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Applied Energy 126 (2014) 113–122
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Applied Energy
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Simulation of a combined heating, cooling and domestic hot watersystem based on ground source absorption heat pump
http://dx.doi.org/10.1016/j.apenergy.2014.04.0060306-2619/� 2014 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. Tel.: +86 10 62785860; fax: +86 10 62773461.E-mail address: [email protected] (X. Li).
Wei Wu, Tian You, Baolong Wang, Wenxing Shi, Xianting Li ⇑Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, China
h i g h l i g h t s
� A combined heating/cooling/DHW system based on GSAHP is proposed in cold regions.� The soil imbalance is effectively reduced and soil temperature can be kept stable.� 20% and 15% of condensation/absorption heat is recovered by GSAHP to produce DHW.� The combined system can improve the primary energy efficiency by 23.6% and 44.4%.
a r t i c l e i n f o
Article history:Received 15 November 2013Received in revised form 29 March 2014Accepted 1 April 2014Available online 24 April 2014
Keywords:Ground source absorption heat pumpThermal imbalanceHeatingDomestic hot waterPrimary energy efficiencyHeat recovery
a b s t r a c t
The amount of energy used for heating and domestic hot water (DHW) is very high and will keep increas-ing. The conventional ground source electrical heat pump used in heating-dominated buildings has theproblems of thermal imbalance, decrease of soil temperature, and deterioration of heating performance.Ground source absorption heat pump (GSAHP) is advantageous in both imbalance reduction and primaryenergy efficiency (PEE) improvement; however, the imbalance is still unacceptable in the warmer parts ofcold regions. A combined heating/cooling/DHW (HCD) system based on GSAHP is proposed to overcomethis problem. The GSAHPs using generator absorber heat exchange (GAX) and single-effect (SE) cycles aresimulated to obtain the performance under various working conditions. Different HCD systems in Beijingand Shenyang are simulated comparatively in TRNSYS, based on which the thermal imbalance, soil tem-perature, heat recovery, and energy efficiency are analyzed. Results show that GSAHP–GAX–HCD is suit-able for Beijing and GSAHP–SE–HCD is suitable for Shenyang. The imbalance ratio can be reduced to�14.8% in Beijing and to 6.0% in Shenyang with an annual soil temperature variation of only 0.5 �Cand 0.1 �C. Furthermore, about 20% and 15% of the total condensation/absorption heat is recovered toproduce DHW, and the PEE can reach 1.516 in Beijing and 1.163 in Shenyang. The combined HCD systemscan achieve a PEE improvement of 23.6% and 44.4% compared with the normal heating/cooling systems.
� 2014 Elsevier Ltd. All rights reserved.
1. Introduction
1.1. Traditional heat supply systems in China
The amount of energy used for heating and domestic hot water(DHW) is very high with heating in urban northern China account-ing for 23.0% of the total building energy consumption, while DHWin China’s urban areas accounted for 23.4% of the total residentialbuilding energy consumption in 2008 [1]. It is predicted that thisconsumption will continue to increase due to the rapid urbaniza-tion of China and obvious improvement of people’s living stan-dards [1]. Currently, most heat supply systems for both heating
and DHW in China are based on fossil fuel burning, which is oflow energy efficiency [2,3]. Moreover, the coal-dominated energystructure (coal ratio of 70%) leads to large quantities of emissionsof CO2, SO2, NOX, and particulate matter, such as PM10 andPM2.5 [4,5]. PM2.5 is also regarded as one of the contributory fac-tors to frequent hazy weather in North China.
1.2. Status of ground source heat pump (GSHP) in cold regions
To overcome the above problems, improvements in energy effi-ciency and the use of renewable energy are supposed to playimportant roles. Ground source electrical heat pumps (GSEHPs)are used extensively throughout the world as a renewable energytechnology for space heating and DHW [6–9]. However, in heat-ing-dominated buildings, the ground thermal imbalance will lead
Nomenclature
h specific enthalpy, kJ/kgm mass flow rate, kg/sQ heat load, kWx solution concentration, kg/kg
AbbreviationsCOP coefficient of performanceDHW domestic hot waterECA externally cooled absorberEHG externally heated generatorGAX generator–absorber heat exchangeGAXA GAX absorberGAXG GAX generatorGSAHP ground source absorption heat pumpGSEHP ground source electrical heat pump
HC heating/coolingHCD heating/cooling/DHWIR underground thermal imbalance ratioLMTD logarithmic mean temperature differencePEE primary energy efficiencySE single-effectSCA solution cooled absorberSHG solution heated generator
Subscriptsc coolingh heatingin inletout outlet
114 W. Wu et al. / Applied Energy 126 (2014) 113–122
to a decrease of soil temperature (Fig. 1) and the deterioration ofheating performance [10,11]. The solutions for these problemsfocus mainly on increasing borehole spacing/depth/number,installing auxiliary heat sources, and utilizing thermal energy stor-age [12]. Among these solutions, increasing borehole spacing/depth/number can alleviate the soil temperature decrease at thecost of higher investment or/and more occupied land, but it cannotfundamentally eliminate the ground thermal imbalance. As for themethod of installing auxiliary heat sources, such as a boiler andheat network, the auxiliary device may be very large when theheating load is much larger than the cooling load, which willreduce the advantages of GSEHP. Regarding the approach of utiliz-ing thermal energy storage, much research has been conducted onsolar energy storage through ground heat exchangers [13,14].Although renewable energy is used in these systems, the require-ment of high initial cost and large installation space may be animportant concern regarding popularizing this system in high-den-sity cities. It will be even more serious in severely cold regions,where the heating load is remarkably higher than the cooling loador where only heating is required, and the heat storage capacityneeds to be very large to maintain the ground thermal balance.
