector Testing..................................................................................................... 29

3.7.3 Model Development................................................................................................ 30

3.7.4 Plant Performance Data Collection and Model Validation..................................... 30

3.7.5 Test Standards......................................................................................................... 30

3.8 Advanced Concepts........................................................................................................ 31

3.8.1 Hybridization .......................................................................................................... 31

3.8.2 Direct Steam Generation......................................................................................... 31

3.8.3 Supercritical CO2 Cycle.......................................................................................... 31

3.8.4 Low-Cost Collector Concepts................................................................................. 31

3.9 New FOAs...................................................................................................................... 31

3.10 Laboratory Support to FOA Contractors.................................................................... 32

4 Spending Plan ........................................................................................................................32

5 Conclusions............................................................................................................................33

Acknowledgments..........................................................................................................................38

References......................................................................................................................................38
iii

List of Figures

Figure 1. Projected LCOE (Real $2009) and associated cost targets. The figure does not include 

projected performance improvements described in detail in Table 4 in this report.........................3

Figure 2. Percentages of total solar field component failure at SEGS III-VI, 1989-2005 average.16

Figure 3. Causes of lost power production at SEGS III-VI, 1999-2001 average. .........................16

Figure 4. Sensitivity of LCOE to potential cost reductions in different areas...............................20

Figure 5. SAM results showing the impact of changes in receiver performance on the real 

levelized cost of electricity. 2009 costs are shown here. ...............................................................23

Figure 6. Impact of collector-related changes on the real levelized cost of electricity. ................25

List of Tables

Table 1. Current cost breakdown for the solar field. .......................................................................5

Table 2. Cost breakdown for the heat transfer fluid piping system. ................................................6

Table 3. Cost breakdown for the thermal storage subsystem. .........................................................7

Table 4. Estimated current and future costs for parabolic trough systems. Representative cases at 

6 and 12 hours of storage are shown..............................................................................................21

Table 5. Multi-year activities and budgets.....................................................................................34
1

Introduction

Line-focus solar collectors, in particular parabolic trough collectors, are the most mature and 

proven technology available for producing central electricity from concentrated solar energy. 

Because this technology has over 25 years of successful operational experience, resulting in a 

low perceived risk, it is likely that it will continue to be a favorite of investors for some time. 

The concentrating solar power (CSP) industry is developing parabolic trough projects that will 

cost billions of dollars, and it is supporting these projects with hundreds of millions of dollars of 

research and development funding. While this technology offers many advantages over 

conventional electricity generation—such as utilizing plentiful domestic renewable fuel and 

having very low emissions of greenhouse gases and air pollutants— it provides electricity in the 

intermediate power market at about twice the cost of its conventional competitor, combined 

cycle natural gas. The purpose of this document is to define a set of activities from fiscal year 

2011 to fiscal year 2016 that will make this technology economically competitive with 

conventional means. Section 1 describes current costs and cost goals and the overall ways that 

cost reductions can occur. Section 2 identifies and discusses specific cost reduction 

opportunities. Section 3 describes specific activities aimed at addressing those opportunities. 

Finally, Section 4 provides a schedule of the activities that will complement and support industry 

efforts including costs and priorities.

1 Cost Reduction Paths

The U.S. Department of Energy (DOE) has set a levelized cost of electricity (LCOE) goal for 

parabolic trough collector technology, and this report proposes activities for meeting or 

surpassing that goal. The goal is to reduce the real levelized cost from today's value of about 

19¢/kWh nominal (15¢/kWh real) (nominal includes inflation) with a 30% investment tax credit 

(ITC) (which is equivalent to 24¢/kWh nominal or 19¢/kWh real if the tax credit were 10%) to a 

value in 2017 of 12¢/kWh nominal (10¢/kWh real) with a 10% ITC. The cost goals were 

developed by estimating the projected cost for fossil generation in target markets. The National 

Renewable Energy Laboratory (NREL) normalized generation costs from several sources 

[References 1-7] to a common set of financial assumptions and examined the resulting predicted 

range of LCOE. The intermediate load market was assumed to be represented by a natural gas 

combined cycle plant operating at a capacity factor of 40%. The estimated costs were 8¢/kWh to 

12¢/kWh in nominal dollars, with the upper end representative of the California market. This 

predicted value of 12¢/kWh nominal (10¢/kWh real) is consistent with the California Market 

Price Referent (MPR) when the MPR is weighted for a solar generation profile. The baseload 

market costs assumed that carbon capture and sequestration (CCS) technologies would be 

deployed for the fossil generators by 2020. The increased cost for CCS was offset by the lower 

generation costs for baseload operation and coincidentally the baseload target was also 12¢/kWh 

nominal or 10¢/kWh real. 

The values above indicate the goal for line-focus systems is to cut the cost over the next seven 

years by 50%. The LCOE can be reduced in two ways: by increasing performance (both initial 

and long-term) and by lowering costs (both capital and operating). This report discusses both of 

these means. 
2

Performance can be increased by:

• improving the solar field optical efficiency

• reducing the solar field thermal losses

• reducing parasitic power consumption

• developing improved configurations that lead to higher utilization and efficiency

• identifying more efficient overall system designs.

Cost reduction can be achieved by:

• reducing equipment capital cost via lower material content, lower-cost materials, 

more efficient design, or less expensive manufacturing and shipping costs

• reducing field assembly and installation costs (via simpler designs and minimization 

and/or ease of field assembly)

• lowering operation and maintenance costs via improved reliability, automation, 

reducing need (as with self-cleaning mirrors), and better techniques

• building larger systems that provide economies of scale, particularly in the power 

block

• deploying more systems to benefit from learning-curve effects.

Figure 1 summarizes the impact of the performance and cost improvements described above. By 

achieving the targets described in the figure, parabolic trough systems can achieve a nominal

levelized cost of energy of 12¢/kWh (10¢/kWh real), achieving DOE’s objective of cost parity 

with conventional intermediate load power plants.
3

Figure 1. Projected LCOE (Real $2009) and associated cost targets. The figure does not include 

projected performance improvements described in detail in Table 4 in this report.

While this report focuses mostly on performance improvements and the first four cost reduction 

means listed above, certain DOE-funded activities will strongly impact the last two cost 

reduction paths. The ultimate goal of the Program is to achieve large-scale field deployment of 

parabolic trough collector systems so that they become major carbon-free contributors to our 

nation's energy supply. Of course, deployment will be encouraged by lower trough system costs, 

higher costs of the competition (for example, resulting from carbon pricing), or a combination of 

the above. But large-scale deployment will also require that utilities and investors observe

successful operation of trough plants. There are currently contracts for approximately 10,000 

MW of new CSP plants in the U.S., and about 5,000 MW of these involve troughs. In addition, 

BLM leases have been sought for approximately 40,000 MW of trough plants. For even a 

fraction of these plants to be financed and built, it is critically important that the first new plants 

be successful. DOE is playing—and should continue to play—an important role in ensuring their 

success by performing testing of evolving components and installations and providing rapid 

feedback to industry. This will foster economies of scale and encourage cost reductions due to 

learning curve effects. 

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

2010 w 30%ITC 2010 w 10%ITC 2015 w 10%ITC 2017 w 10%ITC

Real LCOE (c/kWh)

Present Value of Ins. & Prop. Tax

Present Value of O&M

Indirect Cost

Contingency Cost

Fossil Backup Cost (not used)

Thermal Storage Cost

HTF System Cost

Power Plant Cost

Solar Field Cost

Site Cost $350/m2

$1010 

/kWe

$75/m2

$80/kWt

10%

18.5%

$70/kW-y

$190/m2

$775

/kWe

$40m2

$25/kWt

10%

16%

$45/kW-y

$270/m2

$775

/kWe

$40/kWt

10%

18.5%

$60/kW-y
4

2 Examination of Potential Improvement Opportunities

The costs of electric power generated by a line-focus solar power system are dependent on the 

capital equipment cost and performance, operations and maintenance costs, and parasitic power 

penalties. Innovative plant designs can also impact delivered energy costs. This section will 

discuss where opportunities lie and what is currently being done. Specific recommendations for 

activities are given in Section 3.

2.1 Capital Equipment

The capital equipment for a CSP plant involves solar components (solar collector field, heat 

transfer piping, and storage subsystem) and the more-or-less conventional thermodynamic power 

cycle components. We will focus on the solar components and address opportunities for both 

cost reduction and performance improvement.

2.1.1 Cost

Under contract to NREL, WorleyParsons analyzed the current costs of a typical parabolic trough 

power plant. NREL adjusted the numbers based on costs provided by actual plants, assumed an 

Arizona site (non-union labor), and normalized the data for a 100 MW-net plant with 6 hours of 

thermal storage [10]. The cost breakdowns in terms of dollars per square meter of collector 

aperture area for the solar field, the heat transfer fluid (HTF) piping, and the storage are shown, 

respectively, in Tables 1 through 3. (Note: numbers are rounded to avoid decimals, so totals may 

differ slightly from the sums of individual numbers.)
5

Table 1. Current cost breakdown for the solar field.

Solar Field 

Component

Material Cost 

($/m2)

Labor Cost 

($/m2)

Total Cost 

($/m2)

Mirrors 48 - 48

Receiver Tubes 

& Fittings

70 - 70

Collector 

Frames

79 - 79

Misc. Collector 

Components

2 - 2

Foundations 

and Support 

Structures

18 - 18

Instrumentation 

& Controls

8 0 8

Electrical 2 1 3

Field 

Installation

- 62 62

Fabrication 

Tent

1 1 1

Sun Tracker 4 - 4

Totals 231 64 295
6

Table 2. Cost breakdown for the heat transfer fluid piping system. 

HTF 

Component

Material Cost 

($/m2)

Labor Cost 

($/m2)

Total Cost 

($/m2)

Freeze 

Protection 

System

1 0 1

Ullage System 1 0 1

Pumps 6 0 6

Expansion and 

Blanketing 

Systems

7 1 8

Solar Field 

Piping

34 15 49

Power Block 

Piping

1 0 1

Foundations and 

Supports

1 1 2

Fluid 22 - 22

Totals 73 17 90
7

Table 3. Cost breakdown for the thermal storage subsystem.

