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

Summary Report for 

Concentrating Solar Power 

Thermal Storage Workshop

New Concepts and Materials for Thermal 

Energy Storage and Heat-Transfer Fluids 

May 20, 2011

G. Glatzmaier

Technical Report

NREL/TP- 5500-52134

August 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

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303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Summary Report for 

Concentrating Solar Power 

Thermal Storage Workshop

New Concepts and Materials for Thermal 

Energy Storage and Heat-Transfer Fluids

May 20, 2011

G. Glatzmaier

Prepared under Task No. CP09.2201

Technical Report

NREL/TP- 5500-52134

August 2011
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Acknowledgments

The author would like to thank the following contributors to this report: 

National Renewable Energy Laboratory

Daniel Blake

Nicolas Calvet

Calvin Curtis

Judith Gomez

Joongoo Kang

Chuck Kutscher

Mark Mehos

Michele Olsen

Sulolit Pradhan

Anne Starace

Bill Tumas

Jun Wang

Junyi Zhu

Sandia National Laboratories

Nathan Siegel

U.S. Department of Energy

Joe Stekli
iv

Table of Contents

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

SunShot Initiative...................................................................................................................1

CSP Technology Description................................................................................................2

CSP Thermal Energy Storage ...............................................................................................5

Types of Thermal Energy Storage ........................................................................................... 5

System and Material Challenges for Thermal Energy Storage ..........................................7

Heat-Transfer Fluids ................................................................................................................ 8

Needs and Challenges........................................................................................................ 9

Sensible Energy Storage .......................................................................................................... 9

Cost Benefit from Improved Heat Capacity of Sensible Storage Fluids ........................... 9

Needs and Challenges...................................................................................................... 10

Phase-Change Storage............................................................................................................ 11

Needs and Challenges...................................................................................................... 11

Thermochemical Storage ....................................................................................................... 11

Needs and Challenges...................................................................................................... 12

Research Directions for Thermal Energy Storage ............................................................13

Heat-Transfer Fluids .............................................................................................................. 13

Sensible Storage ..................................................................................................................... 13

Phase-Change Storage............................................................................................................ 14

Thermochemical Storage ....................................................................................................... 14

Summary...............................................................................................................................15

References............................................................................................................................15
1

Introduction

The U.S. Department of Energy (DOE), National Renewable Energy Laboratory (NREL), and 

Sandia National Laboratories hosted a workshop on thermal energy storage for concentrating 

solar power (CSP) on May 20, 2011, at NREL in Golden, Colorado. The objective for this 

workshop was to engage the university and laboratory research communities to identify and 

define research directions for developing new high-temperature materials and systems that

advance thermal energy storage for CSP technologies. Desired outcomes for the workshop 

were to 1) inform the workshop participants of CSP technology challenges, specifically with 

respect to materials, and 2) generate and document new ideas for advancing materials 

development for CSP thermal energy storage.

The workshop agenda featured introductory presentations by DOE, NREL and Sandia staff 

that provided overviews of the DOE CSP Program goals and CSP technologies. Emphasis for 

the presentations was on the role and impact of thermal energy storage when it is incorporated 

into an operating CSP plant. These were followed by featured presentations given by invited 

speakers. Topics for these talks were 1) new heat transfer fluids for CSP technologies, 2) 

sensible thermal energy storage systems, and 3) thermochemical cycles for thermal energy 

storage. The presentations were followed by three parallel breakout sessions that covered 1) 

heat transfer fluids, 2) sensible and latent storage, and 3) thermochemical storage. For each 

session, participants were asked to identify system/material challenges and promising research 

directions for the topic area. The workshop concluded with summary presentations of the 

findings from the breakout sessions. All findings from the workshop are documented in this 

summary report.

SunShot Initiative

This workshop was motivated, in part, by the DOE SunShot Initiative, which was established 

in 2010.1

 This initiative sets a very aggressive cost goal for CSP technologies. The primary 

goal is to reach a levelized cost of energy (LCOE) of 6¢/kWh by 2020 with no incentives or 

credits. Because CSP is the only solar technology that is capable of significant energy 

storage, this cost goal applies to CSP plants that have several hours of thermal energy storage 

(TES) included in their design and operation.2

 As such, the cost and performance of the TES 

system are critical to meeting the overall cost goal for the CSP technology. The target cost 

for the TES system depends on other cost and performance factors for the power plant, but the 

initial cost target for TES system components under SunShot has been established at $15-

$20/kWhth. Furthermore, this cost target assumes the TES system integrates with the solar 

field and power block components in a fashion that does not reduce the efficiency of their 

operation. 

