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Hybrid ground source absorption heat pump in cold regions: Thermal balance keeping and borehole number reduction
Applied Thermal Engineering 90 (2015) 322e334
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research paper
Hybrid ground source absorption heat pump in cold regions: Thermal balance keeping and borehole number reduction Wei Wu, Xianting Li*, Tian You, Baolong Wang, Wenxing Shi Department of Building Science, School of Architecture, Tsinghua University, Beijing, 100084, China
h i g h l i g h t s Hybrid ground source absorption heat pump (HGSAHP) with cooling tower is proposed. HGSAHP maintains soil thermal balance and reduces borehole number in cold regions. Borehole number and land areas are reduced by 37e52% and 20e38% by various cycles. The annual primary energy efficiency can be 17.5% higher than conventional GSHP. Lifecycle cost of HGSAHP over 20 years can be advantageous over conventional GSHP.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2015 Accepted 4 July 2015 Available online 15 July 2015
Thermal imbalance of ground source electrical heat pump (GSEHP) leads to cold accumulation in cold regions. Ground source absorption heat pump (GSAHP) can relieve the thermal imbalance, but may cause heat accumulation in the warmer parts of cold regions. Hybrid GSAHP (HGSAHP) integrated with a cooling tower is proposed to solve this problem. Hourly simulations of single-effect HGSAHP and generator absorber heat exchange (GAX) cycle HGSAHP are conducted, and are compared with hybrid GSEHP (HGSEHP). Results show that the thermal balance can be well kept by HGSAHP, with imbalance ratio reduced from 60e80% to within 20%, and soil temperature variation staying within 3 C after 20 years' operation. Moreover, HGSAHPs are advantageous in heating mode and inferior to HGSEHP in cooling mode. The annual performance of GAX-cycle HGSAHP is very close to that of HGSEHP in Beijing, while being 17.5% higher than that of HGSEHP in colder regions like Shenyang. Compared with HGSEHP, the required borehole number and occupied land areas can be reduced by 37e52% by single-effect HGSAHP and reduced by 20e38% by GAX-cycle HGSAHP. Additionally, the lifecycle cost of GAX-cycle HGSAHP (coal) is the lowest, while GAX-cycle HGSAHP (gas) is cheaper than conventional HGSEHP (gas) assisted by an auxiliary boiler. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Thermal imbalance Absorption heat pump Ground source heat pump Borehole number Soil temperature Primary energy efficiency
1. Introduction 1.1. Problems of ground source heat pump (GSHP) in cold regions GSHPs are widely used for space heating, air conditioning, and domestic hot water due to the advantages of energy saving and emission reduction, as well as the incentives from the government [1e4]. In 2012, GSHP installation reached an estimated 50 GW of capacity; China had the biggest market share and continued to increase at about 10% annually [5]. Under the severe circumstance
* Corresponding author. Tel.: þ86 10 62785860; fax: þ86 10 62773461. E-mail address: [email protected] (X. Li). http://dx.doi.org/10.1016/j.applthermaleng.2015.07.014 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
of hazy weather in China, the urgent necessity of reducing the pollutant emissions of the conventional heating systems offers great opportunities for the clean heating technologies, with GSHP as one of the potential alternatives. Therefore, the GSHP systems are predicted to be more and more popular in the future. However, the large-scale application of GSHP systems brings about three major problems: (1) The conventional heating systems in northern China are mainly based on the burning of fossil fuel because of the coal-dominated energy structure [6,7]. A massive replacement with electricity-based heating systems will bring serious pressure on electricity generation and transmission, and also, it is a big waste of the existing system investment. It
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334
Nomenclature
RGHX
COPc COPh Cannual Cinitial C20 years cp ei Hfuel
RHP tbin,i tbout,i tc,in te,in
coefficient of performance in cooling mode coefficient of performance in heating mode annual operation cost, CNY initial investment of the system, CNY lifecycle total cost over 20 years, CNY specific heat, kJ/(kg C) hourly electricity consumption, kW lower heat value of fuel, kJ/kg for coal and kJ/Nm3 for gas mb,i fluid mass flow rate inside the ground heat exchanger, kg/s Pfuel unit price of fuel, CNY/kg for coal and CNY/Nm3 for gas Pelectricity unit price of electricity, CNY/kWh QAHE accumulated heat extraction during heating season, kWh QAHE,hybrid accumulated heat extraction from soil for hybrid system during heating season, kWh QAHR accumulated heat rejection during cooling season, kWh QAHR,hybrid accumulated heat rejection into soil for hybrid system during cooling season, kWh Qc cooling capacity, kW Qh heating capacity, kW qboiler,i hourly heating load supplied by the auxiliary boiler, kW qi hourly heating or cooling load, kW
may be more practically reasonable to improve energy efficiency and reduce the pollutant emission of the present heating systems, rather than completely replace them with other heating systems attached with new problems. (2) The imbalance between heat extraction and heat rejection will lead to the cold accumulation in heating-dominated buildings [8,9] or heat accumulation in cooling-dominated buildings [10,11]. Cold accumulation is very common in cold regions; the soil temperature will gradually decrease after long-term operation, since the temperature recovery ability of soil is limited, which finally leads to the deterioration of heating performance [12]. Measures to solve the above problems mainly include increasing borehole spacing, depth, and numbers, installing auxiliary heat sources, and utilizing thermal energy storage [13,14]. Among these solutions, increasing borehole spacing or numbers will increase the occupied land and initial investment, besides the disability of fundamentally eliminating the thermal imbalance. Utilizing an auxiliary heat source like a boiler or solar collector will either reduce the energy saving (auxiliary boiler) or increase the initial investment and installation space (auxiliary collector), while thermal energy storage also requires large auxiliary storage devices and high investment [15,16]. (3) The large amount of required boreholes and occupied land is always a major issue that requires the designers to give up GSHP regretfully in their decision-making. A lot of measures have been carried out to reduce the required borehole number and occupied areas, such as increasing the borehole heat exchange rate [17], integrating auxiliary equipment [18], and utilizing a pile-pipe heat exchanger [19]. Among these solutions, the improvement of the heat exchange rate is usually limited; using auxiliary equipment will reduce the
hboiler hdriving hpower
323
ratio of ground heat exchanger rejection to total heat rejection ratio of heat pump heating to total heating hourly borehole inlet fluid temperature, C hourly borehole outlet fluid temperature, C condenser inlet fluid temperature, C evaporator inlet fluid temperature, C boiler efficiency, % driving source efficiency of GSHP power generation efficiency, %
Abbreviations APEE annual primary energy efficiency COP coefficient of performance CPEE cooling seasonal primary energy efficiency CT cooling tower GAX generator absorber heat exchange GHX ground heat exchanger GSAHP ground source absorption heat pump GSEHP ground source electrical heat pump GSHP ground source heat pump HGSAHP hybrid ground source absorption heat pump HGSEHP hybrid ground source electrical heat pump HGSHP hybrid ground source heat pump HPEE heating seasonal primary energy efficiency IR imbalance ratio PEE primary energy efficiency
energy saving or increase initial investment and installation space, while for the pile-pipe heat exchanger, the influence of thermal stress on a building foundation is still unknown.
1.2. Progress of ground source absorption heat pump (GSAHP) To reduce the thermal imbalance of conventional ground source electrical heat pump (GSEHP) systems used in cold regions, a heating/cooling system based on GSAHP was proposed [20]. Compared with GSEHP, GSAHP has lower heating and cooling COP, so it extracts less heat from the soil during the heating season and rejects more heat into the soil during the cooling season, which can effectively reduce the year-round thermal imbalance. Analysis has shown that GSAHP systems have advantageous primary energy efficiency (PEE) over GSEHP systems in heating applications, considering the power generation efficiency of GSEHP [20]. Case studies have indicated that the soil temperature of GSAHP can remain stable in northern parts of northern China (severely cold), but it may increase after long-term operation in southern parts of northern China (cold). In these cases, GSEHP leads to cold accumulation underground, whereas GSAHP will cause heat accumulation. To further reduce the thermal imbalance in this specific region, a combined heating/cooling/domestic hot water system based on GSAHP with heat recovery was put forward. GSAHP with different cycles applied in different regions were simulated comparatively in terms of the thermal imbalance, soil temperature, heat recovery, and energy efficiency [21]. The results showed that GSAHP with a generator absorber heat exchange (GAX) cycle is suitable for Beijing, and GSAHP with a single-effect cycle is suitable for Shenyang. The imbalance ratio can be reduced to 14.8% and 6.0% respectively, with an annual soil temperature variation of only 0.5 C and
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0.1 C. Regarding the rejection heat recovery, about 20% and 15% of the total condensation/absorption heat is recovered to produce domestic hot water. The combined heating/cooling/domestic hot water systems can achieve a PEE improvement of 23.6% and 44.4% compared with the heating/cooling systems. To reduce the thermal imbalance in heating-only systems, the GSAHP integrated with borehole free cooling was proposed to relieve the soil temperature drop and the heating performance deterioration [22]. The soil temperature decrease of heating-only GSAHP can reach 6e7 C after 10 years of operation, but can be reduced to 0e3 C by borehole free cooling. The unguaranteed heating hours of heating-only GSAHP continuously increase year by year, and are reduced dramatically with the aid of free cooling. Additionally, the PEE of GSAHP can be improved from 0.95e1.18 to as high as 1.11e1.56. 1.3. Objectives of this work The above review indicates that the thermal imbalance can be effectively reduced or eliminated by means of GSAHP with competitive PEEs. As mentioned, quite different from the GSEHP with soil cold accumulation in cold regions, the soil heat accumulation is probable for GSAHP in the warmer parts of cold regions. The objective of this work is to study a hybrid GSAHP (HGSAHP) combined with a cooling tower. The hybrid GSEHP (HGSEHP) with a cooling tower has already been widely used in cooling-dominated systems, but there is little research on HGSAHP with a cooling tower in heating-dominated applications. The additional cooling tower can reject the abundant heat of GSAHP to the ambient air rather than to the underground soil, which can obtain better thermal balance, maintain a stable heating performance, reduce the required borehole amounts, as well as lessen the occupied land area. The long-term performance of HGSAHP will be simulated and compared with other conventional GSHP systems, in terms of soil temperature variation, seasonal energy efficiency, borehole numbers, and economic cost. This work can contribute to the soil thermal balance keeping and borehole number reduction of GSHP systems.
