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

Solar-Augment Potential of 

U.S. Fossil-Fired Power Plants

Craig Turchi and Nicholas Langle

National Renewable Energy Laboratory

Robin Bedilion and Cara Libby

Electric Power Research Institute

Technical Report

NREL/TP-5500-50597

February 2011
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

Solar-Augment Potential of 

U.S. Fossil-Fired Power Plants

Craig Turchi and Nicholas Langle

National Renewable Energy Laboratory

Robin Bedilion and Cara Libby

Electric Power Research Institute

Prepared under Task Nos. CP09.2350, SM10.1510

Technical Report

NREL/TP-5500-50597

February 2011
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1

Table of Contents

Table of Contents........................................................................................................................................ 1

Abstract........................................................................................................................................................ 2

Background ................................................................................................................................................. 2

Objectives .................................................................................................................................................... 3

Ranking Criteria .......................................................................................................................................... 4

Age of Plant ...............................................................................................................................4

Capacity Factor..........................................................................................................................5

Annual Average DNI.................................................................................................................5

Amount of Land Available ........................................................................................................5

Topography of the Land.............................................................................................................6

Solar Use Efficiency ..................................................................................................................6

Scoring Methodology ................................................................................................................................. 7

Estimating Solar-Augment Potential......................................................................................................... 8

Solar-Augment Integration ........................................................................................................9

Rating the Plants......................................................................................................................11

Results and Discussion............................................................................................................................ 11

Capacity-Based Results ...........................................................................................................12

Energy-Based Results..............................................................................................................13

Emission Reductions................................................................................................................15

Deployment Impacts................................................................................................................17

Conclusions............................................................................................................................................... 20

References................................................................................................................................................. 21
2

Abstract

Concentrating Solar Power (CSP) systems utilize solar thermal energy for the generation of 

electric power. This attribute makes it relatively easy to integrate CSP systems with fossil-fired 

power plants. The “solar-augment” of fossil power plants offers a lower cost and lower risk 

alternative to stand-alone solar plant construction. This study ranked the potential to add solar 

thermal energy to coal-fired and natural gas combined cycle (NGCC) plants found throughout 16 

states in the southeast and southwest United States. Each generating unit was ranked in six 

categories to create a qualitative overall score of Excellent, Good, or Fair. Plants ranking below 

fair or failing to pass other criteria were scored Not Considered. Separate analysis was performed 

for parabolic trough and power tower technologies due to the difference in the steam 

temperatures that each can generate. The study found a potential for over 11 GWe of parabolic 

trough and over 21 GWe of power tower capacity. Power towers offer more capacity and higher 

quality integration due to the greater steam temperatures that can be achieved. The best sites 

were in the sunny southwest, but each of the sixteen states had at least one site that ranked Good

for augmentation.

Background

In 2009, coal and natural gas provided 2/3 of the nation’s electricity [1]. These fuels are domestic 

and relatively inexpensive, yet they represent finite energy sources that come with environmental 

liabilities. Transitioning to a lower carbon and renewable energy future will require technical 

advances and time to integrate and adapt current systems to new generation sources such as wind 

and solar. Of the wind and solar technologies, CSP is unique in its ability to integrate with 

existing fossil generation systems. Such integration offers a low-risk opportunity to meet 

renewable energy targets while promoting deployment and speeding the learning-curve growth 

that helps drive down the cost of new technologies. 

CSP differs from photovoltaic solar power in that CSP uses the sun’s heat to drive a thermal 

power cycle. This reliance on thermal energy means CSP plants can be backed up with natural 

gas and can supply steam to augment fossil-fired power plants. Such hybridization can allow for 

more reliable and lower cost application of solar power. In 2009, the Electric Power Research 

Institute (EPRI) completed studies examining the best ways to integrate CSP steam into coalfired and NGCC power plants [2, 3]. In this work, EPRI developed models that predicted the 

performance of augmenting coal- or natural gas-fired power plants with steam from CSP 

equipment. NREL assisted by providing data on solar resource and the amount of thermal energy 

that can be supplied by parabolic trough and power tower (also known as central receiver) CSP 

technologies. 

Solar-augment of a fossil power plant offers several advantages to the developer. These 

advantages include:

• Pre-existing steam power block, electrical substation, and other ancillary equipment,

• Pre-existing transmission and grid interconnect,

• Adjacent land for solar field may already be owned by the utility,
3

• Location next to an existing power plant likely minimizes environmental and view-shed 

concerns,

• Solar variability is mitigated by fossil fuel use. 

These features combine to lower the cost and risk associated with the solar project, and also may 

shorten project development timelines. As risk is reduced, indirect costs associated with 

financing costs and project contingencies may also decrease. 

In this follow-on study, EPRI and NREL surveyed the fossil power plants across sixteen US 

states to determine their solar-augment potential. This report documents the solar capacity and 

energy, as well as air emission reductions, that could be achieved by augmenting these existing 

and planned fossil power stations. 

Objectives

The goal of this study is to rank fossil-fired power plants for their suitability to incorporate the 

addition of solar thermal energy. This ranking provides guidance to utilities regarding the 

feasibility of integrating solar thermal energy into their existing fleet and allows analysts to 

estimate the contribution such hybrid plants might make to the nation’s electricity supply. The 

work was carried out through the following steps:

1. Identified all operating, under construction, and planned pulverized coal and NGCC 

power plants in regions of the United States known to have good solar resource as 

defined by their direct normal insolation (DNI). The study region included the western 

states of California (CA), Arizona (AZ), Nevada (NV), New Mexico (NM), Utah (UT), 

Colorado (CO), Texas (TX), and Oklahoma (OK), as well as eastern states of Florida 

(FL), Alabama (AL), Georgia (GA), Louisiana (LA), Mississippi (MS), North Carolina 

(NC), South Carolina (SC), and Tennessee (TN). 

2. Created a qualitative ranking of solar-augment potential for each power plant. Ranking 

was based on augmenting the unit(s) at each power station with the highest potential for 

solar integration. Augment potential was evaluated for both parabolic troughs and power 

tower CSP technologies. Linear Fresnel systems would also be good candidates for this 

application, but performance models were not as well developed as for troughs.

3. Summarized total solar energy provided and emissions reduced by incorporating solar 

thermal energy into the ranked power plants. Categorized potential by plant ranking, 

geography, age, and other criteria as appropriate.

