This article was downloaded by: [Temple University Libraries]On: 22 May 2013, At: 19:05Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number:1072954 Registered office: Mortimer House, 37-41 Mortimer Street,London W1T 3JH, UK

Energy SourcesPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/ueso19

The Viability of ThermalEnergy StorageKamil KaygusuzPublished online: 29 Oct 2010.

To cite this article: Kamil Kaygusuz (1999): The Viability of Thermal EnergyStorage, Energy Sources, 21:8, 745-755

To link to this article: http://dx.doi.org/10.1080/00908319950014489

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions

This article may be used for research, teaching, and private studypurposes. Any substantial or systematic reproduction, redistribution,reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden.

The publisher does not give any warranty express or implied or makeany representation that the contents will be complete or accurate orup to date. The accuracy of any instructions, formulae, and drug dosesshould be independently verified with primary sources. The publishershall not be liable for any loss, actions, claims, proceedings, demand,or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.

The Viability of Therm al Energy Storage

ÇKAMIL KAYGUSUZ

Department of Chemistry

Karadeniz Technical Unive rsity

Trabzon, Turkey

With rising energy costs and an increasing dem and for renewable energy sources,( )therm al energy storage TES system s are becoming an interesting option . TES is a

key component of any successfu l therm al system and a good TES shou ld allow

m inimum therm al energy losses. In this study, various ways of therm al conservation

are outlined and discussed, both theoretical and experimental. In th is respect, the

TES systems and their practical applications and som e selection criteria have also

been given .

Keywords thermal energy storage , sensible he at storage , latent heat storage,phase change material, domestic heating, solar ene rgy

s .Thermal ene rgy storage TE S has always been one of the most critical components

in re sidential solar space he ating applications. Solar radiation is a time -dependent

energy source with an inte rmittent characte r. The heating demands of a re sidential

house are also time dependent. Howeve r, the ene rgy source and the demands of a

building, in ge neral, do not match e ach othe r, e specially in solar he ating applica-

tions. The pe ak solar radiation occurs near noon, but the pe ak he ating demand is

in the late eve ning when solar radiation is not available . Thermal energy storage

provides a re servoir of energy to adjust this mism atch and to mee t the energy

needs at all times. It is used as a bridge to cross the gap between the energy source,

the sun, the application, and the building. So, the rmal ene rgy storage is e ssential in

s .the solar heating system Huang e t al. 1986; KakacË e t al. 1989 .

Thermal energy storage is conside red advanced energy technology, and the re

has been an incre asing intere st in using this essential technique for the the rmal

applications such as he ating, hot wate r, air conditioning, and so on. The se le ction

sof the TES systems mainly depends on the storage pe riod required i.e ., diurnal or

.se asonal , e conomic viability, operating conditions, and the like . In practice , m any

research and deve lopment activitie s related to ene rgy have been concentrated on

e fficient ene rgy use and energy savings, le ading to ene rgy conservation. In this

regard, TES systems appe ar to be one of the most attractive the rmal applications

s .DincË e r e t al. 1996 .

This pre sent work give s both expe rimental and theore tical re sults about

thermal ene rgy storage for re sidential heating applications. In the experimental

study, the viability of the the rmal energy storage concept was tested experimentally

by using phase change materials as a storage medium. The obtained experimental

re sult was used to compare the technical and economical diffe rences between

latent he at storage and sensible energy storage systems.

Received 14 January 1998; accepted 23 February 1998.Address correspondence to K. Kaygusuz, Department of Chemistry, Keradeniz Techni-

cal University, 61080 Trabzon, Turkey. Fax: 0462 325 3195.

745

Energy Sources, 21:745 ] 755, 1999

Copyright Q 1999 Taylor & Francis

0090-8312 r 99 $12.00 q .00

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

19:

05 2

2 M

ay 2

013

K. Kaygu suz746

Types of Therm al Energy Storage

Thermal ene rgy storage basically can be classified according to the way he at is

stored: as sensible he at in hot liquids and solids, as latent he at in me lts and vapor,

as the rmochemical heat appe aring in chemical re actions, and as sorption heat in

adsorption processes. The following sections give de tailed explanations of the se

type s of storage .

