Energy and Buildings 41 (2009) 220–228

Evaluation of ground-source heat pump combined latent heat storage systemperformance in greenhouse heating

Huseyin Benli a,*, Aydın Durmus b

a Department of Technical and Vocational Education, Fırat University, TR-23119 Elazıg, Turkeyb Department of Mechanical Education, Fırat University, TR-23119 Elazıg, Turkey

A R T I C L E I N F O

Article history:

Received 5 May 2008

Received in revised form 12 September 2008

Accepted 16 September 2008

Keywords:

Ground-source heat pump (GSHP)

Horizontal heat exchanger (HHE)

Latent heat storage (LHS)

Phase change material (PCM)

The coefficient of performance (COP)

Greenhouse

A B S T R A C T

The use of renewable energy for greenhouse heating in winter and cold days, helps to save fossil fuels and

conserve green farm environment on the one hand, and on the other hand, enhances the quality of

agricultural products, reduces production costs and limits the release of greenhouse gases. In this study, a

ground-source heat pump-phase change material (GSHP-PCM) latent heat storage system was developed

to use natural energy, to the extent possible, for thermal environment control of the greenhouse. The

coefficient of performance of the heat pump (COPHP), that of the overall system (COPsys) and the energy

capacity of the PCM during the charge–discharge phases has been determined. Based upon the

measurements made in the heating mode from 1 September 2005 till 30 April 2006 in Elazıg, Turkey, the

average heating COPHP of the GSHP unit and the overall system COPsys were obtained to be in the range of

2.3–3.8 and 2–3.5, respectively. These results showed that the utilization of a GSHP-PCM system is a

suitable approach for greenhouse heating in this district.

� 2008 Elsevier B.V. All rights reserved.

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1. Introduction

The greenhouse industry is considered to be one of the fastestgrowing agricultural sectors in Turkey, mainly because of itsfavorable climatic conditions. This sector creates importantemployment opportunities and added value throughout theprocessing and marketing stages of greenhouse products. Theindustry is also very important for creating a demand forsubsectors that provide inputs for greenhouse production suchas seeds, fertilizers, pesticides, glass and so on. The totalgreenhouse area has increased from 1003 ha in 1960 to30,000 ha in 2003, with 23,000 low plastic tunnels (Table 1). In2006, 3 million tons of greenhouse products were produced undercover, of which 5% were exported [1].

Owing to large heating loads and relatively high prices of fossilfuels, alternative energy sources for greenhouse heating has gainedutmost interest. Some of the important alternative sources ofenergy are solar collectors, heat pump and thermal energy storage

* Corresponding author. Tel.: +90 424 2370000/4402; fax: +90 424 2188947.

E-mail address: hbenli@firat.edu.tr (H. Benli).

Abbreviations: GHE, ground heat exchanger; GSHP, ground-source heat pump;

HGSHP, horizontal ground-source heat pump; HHE, horizontal heat exchanger;

LHS, latent heat storage; PC, personal computer; TES, thermal energy storage.

0378-7788/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.enbuild.2008.09.004

(TES) systems using phase change materials (PCMs). As solarenergy is available only during the day, its application requiresefficient thermal energy storage. Therefore, the excess heatcollected during the day is stored for later use during the night.A ground-source heat pump (GSHP) transforms the earth energyinto useful energy to heat and cool greenhouses, buildings, largestructures and so on. It provides low-temperature heat byextracting it from the ground or a body of water and facilitatescooling by reversing this process. Its principal application is spaceheating and cooling, though many also supply hot water, such asfor domestic use. A GSHP system does not directly createcombustion products. It can actually produce more energy thanit uses, as it draws additional free energy from the ground. TheGSHP systems are more efficient than air-source heat pumps,which exchange heat with the outside air, due to the stable,moderate temperature of the ground. They are also more efficientthan conventional heating and air-conditioning technologies, andtypically have lower maintenance costs. In addition, significantenergy savings can be achieved through the use of GSHP in place ofconventional air-conditioning systems and air-source heat pumps.Reductions in energy consumption of 30–70% in the heating modeand 20–50% in the cooling mode can be obtained. Energy savingsare even higher when compared with fossil origin fuels, wood orelectrical resistance heating systems. This potential has led to theuse of GSHP in a variety of applications. The use of GSHPs has other

