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High temperature thermal energy storage
Thermal energy storage (TJZS)
59
WORKING GROUP A HIGH TEMPERATURE THERMAL ENERGY STORAGEt D. E. ELLIOT(Group Leader), Department of Mechanical Engineering, University of Aston, Gosta Green, Birmingham 84 7ET, England, T. STEPHENS(Rapporteur), Building Services, Division of Building Research, National Research Council, Ottawa, Ontario, Canada, hf. F. Bmas, Director, Research and Development Centre, Saskatchewan Power Corporation, Regina, Saskatchewan, Canada, G. BECKMAN,Jacquingasse 55/10, A-1030 Wien, Austria, J. BONNIN,Elect&% de France, 6 quai Watier, 78400Chatou, France, A. BRICARD, C.E.N.G., B.P. 85, DTCE-SIT, Centre de Tri, 38041Grenoble, France, T. D. BRUMLEVE,Solar Energy Technology Division, Sandia Laboratories, Livermore, CA 94550,U.S.A., G. B. DELANCEY,Department of Chemistry and Chemical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030. U.S.A., Ch. A. buWNK, Centraal Laboratorium TNO, P.O. Box 217, Delft, Netherlands, G. A. LANE,The Dow Chemical Company, Midland, MI 48640,U.S.A., W. R. LAWS,Head, Plant 8 Energy Division, Corporate Engineering Laboratory, British Steel Corporation, 140Battersea Park Road, London SW11 4LZ, England, R. F. S. ROBERTSON, Whiteshell Nuclear Research Establishment, Atomic Energy of Canada Ltd., Pinawa, Manitoba ROE lL0, Canada, and J. SCHROEDER, Philips Forschungslaboratorium, Aachen GmbH, D-51 Aachen, Weisshausstrasse, Germany.
tReproduced from a Report of a NATO Science Committee Conference on Thennol Energy Storage held at Turnberry, Scotland, l-5 March 1976.
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1. INTRODUCTION
Thermal energy storage (TES) offers a means of effecting significant cost savings and conservation of premium fuels. By providing thermal storage between the heat source and heat user, cost savings can be realized by improved utilization of capital equipment. Using heat which would otherwise be wasted reduces premium fuel requirements. The basic nature of TES can be described in general as follows. The total heat “Q” that can be stored by a material is Q = n[jr;
C,, dT+/r;
C,,dT+AH,
+AH,],
where n is amount of material in moles, T, and T2 are the lower and upper temperatures, Tf is the melting point, C,, and C,, are heat capacities per mole of the solid and liquid phase, AH, and AH, are transition enthalpies of phase change and chemical change. The four terms in the preceding equation accordingly correspond to the sensible heat of solids and liquids, latent heat of phase change, and chemical heat of reaction, respectively. Technologies based upon these four basic TES modes are discussed here in some detail, for a temperature range of 120-1250°C(which is the presently accepted limit) and, in some cases, to as high as 1500°C. Significant existing or potential applications were identified in the following general areas: transportation and power generation (i.e. heat engines), industrial uses and domestic and commerical uses. Selection of a particular TES mode and basic technology for a given application, and the resultant system performance and economics, will depend on detailed engineering effort. In some cases, the technologies are in an arrested state of development; in others, they are developing rapidly, and basic design and operating experience is inadequate. In general, development of a broad spectrum of thermal storage technology appears to be warranted. II. SENSIBLE
HEAT STORAGE
A. General High temperature energy storage devices using the sensible heat of materials are in widespread industrial use, providing output temperatures ranging from 120to 1250°C.Three basic types exist: storage is in the form of containment of a high temperature working fluid at atmospheric or higher pressures, with subsequent heat recovery by withdrawal of the working fluid; transfer of heat from a working fluid to a storage material at high temperature and at atmospheric or higher pressures, with subsequent energy recovery by reheating of the working fluid; transfer of heat from one fluid to a second fluid via storage in a third material. In this instance, the first and second materials may be at different pressures. Heat storage devices based on sensible heat require special design and operational features to ensure heat extraction within desired temperature limits. The most widely used method is to arrange storage devices in groups of four or five with series/parallel switching on both the charging and discharging cycles. For short term storage, continuous operation is obtained by rotary regenerators and fluidized beds. Switched storage units of large size have been used for many years at pressures of several atmospheres, while continuous regenerators have operated mainly in situations where the pressures of the cold and hot fluid are close to ambient. Although sensible heat storage has been practiced for many years, considerable work remains to be done to increase the amount of stored energy per unit volume (energy density), thus reducing the size and capital cost of a given unit. New configurations, materials and methods of construction need investigation to increase the practical size of installations and to raise maximum working pressures. The reduction of unit costs will act as a stimulus to more widespread application of sensible heat storage devices. B. Solids 1. Packed beds: (a) Above-ground installations. The storage of thermal energy as sensible heat in refractories has been practiced for over a century by the metallurgical industries as a means of raising air temperatures to facilitate the reduction of ores with coke. Technology has
Thermal energy storage (TES)
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advanced to the stage where such storage systems, fired by low calorific value off-gases, preheat air at rates up to 10 tons per minute from ambient to 1250°Cat three atmospheres pressure. For example, a typical large modern unit comprises several storage devices with series/parallel switching and by-pass air bleeds to maintain reasonably constant outlet air temperatures. Each unit can be up to 50 m high and 10 m in diameter, individually housed in insulated steel pressure vessels. The storage matrices are made from specially shaped bricks graded in quality to match the local temperature requirements. The temperature of the bottom support brickwork is approximately 300°Cwhile the top refractory approaches 1500°C.The total refractory weight in a modern installation totals over 20,000 tons and, with existing brick dimensions, cycle times of 45 minutes on and off “blast” are used. Such techniques could be adapted to other applications for storing heat from hot waste gases for subsequent reuse. (b) Underground installations. For some applications requiring storage of large quantities of thermal energy, in which the working medium is a pressurized gas, underground packed-bed TES devices seem technically feasible and economically attractive. In principle, pressure containment is provided by the walls of underground cavities, and thermal storage is provided in the sensible heat of packed solids within the cavity. Heat is transferred by direct contact between the working fluid and the thermal storage material, thereby avoiding the requirement for expensive heat exchange equipment. Economic considerations will usually require siting at minimum depth. Accordingly, the working pressure may be higher than the local overburden pressure and, in general, considerably higher than the hydrostatic pressure of the local ground water. Positive sealing of the cavity will usually be necessary to avoid leakage of the working medium to the surface through overburden fissures. High-pressure grouting may be adequate in some applications and in certain geological formations. In other cases, metal membrane liners for sealing, and lightweight aggregate concrete for thermal insulation and load transfer to the rock walls may be required. Packing material may range from crushed rock to relatively expensive refractory. Potential applications exist in the following systems: -Compressed-air energy storage with heat-of-compression recovery Underground packed-bed TES provides a means of storing the heat-of-compression in compressed air energy storage (CAES) systems. Such adiabatic CAES systems have high (-75%) roundtrip storage efficiencies, and do not require fuel input. They are potentially comparable in cost to pumped hydroelectric storage, with greater siting flexibility. -Gas turbine cycles Open and closed gas turbine cycles using solar, fossil and waste heat sources have been proposed. Underground packed-bed TES systems are directly applicable for source or load leveling in such cycles. -Steam-superheat storage Underground steam accumulators have been proposed as an economic means of accomplishing load-following with nuclear or fossil-fueled electrical power plant. Packed-bed thermal storage can add the feature of superheat storage, at small additional cost. This concept introduces the potential for major increases in storage capacity, and efficiency, and for outlet steam quality. As yet no underground packed-bed TES devices have been built. The mechanical, civil, materials and geological engineering aspects need development, and underground pilot plants will probably be required to prove the concept. 2. Huidized solids. Small solid particles can be fluidized by upward passage of gas stream. Such fluidized solids have several features which make them attractive for heat storage system applications. (a) Advantages-In common with other solid storage materials, particle solids have negligible vapor pressures at temperatures up to at least 15OO”C,and are chemically inert. non-toxic, and non-explosive. -They are easily transported as quasi-liquids. -The large surface area per unit volume which is characteristic of fine particles enables them to absorb sensible heat rapidly from gases or the heat of reaction from combustion or other processes. -High temperature-recovery effectiveness can be readily achieved. -The heat-transfer coefficient between fluidized solids and immersed surfaces is an order of