1.3. Advantages and problems of ground source absorption heat pump(GSAHP)
To reduce the thermal imbalance of conventional GSHP sys-tems deployed in cold regions, a heating/cooling system (HC)
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Ave
rage
Soi
l Tem
pera
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()
Time
GSEHP GSAHP
Heating Cooling Heating
Fig. 1. Soil temperature variation of GSAHP and GSEHP system (in Shenyang).
based on GSAHP was proposed in previous work [15]. Comparedwith GSEHP, GSAHP extracts less heat from the soil during theheating season and rejects more heat into it during the coolingseason, which can effectively reduce the year-round ground ther-mal imbalance in cold regions. Furthermore, GSAHP systems areassessed to have advantageous primary energy efficiency (PEE)over conventional GSEHP systems, considering the current effi-ciency of electricity generation in China [15]. The soil tempera-ture of a GSAHP system may remain stable in northern parts ofnorthern China (severely cold); however, it may increase to someextent after long-term operation in southern parts of northernChina (cold) (Fig. 1). In this region, GSEHP leads to cold accumu-lation underground, whereas GSAHP will cause heat accumula-tion. The redundant heat of GSAHP could be rejected to theambient air via a cooling tower, but this requires an additionalcooling system, a more complex configuration, and higherinvestment.
1.4. Objectives of this work
As is known, DHW is demanded throughout the year in manybuildings and an additional boiler system usually needs to beinstalled to meet this demand. If this requirement can be satisfiedby GSAHP, the thermal imbalance will be reduced because moreheat is extracted throughout the entire year [16,17]. Under suchcircumstances, a novel combined heating/cooling/DHW system(HCD) based on GSAHP is proposed. Moreover, the redundant heatof GSAHP can be recovered to produce DHW in summer, greatlyimproving the energy efficiency of the proposed combined HCDsystem.
This work aims at further reducing the thermal imbalance ofGSAHP as well as improving the energy efficiency of DHW. In orderto choose suitable absorption cycles for different regions, theGSAHP using single-effect (SE) and generator–absorber heatexchange (GAX) cycles are designed and modeled to determinetheir performance under various working conditions. The com-bined HCD system, based on GSAHP of different cycles in differentregions, are simulated in TRNSYS to analyze the thermal imbal-ance, soil temperature variation, heat pump performance, andGSAHP energy efficiency. Essential comparisons between GSAHPand GSEHP are conducted to investigate the advantages of the pro-posed system in maintaining the underground thermal balance.Additionally, GSAHP–HCD is also compared with a GSAHP–HC sys-tem to evaluate the further reduction in thermal imbalance as wellas the improvement in energy efficiency contributed by the pro-posed system.
Table 1Operation modes of the combined HCD system based on GSAHP.
Season Mode Opened valve Closed valve
Winter Heating 1,2,3,4 5,6,7,8,9,10Heating + DHW 1,2,3,4,9 5,6,7,8,10
Summer Cooling 5,6,7,8 1,2,3,4,9,10Cooling + DHW 5,6,7,8,10 1,2,3,4,9
Transition seasons DHW 9 1,2,3,4,5,6,7,8,10
W. Wu et al. / Applied Energy 126 (2014) 113–122 115
2. Description of the combined HCD system based on GSAHP
The schematic diagram of the combined HCD system based onGSAHP is illustrated in Fig. 2. The GSAHP for space heating/cooling(HC unit) and the GASHP for domestic hot water (DHW unit) canoperate either in parallel or in series according to different seasons.The working principles in different seasons are described asfollows:
� In winter, the fluid of the borehole outlet enters the HC unit andthe DHW unit in parallel to provide a low-grade heat source forboth units. Subsequently, the fluids of the evaporator outletsmix together and then return underground to extract furtherheat from the soil. When the DHW production has already beencompleted and only heating is needed, the DHW unit can juststop running.� In transition seasons, only DHW is demanded and therefore, the
HC unit is not in operation and the fluid of the borehole outletenters only the DHW unit.� In summer, the fluid of the borehole outlet enters the HC unit
to remove its heat rejection in the condenser and absorber.The cooling water leaving the condenser and absorber of theHC unit does not entirely return to the ground heat exchang-ers. Instead, part of the cooling water first enters the DHWunit to provide a low-grade heat source and then it mixes withthe other parts of the cooling water. In this way, some of thewaste heat that would have been rejected into the soil canbe recovered by the DHW unit. When the DHW productionhas already been completed and only cooling is needed, theDHW unit can just stop running and all of the cooling waterleaving the HC unit has to flow back to the ground heatexchangers.