2. Principles and methodology 2.1. Configuration of HGSAHP The proposed HGSAHP system with a cooling tower is illustrated in Fig. 1. In the conventional HGSEHP systems, the cooling tower (CT) and the ground heat exchanger (GHX) can be connected in parallel or in series [23e25]. The parallel configuration is adopted due to its flexibility in mode switching between the ground heat exchanger and cooling tower. An intermediate plate heat exchanger is used to prevent the ground heat exchanger from being contaminated by the cooling tower. The GSAHP can be driven by the conventional heating systems like a district heating network and boiler, or be directly fired by natural gas. In winter, the GSAHP operates in heating mode and extracts heat from the soil through the ground heat exchanger. In summer, the GSAHP operates in cooling mode and can reject heat in three different manners: (1) through the ground heat exchanger alone when the building cooling load is low; (2) through the ground heat exchanger and cooling tower together when the cooling load is high; (3) through the cooling tower alone when the ground heat exchanger stops working for better soil recovery in an intermittent operation strategy. The valves that need to be opened and closed in each operation mode are listed in Table 1. The GSAHP in Fig. 1 can be either single-effect or GAX-cycle according to the available driving source temperature. The singleeffect GSAHP (Fig. 2a) usually requires driving source temperature in the range of 100e130 C, and can be well integrated with conventional boilers or district heating networks. The GAX-cycle GSAHP (Fig. 2b) usually requires driving source temperature higher than 160 C, so it is can be driven by high-pressure steam boiler or be directly fired by natural gas. The difference between these two cycles is that the generator and absorber in the GAX-cycle have been divided into several sections, through which it is possible to recover part of the absorption heat for generation, owing to the temperature overlap between the absorber and the generator contributed by high driving source temperatures. Thus, the GAXcycle yields higher performance than single-effect [21].
Cooling tower Plate heat exchanger
Valve 10 Valve 5 Valve 8 Valve 1
Valve 4
GSAHP for space heating/cooling
Building
Valve 2
Valve 3 Valve 6 Valve 7 Valve 9
Ground heat exchanger
Fig. 1. Schematic of HGSAHP with cooling tower.
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334 Table 1 Operation modes of the HGSAHP system. Operation mode
Opened Valve
Closed Valve
Winter extraction: ground heat exchanger Summer rejection: ground heat exchanger Summer rejection: ground heat exchanger þ cooling tower Summer rejection: cooling tower
1, 2, 3, 4, 9
5, 6, 7, 8, 10
5, 6, 7, 8, 9
1, 2, 3, 4, 10
5, 6, 7, 8, 9, 10
1, 2, 3, 4
5, 6, 7, 8, 10
1, 2, 3, 4, 9
2 3 Qc ¼ a þ btc;in þ ctc;in þ dtc;in
(3)
2 3 COPc ¼ a þ btc;in þ ctc;in þ dtc;in
(4)
To evaluate the thermal imbalance degree of GSHP systems, the imbalance ratio (IR) is defined in terms of the heat rejected into the soil in summer and the heat extracted from the soil in winter:
IR ¼
2.2. Performance and evaluation indexes The GSAHP using the single-effect cycle and the GAX-cycle are adopted to make use of different energy sources. NH3eLiNO3 is used for single-effect due to its high efficiency while NH3eH2O is used for GAX-cycle to avoid the possible risk of crystallization that may occur in NH3-salt solutions under higher generation temperatures [26]. The properties of NH3eH2O and NH3eLiNO3 were calculated using the correlations summarized in Ref. [27]. The offdesign performance of GSAHP in heating and cooling mode (Fig. 3) is obtained based on the heat and mass balance models of each component [20,21], which are solved using the software Engineering Equation Solver [28]. These models have been widely used for the analysis of absorption systems with proven accuracy validated in public literatures [29e32]. The conventional GSEHP will also be analyzed for comparison, and the performance data is obtained from the manufacturers, as illustrated in Fig. 4. For more convenient usage in the following system simulation, the heating COP/capacity under different evaporator inlet temperatures and the cooling COP/capacity under different condenser inlet temperatures are correlated in Equations (1)e(4), with the coefficients listed in Table 2. 2 3 Qh ¼ a þ bte;in þ cte;in þ dte;in
(1)
2 3 COPh ¼ a þ bte;in þ cte;in þ dte;in
(2)
325
QAHR QAHE 100% maxðQAHR ; QAHE Þ
(5)
where QAHE and QAHR is the accumulated heat extraction during heating season and accumulated heat rejection during cooling season, kWh, respectively. The accumulated heat extraction from the soil can be calculated as:
QAHE ¼
8760 X
qi
heating;s
1 1 COPh; i
! þ
heating;e X
qi
1
1 1 COPh; i
!
The accumulated heat rejection into the soil was calculated as:
QAHR ¼
cooling;e X
qi 1 þ
cooling;s
1 COPc;i
! (7)
where COPh,i and COPc,i is the hourly heating and cooling COP obtained from hourly simulation; qi is the hourly heating or cooling load, kW; cooling, s and cooling, e is the starting and ending time for cooling; heating, s and heating, e is the starting and ending time for heating. However, in a hybrid GSHP (HGSHP) integrated with a cooling tower, part of the heat is rejected into the air through the cooling tower, while in a HGSHP integrated with an auxiliary boiler, part of the heating load is supplied by the boiler. So the net heat extraction from the soil and the net heat rejection into the soil should be calculated by:
Rectifier
Generator
Rectifier
Condenser GAX Absorber
GAX Generator
Solution-cooled Absorber
Solution-heated Generator
Precooler
Absorber
Condenser
Precooler
Evaporator Externally-heated Generator
(a) Single-effect GSAHP
(6)
Externally-cooled Absorber
(b) GAX-cycle GSAHP
Fig. 2. Schematic of GSAHP with different cycles.
Evaporator
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334
200
1.60
180
1.55
160
COP
1.45
Capacity
100
1.40
80
1.35
COP
140 120
60
1.30
40
1.25
20
1.20
0 -15
-10 -5 0 5 10 15 20 Evaporator inlet fluid temperature
25
0.40
60
20 10 0
0.00 40 35 30 25 Condenser inlet fluid temperature
20
15
2.00
160
1.90
140 COP
120
1.70
100 Capacity
80
1.50
60
1.40
40
1.30
20
1.20
0
1.40
200
180
1.20
160
COP
1.00 COP
COP
30
0.10
Capacity (kW)
180
25
40
0.20
(b) Cooling mode for single-effect 200
-10 -5 0 5 10 15 20 Evaporator inlet fluid temperature
50
Capacity
0.30
45
2.10
-15
80 70
30
2.20
1.60
90
COP
0.50
(a) Heating mode for single-effect
1.80
100
0.60
Capacity (kW)
COP
1.50
0.70
Capacity (kW)
1.65
140 120
0.80
100 0.60
80 60
0.40
Capacity
40
0.20
20 0
0.00 45
30
(c) Heating mode for GAX-cycle
Capacity (kW)
326
40 35 30 25 Condenser inlet fluid temperature
20
15
(d) Cooling mode for GAX-cycle
Fig. 3. Performance of GSAHP in different modes.
cp mb;i tbout;i tbin;i
heating;s
þ
heating;e X
cp mb;i tbout;i tbin;i
(8)
1 cooling;e X
cp mb;i tbin;i tbout;i
(9)
cooling;s
RGHX ¼
3.00
180 150
Capacity
120 2.00
90 60
1.00
30 0.00
0 -5
0 5 10 15 Evaporator inlet fluid temperature
(a) Heating mode
20
25
Capacity (kW)
210
COP
200
5.00
240
4.00
(10)
180
270 5.00
QAHR;hybrid 100% QAHR
6.00
300
6.00
COP
QAHR; hybrid ¼
where cp is the specific heat of fluid, kJ/(kg C); mb,i is the fluid mass flow rate inside the ground heat exchanger, kg/s; tbin,i and tbout,i is the hourly borehole inlet and outlet fluid temperature, C. The ratio of ground heat exchanger rejection to total heat rejection, and the ratio of heat pump heating to total heating are defined as:
Capacity
160 140
4.00
120 COP
3.00
100 80
2.00
60 40
1.00
20 0.00
0 35
30 25 20 Condenser inlet fluid temperature
(b) Cooling mode Fig. 4. Performance of GSEHP in different modes.
15
Capacity (kW)
COP
QAHE; hybrid ¼
8760 X
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334
327
Table 2 The coefficients of GSAHP performance correlations.