Nomenclature

A area (m2)

a heat absorption rate of soil (�)

COM consumption (W)

COP heating coefficient of performance (�)

Cp specific heat (J/kg K)

d diameter (m)

f energy saving rate by the thermal curtain (�)

H enthalpy (J/kg)

h heat transfer coefficient (W/m2 K)

I solar radiation on the horizontal surface (W/m2 h)

LH latent heat of water vaporization (J/kg)

m mass flow rate (kg/s)

Q rate of heat transfer (W)

T temperature (K)

V volume (m3)

W power consumption (W)

X absolute humidity in the greenhouse (�)

Greek lettershi isentropic efficiency of the compressor (�)

hm mechanical efficiency of the compressor (�)

t transmissivity of the greenhouse cover (�)

Subscript1 first condition or evaporator outlet-compressor

inlet

2 second condition or condenser inlet-compressor

outlet

3 condenser outlet, expansion valve inlet

4 expansion valve outlet-evaporator inlet

bw brine water (antifreeze water)

co condenser

comp compressor

cp circulation pump

ev evaporator

fc condenser fan-coil

g greenhouse

HP heat pump

i inner

o outlet

PCM phase change material

rg refrigerant (R-22)

sys system

t greenhouse cover

v ventilation

Table 1Developments of greenhouse production area in Turkey.

Years Glasshouse Plastic house Total

area (ha)

Annular

growth

rate (%)Area (ha) % Area (ha) %

1960 525 52.3 478 47.7 1,003 –

1970 76 38.3 1,572 61.7 2,548 9.8

1980 925 18.5 4,072 81.5 4,997 7.0

1990 2000 23.3 6,600 76.7 8,600 5.6

2000 5656 27.6 14,825 72.4 20,481 9.1

2002 6420 26.25 18,039 73.75 24,458 9.3

H. Benli, A. Durmus / Energy and Buildings 41 (2009) 220–228 221

benefits, too: utilities are promoting the technology as it can helpreduce their internal costs thorough the ability to reduce bothpeak and average load demands. There are also of benefit to thecommunity as the consumption of fossil fuels is reduced and theemission of greenhouse gases and other pollutants is decreasedand centralized. Consequently, this study is devoted to an analysisof the operation of a large-scale GSHP and latent heat storage(LHS) system for greenhouse heating and dehumidificationrequirements analysis of the system. In this experimental study,a heat pump runs continuously for both heating a model-sized

greenhouse of 30 m2 and charging calcium chloride hexahydrate(the PCM) into the system in liquid form. Thermal energy can bestored as sensible heat or latent heat. In most of the storagesystems, it is stored as sensible heat in materials such as water androcks. In air collection systems, rock beds are normally used tostore heat, while water tanks store heat in liquid systems. In LHSsystems, the latent heat arising from the phase change of amaterial is used for TES. PCMs can store large amounts of heat(latent heat of fusion) when transformed from solid to liquidphases. The LHS is one of the most efficient ways of storingthermal energy.

During the 1990s, a great number of investigations wereconducted by many researchers in the design, modeling andtesting of GSHPs and TES techniques [2–6]. Case studies [7–9],handbooks and standards [10–13] are available for the installationprocedures of GSHP systems. These studies which were under-taken on a system basis may be categorized into two main groupsas follows: (i) HGSHP with storage for greenhouse heating and (ii)TES technique using extra heating.

In recent years, TES has been recognized as a potentiallysignificant means by which primary energy consumption can bereduced in domestic, commercial and industrial processes [14].Comprehensive studies have been conducted by many researchersconcerning the application and modeling of LHS systems andthermal properties of PCMs [15–23]. Latent heat thermal energystorage has proved to be an effective method for the utilization ofclean energy, because of its high-energy storage density and smalltemperature variation from storage to extraction. Therefore, anumber of investigations that aimed at improving the efficiency ofLHS systems have been done by some researchers in the design,modeling and testing of LHS systems [24–26]. El-Dessouky andAl-Juwayhel [27] developed the second law analysis for a phasechange TES system using paraffin as a PCM with the latent heattechnique for heating a plastic greenhouse of 180 m2. Benli [28]studied heat distribution in a greenhouse built by him, usinghorizontal GSHP and a PCM (calcium chloride hexahydrate as thePCM).