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magnitude higher than gas/surface coefficients, and by using extended surfaces in contact with the tluidized solids, effective heat transfer coefficients comparable to those for water may be achieved. This permits heat to be transferred in to and out of storage quickly and economically. -With inexpensive solids such as sand, moderately high heat storage capacities can be obtained with moderate temperature cycling. Suitable salts or other materials offer significantly higher thermal capacities and warrant investigation. (b) General considerations. In the systems so far proposed, the solids would be stored in the unfluidized state to save fluidizing pumping power and minimize storage volume and cost. They would be transported around the heat absorption, storage and heat recovery circuit by a combination of gravity, air slide, and a pneumatic or dense phase elevation system. Applications for such systems would include: heat storage at central power stations prior to steam generation to reduce the capital cost of peak load plant; and storage of relatively low temperature waste heat (<400”(Z) in insulated, unpressurized mild steel vessels for use in process steam generation or pressurized hot water production. (c) Research and development needs. The extensive research and development currently underway in fluidized bed combustion of fossil fuels (both atmospheric and pressurized) is generating experimental data which are directly applicable for prediction of the heat transfer conditions in fluidized beds. However, the water-side limitations under very high heat flux conditions in horizontal tubes require clarification. Development of improved dense-phase pumping techniques is desirable to avoid the erosion likely to occur with lean-phase, pneumatic transport. The main uncertainties are: -cost of large-scale containment of hot unfluidized particles at atmospheric pressure (for example, the equivalent of several GWh(t), at temperatures between 800 and 1200°C); -upper temperature limitations for a given system; -temperature and operational limitations due to particle sintering. C. Liquids 1. Pressurized Hot Water Storage: (a) Above-ground
installations. The above-ground storage of hot pressurized water in conventional welded steel vessels has been used extensively for short-term requirements in industrial process heat applications, and to a very limited extent in electric power generation stations. The technical feasibility of such vessels is well established. The major cost component is the pressure vessel itself. Piping and charging devices typically represent less than 5% of the total cost. An example indicating the maximum sizes encountered in current practice is a vessel having a volume of 500m3, with a working pressure of 20 bar. Industrial process-heat users can frequently justify conventional hot water storage due to the high frequency of charging and discharging which is characteristic of such applications. Long-term hot water storage above ground on a large scale for electric utility application (i.e. daily/weekly cycling) has rarely proved to be economical primarily because of the high containment costs. Reduction in vessel costs by 30-40% may be realized by use of the Prestressed Cast-Iron Pressure Vessel (PCIV) concept. The PCIV concept has been technically proven at reduced scale only. However, in principle, PCIVs may be built in very large sizes, since they are assembled from prefabricated elements. With PCIVs the optimum working pressure is higher, which results in savings in piping, valves and, in the case of peak load coverage, savings in the turbine, including the cold end. Prestressed concrete vessels have also been proposed but have not been proven for this application. Such prestressed vessels are not susceptible to catastrophic failure, which makes them more attractive than conventional welded steel vessels. (b) Underground installations. For economical central-power station load following, and possibly for solar-thermal plant heat-source levelling, underground feedwater storage or steam accumulators are technically feasible and may be economically attractive. Constant-pressure feedwater storage might be accomplished economically using techniques in which the hot and cold feedwater is stored at constant-pressure in a free-standing, thin-walled, displacement-type accumulator located in an underground cavern. Because the air in the cavern is regulated at a slight underpressure relative to the hot water in the tank, cavern sealing may be
Thermalenergystorage(TES)
63
required. The feedwater would usually be heated by bled steam. Underground steam accumulators may be designed on the same principle, with the added requirement that relatively large pressure swings must be accommodated. Alternate construction methods may be feasible, and economically more attractive. For example, pressure-balancing may be avoided if metal membrane liners are used for sealing, and the pressure load carried directly to the walls of the cavern through refractory concrete backfill. Storage of hot water at temperatures up to 200°C in aquifers or other underground geological formations is being investigated. No large-scale underground hot pressurized water storage system has yet been built, but there may be significant economic advantages. In order to realize this potential, extensive engineering development and proof-of-concept experiments need to be performed. A demonstration installation would also be necessary. 2. Organic liquids. Various organic liquids such as heat transfer oils can be considered for thermal storage applications up to about 350°C. Their upper temperature is usually limited by decomposition of the material itself rather than reaction with containment materials. Their specific heat tends to be higher, but their volumetric energy density lower, than that of molten salts. While organic liquids are relatively expensive, they do not require pressure containment, and are therefore often used for moderate temperature applications. 3. Inorganic liquids. For higher temperature applications (2~800°C or higher) certain liquid metals and a wide variety of molten salt mixtures can be considered. (a) Liquid metals. A great deal of experience has been gained in the practical application of Na and NaK in some nuclear reactor systems. These liquid metals have excellent heat transfer characteristics, good stability, and are non-corrosive at high temperature when properly contained and maintained, but their specific heats are rather low. Sodium and NaK are highly reactive at high temperature, with both air and water; however, practical methods have been developed for minimizing these hazards. Safety problems must not be underestimated, but cumulative operating experience indicates that liquid metals are not as troublesome as is widely believed. Liquid metals merit consideration for high temperature thermal storage applications, particularly where high heat transfer rates are needed. (b) Molten salts. The class of molten salt mixtures is large, diverse and well suited for certain kinds of high temperature energy storage applications, but the temperature dependence of thermal conductivity, viscosity and density is known for relatively few materials. Information on stability of salts and their compatibility with conventional containment materials at high temperature is also very limited. Notable exceptions include fluoride salts which have been extensively investigated for moderately high temperature nuclear reactor applications, and certain nitrate/nitrite mixtures which have been used extensively as heat transfer fluids and for heat treating in industry. 4. Hybrid systems. In some applications cost reductions or performance improvements can be realized through combintions of storage methods. One example is the increase in energy density through using a material’s latent heat of fusion as well as its sensible heat over a large temperature range. Another example is the use of a packed bed of low cost solids in combination with a sensible heat liquid. In principle this is similar to the more conventional gas/solid packed bed technology, except that the liquid accounts for a significant portion of the sensible heat stored. Particle sizes, void fraction, bed dimensions and flow rates are chosen so that heat exchange between fluid and solid occurs in a narrow moving zone. The result is analogous to the moving thermocline concept in an all liquid system, and a nearly constant output temperature can be maintained during discharge until shortly before the storage is depleted. Investigations of this concept are currently underway in the U.S. solar energy program. but various questions regarding fluid flow, heat transfer, cyclic stresses and material compatibility are not fully resolved. 5. Stratification control in liquid thermal storage. In most applications of sensible heat liquids it is important that the mixing of hot and cold liquids be avoided so as to minimize degradation of energies and to achieve optimum utilization of the storage media. The simplest technique is to store the hot fluid separately from the cold fluid. Because this method requires a total containment volume more than twice that of the storage medium, various methods are being explored for storing hot and cold fluid in the same tank. Techniques range from (a) the use of
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membranes for separating the hot and cold fluids, to (b) the preservation of a comparatively sharp natural thermocline between hot fluid in the upper part of the tank and the cold, more dense fluid in the lower part. Because containment costs for high temperature applications can account for a considerable percentage of the total storage system costs, further investigation is warranted on materials and design aspects for the membrane method, and on the effects of wall-induced convection upon the sharpness of a moving thermocline. movable or collapsible
D. Conclusions Sensible heat storage in liquids appears to be viable in all of the three areas of applications considered by the Working Group. As for any sensible heat storage method, it is intrinsically best suited to applications where the charging energy is obtained by lowering the temperature of a material, and the stored energy is later required for raising the temperature of the same, or another material. It is generally not as well suited as latent heat or some heat-of-reaction storage methods for applications where both the source and the load are isothermal, unless the application has excess availability. Conversely, isothermal types of energy storage tend to be ill-suited to applications where the temperatures of both source and load vary over a wide range. III. LATENT HEAT STORAGE