4. Estimated the benefit to the solar industry and utility sector of deploying CSP systems for 

solar-augmented plants. Benefits were based on investment, manufacturing scale, and 

operational learning. 

5. Incorporated the ranked database of the plants into NREL’s Solar Power Prospector 

webtool.
4

Ranking Criteria

Six criteria were chosen for ranking the solar-augment potential of existing fossil power plants. 

These criteria include: 

• age of the fossil plant, 

• average capacity factor of the fossil plant, 

• DNI resource available at the plant site, 

• amount of available land surrounding the existing plant, 

• topography of that land, and 

• solar-use efficiency that could be expected if the plant were augmented with a solar field. 

A methodology was developed that weighted the importance of these criteria on the overall 

solar-augment potential of the plant and each criterion was assigned a range of scores. From this, 

a numeric score and qualitative ranking of all existing fossil plants was calculated to determine 

those with the highest potential for solar integration. Table 1 shows the weights and ranges 

assigned to the criteria, followed by a discussion of each criterion and the overall scoring 

methodology.

Table 1. Ranking Criteria

Age of 

Plant 

(years)

Capacity 

Factor 

(%)

Annual 

Average DNI 

(kWh/m2

/day)

Amount of 

Land Available 

(acres/fossil 

plant MW)

Topography 

of the Land 

(% slope)

Solar Use 

Efficiency 

(%)

Score/ 

Weighting 5% 20% 35% 15% 15% 10%

Not Considered > 30 < 15 < 4 < 0.05 > 5 --

1 16-30 -- 4-5 0.05-0.2 3-5 < 30

2 -- 15-50 5-6 0.2-0.35 -- 30-32

3 11-15 -- 6-6.5 0.35-0.5 1.5-3 32-35

4 -- -- 6.5-7 0.5-0.65 -- 35-38

5 0-10 ≥ 50 ≥ 7 ≥ 0.65 ≤ 1.5 ≥ 38

Age of Plant

The age of the plant is calculated based on the plant’s in-service date. For plants that are under 

construction or under development, the age of the plant is considered to be zero. Plants that have 

been built more recently receive a higher score because it is expected that they will have a longer 

operating life, increasing the likelihood that the solar plant will be able to operate throughout its 

expected life. Plants that are older than 30 years (built before 1980) are not considered based on 

the assumption that these plants will be closer to retirement and likely have less sophisticated 

controls than the newer plants, which may make incorporating the control logic of the solar field 

integration more difficult. In addition, emissions controls may be limited at these older plants, 

potentially jeopardizing the long-term operation of these plants. Overall, the age of the plant is 

given a 5% weighting as it is only a minor consideration compared to other plant characteristics.
5

Capacity Factor

The capacity factor of the plant indicates how frequently the plant operates. Based on EPRI’s 

standard definitions, a plant with a capacity factor greater than 50% is considered a baseload 

plant, between 20% and 50% is considered intermediate, and less than 20% is considered

peaking. Because solar augmentation can occur only when the fossil plant is operating, a plant 

with a low capacity factor is undesirable. The infrequent operation of a plant with a low capacity 

factor will result in significantly reduced megawatt-hours attributed to solar, which in turn raises 

the cost of solar-generated electricity as there are fewer hours over which to reclaim capital 

costs. However, it is possible that the operation of some peaking plants could correlate well with 

solar resource availability if the peaking units are operated on the hottest, sunniest days to meet 

air-conditioning loads. Baseload plants are most preferable and, therefore, plants with a capacity 

factor of 50% or greater receive a top score. Intermediate and higher capacity peaking plants 

with a capacity factor between 15% and 50% receive low points, but are still considered because 

there is some potential for solar operation. Plants with a capacity factor below 15% are not 

considered further. Due to the strong effect that capacity factor will have on annual megawatthours generated and plant economics, it is given the second highest weighting at 20%.

Annual Average DNI

The solar resource at the plant site, measured as annual average DNI, will significantly affect the 

performance of the solar-augmented fossil plant. A high average DNI will produce more steam 

for augmenting the plant, increasing the number of megawatt-hours attributed to solar and 

reducing fossil fuel consumption. Top points are given to a solar resource of 7 kWh/m2

/day and 

greater, with points dropping incrementally down to a resource of 4 kWh/m2/day. Plants in a 

location with a DNI less than 4 kWh/m2

/day are not considered further. Because the solar 

resource has such a significant effect on the plant performance and economics, it is given the 

highest weighting at 35%.

Amount of Land Available

The amount of land available surrounding the existing fossil plant affects the size of the solar 

field that can be built, which in turn affects the amount of solar steam and solar-generated 

electricity produced. Previous work has shown that existing fossil plants can accept a designpoint maximum of between 10% and 20% of their total plant output from solar steam before 

reaching equipment or other design limitations. Because the amount of solar steam a plant can 

accept will vary based on the plant’s capacity, the amount of available land criterion is calculated 

as acres per fossil plant megawatt. For example, based on the assumption that 1 MWe of solar 

requires 5 acres of land, a 100 MW plant could accept up to 10 to 20 MWe of solar generation, 

which would require 50 to 100 acres of land or 0.5 to 1 acres per fossil plant megawatt. The 

ability to produce a high percentage of the plant output from solar results in larger offsets of 

fossil fuel consumption and plant emissions. Maximum points are given to a plant with enough 

land available to produce roughly 13% or greater of plant output from solar, or greater than 0.65 

acres per fossil plant megawatt. Plants with land available for less than 1% plant output from 

solar, or less than 0.05 acres per fossil plant megawatt, are not considered further. Although 

larger field sizes will benefit somewhat from economies of scale, other economic drivers likely 

have a greater effect. Therefore, an intermediate weighting of 15% is given to this criterion.
6

Topography of the Land

The topography of the land surrounding the plant affects the ease of installation of the solar field 

as well as its performance. Generally, land with less than 3% slope is preferred for concentrating 

solar technologies. Ground slopes greater than 3% require extensive grading for parabolic trough 

installations, and while the power tower technology is less sensitive to the need for flat land, it is 

easier to install on a slope of less than 3%. Plants with a slope greater than 3% will require 

significant regrading, which can notably affect the cost of installation and, therefore, the overall 

plant economics. For this study, any plant with surrounding land that has a slope greater than 5% 

is not considered, while low scores are given to those with a slope of 3% or greater. Plants with a 

slope of 1.5% or less are given maximum points. Because land topography can affect both plant 

performance and installation costs, it is given an intermediate weighting of 15%.

