Computational Analysis of Two Arrangements of a Central Ground-Source Heat Pump System for Residential Buildings

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1 Computational Analysis of Two Arrangements of a Central Ground-Soure Heat Pump System for Residential Buildings Abstrat Ehab Foda, Ala Hasan, Kai Sirén Helsinki University of Tehnology, HVAC Tehnology, FI-25 TKK, Finland ehab.foda@tkk.fi This study aims at obtaining a omputational omparison between the performane of two different ground-soure heat pump (GSHP) entral system arrangements used for heating, ooling and domesti hot water (DHW) energy prodution for a residential area. The required spae heating and ooling energy for the buildings is alulated using dynami building performane simulation, ground heat exhange simulation and a GSHP model reated to study the hourly performane of the system. The system performane is determined and the total required eletrial energy is alulated for the two arrangements. The results obtained from this ase study show that the seasonal performane fator (SPF) for the arrangement that totally uses the GSHP in the DHW energy prodution is onsiderably higher than the partial use of GSHP for that purpose.. Introdution Ground-soure heat pump (GSHP) has beome a popular method for produing heating, ooling and domesti hot water (DHW) energies for residential buildings using entralised or deentralised systems. The objetive of this study is to develop basi onepts and perform omputational omparisons to serve as a starting point for the energy and HVAC system design of a residential area at Nupuri-Espoo, Finland. In this study, the GSHP system performane is analysed based on two different system arrangements, where the main variant is the method of DHW energy prodution. System Arrangement partly uses the GSHP system in the DHW energy prodution along with a loal storage tank in eah house, whih is equipped by a bak-up eletrial heater to provide the DHW at a proper final temperature. System Arrangement 2 totally produes the DHW energy using a 2-stage or asade GSHP system. The simulation of the building energy performane is arried out using IDA ICE 3. (IDA Indoor Climate and Energy) software [] while the ground side simulation is obtained using EED 2. (Earth Energy Designer) software [2]. Then, the output results from IDA ICE 3. and EED 2. are entered to a GSHP model that is reated to analyse the performane of the system. The hourly energy onsumption of the GSHP system is alulated then the oeffiient of performane (COP) and the seasonal performane fator (SPF) for the two arrangements are ompared for better energy performane of the system. 2. Nupuri residential area Nupuri residential area is an area of 4 hetares loated in Hista region at Espoo ity. It onsists of 29 houses of different types (attahed, semi-detahed, luster and single family houses). The buildings are arranged into 7 residential bloks. The number of buildings per eah blok varies from a single building to several buildings inluding servie buildings suh as day-are, gas-station and food stores. The building bloks are divided into 4 groups and

2 eah group has its own energy prodution system. In this study, one of those 4 groups is seleted as a ase study and is named Group. 2. Nupuri house A typial Nupuri house is a two-floor attahed or semi-detahed house. The net area of the houses varies from 8 m² to 58m². The onstrution overall heat transfer oeffiients (Uvalues) are as shown in Table ; these are 3% lower than the maximum values stated in the Finnish building ode for thermal insulations (C3 for the year 27) [3]. Table. U-values of the onstrution. Item U-value (W /m 2 K) External walls.7 External roof. External floor.7 Windows. External doors. 2.2 IDA ICE 3. model house Preliminary arhiteture drawings are used in this study as the basis for the building energy alulations. A typial house is onsidered from those drawings to onstrut the model house on IDA ICE 3. [] and as shown in Fig.. The speifiations of the house are shown in Table 2. The house net area is 28 m 2 with a room height of 2.5 m. The number of oupants is five. The house is divided into five main zones aording to different heating and ooling system settings in eah zone. The fresh and exhaust air are mehanially maintained aording to the ommon approah for residential houses in Finland, whih is supplying the fresh air into bedrooms and living room while the exhaust is taken through the bathrooms and the kithen ventilation hood. Profiles of internal heat gains in the house are assumed for oupants, applianes and lighting, as well as DHW onsumption. IDA ICE 3. hourly simulation results are used to alulate the spae heating and ooling energies per square meter. Then, the heating and ooling systems losses are added aording to the Finnish building ode (D5) [4]. The DHW energy and the eletrial energy onsumption for domesti applianes are diretly estimated aording to the D5 ode [4] (a) Ground Level (b) Upper Level Fig.. Zones in IDA ICE 3. Model House:. Stairase, 2. Living room + kithen + one bedroom, 3. Bathroom, 4. Bathroom, 5. Three bedrooms + orridor.