GSEHP
GSAHP Single-effect
GSAHP GAX-cycle
P8760
heating;s
RHP ¼
Coefficient
Qh
COPh
Qc
COPc
a b c d a b c d a b c d
102.857 6.143 0 0 86.264 2.761 2.429E-3 3.267E-05 87.143 0.853 1.620E-3 1.211E-4
3.103 0.105 0 0 1.506 6.415E-3 1.863E-4 3.368E-06 1.725 1.380E-2 1.921E-05 9.075E-07
201.211 1.644 0 0 93.880 1.587 2.847E-3 2.810E-05 91.124 1.187 0.001.896E-3 1.560E-05
8.036 0.165 0 0 0.643 4.017E-3 1.348E-4 2.680E-06 1.319 4.634E-3 2.151E-4 1.507E-06
P qi qboiler;i þ heating;e qi qboiler;i 1 100% Pheating;e P8760 qi heating;s qi þ 1
The annual operation cost of GSEHP system is calculated by:
0
1 cooling;e heating;e X X q q q i i i A Cannual ¼@ þ þ COPh; i COPc;i COPh; i 1 cooling;s heating;s 0 1 cooling;e heating;e 8760 X X X $Pelectricity þ @ ei þ ei þ ei APelectricity
(11) where qboiler,i is the hourly heating load supplied by the auxiliary boiler, kW. To evaluate the energy performance of different GSHP systems, the heating seasonal primary energy efficiency (HPEE), cooling seasonal primary energy efficiency (CPEE), and annual primary energy efficiency (APEE) are defined:
P8760 HPEE¼
heating;s qi þ
!
P8760
qi ei COPh;i hdriving þ hpower
heating;s
þ
Pheating;e 1
Pheating;e 1
qi
heating;s
CPEE ¼
cooling;s
Pcooling;e cooling;s
qi
qi COPc;i hdriving
!
(13)
heating;s
þ h ei
power
þ
qi þ
Pcooling;e
Pcooling;e cooling;s
cooling;s
qi þ
qi COPc;i hdriving
Pheating;e 1
þ h ei
power
where ei is the hourly electricity consumption, including the pump and fan, kW; hdriving is the driving source efficiency of GSHP unit, which is hdriving ¼ hboiler for GSAHP (70% for coal boiler and 90% for gas boiler) and hdriving ¼ hpower for GSEHP; hpower is the efficiency of power generation, with a typical value of 33% in China. The annual operation cost of GSAHP system is calculated by:
0
cooling;e X qi qi þ COPh; i hboiler COPc;i hboiler cooling;s heating;s 1 heating;e X 3600Pfuel qi A$ þ COPh; i hboiler Hfuel 1 0 1 cooling;e heating;e 8760 X X X þ@ ei þ ei þ ei APelectricity
Cannual ¼@
8760 X
heating;s
Cannual;i
(17)
power
! qi COPh; i hdriving
20 X 1
heating;s
P8760
C20 years ¼ Cinitial þ
þ h ei
P8760 APEE ¼
1
where Hfuel is the lower heat value of fuel, which is 29,271.2 kJ/kg for coal and 38,930.7 kJ/Nm3 for gas [6]; Pfuel is the unit price of fuel, which is 0.8 CNY/kg for coal and 3.0 CNY/Nm3 for gas; Pelectricity is the unit price of electricity, which is 0.8 CNY/kWh. (Note: The current exchange rate is 1 CNY ¼ 0.1609 USD) The lifecycle total cost over 20 years of different GSHP systems are calculated by:
qi ei COPh;i hdriving þ hpower
!
cooling;s
(16)
(12) Pcooling;e
8760 X
cooling;s
! þ
qi
Pheating;e 1
! qi COPh; i hdriving
(14)
þ h ei
power
where Cinitial is the initial investment of the system, mainly including the GSHP unit (1.3CNY/W based on heating capacity for single-effect, and 1.5CNY/W based on heating capacity for GAXcycle), the GSEHP unit (1.0CNY/W based on heating capacity), the boiler (0.4CNY/W based on heating capacity), the ground heat exchanger (10000CNY/borehole), the cooling tower (300CNY/(m3/ h)), the pump (500CNY/(m3/h)) and the plate heat exchanger (400CNY/m2).
2.3. Modeling of different GSHP systems in TRNSYS
1
(15)
In the focused region where the heating load is dominant but the soil heat accumulation exists in a GSAHP system, Beijing and Shenyang are chosen for analysis. The hourly heating and cooling loads of a 5000 m2 hotel building in these two cities are simulated using the software DeST [33]. The hourly loads and ambient drybulb temperatures in these two cities are illustrated in Fig. 5.
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W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334 40
300
30
Beijing Shenyang
100 0 -100 -200
Hourly temperature ( C)
Hourly building load (kW)
200
20 10 0 Beijing Shenyang
-10 -20
-300
-30
-400 1/Jan 1/Feb 1/Mar 1/Apr 1/May 1/Jun 1/Jul 1/Aug 1/Sep 1/Oct 1/Nov 1/Dec Time
(a) load
1/Jan 1/Feb 1/Mar 1/Apr 1/May 1/Jun 1/Jul 1/Aug 1/Sep 1/Oct 1/Nov 1/Dec Time
(b) ambient temperature
Fig. 5. Load and ambient temperatures in Beijing and Shenyang.
The transient system simulation program TRNSYS, which has been widely used in the analysis of GSEHP systems with proven accuracy [25], is used to simulate the hourly operation performance of the HGSAHP systems, with the TRNSYS model demonstrated in Fig. 6. The building loads are imported from the calculated data in Fig. 5, and new TRNSYS components are created for GSAHP and GSEHP based on the correlated performance in Equations (1)e(4). Other components, including the pumps, cooling tower, ground heat exchanger, plate heat exchanger, controlling units and so on, are from the built-in types in TRNSYS libraries. Based on the load characteristic, climate parameters and the GSAHP performance, the HGSAHP are designed and configured in the TRNSYS model. The borehole spacing is 5 m, borehole depth is 100 m, pipe inside velocity is 0.5 m/s, pipe diameter is 26/32 mm, fill conductivity is 2.00 W/(m$K), pipe conductivity is 0.44 W/(m$K), and soil conductivity is 1.50 W/(m$K). To investigate the advantages of the proposed HGSAHP system, different GSHP systems in Table 3 will be comparatively analyzed.
All of these GSHP systems will be modeled and hourly simulated in TRNSYS. 3. Long-term thermal imbalance of different GSHP systems 3.1. Thermal imbalance ratio For a HGSHP system integrated with a cooling tower, the heat rejection in summer is not completely contributed by the ground heat exchanger, while for a HGSHP system integrated with an auxiliary boiler, the heat supply in winter may not be provided by the GSHP individually. The ratio of ground heat exchanger heat rejection to the total heat rejection (RGHX) and the ratio of heat pump heating to total heating (RHP) of different HGSHP systems are calculated in Table 4. It can be seen that the heat rejected through the ground heat exchanger in summer takes a percentage of 92.0%, 24.7% and 40.7% for HGSEHP, single-effect HGSAHP and GAX-cycle HGSAHP in Beijing, with the rest rejected through the cooling
Fig. 6. TRNSYS model of HGSAHP with cooling tower.
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334 Table 3 Characteristics of different absorption and electrical systems. Systems
Characteristics
GSAHP (single-effect, GAX-cycle) HGSAHP (single-effect, GAX-cycle) GSEHP
Summer: reject heat through GHX only Winter: extract heat from GHX Summer: reject heat through GHX and CT Winter: extract heat from GHX Summer: reject heat through GHX only Winter: extract heat from GHX Summer: reject heat through GHX and CT Winter: extract heat from GHX Summer: reject heat through GHX only Winter: extract heat from GHX and use boiler for peak load
HGSEHP (CT, coolingdominant) HGSEHP (boiler, heatingdominant)
tower. What is quite different in Shenyang is that the IR is very negative for GSEHP but very positive for GSAHP, so the HGSEHP should be integrated with an auxiliary boiler while HGSAHP is still integrated with a cooling tower. The heat rejected through the ground heat exchanger in summer occupies 45.3% and 59.8% in single-effect HGSAHP and GAX-cycle HGSAHP, while the heat pump mode of HGSEHP supplies 56.9% of the heating loads in winter, with the rest supplied by an auxiliary boiler. The imbalance ratio (IR) of different GSAHP and GSEHP systems over 20 years are presented in Table 5 and Table 6. In Beijing, though the IR of GSEHP is very small, it can also be modified as HGSEHP to reduce the required borehole number while keeping a small IR. The IR of single-effect GSAHP is as high as 78e79%, and is reduced to 19e13% by single-effect HGSAHP. As for the GAX-cycle, the IR of GAX-cycle GSAHP is 60e59%, and can be effectively reduced to 7e4% when using GAX-cycle HGSAHP. In Shenyang, the IR of GSEHP is as negative as 44 ~ 40%, while is reduced to a very small value around 3%. The IR of single-effect GSAHP reaches 60e59%, and is reduced to 15e12% by singleeffect HGSAHP. As for the GAX-cycle, the IR of GAX-cycle GSAHP is 25e24%, and can be changed to about 16% when using GAXcycle HGSAHP. In summary, the GSEHP and GAX-cycle GSAHP without auxiliary equipment can obtain small IR values in certain conditions, whereas the hybrid systems (including HGSEHP, singleeffect HGSAHP, and GAX-cycle HGSAHP) can always keep good thermal balance for all occasions.
