Thermal Energy Storage for Solar Power Production

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Thermal energy storage for solar power production
Nathan P. Siegel∗
Solar energy is the most abundant persistent energy resource. It is also an intermittent one available for only a fraction of each day while the demand for electric power never ceases. To produce a significant amount of power at the utility scale, electricity generated from solar energy must be dispatchable and able to be supplied in response to variations in demand. This requires energy storage that serves to decouple the intermittent solar resource from the load and enables around-the-clock power production from solar energy. Practically, solar energy storage technologies must be efficient as any energy loss results in an increase in the amount of required collection hardware, the largest cost in a solar electric power system. Storing solar energy as heat has been shown to be an efficient, scalable, and relatively low-cost approach to providing dispatchable solar electricity. Concentrating solar power systems that include thermal energy storage (TES) use mirrors to focus sunlight onto a heat exchanger where it is converted to thermal energy that is carried away by a heat transfer fluid and used to drive a conventional thermal power cycle (e.g., steam power plant), or stored for later use. Several approaches to TES have been developed and can generally be categorized as either thermophysical (wherein energy is stored in a hot fluid or solid medium or by causing a phase change that can later be reversed to release heat) or thermochemical (in which energy is stored in chemical bonds requiring two or more reversible chemical reactions). C 2012 John Wiley & Sons, Ltd.
How to cite this article:

WIREs Energy Environ 2012, 1: 119–131 doi: 10.1002/wene.10

THE POTENTIAL OF SOLAR POWER PRODUCTION
he amount of solar energy striking the earth is vastly larger than the current global consumption of primary energy resources for electric power production. With respect to electric power, about 1.8 × 104 TWhe of electricity is consumed globally on an annual basis. In contrast, the amount of solar power striking the earth is about 1.7 × 105 TW continuously,1 or 1.5 × 109 TWhth annually, the distribution of which is illustrated in Figure 1.2 Not all of this energy can be utilized in a cost-effective manner owing to a number of factors, including the quality of the local solar resource, rough terrain, and restrictions on development. The technically developable solar resource appropriate for concentrating solar power
Correspondence to: [email protected] Department of Mechanical Engineering, Bucknell University, Lewisburg, PA, USA. DOI: 10.1002/wene.10


T

(CSP) has been estimated to be capable of producing 3 × 106 TWhe/year, the global distribution of which is shown in Figure 2.2 An analysis of the resource potential CSP generation in the Southwestern United States indicates that electric production of 2.1 × 103 TWhe/year is possible.3

Meeting Energy Demands at the Utility Scale
In general, solar energy is a good match for intermediate load power even without storage. However, either storage and/or hybridization with fossil energy are needed to accommodate baseload and peaking operation. Figure 3 shows a representative utility load curve for a summer day along with the normalized power output from a CSP plant operating in one case with storage and in the other without storage. Much of the intermediate load profile, which peaks around 5 PM in this example, can be met using solar energy without storage (in this case, ‘profile’ refers to the shape of the load curve, not necessarily its

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F I G U R E 1 | The global distribution of solar energy expressed as an annual averaged sum of direct normal insolation (DNI)2 . This represents the
amount of solar energy that could be intercepted by a collector tracking the sun in two axes, such as a parabolic dish. Source: DLR (www.dlr.de). (Reprinted by permission from Ref. 2. Copyright 2009, F. Trieb.)

F I G U R E 2 | The global distribution of solar energy resources that could be developed for power production. This map has been ‘filtered’ relative
to Figure 1 and shows only those geographic areas that satisfy a number of criteria related to land use, topography, infrastructure, and other issues impacting development2 . Source: DLR (www.dlr.de). (Reprinted by permission from Ref. 2. Copyright 2009, F. Trieb.)