Figure 1 shows qualitatively the collection, conversion, and overall efficiencies for a general 

CSP plant. The temperature at which the overall efficiency reaches its maximum depends on 

many factors, including material properties of the CSP plant components. Increasing the 

operating temperature of the power generation system generally leads to higher thermal-toelectric conversion efficiency. In a CSP system, higher operating temperature leads to greater 

thermal losses. These two effects combine to give an optimal system-level operating 

temperature that may be less than the upper operating temperature limit of system 
2

components. System-level efficiency may be improved by developing materials, power 

cycles, and system-integration strategies that enable operation at elevated temperature while 

limiting thermal losses. This is particularly true for a TES system and its components. 

Meeting the SunShot cost goal will require new materials for the TES system and heattransfer fluid (HTF) that allow the CSP power plants to operate at higher temperatures and 

with greater efficiency than current parabolic trough and power tower plants.

Figure 1. Collection, conversion, and total system efficiencies as functions of temperature.

CSP Technology Description

CSP plants offer an attractive means for near-term, utility-scale, dispatchable, renewable 

electricity generation. More than 400 megawatts (MW) of capacity are currently in place in 

the U.S. southwest.3 With 650 MW currently under construction in Arizona and California,4,5

CSP technologies will play an increasingly significant role in providing sustainable power 

generation. All CSP technologies have similar components – solar collectors, receivers, and 

thermal power conversion systems. They are grouped into two general types according to 

their collector/receiver geometries: point-focus and line-focus.

Point-focus geometries are the power tower and parabolic dish. The power tower consists of 

a single receiver that is located at the top of a tower (Figure 2). The tower is surrounded by a 

field of two-axis tracking mirrors, or heliostats, that reflect and concentrate sunlight to the 

receiver. An HTF circulates to the receiver, collects thermal energy contained in the 

concentrated sunlight, and returns to the power block where it is used to generate steam for 

the turbine power cycle. Power towers using molten salt HTF normally store the hot and cold 

salt in tanks that allow for separation of the solar collection and power generation cycles. 
3

Figure 2. Schematic of power tower with direct, two-tank molten-salt thermal storage.

The most common HTF is molten nitrate salt that is a thermally stable liquid in the 

temperature range of 220° to 565°C. This type of configuration is a direct system because the 

HTF and storage fluids are the same. Some power towers send water to the tower, generate 

steam in the receiver, and send it directly to the turbine.

The parabolic dish is a modular design in which each module consists of a two-axis tracking, 

point-focus concentrator that has a receiver/engine/generator located at its focal point

(Figure 3). A Stirling engine is typically used to convert thermal energy to mechanical power 

for electricity generation. The working fluid is hydrogen or helium and is heated directly in 

the receiver. The Stirling engine operates nominally at 800oC and has a high thermal-tomechanical conversion efficiency. Heat transfer to the working fluid occurs isothermally at 

the operating temperature of the working fluid.

The most common line-focus geometry is the parabolic trough design. Its collector field 

consists of single-axis parabolic mirrors that reflect and concentrate sunlight to a focal line 

(Figure 4). The concentrated sunlight is absorbed by an HTF that flows through receiver 

tubes located at the focal line. Most trough plants use a synthetic oil for the HTF. Thermal 

energy within the heated HTF is used to generate steam for the turbine power cycle or can be 

stored within a separate storage fluid for later use. The storage fluid for this design is molten 

salt and the storage system is referred to as indirect because the HTF and storage fluids are 

distinct and require a heat exchanger to transfer thermal energy between them.
4

Figure 3. Schematic of parabolic dish with Stirling engine.

