Review on sustainable thermal energy storage technologies -HEAT STORAGE MATERIALS AND TECHNIQUES

REVIEW ON SUSTAINABLE THERMAL ENERGY STORAGE TECHNOLOGIES,PART I:HEAT STORAGEMATERIALS AND TECHNIQUESS.M.HASNAINEnergy Research Institute,King Abdulaziz City for Science and Technology,P.O.Box 6086,Riyadh 11442,Saudi Arabia(Received 9July 1997)Abstract ÐThis paper reviews the development of available thermal energy storage (TES)technologies and their individual pros and cons for space and water heating applications.Traditionally,available heat has been stored in the form of sensible heat (typically by raising the temperature of water,rocks,etc.)for later use.In most of the low temperature applications,water is being used as a storage tent heat storage on the other hand,is a young and developing technology which has found considerable interest in recent times due to its operational advantages of smaller temperature swing,smaller size and lower weight per unit of storage capacity.It has been demonstrated that,for the devel-opment of a latent heat thermal energy storage system,the choice of the phase change material (PCM)plays an important role in addition to heat transfer mechanisms in the PCM.Attempts have also been made to utilize technical grade phase change materials as storage media and embedded heat exchange tubes/heat pipes with extended surfaces in order to enhance the heat transfer rate to/from the PCM.#1998Elsevier Science Ltd.All rights reservedSolar energy applications Thermal energy storage Sensible heat storagePhase changematerial Extended surfaces Heat pipeINTRODUCTIONStorage of thermal energy is very important in many engineering applications.For example,among the practical problems involved in solar energy systems is the need for an e ective means by which the excess heat collected during periods of bright sunshine can be stored,preserved and later released for utilization during the night or other periods.Similar problems arise for waste heat recovery systems where the waste heat availability and utilization periods are di er-ent.Heat storage can also be applied in most types of buildings where heating needs are signi®-cant and electricity rates allow heat storage to be competitive with other forms of heating.There are numerous types of commercially available systems,and many new developments in the technology are taking place.Therefore,the scope of this paper is to review sensible heat sto-rage and latent heat storage methods for space and water heating applications.In a second paper (Part II),a state-of-the-art on cool energy storage technologies as a Demand Side Management (DSM)as well as a Supply Side Management (SSM)tool for electric load manage-ment is discussed.The basic types of thermal energy storage techniques can be described as:Sensible heat sto-rage ,in which the temperature of the storage material varies with the amount of energy stored,and latent heat storage ,which makes use of the energy stored when a substance changes from one phase to another by melting (as from ice to water).Typical data of some relevant properties of heat storage materials used in thermal stores are given in Table 1for comparison.The last row in this table gives the relative volume occupied by each of the storage media for a heat sto-rage capacity of 106kJ and a temperature rise of 15K during heat storage [1].SENSIBLE HEAT STORAGESensible heat storage is e ected by raising the temperature of the storage medium.Thus,it is desirable for the storage medium to have high speci®c heat capacity,long term stability under thermal cycling,compatibility with its containment and,most importantly,low cost.Sensible heat storage may be classi®ed on the basis of the heat storage media as Liquid media storageEnergy Convers.Mgmt Vol.39,No.11,pp.1127±1138,1998#1998Elsevier Science Ltd.All rights reservedPrinted in Great Britain 0196-8904/98$19.00+0.00PII:S0196-8904(98)00025-91127(like water,oil based ¯uids,molten salts etc.)and Solid media storage (like rocks,metals and others).Liquid media storageHeat storage liquids are plentiful and economically competitive.The pros and cons of some of the selected media are given below.Storage in water.At low temperature water is one of the best storage media.It has higher speci®c heat than other materials,and it is cheap and widely available.However,due to its high vapor pressure,it requires costly insulation and pressure withstanding containment for high tem-perature applications.Water can be used over a wide range of temperature,say 25±908C.For a 608C temperature change,water will store 250kJ/kg or 2.5Â105kJ/m 3.Hot water is required for washing,bathing etc.,and it is commonly employed in radiators for