This study includes the performance evaluation of an HGSHPwith R-22 as the refrigerant heating mode and LHS with a PCM. AnLHS tank was directly connected to a heat pump. An experimentalset-up, described in Section 2, was constructed and tested for thefirst time on the basis of a university study performed in thecountry. The coefficient of performance (COP) of the heat pump,the TES of the PCM and the whole system were computed from themeasurements.

2. Description of the system

In this study, a GSHP-PCM latent heat storage system wasdeveloped to use natural energy, to the extent possible, for thermalenvironment control of the greenhouse. The schematic arrangementof the GSHP and LHS systems for greenhouse heating is given in

Fig. 1. Experimental equipment of greenhouse heating system.

H. Benli, A. Durmus / Energy and Buildings 41 (2009) 220–228222

Fig. 1. The system consists mainly of five units: (1) GSHP and groundheat exchanger (GHE), (2) LHS unit, (3) heat storage material, (4)experimental greenhouse and (5) heat transfer unit and data loggerunit. The greenhouse heating system utilizes heat pump andseasonal LHS located in the Elazıg, East Anatolia region of Turkey.

2.1. Heat pump and ground heat exchanger

To avoid freezing of water under the working conditions andduring cold winter days, non-toxic propylene glycol solution (30%by weight) was used. The refrigerant circuit was built on theclosed-loop copper tubing and the working fluid was R-22. A GSHPsystem can be used for both heating and cooling. In this study,conversion from the heating to cooling cycle was achieved bymeans of a four-way valve. The performance evaluation method ofthe cooling mode operation of the system will not be discussedhere. During winter, the water–antifreeze solution in the pipesextracts heat from the earth and carries it into the greenhouse,whereas during summer, the system reverses and takes heat fromthe greenhouse storing it in the cooler ground. The heat pump usedin the study is shown in Figs. 2. Fig. 3(a) shows the layout of theGHE and Fig. 3(b) shows how the GHE was covered with soil. Heattransfer from the earth to the heat pump or vice versa ismaintained with the fluid or water–antifreeze solution circulatedthrough the GHE. The fluid transfers its heat to the refrigerant fluidin the evaporator. The refrigerant, which flows through the otherclosed loop in the heat pump, evaporates by absorbing heat fromthe water–antifreeze solution circulated through the evaporator. Itthen enters the hermetic compressor where it is compressedbefore it enters the condenser where it is condensed. A fan blowsacross the condenser to move the warmed air of the greenhouse.

Fig. 2. The view of GSHP unit.

The specifications and characteristics of the main components ofthe GSHP system are given in Table 2.

2.2. The LHS unit

A cylindrical plastic tank with a diameter of 0.60 m and a totalvolume of 0.33565 m3 was used as the seasonal LHS unit. Thewhole surface of the LHS unit was insulated with 0.07 m glass wooland the unit was placed horizontally on the ground surface at adistance of 0.5 m from the experimental greenhouse. The LHS unitwas attached to the heat pump by means of a paddle box. Heatenergy, produced by the GSHP, was transferred to the LHS unit andthe experimental greenhouse. The heat pump was operatedcontinuously all day long for heating the greenhouse and feedingthe PCM to the system. The energy obtained from the GSHP wastransferred to the LHS unit by circulating air through thepolyethylene (PE) pipes. Perforated PE pipes were installed inthe LHS unit as the heat exchanger. The total length of 30perforated PE pipes was 45 m and the diameter was 0.05 m. Fig. 4shows the LHST manufactured for this study along with a view ofthe GHE.