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F I G U R E 3 | An illustration of a typical utility load curve compared with the power output from a solar energy plant operating either with or
without thermal energy storage.

magnitude). Loads between 5 PM and 6 AM, when sunlight is not available, can only be met by including thermal energy storage (TES). The worldwide installed capacity of grid connected photovoltaic (PV) and CSP systems producing electricity for grid consumption is estimated to be 48 TWhe4 assuming a 25% capacity factor. This is roughly 0.3% of current consumption. At this level of market share, energy storage for solar power is not required from a grid operability perspective as conventional power plants can offset lost production from solar at night or during periods of bad weather. However, as solar and other variable generation technologies, such as wind power, begin to make up a larger share of the production market, it will be increasingly more important that power production from these systems be both reliable and dispatchable. The importance of reliability is one that any electric power consumer can appreciate. From the perspective of power utilities, reliability (or firm capacity) is also important and more value is assigned to technologies that can produce power whenever it is needed or at least with a certain amount of predictability.5 Dispatchability is a component of reliable operation as it enables generation to be matched to variable demand. Conventional power systems include a certain amount of dispatchabiltiy to meet intermediate and peak loads. However, the rate at which power generation can be varied is constrained.

Denholm and Hand6 show that as more variable generation from renewables is brought into the market, the ability of the grid to deal with variations in the amount of power produced from these sources will become a technical challenge unless grid flexibility is improved. One solution is to include energy storage which improves the flexibility of the power supply grid in general and enables more production from renewables, thus reducing the level of curtailment seen in wind and solar power systems deployed today.

The Advantages of TES
There are many ways to store energy produced from the sun. Each of them involves some degree of energy loss during both the charge and discharge processes. Keeping these losses small is an important element of any practical energy storage system. The reason is fairly straightforward: power production is tied directly to solar collection area. The greater the losses in the storage system, the larger the collection area must be to make up for those losses. In utilityscale PV systems, the cost of the collector (modules) is ∼60% of the total system cost.7 For CSP, the mirrors used for collection account for 40–60% of the system cost depending on the platform.8 In the case of TES using relatively inexpensive molten nitrate salts, a roundtrip storage efficiency, energy output divided by energy input, of greater than 98% was demonstrated

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F I G U R E 4 | Estimated thermal conversion efficiency (heat to mechanical work) for power cycles either in use or under consideration for concentrating solar power (CSP) systems. The operating temperature range for the three main CSP platforms is highlighted. at the Solar Two project.9 The cycle life of the molten salt is assumed to be sufficient for a 30 year power plant service lifetime, although no solar TES system has operated for this long. In contrast, round trip energy storage efficiency in batteries is roughly 75% for lead acid with a cycle life upto 2000 cycles and nearly 100% for lithium ion with a lifetime of 3000 cycles.10 Thermal storage can be relatively inexpensive compared to other options. Currently, the estimated cost of thermal storage for CSP is $30 per kWhth11 for a central receiver with 9 h of storage. Assuming a conservative thermal to electrical conversion efficiency of 30%, the effective cost of electrical energy storage is $90 per kWhe for a central receiver system. Electrical energy storage systems, including batteries, have a considerably higher cost near $500 per kWhe and will need to be replaced several times over the life of a power plant given this degradation rates of the batteries themselves,12 resulting in a substantial system cost increase. Despite the relatively low cost of TES, it is not yet practical, either technically or economically, to include enough storage to reach a power plant capa city factor much in excess of 70%. This point is made in a study showing that the minimum levelized cost of energy (LCOE) for a CSP plant is realized by including around 12 h of storage (65–70% capacity), which enables additional use of the power block, thus reducing its effective cost.13 Conventional power plants typically have a capacity factor between 70–90%. CSP facilities with thermal storage can reach this level of capacity by including hybridization with natural gas. In such a system, natural gas, instead of solar-derived heat, is used to drive the thermal power plant over the small fraction of the year when the solar resource is insufficient. This approach has been demonstrated successfully in many plants including the solar energy generating station (SEGS) facilities operating in California since the 1980s.14

TES TECHNOLOGIES
The development of TES technologies has been closely tied to both the operational requirements of power cycles suitable for CSP applications, and the operating characteristics of the three main CSP platforms: parabolic trough, parabolic dish, and central receiver. Figure 4 shows the operating envelope (temperature range) of each of these platforms as well as the potential thermal efficiency of power cycles currently under consideration for CSP. These include Rankine cycles using steam or organic fluids operating between 300◦ C–650◦ C, Brayton cycles at 900◦ C and above for air and 600◦ C–800◦ C for supercritical CO2, and Stirling engines at 600◦ C–800◦ C. The general structure of a CSP power plant with TES is the same, in most respects, regardless of the platform or specific power cycle. An illustration of a central receiver plant, such as the commercial scale (100 MWe) power plant currently under development by SolarReserve15 with a TES system, is shown in Figure 5. The key features are as follows: • Collection and focusing of sunlight onto a receiver by an array of mirrors;