space heating.Water can be used as a storage and as a transport medium of energy,for example,in a solar energy system.Consequently,it is the most widely used storage medium today for solar based warm water and space heating applications [2,3].Because of its simplicity,a large amount of pub-lished data are available on the design criteria for water storage media [1±4].Thermal strati®cation or thermocline in a solar water heat storage tank can be established due to the buoyancy forces,which ensure the highest temperature at the top and the lowest tem-perature at the bottom of the tank.Strati®cation is achieved through the elimination of mixing during storage,whereby a two-fold advantage is gained [2,3]:(i)the e ciency with which the energy can be used will be improved if it is supplied to the load at the temperature it was col-lected rather than at a lower mixed storage temperature and (ii)the amount of energy collected may be increased if the collector inlet ¯uid temperature is lower than the mixed storage tempera-ture.Water storage tanks are made from a wide variety of materials,like steel,aluminum,re-inforced concrete and ®ber glass.The tanks are insulated with glass wool,mineral wool or poly-urethane.The sizes of the tanks used vary from a few hundred liters to a few thousand cubic meters.For large scale storage applications,underground natural aquifers have been considered.Aquifers are geological formations containing ground water,o ering a potential way of storing heat for long periods of time.The storage medium in aquifers consists of water saturated gravel or rge storage volumes are available,as typical aquifer sizes range from hundreds of thousands to millions of cubic meters.For example,105m 3of aquifer material can store about 3MJ of heat for each 10K temperature di erence.This type of storage is well suited for seaso-nal storage,i.e.transferring heat from the summer season to the heating season.Because of its bulk nature,aquifer storage is not feasible for small loads,e.g.individual houses.The usefulness of aquifer thermal energy storage has been widely acknowledged,and several experimental and theoretical studies on them have been performed [5,6].However,because of the multitude of di erent systems and physical parameters,a more detailed computer simulation study is needed to determine the optimum system con®guration.The attractiveness of aquifer storage is due to its low cost characteristics,its high input/output rates and its large capacity.Table parison of various heat storage media (stored energy =106kJ =300kWh;D T =15K)Heat Storage MaterialSensible heat storagePhase Change Materials PropertyRock Water Organic Inorganic Latent heat of fusion (kJ/kg)**190230Speci®c heat (kJ/kg) 1.0 4.2 2.0 2.0Density (kg/m 3)224010008001600Storage mass for storing 106kJ (kg)670001600053004350Relative mass **154 1.25 1.0Storage volume for storing 106kJ (m 3)3016 6.6 2.7Relative volume **1162.51.0*Latent heat of fusion is not of interest for sensible heat storage.**Relative mass and volume are based on latent heat storage in inorganic phase change materialsS.M.HASNAIN:REVIEW ON SUSTAINABLE THERMAL ENERGY1128S.M.HASNAIN:REVIEW ON SUSTAINABLE THERMAL ENERGY1129 Storage in salty water.Solar ponds o er a simple and economical method for collecting and storing large amounts of solar energy in the form of low temperature thermal energy(50±958C). They have potential in space heating and cooling applications,in supplying industrial process heat and in electric power generation.Solar ponds can be classi®ed according to four basic fac-tors:(a)convecting or non-convecting,(b)partitioned(multi-layered)or non-partitioned,(c) gelled or non-gelled and(d)separate collector and storage or in-pond storage.However,most of the research e orts are presently concentrated on the non-convecting salt gradient solar pond[7±9].In this type of solar pond,a density gradient is created by using water containing salt(or sea water),the concentration of which increases with depth from the surface.Sodium chloride(NaCl)and magnesium chloride(MgCl2)are the salts most commonly used in this type of pond.The salt gradient pond has a black or dark bottom where the solar radiation is absorbed,with a consequent increase in water temperature up to958C.Extraction of the ther-mal energy stored in the lower layers of the pond can easily be accomplished without disturbing the upper layers.The largest solar pond in the USA is a3355m2experimental facility in El Paso,Texas,which has operated reliably since its start-up in1986.The pond runs a70kWe or-ganic rankine-cycle turbine generator and a5000gallons per day desalting unit,while also pro-viding process heat to an adjacent food