Solar Use Efficiency

Solar use efficiency is the measure of how many megawatts of solar electricity are generated per 

solar thermal megawatt integrated into the fossil plant. Solar-augmented plants can have solaruse efficiencies that exceed stand-alone solar plants; however, solar-use efficiency generally 

decreases with solar contribution. Particularly for troughs, higher percentages of solar 

integration have been shown to result in lower solar-use efficiencies due to the mismatch 

between the solar steam temperature and that of the fossil plant steam cycle at the point of 

integration, see Figure 1. This study seeks to maximize solar energy production, not solar-use 

efficiency. Plants are given a high score for solar-use efficiencies that exceed values for standalone solar plants, middling score for values comparable to stand-alone solar plants, and low 

score if solar-use efficiency is lower. No plants are eliminated from consideration based on their 

solar use efficiency. To avoid penalizing plants with large solar fields and higher percentages of 

solar integration, the solar use efficiency criterion is given an intermediate weighting of 10%.

Figure 1. Solar-use efficiency generally falls with increasing solar contribution. This study seeks 

to maximize solar energy, which sometimes results in lower solar-use efficiency. 

Figure from EPRI [2].

25.0%

27.0%

29.0%

31.0%

33.0%

35.0%

37.0%

39.0%

41.0%

43.0%

0.0% 1.0% 2.0% 3.0% 4.0% 5.0% 6.0% 7.0% 8.0%

Gross Solar-Use Efficiency

Percent of Steam Cycle's Gross Output from Solar
7

Scoring Methodology

EPRI compiled data about the fossil plant size, in-service year, and capacity factor from the SNL 

database [4]. As data were entered into a scoring spreadsheet, each input was checked to see if it 

met the minimum acceptance criteria. If the age of the plant (calculated by subtracting the inservice year from the current year, 2010) or the capacity factor was below the minimum criteria 

the plant scored as Not Considered and no further analysis was performed. Those plant sites that 

passed this initial hurdle were analyzed by NREL to determine annual average DNI, topography 

of the land, and the amount of land available. The amount of land available on an acre-per-fossilplant-megawatt was calculated by dividing the land amount provided by NREL by the fossil 

plant capacity. Sites not meeting minimum criteria for those parameters were also classified as 

Not Considered. The remaining sites were analyzed with EPRI’s solar use performance models 

that were developed for an earlier project [2, 3] to determine solar-use efficiency and overall 

augment potential. As data were input into the spreadsheet, each site was scored based on the 

criteria described above. Figure 2 shows a sample of the scoring spreadsheet.

Figure 2. Scoring sheet with examples of “Not Considered” rankings.
8

Estimating Solar-Augment Potential

Each plant was first screened as a full plant and then evaluated on a unit basis. On the plant level, 

the in-service year of the newest unit of the plant and the highest capacity factor between 2009 

and 2010 was used for the screening. All plants that passed the minimum criteria of newer than 

30 years and greater than 15% capacity factor were submitted to NREL for land and solar 

resource analysis. 

Analysis for the physical location, topography, and average annual solar DNI value for each 

considered power plant was conducted at NREL using ArcMap Geographic Information System 

(GIS) software. The analysis was conducted using the following primary steps:

• Verify the latitude and longitude for each proposed location.

• Analyze the topography for each location and filter results based on usable land 

requirements.

• Calculate the average solar DNI for the plants that meet the usable land requirements.

Proposed plant locations (latitude/longitude) were cross referenced to ensure geographic 

precision and the verified geographic coordinates were analyzed for the land use criteria. The 

analysis was performed initially using a 3.25-km radius to ensure inclusion of all available raster 

grid cells. A 3-km radius was selected based on previous EPRI studies that indicated solar fields 

located further from the plant power block have significantly higher capital cost, greater thermal 

losses and higher auxiliary loads associated with pumping fluid long distances. The land filtering 

techniques eliminated developed land, wetlands, open water, impervious surfaces (i.e. included 

buildings and structures, roadways, etc.), and areas with percent slope greater than 5% [12]. 

Filtered results from the 3.25 km radius were then reduced to the desired 3-km radius by 

converting the raster to vector format and clipping with the more precisely defined radius. The 

contiguous area was then calculated for those areas that remained. Only land parcels of 10 acres 

or larger were retained for final consideration in the solar-augment modeling process.

In order to determine the solar resource for each location the filtered parcels were spatially 

joined to the average solar DNI data. The statistics for each region were calculated to provide the 

average annual DNI for all parcels within the 3 km radius, and the final average was used as the 

proposed plant solar DNI value. Additional statistics included average slope of the region, 

average contiguous land parcel size, as well as minimum and maximum contiguous land parcel 

sizes.

All plants that passed these initial criteria were then further evaluated on the unit level. For the 

coal plants, each steam turbine was evaluated as a separate unit. For the NGCC plants, the units 

were broken into combinations of combustion turbines and steam turbines. This was done as 

logically as possible based on the unit sizes and in-service years of the individual steam and 

combustion turbines and should provide a representative NGCC unit value for the plants. Each 

steam turbine and combustion turbine combination was then given a unique, representative name 

to be used throughout the analysis; however, these are not names that are necessarily used by the 

plants. 
9

A key criterion for a NGCC plant to accept solar steam is spare capacity in its steam turbines to 

accept additional steam without backing off the combustion turbines. The NGCC plants were 

evaluated in a few different ways to determine if there was sufficient steam turbine capacity. The 

EIA 860 database was consulted to determine if the plants had duct burners or not. Typically, if 

a NGCC has a duct burner, there is extra capacity in the steam turbine when the duct burners are 

not firing, assuming that the plant does not operate as a cogeneration unit that generates steam 

not used in the steam turbine. The EIA 860 database and the SNL database were consulted to try 

to determine if a unit was a cogeneration unit or not. Duct burner manufacturers were also 

consulted to learn where they had installed duct burners, though their response rate was low. 

Finally, the ratio of the combustion turbine capacity to the steam turbine capacity was analyzed, 

with the assumption that a typical combined cycle plant without extra steam turbine capacity has 

a 2:1 ratio of combustion turbine to steam turbine capacity. Based on this collection of data, each 

NGCC was analyzed to determine if it likely had sufficient extra steam turbine capacity for solar 

augmentation. Those that did not were not analyzed further. For 68 units, mostly in the planning 

stage, ratio information was not available. These plants were identified as plants that need more 

information before further analysis can be conducted. For coal plants, the EPRI model assumes 

that coal-firing will be reduced in order to accommodate the solar steam. Excess capacity in the 

steam turbine is not necessary.