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F I G U R E 5 | A central receiver power plant with two tank molten nitrate salt thermal energy storage (TES).15 This configuration is identical to
that demonstrated at the Solar Two project sited in Barstow, CA. This is an example of direct TES wherein the heat transfer fluid and thermal storage media are identical. Source: SolarReserve. (Reprinted by permission from Ref. 16. Copyright of Solar Reserve.)

• A receiver that converts solar energy into heat; • A heat transfer material, usually a fluid, that moves heat from the receiver to the storage system; • A TES system containing a storage media that may or may not be the same as the heat transfer material; • A power block that receives energy either directly from the heat transfer material via the receiver or from the TES system. In the case of the central receiver system shown in Figure 5, both the heat transfer material and the storage media are a molten salt composed of potassium nitrate and sodium nitrate. The salt is heated from 280◦ C to 588◦ C in the receiver and stored in the ‘hot’ tank. From there, the molten salt is pumped through the power block to drive a steam turbine and is then discharged into a ‘cold’ tank where it remains until being pumped back to the receiver. The amount of energy stored in the system is a function of both the heat capacity of the salt, the temperature difference over which it is used, and the mass of salt in the system (tank size).

It is possible to configure the system such that the power block runs at full-rated capacity day and night. In this case, the solar collection field and receiver are oversized beyond what is needed to satisfy the instantaneous energy demands of the power cycle so that additional solar energy may be collected and placed into storage. The incremental amount of collection area installed beyond what is required by the power block is referred to as the solar multiple. In a central receiver with 12 h of storage, a solar multiple of 2.8 is required.13

Three General Approaches to TES
Thermal storage systems can be categorized into technologies that utilize either thermophysical or thermochemical energy storage processes. Thermophysical processes involve the storage of energy in one of two ways, either by adding heat to a material (solid or liquid) to cause an increase in temperature or by adding heat to cause a phase change such as melting. The former case is generally called ‘sensible energy storage’ while the latter is known as ‘latent energy storage’. Thermochemical processes use a series of reversible chemical reactions that result in energy stored in the form of chemical reaction products that may be

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reacted later to liberate heat. The amount of thermal energy that can be stored in each of these three cases is expressed in Eqs (1)–(3) with the assumption that the sensible energy is stored in incompressible media: Qsensible = m
Tmp TL TH TL

c p (T )dT,

(1)

Qlatent = m +m

c p (T )dT + m h fusion c p (T )dT ,

T =Tmp

TH Tmp

(2)

Qthermochemical = m +m

TR TL

c p (T )dT + m h reaction
TH

T =TR

c p (T )dT .

(3)

TR

In the case of sensible energy storage, the quantity of energy stored, Qsensible , is a function of the specific heat of the media, cp , the temperature difference over which it is stored, TH -TL , and the total mass of material in storage, m. The most common approach to sensible energy storage in the CSP industry is to use a molten nitrate salt as the heat transfer fluid and storage media.16 , 17 For a central receiver using nitrate salts, the storage temperature range is typically 288◦ C–565◦ C, but could be extended in future systems to 288◦ C–650◦ C.18 In the latter case, the amount of energy stored per unit mass is 0.58 MJ/kg while the energy stored per volume is 1050 MJ/m3 . The advantage of sensible energy storage approaches is simplicity in design. The primary disadvantage is that large amounts of storage media can be required as systems are scaled up, possibly resulting in high capital cost. In a latent energy storage system, the quantity of energy stored, Qlatent , is a function primarily of the enthalpy of fusion, hfusion , and the mass of material, although sensible energy stored in the solid and liquid phases may also contribute in certain systems. A wide range of materials have been developed for latent energy storage for CSP applications, and they almost exclusively involve a solid-to-liquid phase change as opposed to a more energetic liquid to gas phase change due to the difficulty in storing gaseous products. If a liquid-to-vapor transition is used for storage, Eq (2) would need to be modified to include the energy associated with the latent heat of vaporization. Latent energy storage offers two potential advantages over sensible energy storage: increased energy storage density and isothermal energy transfer. The primary disadvantage is that significant losses may be incurred during charge and discharge in the