Storage 

Component

Material Cost 

($/m2)

Labor Cost 

($/m2)

Total Cost 

($/m2)

Total Cost 

($/kWh-t)

Pumps & Heat 

Exchangers

33 2 35 17

Tanks 44 6 50 25

Storage Fluid 72 1 73 36

Piping and 

Fittings

1 1 2 1

Foundations and 

Support 

Structures

0 1 1 0

Instrumentation 

& Controls

3 4 7 3

Electrical - - 0 0

Totals 153 14 167 81

The total collector aperture area is 854,000 m2

. The total cost of this plant was $824,900,000, 

representing $8,250 per kW or $966/m2

. The total direct cost is $658,900,000 or $772/m2. Total 

costs are obtained from direct costs by adding a fixed percentage to each direct cost item to 

account for project indirect costs. So by looking at the percentages of direct costs we obtain a 

result very close to the percentages of total costs. At a cost of $295/m2

, the solar collector field 

(materials plus labor) represents 42% of the total plant capital cost. The solar field cost divides 

out as 78% materials and 22% labor, and virtually all of the labor cost is in the collector 

installation. There was great debate regarding this cost split among the solar field developers; 

WorleyParson’s analysis suggested the labor fraction was much higher, while some developers 

claimed it was as low as 15% of the total cost. It is believed the different percentages result from 

different cost accounting practices and a 22% labor split was adopted as a representative value. 

In any event, field assembly costs are a significant contributor to total cost. The material cost for 

the solar field is dominated by three components: collector frames (34%), receiver tubes and 

fittings (30%), and mirrors (21%). The cost of foundations and support structures (8%) is also 

significant.

The heat transfer fluid system at a total cost of $90/m2 (81% materials and 19% labor) represents 

13% of the total plant capital cost. Fully 54% of the fluid system cost is due to the piping 

(materials and labor). It is unlikely that CSP Program research can have much impact on a 

component as conventional as piping; however, improved designs might be able to reduce the 
8

total amount of piping needed. The fluid itself represents 24% of the fluid system cost and an 

improved fluid can have ramifications beyond its material cost impact.

The thermal storage subsystem cost at $167/m2 or $81/kWh-t (92% materials and only 8% labor) 

represents 24% of the total plant capital cost. The storage subsystem cost is dominated by three

components: the tanks (30% of total storage subsystem cost), pumps and heat exchangers (21%),

and the molten salt (44%). 

In summary, the collector field, the storage tank, and the storage and heat transfer fluids are 

major areas for component cost reduction. This plan focuses on improving the performance/cost 

ratio of these components as well as reducing parasitic power and O&M costs. 

2.1.2 Performance 

The performance of parabolic trough collectors can be described in terms of two efficiency 

components: optical and thermal. The optical efficiency is a measure of the percentage of 

incoming direct normal radiation that is absorbed by the receiver tube. The thermal efficiency is 

the percent of energy absorbed by the receiver tube that is transferred into the heat transfer fluid 

(the rest is lost to the environment). 

The optical performance of the collector determines how much of the sunlight is converted to

heat in the absorber surface. Optical efficiency of the collector is a critical characteristic because 

it is directly proportional to the energy delivered by the collector field to the power cycle. Hence, 

a 5% improvement in optical efficiency (with no increase in capital cost) essentially lowers the 

cost of delivered electricity by the same 5%. The thermal performance determines how much of 

that heat is transferred to the working fluid as opposed to being lost to the environment. Heat loss 

is determined by the emittance of the receiver, the integrity of the receiver vacuum, as well as 

both operational and overnight losses from the piping and storage subsystems. For each major 

component, we will address opportunities to reduce cost and improve performance (both 

optically and thermally).

In the following sections we describe potential opportunities for performance improvement and 

cost reduction in the different component areas. We also address operation and maintenance 

(O&M) costs, parasitic power, and advanced systems options.

2.2 Collector Field Component Improvement Opportunities

2.2.1 Collector Frames

The support structure for the reflectors must support the weight of the mirrors and have sufficient 

strength to keep the mirrors optically aligned with the receivers under wind loading conditions. 

However, survival wind loads, which vary with location, tend to drive the overall frame design.

The frame must also have sufficient torsional rigidity to minimize twisting of modules 

successively further away from the central drive mechanism. Depending on the choice of 

reflector material, the reflector may or may not play a role in the structure. (Glass mirrors are 

mounted onto the structure and do not generally carry any structural load themselves, whereas a 

thin film reflector substrate can provide some structural contribution.) 
9

Several different types of designs have been used. In one design, a steel torque tube (as in the 

Luz LS-2 and more recent designs by Flagsol and Sener, for example) or torque box (as in the 

Eurotrough and Flabeg) carries all of the torque load. Cantilever struts attached to the tube or 

box provide mounting points for the reflectors. Another common alternative is to use an 

aluminum or steel truss, or space frame, structure as in the Acciona or SkyFuel designs. This 

structure consists of many frame elements. It both supports the reflector and provides torsional 

rigidity. While it reduces material content compared to a torque tube, it could, in some cases, 

increase installation time and the associated labor cost. Combinations of the space frame and 

torque tube (or box) are also possible. The torque tube concept may assemble more quickly while 

the space frame can be lighter weight. The torque tube (or box) typically requires fabrication on 

an accurate, on-site jig in an assembly building; space frame concepts are normally assembled 

“in place” at the solar field.

In addition, various attempts have been made to use monocoque designs that employ honeycomb 

or an internal structure to make the reflector similar to an airplane wing. This concept can reduce 

field assembly time and also lend itself to easier shipping. It is possible to use tensioning cables 

in front of the collector, as has been done by IST with their industrial process heat troughs. 

Finally, various stretched membrane concepts have been proposed over the years, including the 

idea of a low-cost inflated design. Work on these various designs is being undertaken by several 

of the Funding Opportunity Announcement (FOA) contractors. There is still a need, however, to 

perform an independent, detailed cost-performance optimization that addresses material content 

and cost, structural performance, number of parts, attachment mechanisms, ease of manufacture, 

and ease of assembly. 

The choice of materials also plays an important role. Steel is stronger and stiffer than aluminum. 

Aluminum is lighter weight, corrosion-resistant, and more easily processed. Different design 

alternatives should be evaluated for each metal, and combinations of the two metals should be 

considered. For example, a space frame made from steel could potentially be less expensive than 

one made of aluminum, but we would need to develop easy-to-manufacture joints and address 

corrosion issues. Just as Alcoa has sought to optimize the design of an all-aluminum collector, 

the steel industry could potentially be interested in developing an all-steel design. Other 

materials such as composites and honeycombs can also be further explored. Potential cost 

savings can be obtained by better understanding the actual wind load experienced by interior 

collectors in a collector field and by taking measures to minimize wind loads, allowing for a 

lower-cost structure and reflector design. Material price trends also influence the choice of 

material, and price fluctuations can be significant.

2.2.2 Receivers

Receivers have both optical and thermal performance characteristics. Receiver tube 

manufacturers use anti-reflection (AR) coatings on the glass envelopes to increase the 

transmittance of sunlight. This is especially important when the sun strikes the receiver tube at an 

angle. NREL utilizes its outdoor test facility to directly measure the overall optical efficiency of 

parabolic troughs at normal solar incidence as well as a range of off-normal sun angles (which an 

array of parabolic trough collectors will actually experience in the field). These tests can help

identify when better optical performance is needed. Current receiver coatings have very high 

absorptance of short-wave radiation (sunlight). A challenge is to reduce their emittance for long-
10

wave (infrared) radiation while maintaining the high absorptance. Industry has made 

considerable progress in improving solar selective coatings.

NREL conducts indoor heat loss tests on receiver tubes. Sandia National Laboratory conducts 

outdoor trough module test measurements on their rotating platform to simultaneously yield both 

optical and thermal performance. (Collector efficiency curves can be generated either by using 

NREL's separate optical and thermal test results or by using Sandia's overall collector 

performance measurements.) Today's high-vacuum receiver tubes are capable of virtually 

eliminating conduction and convection heat losses to the environment, leaving radiation as the 

only important heat loss mechanism. Radiation heat loss from the absorber tube is minimized by 

using a selective coating with a very low long-wave emittance (0.10 or less). Low-cost, highperformance, durable advanced optical materials are necessary to meet the demands of advanced 

system designs and to achieve the cost and performance goals that are needed to commercialize 

CSP technologies. Increasing the operating temperature from 390oC to >450oC or higher can 

increase overall solar-to-electricity efficiency, reduce thermal storage volume, and reduce the 

LCOE. Further decreases in emittance are possible, although this may be nearing the point of 

diminishing returns. Reducing or maintaining low emittance and high absorptance at higher 

operating temperatures is an important goal.

NREL has developed an improved receiver coating that has a low emittance (as low as 0.07) 

while maintaining a high absorptance (0.96) and having good oxidation resistance at high 

operating temperatures even if air leaks into the vacuum space. NREL is working with an 

industry partner to bring this improved absorber coating to market. In addition, infraredreflecting coatings are used on commercial glass envelopes.

While the vacuum in a new receiver tube effectively eliminates conduction and convection heat 

losses, receiver tubes can degrade in the field. NREL has developed and transferred to industry a 

Receiver Infrared Imaging System that allows plant owners to identify degraded receiver tubes in 

the field. On the basis of these tests, FPL Energy has replaced millions of dollars of receiver 

tubes at their SEGS plants.

Many receiver tubes in the field have exhibited a problem of hydrogen (H2) permeation from the 

heat transfer fluid into the vacuum space, resulting in greatly increased heat loss. Manufacturers 

are addressing this by using more getter material, changing the location of getter material, and 

using hydrogen permeation barriers. NREL is investigating alternative ways to solve this 

problem. NREL has shown experimentally that by introducing a small amount of inert gas such 

as xenon (Xe) or argon (Ar) into the vacuum space, these larger molecules will effectively inhibit 

motion of the H2 molecules, thereby reducing heat loss to near-vacuum levels. It is possible that 

injecting argon by blanketing or flushing could obviate vacuum pumping and reduce receiver 

manufacturing cost, albeit with a small performance penalty up front. By avoiding a hard 

vacuum, it might be possible to use a thinner glass envelope. The argon could also avoid the cost 

of getters, which can represent as much as 20% of the receiver cost.

