ARTICLE IN PRESS

0360-1323/$ - se

doi:10.1016/j.bu

Abbreviations

heat exchanger;

source heat pum

assisted heat pu

heat pump syst�Correspond

E-mail addr

basli@bornova1Also for cor

Building and Environment 40 (2005) 1040–1050

www.elsevier.com/locate/buildenv

Performance analysis of a solar-assisted ground-source heat pumpsystem for greenhouse heating: an experimental study

Onder Ozgenera,1, Arif Hepbaslib,�

aSolar Energy Institute, Ege University, 35100, Bornova, Izmir, TurkeybDepartment of Mechanical Engineering, Faculty of Engineering, Ege University 35100 Bornova, Izmir, Turkey

Received 29 May 2003; received in revised form 8 June 2004; accepted 2 August 2004

Abstract

This study investigates the performance characteristics of a solar-assisted ground-source (geothermal) heat pump system

(SAGSHPS) for greenhouse heating with a 50 m vertical 32 mm nominal diameter U-bend ground heat-exchanger. This system was

designed and installed in the Solar Energy Institute, Ege University, Izmir (568 degree days cooling, base: 22 1C, 1226 degree days

heating, base: 18 1C), Turkey. Based upon the measurements made in the heating mode from the 20th of January till 31st of March

2004, the heat extraction rate from the soil is found to be, on average, 57.78 W/m of bore depth, while the required borehole length

in metre per kW of capacity is obtained as 11.92. Design practices in Turkey normally call for U-bend depths between 11 and 13 m/

kW of heating. The entering water temperature to the unit ranges from 8.2 to 16.2 1C, with an average value of 14 1C. The

greenhouse air has a maximum day temperature of 31.05 1C and night temperature of 14.54 1C with a relative humidity of 40.35%.

The heating coefficient of performance of the heat pump (COPHP) is about 2.00 at the end of a cloudy day, while it is about 3.13 at

the end of sunny day and fluctuates between these values in other times. The COP values for the whole system are also obtained to

be 5–20% lower than COPHP. The clearness index during experimental period is computed as average 0.56. At the same period,

Cucumus sativus cv. pandora F1 was raised, and product quality was improved with the climatic conditions in the designed

SAGSHPS. However, experimental results show that monovalent central heating operation (independent of any other heating

system) cannot meet the overall heat loss of the greenhouse if the ambient temperature is very low. The bivalent operation

(combined with other heating system) can be suggested as the best solution in Mediterranean and Aegean regions of Turkey.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Performance analysis; Solar-assisted ground-source heat pump; Greenhouse heating; Izmir

1. Introduction

Good plant-growth conditions can be achieved byusing greenhouses. A greenhouse can be managed to

e front matter r 2004 Elsevier Ltd. All rights reserved.

ildenv.2004.08.030

: EWT, entering water temperature; GHE, ground

GRP, glass reinforced plastics; GSHPS, ground-

p system; LCD, liquid crystal display; SAHPS, solar-

mp system; SAGSHPS, solar-assisted ground-source

em

ing author. Tel.: 90 232 388 40 00; fax: 90 232 388 03 78.

esses: ozgener@bornova.ege.edu.tr (O. Ozgener), hep-

.ege.edu.tr, hepbasli@egenet.com.tr (A. Hepbasli).

respondence.

protect the plants by creating a favourable environment,allowing intensive use of soil, and helping sanitary plantcontrol. From an economic point of view, the mainobjective of horticultural greenhouses is to advance thenormal season production or to obtain a completelyout-of-season production, which corresponds with high-er crop prices. Many variables must be controlled inorder to provide the good environmental conditions.The most important parameters to be controlled inside agreenhouse are temperature, humidity and light [1].Especially, the temperature at night appears as animportant critical variable to be controlled. Althoughdifferent species are cultivated, the requirement is

ARTICLE IN PRESS

Nomenclature

A1;A2 surface area of components (m2)Ac collector surface area (m2)Cp;wa specific heat of the water–antifreeze solution

(kJ/kgK)COPHP heating coefficient of performance of heat

pump (dimensionless)COPs heating coefficient of performance of the

overall system (dimensionless)f c construction type or quality factor (dimen-

sionless)f s system factor (dimensionless)f w wind or exposure factor (dimensionless)FR collector heat removal factor (dimensionless)h specific enthalpy (kJ/kg)H monthly average daily radiation on a hor-

izontal surface (MJ/m2)H0 monthly average daily extraterrestrial radia-

tion (MJ/m2)Ib direct solar radiation arriving on a horizontal

surface (MJ/m2)Id instantaneous diffuse solar radiation on a

horizontal surface (MJ/m2)I instantaneous total radiation on a horizontal

surface (MJ/m2)IT rate of incidence of total radiation on a unit

area of surface (MJ/m2)KT monthly average clearness index (dimension-

less)_mwa mass flow rate of the water/antifreeze solu-

tion kg/s)_mr mass flow rate of refrigerant (R-22) (kg/s)

