Sd Article 32

diameter and the total volume of the steel tank used as the latent heat storage unit were 1.7 m and 11.6

m3, respectively. The LHS unit was lled with 6000 kg of paran, equivalent to 33.33 kg of PCM per

Keywords: Latent heat storage; Paran; Energy and exergy eciency

* Tel.: +90 322 3386408; fax: +90 322 3387165.

E-mail address: [email protected]

www.elsevier.com/locate/enconman

Energy Conversion and Management 46 (2005) 152315420196-8904/$ - see front matter 2004 Elsevier Ltd. All rights reserved.square meter of the greenhouse ground surface area. Energy and exergy analyses were applied in order

to evaluate the system eciency. The rate of heat transferred in the LHS unit ranged from 1.22 to 2.63

kW, whereas the rate of heat stored in the LHS unit was in the range of 0.652.1 kW. The average daily

rate of thermal exergy transferred and stored in the LHS unit were 111.2 W and 79.9 W, respectively. Dur-ing the experimental period, it was found that the average net energy and exergy eciencies were 40.4% and

4.2%, respectively. The eect of the temperature dierence of the heat transfer uid at the inlet and outlet of

the LHS unit on the computed values of the energy and exergy eciency is evaluated during the charging

period.

2004 Elsevier Ltd. All rights reserved.Experimental evaluation of energy and exergy eciency of aseasonal latent heat storage system for greenhouse heating

H. Huseyin Ozturk *

Department of Agricultural Machinery, Faculty of Agriculture, University of Cukurova, Balcali, 01330 Adana, Turkey

Received 7 January 2004; received in revised form 25 March 2004; accepted 18 July 2004

Available online 15 September 2004

Abstract

In the following work, a seasonal thermal energy storage using paran wax as a PCM with the latent

heat storage technique was attempted to heat the greenhouse of 180 m2 oor area. The system consists

mainly of ve units: (1) at plate solar air collectors (as heat collection unit), (2) latent heat storage(LHS) unit, (3) experimental greenhouse, (4) heat transfer unit and (5) data acquisition unit. The external

heat collection unit consisted of 27 m2 of south facing solar air heaters mounted at a 55 tilt angle. Thedoi:10.1016/j.enconman.2004.07.001

Nomenclature

A area, m2

cp specic heat, J/kg Kd diameter, mk heat transfer coecient, W/m2KL length, m_Q rate of heat transfer, Wt time, sT temperature, Ks thickness, m_v volumetric ow rate, m3/s_W power consumption, W

Greek letters_N rate of exergy transfer, WW exergy eciency, %a heat convection coecient, W/m2Kg energy eciency, %k heat conduction coecient, W/mKq density, kg/m3

Subscripts

a ambientb bottomc charginge equivalenth horizontali inlet, or innerl losso outlet, or outerp paranr references storedt transferred1 steel plate2 glass wool3 storage tank

1524 H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542

1. Introduction

Solar energy, an abundant, clean and safe source, is an attractive substitute for conventionalfuels for passive and active heating applications. During the day, excess solar heat is collectedfor short or long term storage, and it is recovered at night in order to satisfy the heating needsof greenhouses. Ecient and economical heat storage is the main factor in utilization of solar en-ergy for agricultural purposes. Solar thermal energy can be stored as sensible heat, latent heat,heat of reaction or a combination of these. In most storage systems, it is stored as sensible heatin materials such as water and rocks. In air collection systems, rock beds are normally used tostore heat, while water tanks store the heat in liquid systems. In latent heat storage (LHS) systems,the latent heat arising from the phase change of a material is used for thermal energy storage. Thephase change materials (PCMs) can store large amounts of heat (latent heat of fusion) in changingphase from solid to liquid. LHS systems using PCM, in general, provide much higher energy stor-age density than systems using sensible heat storage. For solar heating applications, the use ofLHS systems for thermal energy storage has become an attractive design option in terms ofconstruction cost and storage eciency. The eciency of seasonal storage as well as that of dailystorage depends on system congurations, climate conditions and various set points for environ-mental control.Several authors have investigated the present status, technical potentials and regional distribu-