Figure 4. Schematic of parabolic trough power plant with two-tank, molten-salt thermal storage.
5

Line- and point-focus power plants are currently operating at commercial scale in the U.S. 

southwest and Spain. Commercial-scale plants of both types are currently under construction

in the U.S. and Europe. However, broad implementation of CSP technologies historically has 

been limited by the cost of electricity produced by these plants, which is greater than 

electricity costs from conventional power generation. The purpose of the SunShot Initiative is 

to significantly increase broad-base implementation of solar-to-electricity technologies. Key 

thrusts of this initiative are dramatic performance improvements and cost reductions in all of 

the components that make up a CSP plant and the use of TES to increase the plant’s capacity 

factor and dispatchability. Both thrusts will make CSP plants more attractive for utility-scale 

implementation. Power towers in particular are thought to have greater potential for widescale implementation because of their higher thermal conversion efficiency and greater stored 

energy densities. Both of these features are due to their higher operating temperatures.

CSP Thermal Energy Storage

CSP plants are unique among renewable technologies in that they provide utility-scale, 

dispatchable electricity to the power grid. Dispatchable delivery means power is reliably 

available when it is needed to meet the utility load demand. This feature is due to the 

incorporation of TES into the power plants. TES allows electricity to be generated 

consistently at times when sunlight is not available, including momentary cloud transients, 

which otherwise disrupt electricity generation and cause widely varying power output. For 

longer time scales, TES allows CSP plants to generate electricity well into the evening hours 

when electricity is highly valued, making the power plant more cost effective. TES also 

allows greater use of the turbine and other power-block components. These features provide 

an economic incentive for the addition of TES. Without TES, CSP solar power is an 

intermittent power resource that depends on sunlight availability. In addition to enhancing 

CSP dispatchability, TES enables increased deployment of renewables in general by adding 

flexibility to a grid with photovoltaic and wind power systems.

Types of Thermal Energy Storage

Figure 5 lists a variety of TES options for CSP plants. They fall into three general categories: 

sensible, latent, and thermochemical storage. A book published in the mid 1980s provides a 

comprehensive survey of the fundamentals of the storage options, examples of systems, and 

the issues that must be addressed for technologies in the range from low to high 

temperatures.

6

 The only TES system that currently operates with multiple hours of storage is 

the sensible, two-tank, molten-salt system. This system is used because the components 

associated with molten-salt handling—pumps, valves, tanks, and heat exchangers—have 

demonstrated reliable operation at commercial scale.
6

Figure 5. Thermal energy storage options for CSP technologies.

The molten-salt storage fluid is a mixture of NaNO3 and KNO3. This fluid is liquid in both 

the charge and discharge states, so there are minimal heat-transfer limitations, making the 

heat-exchanger design relatively straight-forward. One drawback for this system is the 

relatively low stored energy density, which results in a large storage medium inventory, 

requiring large insulated storage vessels. Implementation of this TES system into parabolic 

trough power plants requires an indirect configuration—distinct heat-transfer and storage 

fluids—because the storage salt has a high freezing point (220oC) and could possibly freeze in 

the solar collectors if used as the HTF. The indirect system requires a heat exchanger for 

transferring thermal energy between the HTF and storage fluid (Figure 4). This heat 

exchanger reduces the performance of the storage system and adds cost to the plant. This 

approach has been demonstrated commercially in Spain at the Andasol plants.

Implementation of this TES system into power towers can use a direct configuration—

common heat-transfer and storage fluids—because steps can be taken to prevent the freezing 

of molten salt in the receiver and transfer lines within the vertical tower that are not possible 

in a parabolic trough configuration. This type of system was demonstrated during the Solar 

Two project in Barstow, California.

The stored energy density of the two-tank system can be increased in two ways. First, 

increasing the maximum operating temperature of the power plant increases the temperature 

drop across the turbine. Higher temperature drops increase the efficiency of the turbine 

power-cycle and the stored energy density of the sensible portion of the TES system. Second, 

increasing the heat capacity of the storage medium also directly increases the stored energy 

density. These effects are important to reducing size, and therefore the capital cost of the 

two-tank TES system.
7

Thermoclines that use low-cost storage materials offer an opportunity for reducing TES costs. 

The benefit of thermoclines is significant, especially if the storage material is self-supporting. 