A. General considerations For practical applications, only solid-liquid phase change materials are useful because they store a relatively large quantity of heat over a narrow temperature range, without a la:qe volume change. 1. Advantages. The advantages of these systems are: (a) The higher energy density, compared with sensible heat storage, allows a smaller storage volume and weight. (b) For processes requiring heat input at constant temperatures (e.g. Stirling engines or evaporating water systems), heat transfer requirements are less stringent. (c) Preliminary estimates indicate that the economics are comparable with those for pumped hydro, at least in large scale units. 2. Problems. Depending on the particular system, some or all of the following problems may occur: (a) Expansion and contraction during melting and freezing. (b) Corrosion. (c) Toxicity. (d) Safety hazards. (e) Impairment of heat transfer due to film freezing or formation of a gap between the heat exchange surface and the solid salt by thermal contraction. 3. Objectives for research and development. (a) Existing tables of relevant data on potential heat storage materials, both simple salts and eutectics, should be extended, especially to provide values of thermal conductivity, expansion coefficient of the solid, and densities of the liquid phase in the temperature range of interest. (b) Optimized designs should be developed, especially with respect to heat exchange and economics. (c) New heat transfer concepts should be investigated, e.g. direct contact of the heat transfer medium with the storage material in order to increase heat exchange surface and overcome contraction gaps. (d) A search should be made for inhibitors, protective coatings, and other means of inhibiting corrosion without unduly impairing heat transfer. (e) Studies must be made of environmental impact, consequences of leaks and disposal problems. B . Materials
Most of the materials proposed for high-temperature (120-1400°C) energy storage are either inorganic salts or metals. Among the metals, aluminium, magnesium and zinc have been mentioned as suitable examples. Most metals can be ruled out by considerations such as high price, low heat of fusion
Thermal energy storage (TES)
h
per unit weight or volume, unfavorable chemical properties, toxicity, safety, etc. Use of metal media may be advantageous where high heat conductivity is required and where cost is of secondary importance. Among the inorganic salts, a large number of compounds and eutectics is available, with melting points covering the whole temperature range from 120 to 1400°C.A number of alkali or alkaline earth fluorides or their eutectics are available with superior heats of fusion per unit weight or volume. Below 450°C however, only salts with much smaller heats of fusion per unit weight or volume are available. In this range, sodium hydroxide and the alkali metal nitrates may be suitable. The latter exhibit a phase change in the solid state, accompanied by a volume change, which may have a destructive effect on the container with frequent cycling. Sulfates, carbonates, phosphates, nitrides, oxides and chlorides of the lighter metals (e.g. sodium, magnesium and aluminium) may also be considered as storage media. In general, fused salts present corrosion problems, and inhibitors are required. In the case of fluorides, aluminum powder has been shown an effective inhibitor. In the high temperature range, the heat capacities (sensible heat) of the fluorides, sodium hydroxide, and the nitrates are of the same magnitude as those of such conventional solid heat storage media as olivine, magnesite, cast iron, etc. Hence, they are especially suitable where the latent heat as well as the sensible heat can be utilized, e.g. in residential storage heating. Many of the potentially useful latent heat storage materials are relatively cheap and abundant, but at the present early stage of development, reliable cost estimates are difficult to make. C. Applications
Latent heat or phase-change storage may find application in the following fields: 1. Heat engines and power plants. Upstream storage of heat is mandatory in solar power plants. Both upstream and downstream storage may be useful in all types of power plants, e.g. for load levelling, spinning reserve, etc., but competitive storage methods are also available. Upstream latent heat storage has the advantage that approximately isothermal conditions can be maintained and matched to the performance of the engine. This is particularly important for the efficiency of Stirling or Rankine engines. Many latent heat storage media have been thoroughly investigated, but further R & D is needed to develop and design practicable systems. 2. Industrial processes. Latent heat storage systems .will be advantageous in those industrial processes where a constant temperature is required, or where energy is available at a roughly constant temperature level. There is a wide variety of possible industrial applications. In the past it has generally been uneconomical to recover waste heat, and an enormous amount of energy is available if better methods of utilization can be developed. Energy storage may enable integration to be achieved between different processes. Much R & D will be necessary to develop practicable systems. 3. Domestic and commercial. While most applications of latent heat systems to homes and buildings involve storage at temperatures below 120°C a few involve higher temperatures. Absorption air conditioning can be best accomplished in the 120-150°C range. Storage for space heating is most practical at higher temperatures, with mixing to the desired lower temperature as needed. In all these cases, latent heat storage has potential benefits of a smaller volume storage unit, and hence lower heat losses and improved control of output conditions due to the constant temperature storage. Either solar or off-peak electrical energy could be used as the heat source, the first leading to a saving of premium fuel resources, and the latter having its major impact on capital requirements. Again, considerable R & D is needed to bring this technology to fruition.
IV. CHEMICAL
ENERGY STORAGE
A. General description Chemical reactions generally offer large energy storage densities due to the highly energetic processes involved in the reversible destruction and reformation of chemical bonds. Catalytic reactions are particularly attractive because the products of the endothermic reaction contain the concommitant thermal energy as chemical bond energy which is retrievable only by the reverse ECY Vol. 2. No. I-E
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reaction over an appropriate catalyst. Consequently, the products are stable at low temperatures and can be transported over long distances. Thermal dissociation reactions are also attractive in this respect if the products of the endothermic dissociation exist as two phases and are easily separable. The separation of reaction products plays the role of the catalyst in catalytic reactions and the insulation in sensible and latent heat storage systems. Research in the area of chemical energy storage has been concentrated on these two classes of reactions, and further work in the low (<12O”C), intermediate (120-4OO”C)and high temperature (400-1000°C) regimes is recommended. 0. Catalytic reactions
Development efforts in this category have been predominantly centered on the steam reforming reaction CH, + H,O=CO + 3H2, which takes place in the endothermic direction in the range of 85&12OO”Cand in the range of 35&7OO”Cfor the exothermic direction. The heat of reaction at 25°C is 49.8 kcal/gmole. A major application is considered to lie in the transmission of high grade thermal energy with storage affected by pressure modulations in the pipelines or by containment either in steel tanks above ground or in underground mined caverns. This application is referred to as the “Chemical-Heat Pipe” and the “Eva-Adam Process” which is under construction at a demonstration stage in Germany.t Preliminary economic studies which have been based upon high grade-energy from a nuclear heat source have been favorable, and indicate, for example, that: (a) onsite chemical storage for utilities is competitive with underground pumped hydrostorage; (b) transmission of thermal energy to large industrial users (>lOO MW,) may compare favorably with user generation by coal combustion with emission controls; (c) it is environmentally attractive and may be economically feasible for utilities to operate their transmission lines in a base load mode with distributed peaking plants in large load centers driven by clean thermal energy which is transmitted underground from the central plant and stored in the chemical heat pipe. The technology associated with the chemical heat pipe is available and could be applied to high temperature nuclear or solar sources or to a topping cycle in industrial boilers. The reversible decomposition of sulfur trioxide, SO&SO, +tO,, has been proposed as a chemical storage technique. SO, is decomposed at 725°C; the SO, is stored as a liquid and O2as a compressed gas. The technology for the oxidation step is well known but the decomposition step needs further study. Laboratory studies are underway. Research is required for reversible reactions that can be driven with lower temperature heat sources such as the light water reactor (-300°C) or the breeder reactor and focusing solar plants (-500°C). Preliminary indications are that organic hydrogenation reactions such as the hydrogenation of benzene are strong contenders, and laboratory studies have been instituted. However, more studies are required on the economics and the detailed reaction kinetics, catalyst stability, and selectivity characteristics of candidate reactions and materials. C. Themal dissociation reactions
Thermal dissociation of solid materials resulting in the formation of solid and gas phases are thermodynamically constrained by monotonic pressure-temperature relationships. Consequently, the design and operation of the resulting thermal storage schemes rely on the modulation of pressures on the store as well as on advantageous thermal effects of the dissociation reaction. Hydration-dehydration reactions are available for application in several temperature regimes. For example, Ca(OH), -+ CaO + HzO, takes place at 520°C and provides a high energy density in storage. Laboratory studies of prototypes are being conducted. Coupled reactions involving metal hydrides appear to offer the very desirable characteristics of rapid kinetics and high storage densities (-360 MJ/m3). A reaction of the type metal hydridessH* + M has considerable promise for heating, for the flexible use of thermal energy cooling, and energy conversion purposes at modest temperature now that alloy hydrides are available decomposing at temperatures which depend on composition. For example, rare tThis expression derives from the acronym EVA of the German description of the rig used for the heat input: EinzelspaltrohrVersuchsAnlage (single cracking-tube experimental rig). The name for the output end (catalytic combination of CO and H,) was prompted by the recognition that every Eve needs an Adam.