The valves that are needed to be opened and closed in eachoperation mode are listed in Table 1.
In this combined HCD system, the underground thermal imbal-ance of GSAHP can be decreased further in two respects: on onehand, more heat is extracted from the soil in winter and transitionseasons to provide additional DHW; on the other hand, less heatis rejected into the ground in summer owing to the heat recoveryfrom the HC unit to the DHW unit. As a result, the difference
GSAHPspace heatin
(HC u
GSAHPdomestic h
(DHW Wat
erta
nk
Tap water
Domestic hot water
Space heating / coolingvalve 3
valve 4
valve 7
valve 8
Building
Fig. 2. Schematic diagram of the combi
between heat extraction and rejection during a entire year becomessmaller and the underground thermal imbalance is reduced. Apartfrom maintaining a better thermal balance, the system’s energy effi-ciency will also be greatly improved with regard to PEE.
3. Methodology
To perform a dynamic simulation of the proposed combinedHCD system, the models of GSAHP are built and the performanceof GSAHP is simulated. Two cities typical of the region in whichGSAHP will lead to heat accumulation underground are chosenfor the analysis. In addition, essential indexes are defined to eval-uate the thermal imbalance and energy efficiency.
3.1. Modeling of GSAHP
As building loads differ greatly in different locations of the coldregions owing to weather characteristics, two kinds of absorptioncycles, SE and GAX, are involved to satisfy the different situationsand to obtain optimum performance. The detailed mathematicalmodels and working principles of the SE cycle can be found in pre-vious work [18,19], and this part will focus on the modeling of theGAX system, as demonstrated in Fig. 3. The temperature overlapbetween the absorber and the generator makes it possible torecover part of the absorption heat for generation in a GAX cycle.This overlapping of heat is an attractive characteristic ofNH3-based working fluids, which cannot be realized in the widelyused H2O–LiBr systems [20]. The GAX cycle essentially appears tobe an SE configuration; however, it provides a higher coefficientof performance (COP) than any other SE cycle owing to the internalheat recovery between the GAX generator (GAXG) and the GAXabsorber (GAXA) [21].
forg/coolingnit)
Ground heat exchanger
forot waterunit)
valve 1
valve 10
valve 2
valve 5
valve 6
valve 9
ned HCD system based on GSAHP.
Fig. 3. Schematic diagram of a GAX absorption heat pump.
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P
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DHW
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DHW
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Inlet Water Temperature (oC)
116 W. Wu et al. / Applied Energy 126 (2014) 113–122
NH3–LiNO3 is adopted here because no rectifier is needed andbecause it has been proven to deliver better performance than con-ventional NH3–H2O [22–24]. The rich solution (rich in ammonia) atthe outlet of the ECA is pumped into the SCA to obtain part of theabsorption heat, and it then flows into the GAXG to be generatedby another part of absorption heat recovered from GAXA by themiddle loop. It then enters the SHG to be further generated bythe poor solution (poor in ammonia) from the EHG, and the finalgeneration is completed in the EHG driven by the externallysupplied heat source. Similarly, the poor solution at the outlet ofthe EHG flows into the SHG to provide part of the generation heat,and it then enters the GAXA after a throttling process to performthe preliminary absorption, before flowing into the SCA for furtherabsorption. The final absorption is completed in the ECA, cooled bythe water from the condenser. The EHG is driven by the boiler, theevaporator extracts low-grade heat stored underground, and thedemanded hot water is produced in the condenser and the ECA.