Sensible Heat Storage

In the sensible he at storage technique , energy is stored by changing the tempera-

ture of the storage medium. The amount of energy stored by a sensible he at device

is proportional to the difference between the storage input and output tempera-

tures, the mass of the storage medium, and the medium’s he at capacity. E ach

medium has its own advantage s and disadvantage s, but advantages of sensible -he at

storage in liquids include pumpability and high-volume utilization. Their pumpabil-

ity suits them for he at transport as well as storage , thereby facilitating he at

exchange , reducing constraints on system ge ometry, and m aking possible positive

s .methods of temperature separation such as diaphragms KakacË et al. 1989 .

W ate r usually is the pre ferred liquid for temperatures between 0 ] 100 8 C. It is

plentiful, e ssentially free , nontoxic, nonflammable, e asily pumped, has excellent

thermal properties, and is only mildly corrosive in the absence of oxygen. It is used

mainly in building applications where storage subsystem concepts and designs

divide conveniently into those most suitable for passive space he ating, domestic

water he ating, diurnal storage in active space heating and cooling systems, and

se asonal storage. Concepts for process he at storage in water tend to be similar to

those for large domestic hot wate r systems and large diurnal space-he ating installa-

tions. Table 1 give s propertie s of some sensible he at storage media.

s .Thermal storage for the short term le ss than one we ek is implemented most.

Examples are warm-wate r boilers in homes, and ice storage for air-conditioning

plants in office s. Short-te rm storage systems lower the maximum demand capacity

on the supply side . As a consequence, power companies can, for example, lowe r the

maximum capacity se tting and operate their power stations more efficiently. They

s .stimulate the application of storage via rate s tariffs .

Table 1

Propertie s of some sensible he at storage media

Density Specific he at Cost3s . s .Material kg rm J rkg.K $ r1000 kg

W ate r 1000 4190 0.5 ] 1.0

Rock 2500 ] 3500 880 4.0 ] 6.0

Iron 7860 500 50 ] 70

Concre te 2250 650 20 ] 30

Engine oil 888 1880 250 ] 400

Therminol T-66 750 2100 700 ] 750

s .Note: Costs are liberal marketing costs in Trabzon 1996 .

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

19:

05 2

2 M

ay 2

013

The Viability of Therm al Energy Storage 747

s .The application of seasonal the rmal energy storage more than three months

is currently much le ss common. This is not due to a smalle r technical potential. On

the contrary, the re is a large amount of surplus heat in summer and surplus cold in

the winter. Further application is mainly de layed by economic factors, and to a

lesse r extent by technical ones.

In the past fifte en ye ars various applications of underground the rmal energy

storage have been studied. Much attention has been given to the deve lopment of

storage technique s. It appears that the deve lopment of a given type of storage

technique gre atly depends on local ge ological conditions. There fore , for example ,

in are as without natural storage structure s, small-scale storage systems in particu-

lar have been studied. The analysis of supply and demand systems has rece ived

little attention. Many we ll-functioning systems have been deve loped in the past few

ye ars which, howe ver, have not been implemented to a gre at extent.

Furthe rmore , it is remarkable that attention in the initial ye ars was primarily

aimed at he at storage . The drop in price s of prime energy and hydrotherm al

problems in storage have caused the number of applications of he at storage to be

re latively small. Attention to the application of cold storage has gre atly incre ased

in the past few ye ars, partly due to the increasing dem and for cooling in buildings

and the increased attention to environmental impacts caused by using chillers

s .UTES 1994 .

Latent Heat Storage

When a mate rial me lts or vaporizes, it absorbs heat; when it changes to a solid

s . s .crystallize s or to a liquid condenses , or m aybe to a gas, it gives back this he at.