3.2. Average soil temperature The thermal imbalance ratio has a great influence on the soil temperature, and the annual average soil temperature variation over a 20-year operation is demonstrated in Fig. 7 and Fig. 8. In Beijing, the soil temperature changes from 14.6 C to 14.4 C for GSEHP, to 14.2 C for HGSEHP, to 30.4 C for single-effect GSAHP, to 16.3 C for single-effect HGSAHP, to 24.8 C for GAX-cycle GSAHP and to 15.2 C for GAX-cycle HGSAHP, with variations of 0.2 C, 0.4 C, 15.8 C, 1.7 C, 10.2 C and 0.6 C, respectively. The stable soil temperatures of GSEHP and HGSEHP indicate that both of these two systems maintain good thermal balance in Beijing. The single-effect GSAHP will lead to a great soil temperature
Table 4 RGHX and RHP for different hybrid GSHP systems. Hybrid system
HGSEHP Single-effect HGSAHP GAX-cycle HGSAHP
Beijing
329
increase due to the severe heat accumulation, while the singleeffect HGSAHP becomes quite close to thermal balance. Compared with the single-effect GSAHP, the soil temperature rise of GAX-cycle GSAHP decreases but still very obviously, whereas the GAX-cycle HGSAHP achieves a more stable soil temperature. As for Shenyang, the soil temperature changes from 10.6 C to 2.3 C for GSEHP, to 9.6 C for HGSEHP, to 19.8 C for single-effect GSAHP, to 13.1 C for single-effect HGSAHP, to 13.6 C for GAXcycle GSAHP and to 9.8 C for GAX-cycle HGSAHP, with variations of 8.3 C, 1.0 C, 9.2 C, 2.5 C, 3.0 C and 0.8 C, respectively. During a long period of 10e20 years, a soil temperature variation within 3e4 C can be regarded as acceptable [25]. The single-effect GSAHP will lead to severe heat accumulation while the GSEHP will lead to severe cold accumulation. After being integrated with a cooling tower or an auxiliary boiler, all of the GSHP systems can maintain good thermal balance and stable soil temperature. 3.3. GHX outlet fluid temperature The soil temperature will affect the outlet fluid temperature of the ground heat exchanger, which finally determines the COP and capacity of GSHP units. The minimum outlet fluid temperature in winter and the maximum outlet fluid temperature in summer during a 20-year operation are illustrated in Fig. 9 and Fig. 10. In Beijing, the minimum outlet fluid temperature of single-effect GSAHP increases from 12.7 C to 27.7 C, and that of GAX-cycle GSAHP increases from 11.2 C to 21.1 C in winter. On the other hand, the corresponding maximum outlet fluid temperature in summer increases, respectively, from 34.6 C to 53.8 C and from 34.4 C to 43.8 C. Though the increase of outlet fluid temperature in winter is helpful in heating mode, the increase of outlet fluid temperature in summer will lead to bad performance and low reliability of the cooling supply. In Shenyang, the minimum outlet fluid temperature of GSEHP drops from 0.7 to 6.1, while that of single-effect GSAHP increases from 8.0 C to 16.8 C in winter. On the other hand, the corresponding maximum outlet fluid temperature in summer respectively drops from 26.8 C to 18.8 C, and increases from 29.8 C to 39.2 C. Though the decrease of outlet fluid temperature in summer is helpful in the cooling mode, the decrease of outlet fluid temperature in winter will cause bad performance and low reliability of heating supply. Therefore, it is favorable to keep a stable GHX outlet fluid temperature to maintain both high efficiency and high reliability in both heating and cooling modes during long-term operations. In terms of the thermal imbalance ratio, average soil temperature, and GHX outlet fluid temperature, it can be concluded that GSEHP, HGSEHP, single-effect HGSAHP, and GAX-cycle HGSAHP are suitable in regions like Beijing, whereas HGSEHP, single-effect HGSAHP, GAX-cycle GSAHP, and GAX-cycle HGSAHP are suitable in regions like Shenyang. 4. Comparisons between novel GSAHP and conventional GSEHP The above suitable GSHP systems with good thermal balance will be comparatively analyzed to investigate the primary energy efficiency, required borehole number, and technical economy. 4.1. Seasonal and annual primary energy efficiency
Shenyang
RGHX
RHP
RGHX
RHP
92.0% 24.7% 40.7%
100% 100% 100%
100% 45.3% 59.8%
56.9% 100% 100%
The average HPEE, CPEE, and APEE values of different GSHP systems over 20 years are shown in Fig. 11 and Fig. 12. In Beijing, the HPEE relationship is GAX-cycle HGSAHP > single-effect HGSAHP > GSEHP > HGSEHP, and the CPEE relationship is
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Table 5 IR of different GSHP systems over 20 years in Beijing. Year
GSEHP
HGSEHP
Single-effect GSAHP
Single-effect HGSAHP
GAX-cycle GSAHP
GAX-cycle HGSAHP
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
2.2% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3%
4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.4% 4.4% 4.4% 4.4% 4.4% 4.5%
78.4% 78.4% 78.4% 78.4% 78.4% 78.4% 78.4% 78.4% 78.4% 78.5% 78.5% 78.5% 78.6% 78.6% 78.6% 78.7% 78.7% 78.8% 78.8% 78.9%
18.7% 17.4% 16.4% 15.6% 15.1% 14.8% 14.5% 14.2% 14.0% 13.9% 13.7% 13.6% 13.5% 13.5% 13.4% 13.3% 13.3% 13.2% 13.2% 13.2%
59.7% 59.6% 59.6% 59.5% 59.5% 59.4% 59.4% 59.4% 59.4% 59.3% 59.3% 59.3% 59.3% 59.3% 59.3% 59.3% 59.3% 59.3% 59.3% 59.3%
7.0% 6.4% 5.8% 5.5% 5.3% 5.1% 4.9% 4.8% 4.7% 4.6% 4.5% 4.5% 4.4% 4.3% 4.3% 4.3% 4.2% 4.2% 4.2% 4.2%
Table 6 IR of different GSHP systems over 20 years in Shenyang. Year
GSEHP
HGSEHP
Single-effect GSAHP
Single-effect HGSAHP
GAX-cycle GSAHP
GAX-cycle HGSAHP
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
44.4% 43.8% 43.4% 43.0% 42.7% 42.3% 42.0% 41.7% 41.5% 41.3% 41.0% 40.8% 40.7% 40.5% 40.3% 40.2% 40.1% 40.0% 39.8% 39.7%
3.5% 3.4% 3.4% 3.4% 3.4% 3.4% 3.4% 3.3% 3.3% 3.3% 3.3% 3.3% 3.3% 3.3% 3.3% 3.3% 3.2% 3.2% 3.2% 3.2%
59.6% 59.5% 59.4% 59.4% 59.3% 59.3% 59.3% 59.2% 59.2% 59.2% 59.2% 59.2% 59.2% 59.1% 59.1% 59.1% 59.1% 59.1% 59.1% 59.1%
15.2% 14.6% 14.0% 13.9% 13.5% 13.3% 13.0% 13.0% 12.9% 12.7% 12.7% 12.6% 12.5% 12.5% 12.5% 12.5% 12.4% 12.3% 12.2% 12.1%
25.4% 25.3% 25.2% 25.1% 25.0% 24.9% 24.9% 24.8% 24.7% 24.7% 24.7% 24.6% 24.6% 24.6% 24.5% 24.5% 24.5% 24.4% 24.4% 24.4%
15.9% 15.7% 15.7% 15.8% 15.9% 15.8% 15.8% 15.8% 15.7% 15.8% 15.7% 15.8% 15.7% 15.8% 15.7% 15.9% 15.9% 16.0% 16.0% 16.0%
Single-effect GSAHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP GSEHP HGSEHP
Annual average soil temperature ( C)
30 28 26
Single-effect GSAHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP GSEHP HGSEHP
22 20 Annual average soil temperature ( C)
32
24 22 20 18 16 14 12
18 16 14 12 10 8 6 4 2
0
5
10
15
20
Year Fig. 7. Soil temperatures of different GSHP systems over 20 years in Beijing.
0
5
10
15
20
Year Fig. 8. Soil temperatures of different GSHP systems over 20 years in Shenyang.
32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2
56
Single-effect GSAHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP GSEHP HGSEHP
331
Single-effect GSAHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP GSEHP HGSEHP
54 52 50 GHX outlet temperature ( C)
GHX outlet temperature ( C)
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334
48 46 44 42 40 38 36 34 32 30
0
5
10
15
20
0
5
Year
10
15
20
15
20
Year
(a) heating mode
(b) cooling mode
24 22 20 18 16 14 12 10 8 6 4 2 0 -2 -4 -6 -8
44
Single-effect GSAHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP GSEHP HGSEHP
Single-effect GSAHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP GSEHP HGSEHP
42 40 38 GHX outlet temperature ( C)
GHX outlet temperature ( C)
Fig. 9. GHX outlet temperatures of different GSHP systems over 20 years in Beijing.
36 34 32 30 28 26 24 22 20 18
0
5
10
15
20
0
5
Year
10 Year
(a) heating mode
(b) cooling mode
Fig. 10. GHX outlet temperatures of different GSHP systems over 20 years in Shenyang.
winter; on the other hand, GAX-cycle HGSAHP needs the operation of the cooling tower and the cooling pump, which leads to higher electricity consumption and lower CPEE in summer. In any case, owing to greater advantages of GSAHP in heating mode and a
1.8 1.6
HPEE CPEE APEE
1.4 Primary energy efficiency
GSEHP > HGSEHP > GAX-cycle HGSAHP > single-effect HGSAHP, and finally, the APEE relationship is GSEHP > HGSEHP > GAX-cycle HGSAHP > single-effect HGSAHP. Though the APEE of HGSEHP is a little lower than that of GSEHP, HGSEHP requires less boreholes with the aid of a cooling tower, which will be presented later on. The GSAHP systems are advantageous in heating mode and inferior to GSEHP systems in cooling mode, and the APEE of GAX-cycle HGSAHP is very close to that of HGSEHP in regions like Beijing. As for Shenyang, with a much higher heating load and lower cooling load, the advantages of GSAHP systems in heating mode will be much more favorable to the annual performance. The HPEE relationship is GAX-cycle HGSAHP > GAX-cycle HGSAHP > singleeffect HGSAHP > HGSEHP, and the CPEE relationship is HGSEHP > GAX-cycle GSAHP > GAX-cycle HGSAHP > single-effect HGSAHP, and finally, the APEE relationship is GAX-cycle HGSAHP > GAX-cycle HGSAHP > HGSEHP > single-effect HGSAHP. The efficiency difference between GAX-cycle GSAHP and GAX-cycle HGSAHP is comprehensively caused by heat pump COP and water pump consumption. Compared with GAX-cycle GSAHP, GAX-cycle HGSAHP has lower heating COP and higher cooling COP due to lower soil temperature (Fig. 8). However, GAX-cycle HGSAHP requires fewer boreholes, and thus a smaller fluid flow rate, so the water pump consumption is smaller and the HPEE is higher in
1.2 1.0 0.8 0.6 0.4 0.2 0.0
GSEHP
HGSEH P
Single-e GAX-cy ffect H GSAHP cle HGSAHP
Fig. 11. Average primary energy efficiency of different GSHP systems in Beijing.
