National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Solar Trough Organic Rankine 

Electricity System (STORES) 

Stage 1: Power Plant 

Optimization and Economics 

November 2000 — May 2005 

E. Prabhu 

Reflective Energies 

Mission Viejo, California 

Subcontract Report 

NREL/SR-550-39433 

March 2006 
Solar Trough Organic Rankine 

Electricity System (STORES) 

Stage 1: Power Plant 

Optimization and Economics 

November 2000 — May 2005 

E. Prabhu 

Reflective Energies 

Mission Viejo, California 

NREL Technical Monitor: H. Price 

Prepared under Subcontract No. NAA-1-30441-05 

Subcontract Report 

NREL/SR-550-39433 

March 2006 

National Renewable Energy Laboratory 

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
This publication was reproduced from the best available copy 

Submitted by the subcontractor and received no editorial review at NREL

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Acknowledgements 

We owe a deep debt of gratitude to Hank Price and Tom Williams of NREL, who were 

willing to bet on a concept that at the time was radical and contrary; Vahab Hassani of 

NREL, whose technical acumen kept me honest; Dave Menicucci and Craig Tyner at 

Sandia, who were willing to listen, and then to act; Tommy Rueckert, Jim Rannels and 

Gary Burch of the DOE, my mentors; John Brugman and Laurie Hardy of Bibb and 

Associates, for their process engineering genius; and Bob Martin of the DOE, who dared 

me to dare. 

 iii
Table of Contents 

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

2 Scope of Work ............................................................................................................ 1 

3 Description of Strategy and Processes Used............................................................... 2 

4 Reporting Details........................................................................................................ 2 

4.1 Steam-ORC Comparison .................................................................................... 4 

4.2 Heat Rejection Study .......................................................................................... 7 

Forced Draft Air-Cooled.......................................................................................... 9 

Natural Draft Air-cooled and Water-Cooled Systems ........................................ 10 

4.3 Working Fluid Evaluations............................................................................... 14 

4.4 Power Plant Optimization................................................................................. 27 

4.5 Power Plant Conceptual Design and Cost Estimate ......................................... 29 

4.6 Field and Storage Evaluation and Conceptual Design Report.......................... 43 

4.7 Host Site Selection and Market Potential Report ............................................. 46 

California Opportunities........................................................................................ 46 

Arizona..................................................................................................................... 48 

Other States............................................................................................................. 49 

Worldwide Opportunities ...................................................................................... 50 

Conclusion: Market Opportunities....................................................................... 50 

4.8 Economic Evaluation........................................................................................ 50 

4.9 Planning for the First STORES Power Plant.................................................... 53 

 iv
Executive Summary 

Preamble 

The National Renewable Energy Lab awarded SUBCONTRACT NO. NAA-1-30441-05

to Reflective Energies as a result of its competitive solicitation to find pathways to solar 

thermal power. The subcontract was to perform a study that would further develop the 

concept of the Solar Trough Organic Rankine Cycle Electricity System (STORES) through a 

Stage 1 Study. The objectives of the proposed work are as follows: 1) develop an 

optimized ORC power cycle for integration with a parabolic trough solar field, 2) develop 

a conceptual design and cost estimate for a 10 MWe STORES system, 3) conduct a 

performance and economic assessment of pilot and mature STORES plants, 4) conduct 

preliminary assessment of STORES market potential, and 5) identify next steps towards a 

demonstration or early commercial STORES project. 

Background 

Solar Trough Power Plants began very successfully, with nine plants and a total of 354 

megawatts on line within a few years in the late 1980s. These plants (known as SEGS1

Plants I through IX) have performed very well over the last twenty years, generating 

power all year, but especially during the peak summer months. Their expected life is at 

lease thirty to forty years. They have generated far more power than all other solar 

technologies combined. Their longevity has also exceeded most other solar. Sadly, 

despite this success, no more plants have been built since then. 

A New Proposition for Solar Trough Power Plants: Smaller is Better 

This study investigated a new pathway towards commercialization of solar trough power 

plants, a system dubbed STORES (Solar Thermal Organic Rankine Electricity System). 

The prevailing wisdom was that in order to be economic and successful, solar trough 

projects had to be larger, or integrated with natural gas plants, creating economies of 

scale. This study examined the opposite proposition. It postulated that a large solar 

 

1

 Solar Electric Generator 

 v
plant would take large capital expenditure and suffer long delays, as witnessed by several 

such projects proposed in many countries, none of which has been built. However, a 

small solar trough power plant could be built quickly, provide local value, meeting a local 

customer’s needs. Rather than generate power for the wholesale market, STORES would 

focus on the retail market, because local value and prices may be higher. With proper 

optimization, such plants could be modularized, thereby achieving economies of 

production. They could be automated, and if the market existed, scaled up to larger sizes 

or scaled down to even smaller sizes, depending on the customer’s needs. 

Technological Barriers 

There were many technical barriers to overcome. The original nine plants were built with 

no precedent to guide them, learning as they went along. Given the circumstances, the 

achievement was all the more creditable. They used conventional steam turbine power 

plants for converting heat to electricity, and addressed the challenges of steam generation 

by going to bigger sizes, finding larger turbines, and fewer employees per megawatt. 

Furthermore, they were allowed to use up to 25% natural gas. Storage was not rewarded 

by higher pricing, and except for SEGS I, storage was not built into these plants. The 

plants were well designed, but power was expensive to produce, and when the strong 

price supports of the 1980 PURPA regulations collapsed, no more power plants were 

constructed. The support industry withered, and only small producers survived, helping 

to replace broken parts for maintenance. 

STORES needed to develop a power plant that could be used for small solar power 

plants. Steam technology was no longer available in small sizes. An alternative 

approach was to consider Organic Rankine Cycle (ORC) power systems that have been 

used very successfully for geothermal power plants for decades. There was also reason 

to examine whether the relatively low temperature of the resource could be better 

harnessed. Several process designs were considered to maximize efficiency, and several 

potential ORC suppliers were considered, particularly those who have previously 

supplied components or systems to the geothermal industry. Geothermal fluid, like the 

solar trough plants, is also relatively low temperature (300 F to 500 F), but there are 

 vi
important differences; geothermal fluid should be drained of all its energy before 

returning it to the earth, whereas solar heating fluids are recirculated through the troughs; 

high residual heat means lower new solar heat required. This study performed an 

optimization between ORC systems and solar troughs. Several important thermodynamic 

variations were identified, with efficiency and operating considerations. In the same vein, 

several solar thermal fluids and several ORC fluids were considered, with several, 

cascade and multiple cascade cycles. Another variable considered was the series-parallel 

arrangement of trough assemblies and the impact they have on the cycles being 

evaluated. 

A STORES plant that is technically optimized is a two tier cascade system, with toluene 

as the upper working fluid and butane as the lower working fluid. The solar heat fluid 

(HTF) with this system is Therminol VP-1, similar to most of the SEGS plants. These 

plants would be capable of automatic startup, safe shutdown, and regulation with varying 

solar conditions. They may not produce as much as manned power plants, but the 

savings in labor would more than compensate for the loss of a few kilowatt hours each 

day. The plants would not require any other fuel, and use only small amounts of 

electricity when they are not in operation. 

The Market for STORES Plants 

An important part of the study was to determine whether there is a market for STORES 

power plants. Without a market, the best technologies will fail. The first attempt was to 

find off-grid communities in the Southwestern US that could benefit from 1 to 10 MW of 

electricity. The plant would include storage, allowing delivery of power around the 

clock. The notion was that off-grid consumers are often willing to pay much higher 

prices for electricity. This effort was not successful. The investigation found that all 

communities that could use a megawatt or more were already hooked up to the grid. 

While there is no market for STORES power plants off-grid in the U.S., it is quite likely 

that in the Saharan region of Africa and in the middle-east, there are several remote 

communities that do not have electricity at all, and would benefit from STORES power 

plants that have built-in storage. 

 vii
The next effort was to find grid-connected customers with large loads who were 

concerned about electric reliability and who were paying high prices for electricity. 

These customers also required large, flat land areas and high quality direct normal solar 

radiation. 

Several potential customers were found. Among them are US military bases, National 

Forests and Parks, and Native American lands. Many of these entities expressed a strong 

interest in STORES power plants of the size proposed, where the power would offset 

their grid usage, saving the most power at peak summer prices. The key considerations 

were dependable power, long term price stability and reasonable costs. With the stellar 

track record of the SEGS power plants, these considerations could be satisfied. 

An alternative approach was to look for customers who were required to meet renewable 

energy portfolio thresholds. One electric utility (APS) interested in meeting its portfolio 

standards through solar was very interested in the STORES technology. APS met several 

times with Reflective Energies, and even visited a geothermal power plant at Reflective 

Energies’ request. Subsequently, APS issued an RFP for a one-megawatt STORES 

power plant. Three bidders were selected, three bids received, from which one was 

chosen. That power plant, known as the Saguaro Power Plant in Arizona, is now under 

construction. APS will consider several more such plants as it ramps up to meet its 

renewable portfolio requirements. Other customers are expected to follow. 

STORES has indeed gone from concept to reality, as envisioned in the proposal. 

Cost Considerations 

The capital and O&M costs for STORES power plants were examined. It was estimated 

that the first plant would cost an estimated $5,000 a kW, or about $27,500,000 for a five 

megawatt plant with an additional $2,500,000 for first-of-a-kind engineering and startup 

of the first plant. Each subsequent plant would drop in cost by an estimated 3 to 5 

percent, as experience and increased production favorably impact costs. 

 viii
Smaller power plants, such as a one-megawatt plant, were estimated to have a higher first 

cost, about $9 to 10 million, with the same first-of-a-kind engineering and startup costs. 

Even at the comparatively high first cost, with low interest, long term loans the annual 

component of these costs is acceptable for power displacing retail pricing. 

There were several challenges related to O&M costs. The current SEGS plants have an 

O&M composite cost range of about $0.05 to 0.07 a kWh. Being much smaller, a 

STORES power plant would have a much higher O&M cost if it used the same staffing. 

If the plant fully was automated, it would have no regular staffing at all. A special effort 

was dedicated to examining this notion. The effort included operators, engineers and 

managers from a SEGS facility. After much debate, each of the skeptics was convinced 

that automated STORES plants were feasible, even desirable. A remote display and 

control station would be available offsite, at a manned local substation or other control 

room. This remote control could be transferred to any computer set up to manage the 

power plant. This automation would probably result in operation slightly less optimal 

than manned operation, and therefore cost a few kilowatt-hours in output each day. 