This process of change of state , or phase change, can be used for storing he at.

s .Materials used for this purpose are called phase change mate rials PCMs . Typical

materials are water r ice , salt hydrate s, paraffin wax, and certain polymers. Energy

densities for latent heat storage are greate r than those for sensible heat storage,

s .re sulting in smaller and lighte r storage devices and lower storage losse s figure 1 .

The re lative ly constant temperature of storage can maximize colle ctor e fficiency

and minimize storage he at loss. Especially in a solar-assisted heat pump system for

domestic heating, the PCM stores energy from the solar colle ctors as a latent he at

s .at a ne arly constant transition temperature during me lting and solidification . It

can be used more preferably as a he at source than the wate r and rock storage for

a he at pump because the energy storage temperature of the PCM is around 25 ]s35 8 C for both calcium chloride hexahydrate and sodium sulfate decahydrate the se

.salt hydrates are the most commonly used . These temperature intervals are

suitable for solar-assisted he at pump applications in moderate climatic regions

s .Gultekin e t al. 1991; Kaygusuz 1988; Kaygusuz 1995 .ÈMany phase change mate rials have been investigated. While many studie s on

phase change he at the rmal ene rgy storage systems have been performed at

s .re latively low temperature s below 100 8 C for heat storage in home heating and

s .cooling units, studie s are sparse for higher temperature heat about 200 8 C used

for solar ene rgy systems and also for inte rmediate-temperature phase change

thermal energy storage. An ideal PCM must have the following feature s: appropri-

ate phase change temperature, high latent heat, low cost, ready availabil ity,

snontoxicity and nonflamm ability, uniform phase change characte ristics no subcool-

.ing or separation , and long life under repeated phase change .

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

19:

05 2

2 M

ay 2

013

K. Kaygu suz748

sFigure 1. Performance comparison of PCM, water, and rock storage systems Huang et al.

.1986

s .Abhat 1983 reviewe d low-temperature PCMs in the temperature range 0 ]120 8 C and inve stigated the ir me lting and freezing behavior. Short-te rm he at

storage systems, utilizing salt hydrates that melt in their crystallization wate r, have

sbeen proposed by many rese arche rs Morrison and Abde l-khalik 1978; Te lke s 1974;

.Jurinak and Abde l-khalik 1979; Huang e t al. 1986 . The most studied PCMs

include Glauber’ s salt, calcium chloride hexahydrate, sodium thiosulfate pentahy-

drate , sodium carbonate decahydrate , and disodium phosphate dodecahydrate . The

first two salt hydrates have especially rece ived conside rable attention because the se

are che ape r and have bette r thermal stability and resistance to corrosion than the

othe r salt hydrate s. The melting point, cost, and storage capacity of some salt

shydrates are given in table 2 Carlsson e t al. 1979; Parisini 1988; Brandste tte r 1988;

.Hahne 1996 .

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

19:

05 2

2 M

ay 2

013

The Viability of Therm al Energy Storage 749

Table 2

Propertie s and cost of some salt-hydrates

6Melting Density Latent he at Cost Storage 10 kJ3s . s . s .Salt-Hydrate point 8 C kg rm kJ r kg $ r 100 kg of $ kg

CaCl .6H O 29 1710 176 6 250 41662 2

Na SO .10H O 32 1460 252 5 170 34002 4 2

Na CO .10H O 33 1440 248 9 230 25552 3 2

Na HPO .12H O 36 1520 273 19 510 26842 4 2

Na S O .5H O 47 1665 210 30 1010 33662 2 3 2

s .Source: Costs are from the Chemical Marketing and Drug Report 1996 .

