thermal energy storage for solar power production

Overview

Thermal energy storage for solarpower productionNathan P. Siegel∗

Solar energy is the most abundant persistent energy resource. It is also an in-termittent one available for only a fraction of each day while the demand forelectric power never ceases. To produce a significant amount of power at the util-ity scale, electricity generated from solar energy must be dispatchable and ableto be supplied in response to variations in demand. This requires energy storagethat serves to decouple the intermittent solar resource from the load and enablesaround-the-clock power production from solar energy. Practically, solar energystorage technologies must be efficient as any energy loss results in an increasein the amount of required collection hardware, the largest cost in a solar electricpower system. Storing solar energy as heat has been shown to be an efficient,scalable, and relatively low-cost approach to providing dispatchable solar elec-tricity. Concentrating solar power systems that include thermal energy storage(TES) use mirrors to focus sunlight onto a heat exchanger where it is convertedto thermal energy that is carried away by a heat transfer fluid and used to drivea conventional thermal power cycle (e.g., steam power plant), or stored for lateruse. Several approaches to TES have been developed and can generally be cate-gorized as either thermophysical (wherein energy is stored in a hot fluid or solidmedium 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 ormore 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 POWERPRODUCTION

T he amount of solar energy striking the earth isvastly larger than the current global consump-

tion of primary energy resources for electric powerproduction. With respect to electric power, about1.8 × 104 TWhe of electricity is consumed globallyon an annual basis. In contrast, the amount of so-lar power striking the earth is about 1.7 × 105 TWcontinuously,1 or 1.5 × 109 TWhth annually, the dis-tribution of which is illustrated in Figure 1.2 Not all ofthis energy can be utilized in a cost-effective mannerowing to a number of factors, including the quality ofthe local solar resource, rough terrain, and restrictionson development. The technically developable solarresource appropriate for concentrating solar power

∗Correspondence to: [email protected]

Department of Mechanical Engineering, Bucknell University,Lewisburg, PA, USA.

DOI: 10.1002/wene.10

(CSP) has been estimated to be capable of producing3 × 106 TWhe/year, the global distribution of whichis shown in Figure 2.2 An analysis of the resourcepotential CSP generation in the Southwestern UnitedStates indicates that electric production of 2.1 × 103

TWhe/year is possible.3

Meeting Energy Demandsat the Utility ScaleIn general, solar energy is a good match for inter-mediate load power even without storage. However,either storage and/or hybridization with fossil energyare needed to accommodate baseload and peaking op-eration. Figure 3 shows a representative utility loadcurve for a summer day along with the normalizedpower output from a CSP plant operating in onecase with storage and in the other without storage.Much of the intermediate load profile, which peaksaround 5 PM in this example, can be met using solarenergy without storage (in this case, ‘profile’ refersto the shape of the load curve, not necessarily its

Volume 1, September /October 2012 119c© 2012 John Wi ley & Sons , L td .

Overview wires.wiley.com/wene

FIGURE 1 | The global distribution of solar energy expressed as an annual averaged sum of direct normal insolation (DNI)2. This represents theamount 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.)

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

120 Volume 1, September /October 2012c© 2012 John Wi ley & Sons , L td .

WIREs Energy and Environment Thermal energy storage for solar power production

FIGURE 3 | An illustration of a typical utility load curve compared with the power output from a solar energy plant operating either with orwithout thermal energy storage.

magnitude). Loads between 5 PM and 6 AM, whensunlight is not available, can only be met by includingthermal energy storage (TES).

The worldwide installed capacity of grid con-nected photovoltaic (PV) and CSP systems produc-ing electricity for grid consumption is estimated to be48 TWhe4 assuming a 25% capacity factor. This isroughly 0.3% of current consumption. At this level ofmarket share, energy storage for solar power is not re-quired from a grid operability perspective as conven-tional power plants can offset lost production fromsolar at night or during periods of bad weather. How-ever, as solar and other variable generation technolo-gies, such as wind power, begin to make up a largershare of the production market, it will be increasinglymore important that power production from thesesystems be both reliable and dispatchable.