case when the storage media has a low thermal conductivity. This is not the case for all materials and the impact of charge/discharge losses can be mitigated by reducing the conduction length by encapsulating the storage media, including finned heat exchangers and/or heat pipes, or augmenting the thermal conductivity of the media with additives.19 , 20 The ability to transfer heat at a constant temperature is advantageous for power cycles, such as the Stirling and Rankine cycles, which require that a significant amount of the total energy input be isothermal to achieve peak efficiency. Latent energy storage in nitrate salts is appropriate for parabolic trough power plants using steam as the working heat transfer fluid in the field (direct steam generation or DSG) and in the power block.19 In this case, both sensible and latent energy may be stored in the same media and used for different purposes, e.g., preheat, evaporation, and superheat. The melting point of solar salt, a near eutectic mixture of sodium and potassium nitrate suitable for DSG systems, is around 220◦ C. The enthalpy of fusion, and gravimetric storage density, is 0.1 MJ/kg. The volumetric storage density is 195 MJ/m3 . Latent energy storage for solar power plants based on the Stirling cycle requires a considerably higher phase change temperature (700◦ C– 800◦ C) that can be achieved with inorganic salts (hydroxides, fluorides, carbonates) or metallic phase change media.21 The energy stored in a thermochemical system, Qthermochemical , is primarily a function of the reaction enthalpy, hreaction , and the mass of the material in storage, although sensible heat may be stored above and below the reaction temperature. One major difference between thermochemical storage and the other two approaches is that in the case of thermochemical storage the temperature at which the system is charged and discharged may be significantly different. This results from the requirements of the individual chemical reactions comprising the storage system. Currently, thermochemical storage is not being actively used for CSP applications although preliminary investigations are underway.22 The advantages of a thermochemical storage approach are potentially high gravimetric storage density and the possibility of energy storage for long periods of time, as stable reaction products, with little energy loss.

TES Media
With respect to performance, the key differentiating characteristics of the wide array of TES media that have been developed over the years are operating temperature range, gravimetric and volumetric

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Media, Both Liquid and Solid, Are Assumed to Have a Storage Temperature Differential of 350◦ C with Respect to the Calculation of Volumetric and Gravimetric Storage Density Storage Medium Sensible Energy Storage—Solids Concrete Sintered bauxite particles NaCl Cast iron Cast steel Silica fire bricks Magnesia fire bricks Graphite Aluminum oxide Slag Specific Heat (kJ/kg-K) 0.9 1.1 0.9 0.6 0.6 1 1.2 1.9 1.3 0.84 Latent or Reaction Heat (kJ/kg) – – – – – – – – – – – – – – – – – – 397 515 196 607 100 215 481 1044 2582 160 Density (kg/m3 ) 2200 2000 2160 7200 7800 1820 3000 1700 4000 2700 1815 750 900 2100 690 960 900 1700 2380 2250 7090 2200 1950 2400 2170 2200 790 2070 Temperature Range (◦ C) Cold Hot 200 400 200 200 200 200 200 500 200 200 300 300 300 450 150 316 300 350 – – – – – – – – – – 400 1000 500 400 700 700 1200 850 700 700 600 400 400 850 316 700 700 1100 660 579 803 726 222 730 801 842 683 320

T A B L E 1 The Physical Properties of Selected Thermal Energy Storage Media. Sensible Energy Storage

Gravimetric Storage Density (kJ/kg) 315 385 315 210 210 350 420 665 455 294 560 875 735 630 980 455 385 735 397 515 196 607 100 215 481 1044 2582 160

Volumetry Storage Density (MJ/m3 ) 693 770 680 1512 1638 637 1260 1131 1820 794 1016 656 662 1323 676 437 347 1250 945 1159 1390 1335 195 516 1044 2297 2040 331

References 23 24 23 25 23 23 25 26 27 28 17 29 23 23 25 25 30 27 28 31, 32 32 32 28 33 33 33 31 31