In this case, the structural requirements of the containment vessel can be reduced. At very 

high temperatures, the cost of the containment vessel(s) for the storage system may negate the 

cost benefit from increased temperature drop. Thermoclines have more complex operating 

requirements than the two-tank, molten-salt system, which creates the potential for utilization 

and performance losses.

Alternatives to the two-tank, molten-salt storage system are being considered to increase the 

stored energy density and, ultimately, to reduce the cost of the TES system. The most 

developed alternative is the use of phase-change materials (PCMs) to increase stored energy 

density. PCMs have both latent and sensible enthalpies that contribute to the stored energy 

density, providing a benefit over purely sensible systems. PCM systems suffer from a 

limitation in heat transfer during the discharge process due to the generally low thermal 

conductivity of the solid phase. This limitation results in low power density for PCM systems 

and will need to be overcome if PCM storage is to become a viable alternative. PCM storage 

is the most compatible storage system for the parabolic dish/Stirling concentrator because 

thermal energy delivery to the engine is isothermal.

The third option, thermochemical storage offers perhaps the greatest benefit due to the large 

quantity of stored energy associated with the heat of reaction. Practical implementation of 

these systems is often limited by the loss of system performance as it is put through many 

charge/discharge cycles. System performance depends on maintaining consistent physical 

and chemical properties of the chemical components and of any solid-phase materials used in 

the system over many cycles. Over time, degradation of these material properties may result 

in reducing both the system heat-transfer rate and storage capacity. In addition, some cycles 

require the handling of gas-phase reactants that may require compression or corrosive 

substances that require special materials of construction. The benefits of very high energy 

densities and the possibility of storing reaction products at ambient temperature keep 

thermochemical storage under consideration for CSP technologies.

System and Material Challenges for Thermal Energy Storage

Meeting the SunShot cost target will require cost and performance improvements in all 

systems and components within a CSP plant. Solar collector field hardware will need to 

decrease significantly in cost with no loss in performance and possibly with performance 

improvements. As higher temperatures are considered for the power block, new working 

fluids, heat-transfer fluids, and storage fluids will all need to be identified to meet these new 

operating conditions. Figure 6 shows thermodynamic conversion efficiency as a function of 

temperature for the ideal Carnot cycle and 75% Carnot, which is considered to be the practical 

efficiency attainable by current power cycles. Current conversion efficiencies for the 

parabolic trough steam cycle, power tower steam cycle, parabolic dish/Stirling, Ericsson, and 

combined air-Brayton cycles are shown at their corresponding operating temperatures. 

Efficiencies for supercritical steam and CO2 are also shown for their operating temperature 

ranges.
8

Figure 6 makes clear the benefit of increased power-cycle operating temperature. Moving in 

this direction requires the use of working fluids other than subcritical steam for parabolic 

troughs and power towers. Supercritical steam and CO2 are options that correspond to power 

cycles in the 600o to 800oC range. Air is being considered for air-Brayton cycles operating 

above 1,200oC. These working fluids will impact the requirements for the heat-transfer fluid. 

In some cases, the HTF and working fluid may by the same, as in the case of a supercritical 

CO2 power cycle that uses a supercritical CO2 receiver or an air-Brayton cycle that uses an air 

receiver. These cycles also dictate the requirements for the heat exchanger and TES system 

that will couple to the power cycle and receiver. A temperature greater than 565oC will be 

required, and whether that temperature falls into the 565o–800oC range or is closer to 1,300oC 

will depend, to a large extent, on the feasibility of developing a TES system able to operate at 

the corresponding temperature while keeping to a minimum the system losses associated with 

high-temperature operation (Figure 1).

Figure 6. Power-cycle efficiencies for CSP technologies.

Participants in the breakout sessions identified the following needs and challenges for thermal 

energy storage.

Heat-Transfer Fluids

Reaching the higher temperatures required for the SunShot goal will require new HTFs that 

possess a very wide liquid temperature range. In the extreme, new HTFs could be identified 

that have liquid temperatures in the range of 0o to 1,300oC. Practically, the temperature range 

will be narrower, depending on the power cycle. In addition to having a wide liquid 

temperature range, a suitable fluid will require combination of the following properties, 

depending on heat-transfer and storage media: 1) low cost, 2) environmentally safe, 3) 
9

compatible with piping and tank wall materials, and 4) high-temperature stability, possibly in 

air. Preferred properties that will make any new HTF more attractive for particular systems

are 1) high heat capacity, 2) high thermal conductivity, 3) low viscosity, 4) high fluid density, 

5) low thermal expansion coefficient, 6) high extinction coefficient, 7) high heat-transfer 

coefficient, and 8) low vapor pressure over the operating range.