tion of renewable energy resources in Turkey [118] and concluded that Turkey has extensiverenewable energy resources that can be developed as a signicant source of energy. Turkey alsohas great solar energy potential due to its location in the Mediterranean Region (36 and 42North latitudes). The sunshine period of Turkey is 2624 h/year with a maximum of 365 h/monthin July and a minimum of 103 h/month in December. The main solar radiation intensity is about3.67 kWh/m2 day. The cumulative total of this is about 1.340 MWh/m2 year. The amount of solarradiation received over all of Turkey, in other words, the gross solar energy potential is 3517 EJ/year [19]. With rising energy costs and an increasing demand for renewable energy sources, ther-mal energy storage (TES) systems are becoming an interesting option. TES is a key component ofany successful thermal system, and a good TES system should allow minimum thermal energylosses [20]. In recent years, TES has been recognized as a potentially signicant means by whichprimary energy consumption can be reduced in domestic, commercial and industrial processes[21]. Comprehensive studies have been conducted concerning the application and modelling ofLHS systems and the thermal properties of PCMs by many researchers [6,2249]. Latent heatthermal energy storage has been proved to be an eective method for utilization of clean energysources, such as solar energy, because of its high energy storage density and small temperaturevariation from storage to extraction. Therefore, a number of investigations that aimed at improv-ing the eciency of LHS systems have been done by some researchers in the design, modeling andtesting of LHS systems [5057]. El-Dessouky and Al-Juwayhel [58] developed second law analysisfor a phase change thermal energy storage system using paran wax and calcium chloride hexa-hydrate as the PCM.Consequently, this study is devoted to an analysis and operation of a large scale LHS system for

greenhouse heating. Energy and exergy analyses were applied for evaluating the eciency of a so-lar thermal latent heat storage application. In this experimental study, solar energy was stored

H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542 1525using paran as a PCM with the latent heat technique for heating a plastic greenhouse of

180 m2. In the present paper, the energy and exergy eciencies of a LHS system, using paran asthe PCM, dened as the ratio of the energy and exergy stored in the LHS unit to the energy andexergy originally delivered to the LHS unit, respectively, are introduced for the rst time. Thus,the performance of the LHS system for greenhouse heating was evaluated by energy and exergyanalyses, and the energy and exergy eciencies of the LHS system were compared during thecharging periods. The objectives of the present study are: (1) to achieve more ecient utilizationof solar energy in greenhouse heating systems; (2) to determine the annual fraction of the green-house heating demand that can be supplied by the LHS system under ambient conditions of theCukurova region in Turkey; (3) to investigate the thermal performance of the LHS system usingparan as a PCM under the operating conditions; (4) to design and build external collection andheat storage units based on paran with a melting temperature range of 4860 C and latent heatof melting of 190 kJ/kg; (5) to compare the energy eciency of the LHS system with its exergyeciency; and (6) to determine the feasibility of using a PCM as a possible alternative to otherstorage techniques for storage of solar thermal energy for the Cukurova climate.

2. Description of the system

Data Acquisition Unit

1526 H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542V E R TOP LAM A V E

KONTROL N TE S

4

5

1 2

3

FANA

FAN

B

IS I D EP O LAM A N TE S

IS I TOP LAM A N TE S

PLAS T K S E RA

IS I DE P OLAM A HATTI

Heat Storage Unit

Valves Valves

Flat Plate Solar Air Collectors

Greenhouse

Heat Storage

Heat RecoveryIn the following work, seasonal thermal energy storage using paran wax as a PCM with thelatent heat storage technique was attempted to heat a greenhouse of 180 m2 oor area. The sche-matic arrangement of the LHS system for greenhouse heating is given in Fig. 1. The system con-sists mainly of ve units: (1) at plate solar air collectors (as heat collection unit), (2) LHS unit, (3)experimental greenhouse, (4) heat transfer unit and (5) data acquisition unit. The greenhouseheating system utilizes seasonal latent heat storage located in the Cukurova region of Turkey.The constructional features of the LHS system are described in detail in the following sections.Fig. 1. The arrangement of the heat storage and greenhouse heating system.

2.1. Flat plate solar air collectors

Flat plate solar air collectors were used to collect solar energy. The external heat collection unitconsisted of 27 m2 of south facing solar air heaters mounted at a 55 tilt angle (Fig. 1). Solar aircollectors that have packed air ow passages were used in the heat collection unit. Each of thesolar air collectors has an absorber plate area of 1.5 m2. The Raschig ring type of packing wasused to increase the heat transfer from the plates to the heat transfer uid underneath the absor-ber plates of the air collectors. The characteristic diameter of the Raschig rings, made of polyvinylchloride (PVC) tube, was 0.05 m. The aluminum based absorber of the air heaters is covered witha 4 mm glass sheet, and the underside is insulated with glass wool. The dimensions of the air col-lectors are 1.9 0.9 m, and 18 of them form the external heat collection system. The absorber sur-face area of the installed air collectors was 0.225 m2 per square meter of the tunnel greenhouse

H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542 1527ground surface area. The heat collection unit was supplied with air from the environment by acentrifugal fan that had a volumetric ow rate of 600 m3/h. The heat collection unit was installedoutside the greenhouse and mounted on a galvanized steel structure.