Thermochemical Energy Storage

The chemical he at storage unit so far store s he at at high or low temperature by

using chemical reactions. Despite a number of proposals regarding chemical

storage systems in chemical engineering rese arch, to our knowledge , the re has

been no bre akthrough in this fie ld so far. Howeve r, the re have been deve loped five

different chemical he at storage technologies: three for high-temperature storage

and two for low-temperature storage . Metal hydrite s have been examined for use in

chemical he at pumps using hydroge n at pressure s substantially lower than the

saturation pressure. Metallic salts combined with ammonia have also been exam-

ined as a me ans of the rmochemical ene rgy storage. Temperatures can be lower

than those of comparable sensible he at systems so that he at losse s during long-te rm

cycles can be reduced. The reve rsible thermal decomposition of several inorganic

s s . .substances e .g., Ca O H , CaCO , ZnSO , and NH HSO has recently been2 3 4 4 4

s .studied JETRO 1989; Casarin and Ibanez 1993 .

Exp erim en tal Setup

The water-based system inve stigated in this experimental study is shown in figure 2,

and the system parameters are listed in table 3. As shown in figure 2, this system

consists of the solar colle ctors, ene rgy storage tank filled by PCM, wate r-to-air he at

Figure 2. Schematic diagram of the base solar energy system

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

19:

05 2

2 M

ay 2

013

K. Kaygu suz750

Table 3

W ater-based system parameters

Collector

Number of glass cove rs 1

Thickness of glass cove r 0.004 m

Refractive index 1.45

Collector plate absorptance 0.90

Colle ctor emittance 0.85

Collector e fficiency factor 0.852

Back and side losse s 1.20 kJ r h m K2

Mass flow rate 40 kg r h m2

Total colle ctor area 30 m

Number of colle ctors 18

System circu it pipe

Length 40 m

Diameter 0.04 m

Heat loss 20 kJ r h K3s .Fluid density water 1000 kg rm

Fluid specific heat 4.197 kJ rkg K

Ambient temperature 18 8 CEnergy storage tank

3V olume 3.65 m

2s .Therm al loss 0.210 W r m . 8 Cs .Shape L r D 2.46

Initial temperature 18 8 C

exchange r, wate r circulating pump, and other me asuring and control equipment. A

de tailed description of the expe rimental se tup was given in previous studie s

s .Kaygusuz e t al. 1993; Kaygusuz 1993 .

Configuration of the Storage Tank

Figure 3 shows the configuration chosen for the storage tank. It consists of a vesse l

packed in the horizontal dire ction with cylindrical tubes. The ene rgy storage

s . smaterial calcium chloride hexahydrate is inside the tubes the tubes or containers

. s .are m ade of PVC plastic , and the heat transfe r fluid wate r flows paralle l to them.

The storage tank contains cylindrical PV C containe rs filled with PCM. The void

s .fraction the ratio between the fluid volume and the storage tank volume is 0.3.

The inside volume and inside surface are a of the ene rgy storage tank are ,

re spective ly, given by V and A . The number of cylindrical PVC containers insidest st

the storage tank is N . The radius of the cylinde r containe rs is r , and the length ofc c

the cylindrical tube containers is given by L. Also, the radius and length of the

energy storage tank are given by R and L , re spective ly. The r rL is 0.01 and thisst st c

ratio is small enough to minimize radial heat conduction in the storage m aterial

s .Kaygusuz 1995 .

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

19:

05 2

2 M

ay 2

013

The Viability of Therm al Energy Storage 751

Figure 3. Schematic configuration of the ene rgy storage tank

Results an d Discu ss ion

Figure 4 shows the temperature variation with time for Na SO .10H O as a PCM2 4 2

for an energy storage application. This figure was obtained during the he ating

pe riod of 1.0 kg of PCM in the sm all cylindrical stainless ste el container. A detailed

s .description of this container is given in a previous study Kaygusuz 1988 .

Figure 4. Temperature variation with time for sodium sulfate decahydrate 1: water inle t

temperature 2: water outle t temperature 3: temperature of PCM

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

19:

05 2

2 M

ay 2

013

K. Kaygu suz752

Figure 5 also shows the temperature variation with time for calcium chloride

hexahydrate as a PCM for an ene rgy storage application. This figure shows the

s .he ating pe riod of the PCM 1.0 kg in the same cylindrical containe r. Figures 4 and

5 show the the rm al behavior during the melting period for pure PCMs obtained

from Merck Chemical Company.