Plants were evaluated on a unit level in the same way that they were screened on a full plant 

scale, except that the individual unit’s in-service year and capacity factor were used when 

available. For plants with multiple units, each unit was initially screened with the assumption 

that it could use all of the land available surrounding the plant. However, in the modeling, the 

full amount of land available was divided among the units. For all plant locations, only the single 

largest contiguous land parcel was used. In some cases, it could be that two or more separate 

parcels could be used to add more solar to a plant, but such a piecemeal configuration could 

increase costs and was not used in this analysis.

Solar-Augment Integration

Units that passed all age, capacity factor, land availability, topography, and DNI screening 

criteria were then evaluated in models developed by EPRI to determine the solar-augment 

potential [2, 3]. EPRI developed detailed performance models using the process simulation 

software IPSEPro to examine a variety of integration points and solar steam conditions. While a 

more detailed analysis would be required before any specific project is undertaken, the modeling 

tool does provide an estimate of the solar-use efficiency and annual solar output of a given plant 

based on its general design, the amount of solar integration assumed and the solar resource 

available at its location. 

Each unit was evaluated for both parabolic trough and power tower integration. The solar steam 

conditions (Table 2) are selected to match the typical steam pressure conditions expected in the 

fossil power plant at the point of integration. Boiler feedwater is extracted from the fossil plant’s 

Rankine cycle and routed through a heat exchanger, where it is heated by the heat transfer fluid 

(HTF) coming from the solar field before being returned to the fossil plant’s steam cycle at a 

location that depends on the steam conditions (Figure 3). The EPRI model was developed for oilHTF trough and molten-salt-HTF power tower systems and assumes a heat exchanger bank is 

used to generate the specified steam. Similar performance results can be expected for other 

working fluids, such as steam, if the same temperatures and pressures of integrated steam are 
10

achieved. However, the cost and land usage of these direct steam generation designs may differ 

from the assumptions used here. 

Table 2. Steam conditions and integration points for solar-augment.

Solar Technology Fossil 

Technology

Preferred Integration 

point

Solar Steam 

Conditions

Parabolic Trough

Coal Before superheaters 165 bar, 371 °C

NGCC Before superheaters 110 bar, 371 °C

Power Tower

Coal After final superheater 165 bar, 538 °C

NGCC After final superheater 110 bar, 538 °C

The maximum amount of solar integration possible was determined for each unit, either based on 

the size of the plant and limits determined in earlier EPRI studies or the amount of land 

available. Limits imposed by the fossil plant included the duct firing capacity for NGCC units 

and the steam pressure limits on coal plants. For plants that were not limited by the amount of 

land available, the field was sized with a solar multiple of 1.2 over the maximum amount of solar 

thermal input the plant could accept. (A solar multiple of 1.0 provides the exact amount of solar 

thermal energy required when running at a specified design-point insolation.) While a solar 

multiple greater than one results in defocusing some mirrors during peak periods of solar 

resource, it results in more hours when the plant operates near full solar capacity. For plants with 

a limited amount of land, it was assumed that the newest units would receive the maximum 

augmentation based on the maximum solar field achievable. Power tower plants were limited to 

a maximum tower height of 300 meters based on input from Sandia National Laboratory and 

NREL. For sites with multiple units, it was assumed that one central receiver could provide solar 

input to multiple units if the tower was within the 300 meter limitation. For these plants, annual 

solar output is reported as a single output for all units fed by the tower. In cases where there were 

multiple units and enough land, it was assumed that more than one tower could be built on the 

same plant site to supply the multiple units. However, it was assumed that only one tower would 

feed an individual unit. 

Once the maximum solar field was determined, NREL’s System Advisor Model (SAM, 

https://www.nrel.gov/analysis/sam/ ) was run for the solar field size and plant location to 

determine the hourly thermal megawatt output of the field for a full year. These data were input 

into the EPRI model and the annual megawatt-hour output due to solar and the solar use 

efficiency were calculated. For NGCC plants, the EPRI model is based on solar steam integration 

into the high pressure stage of the heat recovery steam generator. The coal plant integration 

assumes that the solar steam is integrated with the main steam flow after the boiler and either 

before or after the superheater, depending on the solar technology used. The solar-use efficiency 

result for each unit was input to the database scoring spreadsheet as the final piece in the ranking 

criteria. Each unit was analyzed for maximum solar input based on either the land available or 

the plant size. It has been found that for solar-augmented plants, especially those using parabolic 

troughs, increased solar input can result in decreased solar-use efficiency. As a result, for the 

plants that did not have a land limitation, solar-use efficiency may be lower than those that had a 

reduced amount of solar input. However, these plants will have a higher annual solar output.
11

Figure 3. Simplified schematic of solar steam integration into a coal plant. SH = superheater; RH = 

reheater.

Rating the Plants

Once the plants were fully scored, they were ranked based on total score and assigned a rating of 

Excellent, Good, Fair, and Not Considered. Ratings were assigned on a 5-point scale where a 

score of 4-5 is Excellent, a score of 3-4 is Good, and a score of 2-3 is Fair. Plants and units that 

did not qualify for a complete evaluation are listed as Not Considered with an explanation of 

which criterion the plant failed. A handful of plants passed the initial hurdle but scored less than 

2 in the ratings. Although they were evaluated, they were ultimately placed in the Not 

Considered category. A few plants were dropped for other reasons such as insufficient 

information on duct firing or extra steam turbine capacity.

Results and Discussion

It is important to note that the augment potential was calculated separately for parabolic troughs 

and power towers; thus, cumulative numbers for the two technologies should be considered 

individually and not added together. Technically, power towers offer a better option for 

augmentation because they can achieve higher steam temperatures than troughs and, therefore, 

have greater opportunity for integration with the fossil plant. From a project perspective, this 

technical advantage may be countered by the greater maturity and lower risk associated with the 

use of parabolic trough solar fields.
12

Capacity-Based Results

Table 3 displays the cumulative coal and NGCC nameplate capacity for the 16 states in the 

study. The numbers include existing, under construction, and planned facilities and combine for 

a total fossil-generation capacity of approximately 353 GW. For comparison, total nameplate 

capacity for all sources in the U.S. is 1122 GW [1]. Also shown are the augment potentials in 

MWe for both parabolic trough and power tower CSP technologies. The total augment potential 

for troughs comes to approximately 11 GWe, while towers offer the ability to achieve over 21 

GWe. The augment potential for troughs represents roughly 3.2% of fossil capacity and for 

power towers corresponds to 6.1% of total fossil capacity. Figure 4 shows the cumulative 

augment potential for the two different CSP technologies in graphical form. This format 

highlights the greater potential and higher-quality potential capable with the power tower 

technology. All the Excellent sites are in the southwest, but every state has at least one Good site.