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W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334 1.8 1.6
100
HPEE CPEE APEE
80
1.4 1.2
Borehole number
Primary energy efficiency
96
1.0 0.8 0.6 0.4
60
54 43
40
34
20
0.2 0.0
HGSEH P
Fig. 12. Average primary energy efficiency of different GSHP systems in Shenyang.
bigger percentage of heating load in regions like Shenyang, the annual performance of GAX-cycle HGSAHP becomes the highest among all the GSHP systems, which is 17.5% higher than that of the conventional HGSEHP. 4.2. Required borehole numbers and occupied areas Although the HGSAHP is not absolutely the best selection in terms of APEE, its ability to reduce the required borehole number makes it much more preferable. The design borehole numbers of the aforementioned GSHP systems with good thermal balance are demonstrated in Fig. 13 and Fig. 14. Compared with the currently widely used HGSEHP, the single-effect HGSAHP can reduce the required borehole number by 52% in Beijing and 37% in Shenyang, while the GAX-cycle HGSAHP can reduce the required borehole number by 38% in Beijing and 20% in Shenyang. This is because HGSAHP has lower heating COP, it extracts less heat from the soil and it requires less boreholes for heat exchange. It should be noted that GAX-cycle GSAHP requires more boreholes, though it can keep the soil temperature variation within 3e4 C, so it is much better to use GAX-cycle HGSAHP integrated with an auxiliary cooling tower. Fewer boreholes means not only less borehole construction and investment, but also less occupied land, which is of great significance nowadays in the high-rise buildings in high-density cities. With a borehole spacing of 5 m, each borehole needs 25 m2 area; the total occupied areas required by different GSHP systems are calculated in Table 7. It is obvious that the occupied areas can be effectively reduced by single-effect HGSAHP and GAX-cycle HGSAHP.
100
Borehole number
80
74
60
0
G GAX-cy Single-e ffect H cle GSA AX-cycle HGS GSAHP AHP HP
56
40
HGSEH P
GAX-cy GAX-cy Single-e ffect H cle HGS GSAHP cle GSAHP AHP
Fig. 14. Required borehole number of different GSHP systems in Shenyang.
4.3. Economic analysis The initial investments and 20-year lifecycle costs of the balance-kept GSHP systems using different energy sources are calculated to further evaluate their applicability. The initial investments are presented in Table 8, which are estimated based on the selection of main equipment composing different GSHP systems. Compared with HGSEHP, HGSAHP reduces investment in boreholes and boiler, but increases investment in the GSHP unit, pump, cooling tower, and heat exchanger. Two factors lead to higher investment in an absorption-type GSHP unit: (1) higher unit price per heating capacity of GSAHP; (2) much lower ratio of cooling capacity to heating capacity (Fig. 3), which means that greatly redundant heating capacity has to be selected to guarantee the same cooling capacity. The combination of GSAHP and GSEHP can be further studied in the future to solve this problem. The 20-year lifecycle total costs of different GSHP systems are shown in Fig. 15 and Fig. 16. The cost in the 0th year stands for the initial investment of each system. In Beijing, the lifecycle cost is respectively 300.5 104CNY for GSEHP, 296.4 104CNY for HGSEHP, 300.7 104CNY for single-effect HGSAHP (coal), 469.0 104CNY for single-effect HGSAHP (gas), 236.5 104CNY for GAX-cycle HGSAHP (coal), and 345.8 104CNY for GAX-cycle HGSAHP (gas). As a result, GAX-cycle HGSAHP (coal) needs the lowest lifecycle cost among all the systems, while GSEHP, HGSEHP and single-effect HGSAHP (coal) are very close and rank at the second lowest level. The HGSAHP systems using gas as the energy source is relatively expensive due to the current high price of gas. In Shenyang, the lifecycle cost is respectively 311.7 104CNY for HGSEHP (coal), 374.1 104CNY for HGSEHP (gas), 279.9 104CNY for single-effect HGSAHP (coal), 454.7 104CNY for single-effect HGSAHP (gas), 295.3 104CNY for GAX-cycle GSAHP (coal), and 414.2 104CNY for GAX-cycle GSAHP (gas), 233.4 104CNY for GAX-cycle HGSAHP (coal), and 356.3 104CNY for GAX-cycle HGSAHP (gas). Consequently, GAX-cycle HGSAHP (coal) still requires the lowest lifecycle cost among all the systems. It is worth noting that lifecycle cost of GAX-cycle HGSAHP (gas) becomes
35 27
Table 7 Total occupied areas required by different GSHP systems.
20
0
GSEHP
HGSEH P
GAX-cy Single-e ffect H GSAHP cle HGSAHP
Fig. 13. Required borehole number of different GSHP systems in Beijing.
System
Beijing
Shenyang
GSEHP HGSEHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP
1850 m2 1400 m2 675 m2 e 875 m2
e 1350 m2 850 m2 2400 m2 1075 m2
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334
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Table 8 Initial investments of different GSHP systems. Beijing (unit: 104CNY) Item GSHP unit Boiler Borehole Pump Cooling tower PHX Total
GSEHP 29.3 0.0 74.0 4.0 0.0 0.0 107.3
HGSEHP 29.3 0.0 56.0 3.9 0.3 0.3 89.7
Single-effect HGSAHP 90.1 0.0 27.0 14.0 3.8 3.6 138.4
GAX-cycle HGSAHP 80.4 0.0 35.0 9.0 2.1 2.1 128.5
HGSEHP 29.8 7.2 54.0 7.0 0.0 0.0 98.0
Single-effect HGSAHP 63.6 0.0 34.0 11.7 3.0 2.9 115.1
GAX-cycle GSAHP 64.0 0.0 96.0 7.0 0.0 0.0 167.0
GAX-cycle HGSAHP 64.0 0.0 43.0 5.3 1.4 1.4 115.0
Shenyang (unit: 104CNY) Item GSHP unit Boiler Borehole Pump Cooling tower PHX Total
Note: The current exchange rate is 1 CNY ¼ 0.1609 USD.
500
4
Total cost over 20 years ( 10 CNY)
450 400 350
system efficiency, reduce the required borehole number, and lessen the occupied area. Hourly simulations of the single-effect HGSAHP and GAX-cycle HGSAHP have been carried out and compared with other GSHP systems to investigate the advantages and disadvantages. The following conclusions can be made:
GSEHP HGSEHP Single-effect HGSAHP(coal) Single-effect HGSAHP(gas) GAX-cycle HGSAHP(coal) GAX-cycle HGSAHP(gas)
300 250 200 150 100 50 0
5
10
15
20
Year Fig. 15. Lifecycle cost of different GSHP systems in Beijing.
lower than that of conventional HGSEHP (gas) assisted by an auxiliary boiler. 5. Conclusions HGSAHP combining GSAHP with a cooling tower is proposed to obtain better thermal balance, keep stable soil temperature and 500
Total cost over 20 years ( 10 CNY)
450 400 350 300
References
250 200 150 100 50 5
Acknowledgements The authors gratefully acknowledge the support from the Natural Science Foundation for Distinguished Young Scholars of China (grant No. 51125030) and the National 12th Five-year Science and Technology Support Project of China (grant No. 2011BAJ03B09).
HGSEHP(coal) HGSEHP(gas) Single-effect HGSAHP(coal) Single-effect HGSAHP(gas) GAX-cycle GSAHP(coal) GAX-cycle GSAHP(gas) GAX-cycle HGSAHP(coal) GAX-cycle HGSAHP(gas)
0
(1) The thermal balance can be well kept by HGSAHP, with IR reduced from 60e80% to within 20%, and soil temperature variation staying within 3 C after 20 years' operation; (2) The HGSAHP are advantageous in heating mode and inferior to HGSEHP in cooling mode. The annual performance of GAX-cycle HGSAHP is very close to that of HGSEHP in Beijing, while being the highest, and 17.5% higher than that of HGSEHP in colder regions like Shenyang; (3) Compared with HGSEHP, single-effect HGSAHP can reduce the required borehole number by 37e52%, while the GAXcycle HGSAHP can reduce the required borehole number by 20e38%. The land areas occupied by the boreholes can also be effectively reduced; (4) The lifecycle cost of GAX-cycle HGSAHP (coal) is the lowest in both Beijing and Shenyang, while GAX-cycle HGSAHP (gas) becomes cheaper than conventional HGSEHP (gas) assisted by an auxiliary boiler.
10
15
20
Year Fig. 16. Lifecycle cost of different GSHP systems in Shenyang.