In the modeling of GSAHP, some reasonable assumptionsshould be made [18,25]: the system is in steady state and heat bal-ance; the refrigerant leaving the evaporator and condenser is satu-rated vapor and liquid; the solutions leaving the generator andabsorber are both saturated; the flow resistance, pressure losses,and heat losses in pipes and components are all ignored; and thethrottling in the expansion valves are isenthalpic processes. Themathematical models of GSAHP–GAX can be built based on themass and energy balance of each component:X
mout ¼X
min ð1Þ
Xmoutxout ¼
Xminxin ð2Þ
Q þX
mouthout ¼X
minhin ð3Þ
Q ¼ UA � LMTD ð4Þ
where UA is the product of the heat transfer coefficient and heattransfer area of each heat exchanger, and LMTD is the logarithmicmean temperature difference.
The heating and cooling COPs of GSAHP–GAX are defined as:
COPh ¼QC þ Q ECA
Q SHGð5Þ
COPc ¼Q E
QSHGð6Þ
3.2. Performance of GSAHP
Based on the above models, the performance of GSAHP underdifferent working conditions can be simulated. The water temper-ature for heating, cooling, and DHW is set as 45, 7, and 50 �C,respectively. For a design heating capacity of 100 kW, the off-design heating COP/capacity and cooling COP/capacity of GSAHP–SE and GSAHP–GAX, under different inlet water temperatures,are shown in Figs. 4 and 5, respectively.
Fig. 4 shows that the heating COP of GSAHP–SE is within therange of 1.349–1.618 and the heating capacity is within the rangeof 44.2–167.8 kW as the inlet water temperature varies from �15to 30 �C. If DHW is produced by the same equipment, the heatingperformance is a little worse because the hot water temperatureis set higher. In DHW mode, the heating COP is within the rangeof 1.122–1.648 and the heating capacity is within the range of22.2–183.7 kW as the inlet water temperature varies from �15to 45 �C. It should be noted that the inlet water temperature ofGSAHP–DHW can be very high in the heat recovery mode in sum-mer and thus, the heating performance will be relatively high. Asfor the cooling mode of the same GSAHP–SE, the cooling COP andcooling capacity are within the ranges of 0.494–0.623 and 25.7–86.0 kW, respectively, when the inlet water temperature variesfrom 5 to 45 �C.
Fig. 5 shows that the heating COP of GSAHP–GAX is within therange of 1.530–2.111 and the heating capacity is within the rangeof 85.9–108.9 kW as the inlet water temperature varies from �15to 30 �C. In DHW mode, the heating COP is within the range of1.122–1.648 and the heating capacity is within the range of22.2–183.7 kW as the inlet water temperature varies from �15to 45 �C. In cooling mode, the cooling COP and cooling capacityof the same GSAHP–GAX are within the ranges of 0.745–1.397and 38.3–77.8 kW when the inlet water temperature varies from5 to 45 �C. Comparisons between Figs. 4 and 5 reveal clearly thatthe performance of GSAHP–GAX is better than that of GSAHP–SE,because higher generation temperature has to be used to achievea temperature overlap between the generator and absorber. If theoverlap is insufficient, the GAX becomes an SE cycle. The turningpoint of the heating performance curve in Fig. 5 is just the switchpoint from GAX to SE when the inlet water temperature decreasesto a certain value.
3.3. Simulation of building loads
In the region where GSAHP will lead to heat accumulationunderground, Beijing and Shenyang in northern China are chosenas typical cities to investigate the potential of the proposed
Fig. 4. Performance of GSAHP–SE.
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Performance of NH3-LiNO3 GSAHP-GAX
Heating
DHWHeating
DHW
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Cooling
Fig. 5. Performance of GSAHP–GAX.
Table 2Main configurations of the combined HCD systems based on GSAHP.
Items Beijing Shenyang
Average air temperature (�C) 12.6 8.6Building area (m2) 5000 5000Bed number 300 300Daily DHW consumption (L) 120 120Design heating load (kW) 233 298Design cooling load (kW) 333 297Design DHW load (kW) 69 73Heating season Nov. 15–Mar. 15 Nov. 1–Mar. 31Cooling season Jun. 1–Aug. 31 Jun. 1–Aug. 31DHW season Entire year Entire yearBorehole number 106 132Borehole spacing (m) 5 5Borehole depth (m) 100 100
W. Wu et al. / Applied Energy 126 (2014) 113–122 117
combined GSAHP–HCD systems. The hourly heating/cooling loadsof a hotel building with a total area of 5000 m2 are calculated usingthe dynamic energy simulation tool DeST [26], as shown in Fig. 6.
Based on the dynamic building loads and weather characteris-tics, the main configurations of the combined HCD systems usedin the typical cities are designed in Table 2. Given these parame-ters, both GSAHP–SE and GSAHP–GAX system can be modeled inTRNSYS for dynamic operation simulation.