processing company.The pond has reached and sustained temperatures higher than908C in its heat storage zone,generated more than100kWe during peak power output and produced more than80,000gallons of potable water in a24h period[9].Storage in other¯uids.The most commonly proposed substitutes for water are petroleum based oils and molten salts.The heat capacities are25±40%of that of water on a weight basis. However,these substitutes have lower vapor pressure than water and are capable of operating at high temperatures exceeding3008C.The oils are limited to less than3508C due to stability and safety reasons and can be quite expensive.Some of the oil candidates that have been con-sidered are Therminol and Caloria-HT[2].A few molten mixtures of inorganic salts have been considered for high temperatures(3008C and above).One is sodium hydroxide which has a melting point of3208C and could be used for temperatures up to8008C[7].However,it is highly corrosive,and there is a di culty in containing it at high temperatures.Liquid metals are also mentioned as possible sensible heat storage media.While most of their properties are simi-lar to those of water,they,along with the petroleum based oils,have low speci®c heats and higher potential for reactivity with the container.They do,however,enjoy higher thermal con-ductivity.Type304stainless steel is the most common material used for containment of oils and liquid metals,with special attention usually given to the maintenance of an oxygen and oxide free environment in order to prevent corrosion[10].Solid media storageFor a low as well as high temperature thermal energy storage,solid materials such as rocks, metals,concrete,sand,bricks etc.,can be used.In this case,the energy can be stored at low or high temperatures,since these materials will not freeze or boil.The di culties of the high vapor pressure of water and the limitations of other liquids can be avoided by storing thermal energy as sensible heat in solids.Moreover,solids do not leak from their container.The highest pro-duct in the list of solid materials for sensible heat storage is cast iron,which exceeds the energy density level of water storage[11].However,cast iron is more expensive than stone or brick and,hence,the payback period is much longer.Pebble beds or rock piles are generally preferred as the storage material due to their low cost.Storage in rocks.The pebble bed or rock pile consists of a bed of loosely packed rock material through which the heat transport¯uid can¯ow.The thermal energy is stored in the packed bed by forcing heated air into the bed and utilized again by recirculating ambient air into the heated bed.The energy stored in a packed bed storage system depends,apart from the thermophysical properties of the material,on several parameters,including rock size and shape,packing density, heat transfer¯uid etc.Solar energy can also be stored in rocks or pebbles(packed in insulated vessels),and it is con-venient for use in buildings.This type of storage is used very often for temperatures up to 1008C in conjunction with solar air heaters[12].Typically,the characteristic size of the pieces ofrock used varies from 1to 5cm.An approximate thumb rule followed for sizing is to use 300±500kg of rock per square meter of solar collector area for space heating applications.For a temperature change of 508C,rocks and concrete will store around 36kJ/kg or about 105kJ/m 3.Rock or pebble bed storage can also be used for much higher temperatures,up to 10008C.King and Burns [12]use a number of characteristics,typically particle size,void fraction,bed cross-sectional area and bed length,super®cial air velocity and Reynolds number,in order to describe the thermal and geometric properties of packed beds.Like rock beds,¯uidized beds can be uti-lized for low,intermediate and high temperature solar applications [13].In addition,the rate of heat exchange between the heat carrying ¯uid and the storage medium is much faster in ¯ui-dized beds than in rock beds,which can be an advantage in several applications.The ¯uidized bed thermal storage can also be used for waste heat recovery purposes.Storage in building fabric.Heat storage is applicable to both new and existing buildings and can be integrated with both air and water distribution systems.The most prevalent storage ma-terial is ceramic brick,consisting of olivine,magnesite,microtherm or feolite.Building mass and structural cement can also be used with active or passive storage designs.Improvements in insu-lation and storage materials continue to be made by manufacturers.The most common con-®guration using building mass for thermal storage is ¯oor warming.The ¯oor becomes a large low temperature