2.3. Heat storage material

In general, materials that have a large change in internal energyper unit volume minimize the space needed to store energy. A largenumber of organic and inorganic substances are known to meltwith a heat of fusion in any required temperature range (0–120 8C).Nevertheless, the following properties of the heat storage materialmust be taken into account in the selection of the storage materialfor sensible heat storage systems: heat capacity, density, heatstorage temperature, storage material cost and heat exchange cost,thermal conductivity, vapor pressure, toxicity and corrosiveness.For selecting the PCM for the present LHS system, the followingdesirable properties of calcium chloride hexahydrate were takeninto account: high latent fusion per unit mass, chemical stability,melting in the desired operating temperature range, small volumechanges during phase transition, availability in large quantitiesand low price. Thermal properties of calcium chloride hexahydratewere measured with a differential scanning calorimeter. Themelting temperature range and latent heat of fusion were found tobe 32–45 8C and 190 kJ/kg, respectively. The LHS unit was filledwith 300 kg of calcium chloride hexahydrate, corresponding to10 kg of PCM/m2 of the greenhouse ground surface area.

2.4. Experimental greenhouse

The experiments were conducted in a glass greenhouse thatwas laid in the north–south direction. The greenhouse was framed

Fig. 3. The layout and the covering of the ground heat exchanger.

H. Benli, A. Durmus / Energy and Buildings 41 (2009) 220–228 223

with galvanized steel with mechanical up-or-down ventilationopenings in the sidewalls. The glass material (side wall thickness3 mm and roof thickness 4 mm) contained ultraviolet (UV) andinfrared (IR) stabilizers. The dimensions of the greenhouse werewidth 5 m, length 6 m and height 2.9 m. Warm air from the GSHPunit was distributed by PE pipes (of diameter 0.10 m) lying on thecorner of the ground surface inside the greenhouse. The experi-mental system is illustrated in Fig. 5.

2.5. Heat transfer unit

In this experiment, heat transfer with forced convectionbetween the heat pump unit, the LHS unit and the experimental

Table 2The main components specification and characteristics of the GSHP system studied.

Main circuit Element Technical s

Ground coupling unit HGHE length of 246 m Horizontal

material po

Water–antifreeze solution

circulating pump

Manufactur

and 140 W

Expansion tank 50 l

Refrigerant circuit Compressor Type, herm

rate 9.2 m3

refrigerant

Heat exchanger Manufactur

Condenser for heating Manufactur

area; 45 cm

Dryer Manufactur

Observe glass Manufactur

Fan circuit Fan of air cooled condenser Manufactur

power180 W

Fan of discharging of PCM Manufactur

power 85 W

greenhouse was facilitated with two centrifugal fans (fan A for thecondenser unit and fan B for the LHS unit). During winter, twodifferential thermostats were used to control the charging anddischarging processes. The thermostats were controlled byindependently operating centrifugal fans and at the same time,the GSHP was continuously operated for charging the LHS unit.

During the charge phase, the air obtained from the heat pumpwas passed through the LHS unit. During winter and cold days,when the air temperature of the experimental greenhouse fellbelow a certain value, fan B (discharging fan) was activated duringthe extraction operation. The discharging fan drove the greenhouseair through the LHS unit during the night and returned it to theinterior of the experimental greenhouse through the PE pipes. The

pecification

heat exchanger; pipe distance 0.3 m; pipe diameter 0.016 m; piping depth 2 m;

lyethylene, PX-b Cross Link

er: DAB A50/180x3 speed; speed step (2710, 2540 and 1715 rpm); power (160, 148

); flow rate 1–12 m3/h; pressure head 8 m

etic reciprocating, manufacturer: Tecumseh; model TFH 5532 F volumetric flow

/h; speed 2900 rpm; the rated power of electric motor driving 2.5 HP (1.86 kW);

R-22; capacity 5.484 kW (at cooling/condensing temperatures of 0/46 8C)

er: Altıntas Isı type—ID 23-01; capacity 10 kW heat transfer surface 0.85 m2

er: Azak Sogutma type—AS169 25 model; m2D capacity 11.63 kW; 25 m2 surface

fan diameter

er: DE-NA/233-083 Dry-101 connection 3/8 in.

er: Honeywell S21; connection 3/8 in.

er: Aldag type—SAS 228; diameter 380 mm; air volumetric flow rate 600 m3/h;

er: Bahcıvan motor—BDRKF 180; diameter 200 mm; volumetric flow rate 860 m3/h;

; speed 2350 d/d

Fig. 4. Manufacturing of LHST and the ground heat exchangers buried at 2 m depth.