The importance of reliability is one that anyelectric power consumer can appreciate. From theperspective of power utilities, reliability (or firm ca-pacity) is also important and more value is assignedto technologies that can produce power wheneverit is needed or at least with a certain amount ofpredictability.5 Dispatchability is a component of reli-able operation as it enables generation to be matchedto variable demand. Conventional power systems in-clude a certain amount of dispatchabiltiy to meetintermediate and peak loads. However, the rate atwhich power generation can be varied is constrained.

Denholm and Hand6 show that as more variable gen-eration from renewables is brought into the market,the ability of the grid to deal with variations in theamount of power produced from these sources willbecome a technical challenge unless grid flexibility isimproved. One solution is to include energy storagewhich improves the flexibility of the power supplygrid in general and enables more production from re-newables, thus reducing the level of curtailment seenin wind and solar power systems deployed today.

The Advantages of TESThere are many ways to store energy produced fromthe sun. Each of them involves some degree of en-ergy loss during both the charge and discharge pro-cesses. Keeping these losses small is an important el-ement of any practical energy storage system. Thereason is fairly straightforward: power production istied directly to solar collection area. The greater thelosses in the storage system, the larger the collectionarea must be to make up for those losses. In utility-scale PV systems, the cost of the collector (modules) is∼60% of the total system cost.7 For CSP, the mirrorsused for collection account for 40–60% of the sys-tem cost depending on the platform.8 In the case ofTES using relatively inexpensive molten nitrate salts, aroundtrip storage efficiency, energy output divided byenergy input, of greater than 98% was demonstrated


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FIGURE 4 | Estimated thermal conversion efficiency (heat to mechanical work) for power cycles either in use or under consideration forconcentrating 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 moltensalt is assumed to be sufficient for a 30 year powerplant service lifetime, although no solar TES systemhas operated for this long. In contrast, round trip en-ergy storage efficiency in batteries is roughly 75% forlead acid with a cycle life upto 2000 cycles and nearly100% for lithium ion with a lifetime of 3000 cycles.10

Thermal storage can be relatively inexpensivecompared to other options. Currently, the estimatedcost of thermal storage for CSP is $30 per kWhth11 fora central receiver with 9 h of storage. Assuming a con-servative thermal to electrical conversion efficiency of30%, the effective cost of electrical energy storage is$90 per kWhe for a central receiver system. Electri-cal energy storage systems, including batteries, havea considerably higher cost near $500 per kWhe andwill need to be replaced several times over the life of apower plant given this degradation rates of the batter-ies themselves,12 resulting in a substantial system costincrease. Despite the relatively low cost of TES, it isnot yet practical, either technically or economically,to include enough storage to reach a power plant capacity factor much in excess of 70%. This point is madein a study showing that the minimum levelized cost ofenergy (LCOE) for a CSP plant is realized by includingaround 12 h of storage (65–70% capacity), which en-ables additional use of the power block, thus reducingits effective cost.13 Conventional power plants typi-cally have a capacity factor between 70–90%. CSPfacilities with thermal storage can reach this level ofcapacity 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 overthe small fraction of the year when the solar resourceis insufficient. This approach has been demonstratedsuccessfully in many plants including the solar energygenerating station (SEGS) facilities operating in Cali-fornia since the 1980s.14

TES TECHNOLOGIES

The development of TES technologies has been closelytied to both the operational requirements of powercycles suitable for CSP applications, and the operat-ing characteristics of the three main CSP platforms:parabolic trough, parabolic dish, and central receiver.Figure 4 shows the operating envelope (temperaturerange) of each of these platforms as well as the po-tential thermal efficiency of power cycles currentlyunder consideration for CSP. These include Rankinecycles using steam or organic fluids operating between300◦C–650◦C, Brayton cycles at 900◦C and above forair and 600◦C–800◦C for supercritical CO2, and Stir-ling engines at 600◦C–800◦C.

The general structure of a CSP power plant withTES is the same, in most respects, regardless of theplatform or specific power cycle. An illustration ofa central receiver plant, such as the commercial scale(100 MWe) power plant currently under developmentby SolarReserve15 with a TES system, is shown inFigure 5. The key features are as follows:

• Collection and focusing of sunlight onto a re-ceiver by an array of mirrors;


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FIGURE 5 | A central receiver power plant with two tank molten nitrate salt thermal energy storage (TES).15 This configuration is identical tothat demonstrated at the Solar Two project sited in Barstow, CA. This is an example of direct TES wherein the heat transfer fluid and thermalstorage 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, thatmoves heat from the receiver to the storagesystem;

• A TES system containing a storage media thatmay or may not be the same as the heat trans-fer material;

• A power block that receives energy either di-rectly from the heat transfer material via thereceiver or from the TES system.