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Research paper
Hybrid ground source absorption heat pump in cold regions: Thermal balance keeping and borehole number reduction Wei Wu, Xianting Li*, Tian You, Baolong Wang, Wenxing Shi Department of Building Science, School of Architecture, Tsinghua University, Beijing, 100084, China
h i g h l i g h t s Hybrid ground source absorption heat pump (HGSAHP) with cooling tower is proposed. HGSAHP maintains soil thermal balance and reduces borehole number in cold regions. Borehole number and land areas are reduced by 37e52% and 20e38% by various cycles. The annual primary energy efficiency can be 17.5% higher than conventional GSHP. Lifecycle cost of HGSAHP over 20 years can be advantageous over conventional GSHP.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2015 Accepted 4 July 2015 Available online 15 July 2015
Thermal imbalance of ground source electrical heat pump (GSEHP) leads to cold accumulation in cold regions. Ground source absorption heat pump (GSAHP) can relieve the thermal imbalance, but may cause heat accumulation in the warmer parts of cold regions. Hybrid GSAHP (HGSAHP) integrated with a cooling tower is proposed to solve this problem. Hourly simulations of single-effect HGSAHP and generator absorber heat exchange (GAX) cycle HGSAHP are conducted, and are compared with hybrid GSEHP (HGSEHP). Results show that the thermal balance can be well kept by HGSAHP, with imbalance ratio reduced from 60e80% to within 20%, and soil temperature variation staying within 3 C after 20 years' operation. Moreover, HGSAHPs are advantageous in heating mode and inferior to HGSEHP in cooling mode. The annual performance of GAX-cycle HGSAHP is very close to that of HGSEHP in Beijing, while being 17.5% higher than that of HGSEHP in colder regions like Shenyang. Compared with HGSEHP, the required borehole number and occupied land areas can be reduced by 37e52% by single-effect HGSAHP and reduced by 20e38% by GAX-cycle HGSAHP. Additionally, the lifecycle cost of GAX-cycle HGSAHP (coal) is the lowest, while GAX-cycle HGSAHP (gas) is cheaper than conventional HGSEHP (gas) assisted by an auxiliary boiler. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Thermal imbalance Absorption heat pump Ground source heat pump Borehole number Soil temperature Primary energy efficiency
1. Introduction 1.1. Problems of ground source heat pump (GSHP) in cold regions GSHPs are widely used for space heating, air conditioning, and domestic hot water due to the advantages of energy saving and emission reduction, as well as the incentives from the government [1e4]. In 2012, GSHP installation reached an estimated 50 GW of capacity; China had the biggest market share and continued to increase at about 10% annually [5]. Under the severe circumstance
* Corresponding author. Tel.: þ86 10 62785860; fax: þ86 10 62773461. E-mail address: [email protected] (X. Li). http://dx.doi.org/10.1016/j.applthermaleng.2015.07.014 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
of hazy weather in China, the urgent necessity of reducing the pollutant emissions of the conventional heating systems offers great opportunities for the clean heating technologies, with GSHP as one of the potential alternatives. Therefore, the GSHP systems are predicted to be more and more popular in the future. However, the large-scale application of GSHP systems brings about three major problems: (1) The conventional heating systems in northern China are mainly based on the burning of fossil fuel because of the coal-dominated energy structure [6,7]. A massive replacement with electricity-based heating systems will bring serious pressure on electricity generation and transmission, and also, it is a big waste of the existing system investment. It
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334
Nomenclature
RGHX
COPc COPh Cannual Cinitial C20 years cp ei Hfuel
RHP tbin,i tbout,i tc,in te,in
coefficient of performance in cooling mode coefficient of performance in heating mode annual operation cost, CNY initial investment of the system, CNY lifecycle total cost over 20 years, CNY specific heat, kJ/(kg C) hourly electricity consumption, kW lower heat value of fuel, kJ/kg for coal and kJ/Nm3 for gas mb,i fluid mass flow rate inside the ground heat exchanger, kg/s Pfuel unit price of fuel, CNY/kg for coal and CNY/Nm3 for gas Pelectricity unit price of electricity, CNY/kWh QAHE accumulated heat extraction during heating season, kWh QAHE,hybrid accumulated heat extraction from soil for hybrid system during heating season, kWh QAHR accumulated heat rejection during cooling season, kWh QAHR,hybrid accumulated heat rejection into soil for hybrid system during cooling season, kWh Qc cooling capacity, kW Qh heating capacity, kW qboiler,i hourly heating load supplied by the auxiliary boiler, kW qi hourly heating or cooling load, kW
may be more practically reasonable to improve energy efficiency and reduce the pollutant emission of the present heating systems, rather than completely replace them with other heating systems attached with new problems. (2) The imbalance between heat extraction and heat rejection will lead to the cold accumulation in heating-dominated buildings [8,9] or heat accumulation in cooling-dominated buildings [10,11]. Cold accumulation is very common in cold regions; the soil temperature will gradually decrease after long-term operation, since the temperature recovery ability of soil is limited, which finally leads to the deterioration of heating performance [12]. Measures to solve the above problems mainly include increasing borehole spacing, depth, and numbers, installing auxiliary heat sources, and utilizing thermal energy storage [13,14]. Among these solutions, increasing borehole spacing or numbers will increase the occupied land and initial investment, besides the disability of fundamentally eliminating the thermal imbalance. Utilizing an auxiliary heat source like a boiler or solar collector will either reduce the energy saving (auxiliary boiler) or increase the initial investment and installation space (auxiliary collector), while thermal energy storage also requires large auxiliary storage devices and high investment [15,16]. (3) The large amount of required boreholes and occupied land is always a major issue that requires the designers to give up GSHP regretfully in their decision-making. A lot of measures have been carried out to reduce the required borehole number and occupied areas, such as increasing the borehole heat exchange rate [17], integrating auxiliary equipment [18], and utilizing a pile-pipe heat exchanger [19]. Among these solutions, the improvement of the heat exchange rate is usually limited; using auxiliary equipment will reduce the
hboiler hdriving hpower
323
ratio of ground heat exchanger rejection to total heat rejection ratio of heat pump heating to total heating hourly borehole inlet fluid temperature, C hourly borehole outlet fluid temperature, C condenser inlet fluid temperature, C evaporator inlet fluid temperature, C boiler efficiency, % driving source efficiency of GSHP power generation efficiency, %
Abbreviations APEE annual primary energy efficiency COP coefficient of performance CPEE cooling seasonal primary energy efficiency CT cooling tower GAX generator absorber heat exchange GHX ground heat exchanger GSAHP ground source absorption heat pump GSEHP ground source electrical heat pump GSHP ground source heat pump HGSAHP hybrid ground source absorption heat pump HGSEHP hybrid ground source electrical heat pump HGSHP hybrid ground source heat pump HPEE heating seasonal primary energy efficiency IR imbalance ratio PEE primary energy efficiency
energy saving or increase initial investment and installation space, while for the pile-pipe heat exchanger, the influence of thermal stress on a building foundation is still unknown.
1.2. Progress of ground source absorption heat pump (GSAHP) To reduce the thermal imbalance of conventional ground source electrical heat pump (GSEHP) systems used in cold regions, a heating/cooling system based on GSAHP was proposed [20]. Compared with GSEHP, GSAHP has lower heating and cooling COP, so it extracts less heat from the soil during the heating season and rejects more heat into the soil during the cooling season, which can effectively reduce the year-round thermal imbalance. Analysis has shown that GSAHP systems have advantageous primary energy efficiency (PEE) over GSEHP systems in heating applications, considering the power generation efficiency of GSEHP [20]. Case studies have indicated that the soil temperature of GSAHP can remain stable in northern parts of northern China (severely cold), but it may increase after long-term operation in southern parts of northern China (cold). In these cases, GSEHP leads to cold accumulation underground, whereas GSAHP will cause heat accumulation. To further reduce the thermal imbalance in this specific region, a combined heating/cooling/domestic hot water system based on GSAHP with heat recovery was put forward. GSAHP with different cycles applied in different regions were simulated comparatively in terms of the thermal imbalance, soil temperature, heat recovery, and energy efficiency [21]. The results showed that GSAHP with a generator absorber heat exchange (GAX) cycle is suitable for Beijing, and GSAHP with a single-effect cycle is suitable for Shenyang. The imbalance ratio can be reduced to 14.8% and 6.0% respectively, with an annual soil temperature variation of only 0.5 C and
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0.1 C. Regarding the rejection heat recovery, about 20% and 15% of the total condensation/absorption heat is recovered to produce domestic hot water. The combined heating/cooling/domestic hot water systems can achieve a PEE improvement of 23.6% and 44.4% compared with the heating/cooling systems. To reduce the thermal imbalance in heating-only systems, the GSAHP integrated with borehole free cooling was proposed to relieve the soil temperature drop and the heating performance deterioration [22]. The soil temperature decrease of heating-only GSAHP can reach 6e7 C after 10 years of operation, but can be reduced to 0e3 C by borehole free cooling. The unguaranteed heating hours of heating-only GSAHP continuously increase year by year, and are reduced dramatically with the aid of free cooling. Additionally, the PEE of GSAHP can be improved from 0.95e1.18 to as high as 1.11e1.56. 1.3. Objectives of this work The above review indicates that the thermal imbalance can be effectively reduced or eliminated by means of GSAHP with competitive PEEs. As mentioned, quite different from the GSEHP with soil cold accumulation in cold regions, the soil heat accumulation is probable for GSAHP in the warmer parts of cold regions. The objective of this work is to study a hybrid GSAHP (HGSAHP) combined with a cooling tower. The hybrid GSEHP (HGSEHP) with a cooling tower has already been widely used in cooling-dominated systems, but there is little research on HGSAHP with a cooling tower in heating-dominated applications. The additional cooling tower can reject the abundant heat of GSAHP to the ambient air rather than to the underground soil, which can obtain better thermal balance, maintain a stable heating performance, reduce the required borehole amounts, as well as lessen the occupied land area. The long-term performance of HGSAHP will be simulated and compared with other conventional GSHP systems, in terms of soil temperature variation, seasonal energy efficiency, borehole numbers, and economic cost. This work can contribute to the soil thermal balance keeping and borehole number reduction of GSHP systems.