Needs and Challenges

When considering a fully integrated power plant, use of a single fluid for heat transfer and 

storage is preferred because the heat exchanger and associated thermodynamic penalties can 

be eliminated. This arrangement results in lower power-plant capital costs and greater 

thermodynamic efficiency. Developing a low-cost fluid that meets all the technical 

requirements for both functions may not be possible, in which case the use of two distinct 

fluids may be the only practical configuration for a plant.

There is a need to develop guidelines and standards for high-temperature thermophysical 

properties measurements and a figure of merit for high-temperature performance of new heattransfer fluids. High-temperature measurements were recognized as being very difficult to 

make, and a large amount of high-temperature data already exist in the literature. There is a 

need to better characterize the structure of nanofluids and identify the role of radiative heat 

transfer in high-temperature systems. Additionally, hysteresis of the melting and freezing 

points can generate uncertainty in the behavior of the fluid at low temperatures where freezing 

in the plant piping may occur; this effect needs to be better understood. Compatibility with 

materials of construction must also be determined. The measurement of properties requires 

very careful analysis of the chemical composition of the samples tested and documentation of 

the methods of measurement.

Sensible Energy Storage

The effectiveness of TES systems depends critically on the thermophysical properties of the 

storage materials. Thermal conductivity, density, viscosity, melting and freezing points, and 

enthalpies of fusion or reaction all impact the design, performance, and cost of the TES 

system, as well as the fully integrated power plant. Part of current work is to identify the 

impact of these properties, along with operating conditions, on TES system performance. The 

following analysis for storage-fluid heat capacity is given as an example of the analyses that 

will help define new research directions.

Cost Benefit from Improved Heat Capacity of Sensible Storage Fluids

The simplicity and efficiency of direct two-tank storage makes it an appealing approach to 

TES. A relatively easy way to improve the economics of this system is to improve the heat 

capacity of the storage fluid. The minimum required heat capacity for a sensible storage 

material is a function of the temperature drop across the turbine. The heat capacity and 

temperature drop together determine the stored energy density of the TES system. Other 

factors are the costs of the storage material, tank, and piping materials that are required for the 

TES system. For the two-tank storage system, the low-temperature tank is normally at 300o

C

(as specified by the steam-cycle pinch point), so the temperature drop across the power block 

is determined by the maximum operating temperature. 
10

Analysis was performed in which the cost of TES was estimated as a function of the 

maximum operating temperature from 400o

C to 1,100oC. Heat capacities for the storage fluid 

varied from 1.5 J/g-K, which is the heat capacity of the binary NaNO3/KNO3 mixture that is 

currently used in the two-tank storage system, to 4.5 J/g-K or a factor of 3 greater than the 

binary salt. The cost of the tank wall material varies with temperature. From 400oC to 450oC, 

carbon steel can be used as the wall material for the hot tank. From 450oC to 650oC, stainless 

steel is required and above 650o

C, a nickel-based alloy is required. The cost difference 

between these materials significantly impacts the cost of the TES system as a function of 

operating temperature. The analysis determined the cost of the TES system in the temperature 

range accounting for the increased cost of the tank wall material as temperature increases. 

Results of the analysis indicate that the TES cost goal of $15–$20/kWhth can be reached at 

temperatures in the range of 450o

C and at 650oC if the heat capacity of the storage fluid is 

between 3–4.5 J/g-K. For temperatures greater than 650oC, the high cost of nickel-based 

alloys makes it difficult to meet the cost target using this type of storage system.

This analysis assumes that the storage medium needs to be contained in a two-tank type 

storage system. If thermochemical TES is used, the high-energy products from the charging 

process may be stored at low temperature. The stored energy in the products can then be 

released at high temperature. Also, if the storage medium is self supporting, such as a 

ceramic matrix material in a thermocline, then the requirements for the high-temperature tank

material may not be as strict and its associated cost may be less. In both of these cases, 

operating temperatures well above 650oC still offer performance and cost benefits.