In the case of the central receiver system shownin Figure 5, both the heat transfer material and thestorage media are a molten salt composed of potas-sium nitrate and sodium nitrate. The salt is heatedfrom 280◦C to 588◦C in the receiver and stored inthe ‘hot’ tank. From there, the molten salt is pumpedthrough the power block to drive a steam turbine andis then discharged into a ‘cold’ tank where it remainsuntil being pumped back to the receiver. The amountof energy stored in the system is a function of boththe heat capacity of the salt, the temperature differ-ence over which it is used, and the mass of salt in thesystem (tank size).

It is possible to configure the system such thatthe power block runs at full-rated capacity day andnight. In this case, the solar collection field and re-ceiver are oversized beyond what is needed to satisfythe instantaneous energy demands of the power cycleso that additional solar energy may be collected andplaced into storage. The incremental amount of col-lection area installed beyond what is required by thepower block is referred to as the solar multiple. In acentral receiver with 12 h of storage, a solar multipleof 2.8 is required.13

Three General Approaches to TESThermal storage systems can be categorized into tech-nologies that utilize either thermophysical or ther-mochemical energy storage processes. Thermophysi-cal processes involve the storage of energy in one oftwo ways, either by adding heat to a material (solidor liquid) to cause an increase in temperature or byadding heat to cause a phase change such as melting.The former case is generally called ‘sensible energystorage’ while the latter is known as ‘latent energystorage’. Thermochemical processes use a series of re-versible chemical reactions that result in energy storedin the form of chemical reaction products that may be


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reacted later to liberate heat. The amount of thermalenergy that can be stored in each of these three cases isexpressed in Eqs (1)–(3) with the assumption that thesensible energy is stored in incompressible media:

Qsensible = m∫ TH

TL

cp (T)dT, (1)

Qlatent = m∫ Tmp

TL

cp(T)dT + m�h fusion∣∣T=Tmp

+ m∫ TH

Tmp

cp(T)dT, (2)

Qthermochemical = m∫ TR

TL

cp(T)dT + m�h reaction∣∣T=TR

+ m∫ TH

TR

cp(T)dT. (3)

In the case of sensible energy storage, the quantityof energy stored, Qsensible, is a function of the spe-cific heat of the media, cp, the temperature differenceover which it is stored, TH-TL, and the total mass ofmaterial in storage, m. The most common approachto sensible energy storage in the CSP industry is touse a molten nitrate salt as the heat transfer fluid andstorage media.16,17 For a central receiver using ni-trate salts, the storage temperature range is typically288◦C–565◦C, but could be extended in future sys-tems to 288◦C–650◦C.18 In the latter case, the amountof energy stored per unit mass is 0.58 MJ/kg while theenergy stored per volume is 1050 MJ/m3. The advan-tage of sensible energy storage approaches is simplic-ity in design. The primary disadvantage is that largeamounts of storage media can be required as systemsare scaled up, possibly resulting in high capital cost.

In a latent energy storage system, the quantityof energy stored, Qlatent, is a function primarily of theenthalpy of fusion, �hfusion, and the mass of material,although sensible energy stored in the solid and liquidphases may also contribute in certain systems. A widerange of materials have been developed for latent en-ergy storage for CSP applications, and they almost ex-clusively involve a solid-to-liquid phase change as op-posed to a more energetic liquid to gas phase changedue to the difficulty in storing gaseous products. Ifa liquid-to-vapor transition is used for storage, Eq(2) would need to be modified to include the energyassociated with the latent heat of vaporization.