R ratio of the total radiation on a tilted surfaceto that on a horizontal surface

Rb ratio of the instantaneous direct solar radia-tion on a tilted surface to the instantaneousdirect solar radiation on a horizontal surface

R1;R2 thermal resistance of each component(m2

%o C/W)

_Qu rate of useful heat transfer to water in thesolar collector (kW)

_QGRP rate of greenhouse heat loses (kW)_Qco heat rejection rate in the condenser (kW)_Qe ground heat exchanger load (kW)_Qev heat transfer rate in the evaporator (kW)

Ta ambient temperature (K, 1C )T i fluid inlet temperature, greenhouse inside

design temperature (K, 1C )To fluid outlet temperature, greenhouse outside

design temperature (K, 1C )UL overall loss coefficient of the collector

(W/m2 K)V average phase voltage (V)VLL phase to phase voltage (V)VLN phase voltages (V)_W comp power input to the compressor (kW)_W pumps total power input to the brine and water

circulating pumps (kW)_W fc power input to the fans of the fan-coil unit

(kW)

Greek letters

a absorptance (dimensionless)Z energy efficiency (dimensionless)Zic isentropic compressor energy efficiency

(dimensionless)Zmc mechanical compressor energy efficiency

(dimensionless)b tilt angle of collector plane from the hor-

izontal (dimensionless)d declination angle (dimensionless)r reflectance (dimensionless)t transmittance (dimensionless)o hour angle at sunrise (dimensionless)F latitude angle (dimensionless)SA total current (A)

O. Ozgener, A. Hepbasli / Building and Environment 40 (2005) 1040–1050 1041

always the same: the avoidance of low temperatures atnight. The conventional solution for this problem is theburning of some fossil fuel inside the greenhouse duringdangerous freezing night. Since fuel prices are high, thisoption is very expensive, but it is necessary, consideringthe possibility of the complete destruction of plants.Consumed energy for greenhouses heating purposesdepends on seasons and daily changing climatic condi-tions. In addition to solar energy gain, greenhousesshould be heated during nights and cold days.

Good plant-growth conditions can be achieved byusing greenhouses. A greenhouse can be managed toprotect the plants by creating a favourable environment,

allowing intensive use of soil, and helping sanitary plantcontrol. From an economic point of view, the mainobjective of horticultural greenhouses is to advance thenormal season production or to obtain a completelyout-of-season production, which corresponds with high-er crop prices. Many variables must be controlled inorder to provide the good environmental conditions.The most important parameters to be controlled inside agreenhouse are temperature, humidity and light [1].Especially, the temperature at night appears as animportant critical variable to be controlled. Althoughdifferent species are cultivated, the requirement isalways the same: the avoidance of low temperatures at

ARTICLE IN PRESSO. Ozgener, A. Hepbasli / Building and Environment 40 (2005) 1040–10501042

night. The conventional solution for this problem is theburning of some fossil fuel inside the greenhouse duringdangerous freezing night. Since fuel prices are high, thisoption is very expensive, but it is necessary, consideringthe possibility of the complete destruction of plants.Consumed energy for greenhouses heating purposesdepends on seasons and daily changing climatic condi-tions. In addition to solar energy gain, greenhousesshould be heated during nights and cold days.

The climatic conditions of the region are the primefactors, which effect the development of the plant andthe economics of the greenhouse production. It becomesnecessary to take measures to heat the greenhouseswhen the temperature goes below 12 1C in order toobtain high-quality and high-yield crop, that is espe-cially important for export purpose [2].

A variety of heating systems are being used in presentday greenhouses to meet the heating and coolingrequirements. An example is the steam or hot waterradiation system that makes use of hot water or steamsupplied through pipe networks running through thegreenhouse, and hot air unit heaters. These systems aregenerally able to meet the heating requirements of thegreenhouse, but the temperature distribution patternswithin the greenhouse associated with such systems arereadily influenced by the outdoor weather conditions[3,4]. Further, such systems are usually not able tocontrol and maintain the required humidity levels withinthe greenhouse; another important thermal conditionaffecting the growth of crops. As an alternative, there isgreat potential in employing a heat pump system forgreenhouse air-conditioning based on its ability toperform the multi-function of heating, cooling anddehumidification. The employments of heat pumps inmany thermal operations has shown favourable energysavings ranging from 20% to 50%, and have consider-able energy-conservation potential as compared toconventional heating systems [4,5].