However, the automation would save a lot of money in reduced labor costs, particularly 

for the smaller STORES power plants, more than offsetting the lost production and 

related revenue. The estimated O&M costs are about $0.05 a kWh. 

Conclusion and Recommendations 

STORES power plants offer a new pathway towards commercialization of solar trough 

power plants. Instead of moving to larger power plants or plants that are integrated with 

natural gas, STORES plants would stand alone, providing electricity for customers who 

need it most. Rather than benefit from economies of scale, STORES plants would benefit 

from economies of production and modularity. STORES plants would provide power 

where possible at retail rates that are two to three times wholesale rates. Where feasible, 

STORES plants could use storage, although in the Southwestern U.S. this was not found 

to be of value. It may be vital, however, in developing countries, such as in the Sahara or 

Middle East. 

 ix
As a direct result of this study, the first STORES plant is already in construction. It is the 

first solar trough power plant in the US in twenty years. The customer is interested in 

more such plants, and there are many other potential customers. It is hoped that several 

STORES plants will follow. By providing the building blocks for the revival of solar 

troughs, STORES power plants also offer the potential for larger solar power plants to be 

constructed. 

Solar troughs already provide more solar-derived electricity than any other type of solar 

energy. STORES will help it to grow. Solar power will be able to provide a large 

portion of the energy needs of the southwestern US, and in many other countries, helping 

towards energy independence, reducing emissions and greenhouse gases. 

 x
1 Introduction 

The goal of this study is to develop a new pathway towards resurrecting and 

commercializing Solar Trough power plants. There are several organizations that are 

pursuing the development of large Solar Trough based steam plants (100 megawatts or 

larger). This study examined whether there is a pathway to success that uses smaller, 

modular, solar trough based power plants as an alternate approach to large central plants. 

This report fulfills the requirements of Subcontract No. NAA-1-30441-05 between the 

National Renewable Energy Lab (NREL) and Reflective Energies. Previously, Reflective 

Energies had developed a concept known as STORES (Solar Thermal Organic Rankine 

Electricity System), as part of an earlier subcontract with NREL. Under that earlier 

effort, it was found that the STORES concept had much potential but that several issues 

were still to be resolved. The current work is known as the STORES Stage I study, with 

the goal of developing an optimized ORC Power cycle, a conceptual design and cost 

estimate for a STORES system, and a preliminary assessment of the market potential, and 

setting the stage for the first STORES demonstration plant. 

The objectives of the work are as follows: 1) develop an optimized ORC power cycle for 

integration with a parabolic trough solar field, 2) develop a conceptual design and cost 

estimate for a 10 MWe STORES system, 3) conduct a performance and economic 

assessment of initial and mature STORES plants, 4) conduct preliminary assessment of 

STORES market potential, and 5) identify next steps towards a demonstration or early 

commercial STORES project. 

2 Scope of Work 

The Scope of Work for this subcontract is shown below: 

• A comparison of steam Rankine and organic Rankine power cycles 

• An evaluation of heat rejection options for ORC systems 

• An assessment of working fluids for the solar field and ORC systems 

• The optimization of organic Rankine power cycles for solar resource temperatures 

• Detailed conceptual ORC power plant design 

• Development of an optimized solar field and power plant 

 1
• Identification of a potential demonstration plant and preliminary market assessment 

• Economic and financial assessment of demonstration plant 

• Development of SOW for next steps towards commercialization of STORES power 

plants. 

• Generation of a final report. 

3 Description of Strategy and Processes Used 

There were two basic strategies used in this study, and they were pursued in parallel. 

• The first was to find the most suitable power plant in the range of one to ten 

megawatts, from a technical perspective. This was achieved by combining the 

skills of solar experts and geothermal power plant experts. Geothermal power 

plant experts have long been able to design power plants that take advantage of 

low temperature resources. They began to use the Organic Rankine Cycle (ORC) 

for several geothermal plants a few decades ago. Their expertise was crucial in 

coming up with optimum solar cycles 

• The second strategy was to find potential clients in the sun-belt that would be able 

to use solar energy from Trough type power plants and to determine their needs. 

Rather than build large solar plants for supplying power to the grid at wholesale 

energy prices, this strategy considered smaller power plants that would sell 

electricity to a customer currently paying retail prices, which are much higher 

than wholesale power prices. 

Whereas initially it was anticipated that likely customers would be off-grid, where 

storage would be of value, it was found that most potential customers already received 

grid power, but for several reasons would benefit from solar power as well. This finding 

changed the course of the study significantly, resulting in a new design 

4 Reporting Details 

This study was performed in nine stages, with the tenth stage being the integration of the 

previous nine to reach conclusions and provide a course of action. 

 2
The nine tasks that were to be performed and reported on were drawn from the Contract 

Statement of Work and are as follows: 

Task 4.1: Steam-ORC Comparison Interim Report: The report will summarize 

the results of a comparison between the reference cascade ORC cycle and a steam 

cycle using similar solar resource conditions. The report will include heat 

balances, details of process flow, a list of equipment, and a summary of the 

assumptions for key components. 

Task 4.2:Heat Rejection Study Interim Report. – This report will evaluate the 

feasibility of the cooling systems identified in task 4.2. The report will consider 

first order costs, the practicality for the intended application, and compare the 

impact on reference system parasitics, cycle efficiency, and cost. 

Task 4.3: Working Fluid Study Interim Report. – This report will identify the 

ORC working fluids that are stable, optimized for the cascade cycle. The primary 

focus will be on the higher temperature ORC loop. The report will also evaluate 

various heat transfer fluids that could be used in the solar field. 

Task 4.4: Power Plant Optimization Report – This report will summarize the 

optimization of the cascade ORC cycle for a solar application. It will show a 

comparison of various design options and evaluation based on rough first costs 

verses performance trade-offs. 

Task 4.5: Power Plant Conceptual Design and Cost Estimate – This package will 

include drawing, plant descriptions, renderings, performance curves, and cost 

estimates. 

Task 4.6: Field and Storage Optimization and Conceptual Design Interim Report. 

This report will summarize the design optimization of the solar field and thermal 

storage system. The report will summarize the evaluation of 2-tank vs. 

thermocline storage, optimization of solar field and thermal storage system sizes, 

discuss safety issues, and include detailed solar plant cost and design. 

Task 4.7: Host, Site Selection and Market Potential Interim Report. – This report 

summarizes the selected host application systems requirements, review key 

features of the selected project site and summarizes results of market assessment. 

Task 4.8: Economic Evaluation Interim Report – This report defines the 

methodologies used to evaluate project economics. Includes assessment of 

economics for the selected project. 

Task 4.9: Stage 2 Project Planning Interim Report – The report defines the 

actions to be completed in the next phases of this effort to develop and test a 

demonstration plant. 

As the study progressed, some of the tasks above had to be tailored to match the 

findings of the study. For example, when it was found that storage did not add high 

value for the additional cost, but plant sizing flexibility was important, the study was 

modified to include greater plant flexibility, and to maximize output without storage. 

 3
4.1 Steam-ORC Comparison 

In order to establish a baseline for comparison between steam and organic cycles, the 

meteorology of several potential locations was considered, and the site of the Kramer 

Junction Power Plant was chosen for comparison purposes. 

Figure 4.1.1 shows the annual Insolation Mantle for the Kramer Junction area. 

Figure 4.1.1 Annual insolation mantle for Kramer Junction area 

 4
Temp. Cycle and Fluids

gross net gross net

Steam Simple Cycle vs. ORC 

Simple Cycle Butane 26.6 24.3 15.6 14.5

 Steam Reheat vs. ORC Cascade 

Cycle Cyclohexane/Butane 27.8 25.6 27 19.3

 Steam Reheat vs. ORC Cascade 

Cyele Toluene/butane 27.8 25.6 27.6 21

Steam Reheat+ Extraction vs. 

ORC Recuperative+ Cascade Cycle 

Cyclohexane/Butane 30.3 28.2 30.9 21.1

Steam Reheat+ Extraction vs. 

ORC Recuperative+ Cascade Cycle 

Toluene/Butane 30.3 28.2 31.3 23.9

Steam Simple Cycle vs. ORC 

Simple Cycle Butane 30.3 27.9 15.6 14.5

 Steam Reheat vs. ORC Cascade 

Cycle Steam/Butane 32.2 29.9 32 26.9

 Steam Reheat vs. ORC Cascade 

Cyele Toluene/butane 32.2 29.9 29.2 22.5

Steam Reheat+ Extraction vs. 

ORC Recuperative+ Cascade Cycle 

Steam/Butane 37.3 34.5 33.7 27.6

Steam Reheat+ Extraction vs. 

ORC Recuperative+ Cascade Cycle 

Toluene/Butane 37.3 34.5 35.4 28

735 °F HTF

Thermal Efficiency, %

Steam ORC

560 °F HTF

Fig. 4.1.2: Comparison of Steam and Organic Cycles

Temp. Cycle and Fluids

gross net gross net

Steam Simple Cycle vs. ORC 

Simple Cycle Butane 26.6 24.3 15.6 14.5

 Steam Reheat vs. ORC Cascade 

Cycle Cyclohexane/Butane 27.8 25.6 27 19.3

 Steam Reheat vs. ORC Cascade 

Cyele Toluene/butane 27.8 25.6 27.6 21

Steam Reheat+ Extraction vs. 

ORC Recuperative+ Cascade Cycle 

Cyclohexane/Butane 30.3 28.2 30.9 21.1

Steam Reheat+ Extraction vs. 

ORC Recuperative+ Cascade Cycle 

Toluene/Butane 30.3 28.2 31.3 23.9

Steam Simple Cycle vs. ORC 

Simple Cycle Butane 30.3 27.9 15.6 14.5

 Steam Reheat vs. ORC Cascade 

Cycle Steam/Butane 32.2 29.9 32 26.9

 Steam Reheat vs. ORC Cascade 

Cyele Toluene/butane 32.2 29.9 29.2 22.5

Steam Reheat+ Extraction vs. 