2.2. Latent heat storage unit

A cylindrical steel tank was used as the seasonal LHS unit. The dimensions of the LHS unit andthe placement of the heat exchanger in the LHS unit are given in Fig. 2. The diameter and thetotal volume of the steel tank used as the LHS unit were 1.7 m and 11.6 m3, respectively. Thewhole surface of the LHS unit was insulated with 0.05 m of glass wool and 0.001 m of steel plate,respectively. The LHS unit was placed horizontally on the ground surface at a distance of 2 mfrom the experimental greenhouse. The LHS unit volume per square meter of greenhouse groundsurface area was 0.066 m3/m2, while the storage volume per square meter of the solar air collectorwas about 0.44 m3/m2. The LHS unit was attached to the experimental greenhouse by means ofpolyethylene (PE) pipes. Solar energy, collected by the solar air collectors, was transferred to theLHS unit by circulating air through the PE pipes. Perforated PE pipe as the heat exchanger wasinstalled in the LHS unit (Fig. 2). Perforated PE pipe was used to ensure direct contact betweenthe heat transfer uid and the PCM in the LHS unit. The total length and diameter of the perfo-rated PE pipe were 97 m and 0.1 m, respectively.

850

1700

5200

425

50Fig. 2. The latent heat storage unit and sensor placement; all dimensions in mm.

2.3. Heat storage material

In general, materials that have a large change in internal energy per unit volume minimize thespace needed to store energy. A large number of organic and inorganic substances are known tomelt with a high heat of fusion in any required temperature range (0120 C). However, for theiremployment as heat storage materials in LHS systems, these PCMs must exhibit certain desirablethermodynamic, kinetic and chemical properties. Moreover, economic considerations of cost andlarge scale availability of the materials must be considered. Paran was used as a PCM in thepresent LHS system due to several desirable properties. Parans qualify as heat of fusion storagematerials due to their availability in a large temperature range and their reasonably high heat offusion. Furthermore, they are known to freeze without subcooling. Because of cost considera-tions, however, only technical grade parans may be used as PCMs in LHS systems. Hawladeret al. [59] found that encapsulated paran wax shows a good potential as a storage materials.In the selection of PCM for the present LHS system, the following desirable properties of the

paran as a PCM were taken into account: high latent heat of fusion per unit mass, chemical sta-bility, melting in the desired operating temperature range, small volume changes during the phasetransition, self nucleating and fast phase transition, little subcooling during freezing, availabilityin large quantities, non-corrosiveness to construction materials, easily packaged and inexpensive.The distribution of Catoms of the technical grade paran used in this experiment is C22C45,and its purity (oil content) is 5%. The thermal properties of the paran used as the PCM weremeasured with a dierential scanning calorimeter (DSC): the melting temperature range andthe latent heat of fusion were 4860 C and 190 kJ/kg, respectively. The PCM required for thegreenhouse heating was calculated on the basis of the heat storage capacity per unit volume ofthe selected paran and the total heat requirement of the experimental greenhouse. The LHS unitwas lled with 6000 kg of paran, equivalent to 33.33 kg of PCM per square meter of the green-house ground surface area.

2.4. Experimental greenhouse

The experiments were conducted in a PE greenhouse that was aligned northsouth. The exper-imental greenhouse, consisting of galvanized steel tube, has continuous side openings operated bya rolling mechanism. The openings in the sidewalls, created by rolling a plastic lm up or down,were used for ventilation. The greenhouse was covered with a double skin PE material (thicknessof 0.35 mm) that contains ultraviolet (UV) and infrared (IR) stabilizers. The dimensions of thegreenhouse were: width 12 m; length 15 m and height 3 m. The warm air from the LHS unitwas distributed by perforated PE ducts lying on the ground surface inside the greenhouse. Thediameter of the PE ducts was 0.15 m.

2.5. Heat transfer unit

In this experiment, heat transfer with forced convection between the heat collection unit, LHSunit and the experimental greenhouse was accomplished with two centrifugal fans (fan A and fanB in Fig. 1). Two dierential thermostats were used to control the charging and discharging proc-

1528 H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542ess by controlling the centrifugal fans, which can be operated independently. When the dierence

between the outside air temperature and the LHS system temperature exceeds the preset value,one of the dierential thermostats activates fan A (charging fan). During the summer, heat col-lected by the at plate solar air collectors was used to charge the LHS unit. During the chargingoperation, the air from outside was circulated through the heat collection unit by the charging fanand then passed through the LHS unit. During the winter, when the air temperature of the exper-imental greenhouse fell below a preset value, fan B (discharging fan) was activated during theextraction operation. The discharging fan drove the greenhouse air through the LHS unit duringthe night and then returned it to the interior of the experimental greenhouse via the perforated PEducts. The operation of the electric motor activating the discharging fan was controlled by a timeclock between 22:00 and 02:00 h.