Latent energy storage offers two potential ad-vantages over sensible energy storage: increased en-ergy storage density and isothermal energy transfer.The primary disadvantage is that significant lossesmay be incurred during charge and discharge in the

case when the storage media has a low thermal con-ductivity. This is not the case for all materials and theimpact of charge/discharge losses can be mitigatedby reducing the conduction length by encapsulatingthe storage media, including finned heat exchangersand/or heat pipes, or augmenting the thermal conduc-tivity of the media with additives.19,20

The ability to transfer heat at a constant tem-perature is advantageous for power cycles, such asthe Stirling and Rankine cycles, which require that asignificant amount of the total energy input be isother-mal to achieve peak efficiency. Latent energy storagein nitrate salts is appropriate for parabolic troughpower plants using steam as the working heat trans-fer fluid in the field (direct steam generation or DSG)and in the power block.19 In this case, both sensibleand latent energy may be stored in the same mediaand used for different purposes, e.g., preheat, evap-oration, and superheat. The melting point of solarsalt, a near eutectic mixture of sodium and potassiumnitrate suitable for DSG systems, is around 220◦C.The enthalpy of fusion, and gravimetric storage den-sity, is 0.1 MJ/kg. The volumetric storage density is195 MJ/m3. Latent energy storage for solar powerplants based on the Stirling cycle requires a con-siderably higher phase change temperature (700◦C–800◦C) that can be achieved with inorganic salts(hydroxides, fluorides, carbonates) or metallic phasechange media.21

The energy stored in a thermochemical system,Qthermochemical, is primarily a function of the reac-tion enthalpy, �hreaction, and the mass of the mate-rial in storage, although sensible heat may be storedabove and below the reaction temperature. One majordifference between thermochemical storage and theother two approaches is that in the case of thermo-chemical storage the temperature at which the systemis charged and discharged may be significantly differ-ent. This results from the requirements of the indi-vidual chemical reactions comprising the storage sys-tem. Currently, thermochemical storage is not beingactively used for CSP applications although prelimi-nary investigations are underway.22 The advantagesof a thermochemical storage approach are potentiallyhigh gravimetric storage density and the possibilityof energy storage for long periods of time, as stablereaction products, with little energy loss.

TES MediaWith respect to performance, the key differentiatingcharacteristics of the wide array of TES media thathave been developed over the years are operatingtemperature range, gravimetric and volumetric


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Nota

Se servono temperature molto alte per la sorgente isoterma di calore (come potrebbero forse servire in un ciclo a vapore supercritico) non ci si può ovviamente affidare alle comuni miscele di sali fusi, ma si devono usare sali inorganici, con alta temperatura di fusione. Usando questi sali possono insorgere problemi di corrosione, che sono discussi in un articolo apposito.


TABLE 1 The Physical Properties of Selected Thermal Energy Storage Media. Sensible Energy StorageMedia, Both Liquid and Solid, Are Assumed to Have a Storage Temperature Differential of 350◦C withRespect to the Calculation of Volumetric and Gravimetric Storage Density

Specific Latent or Temperature Gravimetric VolumetryStorage Heat Reaction Density Range (◦C) Storage StorageMedium (kJ/kg-K) Heat (kJ/kg) (kg/m3) Cold Hot Density (kJ/kg) Density (MJ/m3) ReferencesSensible Energy Storage—SolidsConcrete 0.9 – 2200 200 400 315 693 23Sintered bauxite particles 1.1 – 2000 400 1000 385 770 24NaCl 0.9 – 2160 200 500 315 680 23Cast iron 0.6 – 7200 200 400 210 1512 25Cast steel 0.6 – 7800 200 700 210 1638 23Silica fire bricks 1 – 1820 200 700 350 637 23Magnesia fire bricks 1.2 – 3000 200 1200 420 1260 25Graphite 1.9 – 1700 500 850 665 1131 26Aluminum oxide 1.3 – 4000 200 700 455 1820 27Slag 0.84 – 2700 200 700 294 794 28

Sensible Energy Storage—-LiquidsNitrate salts 1.6 – 1815 300 600 560 1016 17

(ex. KNO3-0.46NaNO3)Therminol VP-1 R© 2.5 – 750 300 400 875 656 29Silicone oil 2.1 – 900 300 400 735 662 23Carbonate salts 1.8 – 2100 450 850 630 1323 23Caloria HT-43 R© 2.8 – 690 150 316 980 676 25Sodium liquid metal 1.3 – 960 316 700 455 437 25Na-0.79K metal eutectic 1.1 – 900 300 700 385 347 30Hydroxide salts (ex. NaOH) 2.1 – 1700 350 1100 735 1250 27