ORC Recuperative+ Cascade Cycle 

Steam/Butane 37.3 34.5 33.7 27.6

Steam Reheat+ Extraction vs. 

ORC Recuperative+ Cascade Cycle 

Toluene/Butane 37.3 34.5 35.4 28

735 °F HTF

Thermal Efficiency, %

Steam ORC

560 °F HTF

Fig. 4.1.2: Comparison of Steam and Organic Cycles

A comparison of steam and organic cycles for baseline conditions is shown in Figure 

4.1.2 for two temperature thresholds, a peak of 560 F (the current practical maximum for 

storage) and a peak of 735 F (the practical maximum when storage is not required). For 

this study, it was assumed that the temperature difference between the hot HTF fluid and 

the cold HTF fluid is 85 F, representing a compromise between the conflicting goal of 

maximizing average HTF temperature and minimizing HTF volume. It is seen that even 

though careful design and fluid choice increase the performance of the ORC systems, at 

the average summer high temperature conditions, the steam cycle outperforms the ORC 

by 15% to 25%. However, there are other benefits of the ORC which make it a viable 

alternative to the steam cycle in specific applications. These benefits include: the ability 

to scale to smaller unit sizes, higher efficiencies during cooler ambient temperatures, 

 5
immunity from freezing at cold winter nighttime temperatures, the ability to remain 

above atmospheric pressures, particularly at lower temperatures, and the adaptability to 

semi-attended or unattended operations. 

The potential for using steam as the working fluid for the upper stage of the cascade cycle 

was also considered. Toluene may be able to compete with steam as the upper stage 

working fluid. Although the extraction/reheat steam cycle remains the standard of 

comparison for larger trough solar facilities, innovative ORCs present a viable alternative 

for site-specific cases and approach the steam cycles in performance. Task 4.3, which 

look at working fluids, evaluates a larger variety of ORC options. 

Figure 4.1.3 shows a simple organic cycle, and Figure 4.1.4 shows a cascade organic 

cycle. 

Hot Fluid

“Cold” Fluid

Evaporator

Turbine-Generator

Cooling

Tower/Condenser

Isobutane Working Fluid

Simple Organic

Cycle

Figure 4.1.3

 6
Hot 

Fluid

“Cold”

Fluid

Low Temp. 

“Lower”

Working 

Fluid

Cascade 

Organic Cycle

High Temp. “Upper”

Working Fluid

Figure 4.1.4

Hot 

Fluid

“Cold”

Fluid

Low Temp. 

“Lower”

Working 

Fluid

Cascade 

Organic Cycle

High Temp. “Upper”

Working Fluid

Hot 

Fluid

“Cold”

Fluid

Low Temp. 

“Lower”

Working 

Fluid

Cascade 

Organic Cycle

High Temp. “Upper”

Working Fluid

Figure 4.1.4

4.2 Heat Rejection Study 

An essential part of the Rankine Cycle for power generation is that waste heat must be 

disposed of, known as heat rejection. Heat is rejected into heat sinks. The colder the 

temperature of the heat sink, the more power that can be extracted from the energy 

source. Heat sinks allow power plants to generate more power for the same energy input, 

and thus operate more efficiently. The following is a general description of each of the 

heat sink technologies that will be evaluated. 

 7
Water-cooled (with mechanical draft cooling tower) 

Accumulator

Cooling 

Tower

TurbineGenerator

Cooling Water 

Circulation Pumps

Working

Fluid

Water 

Cooled 

Condenser

Cooling

Water

Air

Figure 4.2.1: Water Cooled Heat Sink

Accumulator

Cooling 

Tower

TurbineGenerator

Cooling Water 

Circulation Pumps

Working

Fluid

Water 

Cooled 

Condenser

Cooling

Water

Air

Figure 4.2.1: Water Cooled Heat Sink

Figure 4.2.1 shows a process flow diagram for a water-cooled heat sink. In this type of 

heat sink, the working fluid from the turbine discharge flows to a water-cooled condenser 

after being discharged from the turbine. As the working fluid condenses, its heat energy 

is transferred to the cooling water. The condensed working fluid drains to the working 

fluid accumulator, whence it is pressurized with “condensate pumps” and “feed pumps”, 

heated and evaporated prior to re-entering the turbine. 

“Circulating” water pumps force cooling water through the condenser and up to the top of 

a mechanical draft cooling tower. The mechanical draft cooling tower has fans that draw 

ambient air past the falling water, cooling it by evaporation similar to a swamp cooler. 

 8
Forced Draft Air-Cooled 

Figure 4.2.2 shows a process flow diagram for a forced draft air-cooled heat sink. 

Accumulator

TurbineGenerator

Working

Fluid

Forced Draft AirCooled Condenser

Air

Figure 4.2.2: Forced Draft Air Cooled Heat Sink 

Air Cooled Sinks do not utilize water as a cooling medium. Instead, the working fluid is 

cooled directly by air. Fans draw ambient air past tube bundles which contain the 

condensing working fluid. This technology requires no water and is simpler to operate 

than a water-cooled heat sink. Like water-cooled systems, forced draft air-cooled systems 

are used in many geothermal ORC power plants. 

 9
Natural Draft Air-cooled and Water-Cooled Systems 

Figure 4.2.3 shows a process flow diagram for a natural draft air-cooled heat sink. 

Accumulator

TurbineGenerator

Working

Fluid

Natural 

Draft Air 

Cooled 

Condenser

Air

Figure 4.2.3 Natural Draft Air-Cooled Heat Sink 

Natural draft air-cooled and water cooled systems require no cooling fans, relying on 

natural air flow through long cooling towers that are symbolic of nuclear power plants. 

They are often used when the use of cooling water and power is expensive. The cooling 

tower must be several hundred feet tall in order to generate the needed draft, making 

these systems particularly expensive for small plants. Induced draft cooling towers 

together with water cooling are common in large nuclear power plants in the Eastern and 

Midwestern U.S. 

 10
METHDOLOGY 

In order to make a valid comparison between the various heat rejection technologies, 

each cycle must be compared at equivalent ambient conditions. The average, summer, 

daytime (i.e., daylight hours only), dry-bulb temperature at nearby Edwards Air Force 

Base was found to be 89 °F. The coincident wet bulb temperature was 62 °F. Each heat 

sink technology was compared at these conditions. 

The power output of an ORC power plant increases as working fluid condensing 

temperature decreases. Organic rankine cycles were modeled using a commercial process 

simulator program, HYSIM. Using HYSIM, a relationship between the condensing 

temperature of the working fluid and the gross cycle power was established. This 

relationship is shown in Figure 4.2.4 for a ten megawatt plant, but would be roughly the 

same for any other Rankine Cycle power plant. 

10,500

11,000

11,500

12,000

12,500

13,000

85 90 95 100 105 110 115 120 125 130 135

Condenser Temperature (F)

Gross Power (kW)

Figure 4.2.4. Cycle Gross Power vs. Condenser Temperature

 11
Each of the heat rejection systems has its own parasitic power requirements, and the 

results were adjusted to compensate for these requirements. 

The cost of water is another variable, and for the study was assumed as a middle-of-theroad $200 an acre-foot. 

Each type of heat sink was then optimized for lowest net present cost taking into account 

each of the before mentioned factors. 

COMPARISON 

Tables 4.2.1A and B summarize the process data and economic data for the optimum 

design for each of the three heat sink systems. Two water-cooled cases are shown. The 

first case is the normal case with water discharged into a sewer. The second case 

includes the cost of zero liquid discharge that has been imposed on some desert plants. 

1 2 3 4 5 6 7 8 910

Cond. Cond. Fan Circ. ZLD Gross Net ∆ Gross ∆ Net Water

CASE Pinch Temp. Pwr Pump Power Power Power Power Power Use

(

o

F) (oF) (kW) (kW) (kW) (kW) (kW) (kW) (kW) (gpm)

Water Cool 3 87 224 355 0 12,741 12,162 345 153 313

Water Cool, ZLD 3 92 149 355 73 12,517 11,940 569 375 274

Air Cool 20 118 244 0 0 11,441 11,197 1645 1118 0

Nat Draft 45 138 0 0 0 10,776 10,776 2310 1539 0

Table 4.2.1A. Heat Sink Process Summary 

12345678

CT/AC Pump Cond ZLD Tot. Tot. Inst. Cost of Oper.

CASE Cost Cost Cost Cost Cost Cost Power Cost

(MM$) (MM$) (MM$) (MM$) ($MM) ($MM) ($MM) ($MM)

Water Cool 0.450 0.126 0.460 0.000 1.035 2.795 0.611 0.280

Water Cool, ZLD 0.300 0.126 0.460 1.346 2.233 6.028 1.498 0.400

Air Cool 1.089 0.000 0.000 0.000 1.089 2.941 4.473 0.000

Nat Draft 2.852 0.000 0.000 0.000 2.852 7.699 6.156 0.000

9

Net Heat

Sink Cost

($MM)

3.686

7.926

7.414

13.856

Table 4.2.1B. Heat Sink Cost Comparison 

The analysis shows that a water-cooled heat sink is the best option for a solar ORC cycle. 

assuming that there are no unusually stringent water disposal requirements such as the 

zero liquid discharge. 

 12
The primary reason that the water-cooled option emerges as the optimum is that it can 

condense the working fluid to a much lower temperature, allowing more work to be 

extracted. 

Forced air cooled power plants are less efficient, and natural draft cooled plants, with or 

without water are far too expensive because of the high cost of building a four hundred 

foot tall cooling tower. 

OPTIMIZATION 

The water-cooled heat sink was optimized for a nominal ten megawatt plant. These 

optimizations will hold true for smaller plants as well. 

0

1,000,000

2,000,000

3,000,000

4,000,000

5,000,000

0123456

Approach Temperature (F)

Cost ($MM)

Net Heat Sink Cost

Condenser Capital Cost

Cost of Power

Figure 4.2.5. Condenser Cost vs. Approach Temperature 

Figure 4.2.5 is a plot of net present costs versus condenser approach temperature. The 

plot shows that the optimum approach temperature for the condenser is roughly 3 oF. 

This is the point where the added benefit of additional power production is balanced by 

the added cost for condenser surface area. This optimum approach temperature is smaller 

than that usually found for other types of power plants because solar power plants are so 

much more expensive than most traditional power plants. 