3.4. Evaluation indexes
To evaluate the difference between the accumulated heatextraction and accumulated heat rejection throughout an entireyear, the thermal imbalance ratio (IR) is defined as:
IR ¼P
Q AHR �P
QAHE
maxP
Q AHR;P
Q AHEð Þ � 100% ð7Þ
whereP
QAHR andP
QAHE are the annual accumulated heat rejec-tion and annual accumulated heat extraction, respectively, in kW h.
To investigate the PEE, the annual PEE of the combined HCDsystem is defined as:
PEE ¼P
Q heating þP
Q cooling þP
Q DHW
ðP
Q GSAHP þP
QboilerÞ=gboilerð8Þ
whereP
Qheating,P
Qcooling, andP
QDHW are the total suppliedheating, cooling, and DHW load, respectively, in one year, inkW h;
PQGSAHP and
PQboiler are the total heat consumption of
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oad
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Fig. 6. Hourly heating/cooling
GSAHP and the boiler (only when DHW is produced individuallyby the boiler), respectively, in kW h; and gboiler is the boiler effi-ciency. A gas boiler is adopted in this work and the boiler efficiencyis taken as 0.90 [1].
3.5. Scheme for comparative analysis
Essential comparisons between GSAHP and GSEHP are con-ducted to investigate the advantages of the proposed system inmaintaining the underground thermal balance. Additionally,GSAHP–HCD is also compared with the GSAHP–HC system (DHWis supplied by the boiler) to evaluate the further reduction in ther-mal imbalance, as well as the improvement in energy efficiencycontributed by the proposed system. The detailed descriptions ofthe different HCD systems and the purpose of each comparisonanalysis are listed in Table 3. This work aims to evaluate the con-tribution of the proposed system to thermal balance. To makethe comparison fair and to reduce the number of influencing fac-tors, the borehole number, borehole depth and borehole spacing(Table 2) have been designed the same for different system typesin the same city.
4. Simulation of the combined HCD systems based on GSAHP
4.1. Analysis of ground thermal imbalance
The thermal imbalance analysis can be an important indexwhen choosing the most suitable cycle for a combined HCD appli-cation. The IR values of the systems described in Table 3 are calcu-lated in Table 4. In Beijing, the IR of GSEHP–HCD is �77.0%, which
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loads of a typical building.
Table 3Comparison scheme of different HCD systems.
System Description Purpose
GSEHP–HCD (forcomparison)
GSEHP is used for heating/cooling/DHW; Heat is recoveredfor DHW in summer
To analyze the advantages of GSAHP–SE–HCD and GSAHP–GAX–HCD in thermal balance
GSAHP–SE–HC (forcomparison)
GSAHP of SE cycle is used for heating/cooling; boiler is usedfor DHW throughout the entire year
To analyze the advantages of GSAHP–SE–HCD in thermal balanceand energy efficiency
GSAHP–GAX–HC (forcomparison)
GSAHP of GAX cycle is used for heating/cooling; boiler is usedfor DHW throughout the entire year
To analyze the advantages of GSAHP–GAX–HCD in thermalbalance and energy efficiency
GSAHP–SE–HCD (proposedsystem)
GSAHP of SE cycle is used for heating/cooling/DHW; Heat isrecovered for DHW in summer
To find a more suitable GSAHP cycle for different regions
GSAHP–GAX–HCD(proposed system)
GSAHP of GAX cycle is used for heating/cooling/DHW; Heat isrecovered for DHW in summer
Table 4Thermal imbalance analysis of GSEHP and GSAHP.
City Cycle Extraction-heating (kW h) Extraction-DHW (kW h) Rejection-cooling (kW h) Imbalance ratio (%)
Beijing GSEHP–HCD 217,863 333,221 126,637 �77.0GSAHP–SE–HC 103,343 0 513,092 79.9GSAHP–SE–HCD 102,498 158,377 462,057 43.5GSAHP–GAX–HC 139,817 0 354,095 60.5GSAHP–GAX–HCD 137,456 197,668 285,539 �14.8
Shenyang GSEHP–HCD 310,453 340,415 85,553 �86.9GSAHP–SE–HC 150,977 0 382,774 60.6GSAHP–SE–HCD 150,121 164,292 334,627 6.0GSAHP–GAX–HC 198,654 0 259,236 23.4GSAHP–GAX–HCD 195,804 207,552 198,499 �50.8
118 W. Wu et al. / Applied Energy 126 (2014) 113–122
means that the accumulated heat extraction is far higher than theaccumulated heat rejection. If GSAHP–SE–HC and GSAHP–GAX–HCare replaced, the IR becomes 79.9% and 60.5%, indicating that theaccumulated heat extraction is obvious lower than the accumu-lated heat rejection in this circumstance. Therefore, serious coldaccumulation turns to serious heat accumulation. However, withthe application of GSAHP–SE–HCD and GSAHP–GAX–HCD, the IRwill be reduced to 43.5% and �14.8%. Thus, GSAHP–GAX–HCDcan obtain the best thermal balance in Beijing.