3 The heating system of the house is under-floor heating system. The water supply temperature from GSHP is linearly proportional to the outdoor air temperature (supply temperatures of 35 C and 2 C at outdoor air temperatures of -26 C and 2 C, respetively). The same heating water is also supplied to the ventilation air handling units. The under-floor water piping is used for ooling during summer, where ooling water is supplied at 8 C by a free ooling heat exhanger. However, this is not appliable for the bathrooms zones whereas the heating is always used in all seasons. The final DHW temperature at onsumers is 55 C that is provided by two different system arrangements. Table 3. IDA ICE 3. Model speifiations Item Speifiations Windows Type Pilkington Sun-ool HP Brilliant 5 Internal shading Blinds between panes 5% open External shading 45 mm windows reess and balony on western façade Heating system Under-floor water heating Heating set point temperature 2 C for rooms and 22 C for bathrooms Cooling system Under-floor water ooling Cooling set point temperature 26 C Internal heat gains Aording to D5 ode [4] Ventilation system Mehanial system (supply air at 8 C) Ventilation flow rate 53.3 l/s =.6 air hange per hour (ACH) Heat reovery system effiieny.7 (Annual average value) Tightness n5.5 ACH Outdoor onditions Helsinki 2 weather file 2.3 EED 2. Ground model EED 2. software is used for the GSHP ground heat exhange simulation. EED software uses algorithms that have been derived from modelling and parameter studies with a numerial simulation model (SBM) resulting in analytial solutions of the heat flow with several ombinations for the bore hole pattern and geometry (g-funtions) [2]. The monthly average heating and ooling loads, the key ground parameters (thermal ondutivity and speifi heat) as well as properties of pipe materials, a preliminary onfiguration (g-funtion) and a heat arrier fluid are the input data to the EED. The borehole thermal resistane is then alulated in the program, using the borehole geometry, grouting material, pipe material and geometry. A fluid temperature onstraint in the software is used to be /5 C as min/max values. Calulations of the boreholes depth (in meters) was arried out by Geologial Survey of Finland (GTK) [5] to fulfil that onstraint. The number of boreholes for Group buildings is estimated to be 4 boreholes with diameter of 4 mm and using single U-tube 4mm with average depth of 89 m and boreholes spaing of 7 m. 3. GSHP System arrangements The GSHP system is studied based on two different arrangements for heating and ooling energy prodution. The main differene between the two systems is related to the DHW prodution. The GSHP ompressor (for both arrangements) is sized to over 8% of the heating power. An auxiliary system will then be needed to over the peak loads. The main reason for that is to avoid over-sizing of the GSHP system and to ensure the performane of the GSHP lose to its full load performane. In addition, overing % of the heating demand by the heat pump may slightly lower the heating ost but the savings may not offset

4 the added ost for a larger GSHP system. The ooling load is planned to be totally overed by free ooling, that is heat exhange with the ground fluid without using the heat pump. 3. Arrangement Arrangement uses a GSHP to produe the spae heating energy and it is sized to produe part of the DHW energy by passing part of the supply water through a DHW heat exhanger. The DHW heat exhanger is used to preheat the ity water. Then, the loal bak-up heater provides the DHW at the fixture temperature (i.e. 55 C). The irulation of the DHW is maintained loally. Arrangement main omponents (as shown in Fig. 2) are: a stationary GSHP that is mainly produing the spae heating energy; a DHW heat exhanger; a free ooling heat exhanger as the main ooling system; pumps for the irulation of heating and ooling water as well as a loal DHW irulation pump at eah onsumer and a bak-up eletrial heater storage tank at eah onsumer. Arrangement is thought to be a flexible arrangement to assure the temperature of the DHW supply. Fig.2. GSHP System Arrangement 3.2 Arrangement 2 Arrangement 2 uses a two-stage or a asade GSHP to produe the spae heating and the DHW energies. The DHW plant supply temperature is designed to be 6 C to ompensate Fig.3. GSHP System Arrangement 2