Table 3. Nameplate Fossil-Fired Capacity and Solar-Augment Potential by State

Figure 4. Augment potential in capacity (GWe) for either troughs (left) or power towers (right). 

State

Coal 

Capacity 

(MW) 

NGCC 

Capacity 

(MW) Excellent Good Fair Total Excellent Good Fair Total

AL 12,620 13,020 - 45 723 767 - 49 918 967

AZ 6,760 15,570 849 253 - 1,102 1,794 97 - 1,890

CA 220 32,880 166 339 360 866 191 702 112 1,006

CO 6,120 4,290 - 173 283 456 - 953 167 1,120

FL 10,760 33,370 - 615 415 1,030 - 1,298 464 1,762

GA 17,300 9,590 - 131 523 654 - 609 701 1,310

LA 3,800 10,310 - 76 420 496 - 484 401 885

MS 2,920 7,680 - 18 363 381 - 215 438 653

NC 14,100 5,650 - - 242 242 - 191 505 696

NM 5,880 1,780 19 322 - 341 105 502 - 607

NV 3,420 7,280 396 154 - 549 553 418 - 971

OK 5,720 10,320 - 111 698 809 - 511 1,150 1,661

SC 6,800 3,470 - 205 121 326 - 457 119 576

TN 9,990 1,840 - 40 - 40 - 45 - 45

TX 26,840 54,960 - 740 2,103 2,843 - 4,453 2,008 6,461

UT 5,090 2,570 - 328 13 341 - 1,031 - 1,031

 138,340 214,580 1,430 3,550 6,264 11,244 2,643 12,015 6,982 21,640

Percentage relative to total fossil capacity = 3.2% 6.1%

Parabolic Trough (MWe) Power Tower (MWe)

Excellent

13%

Good

31%

Fair

56%

Parabolic Troughs

11.2 GWe Total

Excellent

12%

Good

56%

Fair

32%

Power Towers

21.6 GWe Total
13

Energy-Based Results

Table 4 and Figure 5 display the results on the basis of total energy produced. Once again it is 

apparent that power towers provide greater options for augmentation, with slightly more than 

twice the total electricity production than is possible with parabolic troughs. The average 

capacity factor of the fossil systems in this study is approximately 46% for NGCC and 56% for 

coal generators. These values are almost twice that expected for the CSP technologies without 

storage. The corresponding solar contribution to total electricity generation is about one-third

that stated above on a capacity basis. Troughs contribute about 1% to total annual energy 

generation, while power towers can contribute up to 2.2%. Adding thermal storage to the CSP 

system could increase their contribution, but this option was not included in the study. One 

primary benefit of solar-augment projects is their lower project risk and adding thermal storage 

would negate some of that advantage. 

Table 4. Fossil-Fired Electricity Generation and Solar-Augment Potential by State

Figure 5. Augment potential in energy (GWhe) for either troughs (left) or power towers (right).

State

Coal 

(MWhe/yr) 

NGCC 

(MWhe/yr) Excellent Good Fair Total Excellent Good Fair Total

AL 60,225,000 53,370,000 - 56,371 957,685 1,014,056 - 77,454 1,399,077 1,476,531

AZ 45,433,000 58,359,000 1,886,412 573,063 - 2,459,475 4,616,796 262,916 - 4,879,711

CA 1,600,000 189,886,000 392,558 624,209 605,522 1,622,289 519,194 1,500,123 197,589 2,216,906

CO 39,717,000 21,181,000 - 280,856 505,049 785,905 - 1,841,594 429,124 2,270,719

FL 55,587,000 160,378,000 - 897,681 615,040 1,512,722 - 2,094,672 712,369 2,807,040

GA 94,778,000 44,620,000 - 178,546 784,650 963,197 - 1,034,561 1,010,809 2,045,369

LA 25,242,000 38,264,000 - 97,932 605,441 703,373 - 732,134 666,936 1,399,070

MS 14,677,000 22,433,000 - 23,332 493,457 516,789 - 362,735 698,973 1,061,708

NC 70,905,000 30,094,000 - - 318,136 318,136 - 299,961 766,319 1,066,280

NM 42,172,000 7,502,000 42,124 710,663 - 752,788 272,172 1,187,217 - 1,459,389

NV 21,340,000 40,361,000 883,838 307,079 - 1,190,918 1,439,584 910,608 - 2,350,192

OK 36,576,000 48,791,000 - 154,462 1,022,963 1,177,426 - 861,349 1,991,516 2,852,865

SC 33,055,000 13,782,000 - 295,896 164,602 460,498 - 715,880 191,485 907,365

TN 47,561,000 12,095,000 - 51,365 - 51,365 - 71,241 - 71,241

TX 182,256,000 262,224,000 - 1,107,929 3,287,412 4,395,341 - 7,611,358 3,469,448 11,080,806

UT 37,027,000 14,948,000 - 593,496 20,830 614,326 - 2,158,241 - 2,158,241

808,151,000 1,018,288,000 3,204,932 5,952,882 9,380,788 18,538,603 6,847,746 21,722,044 11,533,644 40,103,433

Percentage relative to total fossil-derived electricity = 1.0% 2.2%

Parabolic Trough (MWhe/yr) Power Tower (MWhe/yr)

Excellent

17%

Good

32%

Fair

51%

Parabolic Troughs

18,500 GWhe Total

Excellent

17%

Good

54%

Fair

29%

Power Towers

40,100 GWhe Total
14

The solar-use efficiency (Figure 6) is calculated as the net electric energy attributed to solar 

divided by the solar thermal energy delivered to the power block. For comparison, the net 

thermodynamic conversion efficiency at design point for a solar-only trough or power tower 

system is approximately 33% or 37%, respectively. (This efficiency is calculated as the plant’s 

net electric output divided by thermal energy delivered to the power block.) Although this was 

not a goal of the study, some of the best solar augment cases exceed the efficiency of the standalone solar plants. This study seeks to maximize solar contribution and, as shown in Figure 1, 

greater solar contribution often leads to lower solar-use efficiency.