2.6. Operational procedures

There are many factors that aect the performance of a LHS system, such as the average tem-peratures of the LHS unit and the fractions of time during which the LHS unit is charged anddischarged. These factors depend primarily on the amount of incident solar energy relative to theheating load of the experimental greenhouse. In this experiment, the directions of the heat owbetween the heat collection unit, LHS unit and experimental greenhouse were controlled by vevalves. The ve valves and two fans, shown in Fig. 1, were installed to control the operationmodes of the LHS system. All valves and fans were activated and controlled by the computerprograms according to the data logger records and preset values of the outlet temperature ofthe at plate solar air collectors and the air temperature inside the greenhouse. During thecharging operation, valves 1, 2 and 4 were open, and 3 and 5 were closed. The warm air, comingfrom the heat collection unit, was circulated through the heat exchanger embedded in theparan in the LHS unit. The paran, used as a PCM in the LHS unit, changed its phase(solid-liquid phase transition) and stored heat (heat of fusion) during this process. During thedischarging process in the winter, valves 2, 3 and 5 were open, and 1 and 4 were closed. Duringthe discharging of the LHS unit, the paran released its latent heat and solidied (liquidsolidphase transition).

2.7. Data acquisition unit

All air temperatures at the inlets and outlets of the at plate solar air collectors and the LHS unitand inside the tunnel greenhouse were measured with thermistors. The range of the thermistorswas 20 to +80 C, and the accuracy was 0.2 C over the range 070 C. The sensors consistedof a stainless steel clad thermistor probe with a 5 m cable. The temperatures of the paran, as thePCM, and the circulating air, as the heat transfer uid, were measured with 2 kX thermistors dis-tributed uniformly throughout the LHS unit. Fifteen thermistors were placed in the LHS unit tomeasure the temperature of the paran and heat transfer uid. The thermistors that were used tomeasure the temperature of the paran and heat transfer uid were installed in three rows at 0.425m depth in the LHS unit (Fig. 2). The distance between the thermistor rows was 0.425 m. Thesethermistors were located at the distance of 0.85 m on each of the thermistor rows in the LHS unit.The sensors that were used to measure the temperature of the PCM were positioned in such a way

H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542 1529that the temperature of the paran was measured in the LHS unit. Four thermistors were located

to mavera

made

A

periods can be recorded as single values, representing the average, maximum or minimum readingfor the period. Recording of the temperatures was made at 1 s intervals and averaged over 30 min

1530 H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542in the experiment.

3. Energy and exergy analysis for the charging period

The rate of heat transfer _Qt in W from the heat collection unit to the LHS unit was calculatedduring the charging period by using the following equation:

_Qtt _vqcpT it T ot 1where _v is the volumetric ow rate of the charging uid in m3/s; q is the density of the heat transferuid in kg/m3; cp is the specic heat of the heat transfer uid in J/kg K; Ti and To are the inlet andoutlet temperatures of the heat transfer uid in K; and t is the time in s.The rate of heat stored in the LHS unit _Qs in W was determined with respect to the heat transfer

rate into the LHS unit and the heat losses from the LHS unit for the charging period:

_Qst _Qtt _Qlt 2where _Ql is the rate of the overall heat loss from the LHS unit in W. The rates of the heat lossesfrom the bottom and horizontal surfaces of the LHS unit and the overall heat loss were calculatedfrom:

_Qlt _Qlht _Qlbt 3inputs in the form of voltages, resistances, counts and frequencies. The recorded data were storedin memory for output to a printer or to a computer for storage on disk. Data can be retrievedfrom the logger, and the current readings of the sensors can be examined without interruptingthe logging process. Readings can be taken at regular intervals, which can be dierent for eachchannel. To optimize use of the logger memory, timed readings taken on a channel over specieddata logger was used for taking and storing readings from the sensors; it could accept digitalsors were used to measure air temperature inside the tunnel greenhouse. These sensors were placedat a height of 1.5 m at three dierent locations of the tunnel greenhouse. Two sensors were placedat each of the inlet, the middle and at the end of the tunnel greenhouse. The ow rates of the heattransfer uid were measured at the inlet and outlet of the heat storage unit. The ow rates of the airow were measured with an anemometer (OMEGAHHF7P1) with a range of 0.53.5 m/s and anaccuracy of 1%.of anodized aluminum, which protects the sensor against solar radiation and rain. Six sen-urements of the sensors. Air temperature inside the tunnel greenhouse was measured with 2 kXhermetically sealed thermistors with an accuracy of 0.1 C over the range 080 C; the accuracywas 0.13 C at 20 C. The sensors were mounted in an open cylindrical probe made from amaterial that has a low anity for water and that ts inside a cylindrical louvered radiation screeneasure the temperature of the heat transfer uid at the inlet and outlet of the LHS unit. Thege temperatures of the PCM and heat transfer uid were determined by averaging the meas-_Qlbt 2kbAbT pt T at 4

wherunit i

equa

LHS unit to the heat transfer from the heat collection unit. Then, the total energy eciency duringthe c

chargden

Th _

calcu

Similwere

H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542 1531ar to the energy eciency, the total and net exergy eciencies during the charging period_Ntt _Qtt T ra _vqcp ln T ot 8

where Tra is the reference ambient temperature in K.The rate of thermal exergy stored in the LHS unit _Ns in W was determined in relation to the

exergy transfer rate to the LHS unit and the exergy losses from the LHS unit for the chargingperiod:

_Nst _Ntt _Nlt 9where _Nl is the rate of thermal exergy loss from the LHS unit in W. During the charging period,the rate of thermal exergy losses associated with the heat losses from the LHS unit to the sur-roundings was evaluated as follows [6066]:

_Nlt _Qlt 1T atT pt

10e rate of thermal exergy transfer from the heat collection unit to the LHS unit Nt in W waslated during the charging period from the following equation [6066]:

T itgcnett s_Qtt _W 100 7

where _W is the power of the electric motor in W.ing period is considered, the net energy eciency during the charging period gc(net) in % wased as follows [6066]:

_Q tgctotalt s_Qtt 100 6

When the power consumed by the electric motor that was used to activate the charging fan for theharging period gc(total) in % can be formulated as follows:

_Q tconduction and convection coecients, the thickness of the insulating materials, the inner andouter surface areas of the LHS unit and the temperatures of the PCM and the ambient (seeAppendix A for the related equations).The energy eciency for the charging period was dened as the ratio of the heat stored in thel to that of the PCM. The overall heat loss coecient was calculated on the basis of the heatLHS unit in W/m K; Ab and Ah are the bottom and horizontal surface areas of the LHS unit inm2; Tp and Ta are the temperatures of the paran and the ambient air in K, respectively.It was assumed that heat was transferred from the LHS unit to the surroundings by conduction

and free convection. The temperature of the inner surfaces of the LHS unit was assumed to bee _Qlb and _Qlh are the rates of heat loss from the bottom and horizontal surfaces of the LHSn W; kb and kh are the heat transfer coecients for the bottom and horizontal surfaces of the

2_Qlht khAhT pt T at 5determined by the following equations, respectively.

exergy varied with time during the same charging period. The rate of thermal exergy stored in theLHS unit was in the range of 3.9116.9 W. The average daily rate of thermal exergy transferred

nicadiermal ethe qenergTh

follo

1532 H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542ntly dierent from the rate of heat stored in the LHS unit during the charging periods. Thisence is due to the fact that the quality of energy was taken into account to calculate the ther-xergy transfer [Eq. (9)]. However, in the calculation of thermal energy transfer [Eq. (2)], onlyuantity of the energy was taken into account. In other words, the useless part of the thermaly stored in the LHS unit was ignored in the calculation of the thermal exergy transfer.e dierences between the rates of heat and exergy stored in the LHS unit may be explained asand stored in the LHS unit were 111.2 and 79.9 W, respectively. Thus, the average daily rate ofexergy loss was 31.9 W during the experimental period. The rate of thermal exergy stored was sig-Wctotalt _Nst_Ntt

100 11

Wcnett _Nst

_Ntt _W 100 12

where Wc(net) is the net exergy eciency during the charging period in %; and Wc(total) is the totalexergy eciency during the charging period in %.

4. Results and discussion

There are a lot of factors aecting the eciency of LHS systems as solar energy collection/stor-age systems for greenhouse heating. The eciency of the heat storage system depends on the ther-mal and physical properties of the PCM, the heat storage temperature, the geometry of the heatexchanger and the system conguration. Furthermore, an exergy eciency of the LHS systems isa new approach to determine the net eciency of the system. The results of energy and exergyanalyses of the LHS system were evaluated for the charging periods.

4.1. The results of the energy and exergy analyses during the charging period

The rates of heat and thermal exergy stored in the LHS unit during the charging periods werecalculated by using Eqs. (2) and (9), respectively. The results of the energy and exergy analysesduring the charging periods are explained in the following sections. The changes of the rates ofheat and thermal exergy stored in the LHS unit during the charging periods are shown as a func-tion of time in Fig. 3. The rates of heat and thermal exergy stored in the LHS unit increased as theinlet temperature of the heat transfer uid increased during the charging period.The rate of heat transferred in the LHS unit ranged from 1.22 to 2.63 kW, whereas the rate of

heat stored in the LHS unit was in the range of 0.652.1 kW (Fig. 3a). The rate of heat stored inthe LHS unit, which was only 0.65 kW at 11:00 h, reached its maximum value (2.1 kW) at 14:00 hduring the experimental period. The average daily rates of the transferred and stored heat inthe LHS unit were 2.15 and 1.63 kW, respectively. The rates of heat loss from the LHS unit werein the range of 0.460.57 kW during the charging period. Fig. 3b shows that the rate of thermalws. (1) During the experimental period, the average daily rate of heat stored in the LHS unit

0

0.5

1

1.5

2

2.5

3

11 12 13 14 15 16 17Time, h

Rat

e of

hea

t tra

nsfe

r, W

Transferrred Stored Loss

50

100

150

200

Rat

e of

exe

rgy

trans

fer,

W

(a)