Fig. 5. Various views of greenhouse and heating unit.

H. Benli, A. Durmus / Energy and Buildings 41 (2009) 220–228224

operation of the electric motor activating the discharging fan wascontrolled by a time clock between 17:00 and 04:00 h.

3. Measurements

The following data were regularly recorded at 15-min intervalsduring the experiments:

� M

ass flow rates of the water/antifreeze solution measured by arotameter. � M ass flow rates of the refrigeration solution measured by a flow

meter.

� T emperature of the water–antifreeze solution entering and

leaving the GHE measured by copper-constantan thermocouplesmounted on the water inlet and outlet lines.

� C ondenser and evaporator pressures measured by bourdon-type

manometers.

� A mbient atmospheric pressure measured by a barometer. � O utdoor and greenhouse air temperatures and humidity at the

northern side as well as the southern side measured by using amulti-channel digital thermometer.

� E lectrical power input to the compressor, fans (A and B) and the

circulating pump measured by a wattmeter.

� G round temperatures measured by copper-constantan thermo-

couples.

� In let and outlet temperatures of circulated water–antifreeze

solution through the closed-loop HGHE and WARHEX measuredby copper-constantan thermocouples.

� W

ind velocities at a height of 6 m measured by an anemometermaximum dic-3. � S olar flux inside and outside the greenhouse measured by a Kipp

& Zonnen CM11 pyranometer and Elazıg State MeteorologicalStation weather data from 2005 to 2006.

� T emperatures of the heat storage materials and circulating air as

the heat transfer fluid measured by copper-constantan thermo-couples. Ten thermocouples were uniformly placed in the heatstorage unit to measure the temperature of the heat storagematerial.

4. Energy balance in the greenhouse heating system

As shown in Fig. 6, solar radiation and thermal energy gainedfrom the heat pump were the energy sources for heating thegreenhouse. The crop, air, soil and the PCM were used as the heatstorage media to heat the air inside the greenhouse. However, onlya small amount of this heat source could be used to heat the airinside the greenhouse while a large amount was lost through thegreenhouse cover.

Under daylight, the thermal energy balance can be written asfollows [29]:

Qsolar þ Qsys ¼ Qt þ Qv þ Qsoil þ Qcro p þ Qair þ Q pcm (1)

where solar radiation in the greenhouse (Qsolar) is calculated fromthe following equation:

Qsolar ¼ tI (2)

Fig. 6. Energy balance model in the greenhouse heating system equipped with PCM storage unit and the heat pump.

H. Benli, A. Durmus / Energy and Buildings 41 (2009) 220–228 225

For the cycle calculations for the GSHPS, the followingassumptions were made: (i) the volumetric efficiency of thecompressor was taken to be 90%, (ii) the compressor isentropicefficiency was taken to be 75% and (iii) there were no pressureloses in the cycle.

The rate of heat extracted (absorbed) by the unit in the heatingmode (GHE load) QGHE was calculated from the following equation:

QGHE ¼ mbwC p;bwðTo;bw � Ti;bwÞ (3)

The heat rejection rate in the condenser was calculated by

Qco ¼ mrgðH2 � H3Þ (4)

The heat transfer rate in the evaporator is

Qev ¼ mrgðH1 � H4Þ (5)

The work input rate to the compressor is

Wcom p ¼mrgðH2 � H1Þ

hihm

(6)

Hence, the COP of the GSHP can be calculated as

COPHP ¼Qco

Wcom p

(7)

The coefficient of performance of the overall heating system(COPsys), which is the ratio of the condenser load to total workconsumption of the compressor, brine water, circulation pump andthe condenser fan-coil unit, was computed by the followingequation:

COPsys ¼Qco

Wcom p þ Wc p þ W fc

(8)

The heat gained from the GSHP overall system can be written as

Qsys ¼ COPsysCOMelectric (9)

The heat loss through the greenhouse cover (Qt) is

Qt ¼ Aghtð1� f ÞðTinside � TambientÞ (10)

The heat loss of ventilation from the greenhouse (Qv) is

Qv ¼ AghvðTinside � TambientÞ (11)

The heat absorbed and released by the soil in the greenhouse (Qsoil)is

Qsoil ¼ asoilAgRgtI (12)

where the ratio of soil area not covered by the crop is calculated as

Rg ¼Ag � Acro p

Ag

� �(12a)

The heat absorbed and released by the crop (Qcro p) is

Qcro p ¼ Rcro pacro ptI ¼ ð1� RgÞacro ptI (13)

where the ratio of soil area covered by the crops is calculated as

Rcro p ¼Acro p

Ag

� �(13a)

Air enthalpy in the greenhouse (Qair) is

Qair ¼ fhdryair þ xhva porgmair

¼ C pTinside þ 0:6220pva por

pdryair

ðLHwater þ CvTinsideÞ( )

mair (14)

The heat stored by the PCM (Q) is

QPCM ¼ m pcmsolidC p pcmsolidðT2;solid � T1;solidÞ þ LH pcmm pcm

þ m pcmliquidC p pcmliquidðT1;liquid � T2;liquidÞ (15)

The heat gain from the ground depends on the seasonalconditions. In our study using the pipes, which were laidhorizontally 2 m under the ground, the heat gain per pipe unitlength (m) was found to be 0.00315 kW/m on an average. Heattransfer coefficient of the ground was taken as 1.70 W/m K. Inorder to achieve a better level of heat transfer, pebbles with a heattransfer coefficient of 1.75 W/m K were spread on the ground. InLHS systems, the transferred heat can be calculated instanta-neously with the following equation. The heat transfer during thecharge and discharge of the phase change material is a function oftime:

QPCMðtÞ ¼ mairC pairðTi;air � To;airÞ (16)

This also shows the amount of heat, which appears during thecharge phase. The heat transferred to the chemical material tank by

Fig. 7. The daily variation of various temperatures of HGHE.Fig. 8. Heat storage variation with time of day during charging period.

Fig. 9. Heat storage variation with time of day during discharging period.

H. Benli, A. Durmus / Energy and Buildings 41 (2009) 220–228226

air is assumed to be the heat received by the chemical material:

QPCMðtÞ ¼ QPCMðtÞcharge¼ QPCMðtÞdischarge

(17)

When the heat pump is off and the chemical material storage is letto cool, the heat the chemical material transfers via movement willbe equal to the discharge heat:

QPCMðtÞ ¼ hinsideAsur face; pcmðTsur face; pcm;average � Tinside; pcmÞ (18)

In this study, a heat pump and a PCM heat storage unit weredeveloped in order to limit the use of fossil fuels with increased useof renewable energy for greenhouse heating. Fig. 7 shows dailyvariation of Tin, Tout, Tsoil, Tbw-i and Tbw-o in the case of using HGHE.

4.1. Calculating heat loss from the greenhouse

Heat loss calculation is the first step in determining the heatingsystem capacity before selecting the system and its variouscomponents. The heating system should be properly sized to meetthe needs of the greenhouse under extreme weather conditions.Greenhouse heat loss is determined by the following equation[30]:

Qg ¼A1

R1þ A2

R2þ � � � ðTinside � ToutdoorÞ f wind f construction f sys (19)

Using Eq. (19) and assuming that the construction type factor(fconstruction), the system factor (fsys) and the wind factor (fwind) are1.08, 1.00 and 1.03, respectively, the average heating load of thesolar greenhouse was obtained to be 6.28 kW at the designconditions. In this equation, greenhouse surface area wascalculated to be 29.15 m2, the thermal resistance of greenhouse0.165 m2 8C/W and the difference between the greenhouse insideand outdoor temperatures (Tinside � Toutdoor) to be 10 8C.