Thermal
energystorage033
67
earth-nickel alloys of the AB5 type can be used over a very wide range of temperatures, depending upon composition. By connecting a reactor containing, e.g. lanthanum nickel hydride with a second reactor containing “mischmetall”t nickel hydride and by maintaining each reactor at a separate temperature, hydrogen can be made to flow (with appropriate control systems), from one reactor to the other. The system may be used to store heat or cold, or the hydrogen at high pressure can be made to do mechanical work as it passes from one reactor to the other. The rates of compound formation are very rapid (limited by heat transfer capability) and are reversible. The energy storage density is very high (940 M.J/m3).The feasibility of the system has been proven in the laboratory, and larger scale systems now need developing. The success of this interesting scheme in the next ten years depends upon the ability to reduce the cost of rare earths and rare earth alloys by a factor of about 10. The reaction, CaC12.6NH,$CaCl, . NH, + 5NH, is being investigated for its application to thermal energy transmission. Vehicular transport is envisioned for the solid ammoniates and liquified ammonia. Ammoniated salts are being evaluated for utility power plant applications. V. SUMMARY
AND
CONCLUSIONS
It is concluded that high temperature heat storage offers great potential for reducing capital costs, and saving premium fuels and could be extremely important in solar energy applications. The technologies and areas of application which have been considered are summarized in Table 1. Crosses indicate existing or potential matching between a particular technology and a particular application area. It was generally agreed that the greatest near term potential for energy saving lies in the more efficient utilization of industrial process heat and the integration of power generation, process heat production, and waste heat recovery for space heating. For most effective use of capital, the best near-term potential involves improved storage of heat produced by off-peak power for house heating. The technology closest at hand is sensible heat storage. Based on potential benefits and probability of success, the most promising areas are: -Pressurized water storage -Sensible heat storage in organic liquids for feed water heating -Packed solid beds -Fluidized solids. Above-ground storage of hot water is in use now, but great potential exists for large scale storage in underground strata. Technologies with good long-term prospects involve latent heat storage and chemical storage schemes. The most promising areas involve: -Inorganic phase change materials (e.g. fluoride eutectics) -Chemical heat pipe (e.g. Eva-Adam). Further development of latent heat storage systems, especially work on corrosion prevention, heat transfer problems, and systems development should be encouraged, in order to take advantage of the superior storage capabilities of these materials. To adapt the chemical heat pipe principle to lower temperature sources (e.g. LWR), the development of alternate chemical storage systems should be encouraged. Thermal storage for solar applications
Practical, low cost methods for storing large amounts of high temperature energy appears to be singularly important for solar-thermal conversion to electrical power and other potential industrial processes. In the other high temperature applications discussed, thermal storage is generally aimed either at better utilization of primary fossil fuels (e.g. recovery and use of waste heat) or better use of capital investment (e.g. load leveling). In solar electric conversion. thermal storage is a key technology for successful exploitation of this energy source on a significant scale. Thermal storage will permit the intermittent solar energy to be used later upon demand. t”Mischmetall”-a
naturally-occurring
ore containing
a number of rare earth compounds.
-~_F..
Solar
Nuclear __”___.
Fossil -____.._
Integration
_________
__--
_.-_.-
-_
Y
X ________
X
__._
x __._.____
X -.-_-..
i<
X
X
A
X
~-------.-
x X
-_.
X X
X
---
----
X
-_-----.-~~
--
X
x _..--
X
X
X
X
X
x
-_-__-
X
X
X
-p
X - ---__-_~--
X
----.
X
X
~._..--_~-_~--__
X
X
X
X
__-
___-_____
---------_____---
X
X
X
X
X
X
X __I_-__
_.~
X
X ~_____
X
X
X
X
X
-X ~--__
X
X
X -__
X
~-~~ __-~
X
X
X
X
X
X -_____
X
X
-___-
Dissociative (Set IV C)
X
X
Salts
Catalytic (Set IV B)
X
X
Metals
Metals Salts (Set III B 1) (Set III B 2)
Chemical
x
x
UG
Org. liquid
Liquids (Set 11 C)
Latent
X
.__.__c_I____--“-_I___l_-~
--“---.---~-_
AG
Hot water
Sensible
X
X
UG
Fluidized beds
Solids (Set 11 B)
Packed beds
AG
_....- .___ -_--.__-_-
___
Process-to-PI,ocess --___ _.---__--
Regeneration ---__ .-.._-._-,
--~--_-_---l
Heat
(Heat)
Food
District
Heat
Water
Transport __-_~_---_--..__.-~---_--~AG = Above Groilnd UG = Underground
Engines
Heat
-_~_--~~___._.
Industrial
-_
Domestic and Commercial
Heat
Space
Application
Table I.
Thermal energy storage (TES)
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REFERENCES 1. T. T. Bramlett et al., Survey of high temperature thermal energy storage. Sandia Labs., ISAND 75-8063,US ERDA (March 1976). 2. J. D. Brumleve, Sensible heat storage in liquids. Sandia Labs., SLL-733-0263,US ERDA (Julv 1974). 3. D. E. Elliott, Fluidized-bed boilers. VDI Bericht, Vol. 236. VDI-Verlag Dusseldorf (1975). 4. P. U. Gilli and G. Beckmann, Lines of development in energy engineering. Vol. 236. VDI-Bericht. VDI-Verlag Dusseldorf (1975). 5. I. Glendenning, Storing and energy of compressed air. CEGB Research. CEGB, Marchwood Engng Lab.. Southampton (May 1975). 6. D. C. Golibersuch, F. P. Bundy, P. G. Kosky and H. B. Vakil, Thermal Energy Storage for utility Applications. Proc. of Electrochemical Society Symp. on Energy Storage, Dallas, Texas, (Oct. 1975). 7. C. F. Meyer, Status report on heat storage wells. Water Resources Bulletin (March 1976). 8. P. H. Morgen, Thermal energy storage in rock chambers. Proc. of the 401 Inr. Conf. on the Peaceful Uses of Atomic Energy 4, 177 (Sept. 1971). 9. F. E. Schilling and B. Beine, The prestressed cast iron reactor vessel. Nucl. Engng Design 25, 315 (1973). IO. R. Schulten et ol., Chemical latent heat for transport of nuclear energy over long distances. Proc. of British Nucl. Sot. Inr. Conf. on The High Temperature Reactor and Process Applications (1974). II. M. Tenenbaum et al.. Energy and the U.S. steel industry. Proc. of 5th Annual Conf. IZSI. Toronto (1971).