Latent Energy StorageAluminum 1.2 397 2380 – 660 397 945 28Aluminum alloys 1.5 515 2250 – 579 515 1159 31, 32

(ex. Al-0.13Si)Copper alloys – 196 7090 – 803 196 1390 32

(ex. Cu-0.29Si)Carbonate salts – 607 2200 – 726 607 1335 32

(ex. Li2CO3)Nitrate salts 1.5 100 1950 – 222 100 195 28

(ex. KNO3-0.46NaNO3)Bromide salts (ex. KBr) 0.53 215 2400 – 730 215 516 33Chloride salts (ex. NaCl) 1.1 481 2170 – 801 481 1044 33Flouride salts (ex. LiF) 2.4 1044 2200 – 842 1044 2297 33Lithium hydride 8.04 2582 790 – 683 2582 2040 31Hydroxide salts (ex. NaOH) 1.47 160 2070 – 320 160 331 31

Thermochemical EnergyStorage

SO3(g)↔ SO2(s) + 1/2O2(g) – 1225 – – 650 1225 – 28, 30, 34CaCO3(s)↔CO2(g) + CaO(s) – 1757 – – 527 1757 – 28, 34CH4(g) + CO2(g)↔2CO(g) – 4100 – – 538 4100 – 35

+ 2H2(g)CH4(g) + H2O(g)↔ – 6064 – – 538 6064 – 35

3H2(g) + CO(g)Ca(OH)2(s)↔CaO(s) + H2O(g) – 1351 – – 521 1351 – 28, 30, 34NH3(g)↔1/2N2(g) + 3/2H2(g) – 3900 – – 195 3900 – 36



FIGURE 6 | Operating temperature limit and energy storagedensity for thermophysical (sensible and latent) and thermochemicalenergy storage media. In this chart, the energy storage density forsensible energy media is constrained by the temperature range overwhich energy is stored. This was fixed at 350◦C. In addition, sensibleenergy storage is not included in either the thermochemical or latentenergy storage media calculations.

storage density, and cost. The physical properties ofsome candidate storage media are given in Table 1with gravimetric storage density plotted against max-imum operating temperature in Figure 6. Cost dataare not included in Table 1 as these are closely tied tocurrent market values of the storage media. In addi-tion, storage media costs are not a sufficient indicatorof the resultant storage system cost that may includeexpensive containment vessels, piping, pumps, andother hardware. Figure 6 illustrates the wide variabil-ity in storage density and process temperature thatmay be accommodated by the different storage ap-proaches. It should be noted that in both Table 1and Figure 6 the temperature range over which sen-sible energy storage systems are assumed to operatewas fixed at 350◦C for ease of comparison. In reality,some of these materials are capable of storage over amuch wider temperature range, leading to improvedstorage density. This is particularly true of solid phasesensible energy storage media. However, in all casesthe storage system must be well matched to the pro-cess heat demands, i.e., storing energy over a widertemperature range than required by the process willlikely increase system losses due to increased thermallosses in the solar collection (receiver) and storagesubsystems. While it is true that the thermophysicalproperties of the thermal storage media have a signif-icant impact on the performance of a storage system,they should not be used exclusively as a means to es-timate system performance. It is often the case thatsignificant losses can be incurred during the thermalenergy charge and discharge processes. This is par-

ticularly true of latent energy and thermochemicalenergy storage systems. The impact of a given TESstrategy on overall performance and cost can only beevaluated through a rigorous system level analysis.

Sensible Energy Storage SystemsSensible energy storage is the most commonly usedapproach to TES in CSP applications and the onlytype of TES system that has been deployed commer-cially. There are two configurations generally used forsensible energy storage. In a direct storage system, theheat transfer fluid is also the storage media. An exam-ple of a direct storage system is the two-tank moltensalt storage system demonstrated at Solar Two9,37

and illustrated in Figure 5. In an indirect storage sys-tem, the heat transfer fluid discharges energy to astorage media through a heat exchanger. Indirect stor-age is used in the Andasol parabolic trough plants inSpain16 and shown in Figure 7. In these 50 MWe facil-ities, the heat transfer fluid in the field is synthetic oilwhile the storage media is molten nitrate salt. Thereare two incentives for deploying indirect storage, thefirst being cost. In the case of parabolic trough plants,the synthetic oils are several times more costly thanmolten salt per unit energy stored.38 The second rea-son to deploy an indirect storage system is that thestorage media cannot itself be used as a heat transferfluid. This is the case with high-temperature centralreceivers operating with air as the heat transfer fluidand a solid ceramic thermal storage media, such as the1.5 MWe power tower located in Julich, Germany.39

