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2011 International Conference on Electronic &Mechanical Engineering and Information Technology

Performance Analysis of Seasonal Soil Heat Storage Air Conditioning System inSolar Ground Coupled Heat Pump

Fang Wang1,a, Zhilong Liu', Zhongjian Li2, Yuejun Liu ', Zuomin Wang1, Maoyu Zheng3IHarbin University ofScience and Technology Harbin, Heilongjiang Province, China

2United Technologies Research Center (China) Ltd. Shanghai, China3Harbin Institute ofTechnology Harbin, Heilongjiang Province, China"e-mail: wf69yuan@163.com

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Abstract-Aimed to ground source heat pump's low coefficientof performance and regional requirement, solar-groundcoupled heat pump system with soil heat storage has beenadvanced. Simulation analysis was done for the undergroundheat exchanger using finite element method. Variety trend ofsoil temperature with di fferent depths, d ifferent runningconditions and different properties of soil were revealed. Theanalysis results show that it is especially suitable for severecold areas and provides theoretical basis for residential using.

Keywords-solar-ground coupled heat pump (SGCHP); heatstorage;soil temperature;performanceI. INTRODUCTION

Ground source heat pump (GSHP) as we know has beenwidely used for years as its renewable and environmentalprotecting. The suitable areas to GSHP are those districtswhere the underground soil temperature is between 10C to20 C or more underground heat exchangers may be installedin cold climatic conditions. Solar-ground coupled heat pump(SGCHP) can solve the problem above. The basic objectivewith a SGCHP is to attain higher heating or coolingCoefficient of Performance (COP) in comparison to regularheat pump system.Solar assisted heat pumps have a variety of system buildup due to various components, temperature levels, thermalrequirement, climate and system output [1,2]. Theassessment of SGCHP system performance of variousdesigns and operations has been made by theoreticalsimulations or experimental tests [3,4]. Inalli and Esencompared thermal and economic performance of solarheating systems with seasonal storage [5,6]. Stojanovicstudied long-term performance test of a full-scale solarassisted heat pump. Analysis shows that the system wassuccessfully in full operation fulfilling heating requirementsfor low energy use with unfavorab le building conditions [7].

II. INTRODUCTION OF EXPERIMENTAL SYSTEMThe experimental system was established in severe coldarea in Harbin (lat4541', longI2637'), in China. Buildinggross area is 660m2, and heating area is about 570m2 Thebuilding is a conservation construction, exterior and interiorwall are saving walls, and there is a 150mm thickpolystyrene insulation outside the exterior wall. Windows are

978-1-61284-088-8/11/$26.00 2011 IEEE

three-layer 1.5mx1.8m conservation windows, which hasgood seal performance.Seasonal soil heat storage heating air-conditioning systemconsists of four parts: ceiling solar collecting system,underground embedded heat exchange system (soil heatstorage system), heat pump system and air conditioningterminal unit (floor radiant heating system). Besides there aretwo flat heat exchangers, one is for the heat exchangebetween solar hot water direct heating system and floorradiant heating system, the other is for the heat exchangebetween solar collection system and soil heat storage system.Figure 1 is the schematic diagram of soil storage SGCHPsystem.

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I I -1I-solar collector; II-soil heat exchanger; III-floor radiant system; IV-heat

pump; v" VI-flat heat exchanger; PI" P2" P3" P4-circulation pump; 121magnetic valve.Figure 1. Schematic diagram of soil storage SGCHP system

Solar collecting system is composed of four rows' seriesflat heat e x c h a n ~ e r s connecting parallel. Total collectingareas are 41.4m, which are installed on the 60 degreeobliquity south ceiling.Underground soil heat exchanging system adopted verticalU type heat exchangers embedded under 53depth soil. Singlevertical well with single U-tube connected parallel totally 12,which are 3 rows and 4 lines.Air-conditioning terminal unit used floor radiant heatingsystem, which is a new conservation terminal unit. Floor

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heating temperature range is between 30"-'50C, that makesit possible for solar collector or heat stored undergroundheating inwinter.III. SOIL HEAT EXCHANGER

(1)

Figure 2. Three dimensional diagram of vertical U-tube soil heatexchanger

(4)

(3)

A I - A aT: Ihe a r=r - s a r=rrout rout

-A aTs (z,r,e, r)1 = 0:s ar r=ro

-A aTs(z,r,(J,r )1 = 0s az z=53

I tn: IT -T =_----l2!...(/ he ) r ~ r i n 8r r ~ r i n

ab:

bc(ef):

Boundary condition between U'-type pipe outside walland soil is that:

Where Psis density of soil (kg/m ') ; Cs is specific heat ofsoil (kJ/kg C); T is transient temperature of soil (OC); r istime (s); z is depth of stratum (m); r is distance of onepoint to axes (m); () is tangential component (rad); A isconductivity of soil (W/m C).C. Initial condition and boundary conditionsInitial condition is expressed as following:

Where Tf is fluid temperature inside pipe; t; istemperature of soil heat exchanger wall; T: is temperature ofsurrounding soil.Boundary conditions are expressed as followings:Boundary condition between fluid in pipe and U-type pipeinside wall is that:

Boundaryab (bc,ef) is considered as adiabatic condition,they can be expressed as:

-53.00a bX

~ '" ~ B~ I> -< ~ .tQJlliL AriQQQ

000( " ' )tn

~ r : : s"'-;