5. Results

To run the experiments, 300 kg chemical material and 200 kgwater were put in to the heat tank. About 6 kg potassium nitratewas added, to facilitate nucleation and transformation. Themixture was then slightly heated and mixed in order to providehomogeneity. Then, it was ready for use:

VPCM

Vheating pi pe¼ 0:33565

0:08835¼ 3:8 (20)

The derived experimental equations are valid for situationswhere the ratio of the amount of the chemical material to theheating surface area is three to four times higher. Fig. 8 gives thechange of total heat stored by the chemical material duringthe charge phase with respect to time. Fig. 9 gives the change of

heat dissipated by the chemical material during the dischargephase with respect to time.

Empirical relations as derived separately for the heat pump andthe chemical material charge and discharge conditions were foundto be as follows:

For heat pump during charge

Q pcmcharge ¼ 0:0276 t2 � 0:4488 t þ 1:9143 ðR2 ¼ 0:963Þ (21)

For heat pump during discharge

Q pcmdischarge ¼ 0:0504 t2 � 0:6962 t þ 2:3776 ðR2

¼ 0:947Þ (22)

These empirical equations are valid for the following values:

� 6

h > t > 1 h (time period for charging) and 4 h > t > 1 h (timeperiod for discharging); � 5 0 8C > Ti,air > 35 8C (heat pump chemical material air input

temperature for charging) and 35 8C > To,air > 5 8C (heat pumpchemical material air output temperature for discharging);

Aratio ¼Aheating pi pes

APCM¼ 0:21; Vratio ¼

VPCM

Vheating pi pe¼ 0:33565

0:08835

¼ 3:8; (23)

mPCM = 300 kg chemical material;

� � m ˙ air ¼ 0:27 kg=s:

For the GSHP, the experimental data were converted into tablesand under the light of these data COP values for each system werecalculated and various graphics were obtained. COPsys changes ofthe GSHP according to the hours of the day are given in Fig. 10.COPsys value of the system and a dimensionless u = Tenvironment/Tsoil

number was determined and empirical (experimental) relationswere found as follows. In order to obtain reliable values,Tsoil > Tenvironment should be taken. For this reason u value is alwayssmaller than 1. Otherwise, underground heat exchanger turns outto be harmful rather than beneficial. The change in total electrical

Fig. 10. The daily variation of COPsys of the GSHP.

Fig. 11. Total power consumption (compressor, condenser fan and circulating

pump).

H. Benli, A. Durmus / Energy and Buildings 41 (2009) 220–228 227

energy consumption according to the hours of the day is given inFig. 11.

October 2005 (15 October 2005)

COPsys ¼ �11944u2 þ 23055u � 11122 ðR2 ¼ 0:9804Þ (24)

November 2005 (5 November 2005)

COPsys ¼ 2333:3u2 � 4488:3u þ 2161:3 ðR2 ¼ 0:9738Þ (25)

December 2005 (10 December 2005)

COPsys ¼ �10476u2 þ 20866u � 10387 ðR2 ¼ 0:9761Þ (26)

January 2006 (20 January 2006)

COPsys ¼ �10076u2 þ 19376u � 9312:8 ðR2 ¼ 0:9812Þ (27)

February 2006 (15 February 2006)

COPsys ¼ �16429u2 þ 31491u � 15088 ðR2 ¼ 0:9635Þ (28)

March 2006 (20 March 2006)

COPsys ¼ 43667u2 � 85935u þ 42282 ðR2 ¼ 0:9824Þ (29)

April 2006 (5 April 2006)

COPsys ¼ �857:14u2 þ 1700:3u � 840:14 ðR2 ¼ 0:9765Þ (30)

The uncertainties arising in calculating a result (wR) due to severalindependent variables are given as follows:

WRþ ¼dRþ

dX1w1

� �2

þ dRþ

dX2w2

� �2

þ � � � þ dRþ

dXnwn

� �2 !1=2

(31)

where the result R+ is a given function of the independent variablesX1, X2, . . ., Xn, and w1, w2, . . ., wn are uncertainties in theindependent variables. In this study, the values for COPHP and

COPsys were found to be 3.8 and 3.5, respectively, while themaximum uncertainties associated with COPHP and COPsys were�5.78% and �5.66%, respectively.