5 for the irulation losses and also to obtain peak hours entral storage at 55 C. The DHW irulation is one of the disadvantages of Arrangement 2 sine the water will keep irulating between the plant and the group of houses to maintain the DHW fixture temperature or storage temperature always at 55 C. Arrangement 2 main omponents (as shown in Fig.3) are: st stage GSHP that is mainly produing the spae heating energy; 2 nd stage heat pump to supply the DHW at 6 C; a free ooling heat exhanger as the main ooling system; pumps for the irulation of the heating, ooling and DHW water. 4. GSHP Modelling The GSHP model for eah system arrangement inludes a similar approah in most of the alulations. The hourly load ( Q ) from IDA ICE 3. alulations and the hourly outdoor air temperature ( T o ) from Helsinki 2 weather data are used as hourly input data. The supply water temperature ( T s ) varies linearly with the outdoor air temperature ( T o ). An estimation of the ondutane of the spae heating system ( G r ) is made based on the peak heating power. Then, the return temperature ( T r ) is alulated iteratively with the water mass flow rate ( m ) from G r Tr = Troom + exp ln( Ts Troom ) () m p Q m = ( Ts Tr ) (2) p The mass flow rate is then alulated based on the produed power. The ompressor is sized to over 8% of the peak power. The ondenser ondutane ( G ) is alulated at the peak power by assuming that the ondenser temperature ( T ) is 5 K higher than the water supply temperature ( T s ) exp GC T s Tr m p T = (3) exp GC m p The brine mean temperature is taken from EED 2. results as hourly input data. It is adjusted in the model to have a onstant brine temperature differene ( Tb Tb 2 ) equal to 5 C. The ondutane of the evaporator ( G ) is alulated at the peak power assuming that the e evaporator temperature is 5 K lower than the brine return temperature ( T b2 ). The alulation of the evaporator temperature involves an iterating operation with other parameters as the COP, ompressor power (P) and the brine mass flow rate ( m ) from T e exp Ge T 2 b Tb me pb = exp Ge me pb where C pb is the brine heat apaity in kj/kg. The COP is alulated from e (4)

6 T COP = η T T e where η is the ompressor power fator or yle effiieny (equals.6 aording to Bitzer sroll ompressor data sheet). The ompressor power (P) is alulated from Q P = (6) COP The alulation of the ooling mode is based only on the free ooling. The sizing of the free ooling heat exhanger is based on the peak ooling load. The plant supply temperature of the ooling water is onstant (8 C) that the temperature will be adjusted at eah zone by the 3- way ontrol valve. A similar approah to the heating mode is used to alulate the return temperature. The mass flow rate is diretly alulated from the water temperatures and the ooling load. The pumping energy for the brine iruit is alulated based on two different alternatives for boreholes distribution in paved streets or forest walkways. However, the pumping energy for both options does not muh vary from eah other sine the pressure drop in the iruit is mainly through the borehole U-tube. The alulation is done by first sizing the iruit based on the maximum flow rate and then alulating the iruit onstant that is used to alulate the hourly pressure drop and thereby the pumping energies. The heating and ooling water iruits are alulated in a similar approah. 5. Results Fig. 4 shows results from IDA ICE 3. simulation software for the hourly performane of Group buildings around the year. Fig 4a indiates the sum of spae heating and DHW as well as the ooling power demand while Fig. 4b shows the spae heating, DHW and ooling powers on a duration diagram for the two arrangements. The duration diagram sorts the hourly power data for one year in a desending order. The negative values refer to the ooling power. It shows the power demand for heating and the overed power by the GSHP, where the auxiliary system handles the differene. Fig. 5a shows the results from EED for the monthly brine temperatures between the boreholes and the GSHP evaporator while Fig.5b indiates the brine pumping power for Group buildings based on two different options for the boreholes distribution in surrounding paved streets or forest walkways. Fig. 6 shows the GSHP model results. Fig. 6a gives the ompressors power for the two arrangements in a duration diagram while Fig. 6b shows hourly heating COP for the two arrangements. The omparison between the two system arrangements based on the seasonal performane fator SPF is neessary due to the use of the bak-up eletrial heater in Arrangement. The SPF is basially the sum of the annual system output energies divided by the annual needed eletrial energies on the whole system boundary inluding ompressors, pumps, fans, and eletrial heaters. The SPF is alulated as: QC + QDHW. E. H SPF = (Arrangement ) (7) E + E + E ompressor pump DHW. E. H QC SPF = (Arrangement 2) (8) E + E ompressor pump where, Q C is the total ondenser output for spae heating and DHW in kwh, Q DHW. E. H is the DHW bak-up eletrial heater output in kwh, E ompressor is the eletrial onsumption of the (5)