Taking Turkey’s geothermal energy potential intoaccount, it may be concluded that the use of geothermalenergy in heating greenhouses is very low. At present, itis estimated that only 35.7 ha of greenhouses are heatedby geothermal energy in Turkey, although the countryhas a total greenhouse area of 22,000 ha. The majorityof the geothermal greenhouse applications are in thewestern part of Turkey and heating capacity rangesfrom 3.01 to 101.72 million kJ/h [6].

When the temperature variations of Mediterranean,Aegean and Marmara regions of Turkey, where green-house agriculture is particularly popular, are examined,it is observed that the daily average temperatures staybelow 12 1C especially in December, January andFebruary [7]. That is why the greenhouses need to beheated in certain time of the year in the Mediterraneanregion. However, it was found that almost none of theproducers have regular heating system in their green-

houses since heating cost was on average 60–80% oftotal operating cost. They seem to have some basicmeans of heating in order to prevent the crop from thefrost. This can have a considerable effect on the quality,yield and cultivation time of horticultural products. Inorder to establish optimum growth conditions in green-houses, renewable energy sources should be used asmuch as possible. These sources are mainly solar,geothermal, biomass and wind energies.

Although solar energy is free and available, practicalutilization for greenhouse heating still presents technicaland economical problems. Solar energy that has beenabsorbed inside the greenhouse during the day covers apart of heating energy that is needed during daytime. Touse solar energy for heating the greenhouse during thenight, two problems are to be solved [8]: (i) theconversion of global radiation into thermal energyand, (ii) the storage of thermal energy for heatingpurposes during night. The problems of storage and theutilization of solar energy and the low radiation in theMediterranean region during the production period(November to May) limited its use for greenhousesheating purposes.

Greenhouses also have important economical poten-tial in Turkey’s agriculture. In addition to solar energygain, greenhouses should be heated during nights andcold days. In order to establish optimum growthconditions in greenhouses, renewable energy sourcesshould be utilized as much as possible. Effective use ofheat pump with suitable technology in the moderngreenhouses plays a leading role in Turkey in theforeseeable future. Although in greenhouses not only thepossibility of heating but also the ability of cooling anddehumidification has been recognized, only a restrictednumber of practical applications have been realized [6].

In northern climates where the heating load is thedriving design factor, supplementing the system withsolar heat can reduce the required size of a closed-loopground-coupling system. Solar collectors, designed toheat water, can be installed into the ground-coupledloop (by means of a heat exchanger or directly). Thecollectors provide additional heat to the heat transferfluid. This type of variation can reduce the required sizeof the ground-coupled system and increase heat pumpefficiency by providing a higher temperature heattransfer fluid [9].

During the last decade, a number of investigationshave been conducted by some researchers in the design,modelling and testing of solar-assisted heat pumpsystems (SAHPSs). These studies undertaken on asystem basis may be categorized into three groups asfollows: (i) SAHPSs for water heating [10–12], (ii)SAHPSs with storage (conventional type) for spaceheating [13–17], and (iii) SAHPSs with direct expansionfor space heating [18]. Among them, the studyperformed by Bi et al. [17] is noteworthy since it is

ARTICLE IN PRESSO. Ozgener, A. Hepbasli / Building and Environment 40 (2005) 1040–1050 1043

closely associated with the present study. They per-formed theoretical and experimental studies on a so-called solar ground-source heat-pump system with avertical double-spiral coil ground heat exchanger andconcluded that the utilization of this heat pump systemis feasible. Although various studies were undertaken toevaluate the performance of SAHPSs, as describedpreviously, to the best of authors’ knowledge, no studiespublished on the experimental performance testing of aSAHPS with a 50 m vertical 32 mm nominal diameter U-bend ground heat exchanger (GHE) for greenhouseheating have appeared in the open literature.

The study reported here includes the performanceevaluation of a vertical SAHPS with R-22 as therefrigerant in the heating mode. A flat-type solar collectoris directly installed into the ground-coupled loop. Anexperimental set-up, described in the next section, isconstructed and tested for the first time on the basis of auniversity study performed in the country. The coefficient

Expansion tan

Valve

β

Solar collectorVIII

50 m

Sun

Ground level

Ground heaexchanger

V

Fig. 1. The main components and

Fig. 2. Various views o

of performance (COP) of the heat pump itself and thewhole system is computed from the measurements.

2. System description

2.1. Experimental set-up

A schematic diagram and some photographs of theconstructed experimental system are illustrated in Figs.1 and 2, respectively, while its comprehensive descrip-tion without solar collector and greenhouse is givenelsewhere [19]. This system mainly consists of threeseparate circuits as follows: (i) the ground couplingcircuit with solar collector (brine circuit or water–antifreeze solution circuit), (ii) the refrigerant circuit (ora reversible vapour compression cycle) and (iii) the fan-coil circuit for greenhouse heating (water circuit). Themain characteristics of the elements of the solar-assisted

k VII

GreenhouseXI

X

GSHP unit

Pump IIVI

IV

I

IIII Pump

IIX

II

t

schematic of the SAGSHPS.

f the SAGSHPS.