Sensible Energy Storage—-Liquids Nitrate salts 1.6 (ex. KNO3 -0.46NaNO3 ) 2.5 Therminol VP-1 R Silicone oil 2.1 Carbonate salts 1.8 2.8 Caloria HT-43 R Sodium liquid metal 1.3 Na-0.79K metal eutectic 1.1 Hydroxide salts (ex. NaOH) 2.1 Latent Energy Storage Aluminum Aluminum alloys (ex. Al-0.13Si) Copper alloys (ex. Cu-0.29Si) Carbonate salts (ex. Li2 CO3 ) Nitrate salts (ex. KNO3 -0.46NaNO3 ) Bromide salts (ex. KBr) Chloride salts (ex. NaCl) Flouride salts (ex. LiF) Lithium hydride Hydroxide salts (ex. NaOH) Thermochemical Energy Storage SO3 (g)↔ SO2 (s) + 1/2O2 (g) CaCO3 (s)↔CO2 (g) + CaO(s) CH4 (g) + CO2 (g)↔2CO(g) + 2H2 (g) CH4 (g) + H2 O(g)↔ 3H2 (g) + CO(g) Ca(OH)2 (s)↔CaO(s) + H2 O(g) NH3 (g)↔1/2N2 (g) + 3/2H2 (g) 1.2 1.5 – – 1.5 0.53 1.1 2.4 8.04 1.47

– – – – – –

1225 1757 4100 6064 1351 3900

– – – – – –

– – – – – –

650 527 538 538 521 195

1225 1757 4100 6064 1351 3900

– – – – – –

28, 30, 34 28, 34 35 35 28, 30, 34 36

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ticularly true of latent energy and thermochemical energy storage systems. The impact of a given TES strategy on overall performance and cost can only be evaluated through a rigorous system level analysis.

Sensible Energy Storage Systems
Sensible energy storage is the most commonly used approach to TES in CSP applications and the only type of TES system that has been deployed commercially. There are two configurations generally used for sensible energy storage. In a direct storage system, the heat transfer fluid is also the storage media. An example of a direct storage system is the two-tank molten salt storage system demonstrated at Solar Two9 , 37 and illustrated in Figure 5. In an indirect storage system, the heat transfer fluid discharges energy to a storage media through a heat exchanger. Indirect storage is used in the Andasol parabolic trough plants in Spain16 and shown in Figure 7. In these 50 MWe facilities, the heat transfer fluid in the field is synthetic oil while the storage media is molten nitrate salt. There are two incentives for deploying indirect storage, the first being cost. In the case of parabolic trough plants, the synthetic oils are several times more costly than molten salt per unit energy stored.38 The second reason to deploy an indirect storage system is that the storage media cannot itself be used as a heat transfer fluid. This is the case with high-temperature central receivers operating with air as the heat transfer fluid and a solid ceramic thermal storage media, such as the 1.5 MWe power tower located in Julich, Germany.39 It is also true of parabolic trough systems such as Andasol that cannot use molten nitrate salt as a heat transfer fluid in the field due to its high melting point (∼220◦ C for the salt used at Andasol versus 12◦ C for Therminol R VP-1). Costs for sensible energy storage systems can be reduced by using a hybrid storage configuration that includes both solid- and liquid-phase storage media. This is called a thermocline40 and generally involves a single storage tank wherein some of the typically more expensive liquid media is displaced by a less expensive solid media such as crushed rock. Molten nitrate salts are the most commonly used sensible energy storage media and will likely play a large role in thermal energy storage for both near-term central receivers and advanced parabolic troughs. Many molten nitrate salt formulations have been developed over the last few decades.41 , 42 The most common mixture is known as ‘solar salt’ and is a binary salt composed of 36 mol% KNO3 and 64 mol% NaNO3. The components are mined primarily in Chile, and the mixture can be relatively low in cost, about $0.5 per kilogram.43 Other mixtures have been developed with the aim of reducing