As shown in Figure 7, the solar-use efficiency is slightly higher for NGCC plant augmentation 

than for coal plants. These different efficiencies result from what governed solar-augment 

capacity. EPRI limited NGCC plants by the duct burner or extra steam turbine capacity in the 

plant, while coal plants were limited by when turbine pressure limits were reached. Turbine 

pressure rises as one incorporates lower quality steam and seeks to offset the steam quality by 

increasing mass flow through the turbine to maintain plant output. This is most apparent for 

trough-based augments. Thus, the slightly higher solar-use efficiency in NGCC plants is an 

artifact of the lower solar fractions applied to the NGCC plants. EPRI’s prior work indicates that 

solar-use efficiencies in coal plants can be higher than NGCC facilities when the integration is 

optimized for high solar-use efficiency rather than maximum solar output [2, 3]. 

Figure 6. Higher temperatures allow power towers to have higher solar-use efficiencies 

compared to troughs. 

0

20

40

60

80

100

120

Solar-augment Plants

Solar Use Efficiency (net MWhe/MWht)

Trough Tower
15

Figure 7. The higher solar-use efficiency for NGCC plants is an artifact of their typically lower 

solar-augment fraction due to limiting solar integration to the plant’s duct firing or extra steam 

turbine capacity.

Emission Reductions

Solar-augment of fossil-fired power plants results in the reduction of air emissions associated 

with coal and natural gas combustion. The potential emission savings were quantified by 

examining the annual solar energy contribution in MWhe and assuming fuel consumption was 

diminished according to the reported heat rate for the facility in question. If heat rate was not 

reported, a value of 10,000 BTU/kWh for coal and 7,500 BTU/kWh for NGCC was used. 

Emission savings were calculated using EPA emission factors that are typically listed as pounds 

of pollutant per ton of coal or per standard cubic foot of natural gas [5]. Values were calculated 

for carbon dioxide (CO2), sulfur dioxide (SO2) and nitrogen oxides (NOx). Other pollutants or 

by-products, for example, mercury and fly ash, could be calculated in a similar fashion but are 

not included in this report. The formula used for emissions reduction for each solar augment case 

took the form of:

Reduced air emissions =

(Solar Augment, kWhe/yr)*(Heat rate, BTU/kWhe)*(CO2 emission factor, lb CO2/BTU)*(metric ton/2205 lb);

(Solar Augment, kWhe/yr)*(Heat rate, BTU/kWhe)/(heat value coal, BTU/lb)*(SO2 emission factor, lb SO2/ton 

coal)* (0.9072 metric ton/ton);

(Solar Augment, kWhe/yr)*(Heat rate, BTU/kWhe)/(heat value coal, BTU/lb)*(NOx emission factor, lb NOx/ton 

coal)*(0.9072 metric ton/ton).

Similar expressions were used to evaluate the emissions from natural gas combustion. The 

emission factors and values used for coal and natural gas properties are shown in Table 5 below. 

Values representative of Powder River Basin subbituminous coal were used for all the pulverized 

coal plants in the western states and an Illinois Basin coal was selected to represent coal plants in 

the eastern states. Actual fuel source for the individual plants is not reported. It was assumed that 

all plants utilized low-NOx burners and coal units larger than 400 MW employed some form of 

SO2 scrubbing. (According to the EPA, the average size of units with SO2 scrubbers is slightly 

over 400 MW [11].) Because of these broad assumptions the estimated air emission savings 

should be considered rough approximations. 

-

2,000 

4,000 

6,000 

8,000 

10,000 

12,000 

33-34% 34-36% 36-38% >38%

Solar-Augment Capacity (MWe)

Solar-Use Efficiency (MWhe/MWht)

Coal NGCC
16

Table 5. Emission Factors and Fuel Properties used to Estimate Avoided Air Emissions

Parameter Value Units Source / Comments

Coal heat value (western) 8,600 BTU / lb Representative of Powder River Basin subbituminous coal

Coal heat value (eastern) 11,400 BTU / lb Representative of Illinois Basin, high-sulfur coal

Coal CO2 emission factor 210 lb CO2 / MMBTU http://www.eia.doe.gov/cneaf/coal/quarterly/co2_article/co2.html

Coal SO2 emission factor 

(western)

4.6 lb SO2 / ton [5], assumes 0.6% sulfur coal and 80% scrubber control 

(Units smaller than 400 MW have no scrubber)

Coal SO2 emission factor 

(eastern)

11 lb SO2 / ton [5], assumes 2.9% sulfur coal and 90% scrubber control 

(Units smaller than 400 MW have no scrubber)

Coal NOx emission factor 7.5 lb NO2 / ton [5], assumes low-NOx burners and subbituminous coal

NG heat value 1,000 BTU / SCF Typical value for natural gas, SCF= standard cubic foot

NG CO2 emission factor 0.12 lb CO2 / SCF [5]

NG SO2 emission factor 0.6 lb SO2 / 106 SCF [5]

NG NOx emission factor 140 lb NO2 / 106 SCF [5], assumes low-NOx burners

The annual air emission reductions by state and in total are shown in Table 6. As before, the 

values shown assume solar-augmentation using either parabolic troughs or power towers. The 

total solar contribution for each approach (trough or tower) is provided along with the associated 

estimated air emission reductions. The trough case reduces CO2 emissions by 11.5 million metric 

tons per year and the tower case provides an annual reduction of 30 million metric tons. A check 

of these total emission values is made by comparison with the emission footprint for the electric 

generation industry (by NERC region) published by the Leonardo Academy [6]. Average 

industry-wide emission factors for the five NERC regions relevant for this study are: 1.4 lb 

CO2/kWh, 0.0039 lb SO2/kWh, and 0.0020 lb NOx/kWh. Using these values yields avoided 

emission estimates for CO2 and NOx that are 15% to 30% lower, which can be explained by 

nuclear and renewable power generators, as well as post-combustion NOx controls on some 

plants, lowering the industry-average emission factor. The industry-average estimate for SO2

emissions is within 2%, but when nuclear and renewable generation are accounted for this 

suggests our estimated SO2 savings maybe too low. That is, actual SO2 savings may be greater 

than listed in Table 6.