As for Shenyang, the IR of GSEHP–HCD is as high as �86.9% dueto the larger heating load and lower cooling load there. However, itcan be reduced to 60.6% and 23.4% by GSAHP–SE–HC and GSAHP–GAX–HC, and it can be reduced to 6.0% and �50.8% by GSAHP–SE–HCD and GSAHP–GAX–HCD. The IR value as low as 6.0% makesGSAHP–SE–HCD a promising option for maintaining the best ther-mal balance in Shenyang.
It can be concluded from the thermal imbalance analysis thatGSAHP–GAX–HCD is suitable for cities such as Beijing, whereasGSAHP–SE–HCD is beneficial for cities like Shenyang. Under thiscycle selection, the heat extraction/rejection of the combinedHCD systems based on GSEHP and GSAHP during an entire yearare illustrated in Fig. 7 (Beijing) and Fig. 8 (Shenyang).
It can be seen in Fig. 7 that the maximum heat extraction/rejec-tion rate of GSEHP–HCD in Beijing is 219 kW (winter), 51 kW(transition seasons), and �351 kW (summer). However, the corre-sponding values of GSAHP–GAX–HCD become 136, 30, and�652 kW. Similarly, Fig. 8 indicates that the maximum heatextraction/rejection rate in Shenyang is changed from (263, 53,�293 kW) to (128, 25, �803 kW) by GSAHP–SE–HCD.
Comparisons between GSEHP and GSAHP reveal that GSAHPextracts less heat from the soil and rejects more heat into the soilbecause of its relatively low heating and cooling COP, which ulti-mately, contributes to its better thermal balance in cold regions.
4.2. Variation of soil temperature
To investigate the influence of underground thermal imbalance,the soil temperature variations of different systems during an
entire year are presented in Figs. 9 and 10. In Beijing, the averagesoil temperature decreases from 14.6 to 11.6 �C after one year’soperation of the GSEHP–HCD system; a reduction of 3.0 �C. How-ever, the average temperature will increase from 14.6 to 15.9 �Cfor the GSAHP–GAX–HC system due to an IR value of 60.5%,whereas it will decrease to 14.1 �C for the GSAHP–GAX–HCD sys-tem. The annual soil temperature change can be maintained within0.5 �C by GSAHP–GAX–HCD, of which the IR value is only �14.8%.
In Shenyang, the average soil temperature decreases from 10.6to as low as 7.3 �C for the GSEHP–HCD system with a serious ther-mal imbalance of �86.9%. As for the GSAHP–SE–HC system, thetemperature will increase from 10.6 to 11.7 �C after one year’soperation due to an IR value of 60.6%, whereas it will decrease to10.5 �C for the GSAHP–SE–HCD system. Therefore, the annual soiltemperature can be held stable by GSAHP–SE–HCD with an IRvalue as low as 6.0% and a consequent negligible reduction of only0.1 �C.
It can be concluded that the soil temperature increase ofGSAHP–HC can be managed by the proposed GSAHP–HCD. The soiltemperature variations of GSAHP–GAX–HCD in Beijing and byGSAHP–SE–HCD in Shenyang are simulated further during along-term operation of 10 years, as demonstrated in Figs. 11 and12. In the above analysis, the DHW consumption of 120 L/(bed�day)is used for all the systems. To determine the influence of dailyDHW demands, 100 L/(bed�day)) is also taken into account forcomparison in this part. Fig. 11 indicates that the average soil tem-perature in Beijing decreases from 14.6 to 13.3 �C at a DHW con-sumption of 100 L/(bed�day), whereas it decreases to 11.8 �C at aDHW consumption of 120 L/(bed�day). Fig. 12 shows that the aver-age soil temperature in Shenyang changes from 10.6 to 11.6 �C at aDHW consumption of 100 L/(bed�day), and to 10.5 �C at a DHWconsumption of 120 L/(bed�day). All of these temperature varia-tions are very small for an operation period as long as 10 years.It can be concluded that higher DHW demand leads to an addi-tional reduction on soil temperature, and that the DHW consump-tion of 100 L/(bed�day) is beneficial for GSAHP–GAX–HCD inBeijing, whereas 120 L/(bed�day) would be better for GSAHP–SE–HCD in Shenyang.
(a) GSEHP-HCD (b) GSAHP-GAX-HCD
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Fig. 7. Heat extraction/rejection of GSEHP and GSAHP in Beijing.
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Fig. 8. Heat extraction/rejection of GSEHP and GSAHP in Shenyang.