6. Conclusions

In the region where the current study was carried out, January isthe coldest month of the year. Temperature range is �5 8C to�20 8C for 20–22 days, which provides ideal conditions for a stableoperation of the GSHPs. To prevent freezing problem of brinewater, which is fed into the heat pump, it is necessary to takeadditional measures. The temperature of the ground in the monthof January is around 5–7 8C. During the process of feeding brineinto the nested pipes of the heat exchanger, environmenttemperature leads to sudden drops in the heat exchange systemof F-22 gas, as a result of which freezing problems emerge. Anadditional heat source may be necessary in the case of sudden heatdrops. Placing the heat pump directly inside the greenhouse or in acovered place outside would be a more convenient solution. TheGSHPs can be used in times when the temperature of theenvironment is lower than that of the ground. The GSHP systemsshow better performance than conventional air-source heatingsystems under low environment conditions. The effect of climaticconditions and operating parameters on the system performanceparameters and PCM heat gain were also investigated in this study.Nine empirical correlations were developed to estimate theaverage performance coefficients of the system (COPsys) and theheat transfer rate of PCM during the charge and discharge phases.

Consequently,

1. C

OPHP values of the GSHP were found to be higher than theCOPHP values of the air-source heat pump during winter andcold days. Operation of the compressor was quite stable. Ashermetic compressors demonstrate low risks of malfunction-ing, they are quite suitable for long-period operations. Using ascroll compressor instead of a hermetic compressor would leadto an increase in the COPHP values obtained.

2. W

hen the mass flow of the brine water increases, heat transfer,and thus the COPsys of the system also increase. Brine water hasno risk of freezing until �20 8C. As this risk was calculated, anadditional emergency heating and automatic stop facility weremade available in the system.

3. H

eat pumps must be placed inside the greenhouse or under acover for an efficient use in these systems.

4. S

ince the temperature of water received from the ground ishigher than that of the environment, the compression rate ofthe compressor is rather low, by means of which it needs lessenergy from external sources.

5. W

ith the help of a storage system, the state of the chemicalmaterial (stability and changes) could easily be observed. Sincethe temperature of the chemical material stored was almostconstant, a rational distribution of heat in the greenhouse wasachieved.

6. S

ince a GSHP, which easily operates even at �20 8C was used,freezing problems did not occur.

7. T

he pipes used in the heat exchanger should have a diameter of(25.4 mm) 1 in. or higher.

8. D

uring warm days, it takes a shorter time to charge thechemical material and a longer time to discharge it, whereasthe reverse is true for cold days.

9. T

he performance of the GSHP was found to be between 2.3 and3.8 for COPHP, whereas it was between 2 and 3.5 for COPsys.Depending on the temperature of the environment in thegreenhouse, the heat pump increased the temperature by5–10 8C and the chemical material increased it by 1–3 8C on the

H. Benli, A. Durmus / Energy and Buildings 41 (2009) 220–228228

average (auxiliary heat). The best (peak) COPHP of the GSHP andthe overall system were obtained as 4.3 and 3.8, respectively,on 20 March 2006.

10. E

xperimental results show that univalent central heatingoperation (independent of any other heating system) cannotcompensate for the overall heat loss from the greenhouse if theambient temperature is very low. Bivalent operation (com-bined with another heating system) can be suggested as thebest solution in the Eastern region of Turkey, if peak loadheating can be easily controlled.

Acknowledgments

The authors gratefully acknowledge the financial support fromthe Fırat University Research Fund. Our sincere thanks also goes toMr. Mustafa Ceyhanlı, founder and manager of the ‘‘CeylanlarGreenhouse’’, for his invaluable support (Ceylanlar Greenhouse is aTurkish company based in Adana. Further information about thecompany is available at www.ceylanlar.com.tr.)

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