59
WORKING GROUP A HIGH TEMPERATURE THERMAL ENERGY STORAGEt D. E. ELLIOT(Group Leader), Department of Mechanical Engineering, University of Aston, Gosta Green, Birmingham 84 7ET, England, T. STEPHENS(Rapporteur), Building Services, Division of Building Research, National Research Council, Ottawa, Ontario, Canada, hf. F. Bmas, Director, Research and Development Centre, Saskatchewan Power Corporation, Regina, Saskatchewan, Canada, G. BECKMAN,Jacquingasse 55/10, A-1030 Wien, Austria, J. BONNIN,Elect&% de France, 6 quai Watier, 78400Chatou, France, A. BRICARD, C.E.N.G., B.P. 85, DTCE-SIT, Centre de Tri, 38041Grenoble, France, T. D. BRUMLEVE,Solar Energy Technology Division, Sandia Laboratories, Livermore, CA 94550,U.S.A., G. B. DELANCEY,Department of Chemistry and Chemical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030. U.S.A., Ch. A. buWNK, Centraal Laboratorium TNO, P.O. Box 217, Delft, Netherlands, G. A. LANE,The Dow Chemical Company, Midland, MI 48640,U.S.A., W. R. LAWS,Head, Plant 8 Energy Division, Corporate Engineering Laboratory, British Steel Corporation, 140Battersea Park Road, London SW11 4LZ, England, R. F. S. ROBERTSON, Whiteshell Nuclear Research Establishment, Atomic Energy of Canada Ltd., Pinawa, Manitoba ROE lL0, Canada, and J. SCHROEDER, Philips Forschungslaboratorium, Aachen GmbH, D-51 Aachen, Weisshausstrasse, Germany.
tReproduced from a Report of a NATO Science Committee Conference on Thennol Energy Storage held at Turnberry, Scotland, l-5 March 1976.
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1. INTRODUCTION
Thermal energy storage (TES) offers a means of effecting significant cost savings and conservation of premium fuels. By providing thermal storage between the heat source and heat user, cost savings can be realized by improved utilization of capital equipment. Using heat which would otherwise be wasted reduces premium fuel requirements. The basic nature of TES can be described in general as follows. The total heat “Q” that can be stored by a material is Q = n[jr;
C,, dT+/r;
C,,dT+AH,
+AH,],
where n is amount of material in moles, T, and T2 are the lower and upper temperatures, Tf is the melting point, C,, and C,, are heat capacities per mole of the solid and liquid phase, AH, and AH, are transition enthalpies of phase change and chemical change. The four terms in the preceding equation accordingly correspond to the sensible heat of solids and liquids, latent heat of phase change, and chemical heat of reaction, respectively. Technologies based upon these four basic TES modes are discussed here in some detail, for a temperature range of 120-1250°C(which is the presently accepted limit) and, in some cases, to as high as 1500°C. Significant existing or potential applications were identified in the following general areas: transportation and power generation (i.e. heat engines), industrial uses and domestic and commerical uses. Selection of a particular TES mode and basic technology for a given application, and the resultant system performance and economics, will depend on detailed engineering effort. In some cases, the technologies are in an arrested state of development; in others, they are developing rapidly, and basic design and operating experience is inadequate. In general, development of a broad spectrum of thermal storage technology appears to be warranted. II. SENSIBLE
HEAT STORAGE
A. General High temperature energy storage devices using the sensible heat of materials are in widespread industrial use, providing output temperatures ranging from 120to 1250°C.Three basic types exist: storage is in the form of containment of a high temperature working fluid at atmospheric or higher pressures, with subsequent heat recovery by withdrawal of the working fluid; transfer of heat from a working fluid to a storage material at high temperature and at atmospheric or higher pressures, with subsequent energy recovery by reheating of the working fluid; transfer of heat from one fluid to a second fluid via storage in a third material. In this instance, the first and second materials may be at different pressures. Heat storage devices based on sensible heat require special design and operational features to ensure heat extraction within desired temperature limits. The most widely used method is to arrange storage devices in groups of four or five with series/parallel switching on both the charging and discharging cycles. For short term storage, continuous operation is obtained by rotary regenerators and fluidized beds. Switched storage units of large size have been used for many years at pressures of several atmospheres, while continuous regenerators have operated mainly in situations where the pressures of the cold and hot fluid are close to ambient. Although sensible heat storage has been practiced for many years, considerable work remains to be done to increase the amount of stored energy per unit volume (energy density), thus reducing the size and capital cost of a given unit. New configurations, materials and methods of construction need investigation to increase the practical size of installations and to raise maximum working pressures. The reduction of unit costs will act as a stimulus to more widespread application of sensible heat storage devices. B. Solids 1. Packed beds: (a) Above-ground installations. The storage of thermal energy as sensible heat in refractories has been practiced for over a century by the metallurgical industries as a means of raising air temperatures to facilitate the reduction of ores with coke. Technology has
Thermal energy storage (TES)
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advanced to the stage where such storage systems, fired by low calorific value off-gases, preheat air at rates up to 10 tons per minute from ambient to 1250°Cat three atmospheres pressure. For example, a typical large modern unit comprises several storage devices with series/parallel switching and by-pass air bleeds to maintain reasonably constant outlet air temperatures. Each unit can be up to 50 m high and 10 m in diameter, individually housed in insulated steel pressure vessels. The storage matrices are made from specially shaped bricks graded in quality to match the local temperature requirements. The temperature of the bottom support brickwork is approximately 300°Cwhile the top refractory approaches 1500°C.The total refractory weight in a modern installation totals over 20,000 tons and, with existing brick dimensions, cycle times of 45 minutes on and off “blast” are used. Such techniques could be adapted to other applications for storing heat from hot waste gases for subsequent reuse. (b) Underground installations. For some applications requiring storage of large quantities of thermal energy, in which the working medium is a pressurized gas, underground packed-bed TES devices seem technically feasible and economically attractive. In principle, pressure containment is provided by the walls of underground cavities, and thermal storage is provided in the sensible heat of packed solids within the cavity. Heat is transferred by direct contact between the working fluid and the thermal storage material, thereby avoiding the requirement for expensive heat exchange equipment. Economic considerations will usually require siting at minimum depth. Accordingly, the working pressure may be higher than the local overburden pressure and, in general, considerably higher than the hydrostatic pressure of the local ground water. Positive sealing of the cavity will usually be necessary to avoid leakage of the working medium to the surface through overburden fissures. High-pressure grouting may be adequate in some applications and in certain geological formations. In other cases, metal membrane liners for sealing, and lightweight aggregate concrete for thermal insulation and load transfer to the rock walls may be required. Packing material may range from crushed rock to relatively expensive refractory. Potential applications exist in the following systems: -Compressed-air energy storage with heat-of-compression recovery Underground packed-bed TES provides a means of storing the heat-of-compression in compressed air energy storage (CAES) systems. Such adiabatic CAES systems have high (-75%) roundtrip storage efficiencies, and do not require fuel input. They are potentially comparable in cost to pumped hydroelectric storage, with greater siting flexibility. -Gas turbine cycles Open and closed gas turbine cycles using solar, fossil and waste heat sources have been proposed. Underground packed-bed TES systems are directly applicable for source or load leveling in such cycles. -Steam-superheat storage Underground steam accumulators have been proposed as an economic means of accomplishing load-following with nuclear or fossil-fueled electrical power plant. Packed-bed thermal storage can add the feature of superheat storage, at small additional cost. This concept introduces the potential for major increases in storage capacity, and efficiency, and for outlet steam quality. As yet no underground packed-bed TES devices have been built. The mechanical, civil, materials and geological engineering aspects need development, and underground pilot plants will probably be required to prove the concept. 2. Huidized solids. Small solid particles can be fluidized by upward passage of gas stream. Such fluidized solids have several features which make them attractive for heat storage system applications. (a) Advantages-In common with other solid storage materials, particle solids have negligible vapor pressures at temperatures up to at least 15OO”C,and are chemically inert. non-toxic, and non-explosive. -They are easily transported as quasi-liquids. -The large surface area per unit volume which is characteristic of fine particles enables them to absorb sensible heat rapidly from gases or the heat of reaction from combustion or other processes. -High temperature-recovery effectiveness can be readily achieved. -The heat-transfer coefficient between fluidized solids and immersed surfaces is an order of