NREL has also developed a model of hydrogen diffusion throughout the plant and identified a 

means to remove the hydrogen centrally. An agreement with FPL Energy is in place to test this 

concept at an operating SEGS plant. 
11

Finding ways to reduce the receiver cost and decreasing the number of receivers in the field by 

increasing the trough aperture width might bring further cost reductions, as is being investigated 

by FOA contractors and proposed by Sandia. Because the number of suppliers of receiver tubes

is limited, additional manufacturers could help increase product availability and help lower cost. 

The entry of low-cost suppliers into this market could provide significant downward pressure on 

cost. Greater use of Design for Manufacturing and Assembly (DFMA) techniques by

manufacturers could be extremely helpful and is especially beneficial at the beginning of the 

design process. 

2.2.3 Reflectors

After collector frames and receivers, the reflectors represent the largest solar field cost item. For 

concentrating collectors it is important not only that the surface have a high reflectance but that it 

also be highly specular, i.e., that it reflects sunlight into a narrow cone angle so that that the 

focused sunlight will intercept the receiver tube. These properties must be demonstrably 

maintained for long-term service lifetimes. Currently, parabolic trough power plants use thick 

second-surface silvered glass mirrors. The glass is 4-mm-thick, low-iron (or white) glass with 

high transmittance. These are second-surface mirrors in which the light passes through the glass 

and reflects off of the reflective layer on the back side of the glass. Silver reflective layers are 

used primarily because they have the highest spectral reflectance across the solar spectrum (300

nm–2500 nm). The mirrors are coated by traditional wet-chemistry processes and protected with 

a copper layer and low-lead mirror paint systems. Several companies are working to improve 

glass solar mirrors by improving glass transmittance; bending, finishing, and strengthening;

using thin (1-mm) and laminated glass mirrors; improving the silver back coating; and using nolead paint systems and adhesives. Long-term durability needs to be proven, however.

There are a number of efforts underway, including several FOA contracts, to develop improved 

low-cost polymer thin film reflectors. Specific examples include the ReflecTech silvered 

polymer film developed by NREL and SkyFuel, the silvered polymer reflector developed by 3M 

and NREL, the Alanod anodized aluminum reflector, and the Abengoa front-surface mirror.

Compared to glass mirrors, thin film reflectors are lighter in weight, potentially lower in cost and 

higher in reflectance, and they can serve as part of the structure with an appropriate substrate. 

Elimination of glass facets also removes all of the individual struts and attachments needed for 

the mirrors and can reduce installation costs, although it also eliminates the ability to make field 

adjustments of mirror alignment. Mirror breakage is also avoided. However, some thin reflectors 

may not have sufficient abrasion resistance to allow the vigorous cleaning needed to maintain 

high long-term specular reflectance. The development and successful application of hard coat 

finishes could resolve this issue.

Even for a highly specular mirror surface, the fraction of the reflected direct normal radiation 

that intercepts the receiver tube, or the intercept factor, is also a function of the shape of the 

mirror and the size of the receiver tube. How close the intercept factor is to 1 (100% intercept) 

depends on how closely the reflector surface approaches that of a perfect parabola and how well 

the reflector is aligned to the receiver tube. Irregularities in the mirror facets, the influence of 

structural support elements, deflections due to weight, wind, drive forces, and thermal 

expansion/contraction will all impact the shape. Obtaining a highly accurate reflector shape is a 

critical step in the development of a new parabolic trough collector. NREL's Video Scanning 
12

Hartmann Optical Test (VSHOT) laser ray trace technique can characterize the point-by-point 

slope error of a reflector, and manufacturers have made use of this tool to tune their designs. 

Even with a perfect mirror in terms of shape and reflectivity, if it is not aligned correctly, it will 

not perform well. The next step is to take into account the whole solar collector assembly. There 

are multiple modules on a single drive and one tracking sensor. Sandia's Theoretical Overlay 

Photographic Collector Alignment Technique (TOPCAT) system has been used successfully in 

the field to correct alignment of the mirrors with the receiver tubes, which takes into account 

module-to-module misalignment as well as any receiver misalignment. Optical alignment is 

superior to mechanical means and provides the best option for aligning mirrors. TOPCAT had a 

demonstrated improvement for one loop at SEGS VIII of 3.5%. Better mirror alignment also 

allows the use of higher concentration ratios and higher temperatures, reducing storage cost.

The design of new reflectors must consider that the optical performance of the reflector has a 

one-to-one correspondence with the levelized cost of electricity (e.g., a 5% drop in reflectance

will cause a 5% increase in LCOE). However, the cost of the reflector represents only about 4% 

of the total plant cost. (A reflector design will also impact the collector frames. The collector and 

frame costs together represent about 10% of the total plant capital cost.) So reductions in 

reflector cost may not lower the levelized cost of electricity unless they have very little negative 

impact on reflector performance. (Conversely, even a small increase in optical performance 

could justify an increase in reflector cost.)

2.2.4 Optical Materials Testing

Optical materials testing provides DOE and industry with characterization of the optical 

properties (specular reflectance, absorptance, and emittance), durability performance and cost 

improvements of potential advanced solar mirrors and solar selective receiver coatings. 

Candidate materials are identified based on their potential for low cost and high optical 

performance and durability. Candidate solar reflectors under test include thin, thick, and 

laminated glass, aluminum reflectors, and silvered polymer. Materials are optically characterized 

prior to being subjected to exposure in real and simulated weathering environments. Optical 

durability is quantified by periodically re-measuring hemispherical and specular reflectance of 

solar mirrors and absorptance and emittance of solar selective coatings as a function of exposure 

time to assess optical durability. These materials are subjected to outdoor weathering at a variety 

of geographically diverse exposure sites. In addition, accelerated exposure testing (AET) of these 

materials in parallel under laboratory-controlled conditions is correlated with the outdoor results 

to predict service lifetimes. A historical database of optical durability results is available and 

contains more than 1,000 experiments containing more than 20,000 samples encompassing more 

than 300,000 measurements, which date back over more than 21 years. This data provides the 

confidence needed for financing projects using these materials.

2.2.5 Foundations

Foundations represent about 7% of the collector field cost. Various types of piers, footings, slabs, 

and anchor systems are possible. Costs and performance of these various alternatives will depend 

on collector size and design, as well as local soil conditions. The FOA contractors are exploring 

various foundation options.
13

2.3 Storage Subsystem Improvement Opportunities

As pointed out earlier, the thermal storage subsystem cost at $167/m2 represents 24% of the total 

plant capital cost. The storage subsystem cost is dominated by three components: the tanks (30% 

of total storage subsystem cost), pumps and heat exchangers (21%), and the molten salt (44%). 

Typically, two-tank indirect systems are used with a hydrocarbon working fluid in the collector 

field and molten salt in the hot and cold tanks.

Changes in the collector field can impact the size of storage that is needed. Specifically, 

operating at higher temperature will mean a larger temperature difference for the storage with a 

consequent decrease in the required storage volume. A higher operating temperature also results 

in higher power cycle efficiency. Because the higher temperature also means a greater driving 

potential for heat losses, additional attention must be paid to this by using a higher concentration 

ratio (which puts more stringent requirements on the optical components) or by developing an

absorber surface with lower emittance. But the main challenge in going to a higher collector field 

operating temperature is finding a suitable heat transfer fluid (HTF). Previous research at NREL 

and elsewhere has essentially eliminated consideration of organic HTFs at temperatures greater 

than 393°C due to thermal degradation of the HTF and the resulting generation of hydrogen gas. 

Molten salts currently offer the best option for near-term development of a high-temperature 

HTF. It is ideal if the molten salt storage fluid can be used directly in the collector field. In that 

case, the cost of heat exchangers and the performance penalty associated with the temperature 

drop across the heat exchangers can be eliminated. 

However, while molten salts have the capability to operate over 500°C, their freezing points are 

in the neighborhood of 100°C–200°C. Although the hot salt would be circulated through the field 

at night to prevent freezing, there is still a potential for a freezing of the HTF. It is thus generally 

assumed that a reliable and demonstrated means must be provided for thawing out the field in the 

event of freezing, either by use of electric resistance, steam heating, or some other means. If it is 

not thawed out evenly, piping or receiver damage can result. Sandia, Solar Millennium, and 

Halotechnics are developing salt formulations with lower freezing points (but lowering the 

freezing point tends to also lower the maximum operating temperature of the fluid). 

Additionally, experiments are underway at Sandia to determine how frozen pipe can be thawed 

safely. The Italian National Agency for New Technologies, Energy, and Sustainable Economic 

Development (ENEA) has also done work on this. 

High-temperature fluids other than salt are being investigated. These fluids distinguish 

themselves from the molten salts in that they are not single-phase homogeneous formulations. 

NREL and Texas A&M University are developing nanofluids. These fluids consist of traditional 

homogeneous fluids such as molten salts with added nano-scale particles. These particles have 

unique properties and are also able to modify the thermophysical properties of fluids. As such, 

these fluids are considered heterogeneous but may behave and function as homogeneous HTFs 

with enhanced thermophysical properties. Ongoing work to date has focused on using 

nanoparticles to increase the heat capacity of storage fluids. NREL is considering expanding this 

work to include lowering the melting point and increasing the liquid temperature range of fluids 

that already exhibit high-temperature stability.

The current storage material of choice, molten salt, is very expensive, so there has been interest 

in moving to a single-tank thermocline design in which a filler material (rock or sand) is used, 
14

thus reducing the salt volume by 70% or more. In such a design, the hot fluid resides in the top of 

the tank and the cold fluid is in the bottom. Maintaining a good thermocline (i.e., a sharp 

demarcation between the high and low temperatures) is important to ensure that the lowesttemperature fluid always goes to the collectors and the highest-temperature fluid always goes to 

the steam generator. Various ideas have been proposed for maintaining the thermocline, and 

these represent an important research area. Some (but not all) studies of such thermocline tanks 

have suggested that over many charge and discharge cycles, slumping of the filler material might

occur, causing high hoop stresses at the bottom of the tank and leading to tank failure. Further 

investigation is needed to determine the extent of this problem, and various means could be 

studied for mitigating it. Although thermocline tanks have typically been proposed for a direct 

system, they could also be used in an indirect design if it is decided that running molten salt in 

the collector field is too difficult. Even in an indirect design, the thermocline tank would offer 

the advantage of replacing high-cost molten salt with cheap filler material.