 13
The optimum cooling tower size was found to be roughly 1.5 times that of a standard 

cooling tower cell used in conventional power plants. At that size, the value of the 

incremental gross power produced by using a larger tower is equal to the incremental cost 

of the tower and the extra parasitic power of larger fans. 

CONCLUSION 

The water-cooled heat sink is the best option for a STORES power plant. Air-cooled 

systems would be considered only in locations where water is prohibitively expensive or 

have zero liquid discharge mandates. Generally, the condenser and cooling towers 

should be larger than traditional plants because the improved efficiency will be costeffective. 

4.3 Working Fluid Evaluations 

Organic “working fluids”, are the medium used for the Rankine cycle. They are 

hydrocarbons generally extracted from petroleum. At higher temperatures such as the 

735 F possible with use of therminol as heat transfer fluid, the thermal stability of the 

working fluid becomes an issue. In choosing the right working fluid for a given cycle, the 

fluid should have thermal stability over the range of operating temperatures and a 

minimal degradation rate over time. The working fluids studied in this report are butane, 

heptane, cyclohexane, benzene, toluene, orthoxylene and ethylbenzene. 

Although published thermal stability data is sparse, particularly in the range of 

temperatures and pressures used for organic rankine cycles, it is still possible to estimate 

the relative stabilities of various compounds based on actual operating data and 

theoretical considerations. A summary of this analysis is given in Table 4.3.1. 

Fortunately, more extensive data is available for the fluids of greatest interest for 

STORES applications: butane and toluene. Butane has been used as a low temperature 

organic rankine cycle (ORC) working fluid for nearly two decades at a temperature of 

about 300 o

F. Under these conditions, thermal degradation has not been a problem. 

 14
In the 1980’s, toluene was considered as a high temperature working fluid for both solar 

and space ORC applications (Havens and Rogaller, 1988). Extensive long term testing 

was done at 750 o

F yielding quantitative degradation rates of around 0.3 wt% per year. 

This is an acceptable rate for ORC purposes. 

Table 1 shows that benzene is the most thermally stable of the candidate working fluids. 

Next after benzene is toluene. Compared to the stability of isobutane at 290 o

F (typical 

for geothermal operations), benzene at 700 o

F would have about 1.5% of the degradation 

rate and toluene at 700 oF would have 9,000 times greater degradation. However, this 

would still be less than 0.02 wt% per year based on the Havens and Rogaller report. 

Working Fluid Temperature, °F 

Working Fluid 290 330 550 650 700 

Isobutane 1.59E+05 

Butane 2.48E+04 

Heptane 1.90E-05 3.47E-07 6.08E-08

Cyclohexane 2.03E-02 1.59E-04 1.92E-05

Benzene 3.10E+12 5.05E+08 1.13E+07

Toluene 4.78E+05 3.84E+02 1.72E+01

Orthoxylene 1.19E+02 2.80E-01 2.02E-02

Ethylbenzene 4.19E-01 1.12E-03 8.51E-05

Acceptable 

Marginal 

Unacceptable 

Table 4.3.1. Relative Stability Compared to Toluene at 750oF 

For the bottoming cycle, the range of fluid maximum temperatures is anticipated to be in 

the 300 oF to 350 oF range, and isobutane has been successfully used at such temperatures 

in large and small geothermal plants for decades. This experience with isobutane coupled 

with the comparable thermal stability of n-butane, the fluid of choice for the bottoming 

cycle, obviates any need to further investigate its suitability. 

 15
Thermal Efficiency 

The purpose of this section is to choose the ORC that will be best suited to the STORES 

system. Since the solar field represents a major investment, high thermal efficiency is an 

important characteristic of the organic rankine cycle. We have investigated two ways to 

achieve this. The first is through recuperation; that is, by using the sensible heat of the 

turbine exhaust stream to preheat the working fluid prior to vaporization. Recuperation 

reduces the external heat provided by the HTF, thereby increasing thermal efficiency. 

The second method for improving cycle efficiency is to choose a working fluid that 

operates at a higher turbine inlet temperature. In the previous study, the HTF temperature 

was limited to 560 °F. By allowing the HTF temperature to be as high as 735 °F 

(currently achieved in operating trough solar power plants) greater thermal efficiency can 

be achieved. 

Recuperation 

The recuperator acts as a pre-heater to the vaporizer. It transfers heat from the turbine 

exhaust to the fluid entering the vaporizer. Heat that would otherwise be rejected to the 

condenser is recovered to heat up the working fluid. Figure 4.3.1 is a Temperature-Duty 

diagram for the recuperated cascade cycle. Figure 4.3.2 is a schematic showing the 

arrangement of two recuperators, one each for the top cycle and bottom cycle. 

 16
0

100

200

300

400

500

600

700

0 25 50 75 100 125 150 175

Duty, MMBtu/hr

Heating Fluid

Heavier Working 

Fluid

Lighter Working 

Fluid

Cooling Fluid (Air)

Recuperators

Figure 4.3.1. Recuperator Cascade Cycle Configuration 

 

 17
Heating Fluid 735o

F Heating Fluid 650oF

 

 Vaporizer 

 

 

Top Recuperator Turbine Generator 

 

 

 

 Vaporizer/ 

 Condenser 

 

 Feed Pump 

 Turbine 

 Generator 

 

 

 

 

 Bottom Recuperator 

 

 

 

 

 Cooling water or air 

 

 Condenser 

 

 Feed Pump 

 

 

Figure 4.3.2: Schematic with Two Recuperators, Cascade 

In addition to several aromatic heavy hydrocarbons, steam is also a candidate topping 

fluid for various cycle configurations. Figure 4.3.3 shows the Temperature-Duty diagram 

for a typical steam/hydrocarbon cascade cycle. The steam from the upper cycle will 

usually be superheated. 

 18
0

100

200

300

400

500

600

700

800

0 25 50 75 100 125 150 175 200 225

Duty, MMBtu/hr

Heating Fluid

Hydrocarbon Cycle

Steam Cycle

Cooling Fluid (air)

Figure 4.3.3: Typical Steam Cascade Cycle with 735oF HTF Temperature 

Figure 4.3.4 shows toluene-butane cascade cycle. There are differences between steam 

and toluene (or any similar hydrocarbon) as a topping fluid. First, toluene operates at a 

lower pressure than steam (650 psia vs. 1450 psia). Second, toluene is supercritical at 

these conditions. Therefore, there is no two-phase vaporization (boiling) as there is for 

steam. Furthermore, toluene does not freeze at any ambient temperatures likely to be 

encountered. These differences favor toluene over steam. 

 19
0

100

200

300

400

500

600

700

800

0 25 50 75 100 125 150 175 200

Duty, MMBtu/hr

Heating Fluid

Cooling Fluid

Toluene Cycle

Butane Cycle

Figure 4.3.4: Toluene/Butane Cascade Cycle 

For all of the above cases, the use of recuperation results in even higher cycle thermal 

efficiencies. 

The Maloney-Robertson aqua ammonia cycle was also investigated. It takes a mixture of 

ammonia and water, partially vaporizes it and separates the ammonia-enriched vapor in a 

separator. The vapor passes through the turbine while the water-rich liquid is cooled in 

the recuperator and then mixed with the turbine exhaust stream before it goes into the 

condenser. This allows the condenser pressure to be lower than if only the ammonia rich 

vapor is condensed. This simple Maloney-Robertson cycle can operate as a topping cycle 

but must operate at too high a pressure when the temperature is in the 600 o

F range. 

Therefore, the Maloney-Robertson cycle is considered as a bottoming cycle to an ORC 

topping cycle. 

 20
Figure 4.3.5 shows the Temperature-Duty Diagram for the cascaded Maloney-Robertson 

cycle, and Figure 4.4.6 is the process flow diagram for the cycle. The cascade MaloneyRobertson cycle has a lower thermal efficiency than the comparable cascade ORC cycles. 

0

100

200

300

400

500

600

700

800

0 25 50 75 100 125 150 175 200 225

Duty, MMBtu/hr

Vapor

Liquid 2-phase

Liquid

Heating Fluid

Hydrocarbon Cycle

Ammonia-Water Cycle

Cooling Fluid 

Figure 4.3.5: Typical Cascade Maloney-Robertson Cycle Configuration 

 21
Figure 4.3.6: Recuperated Cascade Maloney-Robertson Cycle Configuration 

 22
A triple cascade cycle was also investigated using butane for the bottoming cycle, 

cyclohexane for the middle cycle and either steam or toluene as the topping cycle. Figure 

4.3.7 shows the Temperature-Duty Diagram for the 3-cascade cycle and Figure 4.3.8 

shows the corresponding process flow diagram. 

Neither 3-cascade cycle is more efficient than the most efficient 2-cascade cycles. This is 

due to the additional internal losses of the third cycle. The additional complexity and cost 

of the triple cascade cycle dismiss it from further consideration. 

0

100

200

300

400

500

600

700

800

0 25 50 75 100 125 150 175 200

Duty, MMBtu/hr

Heating Fluid

Upper Cycle

Middle Cycle

Lower Cycle

Cooling Fluid (air)

Figure 4.3.7. Typical Triple Cascade Cycle with 735oF HTF Temperature

 23
 Figure 4.3.8: Process Flow Diagram for 3-Cascade Cycle

 24
Table 4.3.2 summarizes the thermal efficiency of the different cycles with and without 

recuperation. 

Thermal Efficiency, % 

Cascade Recuperated Cascade 

Gross Net Gross Net 

560 °F HTF 

Steam/Butane 27.1 22.4 29.2 24.1 

Benzene/Butane 27.3 21.6 30.1 23.9 

Toluene/Butane 26.4 21.3 29.2 23.6 

Cyclohexane/Butane 26.6 20.2 29.4 22.3 

Cyclohexane/ Maloney-Robertson 24.3 19.9 24.9 20.3 

735 °F HTF 

o-Xylene/Butane 29.0 24.3 36.0 30.5 

Toluene/Butane 29.4 24.4 36.5 30.4 

Benzene/Butane 29.9 24.4 36.8 30.0 

Steam/Butane 31.5 27.1 33.1 28.4 

Toluene/Cyclohexane/Butane 32.7 25.9 36.2 27.5 

Steam/Cyclohexane/Butane 28.7 22.8 31.2 24.6 

Table 4.3.2: Cascade Cycle Thermal Efficiencies 

 

From this table it can be seen that the Maloney-Robertson cycle is inferior to the 

recuperated butane cycle as a bottoming cycle. Increasing the HTF temperature from 

560o

F to 735oF improves the cycle thermodynamic efficiency by about 28%. The triple 

cascade cycles are less efficient than the comparable dual cascade cycles. The highest 

efficiency for an HTF temperature of 735o

F is the o-xylene/butane cycle followed closely 

by the toluene/butane and benzene/butane cycles. However, since o-xylene is less stable 

than toluene at the cycle operating temperature, toluene would be the recommended 

working fluid. 