Figure 6 shows the temperatures of the indoor and outdoor air and outlet

water of the storage and total solar insolation with time of day for TES applica-

tion. As shown in figure 6, the total solar insolation was at a maximum value of

900 W rm2

at a local time of 12:00.

Figure 7 also shows the temperature variation of calcium chloride hexahydrate

with time of day in the storage tank shown in figure 3. As shown in figure 7, the re

Figure 5. Temperature variation with time for calcium chloride hexahydrate

Figure 6. Temperature and insolation variations with time of day 1: insolation 2: outle t

water temperature of storage 3: indoor air temperature 4: outdoor air temperature

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

19:

05 2

2 M

ay 2

013

The Viability of Therm al Energy Storage 753

Figure 7. Temperature variation of calcium chloride hexahydrate with time of day in

storage tank

T1: upper temperature

T2: middle temperature

T3: bottom temperature

is a temperature stratification in the tank, but differences between the three points

are small, so we say there is semistratification in the tank.

Figure 8 shows the variation of Ta, Tind, Tmean, Tcol2, Tcol1, and Tout with

time of day for the same TES system. Around solar noon, the temperatures of

storage outle t and collector inlet routlet are roughly at m aximum value because the

solar insolation is at maximum at solar noon. This re sult shows that the expe rimen-

tal re sults agree with theoretical re sults.

Con clus ions

Energy is a commodity for which we shall continue to pay an increasing price .

Furthe rmore, the inve stment and capital costs of powe r plants are high. In

addition, the environmental damage due to thermal power gene ration is an

important consideration for Turkey and all othe r countrie s. There fore , TES is a

key component for any the rmal system, and a well-designed TES system should

allow minimal the rmal energy losse s, le ading to energy savings, while permitting

the highe st possible extraction efficiency of the stored thermal energy.

From the experimental and theore tical inve stigations, we conclude that the r-

mal ene rgy storage is an important component in cold and moderate climatic

conditions such as Erzurum and Trabzon in Turkey, re spectively. In Erzurum, the

winter se ason is ve ry cold but has more sunny days. Therefore, using solar energy

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

19:

05 2

2 M

ay 2

013

K. Kaygu suz754

Figure 8. Temperature variation with time of day

Ta: ambient air temperature

Tind: indoor air temperature

Tm: mean temperature of storage

Tc2: collector outlet water temperature

Tc1: collector inle t water temperature

Tout: storage outlet temperature

with the TES system will give more ene rgy for domestic he ating. O n the other

hand, in the case of Trabzon, the winter se ason is mild but has more cloudy days.

There fore , a solar assisted he at pump system should be used with ene rgy storage

for residential he ating.

The following concluding remarks can be given from this study:

v Phase-change energy storage technology is still in the re search and deve lop-

ment stage and should not be ove rlooked because of its pre sent high cost

compared with water and rocks.v This technology could also be used to leve l the peak output of the utility

powe r ge neration and off-peak storage for residential heating applications

with air and water he at pumps.v The se lection of the thermal energy storage systems mainly depends on the

s .storage period required e.g., daily or se asonal , e conomic viability, and

operating conditions.v From the viewpoint of the rm al stability, corrosion e ffe ct, and economy,

sodium sulfate decahydrate and calcium chloride hexahydrate have more

advantage s compared with othe r phase change mate rials.v Substantial energy savings can be re alized by taking advantage of TES when

implementing techniques such as using waste ene rgy and surplus he at,

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

19:

05 2

2 M

ay 2

013

The Viability of Therm al Energy Storage 755

avoiding he ating and air conditioning equipment purchase s, and reducing

e le ctrical demand charge s.v Thermal energy storage can reduce the time or rate mismatch between

energy supply and energy dem and, the reby playing a vital role in energy

savings. Also, TES can play a vital role in mee ting socie ty’s needs for more

e fficient, environmentally benign energy use in various sectors.