ARTICLE IN PRESSO. Ozgener, A. Hepbasli / Building and Environment 40 (2005) 1040–10501044

ground-source (geothermal) heat pump system(SAGSHPS)are given in Table 1, where the numbersin parentheses correspond to these elements as depictedin Fig. 1. Conversion from the heating cycle to thecooling cycle is obtained by means of a four-way valve.To avoid freezing the water under the working conditionand during the winter, a 10% ethyl glycol mixture byweight was prepared. The refrigerant circuit was built onthe closed-loop copper tubing. The working fluid is R-22. The SAGSHPS studied was installed at Solar EnergyInstitute of Ege University (latitude 381240N, longitude271500E), Izmir, Turkey. Solar greenhouse was posi-tioned towards the south along south–north. Thegreenhouse will be conditioned during the summer andwinter seasons according to the type of the agriculturalproducts to be raised in it.

2.2. Measurements

The following data were regularly recorded with atime interval of 15 min:

(a)

Tab

Mai

Mai

Refr

Gro

Fan

Measurement of mass flow rates of the water/antifreeze solution by a rotameter.

(b)

Measurement of mass flow rates of the refrigerationby a flowmeter.

(c)

Measurement of temperature of the water/antifreezesolution entering and leaving the GHE and solarcollector by copper–constantan thermocouplesmounted on the unit water inlet and outlet lines.

le 1

n characteristics of the SAGSHP

n circuit Element Technical s

igerant circuit Compressor (I) Type: herm

volumetric

driving: 2 H

condensing

Heat exchanger (II)

� Condenser for heating

� Evaporator for cooling

Manufactu

surface: 0.5

Capillary tube (III) Copper cap

Heat exchanger (IV)

� Evaporator for heating

� Condenser for cooling

Manufactu

surface: 0.8

und coupling circuit Ground heat exchanger (V) Vertical-sin

of a bore d

polyethylen

Brine circulating pump

(VI)

Manufactu

m3/h; press

Expansion tank (VII) Manufactu

Solar collector (VIII) 1.82 m2, fla

-coil circuit Water circulating pump

(IX)

Manufactu

m3/h; press

Fan-coil unit (X) Manufactu

flow rate: 6

Greenhouse (XI) GRP surfac

(d)

pecifi

etic;

flow

P (1

tem

rer: A

5 m2

illar

rer: A

0 m2

gle U

iame

e

rer: M

ure h

rer: Z

t-typ

rer: M

ure h

rer: A

00 m

e ar

Measurement of condenser and evaporator pres-sures by Bourdon-type manometers.

(e)

Measurement of ambient atmospheric pressure by abarometer.

(f)

Measurement of outdoor and greenhouse air tem-peratures and humidities by using multi-channelcable-free thermo-hygrometer.

(g)

Measurement of electrical power input tothe compressor and circulating pump by awattmeter.

(h)

Measurement of inlet water temperature to and exitwater temperature from fan-coil unit by copper–constantan thermocouples.

(i)

Measurement of surface temperatures of glassreinforced plastics (GRP) greenhouse by inferredthermometer.

(j)

Measurement of wind velocities at the height of 12 mby the Vantage Pro Meteorological Station installedin the Solar Energy Institute of Ege University,respectively.

(k)

Measurement of solar flux inside and outsidegreenhouse by an Eppley Black and White pyran-ometer and the Vantage Pro Meteorological Stationinstalled in the Solar Energy Institute of EgeUniversity, respectively.

(l)

Measurement and monitoring on a LCD displayof instantaneous power consumptions of theheat pump compressor, the pumps and allelectrical parameters by using electronic energyanalyser.

cation

reciprocating; manufacturer: Tecumseh; model: TFH 4524 F;

rate: 7.5 m3/h; speed: 2900 rpm; the rated power of electric motor

.4 kW); refrigerant: R-22; capacity: 4.134 kW (at evaporating/

peratures of 0/45 1C);

lfa Laval; model: CB 26-24; capacity: 6.66 kW; heat transfer

y tube; 1.5 m long; inside diameter: 1.5 mm

lfa Laval; model: CB 26-34; capacity: 8.2 kW; heat transfer

-bend type; bore diameter: 105 mm; Diameter of U-bends: 32 mm;

ter with a boring depth of 50 m; boring depth: 50 m; material:

arina; type: KPM 50; range of volumetric flow rate: 0.36–2.4

ead: 41–8 m of water column, power: 0.37 kW; speed: 2800 rpm

immet; type: 541/L; capacity: 12 l; precharge: 1 bar

e

arina; type: KPM 50; range of volumetric flow rate: 0.36–2.4

ead: 41–8 m of water column; power: 0.37 kW; speed: 2800 rpm

ldag; type: SAS 28; Cooling/heating capacities: 3.25/9.3 kW; air3/h

ea: 48.51 m2

ARTICLE IN PRESSO. Ozgener, A. Hepbasli / Building and Environment 40 (2005) 1040–1050 1045