 25
The most efficient cycle for an HTF temperature of 560o

F is the steam/butane cycle 

followed closely by the benzene/butane and toluene/butane cycles. For the reasons 

mentioned above, the ORC would be the preferred topping cycle. The slight 

thermodynamic advantage of steam would not compensate for its other disadvantages. 

In all cases, recuperated cycles are significantly more efficient than the non-recuperated 

cycles. 

Fluids and Cycle Digest 

Of the potential organic working fluids reviewed in this study, only the aromatic 

hydrocarbons have sufficient thermal stability to be used as the topping cycle working 

fluid in a cascade solar thermal power plant. Light hydrocarbons such as butane have 

sufficient thermal stability to be used as bottoming cycle working fluids. 

The most efficient solar organic rankine cycle is the recuperated orthoxylene/butane 

cascade cycle with a net efficiency of 30.5% and a gross efficiency of 36.0%. However, 

considering thermal stability, availability, and prior experience, the toluene/butane 

cascade cycle, with almost the same efficiency, would be the recommended choice. This 

cycle offers high thermal efficiency over a range of HTF temperatures. 

The recuperated cascade cycle is the most thermally efficient organic rankine cycle. The 

best organic rankine cycles, even with recuperation, are less efficient than extraction 

steam cycles. However, they have other advantages which allow for lower operation and 

maintenance, making them suitable for small installations (1-10 MW). 

 26
4.4 Power Plant Optimization 

This section addresses other factors that must be considered for power plant optimization. 

It has already been shown that a cascade cycle the best choice and that water cooled heat 

sinks are the best choice. The working fluids are toluene for the upper cycle and butane 

for the lower cycle. 

Field Configuration 

Additional efficiency improvements may be gained by configuring the solar field such 

that while the peak temperature of the HTF remains at 735 F, the average HTF 

temperature is significantly higher than at the SEGS power plants. This is achieved by 

modifying the series/parallel arrangement of the troughs such that fewer troughs are in 

series. For this task, the HTF hot-cold temperature difference was assumed to be 85 F, 

with an average temperature of 692 F. This is significantly lower than that of the current 

SEGS power plants. The toluene/butane cycle is able to take best advantage of this 

higher average HTF temperature. 

Heat Transfer Fluid 

The Heat Transfer Fluid (HTF) that makes the most sense for a STORES power plant is 

Therminol VP-1 because it can withstand higher temperatures than Caloria, and the 

largest experience to date has been with Therminol. While other fluids may make more 

sense in the future, others are researching this issue, and at the present these are not 

considered. 

 27
Other Considerations 

There are several other considerations that may affect the selections. The supply chain 

that resulted in the original STORES plants have dwindled, and must be re-established. 

The expertise that resulted in several geothermal power plants has also dwindled. There 

are other suppliers that do build Organic Rankine systems for specialized uses, and these 

suppliers make lower temperature systems. If such systems are used, the plant 

performance may not be optimal, and this would need a trade-off between practicality 

and optimization. 

Cost Trade-Offs 

CAPITAL COSTS 

The capital cost of the solar field is directly proportional to the efficiency. As plant 

efficiency goes up, the size of the solar field is reduced. Furthermore, as plant efficiency 

increases, the size and cost of the condenser decreases. The three most expensive part of 

the power plant are the solar field, the turbine-generator and the condenser. Turbinegenerator costs tend to remain relatively constant for the same power output, which 

means that the two governing costs are solar field and condenser. 

While there are some differences in the cost of working fluids, the overall cost of 

working fluid is minuscule relative to the overall cost of the plant and they may therefore 

be disregarded. 

O&M COSTS 

Another vital component of a STORES power plant is the O&M cost. In the U.S., by far 

the dominating O&M cost for the SEGS power plants is the cost of labor. A significant 

effort was put into reducing the O&M costs for a STORES power plant. The ORC 

Cascade cycle and the solar field both lend themselves to fully automatic operation with 

remote surveillance and control for handling problems. This automation will increase the 

capital costs but fortunately the costs related to Distributed Controls Systems and remote 

control have plummeted in the last few years, and will not be a major factor in the first 

STORES power plant. It is vital to the success of future STORES plants in the US that 

unlike the original SEGS plants, STORES plants must be fully automated. 

 28
Optimization Summary 

The optimized STORES plant relative to this study will have the following features: 

• A recuperated cascade cycle with toluene as the topping fluid and butane as the 

bottoming fluid. 

• A field configuration that keeps the high and low HTF temperatures as close as 

practical. 

• A water-cooled heat sink. 

• A large cooling tower consistent with high incremental cost of electricity 

• A large condenser consistent with high incremental cost of electricity 

• A fully automated power plant, with no operators normally on site on a daily basis 

4.5 Power Plant Conceptual Design and Cost Estimate 

The plant conceptual design comprises the following: 

• Power Plant General Arrangement and Plot Plan 

• Solar Field General Arrangement 

• Process Flow Diagrams for the power plant under various operating conditions 

General Arrangement and Plot Plan 

Figure 4.5.1 and 4.5.2 are the Plot Plan and General Arrangement Drawings for the 5-

megawatt STORES power plant and solar field. The power plant will be area will 

roughly 250 feet 200 feet, or 50,000 square feet. The turbines are located together on one 

side of the plant to facilitate the electrical connections. The heat exchangers are located 

adjacent to each other in such a way as to minimize pipe runs between them. Also, they 

are arranged so tubes can be removed for cleaning. The cooling tower is located to one 

side of the plant near the condenser. This minimizes cooling water pumping parasitic 

power. 

 29
Figure 4.5.2 shows the solar field as well as the power plant. As the plot plan shows, the 

solar field takes up considerably more area than the power plant. It will be about 1800 

feet wide by 640 feet long, or about 1.2 million square feet. 

 30
E101 Cooling 

Tower

E105A&B Condensers

E104A-C LT Recuperator

TurbineGenerator TG101 Toluene

TurbineGenerator TG102 Butane

E102 A-C HT Recuperator

V 103 Butane Accumulator

P103 A&B Butane Pumps

V102 Toluene Accumulator

V 101 HTF Accumulator

P 101 A&HTF Pum

P102 A&B Toluene Pumps

Figure 4.5.1 Arrangemen5-MW STORPower Plant

E101 Cooling Tower

E105A&B Condensers

E104A-C LT Recuperator

TurbineGenerator TG101 Toluene

TurbineGenerator TG102 Butane

E102 A-C HT Recuperator

V 103 Butane Accumulator

P103 A&B Butane Pumps

V102 Toluene Accumulator

V 101 HTF Accumulator

P 101 A&HTF Pum

P102 A&B Toluene Pumps

Figure 4.5.1 Arrangemen5-MW STORPower Plant

31
80 M

50 M

250 M

Hot 50 MHTF

15M

Cold HTF

700 M

250 M

350 M 

195 M

Power Plant

60 M Fig. 4.5.2 General Arrangement

Five Megawatt STORES Power 

Plant Reflective Energies

April 26, 2002

1-LS2

80 M

50 M

250 M

Hot 50 MHTF

15M

Cold HTF

700 M

250 M

350 M 

195 M

Power Plant

60 M Fig. 4.5.2 General Arrangement

Five Megawatt STORES Power 

Plant Reflective Energies

April 26, 2002

1-LS2

32
Process Description 

Figures 4.5.3 shows the process flow diagrams for the 5 –MW STORES and Figures 

4.5.4 shows the process flow diagrams for the 1 MW systems. These are the summer 

conditions. As shown in the process flow diagrams, the heat energy is transferred from 

the source, the heat transfer fluid, to the sink, the air, in a series of heat transfer steps. 

Heat transfer fluid circulating pumps pump the heat transfer fluid through a parabolic 

trough solar field. The fluid then returns with heat energy and enters a toluene vaporizer. 

There, the heat transfer fluid transfers its heat energy to the toluene working fluid. Then, 

the heat transfer fluid returns to an accumulator. 

The toluene vapor flows to a high temperature turbine and expands to produce 

mechanical work. A generator converts this mechanical work into electricity. The 

toluene then exits the turbine and flows to a toluene recuperator. There, the heat from the 

turbine exhaust stream is used to pre-heat the toluene prior to its entry into the vaporizer. 

The toluene then flows to a toluene-butane condenser-vaporizer. In this heat exchanger, 

the heat energy from the toluene is ‘cascaded’ down to a lower temperature butane cycle. 

The toluene is condensed as the butane is vaporized. The toluene then flows to a toluene 

accumulator where it then is pumped out by toluene circulating pumps. 

The butane vapor flows from the condenser-vaporizer to a lower temperature turbine 

where it expands to produce more mechanical work. Another generator converts this 

mechanical work into additional electricity. The butane then exits the turbine and flows to 

 33
a butane recuperator. There, the heat from the turbine exhaust stream is used to pre-heat 

the butane prior to its entry into the condenser-vaporizer. 

The butane then flows to a condenser where it is condensed by a stream of cold water 

from the cooling tower. The butane then collects in a butane accumulator where it is 

pumped out by butane circulating pumps. 

The cooling water circulating pumps pump the cold cooling water through the condenser 

and back to the cooling tower. In the cooling tower, a fan draws in ambient air allowing 

it to contact the cooling water. Some of the cooling water evaporates and leaves with the 

ambient air. Since some of the cooling water evaporates, a make-up source of cooling 

water is required. In order to avoid excessive concentration of the cooling water, a 

blowdown stream of cooling water must be discharged. 