Referen ces

Abhat, A. 1983. Low temperature latent heat thermal ene rgy storage: Heat storage materi-

s .als. Solar Energy 30 4 :313 ] 31.

Brandste tte r, A. 1988. On the stability of calcium chloride hexahydrate in thermal storage

s .systems. Solar Energy 41 2 :183 ] 91.

Carlsson, B., H. Stymne, and G. Wettermark. 1979. An incongruent he at-of-fusion

system } CaCl .6H O } made congruent through modification of the chemical compo-2 2

sition of the system. Solar Energy 23:343 ] 50.

Casarin, C., and G. Ibanez. 1993. Experimental demonstration of the principle s of thermal

s .ene rgy storage and chemical heat pumps, Journ al of Chem ical Education 70 2 :158 ] 62.

DincË er, I., S. Dost, and X. Li. 1996. Thermal ene rgy storage systems and energy savings.

Proceedings of the First Trabzon Internation al Energy and Env ironm ent Symposium , July

29 ] 31, Trabzon, Turkey.

Gultekin, N., T. Ayhan, and K. Kaygusuz. 1991. Heat storage chemical materials which canÈbe used for domestic he ating by he at pumps. Energy Conversion Management

s .32 4 :311 ] 17.

Hahne, E. 1996. Thermal conservation technologies. Proceedings of the First Trabzon Interna-

tional Energy and Environment Sym posium , July 29 ] 31, Trabzon, Turkey.

Huang, B. K., M. Toksoy, and Y. A. CË enge l. 1986. Transient response of latent heat storage

s .in greenhouse solar system. Solar Energy 37 4 :279 ] 92.

s .Japan External Trade O rganization JETRO . 1989. State of he at pump research and

deve lopment in Japan. Chemical Heat Pump, Special Issue , 1 ] 16.

Jurinak, J. J. and S. I. Abde l-khalik. 1979. Sizing phase -change energy storage units for

air-based solar he ating systems. Solar Energy 22:355 ] 59.

KakacË , S., E. PaykocË , and Y. Yener. 1989. Energy storage systems. NATO ASI Series, p. 129.

Dordrecht: Kluwer Academic.

Kaygusuz, K. 1988. Determination of the best suitable he at storage chemical materials which

can be used for domestic he ating by heat pumps. M.Sc. thesis, Department of Chem-

istry, Karadeniz Technical University, Trabzon, Turkey.

Kaygusuz, K. 1993. Investigation of the residential he ating by the solar assisted he at pumps

in the Black Sea region. Ph.D. thesis, Department of Chemistry, Karadeniz Technical

University, Trabzon, Turkey.

Kaygusuz, K. 1995. Experimental and theore tical investigation of latent heat storage for

s .water-based solar heating systems. Energy Conversion Managem ent 36 5 :315 ] 23.

Kaygusuz, K., N. Gultekin, and T. Ayhan. 1993. Solar-assisted he at pump and energy storageÈs .for domestic he ating in Turkey. Energy Conversion Man agem ent 34 5 :335 ] 46.

Morrison, D. J., and S. I. Abde l-khalik. 1978. Effects of phase -change energy storage on the

performance of air-based and liquid-based solar heating systems. Solar Energy 20:57 ] 67.

Parisini, F. C. 1988. Salt hydrates used for latent heat storage: Corrosion of metals and

s .re liability of thermal performance . Solar Energy 41 2 :193 ] 97.

Telkes, M. 1974. Solar energy storage. ASHRAE Journal :34 ] 44.

s .Underground Thermal Energy Storage UTES . 1995. State of the art 1994. Report published

by The Netherlands Agency for Energy and the Environment and the Swedish Council

for Building Rese arch.

Dow

nloa

ded

by [

Tem

ple

Uni

vers

ity L

ibra

ries

] at

19:

05 2

2 M

ay 2

013