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magnitude higher than gas/surface coefficients, and by using extended surfaces in contact with the tluidized solids, effective heat transfer coefficients comparable to those for water may be achieved. This permits heat to be transferred in to and out of storage quickly and economically. -With inexpensive solids such as sand, moderately high heat storage capacities can be obtained with moderate temperature cycling. Suitable salts or other materials offer significantly higher thermal capacities and warrant investigation. (b) General considerations. In the systems so far proposed, the solids would be stored in the unfluidized state to save fluidizing pumping power and minimize storage volume and cost. They would be transported around the heat absorption, storage and heat recovery circuit by a combination of gravity, air slide, and a pneumatic or dense phase elevation system. Applications for such systems would include: heat storage at central power stations prior to steam generation to reduce the capital cost of peak load plant; and storage of relatively low temperature waste heat (<400”(Z) in insulated, unpressurized mild steel vessels for use in process steam generation or pressurized hot water production. (c) Research and development needs. The extensive research and development currently underway in fluidized bed combustion of fossil fuels (both atmospheric and pressurized) is generating experimental data which are directly applicable for prediction of the heat transfer conditions in fluidized beds. However, the water-side limitations under very high heat flux conditions in horizontal tubes require clarification. Development of improved dense-phase pumping techniques is desirable to avoid the erosion likely to occur with lean-phase, pneumatic transport. The main uncertainties are: -cost of large-scale containment of hot unfluidized particles at atmospheric pressure (for example, the equivalent of several GWh(t), at temperatures between 800 and 1200°C); -upper temperature limitations for a given system; -temperature and operational limitations due to particle sintering. C. Liquids 1. Pressurized Hot Water Storage: (a) Above-ground
installations. The above-ground storage of hot pressurized water in conventional welded steel vessels has been used extensively for short-term requirements in industrial process heat applications, and to a very limited extent in electric power generation stations. The technical feasibility of such vessels is well established. The major cost component is the pressure vessel itself. Piping and charging devices typically represent less than 5% of the total cost. An example indicating the maximum sizes encountered in current practice is a vessel having a volume of 500m3, with a working pressure of 20 bar. Industrial process-heat users can frequently justify conventional hot water storage due to the high frequency of charging and discharging which is characteristic of such applications. Long-term hot water storage above ground on a large scale for electric utility application (i.e. daily/weekly cycling) has rarely proved to be economical primarily because of the high containment costs. Reduction in vessel costs by 30-40% may be realized by use of the Prestressed Cast-Iron Pressure Vessel (PCIV) concept. The PCIV concept has been technically proven at reduced scale only. However, in principle, PCIVs may be built in very large sizes, since they are assembled from prefabricated elements. With PCIVs the optimum working pressure is higher, which results in savings in piping, valves and, in the case of peak load coverage, savings in the turbine, including the cold end. Prestressed concrete vessels have also been proposed but have not been proven for this application. Such prestressed vessels are not susceptible to catastrophic failure, which makes them more attractive than conventional welded steel vessels. (b) Underground installations. For economical central-power station load following, and possibly for solar-thermal plant heat-source levelling, underground feedwater storage or steam accumulators are technically feasible and may be economically attractive. Constant-pressure feedwater storage might be accomplished economically using techniques in which the hot and cold feedwater is stored at constant-pressure in a free-standing, thin-walled, displacement-type accumulator located in an underground cavern. Because the air in the cavern is regulated at a slight underpressure relative to the hot water in the tank, cavern sealing may be
Thermalenergystorage(TES)
63
required. The feedwater would usually be heated by bled steam. Underground steam accumulators may be designed on the same principle, with the added requirement that relatively large pressure swings must be accommodated. Alternate construction methods may be feasible, and economically more attractive. For example, pressure-balancing may be avoided if metal membrane liners are used for sealing, and the pressure load carried directly to the walls of the cavern through refractory concrete backfill. Storage of hot water at temperatures up to 200°C in aquifers or other underground geological formations is being investigated. No large-scale underground hot pressurized water storage system has yet been built, but there may be significant economic advantages. In order to realize this potential, extensive engineering development and proof-of-concept experiments need to be performed. A demonstration installation would also be necessary. 2. Organic liquids. Various organic liquids such as heat transfer oils can be considered for thermal storage applications up to about 350°C. Their upper temperature is usually limited by decomposition of the material itself rather than reaction with containment materials. Their specific heat tends to be higher, but their volumetric energy density lower, than that of molten salts. While organic liquids are relatively expensive, they do not require pressure containment, and are therefore often used for moderate temperature applications. 3. Inorganic liquids. For higher temperature applications (2~800°C or higher) certain liquid metals and a wide variety of molten salt mixtures can be considered. (a) Liquid metals. A great deal of experience has been gained in the practical application of Na and NaK in some nuclear reactor systems. These liquid metals have excellent heat transfer characteristics, good stability, and are non-corrosive at high temperature when properly contained and maintained, but their specific heats are rather low. Sodium and NaK are highly reactive at high temperature, with both air and water; however, practical methods have been developed for minimizing these hazards. Safety problems must not be underestimated, but cumulative operating experience indicates that liquid metals are not as troublesome as is widely believed. Liquid metals merit consideration for high temperature thermal storage applications, particularly where high heat transfer rates are needed. (b) Molten salts. The class of molten salt mixtures is large, diverse and well suited for certain kinds of high temperature energy storage applications, but the temperature dependence of thermal conductivity, viscosity and density is known for relatively few materials. Information on stability of salts and their compatibility with conventional containment materials at high temperature is also very limited. Notable exceptions include fluoride salts which have been extensively investigated for moderately high temperature nuclear reactor applications, and certain nitrate/nitrite mixtures which have been used extensively as heat transfer fluids and for heat treating in industry. 4. Hybrid systems. In some applications cost reductions or performance improvements can be realized through combintions of storage methods. One example is the increase in energy density through using a material’s latent heat of fusion as well as its sensible heat over a large temperature range. Another example is the use of a packed bed of low cost solids in combination with a sensible heat liquid. In principle this is similar to the more conventional gas/solid packed bed technology, except that the liquid accounts for a significant portion of the sensible heat stored. Particle sizes, void fraction, bed dimensions and flow rates are chosen so that heat exchange between fluid and solid occurs in a narrow moving zone. The result is analogous to the moving thermocline concept in an all liquid system, and a nearly constant output temperature can be maintained during discharge until shortly before the storage is depleted. Investigations of this concept are currently underway in the U.S. solar energy program. but various questions regarding fluid flow, heat transfer, cyclic stresses and material compatibility are not fully resolved. 5. Stratification control in liquid thermal storage. In most applications of sensible heat liquids it is important that the mixing of hot and cold liquids be avoided so as to minimize degradation of energies and to achieve optimum utilization of the storage media. The simplest technique is to store the hot fluid separately from the cold fluid. Because this method requires a total containment volume more than twice that of the storage medium, various methods are being explored for storing hot and cold fluid in the same tank. Techniques range from (a) the use of
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Proceedingsof a NATOconference
membranes for separating the hot and cold fluids, to (b) the preservation of a comparatively sharp natural thermocline between hot fluid in the upper part of the tank and the cold, more dense fluid in the lower part. Because containment costs for high temperature applications can account for a considerable percentage of the total storage system costs, further investigation is warranted on materials and design aspects for the membrane method, and on the effects of wall-induced convection upon the sharpness of a moving thermocline. movable or collapsible
D. Conclusions Sensible heat storage in liquids appears to be viable in all of the three areas of applications considered by the Working Group. As for any sensible heat storage method, it is intrinsically best suited to applications where the charging energy is obtained by lowering the temperature of a material, and the stored energy is later required for raising the temperature of the same, or another material. It is generally not as well suited as latent heat or some heat-of-reaction storage methods for applications where both the source and the load are isothermal, unless the application has excess availability. Conversely, isothermal types of energy storage tend to be ill-suited to applications where the temperatures of both source and load vary over a wide range. III. LATENT HEAT STORAGE