There is a general need to increase the stored energy density of thermal storage systems. One

approach is to identify storage materials with lower costs and better thermal properties than those 

typically used for storage. NREL is using the FactSage software to evaluate the thermal 

properties and phase behavior of new salt formulations. NREL has identified a formulation that 

has a higher heat capacity and lower cost than solar salt that can reduce storage volume by about 

20%. To complement this modeling effort, NREL has built a materials laboratory to measure and 

validate the thermophysical properties of such fluids.

The use of phase-change enthalpies to increase the stored energy density has potential to reduce 

the energy storage inventory by a factor of two or more. Implementation of phase-change 

storage requires innovative solutions to several practical barriers. Energy transfer due to phase 

change occurs at a single temperature and therefore does not match well to the sensible enthalpy 

of a single-phase HTF. This mismatch can be addressed by using a set of cascading phasechange materials with varying transition temperatures. NREL is modeling phase-change storage 

and using FactSage to identify salt formulations that possess similar physical properties and 

uniformly-spaced transition temperatures for efficient energy transfer in phase-change storage 

systems.

The most significant practical barrier to the use of phase-change storage is the solid-phase 

thermal conductivity, which limits heat transfer during discharge and the power density of the 

storage system. Many solutions to this issue are being investigated at NREL, Sandia, 

universities, and industry. One approach is the use of encapsulated phase-change particles or 

pellets. Their size range varies from nanometer, which is being investigated as an additive to 

storage fluids to increase heat capacity, to millimeter, which is being investigated as the storage 

medium for a packed bed, thermocline design. In any case, the size of the encapsulated phasechange material is such that heat transfer is not limited by the thermal conductivity of the solid 

phase. Another approach is to enhance the conductivity of the solid phase with heat pipe 

modules. Still another is to develop methods to prevent the formation of the solid phase onto the 

heat transfer surface or remove it as it forms. Variations of all of these approaches are currently 

being investigated.

Thermochemical energy storage perhaps offers the greatest improvement in storage energy 

density. This type of storage uses the enthalpy associated with a reversible chemical reaction. 
15

The enthalpy can exceed the sensible enthalpies of storage materials by an order of magnitude. 

Research and development in this area is considered to be long term because there are several 

practical limitations to its implementation. The reactions tend to occur at temperatures (1,000°C) 

greater than those currently being used or considered in CSP technologies, and the substances 

that undergo reaction can be very corrosive. Lastly, even though the intrinsic reaction rates are 

appreciable and reversible, morphological changes in the solid phases of the reaction mixture 

tend to decrease surface areas and limit the mass transfer that is required to sustain the reactions 

at required rates. Currently, one company is performing research in thermochemical conversion 

for CSP thermal energy storage.

Various storage means have been explored in Europe, including concrete thermocline storage 

and phase change storage. The latter is especially well suited to trough systems that boil water in 

the collector field, as the phase-change temperature can be matched to the field operating 

temperature. (Where heat must be transferred over a temperature range, multiple phase change 

tanks having different phase-change temperatures can be employed; however, this increases the 

technical challenge as each phase-change system must be made to operate successfully.) Both 

concrete and phase-change storage have issues associated with conducting heat into and out of 

material in a solid state. 

2.4 Overall Collector Design Improvements

Existing troughs rotate around an imaginary axis between the receiver and the collector. Other 

axes of rotation are possible, specifically around the receiver or around the torque tube. Rotating 

around the receiver would mean that the receiver is fixed and so many ball joints or flex hoses 

could be eliminated. Increasing the aperture width of the collector decreases the number of 

receiver tubes, pylons, and drive motors per square meter of collector. Sandia has proposed a 2X 

trough in which the aperture is doubled with the same receiver tube diameter, thus doubling the 

concentration ratio. Clearly there are potential cost savings associated with going with a larger 

aperture, although it is not clear that a factor of two increase in the aperture is the optimum or 

that it is optimum to double the concentration ratio. Lower concentration ratios result in lower 

parasitic pumping power due to larger receiver tube diameters, as described earlier, and modern 

receivers already have very low heat loss (at least when new). The module length can also be 

increased. When making the collector bigger, one must account for increased wind loads and 

potential difficulties associated with transporting the collector components and assembling them 

in the field. Detailed analysis by the laboratories covering the various degrees of freedom is 

critical to identify optimum designs.

2.5 O&M Improvement Opportunities

The estimated fixed O&M costs for a 100-MW plant in southwest Arizona were $8,500,000, 

(approx. $70/kW-yr) based on shifting the WorleyParsons analysis to that location [10]. Variable 

O&M costs (for utilities and water) are estimated at $2.5/MWh. Total O&M costs equate to 

about 1.5¢/kWh. This cost is consistent with the most recent data from the SEGS plants. At an 

LCOE of 14 cents per kWh, this represents 11% of the cost of the delivered electricity and is 

thus a very important item. 

Of the $8.5 million annual O&M cost, most is for plant operation. However, solar field 

maintenance costs (labor plus materials) were estimated at $1.6 million, which is 19% of the 

O&M cost. Including mirror washing costs brings the total to $1.9 million or about 23% of the 
16

total plant O&M cost. O&M data from the latest plants is not available and so we currently must 

rely on results from the older SEGS plants. A 2009 review by NREL focusing on the SEGS 

plants assessed the various items affecting both maintenance costs and failure rates. The relative 

failure rates of different items are shown in Figures 2 and 3. 

Figure 2. Percentages of total solar field component failure at SEGS III-VI, 1989-2005 average [8].

Figure 3. Causes of lost power production at SEGS III-VI, 1999-2001 average [9].

58%

30%

12%

Reflector Panel

HCE

Other

20%

18%

10%

9%

8%

8%

7%

3%

3%

2%

12%

Field Reflectivity 

HCEs: Broken 

Other (Clouds) 

SF Availability/Tracking 

Wind 

Maintenance Outages 

HCEs: Lost vacuum 

Forced Outage - Solar Steam 

HCEs: Fluorescent 

Broken RPs 

Other
17

As shown in Figure 2, broken reflector panels have historically been the biggest problem. These 

involved panels cracking, breaking, and becoming separated from their mounting pads. The glass 

breakage and cracking was primarily due to wind damage, and the pad separation was primarily 

due to adhesive failure. Thermal expansion of the metal pads relative to the glass panels was a 

major contributor, causing Flabeg to change to ceramic pads. Wind damage was especially 

problematic on rows at the edge of a field, which were not sheltered by other rows. Figure 3

shows that loss of reflectance accounts for a significant fraction of lost power production.

Receiver breakage was another major failure item. Failures involved vacuum loss, hydrogen 

infiltration, glass envelope breakage, and degradation of the coating. All of these generate 

replacement costs—and until replacements are made, performance degradation. This is 

especially the case for broken glass receiver tubes, which can have high heat losses on windy 

days. The most recent data for SEGS indicated that receiver tube failures had decreased to 3.37% 

of the total field receivers per year. Of these failures, 55% were reported to involve broken glass 

and 29% involved loss of vacuum, in most cases due to the failure of glass/metal seals, but also 

due to bowing tubes. Tubes exposed to even just one sun can bow due to differential heating if 

they do not have heat transfer fluid flowing through them, and this problem has become even 

greater as receiver coatings have improved.

Mirror breakage can be addressed by using thin film reflectors, although issues such as buckling 

may need to be avoided. Also, laminated mirrors that allow mirrors to crack without falling off 

could maintain reflective performance. Wind breaks and reinforcing perimeter collectors can 

help prevent wind damage. To maintain high reflectance, lower-cost and more effective cleaning 

systems and anti-soiling coatings are being investigated. Developing an automated contact 

washing system, possibly integrated into the trough itself, could reduce cleaning costs and ensure 

continuous high performance. 

A receiver tube breakage rate of 3.4% per year seems unacceptably high, given that the receivers 

themselves represent 30% of the solar field material cost and would require additional labor to 

replace. Data from the latest plants is needed to determine the current failure rate. In general, 

data collection is needed to investigate what the latest failure mechanisms are and identify 

potential means to prevent them. Loss of vacuum and hydrogen infiltration in receiver tubes can 

lead to significant performance losses and has been discussed in Section 2.2.2. 

2.6 Parasitic Power Reduction Opportunities

Analyses by WorleyParsons indicate that the parasitic power as a percentage of gross turbine 

output ranges from 13% for a water-cooled plant to 15% for an air-cooled plant. Approximately 

half of the parasitic power is used to pump the heat transfer fluid. Runs of a piping model 

indicate that about half of this (or about one-quarter of the total parasitic power) results from the 

pressure drop in the receiver tubes. The pumping power required for the receiver tubes varies 

inversely with the diameter of the receiver tube raised to the fifth power. So a small increase in 

receiver tube diameter can result in a greatly reduced pumping power. The larger receiver can 

also compensate for optical inaccuracies in the mirrors and the mirror-receiver alignment. 

This savings in pumping power must be weighed against an increase in the cost of the larger 

receiver and somewhat increased heat loss. Modern receivers have very high vacuums (<0.1 Pa) 

and their solar selective coatings have low emittance (<0.10) such that overall heat loss from 
18

them is very low, meaning that a small percentage increase in this heat loss will not necessarily 

have a significant impact on overall collector efficiency. Because the power required to pump the 

heat transfer fluid through the receivers represents 3%–4% of the plant gross power, it may make 

sense to evaluate the use of larger-diameter receiver tubes than the ones that have typically been 

used. (However, header pipe sizes must be carefully chosen to ensure adequate flow uniformity 

if the pressure drop in the collectors is reduced.) A larger aperture width collector also helps by 

reducing the number of receiver tubes in the field. Another opportunity for reducing parasitic 

power relates to the properties of the circulating HTF. Reducing HTF viscosity will reduce 

pumping power. Increasing the HTF volumetric heat capacity (ρCp) will reduce fluid velocity 

within the receiver tubes and also reduce pumping power.