The tests were conducted on the SAGSHPS understeady-state conditions in the heating mode over theperiod from 20th of January till 31st of March 2004.Daily average values of 37 measurements from 8.30 a.m.to 4.00 p.m. with an interval of 15 min are given inTable 2.

Uncertainty analysis is needed to prove the accuracyof the experiments. An uncertainty analysis wasperformed using the method described by Holman[20]. In the present study, the temperatures, flow rates,pressure drops, voltages and amperes were measuredwith appropriate instruments explained previously.Total uncertainties of these measured parameters arepresented in Table 2.

3. Analysis

When designing a heating system for a greenhouse,the first step is calculating how much heat energy mustbe supplied. The amount of heat necessary to keep the

Table 2

Measured parameters in average and their total uncertainties in average

Item

Average energy consumption of all systems

Average power input to the compressor

Power input to the brine circulating pump

Power input to the water circulating pump

Total power input to the fan of the fan-coil unit

Current of antifreeze solution circulating pump at ground heat exchanger (G

Current of water circulating pump at fan-coil side

Maximum phase to phase voltage (VLL)

Maximum phase voltages (VLN)

Average phase voltage (V)

Average phase to phase voltage

Total maximum current (SA)

Frequency

Average power factor (cosC)

Evaporation (low) pressure

Condensation (high) pressure

Condensing temperature

Evaporating temperature

Temperature of water at GHE inlet

Temperature of water at GHE outlet (solar collector inlet)

Temperature of water at solar collector outlet

Supply water temperature of fan-coil unit

Return water temperature of fan-coil unit

Volumetric flow rate of brine

Volumetric flow rate of refrigerant

Outdoor temperature

Outdoor relative humidity

Greenhouse inside design relative humidity

Solar radiation inside the greenhouse

Solar radiation outside the greenhouse

Wind velocity at a height of 12 m

Length (width) of the collector

greenhouse warm can be determined by adding up theamount of heat that is lost and must be replaced. Thecalculation is made for heat loss under the averagehighest heat loss conditions. The average winter lowtemperature for the coldest month is used. The secondstep is selecting an economical and efficient heatingsystem to supply the heat energy [21].

The present work is geared towards studying theperformance of a ground-source heat pump coupled to amodel-sized greenhouse to meet its heating, cooling anddehumidification requirements.

3.1. Useful solar energy

The solar radiation equations given below are takenfrom Duffie and Beckman [22,23]. Ratio of theinstantaneous direct solar radiation on a tilted surfaceto the instantaneous direct solar radiation on ahorizontal surface is calculated by the equation

Rb ¼cosðf� bÞ cos d cos oþ sinðf� bÞ sin d

cos f cos d cos oþ sin f sin d: (1)

Nominal value Unit Total uncertainty (%)

1.335 kW 71.02

0.806 kW 71.02

0.059 kW 71.02

0.059 kW 71.02

0.048 kW 71.02

HE) side 0.33 A 71.02

0.33 A 71.02

407 V 71.02

232 V 71.02

220 V 71.02

380 V 71.02

4.93 A 71.02

50 Hz 71.02

0.80 Dimensionless 71.02

0.425 MPa 73.32

2.8 MPa 73.32

66.75 1C 73.33

�4.74 1C 73.33

14 1C 71.59

17.2 1C 71.59

17.7 1C 71.59

52 1C 71.59

42 1C 71.59

0.0002 m3/s 73.01

0.0186� 10�3 m3/s 71.50

13.34 1C 71.59

60.47 % 71.02

43 % 71.02

110.87 W/m 71.81

138.59 W/m 70.80

3.39 m/s 72.00

1.94 (0.94) m 71.51

ARTICLE IN PRESSO. Ozgener, A. Hepbasli / Building and Environment 40 (2005) 1040–10501046

The total irradiance on a tilted surface under clearsky conditions is computed from the followingequation:

IT ¼ RbIb þ Id1 þ cos b

2

� �þ rI

1 � cos b2

� �: (2)

Ratio of the total radiation on a tilted surface to thaton a horizontal surface is expressed as

R ¼IT

I¼ Rb 1 �

Id

I

� �þ

Id

I

1 þ cos b2

� �

þ r1 � cos b

2

� �: ð3Þ

A flat-plate solar collector was used in the experi-mental set-up. It was positioned towards the south alongsouth–north at a tilt angle of latitude plus 151(531),which is the optimum angle for winter application inIzmir. Optimum tilt angle is given as f 15 in thenorthern hemisphere by Duffie and Backman [23].However, different recommendations such as f 10

have been made for the optimum tilt angle in the heatingseason depending on the latitude [24,25].