Similar analyses have been performed for the other thermophysical properties including 

conductivity, density, and viscosity. The impacts of these properties on the performance of 

TES systems and the complete plant have been characterized. High viscosities for either the 

HTF or storage fluid lead to unacceptably high pumping power within the plant. This power 

requirement is subtracted from the total generated power to get the net power produced. 

Limits for viscosity have been established to minimize this parameter. Increasing HTF and 

storage-fluid densities actually improve plant performance and cost. Higher storage-fluid 

density decreases the required storage volume and decreases tank costs. Higher HTF density 

reduces HTF velocities in the piping, thereby reducing frictional losses and pumping power. 

Minimum values for storage fluid and HTF thermal conductivities have also been established 

to ensure adequate heat transfer within the receiver and heat exchangers.

Needs and Challenges

There is significant overlap of needs in sensible-heat storage materials and heat-transfer 

fluids. These include the need for integrated systems modeling to determine costs, 

requirements, and relative needs for various fluids and components within an operating power 

plant. The point was made that the requirements for the HTF vary depending on the 

specifications and performance of other components within the plant. Fully integrated system 

models are able to determine the dependence and trade-offs between these factors and set 

bounds of the properties that are required for the HTF and other components. 

There is a need to learn more from members of the nuclear community because they have 

been working in this area of high-temperature HTFs for many years and have accumulated 

much information on this topic. The laboratories need to develop high-temperature materials 
11

characterization methods so new fluids can be accurately characterized. These methods 

include high-temperature materials compatibility testing and properties measurements.

Several needs were identified relating to systems and components for the heat-transfer fluid. 

Eliminating the heat exchanger between either the HTF and storage fluid or HTF and working 

fluid reduces capital cost and exergy losses. For the air-Brayton power cycle, means for 

efficient heat transfer between the air and storage medium need to be modeled and developed. 

New alloys need to be developed that are inexpensive, strong at high temperatures, and 

compatible with potential heat-transfer fluids.

Phase-Change Storage

Needs and Challenges

The same capability needs that were identified for sensible storage are applicable to phasechange storage, as well. For phase-change storage, it is particularly important to develop new 

materials with high thermal conductivity to improve heat-transfer rates upon discharging 

(freezing) of the PCM. Fully integrated systems modeling is essential to determine the 

specifications and performance for PCM storage because its behavior is more complex than 

that of sensible-storage systems. Of specific interest is the need to better model heat transfer 

between the heat transfer fluid and PCM in complex geometries. New approaches must be 

found to overcome the losses due to build up of solid phase at heat-exchange surfaces. 

Encapsulation of phase-change media at scales from nano to macro must be developed to 

improve heat transfer and construction of a cascade of PCMs that cover the operating range of 

the power cycle. High-temperature materials characterization is also essential for PCM 

storage.

PCM storage matches well to Stirling engines because energy transfer to the Stirling engine is 

isothermal. There is a need to develop PCMs that are specific to this application. Also, the 

need for high heat-transfer rates and adequate power density is required and may be met by 

the use of heat pipes integrated with the receiver and Stirling engine components.

Thermochemical Storage

Thermochemical storage offers the greatest volumetric stored energy density of any of the 

thermal energy storage options.7, 8, 9

 In some cases, the stored energy density may be an order 

of magnitude greater than that available from sensible or phase-change storage systems. In 

thermochemical storage systems, thermal energy from sunlight is absorbed by a forward 

reaction, A  B (Figure 8). The reaction product, B, preheats the reactant, A, and is stored at 

ambient temperature. To recover thermal energy, the reverse reaction generates product A 

and is used to preheat the reactant B for this reaction. Because A and B can be stored at 

ambient temperature, this approach creates the potential for long-term storage.

For any thermochemical cycle, design considerations must include the minimization of exergy 

losses. For the general storage system in Figure 7, the heat exchangers should be designed to 

have minimal temperature drop. The reverse reaction, B A, should occur at a temperature 

as close as practical to the temperature of the forward reaction, A  B. Parasitic losses due to 

pumping and/or compressing reactants and/or products also need to be minimized. 
12

Figure 7. Schematic of generalized thermochemical storage cycle for CSP technologies.