In air-cooled plants, fan power represents 24% of the parasitic power, or 3% of the gross power. 

Air-cooled condenser arrays typically use two-speed fans, and this was assumed in the 

WorleyParsons analysis. Because fan power is highly nonlinear as a function of air volumetric 

flow rate (it varies with the cube of flow rate), the use of a variable frequency drive could cut 

down considerably on fan power. Variable frequency drives (VFDs) are not typically used in 

conventional air-cooled power plants because of their cost, but for a parabolic trough collector 

system that is dominated by collector field capital costs, the use of VFDs to reduce fan power 

consumption may be worth considering. 

2.7 Advanced Systems Concepts

2.7.1 Direct Steam Generation (DSG)

Current trough collector fields are limited to about 400°C outlet temperature because that is the 

upper temperature limit of the oil. Molten salt would allow higher temperatures but at the 

expense of a high freezing point. Another option is to run pressurized water in the collector field. 

An early study by Murphy and May at NREL showed that boiling water directly in a parabolic 

trough collector field was an attractive option for providing industrial process steam. Researchers 

from the German Aerospace Center (DLR) have analyzed direct steam generation in a trough 

field and found that there are some challenges involved in maintaining balanced steam pressures. 

There is also the issue of coming up with a suitable storage system for DSG designs. 

Nevertheless, because of the potential advantages associated with operating at higher 

temperatures, this remains a promising option.

2.7.2 Hybrid Designs

Recent changes in the natural gas industry make it more attractive to combine natural gas 

burning with solar electricity production. Large reserves of gas shales have recently become 

available. In addition, because the use of natural gas to produce electricity releases less carbon 

per MWh than a coal plant, the natural gas industry has taken the position of strongly supporting 

carbon reduction legislation. They see natural gas as an enabling technology for renewables. 

Natural gas is burned to provide up to 25% of the power at SEGS. Various studies have looked at 

integrated solar-combined cycle (ISCC) plants. NREL has been investigating a new 

configuration, and preliminary cost estimates look promising compared to typical ISCC 

configurations. A preliminary analysis has indicated that such a design could lower the real 

LCOE (compared to an all-solar parabolic trough plant) by about 5¢/kWh for a design that is 

about 46% solar/54% gas. (Note, however, that this lower cost is due in part to the fact that 

natural gas is being burned. Analysis is underway to separate out the solar-specific cost.) 
19

Integration and transmission issues will be minimized with solar/fossil designs. While a reduced

solar fraction may sound less attractive, it provides a path to support manufacturing of solar 

fields and familiarity for utilities that will lay the groundwork for dedicated CSP plants.

2.7.3 Supercritical Brayton Cycle

Consideration has also been given to the use of supercritical CO2 Brayton cycles. The potential 

for s-CO2 is longer term, because no one yet builds s-CO2 turbines. Most references suggest the 

s-CO2 cycle could be slightly more efficient than superheated steam cycles at roughly equal 

temperature and slightly higher pressure. The turbomachinery is projected to be 20% less 

expensive. Such a power cycle would be more compatible with molten salt storage than a steam 

cycle. In theory this could lower LCOE by about 1¢/kWh.

2.8 Environmental Issues

Water usage is a major concern with parabolic trough power plants because they are located in 

arid regions. A typical water-cooled plant uses 800 to 900 gallons of water per MWh. NREL has 

conducted analyses on the use of dry and hybrid (wet/dry) cooling to greatly reduce water 

consumption. Further analysis is needed to determine the impact of time-of-day electricity rates. 

Also, there is a need to study potential ways to recycle the water used for mirror washing and 

steam cycle make-up (needed to replace boiler blowdown water used to remove mineral buildup).
20

3 Recommended Activities

In this section we list the specific proposed activities needed to achieve the full range of cost 

reductions. These activities include some that are currently underway by the laboratories, FOA 

contractors, and industry, as well as proposed new activities. SAM computer simulations were 

made to determine the impacts of various improvements on the levelized cost of electricity. 

Figure 4 shows the results of a sensitivity analysis for the various cost reduction areas. This 

clearly shows that reducing collector field cost is a key activity. Table 4 shows scenarios for 

specific cost reductions using improvements that are "in the pipeline" (shown for the year 2015) 

as well as a combination of improvements which, if made by 2017, could achieve 2017 cost 

goals. Note that it is assumed that there will only be a 10% ITC in 2017. Thus today's costs are 

shown not only with today's 30% ITC but also what they would be with a 10% ITC so that the 

extent of needed improvement can be seen.

Figure 4. Sensitivity of LCOE to potential cost reductions in different areas.
21

Table 4. Estimated current and future costs for parabolic trough systems. Representative cases at 6 and 12 

hours of storage are shown.

2010 2010 2015 2015 2020

Design Inputs:

Turbine MWe (gross/net) 111/100 110/100 280/250 110/100 280/250

HTF Syn. Oil Syn. Oil Syn. Oil Salt Salt

Solar Field Temperature (°C) 391 391 391 450 500

Solar Multiple 1.3 2.0 2.0 2.0 2.8

Thermal Storage Hours 0 6 6 6 12

Cost & Performance Inputs:

System Availability 94% 94% 96% 96% 96%

Turbine Efficiency (cooling 

method)

0.377

(wet)

0.377

(wet)

0.356

(dry)

0.379

(dry)

0.397

(dry)

Collector Reflectance 0.935 0.935 0.95 0.95 0.95

Solar Field ($/m2) 295 295 245 245 190

HTF System ($/m2) 90 90 90 50 50

Thermal Storage ($/kWh-t) - 80 80 50 25

Power Block ($/kWe - gross) 940 940 875 1140 875

O&M ($/kW-yr) 70 70 60 60 45

Cost & Performance outputs:

Capacity Factor 26% 41% 43% 43% 60%

Installed Cost ($/W) 4.6 8.0 7.9 6.6 6.5

LCOE (cents/kWh, real) 17.3 17.9 16.5 14.2 9.9

3.1 Items Affecting the Collector Field

3.1.1 Overall Collector Design

Industry is moving toward larger-aperture troughs, as these will reduce the number of collectors 

and receivers per megawatt with a corresponding reduction in overall piping and number of 

drives. The upper limit on size is probably based on survival wind loading, transportation issues, 

and installation difficulty. Sandia has proposed to double the aperture while maintaining the 

same diameter receiver tube. This increase in aperture size is a move in the right direction, but a 

detailed systems analysis study is needed to determine the optimum combination of parameters 

that minimizes the overall LCOE including initial costs, performance, and parasitics. Sandia and 

NREL should work together to investigate this, using contractor support to get the best cost 

information. Results should be shared with industry to obtain their feedback. Because some 

similar exercises are underway in the FOA contracts, the awardees should be urged to share their 

results on a confidential basis with the DOE labs. This information can then be used in a generic 

way. Higher concentration ratios may require a more accurate tracker, and Sandia should pursue 

its investigation of an improved closed-loop tracker. 

3.1.2 Collector Structure Improvement/Wind Load Investigation

Considerable work is being done to improve the collector support frame by FOA contractors and 

other manufacturers. All of the main methods are being investigated: torque tube and torque box, 

space frame (or truss), and monocoque construction. DOE should continue to support these 
22

efforts and carefully monitor the progress in cost reduction. The cost evaluations shall provide 

detail on materials costs and content, assembly costs, and installation costs. The national labs 

will also help by providing test results to ensure that the structures meet optical and wind load 

requirements. Because wind loads tend to drive the structural requirements, the labs will measure 

wind loads in an actual collector field and investigate innovative ways to reduce those loads. 

Various ideas have been suggested for decreasing wind loads on trough fields, and the 

laboratories should work with industry to evaluate these. Finite element analysis will be used to 

study how wind and gravity loads affect the collector.

3.1.3 Receiver Tubes

Receiver tubes and fittings represent 30% of the collector field material cost. SAM simulations 

show that the LCOE from the plant can vary by more than ±2¢/kWh depending on how well the 

receivers absorb and retain the heat from the sun. Although great strides have been made in 

improving receiver performance, there is room for improvement in both the areas of new 

receiver tube performance and (especially) long-term performance.

3.1.3.1 Advanced Receiver Coating Development

SAM analysis demonstrates that a 10% improvement in the solar field performance for a 

parabolic trough system lowers the nominal LCOE for the 2009 baseline costs by 1.2¢/kWh,

whereas a 10% cost reduction in the same solar field will reduce the nominal LCOE by 

0.8¢/kWh. Thus performance improvements have a large impact on the overall economics. The 

overall objective of this activity is to develop new, more efficient selective coatings with both 

high solar absorptance (α > 0.96) and low thermal emittance (ε < 0.07 at 450oC) that are 

thermally stable above 550oC, ideally in air, with improved durability and manufacturability and 

reduced cost. The resulting reduction in LCOE by moving to higher temperatures flattens out 

above 450°C, but a coating that is durable in air allows the receiver coating to be functional if the 

vacuum is breached in an evacuated tube for parabolic trough applications. In addition, a coating 

stable in air can be used in non-evacuated linear Fresnel receivers and in towers. 

3.1.3.2 Hydrogen Mitigation

SAM analysis predicts that a parabolic trough plant with no hydrogen (no hydrogen-filled 

receivers) will generate electricity for 3¢/kWh less than the same plant that has hydrogen in 50% 

of its receivers. This is an enormous penalty, so it is no wonder that FPL Energy has replaced 

millions of dollars of receiver tubes due to hydrogen infiltration. Newer receiver designs 

incorporate more getter capacity and use hydrogen diffusion barriers. Nevertheless, the latest 

field results suggest that this problem is still occurring even in new tubes. Mineral oil heat 

exchanger fluids will likely continue to be used in new installations for a number of years until 

an alternative high-temperature fluid is developed and proven, and so addressing the hydrogen 

problem should be a high program priority. In addition to the changes that the manufacturers 

have made, potential solutions to be explored are:

• removing hydrogen centrally

• using a large-molecule inert gas like argon or xenon or nitrogen to block the motion 

of the hydrogen molecules

• improved hydrogen barrier coatings on the receiver tube
23

• developing a new heat transfer fluid that does not generate hydrogen.