A. General considerations For practical applications, only solid-liquid phase change materials are useful because they store a relatively large quantity of heat over a narrow temperature range, without a la:qe volume change. 1. Advantages. The advantages of these systems are: (a) The higher energy density, compared with sensible heat storage, allows a smaller storage volume and weight. (b) For processes requiring heat input at constant temperatures (e.g. Stirling engines or evaporating water systems), heat transfer requirements are less stringent. (c) Preliminary estimates indicate that the economics are comparable with those for pumped hydro, at least in large scale units. 2. Problems. Depending on the particular system, some or all of the following problems may occur: (a) Expansion and contraction during melting and freezing. (b) Corrosion. (c) Toxicity. (d) Safety hazards. (e) Impairment of heat transfer due to film freezing or formation of a gap between the heat exchange surface and the solid salt by thermal contraction. 3. Objectives for research and development. (a) Existing tables of relevant data on potential heat storage materials, both simple salts and eutectics, should be extended, especially to provide values of thermal conductivity, expansion coefficient of the solid, and densities of the liquid phase in the temperature range of interest. (b) Optimized designs should be developed, especially with respect to heat exchange and economics. (c) New heat transfer concepts should be investigated, e.g. direct contact of the heat transfer medium with the storage material in order to increase heat exchange surface and overcome contraction gaps. (d) A search should be made for inhibitors, protective coatings, and other means of inhibiting corrosion without unduly impairing heat transfer. (e) Studies must be made of environmental impact, consequences of leaks and disposal problems. B . Materials
Most of the materials proposed for high-temperature (120-1400°C) energy storage are either inorganic salts or metals. Among the metals, aluminium, magnesium and zinc have been mentioned as suitable examples. Most metals can be ruled out by considerations such as high price, low heat of fusion
Thermal energy storage (TES)
h
per unit weight or volume, unfavorable chemical properties, toxicity, safety, etc. Use of metal media may be advantageous where high heat conductivity is required and where cost is of secondary importance. Among the inorganic salts, a large number of compounds and eutectics is available, with melting points covering the whole temperature range from 120 to 1400°C.A number of alkali or alkaline earth fluorides or their eutectics are available with superior heats of fusion per unit weight or volume. Below 450°C however, only salts with much smaller heats of fusion per unit weight or volume are available. In this range, sodium hydroxide and the alkali metal nitrates may be suitable. The latter exhibit a phase change in the solid state, accompanied by a volume change, which may have a destructive effect on the container with frequent cycling. Sulfates, carbonates, phosphates, nitrides, oxides and chlorides of the lighter metals (e.g. sodium, magnesium and aluminium) may also be considered as storage media. In general, fused salts present corrosion problems, and inhibitors are required. In the case of fluorides, aluminum powder has been shown an effective inhibitor. In the high temperature range, the heat capacities (sensible heat) of the fluorides, sodium hydroxide, and the nitrates are of the same magnitude as those of such conventional solid heat storage media as olivine, magnesite, cast iron, etc. Hence, they are especially suitable where the latent heat as well as the sensible heat can be utilized, e.g. in residential storage heating. Many of the potentially useful latent heat storage materials are relatively cheap and abundant, but at the present early stage of development, reliable cost estimates are difficult to make. C. Applications
Latent heat or phase-change storage may find application in the following fields: 1. Heat engines and power plants. Upstream storage of heat is mandatory in solar power plants. Both upstream and downstream storage may be useful in all types of power plants, e.g. for load levelling, spinning reserve, etc., but competitive storage methods are also available. Upstream latent heat storage has the advantage that approximately isothermal conditions can be maintained and matched to the performance of the engine. This is particularly important for the efficiency of Stirling or Rankine engines. Many latent heat storage media have been thoroughly investigated, but further R & D is needed to develop and design practicable systems. 2. Industrial processes. Latent heat storage systems .will be advantageous in those industrial processes where a constant temperature is required, or where energy is available at a roughly constant temperature level. There is a wide variety of possible industrial applications. In the past it has generally been uneconomical to recover waste heat, and an enormous amount of energy is available if better methods of utilization can be developed. Energy storage may enable integration to be achieved between different processes. Much R & D will be necessary to develop practicable systems. 3. Domestic and commercial. While most applications of latent heat systems to homes and buildings involve storage at temperatures below 120°C a few involve higher temperatures. Absorption air conditioning can be best accomplished in the 120-150°C range. Storage for space heating is most practical at higher temperatures, with mixing to the desired lower temperature as needed. In all these cases, latent heat storage has potential benefits of a smaller volume storage unit, and hence lower heat losses and improved control of output conditions due to the constant temperature storage. Either solar or off-peak electrical energy could be used as the heat source, the first leading to a saving of premium fuel resources, and the latter having its major impact on capital requirements. Again, considerable R & D is needed to bring this technology to fruition.
IV. CHEMICAL
ENERGY STORAGE
A. General description Chemical reactions generally offer large energy storage densities due to the highly energetic processes involved in the reversible destruction and reformation of chemical bonds. Catalytic reactions are particularly attractive because the products of the endothermic reaction contain the concommitant thermal energy as chemical bond energy which is retrievable only by the reverse ECY Vol. 2. No. I-E
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reaction over an appropriate catalyst. Consequently, the products are stable at low temperatures and can be transported over long distances. Thermal dissociation reactions are also attractive in this respect if the products of the endothermic dissociation exist as two phases and are easily separable. The separation of reaction products plays the role of the catalyst in catalytic reactions and the insulation in sensible and latent heat storage systems. Research in the area of chemical energy storage has been concentrated on these two classes of reactions, and further work in the low (<12O”C), intermediate (120-4OO”C)and high temperature (400-1000°C) regimes is recommended. 0. Catalytic reactions
Development efforts in this category have been predominantly centered on the steam reforming reaction CH, + H,O=CO + 3H2, which takes place in the endothermic direction in the range of 85&12OO”Cand in the range of 35&7OO”Cfor the exothermic direction. The heat of reaction at 25°C is 49.8 kcal/gmole. A major application is considered to lie in the transmission of high grade thermal energy with storage affected by pressure modulations in the pipelines or by containment either in steel tanks above ground or in underground mined caverns. This application is referred to as the “Chemical-Heat Pipe” and the “Eva-Adam Process” which is under construction at a demonstration stage in Germany.t Preliminary economic studies which have been based upon high grade-energy from a nuclear heat source have been favorable, and indicate, for example, that: (a) onsite chemical storage for utilities is competitive with underground pumped hydrostorage; (b) transmission of thermal energy to large industrial users (>lOO MW,) may compare favorably with user generation by coal combustion with emission controls; (c) it is environmentally attractive and may be economically feasible for utilities to operate their transmission lines in a base load mode with distributed peaking plants in large load centers driven by clean thermal energy which is transmitted underground from the central plant and stored in the chemical heat pipe. The technology associated with the chemical heat pipe is available and could be applied to high temperature nuclear or solar sources or to a topping cycle in industrial boilers. The reversible decomposition of sulfur trioxide, SO&SO, +tO,, has been proposed as a chemical storage technique. SO, is decomposed at 725°C; the SO, is stored as a liquid and O2as a compressed gas. The technology for the oxidation step is well known but the decomposition step needs further study. Laboratory studies are underway. Research is required for reversible reactions that can be driven with lower temperature heat sources such as the light water reactor (-300°C) or the breeder reactor and focusing solar plants (-500°C). Preliminary indications are that organic hydrogenation reactions such as the hydrogenation of benzene are strong contenders, and laboratory studies have been instituted. However, more studies are required on the economics and the detailed reaction kinetics, catalyst stability, and selectivity characteristics of candidate reactions and materials. C. Themal dissociation reactions
Thermal dissociation of solid materials resulting in the formation of solid and gas phases are thermodynamically constrained by monotonic pressure-temperature relationships. Consequently, the design and operation of the resulting thermal storage schemes rely on the modulation of pressures on the store as well as on advantageous thermal effects of the dissociation reaction. Hydration-dehydration reactions are available for application in several temperature regimes. For example, Ca(OH), -+ CaO + HzO, takes place at 520°C and provides a high energy density in storage. Laboratory studies of prototypes are being conducted. Coupled reactions involving metal hydrides appear to offer the very desirable characteristics of rapid kinetics and high storage densities (-360 MJ/m3). A reaction of the type metal hydridessH* + M has considerable promise for heating, for the flexible use of thermal energy cooling, and energy conversion purposes at modest temperature now that alloy hydrides are available decomposing at temperatures which depend on composition. For example, rare tThis expression derives from the acronym EVA of the German description of the rig used for the heat input: EinzelspaltrohrVersuchsAnlage (single cracking-tube experimental rig). The name for the output end (catalytic combination of CO and H,) was prompted by the recognition that every Eve needs an Adam.