 34
Figure 4.5.3: 5 MW STORES Process Flow: Summer 

35
Figure 4.5.4: 1 MW STORES Power Plant Process Flow: Summer

36
Performance Curves 

The plant output will vary with the wet bulb temperature. In general, as wet bulb 

temperature decreases the plant gross and net power output increase. A colder wet bulb 

condition allows the cooling water to be cooled to a lower temperature. This, in turn, 

allows the working fluid to be condensed at a lower temperature and expanded to a lower 

pressure. This additional expansion produces more power. 

Figure shows the predicted power output for the solar ORC plant as a function of wet 

bulb temperature. The data points are taken from the heat balances in Appendix A. The 

four points represent the average, daytime, wet bulb temperatures in winter, spring, fall, 

and summer from left to right on the plot. The plot shows that the plant will make a high 

of 5.345 MW on an average winter day, and a low of 5.053 MW on an average summer 

day at Kramer Junction. 

Figure 4.5.5 shows performance curves for a 5 WM STORES power plant. It is seen that 

gross and net efficiency are a function of wet bulb temperature. The plant runs more 

efficiently at colder temperatures meaning it produces more gross and net power for the 

same heat input. The plot shows that the net efficiency in winter is 32.3 percent and the 

net efficiency in summer is 31.9 percent. 

Figure 4.5.6 is a performance curve for a 1 MW STORES power plant. 

 37
5 MW Plant Performance Curve

0

1,0002,0003,0004,0005,0006,0007,000

40 45 50 55 60 65

Wet Bulb Temperature

Power (kW)

30%32%34%36%38%40%

Efficiency (%) 

D

av

c

Win

Figure 4.5.5: Performance Curves 5 MW STORES

38
Figure 4.5.6 Performance Curves 1 MW STORES Plant

1 MW Plant Performance Curve

0

200400600800

1,0001,2001,4001,600

40 45 50 55 60 65

Wet Bulb Temperature

Power (kW)

30%32%34%36%38%40%

Efficiency (%) Gr

NePaGrNe

Data p

averag

cond

Winter, S

& S

Left 

39
Cost Estimate 

CAPITAL COSTS 

Table 4.5.1 is a listing of the capital costs of the major equipment for the nominal 5 MW 

plant. All costs for this study were computed in 2003 U.S. dollars. 

 

735 F Therminol VP1 Toluene Cycle

Tag # QTY. Cap. Ea. Description Flow Units HHP

E-101 1 100% Vaporizer 55 MMBtu/hr total 68,000

E-102A-C 3 33% HT Recuperator 32 MMBtu/hr total 354,000

E-103 1 100% Condenser/Vaporizer 43 MMBtu/hr total 129,000

E-104A-C 3 33% LT Recuperator 8 MMBtu/hr total 128,000

E-105A-B 2 50% Condenser 35 MMBtu/hr total 300,000

CT-101 1 100% Cooling Tower 100 MMBtu/hr total 150,000

P-101A&B 2 50% HTF Pumps 2530 gpm 75 148 118,000 

P-102A&B 2 50% Toluene Circ. Pumps 417 gpm 699 227 153,000 

P-103A&B 2 50% Butane Circ. Pumps 398 gpm 561 174 130,000 

P-104A&B 2 50% Cooling Water Pumps 1850 gpm 45 50 33,000 

TG-101 1 100% Toluene T-G Package 2,040,000

TG-102 1 100% Butane T-G Package 1,675,000

V-101 1 100% HTF Accumulator 56,000

V-102 1 100% Toluene Accumulator 86,000

V-103 1 100% Butane Accumulator 71,000

H-301 1 100% HTF Heater 7 MMBtu/hr total 76,000

Total Equipment Cost 5,567,000

Fluid Cost of fluid 357,000

Total Plant Installed Cost 15,388,000

Equipment Cost

Design Head 

psi Total Price

 Table 4.5.1: Capital Cost of a 5 MW STORES Power Plant 

The total cost of the major plant equipment is $5,567,000 or roughly $1100 per kW. For 

binary geothermal power plants, the total installed cost is roughly 2.7 times the major 

equipment capital cost. This works out to a total installed cost of $15,388,000 or $3,050 

per kW. These costs may be considered to be the cost of power plants once the 

technology is mature. In discussions with suppliers of solar trough fields, the cost of the 

field, installed, is anticipated to be about $2,000 a kW, or between $10,000,000 and 

 40
$12,500,000 in total. This brings the total cost of, the cost of the first STORES plant will 

be in the range of $25,000,000 to $26,000,000. For the first plant, it is anticipated that 

additional “first time” engineering costs will be needed, and the cost of the plant is 

anticipated to be closer to $27,000,000 to 30,000,000. For the economic analysis, a midrange figure of $27,500,000 was used. 

735 F Therminol VP1 Toluene Cycle

Tag # QTY. Cap. Ea. Description Flow Units HHP

E-101 1 100% Vaporizer 3 MMBtu/hr total 15,000

E-102A-C 3 33% HT Recuperator 7 MMBtu/hr total 79,000

E-103 1 100% Condenser/Vaporizer 10 MMBtu/hr total 29,000

E-104A-C 3 33% LT Recuperator 2 MMBtu/hr total 28,000

E-105A-B 2 50% Condenser 8 MMBtu/hr total 67,000

CT-101 1 100% Cooling Tower 22 MMBtu/hr total 61,000

P-101A&B 2 50% HTF Pumps 562 gpm 75 32 48,000 

P-102A&B 2 50% Toluene Circ. Pumps 93 gpm 699 50 62,000 

P-103A&B 2 50% Butane Circ. Pumps 88 gpm 561 38 53,000 

P-104A&B 2 50% Cooling Water Pumps 411 gpm 45 11 13,000 

TG-101 1 100% Toluene T-G Package 827,000

TG-102 1 100% Butane T-G Package 679,000

V-101 1 100% HTF Accumulator 12,000

V-102 1 100% Toluene Accumulator 19,000

V-103 1 100% Butane Accumulator 16,000

H-301 1 100% HTF Heater 1.5 MMBtu/hr total 31,000

Total Equipment Cost 2,039,000

Fluid Cost of fluid 79,000

Total Plant Installed Cost 5,584,300

Equipment Cost

Design Head 

psi Total Price

Table 4.5.2: Capital Cost of a 1 MW STORES Power Plant 

For the 1 MW nominal alternative, the total installed plant cost will be approximately 

$5,584,300, as shown in Table 4.5.2. The increase in cost per kilowatt is because the 

fixed costs for a smaller plant a smaller plant are the same as for a larger plant, and the 

equipment cost is proportionately larger, making the per-kilowatt costs higher. The cost 

 41
for the solar field is estimated to be $2,500 a kW, or $2,500,000. When additional costs 

associated with first-time effort and other uncertainties are added, the cost of the first 

STORES plant at 1 MW is anticipated to be between $9,000,000 and $10,000,000. These 

additional costs will be for the first plant, whether it is one megawatt or five megawatts. 

O&M COSTS 

Unlike the current Solar Thermal Trough based plants, STORES plants will be designed 

to run automated, including automated start-up and shut-down. This significantly 

reduces labor costs, which is important, because for the current plants, labor is spread 

over considerably larger power production. The O&M costs are anticipated to be about 

0.05 a kW for a STORES plant. Figure 4.5.3 provides the table for O&M costs. 

123 4 5 67 8 9

Megawatts Labor (Equiv. 

Persons) Rate $/hr Ann. Labor $

Ann. Solar 

Field NonLabor O&M 

Ann. Power 

Plant NonLabor O&M

Total Ann.. 

O&M costs

Annual Output 

KWH $/KWH

 5 2 40 160,000 250,000 160,000 12,000,000 570,000 0.048

 1 1 40 40,000 50,000 40,000 130,000 2,400,000 0.054

Figure 4.5.3: Stabilized O&M cost for 5 MW & 1 MW STORES Power Plant

 42
4.6 Field and Storage Evaluation and Conceptual Design Report 

Background 

At the time that this work was initiated, it was estimated that customers for STORES 

power plants would be generally off-grid, and would require a significant amount of 

storage in order to obtain electricity in the evenings and nights. STORES plants were 

initially therefore designed to use a fluid with a very low vapor pressure at high 

temperatures. The fluid of choice was Caloria, a commercially available high 

temperature fluid. Caloria is stable at 560 F, and there is a lot of experience with use of 

Caloria for thermal storage. 

Themocline Storage 

The thermocline system relies on and enhances the inherent low convection currents in 

the fluid. It comprises “hot” and “cold” fluid zones that are broad, indistinct divisions 

inside the tank. When the tank is fully charged, the hot zone comprises all of its useful 

storage capacity. When the tank is fully discharged, the cold zone expands to include the 

entire capacity of the tank. The tank is charged by drawing cold fluid from the tank, and 

heating it by circulating it through the solar collectors, then returning it to the hot zone of 

the same tank, increasing the average temperature of the tank, and enlarging the hot zone 

within the tank. A fully charged tank would comprise only hot fluid. When power is 

needed, hot fluid is drawn from the tank, and used to generate steam or another vapor 

(depending on the working fluid) and “cold” fluid is returned to the same tank. 

 

Dual Tank Storage 

Dual tank storage is the most common type of storage for hot and cold fluids. It 

comprises a hot fluid tank and a cold fluid tank, each with a capacity of the entire useful 

inventory of heat transfer fluid. During sunny days, fluid is drawn from the cold fluid 

tank, heated by running it through the solar collectors, and then it is delivered to the hot 

fluid tank. When power is needed, fluid is drawn from the hot fluid tank, where its heat 

is used to vaporize steam or other working fluid, after which it is sent to the cold fluid 

tank. The SEGS I power plant used a two tank storage system. 

 43
Potential STORES Customers: The Size Vs. Cost Dilemma 

It was hypothesized that customers for STORES power plants would be remote 

customers that are completely off-grid, and these customers would require storage of 

energy to enable power on demand. At that time, it was assumed that there would be 

several sites with peak demands of ten megawatts or more. Hence storage was initially 

an integral part of STORES design. However, in the U.S. such customers were not 

found. The electric power grid is ubiquitous, and most communities have access to it. 