3.1.3.3 Development of Lower-Cost Receivers

Use of an inert gas instead of a vacuum could eliminate the need for getters and reduce 

manufacturing cost. There are low-cost evacuated receivers manufactured abroad that could be 

suitable for trough application, and this would be worth exploring. A FOA or other means to 

stimulate more American manufacturers to enter this market will help spur competition and 

reduce cost.

3.1.3.4 Receiver Heat Loss Measurement

In order to evaluate the effectiveness of new receiver advances, receiver performance must be 

accurately measured. Results for the NREL test loop are very useful to manufacturers as they 

develop higher-performance improved designs. The tests also serve as an independent 

assessment of the technology, which can be given to investors and due-diligence engineers 

involved with plant financing, thereby aiding deployment. Technical reports that result from 

these tests show manufacturers the state-of-the-art and encourage other manufacturers to enter 

the market if they think they can make something that performs as well or better. This testing 

support should continue. NREL also uses an infrared camera system to rapidly measure and 

record heat loss from all receivers in the field to determine those that are losing too much heat 

and are in need of replacement. This technology has been transferred to FPL Energy and should 

be made available to other CSP industry members.

Figure 5 summarizes the results of Solar Advisor Model sensitivity runs that indicate the impact 

of receiver-related changes on the real levelized cost of electricity. DOE Program work is not 

only aimed at improving cost but also eliminating problems (such as hydrogen infiltration in 

receivers) that can decrease performance. Hence this graph shows the impact on costs that would 

occur if corrective measures are not taken, as well as the impact of improvements over the 

design values.

Figure 5. SAM results showing the impact of changes in receiver performance on the real levelized 

cost of electricity. 2009 costs are shown here.

0.0 0.1 0.1 0.2 0.2 0.3

Improved Receiver Cleanliness (+1%)

Improved Receiver Absorptivity (+1%)

Reduced Receiver Bellows Shading (-2%)

Reduced Receiver Thermal Losses (-20%)

Reduction in Levelized Cost of Energy (cents/kwh)
24

3.1.4 Improved Mirrors

As discussed earlier, energy output is directly proportional to optical efficiency; therefore high 

optical performance is critical to minimizing the levelized cost of electricity. Work is needed to 

lower mirror costs, maximize reflectance, ensure long-term durability, and promote cleanability

with minimal water use. Mirrors have a very high markup because of the limited number of 

suppliers, so efforts should be made to stimulate competition. Another reason glass mirrors are 

expensive is that there are not dedicated production lines for the low-iron glass needed for solar 

applications. There are costs incurred in switching back and forth between the production of 

regular green glass and low-iron glass. In other cases, dedicated plants may not be running at 

capacity, so the cost of manufacturing capability must be spread over a small number of sales. In 

any case, as mirror volume production increases, costs should drop significantly. Thus support 

for large, successful projects should help drive costs down. The Program should consider 

providing incentives for dedicated low-iron glass production. There are various efforts underway 

to develop lower-cost, higher-performing reflective surfaces, and these should all continue. 

These efforts are as follows:

• A collaborative NREL-industry effort has developed ReflecTech thin film reflector 

material that is applied to a thin aluminum substrate. Current efforts are aimed at 

further improvements.

• 3M is developing a silvered polymer reflective film as an improvement over its 

earlier ECP-305+ film and under a FOA is coating it with an anti-soiling hard coat.

• Abengoa has a FOA contract to develop a high-performance front surface mirror.

• PPG has a FOA contract to develop an encapsulated glass mirror.

• Alanod has developed an anodized aluminum mirror with enhanced reflectivity.

• The government has provided $38 million of economic stimulus funds to mirror 

manufacturers to reduce mirror costs.

Other industry efforts are aimed at developing thin glass, tempered glass, and laminated glass

and improving the wet chemistry coatings. In the long run, it appears that thin film reflectors 

have the greatest chance of significantly reducing collector cost because they can serve as part of 

the structure and they eliminate the need to install individual glass facets. They also result in a 

much lighter-weight collector, which should lower installation and transportation costs. 

Improved mirror facets should be a top program priority. The dish/Stirling industry has adopted 

the use of structural facets involving a composite structure with thin glass mirrors. Structural or 

composite mirrors should be investigated for use in parabolic troughs.

3.1.5 Optical Materials Testing

The Program should continue to conduct tests of optical properties (specular reflectance, 

absorptance, and emittance), durability performance, and cost improvements of potential 

advanced solar mirrors and solar selective coatings. Maintaining a database of durability results 

for a wide variety of reflector and receiver materials is vital to ensuring high confidence on the 

part of project investors and successful long-term field operation.
25

3.1.6 Foundations and Support Structures

The specifications for these will depend on collector and site characteristics. Several FOA 

contractors are investigating this, and it is important that they report to DOE the details of the 

improvements being made and their impacts on cost. The laboratories should provide support in 

the form of finite element structural analysis for contractors that do not have this capability. 

Figure 6 summarizes the potential impact on LCOE of changes in collector-related parameters. 

Mirror optical performance has a major impact on cost.

Figure 6. Impact of collector-related changes on the real levelized cost of electricity.

3.2 Heat Transfer Fluid

Currently the operating temperature of a trough field is limited to 400°C. Higher operating 

temperature would provide higher cycle efficiency, but it will also increase heat loss. The main 

advantage, however, is the smaller storage volume that would result. Advanced heat transfer 

fluids could also reduce parasitic pumping power, and because the fluid itself represents almost 

2% of the total plant cost, research should also be aimed at identifying fluids with a higher heat 

capacity and a lower materials cost. Analyses by the laboratories and industry partners estimate 

that an HTF that operates at 500°C but requires freeze protection will reduce LCOE by $0.0125–

$0.0175/kWh, depending on the amount of heat tracing. An HTF that is capable of operating at 

500°C without freeze protection will reduce LCOE by $0.0225/kWh.

Using molten salt in the collector field is still a potential mid-term option. Sandia should 

continue its efforts to develop salts with a lower freezing point and continue to run tests to 

determine how frozen salt can be safely thawed. Their test results will also determine the 

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4

Increased Mirror Degradation Rate (-0.25%/year)

Reduced SCA Tracking Error and Twist Error (-1%)

Improved Solar Field Availability (+1%)

Improved Optical Intercept (+2%)

Improved Mirror Cleanliness (+2%)

Improved Mirror Reflectivity (+3%)

Reduction in Levelized Cost of Energy (cents/kwh)
26

compatibility of molten salt with various piping components. NREL intends to identify and test 

new materials for joint seals that are compatible with molten salt. Without such test results, loan 

rates will be much higher. 

Long-term R&D at the labs should focus on modeling and experimental research to identify 

promising fluids and test promising formulations to evaluate their properties. Current work on 

nanofluids should be expanded to investigate nanomaterials that lower the melting point and 

increase the liquid temperature range for molten salts and other liquids. Other innovative 

approaches to developing an appropriate HTF should also be pursued.

3.3 Storage Cost Reduction

There are a number of FOA contracts that are working on storage cost reduction, including HTFs 

with improved properties, storage media with greater stored energy density, and advanced 

storage concepts. Work in this area should continue to focus on improved HTFs and storage 

media and advanced storage concepts. Development of improved storage media should be 

directed at increasing the stored energy density as well as the power density.

3.3.1 Storage Fluid

Near-term R&D should utilize computer modeling software to identify fluids and fluid 

combinations that have a high heat capacity, are low in cost, and are stable at temperatures up to 

500°C. Long-term R&D should continue to investigate innovative approaches to dramatically 

increasing the heat capacity of storage fluids. Nanomaterials research has recently made progress 

in demonstrating the basic approach of increasing fluid heat capacity by adding phase-change 

nanoparticles to the fluid. This research is fundamental and requires the development of new 

methods to synthesize nanoparticles with the desired properties and stabilize these particles in 

the storage fluid. SAM analysis indicates that a two-tank storage system that uses a storage fluid 

with double the current heat capacity will reduce LCOE by $0.013/kWh.

3.3.2 Thermocline Storage

Thermocline tanks have the potential to greatly reduce the fluid inventory, but they present 

challenges in terms of hoop stress, thermocline control, and storage efficiency. Further computer 

modeling is needed to address these issues, and if the results are promising, a prototype tank 

should be built and tested. Laboratory work should be coordinated with FOA work to avoid 

duplication. Innovative variations of the basic thermocline design that eliminate thermocline 

spread and hoop stress should also be modeled initially to predict performance. Additionally, 

thermocline concepts that are based on encapsulated phase-change materials should also be 

modeled. Promising concepts should be validated with experimental prototypes. Several analyses 

by the laboratories and industry partners estimate that a properly performing thermocline will 

reduce LCOE by $0.009/kWh.

3.3.3 PCM Storage

PCM storage probably offers the best mid-term opportunity for significantly increasing stored 

energy density. The greatest barrier to this approach is the limited conductivity of the solid 

phase. This barrier limits the power density and performance of the storage system. Several FOA 

projects are currently investigating methods and geometries for overcoming this barrier. The 

laboratories should work with university and industry partners to provide support in the form of 

transport modeling and experimental testing and evaluation of PCM-based storage systems. 
27

Completely new storage concepts based on PCMs should also be modeled and evaluated. SAM 

and industry analyses show that PCM storage will reduce LCOE by $0.008/kWh.

3.3.4 Thermochemical Storage

Thermochemical storage represents the best option for storage systems that possess very high 

stored energy densities. Improvement in stored energy density of an order of magnitude over 

current technologies is possible. Development of thermochemical storage as a reversible process 

requires the solution of several practical problems. The reversible reactions typically occur at 

temperatures higher than those achieved by current CSP technologies. Reactants may be 

corrosive to reactor walls at the operating temperature. Changes in the morphology of the solid 

reactant phases may result in long-term degradation of reaction rates and power performance. All 

of these issues are fundamental to the process, so solutions to these barriers will require a longterm R&D effort.