10

Needs and Challenges

Comparisons were made to the development of thermochemical cycles for the nuclear 

industry and high-temperature solar fuels applications. These are predominantly directed to 

water splitting to produce hydrogen as an energy carrier.

11, 12 The multi-step thermochemical 

cycles proposed in the temperature range of 600o to >1,000oC will present a challenge to 

integrate into CSP technologies where operation may be subject to repeated start up and shut 

down. Further, integration of thermochemical reactions into CSP systems will require 

receivers that are quite different compared to current receiver designs, which have HTF

flowing through them. Systems will have to be designed that can meet the requirements for 

both generating electricity and charging the storage system with high efficiency. 

Thermochemical cycles will need to be simpler and unaffected by the variable energy input 

that is characteristic of solar power generation.

Much discussion related to the need to always evaluate any thermochemical system from the 

standpoint of exergy losses. On an energy basis, TES efficiency may be very high, but if the 

thermal energy ultimately is delivered to the power block at a low temperature, then the 

exergy efficiency will be unacceptably low. Exergy losses can be minimized by minimizing 

heat-exchange steps in the system and operating the reverse chemical process at the highest 

possible temperature.

Other practical challenges are the need to minimize loss of surface area or activity for 

catalysts or solid-phase reactants, if present, over many thermal cycles due to sintering or 

poisoning. Reaction kinetics are proportional to these properties, so they need to be preserved 

to maintain adequate power densities. There is a need to identify chemical reaction rates that 

are fast enough to match the solar or heat flux that is supplied to the chemical reactors—
13

whether the chemical reactor is directly driven by the solar heat in a dedicated receiver 

designed for the chemical process (direct solar thermal chemistry) or is driven by some 

fraction of heat from the same HTF stream used for driving the power cycle (an indirect solar 

thermal process). Gas-phase reactants or products from the chemical system may require 

compression for storage and contribute to parasitic losses. For example, identifying chemical 

cycles in which gas-phase reactants are oxygen or water would be advantageous because the 

system could be open and not require compression for storage of these gases.

Research Directions for Thermal Energy Storage

Participants in the breakout sessions identified the following research directions for thermal 

energy storage.

Heat-Transfer Fluids

1. Develop advanced capabilities in modeling of mechanistic, multi-physics systems and 

new standards for high-temperature stability and properties measurements. Both of 

these efforts will help define the requirements for performance and reliability for 

HTFs in fully integrated power plants.

2. Develop single heat-transfer/storage fluid, preferably a fluid that has improved 

radiative properties. Possible fluid types include liquid metals (Na, Al/Sn), gases 

(nitrogen), nanofluids, high-temperature non-nitrate salts, ionic liquids, or sulfur. 

Liquid Na has already been studied for use in nuclear power plants, so there are

existing data that may be applicable to CSP.

3. Explore surface modification for improved heat transfer to fluids . Characterization of 

the structure of nanofluids is important for understanding enhancements due to adding 

nanoparticles to fluids.

4. Examine the use of particles for direct absorption of solar radiation. Such systems 

could use the same particles for TES.

5. Develop barrier coatings to prevent hydrogen permeation for Stirling engines and 

evaluate variations of the HTF for Brayton systems that include particles, air, liquid, 

or helium.

Sensible Storage

New directions for sensible storage include:

1. Additives that will lower the freezing point of molten salts to near-ambient

temperature. 

2. Lower-cost containment materials for high-temperature storage. One approach is to 

develop protective coatings that allow containment of potentially corrosive storage 

fluids with lower-cost wall materials.
14

3. Consideration of materials that have not previously been considered for storage 

applications. These materials include inter-metallic materials, nanofluids, and hightemperature storage materials derived from natural sources such as lava, rocks, sand,

and cements.

Phase-Change Storage

New directions in phase-change storage materials include:

1. Investigation of metal alloy PCMs. There is also a need to fully characterize 

chlorates, sulfates, and carbonates for high-temperature storage and to develop 

materials or composites with higher thermal conductivity, particularly in the solid 

state. 

2. Methods to encapsulate micro- or nano-sized PCMs that may be integrated into an 

HTF. Encapsulation of PCMs is needed to reduce heat-transfer losses are encountered 

in the charging and discharging of the storage material.