Figure 8 highlights the fact that solar-augment of coal plants yields much greater air emission 

savings than the augment of NGCC power plants. This is due to the greater emissions that result 

from coal combustion compared to natural gas use. If reduction of air emissions is the primary 

goal, solar-augment of coal plants is an effective pathway. 
17

Table 6. Annual Air Emission Reductions Assuming Deployment of Parabolic Trough or Power 

Tower Systems to Augment Fossil Generators

Figure 8. Greater air emissions savings are achieved by augmenting coal plants versus NGCC 

plants. Data for power tower augment shown. SO2 emissions from NGCC are less than 33 MT/yr.

According to the US EPA, the average coal plant emitted 3.85 million metric tons of CO2 in 

2005; therefore, the estimated CO2 savings of 30 million metric tons per year is roughly 

equivalent to elimination of 8 average-size coal power plants in the U.S. [10].

Deployment Impacts

Figure 9 shows the deployment of solar-augment systems by state for the parabolic trough and 

power tower cases. Texas displays the greatest potential due to its good solar resource and 

numerous fossil power plants. The sunny southwest holds the only states with opportunities that 

score Excellent, but every state has at least one application that ranks Good. 

State

Solar Power 

Generation 

(MWhe)

CO2 avoided 

(metric ton)

SOx avoided 

(metric ton)

NOx avoided 

(metric ton)

Solar Power 

Generation 

(MWhe)

CO2 avoided 

(metric ton)

SOx avoided 

(metric ton)

NOx avoided 

(metric ton)

AL 1,014,000 549,000 1,200 700 1,477,000 819,000 3,400 1,100

AZ 2,459,000 1,237,000 800 1,800 4,880,000 3,189,000 3,800 5,600

CA 1,622,000 658,000 100 800 2,217,000 925,000 500 1,100

CO 786,000 544,000 900 1,000 2,271,000 1,884,000 3,800 3,700

FL 1,513,000 1,004,000 4,500 1,400 2,807,000 2,094,000 13,500 3,100

GA 963,000 748,000 1,500 1,100 2,045,000 1,655,000 3,800 2,500

LA 703,000 511,000 900 700 1,399,000 1,144,000 2,200 1,700

MS 517,000 289,000 300 400 1,062,000 759,000 1,200 1,100

NC 318,000 250,000 1,700 400 1,066,000 959,000 7,300 1,500

NM 753,000 526,000 700 1,000 1,459,000 1,169,000 2,200 2,200

NV 1,191,000 625,000 1,100 900 2,350,000 1,588,000 5,100 2,800

OK 1,177,000 727,000 800 1,200 2,853,000 2,238,000 3,300 4,200

SC 460,000 305,000 2,100 400 907,000 657,000 7,600 1,000

TN 51,000 21,000 - - 71,000 29,000 - -

TX 4,395,000 3,014,000 3,300 5,400 11,081,000 9,058,000 12,600 17,600

UT 614,000 486,000 500 900 2,158,000 1,903,000 2,300 3,900

 18,536,000 11,494,000 20,400 18,100 40,103,000 30,070,000 72,600 53,100

Parabolic Trough Augment Power Tower Augment

23,800,000 23,500,000 

72,600 

45,400 

16,300,000 

6,610,000 

7,700 

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+08

Solar Augment 

(MWhe)

CO2 (MT/yr) SO2 (MT/yr) NOX (MT/yr)

Energy Generated or Emissions Avoided

Coal NGCC
18 

Figure 9. Solar-augment capacity by state for parabolic trough (top) and power tower (bottom) 

technologies. Note different y-axis scale. 

The improvement of new technology cost with greater production volume is often tracked using 

a progress ratio (PR), where: 

൬

2ݐݏ݋ܥ

ݐݏ݋ܥ1൰ ൌ ሺܴܲሻሺ௡௨௠௕௘௥ ௢௙ ௩௢௟௨௠௘ ௗ௢௨௕௟௜௡௚௦ሻ

Assuming a progress ratio of 0.90, as suggested by [7], and accounting for world-wide parabolic 

trough capacity of ~2000 MW, the US solar-augment market alone could drive a 40% cost 

reduction in parabolic trough technology due to deployment, while taking advantage of the lower 

risk and cost associated with such projects. 

-

500 

1,000 

1,500 

2,000 

2,500 

3,000 

3,500 

AL AZ CA CO FL GA LA MS NC NM NV OK SC TN TX UT

Solar Capacity (MWe)

Parabolic Troughs Fair

Good

Excellent

-

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

7,000 

AL AZ CA CO FL GA LA MS NC NM NV OK SC TN TX UT

Solar Capacity (MWe)

Power Towers Fair

Good

Excellent
19

At present there is only one 5-MW power tower demonstration in the US, although 

BrightSource’s 370-MW Ivanpah project is under construction. The lower maturity of the 

technology and greater size of the solar-augment market means the opportunity for cost 

reductions in power towers is even greater than for troughs. Direct steam generation towers, such 

as offered by BrightSource, are ideally suited for solar-augment applications because steam is the 

desired final product. Steam towers are also simpler than molten salt designs, further reducing 

project risk. 

It is worth mentioning that several developers are investigating direct steam generation troughs. 

The deployment of 500°C steam troughs would improve the economics of trough augment 

systems by elimination of the oil-to-steam heat exchanger bank and improve augment potential 

by providing higher temperature steam than is possible with current 390°C oil troughs.

If the full solar-augment capacity were developed, the estimated cost and land requirements are 

listed in Table 7. The trough field is assumed to require 5 acres per MW and the tower heliostat 

field is larger at 9 acre per MW. This power tower land area is based on values for large, singletower designs. Modular tower designs promoted by some developers are reported to have a land 

use of roughly 50% smaller, i.e., comparable to the trough footprint. Investment costs are based 

on current estimated costs for parabolic trough and power tower systems minus the power block, 

but including costs for integration with the fossil plant. These are assumed to be approximately 

$4000/kW for troughs [8] and $3400/kW for steam towers [9]. As noted above, costs are 

expected to fall as deployment volume increases. 