7 GSHP ompressor in kwh, E pump is the eletrial onsumption of the brine and water pumps in kwh and E DHW. E. H is the DHW bak-up eletrial heater onsumption in kwh Maximum heating and DHW power demand = 282 kw Maximum ooling powr demand = 68 kw SPACE HEATING + DHW COOLING 3 2 Arrangement : - Needed auxillary power = 36kW. - The heat pump is always on for bathrooms heating in summertime and partly heat the DHW. - The loal eletri heater overs about 6% of the DHW energy. Arrangement 2: - Needed auxillary power = 34kW. - The heat pump is always on for bathrooms heating in summertime. - DHW energy is totally produed by the heat pump and stored at entral storage. POWER (kw) 5 POWER (kw) DHW eletrial heater Arrangement Arrangement Heat Demand Arr.2 Heat Covered Arr.2 Heat Demand Arr. Heat Covered Arr. Cooling Demand DHW by Eletriity in Arr. Free ooling: - Maximum power =68kW - Cooling Energy 52MW.h/a (a) (b) Fig.4. IDA ICE simulation results (a) Spae heating + DHW and ooling power demand, (b) Spae Heating, DHW and ooling power demand/overed in a duration diagram Cooling 2 8 Temperature (C) Mean Return to boreholes Supply to HP POWER (kw) Option : (Forest walkway) Brine pumping energy in ase of : - Heating by Arrangement =.27MWh/a - Heating by Arrangement 2 = 2.9MWh/a - Cooling =.2MWh/a Option 2: (Street) Brine pumping energy in ase of : - Heating by Arrangement =.5MWh/a - Heating by Arrangement 2 = 2.6MWh/a - Cooling =.7MWh/a Cooling opt Heating Arr.2 opt Cooling opt2 Heating Arr.2 opt2 Heating Arr. opt Heating Arr. opt Month (a) (b) Fig.5. EED output and Brine pumping (a) Brine monthly temperatures from EED simulation, (b) Brine pumping powers in a duration diagram 6 8. POWER (kw) Arrangement 2: - st and 2nd stages Max powers are 46kW and 3kW respetively. - The arrangement maximum power equals 5kW and it overs about 8% of the peak load. Arrangement : - Compressor Max. power is 48kW and it overs about 8% of the peak load. - The load inludes partly raising up the DHW upto C. COP Arrangement : Avg.annual COP = 6.4 Arrangement 2: Avg.annual COP = Arrangement 2 Arrangement (a) (b) Fig.6. GSHP model output (a) Compressors power for both arrangements in a duration diagram, (b)hourly heating COP for both arrangements. Arrangement 2 Arrangement

8 Fig.7. Eletrial energy onsumption for the two system arrangements The eletrial energy onsumption for the two arrangements is illustrated by the pie-harts shown in Fig. 7. It is found that, the total eletrial energy onsumption for Arrangement is more than twie the onsumption for Arrangement 2. Additionally, it an be noted that the eletrial energy onsumption in Arrangement by the eletrial heater is also more than twie by the ompressor. Calulation of SPF for both system arrangements shows that SPF for Arrangement 2 equals 5.9, while SPF for Arrangement equals Conlusions This study presents the dynami performane of the GSHP when linked to the building load on one side and the ground heat exhanger on the other side. This analysis has shown a onsiderably higher heating seasonal performane fator SPF with the system Arrangement 2 (two-stage or asade heat pumps) ompared to Arrangement (DHW is partly heated by GSHP before it is finally heated by an eletrial heater in a loal water tank). Arrangement 2 seasonal performane fator value is near to its average annual COP sine the system eletrial energy onsumption is mainly by the heat pump ompressors. System Arrangement 2 is therefore the preferable system arrangement from the energy onservation perspetive. The irulation of the domesti hot water between the onsumers and the entral plant and related losses would not affet the SPF of the system beause it is minor energy onsumption. However, to ensure that Arrangement 2 will over the instantaneous DHW peak demands and to redue the DHW heat pump size and operation yling, the use of entral storage tanks for groups of houses may be neessary. Further studies may inlude an optimization study to identify eah omponent relative importane with respet to operational onditions. Referenes [] IDA ICE (IDA Indoor Climate and Energy) [2] Earth energy designer EED 2. manual. [3] Finnish ode for thermal insulation C3 27, Ministry of the Environment. [4] Finnish building ode D5, Ministry of the Environment. [5] Geologial survey of Finland GTK, Nupuri report 28.