F I G U R E 6 | Operating temperature limit and energy storage density for thermophysical (sensible and latent) and thermochemical energy storage media. In this chart, the energy storage density for sensible energy media is constrained by the temperature range over which energy is stored. This was fixed at 350◦ C. In addition, sensible energy storage is not included in either the thermochemical or latent energy storage media calculations. storage density, and cost. The physical properties of some candidate storage media are given in Table 1 with gravimetric storage density plotted against maximum operating temperature in Figure 6. Cost data are not included in Table 1 as these are closely tied to current market values of the storage media. In addition, storage media costs are not a sufficient indicator of the resultant storage system cost that may include expensive containment vessels, piping, pumps, and other hardware. Figure 6 illustrates the wide variability in storage density and process temperature that may be accommodated by the different storage approaches. It should be noted that in both Table 1 and Figure 6 the temperature range over which sensible energy storage systems are assumed to operate was fixed at 350◦ C for ease of comparison. In reality, some of these materials are capable of storage over a much wider temperature range, leading to improved storage density. This is particularly true of solid phase sensible energy storage media. However, in all cases the storage system must be well matched to the process heat demands, i.e., storing energy over a wider temperature range than required by the process will likely increase system losses due to increased thermal losses in the solar collection (receiver) and storage subsystems. While it is true that the thermophysical properties of the thermal storage media have a significant impact on the performance of a storage system, they should not be used exclusively as a means to estimate system performance. It is often the case that significant losses can be incurred during the thermal energy charge and discharge processes. This is par-

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F I G U R E 7 | A parabolic trough power plant with thermal energy storage (TES). This is an illustration of an indirect TES configuration that would
likely use a synthetic oil heat transfer fluid in the collector field and a nitrate salt media in the storage system. A heat exchanger is used to move heat between the two fluids. This configuration is currently in use at Andasol I and II in Spain. Source: NREL. (Reprinted by permission. Copyright 2011, NREL.)

the melting point to enable deployment in parabolic troughs where freezing in the field would be very problematic.43 , 14 These are typically ternary or quaternary eutectic compounds containing, in addition to KNO3 and NaNO3 , Ca(NO3 )2 and/or LiNO3 . Sandia National Laboratories has developed quaternary nitrates that melt around 90◦ C42 and other mixtures with nitrite salts that melt around 70◦ C.44 Melting point is relatively less important for central receivers that operate at higher temperatures than parabolic troughs. The limiting factor for the use of nitrates in these systems is their thermal stability limit. Nitrate salts undergo a complex series of decomposition reactions, some irreversible, at temperatures in excess of 500◦ C.45 The products of these reactions, which can include alkali oxides and nitrogen oxide gases, are corrosive to metallic system components.46 , 47

portant as the entire power block and TES system must be supported at the focal point of the parabolic dish concentrator, potentially requiring a very robust structure and drive system. Other phase change systems based on metallic compounds have also been investigated. Systems such as Al–Si,51 Cu–Ca,52 and Si–Mg,53 can operate at the relatively high temperatures required for Brayton power cycles and advanced Rankine cycles. The vast majority of work on metallic latent energy systems has focused on eutectic compositions in which all of the heat is discharged at a constant temperature. However, off-eutectic compounds can also be used provided that a variable discharge temperature can be accommodated in the system.

Thermochemical Energy Storage Systems
Thermal energy storage in the form of chemical bonds offers two significant advantages relative to other approaches: energy storage density can be significantly greater and energy can potentially be stored for extended periods of time at minimal loss in the form of stable reaction products. Despite these advantages a practical thermochemical energy storage system, one that competes with other options on the bases of roundtrip efficiency and cost is yet to be developed. This is not due to lack of effort. From 1976 to 1982, the United States Department of Energy (DOE) sponsored multiple projects focusing on the development of physical and chemical energy storage technologies

Latent Energy Storage Systems
Latent energy storage systems have the potential to achieve high gravimetric storage density and also discharge heat at a constant temperature. Past (and current) research has focused on developing nitrate or carbonate salt-based48 storage systems for integration with either steam power cycles for parabolic troughs and towers, and hydride,49 fluoride,50 or chloride50 salt for energy storage onboard a parabolic dish Stirling power system. In the latter case, both gravimetric and volumetric storage density are critically im-

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The associated technical challenges can be avoided by moving thermal energy off of the dish in the form of lower temperature chemical reaction products. This concept is sometimes called a thermochemical heat pipe or thermochemical transport35 , 59 , 60 and is illustrated in Figure 8. Using this approach, energy is added to the system via a reversible endothermic reaction to produce heated reaction products. The reaction products are cooled in a counterflow heat exchanger, while preheating the chemical reactants, and then sent to storage until the chemical reaction is reversed to produce heat and drive a thermal power cycle.