radiating surface with the concrete acting as the heat storage medium.The ¯oor can be warmed with a heat transfer ¯uid,such as water,with direct electric resistance wir-ing or with air ducts.Water is the most common heat transfer medium.Many buildings with ¯oor warming do not utilize o peak electricity to charge the storage,although they could be retro®tted with a control system to utilize o peak heating.An advanced phase change material for ¯oor storage heating has been developed by a Swiss manufacturer,with applications in the UK,Spain,Korea and Japan [14].Two other categories of building mass storage are building inertia and hollow core construc-tion.Controls to minimize electricity costs for heating by using the overall mass of the building to decrease heating during on peak periods are being developed [15,16].Use of inertia in build-ing mass is a technique where no additional HVAC equipment is installed but special controls are required.Hollow core ventilation systems were developed in Sweden,in which the supply air is distributed through the ¯oor slabs,coupling it to the building mass [17].Storage in metals.Most of the materials proposed for high temperature (120±14008C)energy storage are either inorganic salts or metals [11,18,19].Among the metals,aluminium,mag-nesium and zinc have been mentioned as suitable examples.The use of metal media may be ad-vantageous where high thermal conductivity is required and where cost is of secondary importance.Solid industrial wastes like copper slag,iron slag,cast iron slag,aluminium slag and copper chips could be used as storage material for energy storage.LATENT HEAT STORAGELatent heat storage is a particularly attractive technique,since it provides a high energy sto-rage density and has the capacity to store heat as latent heat of fusion at a constant temperature corresponding to the phase transition temperature of the phase change materials (PCMs).For example,in the case of water,80times as much energy is required to melt 1kg of ice as to raise the temperature of 1kg of water by 18C.This means that a much smaller weight and volume of material is needed to store a certain amount of energy.PCMs undergo solid±solid,liquid±gas,and solid±liquid phase transformations.Relatively few solid±solid PCMs have been identi®ed that have heats of fusion and transition temperatures suitable for thermal storage applications.Liquid±gas PCMs usually have high heats of trans-formations,however,due to the large volume change during transformation,they are not usually considered for practical applications.Solid±liquid PCMs are useful because they store a relatively large quantity of heat over a narrow temperature range,without a corresponding large volume change [20].Typically,the PCM is placed in long thin tubes stacked in a container.During a heat storage cycle,the collected solar heat from the collector is circulated through narrow spaces between the tubes and melts the PCM by storing the heat as sensible heat as well as latent heat of fusion.During the heat recovery cycle,the circulation of low temperature airS.M.HASNAIN:REVIEW ON SUSTAINABLE THERMAL ENERGY1130S.M.HASNAIN:REVIEW ON SUSTAINABLE THERMAL ENERGY1131 would pick up the stored energy from the PCM and transport it to the heat load.Therefore,the latent heat storage units utilize the sensible heat in the solid and liquid phases and additionally the latent heat due to a phase change during melting or freezing of the storage medium.Any latent heat thermal energy storage system must possess at least the three following basic components[11,20]:(a)a heat storage substance that undergoes a solid-to-liquid phase tran-sition in the required operating temperature range and where the bulk of the heat added is stored as the latent heat of fusion,(b)a container for holding the storage substance and(c)a heat exchanging surface for transferring heat from the heat source to the PCM and from the lat-ter to the heat sink.The development of a latent heat thermal energy storage(LHTES)system, therefore,involves an understanding of two essentially diverse subjects:heat storage materials (or PCMs)and heat exchangers.These two main subjects will be discussed in the following two sections.Phase change materials(PCMs)A large number of latent heat storage materials have been reported in the literature for ther-mal storage applications in temperature ranges suitable for heating and cooling[20±38].Some of them according to their temperature range are listed in Table2.PCMs can be classi®ed into the following major categories:inorganic compounds,organic compounds and eutectics of inor-ganic and/or organic compounds.Inorganic compounds include salt hydrates,salts,metals