Communities that are totally off-grid are usually very small, with peak demands of no 

more than a few kilowatts. For such customers, Photovoltaics (PV) is a much better 

option. STORES systems are just not practical in sizes of a few kilowatts. However, 

even though no customers were found that could use ten-megawatt, or even onemegawatt STORES power plants, there was found to be a significant demand for 

STORES power plants located on-grid. While the profile of these customers differs 

widely, some of the common traits include the following: 

• High cost of traditional electricity 

• Summer peaking, mainly air-conditioning 

• At the end of long, weak feeders, perhaps overloaded during summer peaks 

• Need for diversity of fuel source 

• Pressure to increase renewable energy use 

o Portfolio standards 

o Buydowns available for solar power 

o Mandates or targets to increase renewable energy use 

• Energy security needs 

• The need for ultra-clean on-site power 

• Summer peaks of ten to twenty megawatts or higher 

• Large tracts of land available with high-quality direct normal solar radiation 

• A commitment to solar power 

These customers are already receiving large amounts of power from the grid. They are 

located in the Southwestern desert, and are most commonly government institutions. 

They include Army and Air Force bases, weapons stations, US Forest Service stations, 

National Parks services, University outposts, and so on. Some of these institutions will 

be able to use ten megawatts of summer peaking power, but others would be better served 

 44
with power plants that are smaller, in the size range of 1 to 5 megawatts. None of them 

would benefit to any extent were storage available. 

Plants without Storage: Efficiency Considerations 

When storage was considered vital to STORES power plants, the maximum temperature 

available was 560 F, the thermal stability limit for Caloria. Caloria at 560 F has a very 

low vapor pressure, and may be stored in atmospheric tanks. Most SEGS plants use 

Theminol VP-1, that is thermally stable up to 735 F, but has higher vapor pressure at that 

temperature and must therefore be pressurized. Large pressurized tanks are expensive 

and complex. However, with no need for storage, it becomes feasible to consider the 

increase in thermal efficiency that can be achieved with higher temperatures, and the 

economic considerations related to doing so. 

The previous sections (4.1 through 4.5) addressed the various changes to the 

thermodynamic cycle that the higher temperatures can achieve. A dual cascade cycle 

using toluene as the upper working fluid and butane as the lower working fluid were 

selected over the other options: a simple Rankine cycle and a triple cascade cycle with 

three working fluids. 

The Two Sizes Selected: Five megawatts and One megawatt 

Whereas originally it was anticipated that the STORES standard plant would be 10 MW, 

it was found that customers were more interested in smaller plants, in the size range of 1 

to 5 MW. The study therefore reflects the change the STORES design to two choices: 

One megawatt and five megawatts. This was a difficult decision to make, because while 

the solar field is largely modular, a larger power plant has significantly lower cost per 

kilowatt than a smaller power plant. On the other hand, it is well known that no new 

Trough power plants have been built in the last fifteen years. The trend to larger plants 

(as witnessed by the progression from a fifteen megawatt SEGS I to 80 megawatt SEGS 

VIII and IX did not continue. Rather than continue towards larger plants, it is considered 

more prudent to consider market forces and develop a power plant that meets the needs of 

the market. 

 45
Field Design 

It is possible to increase the average temperature of the HTF without exceeding its 

maximum temperature by selecting the series-parallel patterns in which solar collectors 

assemblies are arranged. For the same total number of collectors, the fewer collectors in 

series the higher the average temperature. Should STORES power plants reach full 

commercial potential, this consideration is anticipated to play an important part in 

optimizing the efficiency and output. 

4.7 Host Site Selection and Market Potential Report 

California Opportunities 

Ultimately, STORES systems must have willing, supportive, even enthusiastic clients in 

order to succeed. Solar energy is intermittent and seasonal, and needs grit and 

determination to move forward. It is also vital to provide high value to the client. Several 

attempts were made to find such clients. It was initially felt that the best marketplace in 

the US would be off-grid customers who needed electricity. This approach was not 

successful. Despite much effort, no potential off-grid customers could be found in the 

U.S. who would benefit from 1 to 10 MW of solar electric power. The U.S. grid is 

generally robust and delivers power to any community that has a sustained demand for 

one megawatt of power. While this finding is good for U.S. customers, it was a blow to 

the project. The search included meetings with Sandia National Labs, to help seek Native 

American communities who may benefit from solar electric power. During the meeting, 

Dave Menicucci of Sandia suggested that there may be on-grid customers who would be 

very interested in STORES power plants connected to the grid, but on the customer side 

of the meter. These customers would be large US government institutions located in the 

Southwest desert. The U.S. government is the largest single landowner in the 

Southwestern U.S. Many of these institutions are part of the U.S. Department of 

Defense, with military bases, weapons stations, Forts and Air Force bases strategically 

located in many areas. Several of these locations have peaking loads of 10 to 20 

megawatts, and peak load occurs on hot summer days. This is an excellent match with 

solar thermal power because the power plant will also peak during those same hot days 

for the same reason. 

 46
Many of these remote DOD locations are served by long feeders from utility systems. 

After September 11, 2001, there is increasing concern that the security of power supply to 

these remote stations may be inadequate. On-site power generation, even intermittent 

solar power, will help increase security. If the fuel supply is local, such as solar power, 

another risk factor is reduced. As an example, Fort Irwin has a peak load of about 20 

megawatts, pays $0.19 a kWh for summer peak power, $23 per month for peak demand, 

and is at the end of a long line. Fort Irwin is very interested in a STORES power plant. 

Several other military bases have shown initial interest. 

The California Power Authority (CPA) created a parallel market opportunity. The CPA’s 

mandate is broadly to ensure that stable power is available from diverse resources. The 

CPA issued a Request For Bids (RFB) for PV systems on rooftop and open field systems. 

It subsequently amended its RFB to include solar thermal electric systems up to 5 MW. 

This opportunity was very important for the STORES power plants. Reflective Energies 

(through its sister organization, FlexEnergy) submitted a proposal for a total of ten 5-

megawatt STORES power plants all located on the customer’s property. The base 

proposal included five such plants and the optional proposal included another five plants. 

All of these plants would be essentially identical, delivering power directly to the grid, 

but by providing the power on the customer side of the meter, the power produced would 

offset retail power pricing. Retail prices for electricity are significantly higher than 

wholesale prices in California. 

The California Power Authority approved the bid submitted by FlexEnergy and the CPA 

placed FlexEnergy on the approved bidders list for future solar power offerings. Under 

the terms of the award, the CPA will find a host for the power plants who will provide the 

land and the water. The CPA may own and operate the power plants, transfer them to the 

host, or purchase the electricity from the plants. 

A second aspect of this opportunity is the increased energy security requirements 

resulting from the events of September 11, 2001. Congress has commissioned a study 

that is being performed by URS to determine ways to increase the reliability of power for 

 47
vital U.S. defenses. Many U.S. defense bases in the Southwest receive their power supply 

from just a single, vulnerable, long distance transmission line. These bases often have no 

significant on-site generation, and are also limited by fuel storage options. Among the 

solutions being considered is renewable energy. Renewable energy, particularly solar 

power can significantly improve reliability of such sites, even without storage. Solar 

power is locally available, in the Southwest matches the summer peak very well, and is 

largely immune from supply problems. Multiple plants scattered through the bases can 

increase reliability further because even if one site is incapacitated, others will continue 

to function. California Bases with high summer peaks and high electricity costs 

especially for summer peaking power include Fort Irwin, Twenty Nine Palms, Edwards 

Air Force Base and others. 

In summary, the STORES research and development work has resulted directly in two 

major opportunities for Trough systems that were not previously conceived. Rather than 

follow the pathway of the SEGS power plants, where larger was considered better, the 

STORES study created the pathway to smaller power plants. STORES takes a page 

from Distributed Generation, and another from strategies for large Photovoltaic power 

plants. Instead of making low cost electricity, STORES seeks to make high value 

electricity, creating a new marketplace for Trough systems. 

Arizona 

There are many other similar opportunities in Arizona and the Southwestern US. 

Reflective Energies arranged a meeting with Pinnacle West, the Arizona Energy 

Company that is the parent of Arizona Public Service Company. Also in attendance at 

the meeting were energy representatives of the US Marine Corps Air Base at Yuma 

Arizona. The purpose of the meeting was to convince Pinnacle West that trough-based 

solar thermal power plants can be constructed in small modular sizes, and that such plants 

can be used to provide Distributed Generation benefits. A project was considered 

between Pinnacle West, the Marine Air Base and Reflective Energies whereby a 

STORES power plant would be installed and deliver power to offset peak power for the 

Base. Pinnacle West had several questions related to the STORES technology. As it 

learned more about the technology, Pinnacle West became more confident that the 

 48
technology has potential. In February of 2002, at the request of Reflective Energies, a 

team from Pinnacle West visited the Mammoth Geothermal Power Plant. Mammoth uses 

an Organic Rankine Cycle to generate electricity. Pinnacle West intention in making the 

visit was to evaluate the potential for the Organic Rankine Cycle for a STORES power 

plant. The meeting was successful. Pinnacle West’s interest in Trough Based Power 

Plants continued to increase. With the renewable energy portfolio standard in effect in 

Arizona, Pinnacle West must install solar power plants to meet mandated requirements. 

Pinnacle West believes that small modular STORES-type solar thermal power plants can 

play a major role in helping it meet its portfolio standard goals. This conclusion by 

Pinnacle West is directly the result of Reflective Energies’ STORES project 

developmental work. The STORES effort has already produced results far in excess of 

the original plan. 

In April of 2002, Pinnacle West issued a Request for Proposals for a One-Megawatt 

Trough Based Organic Rankine Power Plant. This RFP itself provides further evidence 

that STORES power plants do make sense, that there is a market for such plants. The 

power plant, known as the Saguaro Solar Power Plant, is now under construction. 

The RFP from APS had some changes from the concept originally proposed by 

Reflective. APS is now looking to generate power from solar to compete with wholesale, 

rather than retail prices and this poses a somewhat bigger challenge than the previous 

plan. Furthermore, APS decided that the first STORES plant to be only one megawatt; 

STORES already downsized from ten megawatts to five megawatts, and a further 

downsizing to one megawatt makes the overall project far more expensive on a dollarsper-kilowatt basis. The design of the power plant has been adapted to suit the partners in 

the project. 

Other States 

There are many other opportunities in the Southwest. Colorado and New Mexico are 

considering or implementing portfolio standards, and STORES power plants stand an 

excellent chance of being selected for meeting those portfolio requirements. STORES 

plants can be expanded from military bases to other governmental sites such as 

 49
Universities, Forest Service sites, Desert Laboratories and other such applications. The 

door has opened to consideration of Trough power plants as Distributed Generation. The 

STORES effort has been successful. 