The performance of SAGSHPS is studied during thecold periods. However, only clear skies and monthlyaverage clearness index base equations are given in thepresent study. Monthly average clearness index (KT) isthe ratio of monthly average daily radiation on ahorizontal surface (H) to the monthly average dailyextraterrestrial radiation (H0), that is, [23,26]

KT ¼H

H0: (4)

Cloudy or partially cloudy skies effects take intoaccount. Average clearness index was found as about0.56 for this experimental period.

The instantaneous usable energy collected by solarcollector is calculated by the following equation:

_Qu ¼ FRAC ITðtaÞ � ULðT i � TaÞ½ � ¼ _mCp;waðTo � T iÞ:

(5)

The collector’s instantaneous efficiency is computed asfollows:

Zc ¼_Qu

AcIT: (6)

3.2. Solar greenhouse

A heat loss calculation is the first step in determiningheating system capacity before selecting the system andits various components. The heating system should beproperly sized to needs of greenhouse under extremeweather conditions. The rate of heat loss from the

greenhouse is calculated by the following equation [27]:

_QGRP ¼A1

R1þ

A2

R2þ

� �ðT i � ToÞðf wÞðf cÞðf sÞ: (7)

Using Eq. (6) and assuming that the construction typefactor ðf cÞ; the system factor ðf sÞ; and the wind factorðf wÞ are 1.08, 1.00, and 1.13, respectively, the averageheating load of the prototype solar greenhouseconsidered is obtained to be 7.4 kW at designconditions. In this calculation, GRP surface area ofgreenhouses is 48.51 m2, the thermal resistance of GRPis 0.16 m2

1C/W and the temperature difference betweengreenhouse inside and outdoor temperatures ðT i2ToÞ is20 1C.

3.3. Ground-source heat pump system

During the cycle calculations for the GSHPS,the following assumptions were made: (i) thevolumetric efficiency of the compressor was taken tobe 85%; (ii) compressor isentropic efficiency was takento be 71%, and (iii) there were no pressure losses in thecycle.

The rate of heat extracted (absorbed) by the unit inthe heating mode (GHE load) _Qe is calculated from thefollowing equation:

_Qe ¼ _mwaCp;waðTo;wa � T i;waÞ; (8)

where Cp;wa is the specific heat of the water–antifreezesolution, _mwa is the mass flow rate of the water/antifreeze solution and ðTo;wa2T i;waÞ is the temperaturedifference between the outlet and inlet of the GHE.

The heat rejection rate in the condenser is calculatedby

_Qco ¼ _mrðh2 � h3Þ: (9)

The heat transfer rate in the evaporator is

_Qev ¼ _mrðh1 � h4Þ: (10)

The work input rate to the compressor is

_W comp ¼_mrðh2s � h1Þ

ZicZmc

: (11)

Hence, the COP of the GSHP can be calculated as

COPHP ¼_Qco

_W comp

: (12)

The coefficient of performance of the overall heatingsystem ðCOPsÞ; which is the ratio of the condenser loadto total work consumptions of the compressor, the brineand water circulation pumps, and the fan-coil unit, iscomputed by the following equation:

COPs ¼_Qco

_W comp þ _W pumps þ _W fc

: (13)

ARTICLE IN PRESS

0

50

100

150

200

250

300

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70Days

Sola

r ra

diat

ion

(W/m

2 )

Total solar radiation outside greenhouse Total solar radiation inside greenhouse

Fig. 5. Daily average hourly solar radiation on a horizontal surface

ambient and greenhouse inside.

O. Ozgener, A. Hepbasli / Building and Environment 40 (2005) 1040–1050 1047

4. Results and discussion

In the present study, the results obtained from theexperiments over the heating period from 20th ofJanuary till 31st of March 2004 were evaluated todetermine the performance characteristics of the systemstudied. Figs. 3–7 show the variation of daily averagevalues of outside air and greenhouse temperatures,relative humidity, solar radiation data on the horizontalsurface, wind velocity at a height of 12 m and thegreenhouse level for this period, and ground surfacetemperatures outside and inside greenhouse. Totaluncertainties of the calculated parameters are illustratedin Table 3. The ambient air temperatures varied from�3.32 to 21.05 1C, the greenhouse average temperaturesfrom 6.68 to 31.05 1C, and the solar insolation incidenton the horizontal surface at the inside and outside of thegreenhouse from 0 to 199.10 W/m2 and from 0 to248.88 W/m2, respectively. The average values of thetemperature and the relative humidity for the ambientair and the greenhouse are obtained to be 10 1C and60.47%, and 20 1C and 35.74%, respectively. Theground surface temperatures at the inside and outsideof the greenhouse ranged from 4.68 to 26.35 1C, andfrom �5.32 to 16.33 1C, respectively.