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ture
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GSEHP-HCD GSAHP-GAX-HC GSAHP-GAX-HCD
Heating Cooling Heating
Fig. 9. Variation of soil temperature of different systems (Beijing).
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1-Jan 1-Feb 1-Mar 1-Apr 1-May 1-Jun 1-Jul 1-Aug 1-Sep 1-Oct 1-Nov 1-Dec
Ave
rage
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l Tem
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Fig. 10. Variation of soil temperature of different systems (Shenyang).
W. Wu et al. / Applied Energy 126 (2014) 113–122 119
4.3. Analysis of heat recovery
For GSAHP–GAX–HCD applied in Beijing and GSAHP–SE–HCDapplied in Shenyang, the heat recovered by DHW in summer isshown in Fig. 13. The hourly heat recovery rate from the HC unitto the DHW unit in Fig. 2 is within the range of 30.0–36.8 kW inBeijing with a total heat recovery in summer amounting to
71,144 kW h. From Table 4, it can be seen that the heat rejectionof GSAHP–GAX–HCD in Beijing is 285,539 kW h. Therefore, about20% of the total condensation/absorption heat is recovered to pro-duce DHW. With regard to Shenyang, the hourly heat recovery rateis within the range of 25.0–25.9 kW, and accumulates to be56,447 kW h in one summer. As the heat rejection of GSAHP–SE–HCD in Shenyang is 334,627 kW h (Table 4), some 15% of the totalcondensation/absorption heat is recovered to produce DHW.
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Fig. 11. Variation of soil temperature during long-term operation (Beijing).
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Fig. 12. Variation of soil temperature during long-term operation (Shenyang).
120 W. Wu et al. / Applied Energy 126 (2014) 113–122
4.4. Investigation of energy efficiency
The hourly COP in both HC and DHW modes of both GSAHP–GAX and GSAHP–SE are obtained through the above dynamic sim-ulation and the results are presented in Fig. 14.
In the HC mode of GSAHP–GAX–HCD, the heating COP is around1.90 and the cooling COP can approach 1.28, as indicated inFig. 14(a). In the DHW mode of GSAHP–GAX–HCD, COP is around1.80 in winter and the transition seasons, while it can reach as highas 2.25 during the heat recovery process in summer. Such a high
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Fig. 13. Heat recovery b
COP can be explained by the higher-temperature cooling waterthat comes from the condenser and absorber of the HC unit andwhich enters the evaporator of the DHW unit, as shown in Fig. 2.
It can be seen in Fig. 14(b) that in the HC mode of GSAHP–SE–HCD, the heating COP is about 1.55 and the cooling COP is near0.62. Furthermore, in the DHW mode of GSAHP–SE–HCD, COP isaround 1.51 in winter and the transition seasons, while it remainsaround 1.54 in summer. The COP of GSAHP–SE–HCD is much stea-dier than that of GSAHP–GAX–HCD, owing to the COP characteris-tics shown in Figs. 4 and 5, i.e., the COP of GSAHP–GAX is muchmore sensitive to the inlet water temperature compared withGSAHP–SE.
From the viewpoint of year-round COP, the thermal COP (thetotal heating/cooling/DHW supply divided by the total heat con-sumption) and electric COP (the total heating/cooling/DHW supplydivided by the total electricity consumption of solution pump andwater pump) of the GSAHP systems with and without DHW arecalculated in Table 5. It is found that both the thermal COP andelectric COP can be obviously improved by the integration ofDHW due to the heat recovery in summer.
To assess further the potential of the combined HCD systemsbased on GSAHP, the annual energy efficiency of both GSAHP–SE–HCD and GSAHP–GAX–HCD are evaluated with respect toPEE. The hourly heat consumption of GSAHP in the two cities isdemonstrated in Fig. 15. It is obvious that the heat consumptionsof GSAHP–GAX–HCD in Beijing and of GSAHP–SE–HCD in Sheny-ang are respectively lower than those of the GSAHP–GAX–HC andGSAHP–SE–HC systems. This can be explained in two respects:(1) GSAHP is used for DHW production in the combined HCD sys-tems, whereas a boiler is used in the HC systems; (2) heat recoveryfrom condensation and absorption is realized to produce DHW insummer in the combined HCD systems.
The heat consumption of GSAHP is converted into primaryenergy consumption, considering an energy efficiency of 0.90 forgas boiler. The results of the PEE analysis are presented in Table 6.In Shenyang, the PEE of GSAHP–SE–HC is 0.941, whereas it is 1.163for GSAHP–SE–HCD. The combined HCD system can achieve animprovement in PEE of 23.6%. In Beijing, the PEE of GSAHP–GAX–HC is 1.050, whereas it can reach as high as 1.516 for GSAHP–GAX–HCD. The combined HCD system can achieve an improve-ment in PEE of 44.4% in this case.