It is also true of parabolic trough systems such asAndasol that cannot use molten nitrate salt as a heattransfer fluid in the field due to its high melting point(∼220◦C for the salt used at Andasol versus 12◦C forTherminol R© VP-1). Costs for sensible energy storagesystems can be reduced by using a hybrid storage con-figuration that includes both solid- and liquid-phasestorage media. This is called a thermocline40 and gen-erally involves a single storage tank wherein some ofthe typically more expensive liquid media is displacedby a less expensive solid media such as crushed rock.

Molten nitrate salts are the most commonlyused sensible energy storage media and will likelyplay a large role in thermal energy storage for bothnear-term central receivers and advanced parabolictroughs. Many molten nitrate salt formulations havebeen developed over the last few decades.41,42 Themost common mixture is known as ‘solar salt’ andis a binary salt composed of 36 mol% KNO3and 64 mol% NaNO3. The components are minedprimarily in Chile, and the mixture can be relativelylow in cost, about $0.5 per kilogram.43 Other mix-tures have been developed with the aim of reducing


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FIGURE 7 | A parabolic trough power plant with thermal energy storage (TES). This is an illustration of an indirect TES configuration that wouldlikely 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 moveheat between the two fluids. This configuration is currently in use at Andasol I and II in Spain. Source: NREL. (Reprinted by permission. Copyright2011, NREL.)

the melting point to enable deployment in parabolictroughs where freezing in the field would be veryproblematic.43,14 These are typically ternary or qua-ternary eutectic compounds containing, in addition toKNO3 and NaNO3, Ca(NO3)2 and/or LiNO3. San-dia National Laboratories has developed quaternarynitrates that melt around 90◦C42 and other mixtureswith nitrite salts that melt around 70◦C.44 Meltingpoint is relatively less important for central receiversthat operate at higher temperatures than parabolictroughs. The limiting factor for the use of nitrates inthese systems is their thermal stability limit. Nitratesalts undergo a complex series of decomposition re-actions, some irreversible, at temperatures in excessof 500◦C.45 The products of these reactions, whichcan include alkali oxides and nitrogen oxide gases,are corrosive to metallic system components.46,47

Latent Energy Storage SystemsLatent energy storage systems have the potential toachieve high gravimetric storage density and also dis-charge heat at a constant temperature. Past (and cur-rent) research has focused on developing nitrate orcarbonate salt-based48 storage systems for integrationwith either steam power cycles for parabolic troughsand towers, and hydride,49 fluoride,50 or chloride50

salt for energy storage onboard a parabolic dish Stir-ling power system. In the latter case, both gravimet-ric and volumetric storage density are critically im-

portant as the entire power block and TES systemmust be supported at the focal point of the parabolicdish concentrator, potentially requiring a very robuststructure and drive system. Other phase change sys-tems based on metallic compounds have also beeninvestigated. Systems such as Al–Si,51 Cu–Ca,52 andSi–Mg,53 can operate at the relatively high tempera-tures required for Brayton power cycles and advancedRankine cycles. The vast majority of work on metalliclatent energy systems has focused on eutectic compo-sitions in which all of the heat is discharged at a con-stant temperature. However, off-eutectic compoundscan also be used provided that a variable dischargetemperature can be accommodated in the system.

Thermochemical Energy Storage SystemsThermal energy storage in the form of chemical bondsoffers two significant advantages relative to other ap-proaches: energy storage density can be significantlygreater and energy can potentially be stored for ex-tended periods of time at minimal loss in the form ofstable reaction products. Despite these advantages apractical thermochemical energy storage system, onethat competes with other options on the bases ofroundtrip 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) spon-sored multiple projects focusing on the developmentof physical and chemical energy storage technologies


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Perchè? Perchè è più facile drenare i sali fusi e mandarli nella storage tank in cui non si scende a temperatura pericolosamente bassa?

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Ecco un motivo per cui il degrado termico dei sali fusi ad alta temperatura è pericoloso.