While this study seeks to identify the most favorable solar-augment sites, is does not address the 

specific economics of solar-augmentation. It is important to note that EPRI’s latest analyses [2, 

3] indicate solar-augment of fossil power stations is not cost-effective based purely on the cost of 

energy and using the current cost of parabolic trough and power tower hardware. However, the 

solar-augment option is believed to be less expensive and contain less project risk than a standalone solar plant. For utilities grappling with renewable portfolio standards, solar-augment of

existing assets could be a valuable component to their renewable generation mix. Furthermore, 

aside from the ability to generate steam, the benefits of solar-augment projects are not tied to a 

specific technology. As CSP technology costs decrease, the solar-augment option is likely to be 

the first approach to achieve true cost parity with the traditional generation technologies. 
20

Table 7. Estimated Solar Augment Capacity by State and the 

Associated Land and Investment Requirements

Conclusions

Solar-augment of existing fossil power plants offers a lower cost and lower risk alternative to 

stand-alone solar plant construction. This study ranked the potential to add solar thermal energy 

to each of the coal-fired and natural gas combined cycle plants found throughout the 16 states 

representing the southern half of the United States. Each generating unit was ranked on a fourtiered scale ranging from Excellent to Not Considered. Separate analysis was performed for 

parabolic trough and power tower technologies due to the difference in the steam temperatures 

that each can generate. The study found a potential for over 11 GWe of parabolic trough and 

over 21 GWe of power tower capacity. Power towers offer more capacity and higher quality 

integration due to the greater steam temperatures that can be achieved. The best sites were in the 

sunny Southwest, but all states had at least one site that ranked Good for augmentation.

The study sought to maximize solar energy generation. Even so, the solar-use efficiency for the 

solar field sizes evaluated is comparable to stand-alone CSP plant designs. Smaller solaraugment fraction may yield higher solar-use efficiencies, if that is the dominant design criterion. 

The cost of technology deployment and project risk are lower than stand-alone CSP plants, 

suggesting that solar-augmentation is an attractive option for near-term deployment of solar 

power to meet renewable portfolio standards and reduce greenhouse gases and other air 

pollutants. These studies assume solar-augment to offset fuel usage at the fossil power stations. 

Accordingly, replacing fuel used for duct firing in NGCC plants will yield the best economics; 

however, the reduction of air emissions is much greater when coal plants are augmented. 

State

Capacity 

(MWe) Land (acres)

Investment 

($M)

Capacity 

(MWe) Land (acres)

Investment 

($M)

AL 767 3,840 3,100 967 8,700 3,300

AZ 1,102 5,510 4,400 1,890 17,010 6,400

CA 856 4,280 3,400 967 8,700 3,300

CO 456 2,280 1,800 1,120 10,080 3,800

FL 1,030 5,150 4,100 1,762 15,860 6,000

GA 654 3,270 2,600 1,310 11,790 4,500

LA 496 2,480 2,000 885 7,960 3,000

MS 381 1,910 1,500 653 5,870 2,200

NC 242 1,210 1,000 696 6,260 2,400

NM 341 1,710 1,400 607 5,460 2,100

NV 549 2,750 2,200 971 8,740 3,300

OK 809 4,050 3,200 1,661 14,950 5,600

SC 326 1,630 1,300 576 5,190 2,000

TN 40 200 200 45 400 200

TX 2,843 14,210 11,400 6,461 58,150 22,000

UT 341 1,710 1,400 1,031 9,280 3,500

 11,235 56,190 45,000 21,601 194,400 73,600

Parabolic Trough Augment Power Tower Augment
21

References

1. Energy Information Administration, http://www.eia.doe.gov/

2. Solar Augmented Steam Cycles for Coal Plants: Conceptual Design Study, Report 1018648, 

Project Manager: Cara Libby, Electric Power Research Institute, Palo Alto, CA, April 2010.

3. Solar Augmented Steam Cycles for Natural Gas Plant: Conceptual Design Study, Report 

1018645, Project Manager: Cara Libby, Electric Power Research Institute, Palo Alto, CA, 

July 2009.

4. SNL Energy, http://www2.snl.com, part of SNL Financial LC, Charlottesville, VA, USA.

5. US EPA, Clearinghouse for Inventories & Emissions Factors, AP42, Fifth Edition, Volume I, 

Chapter 1: External Combustion Sources, http://www.epa.gov/ttn/chief/ap42/ch01/index.html

6. “Emission Factors and Energy Prices,” Leonardo Academy Inc., www.leonardoacademy.org, 

Madison, WI, April 21, 2009.

7. “Assessment of Parabolic Trough and Power Tower Solar Technology Cost and 

Performance Forecast,” Sargent & Lundy LLC Consulting Group, Chicago, IL, NREL/SR550-34440, October 2003.

8. Turchi, C., M. Mehos, C. Ho, and G. Kolb, “Current and Future Costs for Parabolic Trough 

and Power Tower Systems in the US Market,” NREL/CP-5500-49303, October, 2010.

9. Kolb, G., internal analysis based on costs in Power Tower Roadmap, Sandia National 

Laboratories, 2010. 

10. http://www.epa.gov/cleanenergy/energy-resources/calculator.html

11. Steam Electric Power Generating Point Source Category: 2007/2008 Detailed Study Report 

U.S. Environmental Protection Agency, Report 821-R-08-011, Washington, D.C., August 

2008.

12. Land classification and impervious surfaces determined by Multi-Resolution Land 

Characteristics Consortium using NLCD 2001 data. Percent slope is calculated using the U.S. 

Geological Survey National Elevation Dataset at a 1 arc second (nominally 90 m) resolution.
F1147-E(10/2008)

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Solar-Augment Potential of U.S. Fossil-Fired Power Plants

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14. ABSTRACT (Maximum 200 Words)

Concentrating Solar Power (CSP) systems utilize solar thermal energy for the generation of electric power. This 

attribute makes it relatively easy to integrate CSP systems with fossil-fired power plants. The “solar-augment” of fossil 

power plants offers a lower cost and lower risk alternative to stand-alone solar plant construction. This study ranked 

the potential to add solar thermal energy to coal-fired and natural gas combined cycle (NGCC) plants found 

throughout 16 states in the southeast and southwest United States. Each generating unit was ranked in six 

categories to create an overall score ranging from Excellent to Not Considered. Separate analysis was performed for 

parabolic trough and power tower technologies due to the difference in the steam temperatures that each can 

generate. The study found a potential for over 11 GWe of parabolic trough and over 21 GWe of power tower 

capacity. Power towers offer more capacity and higher quality integration due to the greater steam temperatures that 

can be achieved. The best sites were in the sunny southwest, but all states had at least one site that ranked Good for 

augmentation.

15. SUBJECT TERMS

Solar-augment; CSP; solar; hybrid

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