H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542 1533was higher than that of the thermal exergy. Since the rate of exergy depends on the temperature ofthe heat transfer uid and its surrounding, the rate of exergy increased as the dierence betweenthe inlet and outlet temperatures of the heat transfer uid increased during the charging periods.(2) The rate of exergy transfer is higher at high temperatures than that at low temperatures. (3)Moreover, the percentage increase in the thermal exergy at high temperatures was higher in com-parison with the thermal energy. In this study, it was also found that the percentage increase in thethermal exergy was higher than that of the thermal energy during the charging periods. These re-sults may be simply explained in terms of heat transfer considerations. The possible amount ofthermal energy stored in the LHS unit decreased as the dierence between the inlet and outlet tem-peratures of the heat transfer uid decreased during the charging periods. The percentage decreasein the possible amount of thermal energy stored in the LHS unit was higher in comparison withthe thermal exergy during the charging periods. The dierences between the thermal energy andexergy stored in the LHS unit are attributed to the dierences in the inlet and outlet temperaturesof the heat transfer uid and the ambient temperature.

4.2. Energy and exergy eciency during the charging period

The total and net energy and exergy eciencies of the heat storage system during the chargingperiod were calculated by using Eqs. (6), (7), (11) and (12), respectively. The changes of the energy

011 12 13 14 15 16 17

Time, hTransferred Stored Loss(b)

Fig. 3. The rates of heat and thermal exergy stored in the heat storage unit during charging.

and exergy eciencies during the experimental period are shown as a function of time in Fig. 4.The energy and exergy eciencies of the LHS system increased as the inlet temperature of the heattransfer uid increased.The total energy eciency ranged from 53.7% to 79.6%, while the net energy eciency was in

the range of 22.147.9% during the rst charging period (Fig. 4a). While the net exergy eciencyat 11:00 h was only 22.1%, it reached its maximum value (47.9%) at 14:00 h. During the experi-mental period, it was found that the average daily total and net energy eciencies were 74.3% and40.4%, respectively. On the other hand, the total exergy eciency ranged from 10.5% to 80.6%,whereas the net exergy eciency was in the range of 0.26.2% during the same charging period(Fig. 4b). The average daily total and net exergy eciencies were 65.2% and 4.2%, respectively.When the net energy eciency is compared with net exergy eciency during the charging per-

iod, the following results can be drawn: (1) The net energy eciency was always higher than thenet exergy eciency. This is expected because the total energy content of the hot air used as heattransfer uid is taken into account in order to calculate the net energy eciency. In other words,to calculate the net energy eciency, the quantity of the energy transferred is taken into account,and the quality of the energy transferred is neglected. (2) For all charging periods, similar resultswere obtained in terms of the net energy eciency of the LHS system. The average daily netenergy eciency of the LHS system remained nearly constant (approximately 75%) during thecharging periods. The net energy eciency of the heat storage system did not change much during

80

100

ncy,

%


20

40

60

11 12 13 14 15 16 17Time, h

Ener

gy e

ffici

e

Total efficiency Net efficiency

0

20

40

60

80

100

11 12 13 14 15 16 17Time, h

Tota

l exe

rgy

effic

ienc

y, %

0

2

4

6

8

10

Net

exe

rgy

effic

ienc

y, %

Net efficiency Total efficiency

(a)

(b)

Fig. 4. The change of eciency of the latent heat storage system during the charging period: (a) energy eciency;(b) exergy eciency.

the charging periods. However, the net exergy eciency of the LHS system changed during thecharging periods. The above results indicate that the net exergy eciency of the LHS systemwas always lower than the net energy eciency at the higher temperatures. (3) It was found thatthe average net exergy eciency was only 4.2% during the charging period. This result indicatesthat the heat storage system investigated in this study is inecient in terms of the exergy eciency.The eect of the temperature dierence of the heat transfer uid at the inlet and outlet of the

LHS unit on the computed values of the net energy and exergy eciencies during the chargingperiod is presented in Fig. 5. A linear regression of the plotted points was used to nd the rela-tionship between the net energy and exergy eciencies and temperature dierence in terms ofintercept (%) and slope (%/K). The temperature dierence in the LHS unit was observed to havemuch inuence on the net energy and exergy eciencies of the LHS system, their values beinghigher for higher temperature dierences. In other words, the net energy and exergy ecienciesincreased as the temperature dierence increased during the charging period. Initially, the temper-ature dierence of the heat transfer uid at the inlet and outlet of the LHS unit was small, but it