,A ~ ' l '~

A. Physical Model ofSoil Heat ExchangerSoil heat exchanger is main component of the system.Heat transfer effect of vertical V type heat exchangeraffected the coefficient of performance (COP) directly.Actual heat transfer between soil heat exchanger andsurrounding soil is complicated and unsteady process. Inorder to analyze the problem conveniently, essentialsimp lifications must be done. The physical model wassimplified as following [8-10]: 1) Variety of conductivity isneglected caused by thermo coupled humid transfer effectand underground water advection heat transfer effect. Heat

transfer between soil and embedded exchangers isconsidered as pure conduction. Soil is stratified according tothe depth. Conductivity of each layer is constant. 2) No heattransfer is thought between the bottom of the vertical welland the soil. The boundary can be considered as adiabatic. 3)Thermal physical parameters of soil, grout, embedded pipesand the fluid in the pipes are not changed when heat transferhappened.In theory, effect of embedded pipe to the surrounding soilcan reach infmite area. In actual, with the increase ofdistance, influence of the U'-tube to the soil temperature fieldis smaller and smaller, it can be neglected when it is fardistance. Figure 2 is the regime considered in actualcalculation.r--A

Boundary cd is the third type condition, it can bedescribed as:

Boundary ad is the second type condition, it can becharacterized as:

B. MathematicalModel ofSoil Heat ExchangerSurrounding soil temperature of vertical U'-type embeddedpipe is a columnar temperature field, columnar coordinatesystem is selected. According to the assumption above,control equation of three dimensional temperature field ofsurrounding soil is given below:

tn; (z,r, e, t ) I [ ] (r)-As =ha ~ e - T : ( O , r , e , r ) 3aZ z=o(7)

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IV. ANALYSIS OF UNDERGROUND SOIL TEMERATUREAccording to the contro I equation, initial condition andboundary conditions, transient numerical simulation wasdone for soil temperature field. It was begun from Nov. ofthe first year. Nov. to nex t Apr. is heating period, Apr. toJune is heat s torage period, June to Sep. is air condit ioningperiod and Sep. to Nov. is the other heat storage period.Heating load is far higher than cooling load in severe coldareas. Soil stores heat by solar collection system intransitional seasons and summer. Heat is extracted form thesoil to meet the demand of great heating load in winter. Soiltemperature field was studied in succession for two years to

the layers of5m, 30m and 50m underground. Figure 3 is Soiltemperature variety surrounding heat exchangers.A-heating period; B- transitional period; C-air conditioning period

Figure 4. Variety of soil temperature in different operation condition intwo years' runningSoil is complex porous medium composed of solid, liquidand gas. Heat transfer process of saturation region of soil isconduction-convection coupled process. The conductivityand specific heat are two main affect factors of heat transfer

of underground embedded pipes. Three different propertiesof soils were selected for comparing. Table I is the propertiesof the soils.

TABLE!. PROPERTY OF SOIL IN DIFFERENT WATER CONTENT

A-heating period; B- heat storage period; C-air conditioning periodFigure 3. Variety of surrounding soil temperature for two years

It can be shown that vibration of soil temperature varietyis smaller in 30m and 50m depths than in 5m depth andtemperatures in these three layers are higher in the secondrunning cycle than in the first one. It can be gotten that heatmay be stored less in the second running year and the effectcan be the same with the first year.Soil heat s torage benefits to keeping soil heat balance,while soil temperature reduces after extract heat in winterheating period in GSHP or SGCHP system especially insevere cold areas. Figure 4 is soil temperature variety in 50mdepth in three running conditions. Differences among theseconditions are whether it has heat storage in transitionalperiod and it is SGCHP system or GSHP system.It was presented that soil temperature became the lowestin next Apr. Soil temperature resumed before the nextheating period. The effect of the resumption from good tobad is SGCHP with heat storage in transitional period,SGCHP without heat storage in transitional period andGSHP in tum. Soil temperature 's trend is rising only in thefirst condition; the trend of soil temperature is falling in theother two conditions. It cannot meet the demand of heatingin winter in severe cold areas in only GSHP system.

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Water Property of SoilContent Conductivity SpecificHeat Density(W/moC) (J/kgoC) (kg/nf):::;30% 1.89 1800 2125:::;40% 2.44 2120 1975>40% 3.02 2430 1825

A-heating period; B- heat storage period; C-air conditioning periodFigure 5. Variety of soil temperature in different soil properties

Figure 5 reveals the variety of soil temperature in differentsoil properties. It shows that conductivity of soil of water

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content bigger than 40 percent is the highest; heat exchangeis the most between soil and heat exchanger and thetemperature of soil rises faster in transitional period. Thus itis helpful for heating in winter in SGCHP system with heatstorage.

v. CONCLUSIONThis paper presents the long-term performance of aSGCHP system with soil heat storage for residential heatingin Harbin, a severe cold city in north-east in China. Thesystem consists of four parts, and the underground heatexchanging system is a part which influents the effect ofheating directly. Simulation was done of vertical U-tube heatexchanger by finite element method. It can be concluded asfollowing:1) Compared among 5m depth, 30m depth and 50m depth,the deeper the soil, the better storage property and stability of

soil temperature it has. It reveals that vertical U-tube heatexchanger had better have the depth over 50 meters.2) The soil temperature is higher than initial value withtwo years' running in SGCHP systemwith soil heat storage.Compared with no heat storage SGCHP and GSHP system, itindicates that heat storage in transitional period is essentialand helpful.3) Soil is a complex medium, its properties affects the heattransfer effect. Three kinds of soil were contrasted that highwater content soil has bigger conductivity, better heattransfer and faster restorability of soil temperature.SGCHP system with underground heat storage is fit toheating system in severe cold areas. It can satisfy therequirement of heating in long winter. It provides theoreticalbasis for residential using.

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ACKNOWLEDGMENTThis work was supported by the Program ofHeilongjiangProvince Educational Department (No.11531038)

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