4.5. Estimation of payback time
Considering that the GSAHP system is proposed to solve theimbalance problem of the traditional GSEHP system, the systemGSEHP plus gas boiler (GSEHP for heating/cooling, gas boiler forDHW) is taken as baseline to calculate the payback time. According
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y DHW in summer.
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Fig. 14. Dynamic COP of GSAHP.
Table 5Thermal and electric COP of GSAHP.
City System Total supply (kW h) Heat consumption (kW h) Electricity consumption (kW h) Thermal COP Electric COP
Solution pump Water pump
Shenyang GSAHP–SE–HC 573,168 520,251 2850 17,792 1.10 27.77GSAHP–SE–HCD 1,212,648 938,426 6316 23,119 1.29 41.20
Beijing GSAHP–GAX–HC 477,580 323,162 1099 16,464 1.48 27.19GSAHP–GAX–HCD 1,082,020 642,257 2558 22,557 1.68 43.08
(a) Beijing (b) Shenyang
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Fig. 15. Hourly heat consumption of GSAHP.
Table 6Primary energy efficiency analysis of GSAHP.
City System Supply(kW h) Heat consumption (kW h) Primary energy consumption (kW h) PEE
Heating Cooling DHW GSAHP Boiler GSAHP Boiler
Shenyang GSAHP–SE–HC 431,521 141,647 639,480 520,251 639,480 578,056 710,533 0.941GSAHP–SE–HCD 431,521 141,647 639,480 938,426 0 1,042,696 0 1.163
Beijing GSAHP–GAX–HC 294,781 182,799 604,440 323,162 604,440 359,069 671,600 1.050GSAHP–GAX–HCD 294,781 182,799 604,440 642,257 0 713,619 0 1.516
W. Wu et al. / Applied Energy 126 (2014) 113–122 121
to the product information, engineering experiences and local pol-icies, the following prices are used: electricity price is 0.8 CNY/kW h; GSEHP price is 1000 CNY/kW; gas boiler price is 500 CNY/kW; GSAHP is 2000 CNY/kW; borehole investment is 50,000 CNY/borehole. Taking Beijing as an instance, the payback time of GSAHPis estimated in Table 7. Compared with the conventional GSEHP,the payback time of the proposed system is about 3.5 years.
5. Conclusions
The combined HCD system based on GSAHP is put forward toreduce the thermal imbalance in cold regions. GSAHP–SE andGSAHP–GAX are modeled and simulated to obtain the heating/cooling/DHW COP and capacity under various working conditions.Different HCD systems in Beijing and Shenyang are simulated
Table 7Estimation of Payback time of GSAHP.
Items GSEHP + boiler GSAHP–GAX–HCD
Annual gas consumption (m3) 69,005 65,990Annual electricity consumption (kW h) 108,529 0Annual operation cost (�104 CNY) 27.1 17.6Capacity for heating (kW) 233 233Capacity for DHW (kW) 69 69Borehole number 106 106Initial cost (�104 CNY) 79.8 113.4Payback time (Year) – 3.5
122 W. Wu et al. / Applied Energy 126 (2014) 113–122
comparatively in TRNSYS, based on which the thermal imbalance,soil temperature, heat recovery, and energy efficiency are analyzed.The following conclusions can be draw from the above analysis:
(1) The proposed combined HCD system based on GSAHP canreduce the thermal imbalance in the warmer parts of north-ern China effectively. GSAHP–GAX–HCD is suitable for Bei-jing and GSAHP–SE–HCD is suitable for Shenyang.
(2) IR can be reduced to �14.8% by GSAHP–GAX–HCD in Beijingand to 6.0% by GSAHP–SE–HCD in Shenyang with an annualsoil temperature variation of only 0.5 �C and 0.1 �C. The soiltemperature over a 10-year period can also be held stable.
(3) About 20% and 15% of the total condensation/absorptionheat is recovered to produce DHW in Beijing and Shenyang;
(4) The PEE of GSAHP–SE–HCD in Shenyang is 1.163, whereasthat of GSAHP–GAX–HCD in Beijing can reach as high as1.516. The combined HCD systems can achieve an improve-ment in PEE of 23.6% and 44.4% compared with the HCsystems.
Overall, the proposed combined HCD system based on GSAHP ispromising in terms both of thermal imbalance reduction andenergy efficiency improvement, and could be a potential alterna-tive to heat supply systems in cold regions.
Acknowledgements
The authors gratefully acknowledge the support from the Natu-ral Science Foundation for Distinguished Young Scholars of China(Grant No. 51125030) and the National 12th Five-year Scienceand Technology Support Project of China (Grant No. 2011BAJ03B09).
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