Worldwide Opportunities 

There are many locations around the world that would greatly benefit from STORES 

plants as Distributed Generation for local consumption. The entire Sahara Desert, as 

large as the continental US, with its dozens of nations, holds great potential for STORES 

plants, grid connected and stand-alone. The entire Middle East with its large number of 

nations, India, Mexico and other countries. For the present, it is vital that the first few 

plants be built in the US satisfying important local needs before the technology is 

exported to other nations, because the complexity of export alone is challenge enough 

without adding the complexity of technological risk, expertise shortage and so on. 

Conclusion: Market Opportunities 

In conclusion, it appears that there is a real, potentially cost-effective market for 

STORES systems here in the U.S. and also in other countries. The STORES system 

should be designed and operated in a manner that meets the client’s needs. The first 

STORES power plant is already under construction. It will help Pinnacle West to meet 

the Arizona Portfolio standard and will also provide the pathway towards low cost power 

from STORES systems. 

4.8 Economic Evaluation 

In order to perform this task, a Pro Forma for the costs, revenues and economics for 

STORES power plants was created. This Pro Forma was constructed to be flexible, 

allowing several assumptions to be input as variables, so that the effect of each variable 

could be examined, and the cumulative effect of multiple variables could also be 

examined. The generalized Pro Forma was created in a Lotus 123 format, and is 

available as a separate document. In order to gain some economies of scale the Pro 

Forma was set up to include the first three STORES plants in sequence,. The 

assumptions used were generally very conservative. The estimated overall cost of the 

first 5-megawatt STORES power plant was set at about $5.50 /kW installed, making the 

 50
total power plant cost $27,500,000. Subsequent plants were considered to be lower in 

cost because of our efforts to flatten the learning curve and because much of the design 

and developmental work would not have to be repeated. The power output from each 

plant would be about 2,560 kWh per installed kilowatt per year, or a total of 11,213,000 

kWh/year for a five-megawatt power plant. This represents a capacity factor of 25.6% on 

an annual basis. The output could be increased somewhat by building a larger solar 

collector system and storage for some increase in cost. There are some tradeoffs between 

storage and maximizing efficiency. Exhibit 1 of the Pro Forma shows the first three 

STORES plants at the same site, thus simplifying the licensing, approvals, engineering 

and other costs for plants two and three. The Pro Forma also examines subsequent power 

plants. Exhibit 2 of the Pro Forma for the tenth STORES plant installed on a site all by 

itself. 

The O&M costs of a STORES power plant are anticipated to be in the range of $0.03 to 

$0.05 /kWh. For the Pro Forma, because there are a total of three plants at the first site, a 

composite cost for all three plants of $0.04 /kWh was assumed. While these O&M costs 

are lower than the existing SEGS power plants, they are not unreasonable. The SEGS 

plants were built during the eighties, with the Standard Offer contracts were in place, and 

natural gas use was allowed. Furthermore, running the power plants, especially in the 

early years, was a significant challenge. Each plant was unique in its own way; the 

technologies for design, installation, O&M were just being developed, and failures were 

much more commonplace. By contrast, STORES plants will be pure solar, have a much 

simpler operating philosophy, and will benefit from the lessons from the nine SEGS 

plants. The Pro Forma allows evaluation of various first costs and various O&M costs. 

One means to examine the reasonableness of costs is to consider the cost of alternative 

generation. It was originally envisaged that STORES power plants would be off-grid; in 

this context alternative generation would be the cost of fossil fuel fired generators or 

other solar generators, such as PV, at the site. As the study progressed, it was found that 

the best application for early STORES power plants was on-grid, on the customer’s side 

of the meter. Alternate generation in this context represents the collective generators that 

 51
together provide power for the grid. A practical surrogate for this is the power delivered 

by the grid to the customer. This power may be wholesale, which is the power purchased 

by the Utility for resale to its customers, or retail, which is the power sold by the Utility 

to its customers. STORES power plants, unlike the large scale SEGS solar thermal 

plants, are small scale, Distributed Generation power plants that may be placed on the 

customer’s side of the meter, thus commanding retail electricity pricing that is much 

more attractive than wholesale pricing. STORES power plants are intended to sell power 

directly to the user, displacing expensive retail Utility power. This is particularly 

attractive during peak summer months when electricity prices are at their highest. This 

strategy dramatically changes the notion of the cost of alternate generation. 

Pro Forma Results Summary 

The Pro Forma analysis shows the following: 

Basis: 

Size: 5 MW 

No. of Plants: 3 

Capital Cost: $5:50 per kW installed 

Capacity Factor: 25.6% 

Loan Amount: 80% debt-to-equity ratio at 5.0%/year interest rate. 

Energy Credit: 0 

Production Tax credits: $0.017/kEh 

O&M Costs: $0.04/kWh 

Energy Pricing (Based on SCE’s then TOU rates): 

Summer Peak: $0.19544/kWh 

Mid-Peak: 0.10897/kWh 

Off-Peak: $0.08808/kWh 

Annual Energy Price Escalation Factor: 5% 

With the assumptions above, the anticipated rate of return on capital is 4.77%. 

If there is a $0.015/kWh energy credit, the rate of return is 18.39%. 

If in addition, there is also an energy tax credit of $0.03/kWh, the rate of return is 

22.18%. 

 52
STORES economics are particularly sensitive to capital cost as well. If the cost of the 

plants can be reduced to $5 /W, for the same returns, energy prices could drop by 11.6%. 

If costs can be brought to $4 /W, energy prices could drop by 34.7% for the same returns. 

It is not at all unreasonable to posit that STORES power plants could be built in the 

future for $4/W, especially if more than one plant is built at the same time, allowing for 

larger volumes of solar collector production. 

A key factor in the equation is low interest rates that have been prevalent for the last five 

years. For capital-intensive industries, such as STORES, low interest rates provide a 

major benefit. In this era of low interest rates and high energy prices there is a definite 

market for STORES systems providing power to end-users here in the U.S. 

4.9 Planning for the First STORES Power Plant 

Integrated Combined Cycle Solar power plants have been actively pursued for about 15 

years, at a cumulative cost of hundreds of millions of dollars, but the first such plant is 

yet to break ground. The STORES concept was intended to find a shorter, less complex 

path towards resurrecting trough technology. Under this task Reflective Energies 

pursued the planning for a demonstration plant. As a result of its early investigations into 

possible paths towards demonstration, Reflective Energies pursued two routes: a plant in 

California, and a plant in Arizona. The California plant would include the California 

Power Authority (CPA) and a large consumer of power such as a major military base in 

the desert. The Arizona plant would be a demonstration plant for Arizona Public Service 

Co (APS) who is seeking to meet its renewable energy portfolio requirements. Both 

entities had a strong need for power, and particularly for large quantities of solar power. 

Reflective decided to develop plans for each of the two possibilities. Each of the entities 

took a different path towards the first demonstration. 

The first path was in California. The CPA issued an RFP for solar power. Initially, this 

RFP did not include solar thermal power plants, but as a result of questions raised during 

meetings with potential bidders, CPA decided to include solar thermal in its RFP. 

FlexEnergy (a sister organization to Reflective Energies) submitted a proposal to the 

 53
CPA through which it would build the first demonstration plant and then subsequent 

STORES plants. The proposal stated that FlexEnergy was prepared to build a series of 

solar thermal STORES power plants, and provided a cost, rudimentary design, and power 

generation information. Each plant would be five megawatts in size, and each 

subsequent plant would be less expensive, both for capital and for O&M. The plants 

would initially be owned by FlexEnergy, supplying power to a retail electric consumer, 

and the CPA would have the option to take over the power plant in time. Alternately, 

FlexEnergy was ready to build plants for the CPA, should that be the CPA’s desire. The 

proposal was successful, and CPA selected FlexEnergy as a supplier of solar power 

plants that the CPA was planning to develop in California. The CPA has since become 

less of a force in California, and the promise of supporting the construction of power 

plants has not materialized so far. 

The second path was in Arizona. Arizona utilities are required to meet a portfolio 

standard, and generate increasing amounts of electricity from renewables. APS is a 

leader in developing new forms of solar electricity, and as a result of Reflective’s efforts, 

became very interested in STORES technology. Following meetings in Arizona, APS 

decided to investigate the reliability of ORC technology such as would be used in a 

STORES power plant. Reflective arranged a tour of the Mammoth geothermal power 

plant that has an ORC power system. APS interest in a STORES demonstration 

STORES plant increased sharply. It asked several questions regarding O&M, and 

requested that Reflective develop a means to significantly reduce O&M costs. Reflective 

organized a meeting at the SEGS III through VII power plants run by KJC Operating Co. 

The meeting was very successful, and a plan was developed to build an automated five 

megawatt STORES power plant with O&M costs significantly below those of the SEGS 

power plants, despite the smaller size of the STORES plant. 

As a result of Reflective’s efforts, APS then decided to go out to bid for a STORES plant, 

to a list of suppliers provided by NREL. APS issued an RFP, but instead of its original 

desire to build a five megawatt power plant, requested that bids be supplied to build a one 

megawatt solar thermal power plant, largely along the lines Reflective had provided to it. 

 54
APS received three bids and following a review, selected Solargenix as its supplier. The 

scope of work was negotiated, and a one-megawatt demonstration STORES plant known 

as the Saguaro Solar Power Plant broke ground on March 24, 2004 and is now under 

construction,. APS had considered the Saguaro site for a power plant for at least twenty 

five years, considering several technologies including the molten salt power tower. 

However, it was ultimately the STORES technology that brought the idea to fruition. 

In summary, the STORES concept was so successful that a demonstration STORES 

power plant was initiated as a result of Reflective’s efforts even as the plan for such a 

demonstration plant could be established. 

 55
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Subcontract Report 

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5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

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4. TITLE AND SUBTITLE

Solar Trough Organic Rankine Electricity System (STORES) 

Stage 1: Power Plant Optimization and Economics 

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NREL/SR-550-39433 

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CP06.1001 

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Prabhu, E. 

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Reflective Energies 

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Mission Viejo, CA 

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Report regarding a Stage 1 Study to further develop the concept of the Solar Trough Organic Rankine Cycle 

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19b. TELEPONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18