and alloys,whereas organic compounds are comprised of para ns,non-para ns and polyalcohols. Salt hydrates.A number of salt hydrates,such as sodium sulfate decahydrate(Glauber's salt), calcium chloride hexahydrate,sodium thiosulphate pentahydrate,sodium acetate trihydrate and barium hydroxide octahydrate,were investigated largely because of their low cost[23,24,27,30,31,35±39].Some of the literature and measured thermophysical properties of important PCMs are listed in Tables3and4.The contents of these tables show a di erence in literature and measured thermophysical properties of salt hydrates.It is worth noting that most manufacturers do not perform any tests on thermophysical properties of technical grade ma-terials,but instead,they rely on the available data for analytical grade materials.Major problems with most of the salt hydrates are supercooling and phase segregation. Supercooling has been prevented in several salt hydrates by adding nucleating agents or by pro-moting nucleation by rough container walls or using the cold®nger technique[24,28,30,34].To prevent phase segregation,several techniques,such as use of thickening agents,rotating storage devices and direct contact heat transfer,have been used[34,38].The encapsulation of PCMs in metal and plastic matrices has been tried extensively.The macro-encapsulation of PCMs can(i) avoid large phase separations,(ii)increase the rate of heat transfer and(iii)provide a self sup-porting structure for the PCM.The most cost e ective containers are plastic bottles and tin pla-ted food cans/mild steel cans[31].However,corrosion could lead to disastrous consequences if internal and external lacquer®nishes are not applied properly to the mild steel metal cans.tent heat storage materials and their temperature rangesTemperature Range(8C)Material Transition Temperature(8C)Heat of Fusion(kJ/kg)0±100Water0335Para n20±60140±280Salt hydrate30±50170±270100±400AlC13192280LiNO3250370Na2O2360314400±80050LiOH/50LiF427512KC1O45271253LiH6992678800±1500LiF868932NaF993750MgF21271936Si14151654T a b l e 3.M e l t i n g t e m p e r a t u r e s a n d l a t e n t h e a t o f f u s i o n o f s e l e c t e d t e c h n i c a l g r a d e p h a s e c h a n g e m a t e r i a l s (P C M )u s i n g D S C -2i n s t r u m e n t [38]O r i g i n a l p h a s e c h a n g e m a t e r i a l s C h e m i c a l S u p p l i e r sM e l t i n g t e m p e r a t u r e T m (8C )D T m =T m e x p e r .ÀT m l i t r a t .(8C )H e a t o f f u s i o n D H f (J /g )d D H fD H f e x p e r X ÀD H f l i t r a t X Â100D H f l i t r a t XL i t r a t .E x p e r .O n s e tP e a k O n s e t P e a k L i t r a t .E x p e r .(%)P a r a n W a x B D H c h e m i c a l s 4944.1651.64À4.842.64210186À11.43C a l c i u m c h l o r i d e h e x a h y d r a t e B D H c h e m i c a l s 2927.4530.05À1.551.05170161.15À5.2D i s o d i u m h y d r o g e n p h o s p h a t e d o d e c a h y d r a t e J o h n s o n M a t h e y c h e m i c a l s 3638.6541.552.655.55280269.7À3.68S o d i u m t h i o s u l p h a t e p e n t a h y d r a t e B D H c h e m i c a l s4849.8551.351.853.35200217.28.6L i t r a t .=L i t e r a t u r e v a l u e s ;E x p e r .=E x p e r i m e n t a l v a l u e s ;B D H =B r i t i s h D r u g H o u s e sT a b l e 4.T h e r m o p h y s i c a l p r o p e r t i e s o f s a l t h y d r a t e s m a t e r i a l sM e l t i n g p o i n t r a n g e (8C )H e a t o f f u s i o n (k J /k g )M a t e r i a ll i t e r a t u r em e a s u r e d a v e r a g e l i t e r a t u r e m e a s u r e da v e r a g eD e n s i t y (k g /m 3)C a l c i u m c h l o r i d e h e x a h y d r a t e 29.7[36]28.5±30.2[36]29[36]171[36]140±180[36]160[36]1634(C a C 12Á6H 2O )29*27.45±30.05[38]170*161.15[38]1630*s 1500*l S o d i u m c a r b o n a t e d e c a h y d r a t e (N a 2C O 3Á10H 2O )32±36±±247±±1442D i s o d i u m p h o s p h a t e d o d e c a h y d r a t e 34±372651522*s (N a 2H P O 4Á12H 2O )36**38±41.5[38]280**269.7[38]1520**1442**S o d i u m s u l f a t e d e c a h y d r a t e (N a 2S O 4Á10H 2O )31±32±±251±±1534s S o d i u m t h i o s u l f a t e p e n t a h y d r a t e 4848.5±49[36]48.7[36]201190±200[36]195[36]1666s (N a 2S 2O 3Á5H 2O )48*49.8±51.3[38]200*217.2[38]1730*s 1660*l*B r i t i s h D r u g H o u s e s (B D H )C h e m i c a l s .