2. Principles and methodology 2.1. Configuration of HGSAHP The proposed HGSAHP system with a cooling tower is illustrated in Fig. 1. In the conventional HGSEHP systems, the cooling tower (CT) and the ground heat exchanger (GHX) can be connected in parallel or in series [23e25]. The parallel configuration is adopted due to its flexibility in mode switching between the ground heat exchanger and cooling tower. An intermediate plate heat exchanger is used to prevent the ground heat exchanger from being contaminated by the cooling tower. The GSAHP can be driven by the conventional heating systems like a district heating network and boiler, or be directly fired by natural gas. In winter, the GSAHP operates in heating mode and extracts heat from the soil through the ground heat exchanger. In summer, the GSAHP operates in cooling mode and can reject heat in three different manners: (1) through the ground heat exchanger alone when the building cooling load is low; (2) through the ground heat exchanger and cooling tower together when the cooling load is high; (3) through the cooling tower alone when the ground heat exchanger stops working for better soil recovery in an intermittent operation strategy. The valves that need to be opened and closed in each operation mode are listed in Table 1. The GSAHP in Fig. 1 can be either single-effect or GAX-cycle according to the available driving source temperature. The singleeffect GSAHP (Fig. 2a) usually requires driving source temperature in the range of 100e130 C, and can be well integrated with conventional boilers or district heating networks. The GAX-cycle GSAHP (Fig. 2b) usually requires driving source temperature higher than 160 C, so it is can be driven by high-pressure steam boiler or be directly fired by natural gas. The difference between these two cycles is that the generator and absorber in the GAX-cycle have been divided into several sections, through which it is possible to recover part of the absorption heat for generation, owing to the temperature overlap between the absorber and the generator contributed by high driving source temperatures. Thus, the GAXcycle yields higher performance than single-effect [21].
Cooling tower Plate heat exchanger
Valve 10 Valve 5 Valve 8 Valve 1
Valve 4
GSAHP for space heating/cooling
Building
Valve 2
Valve 3 Valve 6 Valve 7 Valve 9
Ground heat exchanger
Fig. 1. Schematic of HGSAHP with cooling tower.
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334 Table 1 Operation modes of the HGSAHP system. Operation mode
Opened Valve
Closed Valve
Winter extraction: ground heat exchanger Summer rejection: ground heat exchanger Summer rejection: ground heat exchanger þ cooling tower Summer rejection: cooling tower
1, 2, 3, 4, 9
5, 6, 7, 8, 10
5, 6, 7, 8, 9
1, 2, 3, 4, 10
5, 6, 7, 8, 9, 10
1, 2, 3, 4
5, 6, 7, 8, 10
1, 2, 3, 4, 9
2 3 Qc ¼ a þ btc;in þ ctc;in þ dtc;in
(3)
2 3 COPc ¼ a þ btc;in þ ctc;in þ dtc;in
(4)
To evaluate the thermal imbalance degree of GSHP systems, the imbalance ratio (IR) is defined in terms of the heat rejected into the soil in summer and the heat extracted from the soil in winter:
IR ¼
2.2. Performance and evaluation indexes The GSAHP using the single-effect cycle and the GAX-cycle are adopted to make use of different energy sources. NH3eLiNO3 is used for single-effect due to its high efficiency while NH3eH2O is used for GAX-cycle to avoid the possible risk of crystallization that may occur in NH3-salt solutions under higher generation temperatures [26]. The properties of NH3eH2O and NH3eLiNO3 were calculated using the correlations summarized in Ref. [27]. The offdesign performance of GSAHP in heating and cooling mode (Fig. 3) is obtained based on the heat and mass balance models of each component [20,21], which are solved using the software Engineering Equation Solver [28]. These models have been widely used for the analysis of absorption systems with proven accuracy validated in public literatures [29e32]. The conventional GSEHP will also be analyzed for comparison, and the performance data is obtained from the manufacturers, as illustrated in Fig. 4. For more convenient usage in the following system simulation, the heating COP/capacity under different evaporator inlet temperatures and the cooling COP/capacity under different condenser inlet temperatures are correlated in Equations (1)e(4), with the coefficients listed in Table 2. 2 3 Qh ¼ a þ bte;in þ cte;in þ dte;in
(1)
2 3 COPh ¼ a þ bte;in þ cte;in þ dte;in
(2)
325
QAHR QAHE 100% maxðQAHR ; QAHE Þ
(5)
where QAHE and QAHR is the accumulated heat extraction during heating season and accumulated heat rejection during cooling season, kWh, respectively. The accumulated heat extraction from the soil can be calculated as:
QAHE ¼
8760 X
qi
heating;s
1 1 COPh; i
! þ
heating;e X
qi
1
1 1 COPh; i
!
The accumulated heat rejection into the soil was calculated as:
QAHR ¼
cooling;e X
qi 1 þ
cooling;s
1 COPc;i
! (7)
where COPh,i and COPc,i is the hourly heating and cooling COP obtained from hourly simulation; qi is the hourly heating or cooling load, kW; cooling, s and cooling, e is the starting and ending time for cooling; heating, s and heating, e is the starting and ending time for heating. However, in a hybrid GSHP (HGSHP) integrated with a cooling tower, part of the heat is rejected into the air through the cooling tower, while in a HGSHP integrated with an auxiliary boiler, part of the heating load is supplied by the boiler. So the net heat extraction from the soil and the net heat rejection into the soil should be calculated by:
Rectifier
Generator
Rectifier
Condenser GAX Absorber
GAX Generator
Solution-cooled Absorber
Solution-heated Generator
Precooler
Absorber
Condenser
Precooler
Evaporator Externally-heated Generator
(a) Single-effect GSAHP
(6)
Externally-cooled Absorber
(b) GAX-cycle GSAHP
Fig. 2. Schematic of GSAHP with different cycles.
Evaporator
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334
200
1.60
180
1.55
160
COP
1.45
Capacity
100
1.40
80
1.35
COP
140 120
60
1.30
40
1.25
20
1.20
0 -15
-10 -5 0 5 10 15 20 Evaporator inlet fluid temperature
25
0.40
60
20 10 0
0.00 40 35 30 25 Condenser inlet fluid temperature
20
15
2.00
160
1.90
140 COP
120
1.70
100 Capacity
80
1.50
60
1.40
40
1.30
20
1.20
0
1.40
200
180
1.20
160
COP
1.00 COP
COP
30
0.10
Capacity (kW)
180
25
40
0.20
(b) Cooling mode for single-effect 200
-10 -5 0 5 10 15 20 Evaporator inlet fluid temperature
50
Capacity
0.30
45
2.10
-15
80 70
30
2.20
1.60
90
COP
0.50
(a) Heating mode for single-effect
1.80
100
0.60
Capacity (kW)
COP
1.50
0.70
Capacity (kW)
1.65
140 120
0.80
100 0.60
80 60
0.40
Capacity
40
0.20
20 0
0.00 45
30
(c) Heating mode for GAX-cycle
Capacity (kW)
326
40 35 30 25 Condenser inlet fluid temperature
20
15
(d) Cooling mode for GAX-cycle
Fig. 3. Performance of GSAHP in different modes.
cp mb;i tbout;i tbin;i
heating;s
þ
heating;e X
cp mb;i tbout;i tbin;i
(8)
1 cooling;e X
cp mb;i tbin;i tbout;i
(9)
cooling;s
RGHX ¼
3.00
180 150
Capacity
120 2.00
90 60
1.00
30 0.00
0 -5
0 5 10 15 Evaporator inlet fluid temperature
(a) Heating mode
20
25
Capacity (kW)
210
COP
200
5.00
240
4.00
(10)
180
270 5.00
QAHR;hybrid 100% QAHR
6.00
300
6.00
COP
QAHR; hybrid ¼
where cp is the specific heat of fluid, kJ/(kg C); mb,i is the fluid mass flow rate inside the ground heat exchanger, kg/s; tbin,i and tbout,i is the hourly borehole inlet and outlet fluid temperature, C. The ratio of ground heat exchanger rejection to total heat rejection, and the ratio of heat pump heating to total heating are defined as:
Capacity
160 140
4.00
120 COP
3.00
100 80
2.00
60 40
1.00
20 0.00
0 35
30 25 20 Condenser inlet fluid temperature
(b) Cooling mode Fig. 4. Performance of GSEHP in different modes.
15
Capacity (kW)
COP
QAHE; hybrid ¼
8760 X
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334
327
Table 2 The coefficients of GSAHP performance correlations.
GSEHP
GSAHP Single-effect
GSAHP GAX-cycle
P8760
heating;s
RHP ¼
Coefficient
Qh
COPh
Qc
COPc
a b c d a b c d a b c d
102.857 6.143 0 0 86.264 2.761 2.429E-3 3.267E-05 87.143 0.853 1.620E-3 1.211E-4
3.103 0.105 0 0 1.506 6.415E-3 1.863E-4 3.368E-06 1.725 1.380E-2 1.921E-05 9.075E-07
201.211 1.644 0 0 93.880 1.587 2.847E-3 2.810E-05 91.124 1.187 0.001.896E-3 1.560E-05
8.036 0.165 0 0 0.643 4.017E-3 1.348E-4 2.680E-06 1.319 4.634E-3 2.151E-4 1.507E-06
P qi qboiler;i þ heating;e qi qboiler;i 1 100% Pheating;e P8760 qi heating;s qi þ 1
The annual operation cost of GSEHP system is calculated by:
0
1 cooling;e heating;e X X q q q i i i A Cannual ¼@ þ þ COPh; i COPc;i COPh; i 1 cooling;s heating;s 0 1 cooling;e heating;e 8760 X X X $Pelectricity þ @ ei þ ei þ ei APelectricity
(11) where qboiler,i is the hourly heating load supplied by the auxiliary boiler, kW. To evaluate the energy performance of different GSHP systems, the heating seasonal primary energy efficiency (HPEE), cooling seasonal primary energy efficiency (CPEE), and annual primary energy efficiency (APEE) are defined:
P8760 HPEE¼
heating;s qi þ
!