CONCLUSIONS AND OUTLOOK
F I G U R E 8 | A schematic of a thermochemical transport and storage system that could be used in conjunction with a parabolic dish or central receiver collection system, enabling more efficient solar collection for high-temperature power conversion processes such as those based on the Stirling or Brayton cycles.59 Reaction products may be placed in storage at a fairly low temperature, enabling long-term storage are relatively high efficiency. including several thermochemical energy storage projects targeting CSP applications. These projects are detailed in a series of DOE conference proceedings containing individual contractor reports.31–33 , 54–57 An extensive survey of 550 prospective thermochemical cycles conducted by Rocket Research Company led to the identification of 12 likely candidates after applying several high-level screening criteria. The estimated roundtrip efficiency of an energy storage system based on these 12 candidates ranged from 20– 50% on a first law basis.34 In addition, the temperature required for the ‘charging’ reaction is, in some cases, greater than the temperature of the thermal energy released in the discharging reaction. This represents a loss in exergy as well as energy. Today, the DOE is again funding projects in solar thermochemical storage. One is currently underway at General Atomics and leverages recent efforts in the area of solar fuels production58 to develop storage options for central receivers. Thermochemical storage is also a candidate for parabolic dish systems that operate more efficiently at elevated temperature (∼800◦ C) than do central receivers or parabolic troughs. The challenge of incorporating storage with a parabolic dish is that the energy either needs to be stored on the dish, potentially resulting in prohibitively large and expensive support structures, or on the ground, which requires that high-temperature thermal energy be moved efficiently off of the dish through two rotary joints or flexible couplings to the storage system. Energy storage technologies must be developed if renewable energy from solar and wind resources is to play a significant role in future electrical power generation. Without storage, power distribution grids will likely be overtaxed in dealing with the intermittent nature of power produced from variable wind and solar generation systems, effectively limiting the deployment scale of these technologies. Thermal energy storage integrated with concentrating solar power plants is a commercially demonstrated, relatively low cost solution appropriate for the utility-scale storage of renewable energy. Systems being deployed today are capable of storing enough energy for several hours of operation when the solar resource is not available. Future systems will accommodate sufficient storage to run around the clock on most days of the year and include natural gas hybridization to remain operational during periods of low solar resource, thus providing firm generating capacity. Thermal energy storage technologies have been under development for decades. Over this time, a wide range of prospective thermal storage media have been discovered and matched with solar power generation hardware. Current commercial thermal energy storage approaches using molten nitrate salts are appropriate for Rankine power cycles operating at temperatures up to ∼580◦ C. This type of relatively simple two-tank direct storage system can be deployed at power levels in excess of 100 MWe with 12 h of storage and including fossil hybridization for increased capacity, effectively making utility-scale solar power indistinguishable from more conventional generation. In near future, the cost of solar power systems with thermal energy storage will be reduced through technical developments including an increase in power cycle operating temperature that will enable more efficient electricity production. This transition to more advanced power cycles will be made possible

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through targeted research in the areas of heat transfer fluids and thermal storage media (materials development), hardware development, and system design. This information was prepared by the National Renewable Energy Laboratory for the U.S. Department of Energy.

The abstract figure and figure 7 have been reprinted from the National Renewable Energy Laboratory’s publication TP/5500-52134 “Summary Report for Concentrating Solar Power Thermal Storage Workshop” authored by Greg Glatzmaier: http://www.nrel.gov/docs/fy11osti/52134.pdf, Accessed December 9, 2011.

ACKNOWLEDGMENTS
A portion of this paper was written while the author was employed by Sandia National Laboratories. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC0494AL85000.

REFERENCES
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FURTHER READING
Many of the references in the document are reports generated by Sandia, NREL, or through government contracts. As such, they are not always readily available. The following resources may be used to locate many of these reports: • NREL Troughnet: http://www.nrel.gov/csp/troughnet/; • The Office of Scientific and Technical Information: http://www.osti.gov/bridge/; • The National Technical Information Service: http://www.ntis.gov/.

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