FIGURE 8 | A schematic of a thermochemical transport andstorage system that could be used in conjunction with a parabolic dishor central receiver collection system, enabling more efficient solarcollection for high-temperature power conversion processes such asthose based on the Stirling or Brayton cycles.59 Reaction products maybe placed in storage at a fairly low temperature, enabling long-termstorage are relatively high efficiency.

including several thermochemical energy storageprojects targeting CSP applications. These projects aredetailed in a series of DOE conference proceedingscontaining individual contractor reports.31–33,54–57

An extensive survey of 550 prospective thermochem-ical cycles conducted by Rocket Research Companyled to the identification of 12 likely candidates af-ter applying several high-level screening criteria. Theestimated roundtrip efficiency of an energy storagesystem based on these 12 candidates ranged from 20–50% on a first law basis.34 In addition, the temper-ature required for the ‘charging’ reaction is, in somecases, greater than the temperature of the thermalenergy released in the discharging reaction. This rep-resents a loss in exergy as well as energy. Today, theDOE is again funding projects in solar thermochem-ical storage. One is currently underway at GeneralAtomics and leverages recent efforts in the area ofsolar fuels production58 to develop storage optionsfor central receivers. Thermochemical storage is alsoa candidate for parabolic dish systems that operatemore efficiently at elevated temperature (∼800◦C)than do central receivers or parabolic troughs. Thechallenge of incorporating storage with a parabolicdish is that the energy either needs to be stored onthe dish, potentially resulting in prohibitively largeand expensive support structures, or on the ground,which requires that high-temperature thermal energybe moved efficiently off of the dish through two ro-tary joints or flexible couplings to the storage system.

The associated technical challenges can be avoided bymoving thermal energy off of the dish in the form oflower temperature chemical reaction products. Thisconcept is sometimes called a thermochemical heatpipe or thermochemical transport35,59,60 and is il-lustrated in Figure 8. Using this approach, energyis added to the system via a reversible endothermicreaction to produce heated reaction products. Thereaction products are cooled in a counterflow heatexchanger, while preheating the chemical reactants,and then sent to storage until the chemical reaction isreversed to produce heat and drive a thermal powercycle.

CONCLUSIONS AND OUTLOOK

Energy storage technologies must be developed if re-newable energy from solar and wind resources is toplay a significant role in future electrical power gener-ation. Without storage, power distribution grids willlikely be overtaxed in dealing with the intermittentnature of power produced from variable wind and so-lar generation systems, effectively limiting the deploy-ment scale of these technologies. Thermal energy stor-age integrated with concentrating solar power plantsis a commercially demonstrated, relatively low costsolution appropriate for the utility-scale storage ofrenewable energy. Systems being deployed today arecapable of storing enough energy for several hoursof operation when the solar resource is not available.Future systems will accommodate sufficient storage torun around the clock on most days of the year and in-clude natural gas hybridization to remain operationalduring periods of low solar resource, thus providingfirm generating capacity.

Thermal energy storage technologies have beenunder development for decades. Over this time, a widerange of prospective thermal storage media have beendiscovered and matched with solar power generationhardware. Current commercial thermal energy stor-age approaches using molten nitrate salts are appro-priate for Rankine power cycles operating at tem-peratures up to ∼580◦C. This type of relatively sim-ple two-tank direct storage system can be deployedat power levels in excess of 100 MWe with 12 hof storage and including fossil hybridization for in-creased capacity, effectively making utility-scale solarpower indistinguishable from more conventional gen-eration. In near future, the cost of solar power systemswith thermal energy storage will be reduced throughtechnical developments including an increase inpower cycle operating temperature that will enablemore efficient electricity production. This transitionto more advanced power cycles will be made possible



through targeted research in the areas of heat trans-fer fluids and thermal storage media (materialsdevelopment), hardware development, and systemdesign.

This information was prepared by the NationalRenewable Energy Laboratory for the U.S. Depart-ment of Energy.

The abstract figure and figure 7 have beenreprinted from the National Renewable EnergyLaboratory’s publication TP/5500-52134 “Sum-mary Report for Concentrating Solar Power Ther-mal Storage Workshop” authored by Greg Glatz-maier: 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 Labo-ratories. Sandia National Laboratories is a multiprogram laboratory managed and operated bySandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S.Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

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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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