**J o h n s o n s ±M a t h e y C h e m i c a l s s =s o l i d ;l =l i q u i d .R e f e r e n c e s i n s q u a r e b r a c k e t sS.M.HASNAIN:REVIEW ON SUSTAINABLE THERMAL ENERGY1132S.M.HASNAIN:REVIEW ON SUSTAINABLE THERMAL ENERGY1133 Para ns.Para ns have been found to exhibit many desirable characteristics as PCMs for sto-rage applications,such as high heat of fusion,negligible supercooling,low vapor pressure in the melt,chemically inert and stable,self nucleating,no phase segregation and commercial avail-ability at reasonable cost[20,34,36±38].However,in spite of their many desirable properties, they have some undesirable properties,such as low thermal conductivity and large volume change during phase transition.Metallic®llers,metal matrix structures,®nned tube and alu-minium shavings were used to improve their thermal conductivity[38,40].To overcome the volume change on melting and freezing,plastic containers and di erent geometry of containers were used.Pure para n waxes are very expensive,therefore only technical grade para ns can be used for latent heat storage.Technical grade para ns are mixtures of many hydrocarbons and,therefore,have a melting temperature range rather than a sharp melting point.Some of the para ns investigated for heat storage applications include commercial waxes,n-eicosane and n-octadecane[37,38,40±42].Degradation of commercial grade para n wax was checked using Di erential Scanning Calorimetry(DSC)analysis after the completion of thermal cycling of wax in di erent types of tubes[38].Some of the results obtained from this study are reproduced in Table5.Quantitatively,the results of this study show that the addition of extended surfaces (aluminium honeycomb)in a tube reduces the melting temperature range by0.238C,but the as-sociated heat contents of the solid±liquid transition are increased by2.3%as compared to the original sample.The reason for this distortion could be the presence of impurities in the sample and,after extensive thermal cycling of the wax with metallic surfaces,might create hetero-geneous nucleation.Nevertheless,the(DSC)results with the technical grade para n wax did not show any indication that thermal cycling or contact with metal could degrade its thermal performance appreciably.The reported results showed[37,38]that one can not simply consider any available data on technical grade PCMs for designing an e ective heat storage device,as thermophysical properties vary from manufacturer to manufacturer,mainly due to di erent levels of impurities in technical grade PCMs.It is,therefore,recommended by Hasnain[38]to measure thermophysical properties of technical grade PCMs and not rely on published data unless at least the name of the supplier and purity level of the PCM are speci®ed.Non-para n organics.The non-para n organics include a wide variety of organic materials such as fatty acids,esters,alcohols and glycols.Hale et al.[22]reported about70non-para n organics that have melting points in the range of7to1878C and heats of fusion in the range of 42to250kJ/kg.Fatty acids have melting points suitable for heating applications and have heat of fusion values comparable to those of para ns and salt hydrates[43±45].They exhibit excel-lent melting/freezing characteristics without any supercooling.Their major drawback is their cost which is about three times greater than para ns.Feldman et al.[46,47],Galen and Brink[48],Hasan and Sayigh[49]and Hasan[50]performed investigations on a number of fatty acids by DSC measurement techniques.Some of the experimentally tested fatty acids are listed in Table6.This table reveals the di erent melting range of the same compound,probably due to di erent suppliers and di erent purity levels.The names of the suppliers and purity levels of the tested compounds are lacking in the literature.Fatty acid esters such as butyl stearate, vinyl stearate and methyl±12hydroxystearate,can be considered suitable for passive heat sto-rage.The compatibility of fatty acids with conventional materials of construction is another area which requires further investigation.Eutectics.Eutectics are mixtures of two or more salts which have de®nite melting/freezing points.Their behavior is analogous to congruent melting salt hydrates and have great potential for thermal energy storage applications.A large number of eutectics of inorganic and organic compounds have been reported in[7,34,39],and they can be classi®ed as inorganic eutectics,or-ganic eutectics and organic±inorganic eutectics.Some of the eutectics are listed in Table7. These eutectics can be used for cool and passive solar storage with low or medium temperature ¯at plate collectors,according to their melting point.Solid±solid PCMs,such as pentaerythritol(C5H12O4),pentaglycerine(CH3±C±(CH2±OH3), neopentyl glycol((CH3)2±C±(CH2OH)2)and their mixtures,were used as potential candidates for space heating and process heat applications[34].A highly crystalline polymer,such as high density polyethylene(HDPE)o ers de®nite advantages as a potential thermal energy storage material if it is rendered form stable by crosslinking.Salyer and Botham[51]demonstrated the。