Thermal
energystorage033
67
earth-nickel alloys of the AB5 type can be used over a very wide range of temperatures, depending upon composition. By connecting a reactor containing, e.g. lanthanum nickel hydride with a second reactor containing “mischmetall”t nickel hydride and by maintaining each reactor at a separate temperature, hydrogen can be made to flow (with appropriate control systems), from one reactor to the other. The system may be used to store heat or cold, or the hydrogen at high pressure can be made to do mechanical work as it passes from one reactor to the other. The rates of compound formation are very rapid (limited by heat transfer capability) and are reversible. The energy storage density is very high (940 M.J/m3).The feasibility of the system has been proven in the laboratory, and larger scale systems now need developing. The success of this interesting scheme in the next ten years depends upon the ability to reduce the cost of rare earths and rare earth alloys by a factor of about 10. The reaction, CaC12.6NH,$CaCl, . NH, + 5NH, is being investigated for its application to thermal energy transmission. Vehicular transport is envisioned for the solid ammoniates and liquified ammonia. Ammoniated salts are being evaluated for utility power plant applications. V. SUMMARY
AND
CONCLUSIONS
It is concluded that high temperature heat storage offers great potential for reducing capital costs, and saving premium fuels and could be extremely important in solar energy applications. The technologies and areas of application which have been considered are summarized in Table 1. Crosses indicate existing or potential matching between a particular technology and a particular application area. It was generally agreed that the greatest near term potential for energy saving lies in the more efficient utilization of industrial process heat and the integration of power generation, process heat production, and waste heat recovery for space heating. For most effective use of capital, the best near-term potential involves improved storage of heat produced by off-peak power for house heating. The technology closest at hand is sensible heat storage. Based on potential benefits and probability of success, the most promising areas are: -Pressurized water storage -Sensible heat storage in organic liquids for feed water heating -Packed solid beds -Fluidized solids. Above-ground storage of hot water is in use now, but great potential exists for large scale storage in underground strata. Technologies with good long-term prospects involve latent heat storage and chemical storage schemes. The most promising areas involve: -Inorganic phase change materials (e.g. fluoride eutectics) -Chemical heat pipe (e.g. Eva-Adam). Further development of latent heat storage systems, especially work on corrosion prevention, heat transfer problems, and systems development should be encouraged, in order to take advantage of the superior storage capabilities of these materials. To adapt the chemical heat pipe principle to lower temperature sources (e.g. LWR), the development of alternate chemical storage systems should be encouraged. Thermal storage for solar applications
Practical, low cost methods for storing large amounts of high temperature energy appears to be singularly important for solar-thermal conversion to electrical power and other potential industrial processes. In the other high temperature applications discussed, thermal storage is generally aimed either at better utilization of primary fossil fuels (e.g. recovery and use of waste heat) or better use of capital investment (e.g. load leveling). In solar electric conversion. thermal storage is a key technology for successful exploitation of this energy source on a significant scale. Thermal storage will permit the intermittent solar energy to be used later upon demand. t”Mischmetall”-a
naturally-occurring
ore containing
a number of rare earth compounds.
-~_F..
Solar
Nuclear __”___.
Fossil -____.._
Integration
_________
__--
_.-_.-
-_
Y
X ________
X
__._
x __._.____
X -.-_-..
i<
X
X
A
X
~-------.-
x X
-_.
X X
X
---
----
X
-_-----.-~~
--
X
x _..--
X
X
X
X
X
x
-_-__-
X
X
X
-p
X - ---__-_~--
X
----.
X
X
~._..--_~-_~--__
X
X
X
X
__-
___-_____
---------_____---
X
X
X
X
X
X
X __I_-__
_.~
X
X ~_____
X
X
X
X
X
-X ~--__
X
X
X -__
X
~-~~ __-~
X
X
X
X
X
X -_____
X
X
-___-
Dissociative (Set IV C)
X
X
Salts
Catalytic (Set IV B)
X
X
Metals
Metals Salts (Set III B 1) (Set III B 2)
Chemical
x
x
UG
Org. liquid
Liquids (Set 11 C)
Latent
X
.__.__c_I____--“-_I___l_-~
--“---.---~-_
AG
Hot water
Sensible
X
X
UG
Fluidized beds
Solids (Set 11 B)
Packed beds
AG
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___
Process-to-PI,ocess --___ _.---__--
Regeneration ---__ .-.._-._-,
--~--_-_---l
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Food
District
Heat
Water
Transport __-_~_---_--..__.-~---_--~AG = Above Groilnd UG = Underground
Engines
Heat
-_~_--~~___._.
Industrial
-_
Domestic and Commercial
Heat
Space
Application
Table I.
Thermal energy storage (TES)
69
REFERENCES 1. T. T. Bramlett et al., Survey of high temperature thermal energy storage. Sandia Labs., ISAND 75-8063,US ERDA (March 1976). 2. J. D. Brumleve, Sensible heat storage in liquids. Sandia Labs., SLL-733-0263,US ERDA (Julv 1974). 3. D. E. Elliott, Fluidized-bed boilers. VDI Bericht, Vol. 236. VDI-Verlag Dusseldorf (1975). 4. P. U. Gilli and G. Beckmann, Lines of development in energy engineering. Vol. 236. VDI-Bericht. VDI-Verlag Dusseldorf (1975). 5. I. Glendenning, Storing and energy of compressed air. CEGB Research. CEGB, Marchwood Engng Lab.. Southampton (May 1975). 6. D. C. Golibersuch, F. P. Bundy, P. G. Kosky and H. B. Vakil, Thermal Energy Storage for utility Applications. Proc. of Electrochemical Society Symp. on Energy Storage, Dallas, Texas, (Oct. 1975). 7. C. F. Meyer, Status report on heat storage wells. Water Resources Bulletin (March 1976). 8. P. H. Morgen, Thermal energy storage in rock chambers. Proc. of the 401 Inr. Conf. on the Peaceful Uses of Atomic Energy 4, 177 (Sept. 1971). 9. F. E. Schilling and B. Beine, The prestressed cast iron reactor vessel. Nucl. Engng Design 25, 315 (1973). IO. R. Schulten et ol., Chemical latent heat for transport of nuclear energy over long distances. Proc. of British Nucl. Sot. Inr. Conf. on The High Temperature Reactor and Process Applications (1974). II. M. Tenenbaum et al.. Energy and the U.S. steel industry. Proc. of 5th Annual Conf. IZSI. Toronto (1971).