The

H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542 1535were 98% and 96%, respectively. The results of the variance analysis (ANOVA) for the relation-ships between the net energy/exergy eciency and temperature dierence are given in Table 1. Therelationships between the net energy/exergy eciency and temperature dierence are statisticallyimportant (P = 2.59 105 and P = 1.29 104) at the 95% probability level. For that reason,the obtained regression equations (Eqs. (13) and (14)) represent the relationships between the

y = 1.1243x + 5.2961R2 = 0.9775

y = 0.2629x - 4.0143R2 = 0.9574

0

10

20

30

40

50

60

0 10 20 30 40 50

Temperature difference, K

Effic

ienc

y, %

Energy efficiency Exergy efficiencycnet

Wcnet 4:0143 0:2629DT R2 0:96 14linear regression coecient of determination (R2) for the net energy and exergy ecienciesincreased with time. It was obtained that the highest temperature dierence was 40 K in the LHSunit during the charging period. While the temperature dierence was only 16.1 K in the morning,it reached 40 K in the early afternoon. For the period of time covered by Fig. 4, the average tem-perature dierence in the LHS unit was 31.2 K. The net energy and exergy eciencies (%) as afunction of temperature dierence (K) during the charging period were (Fig. 5):

g 5:2961 1:1243DT R2 0:98 13Fig. 5. Eect of temperature dierence on eciency of the latent heat storage system.

Dierence 5 10.08 2.01 5 1.06 0.213

Sum 6 448.9 6 25.06

1536 H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542variables at the 5% signicant level. The results of the uncertainty analysis for any calculated re-sults are given in Table 2.The values for the net energy and exergy eciencies (%) were computed for a temperature dif-

ference of 50 K using the above determined relationships for the LHS unit. The net energy andexergy eciencies at a temperature dierence of 50 K were estimated to be 61.5% and 9.1% forthe LHS unit.

Table 2

The results of the regression analysis

For Equation (13) For Equation (14)

Coecients Standard error t Stat P value Coecients Standard error t Stat P value

a 5.29614 2.43997 2.170575 0.082083 4.01428 0.793599 5.05832 0.003906b 1.124323 0.076219 14.75128 2.59E-05 0.2629 0.02479 10.60504 0.000129

Standard error of regression equation 1.420112 0.461891Table 1

The results of the variance analysis (ANOVA)

For Eq. (13) For Eq. (14)

df SS MS F P df SS MS F P

Regression 1 438.8 438.8 217.6 2.59E 05 1 23.9 23.99 112.4 0.000129The PCM and the conguration of the heat exchanger dene the theoretical maximum limit ofthe net eciency of a LHS system. In fact, the eciency of a LHS system is more sensitive to theproperties of the PCMs than to its thermal losses. The dierences between the energy and exergyeciencies of the present LHS system may be explained as follows: (1) the exergy eciency of theLHS system increased at a fast rate until the temperature dierence was 40 K. The exergy e-ciency decreased sharply as the temperature dierence decreased during the charging period; (2)it was also found that the percentage increase in the exergy eciency of the LHS system was high-er than that of its energy eciency; (3) for the maximum temperature dierence in the LHS unit,the maximum net energy eciency of the LHS system was 47.9%, whereas its maximum net ex-ergy eciency was 6.2% during the experimental period. These results may be simply explained interms of heat transfer considerations.

5. Conclusions

The LHS system for greenhouse heating was analysed, and its energy and exergy eciencieswere obtained. The eects of various factors on the net energy and exergy eciencies were exam-ined. The following conclusions are general in nature and are expected to be valid, independent oflocation and weather.

5.1. Greenhouse heating with solar energy

The use of solar energy for greenhouse heating has gained an increasing acceptance during recentyears. The main problem is related to the selection and sizing, or more appropriately the passive oractive technology, for the specic application. Active solar systems applied to greenhouses can sup-ply a signicant part of the heating requirements. However, there are some problems related to thecost of the heat collection unit, of the occupied land, the backup system and the heat storage meth-ods. A signicant investment cost is necessary with active solar systems; especially for the metallicglazed collectors used as the heat collection unit. The investment cost of the present system was$US 3500. Basic and applied research and development is needed to improve performance analysis,reduce operation cost of installed systems, assure their long term trouble free operation and im-prove their eciency. The economics and thermal eciency of each new system must be carefullyevaluated in a total system context; otherwise, it is not certain if a concept has sucient merit towarrant further development for solar application. In each application, system studies with realisticcost estimates are needed to dene the economic worth of storage in the intermediate temperaturerange. An active solar system for greenhouse heating can be coupled with a heat pump in order toincrease its eciency. In addition, the storage volume and capacity of the heat storage materialshould be chosen carefully. Research and development on active solar systems for greenhouse heat-ing have given a certain number of technical solutions aimed at overcoming the above problems.