3.3.5 Other Advanced Storage Concepts

Work in Europe and FOA contracts are covering phase-change storage and the use of solid 

media including concrete, ceramics, and sand. An overall systems analysis evaluation is needed 

to determine which of these methods holds promise for significant cost reduction. This analysis 

should utilize cost estimates generated in the FOA awards. Laboratory testing should be 

performed on the most promising concepts.

3.4 Parasitic Power Reduction

Electric power is an expensive, high-value form of energy. Any electric power used to move heat 

transfer fluid or perform other functions directly robs high-value solar-generated power and so 

must be reduced to an absolute minimum. Losing 13%–15% of gross generated power to 

parasitics as indicated in a recent WorleyParsons study (resulting in an equivalent increase in the 

cost of generated electricity) seems excessive and presents a major opportunity for improvement. 

(Some in industry believe this number is only 10%, but even that means there is considerable 

opportunity for improvement.) A thorough analysis of all parasitic power consumption should be 

performed to identify where parasitic losses occur and where the greatest opportunities for 

reductions are. The ways of reducing each of those should be explored and their costs weighed 

against the cost of electricity saved. The use of larger-diameter receiver tubes should be 

considered as part of a broader trough and trough field optimization effort. Attention should be 

focused on minimizing pipe elbows, valves, and other minor losses. Variable speed drives should 

be considered in place of throttling valves and single-speed or two-speed fans. This analysis 

should be performed by the laboratories, possibly in conjunction with a company having 

experience in plant energy efficiency studies (e.g., RMI). HTFs with lower viscosities and 

greater volumetric heat capacities should also be developed as part of the advanced HTF 

development effort.

3.5 O&M Cost Reduction

Because O&M costs represent about 20% of the delivered electricity cost, this should also be a 

high-priority area. Larger system sizes and the development of power parks will decrease the 

man-hours of operating labor needed per megawatt. It is also worthwhile to investigate potential 

areas for automation, such as mirror cleaning, where a trough-integrated system might be 

considered. Because keeping mirrors clean is critical, a system that automatically monitors 

mirror reflectance could help ensure that cleaning occurs only when it is most needed. Mirror 
28

breakage can be addressed by developing laminated glass mirrors or thin film reflectors. The 

latest SEGS data indicated that approximately 2% of the receiver glass breaks each year. This 

seems unacceptably high and should be studied. Prevention of breakage might require improved 

operating procedures. In any case, DOE should work with industry to collect the latest O&M 

data so that failure modes can be avoided or reduced.

3.6 CSP Environmental Impact

Environmental concerns have become a significant issue for CSP. The labs should continue to 

support the BLM PEIS effort by developing best practices for siting on BLM land. In addition, 

the life-cycle impact of trough systems needs to be assessed. This will remove barriers to 

deployment by reducing indirect costs such as the development of environmental impact 

statements. The specific cost savings need to be evaluated as part of this work.

A particularly important issue is water use. Analysis of dry- and hybrid-cooled plants indicates 

that water use can be reduced from 800 gal/MWh to 80 gal/MWh. A dry-cooled plant increases 

the LCOE from 3% to 8%, depending on location. Hybrid cooling systems will have a somewhat 

lower impact on the LCOE, depending on the ratio of dry to wet cooling, but they save less 

water. Further analysis will be conducted to determine the impact of time-of-day electricity rates 

on the cost effectiveness of air and hybrid cooling systems. Of the 80 gallons, 20 are used for 

mirror washing and 60 for steam cycle make-up water. A commercial product that spins water to 

remove particulates can possibly be used to recycle wash water and will be investigated. 

Removing dissolved solids from steam make-up water is more costly, but options will be 

evaluated.

3.7 Collector and System Performance Assessment

Any improvements made in the collector field must be checked via testing. A comprehensive 

testing program is needed to support cost reduction efforts. As described earlier, this type of 

support is needed to ensure success in the field, as it fosters economies of scale and learningcurve improvements. 

Modern parabolic trough collectors have high-performance receivers with minimal heat loss and 

thus have very flat efficiency curves. This means the performance is strongly dictated by the 

optical efficiency. A 1% drop in optical efficiency translates into a 1% increase in the cost of 

electricity. As a consequence, ensuring that deployed collectors have and maintain the highest 

optical performance is critical to success. It is also important that new, lower-cost collector 

concepts get the full benefit of both experimental characterization tools and optical models to 

achieve the highest possible performance. While secondary to optical performance, it is also 

important to measure thermal performance to ensure that cost reduction efforts do not unduly 

compromise performance at operating temperatures.

The following activities support a robust testing effort.

3.7.1 Mirror Measurement Methods

NREL has been utilizing the VSHOT method for determining slope error of mirrors. This has 

been the gold standard used by Acciona, SkyFuel, Abengoa, and Alcoa for determining mirror 

slope error and identifying where improvements are needed. Optical efficiency improvements of 

over 20% have been achieved using VSHOT to help develop new collectors. SAM runs indicate 
29

that for a 100-MW LS2 parabolic trough plant without storage in Daggett, CA with a 30% ITC, a 

20% improvement in optical performance results in a drop in LCOE of approximately 2¢/kWh. 

In fact, both Acciona and SkyFuel were able to make major improvements in their designs, and 

hence lower their costs of electricity, as a result of VSHOT testing. Because performing VSHOT 

tests is time-consuming and NREL cannot meet all the demands for new testing, an effort is 

needed to increase the speed of VSHOT testing and identify other optical techniques, such as 

deflectometry and a Distant Observer technique, that have the potential to take data more rapidly. 

This task will allow more collectors to be tested more quickly so that performance in the field 

can be ensured. SOFAST, a new system developed at Sandia for use with parabolic dishes, uses 

a digital video camera and a projected fringe pattern to produce the same data as VSHOT with 

much higher resolution as well as other information such as mirror twist. It is also faster to set up 

(about 1 hour), and the data can be processed in only about 10 seconds. Its extension to parabolic 

troughs will be investigated.

The TOPCAT alignment system developed by Sandia can be used at an installed collector field 

or at the end of production to determine the extent to which mirrors are out of alignment and

allow mirrors to be realigned to restore proper performance. As is the case with VSHOT, work is 

needed to increase the operational speed. A loop at SEGS VIII was aligned with a 5°C increase 

in its operating temperature as compared to before alignment.

The distant observer technique being developed by NREL involves the use of a camera from an 

aerial platform to determine the optical intercept factors and other optical information (such as 

surface errors and misalignments) for each collector in an entire field. If proven viable, this 

would greatly increase measurement speed compared to VSHOT and would allow an entire field 

to be measured in a short period of time. It is expected that using this tool to identify the main 

causes of optical errors at parabolic trough power plants will enable the plant operators to 

address those optical errors and thus improve their optical efficiency in the range of 2%–5%,

resulting in a decrease in LCOE of about 0.5¢/kWh. Distant observer can also be used to 

determine the impact of operational wind speeds on collector field optics. It can also include an 

infrared camera allowing rapid measurement of an entire field of receiver tubes and identifying

those in need of field repair or replacement. An NREL analysis indicated that if 60% of the 

receivers in a collector field suffer from hydrogen infiltration, receiver replacement will reduce 

the LCOE by about 3¢/kWh.

NREL and Sandia are both developing expanded laboratory space to develop and test new 

optical characterization concepts, including new facets. This includes the capability to apply 

loads to determine how collector shape is affected. Because optical testing is vital to the success 

of new collector concepts, lab development should be a high priority.

3.7.2 Collector Testing

NREL measures collector overall optical efficiency at an outdoor test loop. The measurement

accounts for mirror reflectance, mirror slope error, intercept factor, receiver glazing 

transmittance, and receiver absorptance. Tests of the SkyFuel SkyTrough collector revealed 

areas for improvement, which was very helpful in their final design. Test results are obtained at 

normal incidence and at several different angles. Sandia's outdoor rotating platform is used to 

measure the overall efficiency of parabolic trough collectors (including optical efficiency) at a 

range of operating temperatures and can be used to generate an efficiency curve. It can measure 
30

the performance improvement resulting from such changes as an improved tracking system. All 

new collectors should undergo one or both tests before being deployed in the field. Testing in 

FY10 should indicate the extent to which efficiency curves developed by the two different 

approaches agree. The data collected from these tests allow DOE to assess the optical and 

thermal performance of advanced collector designs funded by the CSP program. Developers 

need the data to predict solar field performance associated with advanced collector designs. The 

data is also critical for attracting the debt and equity necessary to finance projects based on 

advanced designs. 

3.7.3 Model Development

Modeling plays a pivotal role in cost-effectively developing efficient, low-cost parabolic trough 

system designs. Without a suite of modeling tools, the development path and time would be 

considerably longer and more costly. The CSP industry has access to and uses a variety of 

commercial or public (laboratory-developed) models designed to assess component and overall 

system performance. These vary in scope from simplistic models used for scoping analysis to 

more sophisticated tools for detailed engineering design and analysis. Many commercially 

available models are prohibitively expensive and/or are not conducive to CSP-specific 

applications. It is important that DOE continue to support the development of detailed 

component models (e.g., collector, receiver, storage, power plant) that allow both public and 

private researchers to understand and improve the performance of these subsystems. These 

models can be used in conjunction with system-level tools (e.g., Solar Advisor Model, IPSEpro, 

Gate Cycle) to better understand how various subsystems affect the overall system performance 

and to accurately predict the annual performance and levelized cost of energy of current and 

future parabolic trough systems. This information not only supports R&D efforts within the solar 

community, but is used heavily by the financial community in assessing the viability of 

commercial projects.

Optical models are important for developing new collector ideas. NREL's SolTrace should be 

improved to increase its speed and improve its user interface. Sandia's CIRCE also provides 

valuable optical modeling capability. Both tools should be kept available and improved.

3.7.4 Plant Performance Data Collection and Model Validation

This activity evaluates the total performance of operating plants and uses the data to validate 

performance models. This effort involves the development of a non-invasive test capability to 

measure fluid flow rates and temperatures in order to evaluate overall field performance. It can 

also include training a company to take a wide range of collector field measurements.