The pumping brine flow rate was found to be0.185 m3/h per kW of heating capacity, with a pumpingpower of 14.06 W/kW of heating. Table 4 illustrates

-10-50 5

101520253035

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70

Days

Tem

pera

ture

s (º

C)

Ambient temperature Greenhouse temperature

Fig. 3. Daily average hourly values of ambient and greenhouse

temperatures.

0

10

20

30

40

50

60

70

80

90

Rel

ativ

e h

um

idu

ty (

%)

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70

Days

Ambient humiduty Greenhouse humiduty

Fig. 4. Daily average hourly values of ambient and greenhouse

humidity.

benchmarks for GSHP system pumping efficiencyrequired pump power to cooling capacity [28,29]. It isclear from this table that the circulator wattage for theclosed loop can be categorized as acceptable systemswith a good grade.

The entering water temperature (EWT) to the unit(the temperature of the water/antifreeze solution leavingthe earth coil) will be lower than the normal temperatureof the earth. This is due to the heat extraction from theearth to the circulating water [30]. The EWT is perhapsthe single most representative parameter of the groundcoupling effectiveness and heat pump loading. In other

02468

10

1214

16

1 4 7 10 13 16 19 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70

Days

Win

d ve

loci

ty (

m/s

)

Wind velocity at 12 m Wind velocity at greenhouse level

22

Fig. 6. Daily average hourly wind velocity values of greenhouse level

and at 12 m.

-10-505

1015202530

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70

Days

Tem

pera

ture

(ºC

)

Ground surface temperature outside greenhouse

Ground surface temperature inside greenhouse

Fig. 7. Daily minimal hourly values of ground surface temperatures at

the inside and outside of the greenhouse.

ARTICLE IN PRESS

Table 3

Total uncertainties of the calculated parameters in average

Item Nominal value Unit Total uncertainty (%)

Mass flow rate of refrigerant (R-22) 0.02 kg/s 71.51

Mass flow rate of brine 0.21 kg/s 73.02

Solar collector efficiency 57.31 % 74.01

Pumping flow rate of the brine circulator per kW of heating capacity 14.06 W/kW 73.57

Heating load of the greenhouse tested 4.19 kW 73.42

Heat extraction rate from the ground 2.89 kW 73.42

Heat extraction rate per meter of bore depth 57.78 W/m 73.74

GHE borehole length in meter per kW of heating 11.92 m/kW 73.74

Heating COP range of the heat pump 2.00–3.13 Dimensionless 73.57

Overall heating COP range of the system 1.70–2.60 Dimensionless 73.42

Collector area 1.82 m2 72.14

Uncertainty in reading values of table 70.20

Table 4

Benchmarks for GSHP system pumping efficiency required pump

power to cooling capacity [25,26]

Watts input Performance

Per ton Per kW Efficiency Grade

p50 p14 Efficient systems A: excellent

50–75 14–21 Acceptable systems B: good

75–100 21–28 Acceptable systems C: medium

100–150 28–42 Inefficient systems D: poor

4150 442 Inefficient systems E: bad

O. Ozgener, A. Hepbasli / Building and Environment 40 (2005) 1040–10501048

words, the actual performance of the equipment is afunction of the water temperature produced by theGHE. The average EWT over this period was approxi-mately 14 1C. The average temperature difference ofwater–antifreeze solution between the inlet and outlet ofthe GHE was obtained to be approximately 3.2 1C.

Heat extraction rate is the key parameter for the GHElayout is the specific performance, i.e. the heat extrac-tion rate in Watt per metre of borehole length. Duringthe heating season, the rate at which heat is extractedfrom the ground (GHE load) was found to be onaverage 2.889 kW from Eq. (7). This corresponds to aheat extraction rate of 57.78 W/m of bore depth. Bycomparison, Sanner [31] reported that the heat rejectionrate ranges from 40 to 100 W/m with a typical average of55–70 W/m in Mid Europe. This obviously representsthat the values of the heat extraction rate obtained fromthis study remain the range reported by Sanner.