5.2. Heat storage material for greenhouse heating

For intermediate temperature sensible heat storage, the commercially available liquids are generallyexpensive. Research directed toward improving the lifetime of sensible heat storage uids could be use-ful. Solidmaterials are economicallymore attractive for high temperature storage than uids, and theirvolume requirements are nearly comparable. However, research and development are needed to ndheat transfer uids that can be used in direct contact with the solids over long time periods. Develop-ment and low cost containment and direct heat technology could signicantly reduce the cost of green-house heating systems with solar energy in the intermediate temperature range. Direct contact betweenthe solid storage media and a heat transfer uid is vital to minimise the cost of heat exchange in a sen-sible heat storage system. If the same heat exchanger is used for the charging and discharging proc-esses, considerable attention should be given to the design of the heat exchanger. In this case,technical limitations should be taken into account. LHS using PCMs, in general, provides much higherenergy storage density than systems using sensible heat storage. High energy storage densities over anarrow temperature range make PCMs attractive for greenhouse heating. There are problems withrepeatable cycling, heat transfer rate and containment that need to be solved before LHS systemscan become commercially and economically viable for intermediate temperatures. Future studiesshould focus on: LHS for greenhouse heating and modelling the eciency of the heat storage systems.Such experimental studies will be very useful to optimise the management of the heat storage systems.

5.3. Evaluation of the system eciency

The results of this study show that the dierence between the results of energy and exergy

H.H. Ozturk / Energy Conversion and Management 46 (2005) 15231542 1537analyses is signicant. Since exergy is a measure of the quality of energy, exergy eciency is

moretioncounreecdesigysis.desig

sive driving forces. Analyses of the thermal performance of systems, the costs of solar equipmentandtemandto deconcanalysed in order to optimise system eciency.

Equations for the heat transfer coecients for bottom and horizontal surfaces of the LHS unit


kbAb s3

k3Ab3e s2

k2Ab32e s1

k1Ab21e 1

aoAb1o

1

AhAh s3

k3Ah3e s2

k2Ah32e s1

k1Ah21e 1

aoAh1o

Equations for the equivalent surface areas for bottom and horizontal surfaces of the LHS unitFor the bottom surfaces:

Since the cross section of the LHS unit is a circle, the equivalent surface areas for the bottomsurfaces were calculated on the basis of the geometric mean of the inner and outer surface areas ofthe bottom surfaces of the LHS unit as follows:

Ab3e Ab3i Ab3o2

Ab3i pd23i=4 Ab3o pd3i 2s32=4

Ab32e Ab2o Ab3o2

Ab2o pd3o 2s3 s22=4

Ab21e Ab1o Ab2o Ab1o pd3o 2s3 s2 s12=4Appendix Athe costs of auxiliary energy systems can be used to determine the optimum size of the sys-components for a particular application. Exergy analysis is essential to cost eective designmanagement of the thermal energy storage systems. Therefore, exergy analysis must be usedsign thermal energy storage systems with the highest possible thermodynamic eciencies. Inlusion, the charging and discharging processes of a thermal energy storage system must bestorage system is not to store energy, but to store exergy. According to the results of the exper-iments, LHS systems are inherently inecient devices in terms of the exergy eciency. Exergyloss and auxiliary energy consumptions for the charging and discharging processes could be re-duced to improve the exergy eciency of the thermal energy storage systems for a particularapplication. The unit should be capable of receiving energy at the maximum rate without exces-signicant than energy eciency, and exergy analysis should be considered in the evalua-and comparison of thermal energy storage systems. Exergy analysis clearly takes into ac-t the loss of availability of heat in storage operations, and hence, it more correctlyts the thermodynamic and economic value of the storage operation. Optimisation of then and exploitation of thermal energy storage systems can be made by means of exergy anal-When optimising the thermodynamic eciency of a thermal energy storage system, bothn and operational parameters must be considered. The real purpose of a thermal energy2

Energy Convers Mgmt 1993;34(5):33546.

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a

[8] E

1

[9] E

(T

[10] S2

[11] D

T

[12] A

T

[13] Demirbas A. Turkeys energy overview beginning in the twenty-rst century. Energy Convers Mgmt

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urkey. Energy Convers Mgmt 2002;43(4):57589.For the horizontal surfaces:

Since the shape of the LHS unit is a cylinder, the equivalent surface areas for the horizontalsurfaces were calculated on the basis of the logarithmic mean of the inner and outer surface areasof the horizontal surfaces of the LHS unit as follows:

Ah3e Ah3o Ah3iln Ah3oAh3i

Ah3o pd3i 2s3L3 Ah3i pd3iL3

Ah32e Ah2o Ah3oln Ah2oAh3o

Ah2o pd3o 2s3 s2L3

Ah21e Ah1o Ah2oln Ah1oAh2o

Ah1o pd3o 2s3 s2 s1L3

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Experimental evaluation of energy and exergy efficiency of a seasonal latent heat storage system for greenhouse heatingIntroductionDescription of the systemFlat plate solar air collectorsLatent heat storage unitHeat storage materialExperimental greenhouseHeat transfer unitOperational proceduresData acquisition unit

Energy and exergy analysis for the charging periodResults and discussionThe results of the energy and exergy analyses during the charging periodEnergy and exergy efficiency during the charging period

ConclusionsGreenhouse heating with solar energyHeat storage material for greenhouse heatingEvaluation of the system efficiency

Appendix AReferences

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