3.7.5 Test Standards

The development of an acceptance test standard for parabolic trough systems will increase 

confidence among members of the financial community and thus support financing. NREL 

should continue to develop this while supporting a parallel (but longer-term) effort underway by 

ASME. Work is also needed on mirror durability methodology development and standards 

development at the material, collector, and system levels. Work should also be done in 

conjunction with SolarPACES to develop a uniform receiver heat loss test standard.
31

3.8 Advanced Concepts

3.8.1 Hybridization

This task takes advantage of the strong interest expressed by the natural gas industry to partner 

with renewable energy. The laboratories should perform the systems analysis needed to find 

those combinations that maximize the efficiency of both solar energy collection and natural gas 

combustion and yield the highest rate of return for new trough installations. By combining solar 

with storage and natural gas, CSP has the potential to directly displace base load coal plants, 

thereby making an enormous contribution to carbon emissions reductions. It is recommended 

that the DOE laboratories develop partnerships with EPRI and the natural gas industry to 

accelerate this work. NREL has already identified a promising hybrid (solar/natural gas) concept 

and has filed a record of invention.

3.8.2 Direct Steam Generation

The Europeans have taken the lead on direct steam generation, but the laboratories should 

thoroughly evaluate the potential for this approach. While it allows for higher operating 

temperatures, coupling direct steam generation with thermal storage is a challenge. Direct steam 

generation also requires higher pressure, thick-walled receivers, affects solar field assembly due 

to the ASME standards for high pressure or boiler piping, and has other technical issues in its 

implementation. The labs should evaluate the European work on direct steam generation and 

decide if DOE-funded work in this area is warranted. 

3.8.3 Supercritical CO2 Cycle

As stated previously, a Brayton cycle using supercritical CO2 could potentially lower LCOE by 

about 1 ¢/kWh. Systems analysis modeling will be directed at providing a thorough investigation 

of the potential for this concept. Issues such as increased piping cost, parasitic pumping power, 

and reduced heat transfer coefficients should be considered.

3.8.4 Low-Cost Collector Concepts

The laboratories should consider exploring long-term line-focus collector options. SkyFuel 

currently has a FOA to investigate high-temperature linear Fresnel collectors using molten salt 

heat transfer fluid. Compared to troughs, this technology has a smaller number of receivers and 

the receivers are fixed, thus making easier the prospect of running molten salt in the collector 

field. The second phase of the SkyFuel study should determine whether this is a concept worth 

pursuing. If it is, the laboratories should consider providing further support for the development. 

Stretched membrane and inflated concepts have been proposed in the past. A study of these kinds 

of ultra-low-cost ideas is warranted as a high-risk but potentially high-return complement to the 

near-term development emphasized by the Program.

3.9 New FOAs

Because of the success of FOA contracts to date, this plan supports a new round of FOAs. These 

will include next-generation line-focus collectors with special emphasis on manufacturability, 

low-cost receivers (aimed primarily at encouraging new players to enter the market), hightemperature fluids and components (primarily to allow smaller storage volume), and advanced 

storage (to lower storage costs). NREL and Sandia will work with DOE to develop the detailed 

solicitations and then provide technical advisor support to the FOA winners.
32

3.10 Laboratory Support to FOA Contractors

NREL and Sandia staff serve as technical advisors on the FOA contracts in the areas of 

Collectors, Storage/Heat Transfer Fluid, Advanced Reflector, and Baseline Power. This activity 

serves as a vital link between the DOE laboratories and industry and helps ensure high quality in 

the work being performed. It is a vehicle for providing various modeling and testing support. It 

also helps keep laboratory staff up-to-date on the latest status of the technology. Funding for this 

needs to be increased to promote success and be continued with the new FOAs.

4 Spending Plan 

A proposed spending plan is shown in Table 5 (available as an Excel spreadsheet). It covers 

FY10 through FY16, the year at the end of which the 30% investment tax credit is currently 

scheduled to expire. Table 5 includes for each activity:

• the activity title

• the activity participants

• whether it is new (N) or existing (E)

• the relevant section of this plan

• the priority of the activity: high (H), medium (M), or low (L)

• an appropriate metric for the activity

• the potential improvement in that metric

• the potential impact on the levelized cost of electricity (LCOE)

• the time frame

• the needed funding for each fiscal year through FY16 (including the funding, if any, 

in FY10)

• a description of the activity.

It should be noted that each activity is evaluated individually. In reality, there would be overlaps 

in contributions from the various activities, so the potential improvements in the metrics and 

LCOE cannot be added together. Staff at NREL and Sandia and industry members were asked to

rank the various activities. These rankings were used to identify task priorities as high (H), 

medium (M), and low (L), with industry input being given somewhat more weight. These 

priorities are shown in Table 5 to aid DOE in allocating a finite budget. There was general 

agreement among industry members, however, that because reducing the cost of electricity 

generated from line-focus solar collectors by 50% is an ambitious goal, all of the activities 

described in this document can make important contributions to that effort.

Overall, we propose a near-term funding surge to maximize the chances of achieving the cost 

goal and to ensure that products and projects being built perform successfully, thus encouraging 

additional financing and greater deployment. New projects include overall collector design and 
33

frame improvement, development of low-cost receivers, field wind load measurement and 

mitigation, efforts to reduce parasitic power and O&M costs, expanded emphasis on reducing 

total plant water consumption, development of advanced hybrid system concepts, and 

investigation of ultra-low-cost collector concepts. 

5 Conclusions

Achieving a roughly 50% reduction in LCOE in six years is clearly recognized as a considerable 

challenge by most of the CSP industry. To reach that goal, it is important and necessary to 

simultaneously pursue all available avenues. This includes R&D to improve performance and 

lower costs of all components, studies to reduce labor and maintenance costs, encouragement of 

additional manufacturers and product sources, reduction of parasitic power requirements, 

analysis to explore new overall system design approaches, lower-cost collector designs, 

innovative financing options, and testing to ensure high quality in new systems. 

Fortunately, the Program is already fairly well focused on most of these activities, so a major 

change in direction is not needed. FOA projects have made significant progress, so these should 

be completed, and a new round of FOAs focusing on collector development, lowering of receiver 

costs, and storage is warranted. Recognizing the importance of maintaining high optical 

performance, optical characterization work will continue but be consolidated and transferred to 

industry as it is developed. 

There are a number of circumstances that would appear to increase the chances of success. 

Renewable portfolio standards and government solicitations have encouraged the development 

and deployment of new products. The line-focus industry has a good track record and significant 

engineering talent. A close partnership between industry and the DOE laboratories can lead to 

the successful deployment of large-scale systems, which will achieve economies of scale and 

foster learning curve cost reductions. One challenge will be to find the right balance between 

protecting individual commercial interests while at the same time sharing information that is 

valuable to the industry as a whole. Along these lines, DOE should consider establishing an 

industry-laboratory working group to promote mutually collaborative activities and information

sharing. 
34

Table 5. Multi-year activities and budgets.
35
36
37
38

Acknowledgments

The authors would like to thank Greg Kolb of Sandia National Laboratories for his input as well 

as the following industry members: Gilbert Cohen, Randy Gee, Ulf Herrmann, Chris Jensen, 

Dave Kearney, Bruce Kelly, Andrew McMahan, Alex Marker, Paul Nava, Hank Price, Adam 

Schaut, and David White. Finally, we appreciate the input from our colleagues at NREL. 

However, the authors are solely responsible for the final contents of this document.

References

1. 20% Wind Energy Penetration in the United States, A Technical Analysis of the Energy 

Resource, Final Report, October 2007, Black & Veatch Project: 144864, Prepared for 

American Wind Energy Association, October 2007.

2. Comparative Costs of California Central Station Electricity Generation Technologies, 

Final Staff Report, California Energy Commission, CEC-200-2007-011-SF, December 

2007.

3. Renewable Energy Transmission Initiative Phase 1A Final Report, Black & Veatch 

Project: 149148, Contract Manager University of California, Office of the President, 

California Institute for Energy and the Environment, April 2008.

4. Program on Technology Innovation: Integrated Generation Technology Options, Report 

1018329, Technical Update, Electric Power Research Institute, Palo Alto, CA, November 

2008.

5. Cost and Performance Baseline for Fossil Energy Plants, Volume 1: Bituminous Coal and 

Natural Gas to Electricity, Final Report Revision 1, DOE/NETL-2007/1281, National 

Energy Technology Laboratory, August 2007.

6. Assumptions to the Annual Energy Outlook 2008, DOE/EIA-0554(2008), Energy 

Information Administration, June 2008.

7. Assessment of Parabolic Trough and Power Tower Solar Technology Cost and 

Performance Forecasts – 2008, SL-009682, Final DRAFT report to Sandia National 

Laboratories, Sargent & Lundy, LLC, December 2008.

8. KJCOC, 2005, “FRACAS: Failure Reporting and Corrective Action System,” KJC 

Operating Company, Maintenance Log from 1989-2005. 

9. KJCOC, 2002, “SEGS Data and Acquaintance Package: SEGS III - VII,” KJC Operating 

Company.

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

Model (SAM),” NREL/TP-550-47605, July 2010.
F1147-E(10/2008)

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Line-Focus Solar Power Plant Cost Reduction Plan

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14. ABSTRACT (Maximum 200 Words)

Line-focus solar collectors, in particular parabolic trough collectors, are the most mature and proven technology 

available for producing central electricity from concentrated solar energy. Because this technology has over 25 years 

of successful operational experience, resulting in a low perceived risk, it is likely that it will continue to be a favorite of 

investors for some time. The concentrating solar power (CSP) industry is developing parabolic trough projects that 

will cost billions of dollars, and it is supporting these projects with hundreds of millions of dollars of research and 

development funding. While this technology offers many advantages over conventional electricity generation—such 

as utilizing plentiful domestic renewable fuel and having very low emissions of greenhouse gases and air pollutants—

it provides electricity in the intermediate power market at about twice the cost of its conventional competitor, 

combined cycle natural gas. The purpose of this document is to define a set of activities from fiscal year 2011 to 

fiscal year 2016 that will make this technology economically competitive with conventional means. 

15. SUBJECT TERMS

CSP; solar collector; line focus; development plan; costs; cost reduction

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