The heating capacity of the heat pump system wasobtained to be 4.194 kW. The required borehole lengthsin metre per kW of heating capacity were found to be11.92. By comparison, based on the installations inIstanbul, Turkey (1933 degree days heating, base: 18 1C;135 days cooling, base: 22 1C) for horizontal loopsystems, the heat exchanger circuit pipe length rangedfrom 39.5 to 46.7 m/kW with an average of 43.1 m/kW[32,33] as confirmed by Healy and Ugursal [34] with

values of 45–55 m/kW of heat pump capacity. Inaddition, based on a study that covered 22 systemsand was conducted by Bloomquist [35] in the UnitedSates, for closed-loop systems, the heat exchanger circuitpipe length ranged from 20.5 to 52.0 m/kW, with anaverage of 39.3 m/kW. Of those with vertical bores, therange was 14.4–17.7 m of bore per kW. As indicatedabove, the heat exchanger length used may maintain theheating capacity needed. However, the heat pumpdesign should be carefully examined in order to achievethis.

Fig. 8. shows the collector efficiency as a function ofthe ratio of the difference between the fluid inlet andoutside air temperatures to the solar insolation incidenton the solar collector surface. It is obvious from thisfigure that the collector efficiency varied between about0.35 and 0.70. The daily variations of COPHP and COPs

for the period studied are illustrated in Fig. 9. Thevalues for COPHP and COPs were found to be in a rangeamong 2.00–3.13 and 1.7–2.6, respectively, while themaximum uncertainties associated with COPHP andCOPs were 75.83% and 75.81%, respectively.

By comparison, in a study performed by Bi et al. [17],the total average performance in the heating season isfound as follows: the heating load is 2334 W and theCOP is 2.73 for the solar energy-source heat-pumpsystem; the heating load is 2298 W and the COP is 2.83for the ground-source heat-pump system; and theheating load is 2316 W and the COP is 2.78 for thesolar ground-source heat-pump system. It may beconcluded that the COP values obtained from thepresent study are fairly close to those reported by Biet al. [17].

5. Conclusions

An experimental system was installed for investigatingthe thermal performance of a SAGSHPS for greenhouseheating. It has been satisfactorily operated without any

ARTICLE IN PRESS

0

0.2

0.4

0.6

0.8

1

0.0000 0.0100 0.0200 0.0300 0.0400 0.0500 0.0600(Ti-Ta) / IT

Col

lect

or e

ffic

ienc

y,η c

Fig. 8. Effect of collector fluid temperature on the collector efficiency.

-505

10152025

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70

Days

Tem

pera

ture

s (

ºC)

0

1

2

3

4

CO

P

Ambient temperature COPs COPHP

Fig. 9. Daily average values of COPHP and COPs over a period

between 20 January and 31 March 2004.

O. Ozgener, A. Hepbasli / Building and Environment 40 (2005) 1040–1050 1049

serious defects in the (2003/2004) heating season.The results obtained during the heating period of 20thof January till 31st of March 2004 were given anddiscussed. The effects of climatic conditions andoperating parameters on the system performance para-meters were also investigated. The experimental resultsindicate that this SAGSHPS can be used for greenhouseheating in the Mediterranean and Aegean regions ofTurkey. The conclusions drawn from the present studymay be summarized as follows:

1.

The energy performance of a GSHP system isinfluenced by three primary factors: (a) the heatpump unit, (b) the circulating pump or well pumps,and (c) the GHE.(a) The heat pump unit: The values for COPHP varied

from 2.00 to 3.13, while those for COPs wereapproximately 5–20% lower than COPHP. Usinga scroll compressor instead of an hermeticcompressor in this study will lead to the increasein the COP values obtained.

(b) The circulating pump: The pumping brine flowrate was found to be 0.185 m3/h per kW ofheating capacity. Kavanaugh suggests that theoptimum pumping rates for the circulating pumpshould range from 0.162 to 0.192 m3/h per kW ofheating capacity [33]. It may be concluded thatthe pump selected falls into the acceptable limitswith a good grade.

(c) The GHE: Design practices in Turkey normallycall for U-bend depths between 11 and 13 m/kWof heating, while the required borehole length inmetre per kW of heating capacity was obtained tobe 11.92. These values are close to the lower limitquoted by Kavanaugh [36, 37], with a range of15–25 m/kW of cooling.

2.

Experimental results show that monovalent centralheating operation (independent of any other heatingsystem) cannot be met overall heat loss of greenhouseif ambient temperature is very low. The bivalentoperation (combined with other heating system) canbe suggested as the best solution in Mediterraneanand Aegean region in Turkey, if peak load heatingcan be easily controlled.

Acknowledgements

The authors gratefully acknowledge financial supportfrom the Ege University Research Fund, State PlanningOrganization of Turkey (DPT), Arges Inc., Entes Inc.,Guz Technical Service Co., and Golden Seed Inc.,Turkey. In addition, the valuable comments of thereviewers are gratefully acknowledged.

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