3. The development of PCMs specific to dish/Stirling applications. Also, the 

development of heat pipes integrated with the receiver and Stirling engine components 

will increase the high heat-transfer rate and power density required for this system.

Thermochemical Storage

Many thermochemical cycles were discussed and identified as potential candidates for solar 

thermochemical storage. These cycles include the following:

1. Solid-based reactant plus a gas-phase reactant: MO + CO2 = MCO3

2. Reactions that use other gases: SO2, SO3, H2O

3. Systems that have liquid reactant(s) and liquid product(s)

4. Organic reactions that could be possible below 400oC, e.g., depolymerization = 

polymerization conversion

5. Inorganic reactants possible at higher temperatures: polymerization/depolymerization

reactions based on siloxane chemistry [-S(CH3)2O-]n

6. Polymerization/depolymerization of sulfur

7. Metallurgical conversions involving molten metals and metal oxides 

8. Gas reactants to liquid or gas products

9. Reforming reactions and the corresponding reverse

10. Methanation with catalysts from 600o

‒700oC
15

11. Gas reactants to gas products:

SO2 + 1⁄2O2 = SO3

CO + 1⁄2O2 = CO2

Identifying and screening new cycles for solar thermal energy storage will require a general 

capability that 1) uses high-level chemical process modeling software for screening chemical 

storage cycles and 2) takes into account thermodynamics, kinetics, by-products, heat 

recovery, separations, and more.

Summary

CSP technologies need to make significant cost and performance improvements to meet the 

SunShot Initiative cost target of 6¢/kWh LCOE. In addition, thermal energy storage will be 

an essential component of next-generation power plants because these plants will need to 

deliver reliable, consistent power during daylight hours and into the evening. Meeting the 

cost target will require significant performance improvements and cost reductions for all 

components and subsystems that make up a CSP plant. With respect to TES, new heattransfer fluids and storage materials will be required that are stable at high temperature and 

have high stored energy density due to high heat capacities and/or multiple phase changes. 

Thermochemical storage may provide the ultimate solution to the TES challenge if a suitable 

system can be identified and developed for high-temperature CSP applications.

References

1. U.S. Department of Energy. (2011). “SunShot Initiative.” Website. 

www1.eere.energy.gov/solar/sunshot/. Accessed July 7, 2011.

2. U. Herrmann, D. Kearney. (2002). J. Solar Energy Eng. 124, 145.

3. Acciona North America. (2011). “Nevada Solar One.” Website. www.accionana.com/About-Us/Our-Projects/U-S-/Nevada-Solar-One.aspx. Accessed July 7, 2011.

4. Abengoa Solar. (2011). “Solana: The world’s largest solar plant.” Website. 

www.abengoasolar.com/corp/web/en/our_projects/solana/. Accessed July 7, 2011.

5. BrightSource Energy. (2011). Ivanpah Project Overview. Website. 

www.brightsourceenergy.com/projects/ivanpah. Accessed July 7, 2011.

6. H.P. Garg, S.C. Mullick, A.K. Bhargava; (1985). Solar Thermal Energy Storage, D. 

Reidel Publishing Company, Dordrecht, Holland 642 pp.

7. R.D. Smith. (1979). Chemical Energy Storage for Solar Thermal Conversion, Final 

Report. SAND79-8198, Livermore: Sandia National Laboratories, 190 pp.

8. T.T. Bramlette, R.M. Green, J.J. Bartel, D.K. Ottesen, C.T. Schafer, T.D. Brumleve.

(1976). Survey of High Temperature Thermal Energy Storage, SAND75-8063,

Livermore: Sandia National Laboratories.

9. E.A. Fletcher. (2001). J. Solar Energy Eng. 123, 63-74 
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10. A. Steinfeld, R. Palumbo (2001). Solar Thermochemical Process Technology, 

Encyclopedia of Physical Science and Technology, 237-256.

11. S. Yalcin. (1989). A Review of Nuclear Hydrogen Production, International Journal of 

Hydrogen Energy 14(8), 551-561.

12. A. Steinfeld. (2005). Solar Thermochemical Production of HydrogenA Review, Solar 

Energy 78(5), 603-615.