P8760
qi ei COPh;i hdriving þ hpower
heating;s
þ
Pheating;e 1
Pheating;e 1
qi
heating;s
CPEE ¼
cooling;s
Pcooling;e cooling;s
qi
qi COPc;i hdriving
!
(13)
heating;s
þ h ei
power
þ
qi þ
Pcooling;e
Pcooling;e cooling;s
cooling;s
qi þ
qi COPc;i hdriving
Pheating;e 1
þ h ei
power
where ei is the hourly electricity consumption, including the pump and fan, kW; hdriving is the driving source efficiency of GSHP unit, which is hdriving ¼ hboiler for GSAHP (70% for coal boiler and 90% for gas boiler) and hdriving ¼ hpower for GSEHP; hpower is the efficiency of power generation, with a typical value of 33% in China. The annual operation cost of GSAHP system is calculated by:
0
cooling;e X qi qi þ COPh; i hboiler COPc;i hboiler cooling;s heating;s 1 heating;e X 3600Pfuel qi A$ þ COPh; i hboiler Hfuel 1 0 1 cooling;e heating;e 8760 X X X þ@ ei þ ei þ ei APelectricity
Cannual ¼@
8760 X
heating;s
Cannual;i
(17)
power
! qi COPh; i hdriving
20 X 1
heating;s
P8760
C20 years ¼ Cinitial þ
þ h ei
P8760 APEE ¼
1
where Hfuel is the lower heat value of fuel, which is 29,271.2 kJ/kg for coal and 38,930.7 kJ/Nm3 for gas [6]; Pfuel is the unit price of fuel, which is 0.8 CNY/kg for coal and 3.0 CNY/Nm3 for gas; Pelectricity is the unit price of electricity, which is 0.8 CNY/kWh. (Note: The current exchange rate is 1 CNY ¼ 0.1609 USD) The lifecycle total cost over 20 years of different GSHP systems are calculated by:
qi ei COPh;i hdriving þ hpower
!
cooling;s
(16)
(12) Pcooling;e
8760 X
cooling;s
! þ
qi
Pheating;e 1
! qi COPh; i hdriving
(14)
þ h ei
power
where Cinitial is the initial investment of the system, mainly including the GSHP unit (1.3CNY/W based on heating capacity for single-effect, and 1.5CNY/W based on heating capacity for GAXcycle), the GSEHP unit (1.0CNY/W based on heating capacity), the boiler (0.4CNY/W based on heating capacity), the ground heat exchanger (10000CNY/borehole), the cooling tower (300CNY/(m3/ h)), the pump (500CNY/(m3/h)) and the plate heat exchanger (400CNY/m2).
2.3. Modeling of different GSHP systems in TRNSYS
1
(15)
In the focused region where the heating load is dominant but the soil heat accumulation exists in a GSAHP system, Beijing and Shenyang are chosen for analysis. The hourly heating and cooling loads of a 5000 m2 hotel building in these two cities are simulated using the software DeST [33]. The hourly loads and ambient drybulb temperatures in these two cities are illustrated in Fig. 5.
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W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334 40
300
30
Beijing Shenyang
100 0 -100 -200
Hourly temperature ( C)
Hourly building load (kW)
200
20 10 0 Beijing Shenyang
-10 -20
-300
-30
-400 1/Jan 1/Feb 1/Mar 1/Apr 1/May 1/Jun 1/Jul 1/Aug 1/Sep 1/Oct 1/Nov 1/Dec Time
(a) load
1/Jan 1/Feb 1/Mar 1/Apr 1/May 1/Jun 1/Jul 1/Aug 1/Sep 1/Oct 1/Nov 1/Dec Time
(b) ambient temperature
Fig. 5. Load and ambient temperatures in Beijing and Shenyang.
The transient system simulation program TRNSYS, which has been widely used in the analysis of GSEHP systems with proven accuracy [25], is used to simulate the hourly operation performance of the HGSAHP systems, with the TRNSYS model demonstrated in Fig. 6. The building loads are imported from the calculated data in Fig. 5, and new TRNSYS components are created for GSAHP and GSEHP based on the correlated performance in Equations (1)e(4). Other components, including the pumps, cooling tower, ground heat exchanger, plate heat exchanger, controlling units and so on, are from the built-in types in TRNSYS libraries. Based on the load characteristic, climate parameters and the GSAHP performance, the HGSAHP are designed and configured in the TRNSYS model. The borehole spacing is 5 m, borehole depth is 100 m, pipe inside velocity is 0.5 m/s, pipe diameter is 26/32 mm, fill conductivity is 2.00 W/(m$K), pipe conductivity is 0.44 W/(m$K), and soil conductivity is 1.50 W/(m$K). To investigate the advantages of the proposed HGSAHP system, different GSHP systems in Table 3 will be comparatively analyzed.
All of these GSHP systems will be modeled and hourly simulated in TRNSYS. 3. Long-term thermal imbalance of different GSHP systems 3.1. Thermal imbalance ratio For a HGSHP system integrated with a cooling tower, the heat rejection in summer is not completely contributed by the ground heat exchanger, while for a HGSHP system integrated with an auxiliary boiler, the heat supply in winter may not be provided by the GSHP individually. The ratio of ground heat exchanger heat rejection to the total heat rejection (RGHX) and the ratio of heat pump heating to total heating (RHP) of different HGSHP systems are calculated in Table 4. It can be seen that the heat rejected through the ground heat exchanger in summer takes a percentage of 92.0%, 24.7% and 40.7% for HGSEHP, single-effect HGSAHP and GAX-cycle HGSAHP in Beijing, with the rest rejected through the cooling
Fig. 6. TRNSYS model of HGSAHP with cooling tower.
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334 Table 3 Characteristics of different absorption and electrical systems. Systems
Characteristics
GSAHP (single-effect, GAX-cycle) HGSAHP (single-effect, GAX-cycle) GSEHP
Summer: reject heat through GHX only Winter: extract heat from GHX Summer: reject heat through GHX and CT Winter: extract heat from GHX Summer: reject heat through GHX only Winter: extract heat from GHX Summer: reject heat through GHX and CT Winter: extract heat from GHX Summer: reject heat through GHX only Winter: extract heat from GHX and use boiler for peak load
HGSEHP (CT, coolingdominant) HGSEHP (boiler, heatingdominant)
tower. What is quite different in Shenyang is that the IR is very negative for GSEHP but very positive for GSAHP, so the HGSEHP should be integrated with an auxiliary boiler while HGSAHP is still integrated with a cooling tower. The heat rejected through the ground heat exchanger in summer occupies 45.3% and 59.8% in single-effect HGSAHP and GAX-cycle HGSAHP, while the heat pump mode of HGSEHP supplies 56.9% of the heating loads in winter, with the rest supplied by an auxiliary boiler. The imbalance ratio (IR) of different GSAHP and GSEHP systems over 20 years are presented in Table 5 and Table 6. In Beijing, though the IR of GSEHP is very small, it can also be modified as HGSEHP to reduce the required borehole number while keeping a small IR. The IR of single-effect GSAHP is as high as 78e79%, and is reduced to 19e13% by single-effect HGSAHP. As for the GAX-cycle, the IR of GAX-cycle GSAHP is 60e59%, and can be effectively reduced to 7e4% when using GAX-cycle HGSAHP. In Shenyang, the IR of GSEHP is as negative as 44 ~ 40%, while is reduced to a very small value around 3%. The IR of single-effect GSAHP reaches 60e59%, and is reduced to 15e12% by singleeffect HGSAHP. As for the GAX-cycle, the IR of GAX-cycle GSAHP is 25e24%, and can be changed to about 16% when using GAXcycle HGSAHP. In summary, the GSEHP and GAX-cycle GSAHP without auxiliary equipment can obtain small IR values in certain conditions, whereas the hybrid systems (including HGSEHP, singleeffect HGSAHP, and GAX-cycle HGSAHP) can always keep good thermal balance for all occasions.
3.2. Average soil temperature The thermal imbalance ratio has a great influence on the soil temperature, and the annual average soil temperature variation over a 20-year operation is demonstrated in Fig. 7 and Fig. 8. In Beijing, the soil temperature changes from 14.6 C to 14.4 C for GSEHP, to 14.2 C for HGSEHP, to 30.4 C for single-effect GSAHP, to 16.3 C for single-effect HGSAHP, to 24.8 C for GAX-cycle GSAHP and to 15.2 C for GAX-cycle HGSAHP, with variations of 0.2 C, 0.4 C, 15.8 C, 1.7 C, 10.2 C and 0.6 C, respectively. The stable soil temperatures of GSEHP and HGSEHP indicate that both of these two systems maintain good thermal balance in Beijing. The single-effect GSAHP will lead to a great soil temperature
Table 4 RGHX and RHP for different hybrid GSHP systems. Hybrid system
HGSEHP Single-effect HGSAHP GAX-cycle HGSAHP
Beijing
329
increase due to the severe heat accumulation, while the singleeffect HGSAHP becomes quite close to thermal balance. Compared with the single-effect GSAHP, the soil temperature rise of GAX-cycle GSAHP decreases but still very obviously, whereas the GAX-cycle HGSAHP achieves a more stable soil temperature. As for Shenyang, the soil temperature changes from 10.6 C to 2.3 C for GSEHP, to 9.6 C for HGSEHP, to 19.8 C for single-effect GSAHP, to 13.1 C for single-effect HGSAHP, to 13.6 C for GAXcycle GSAHP and to 9.8 C for GAX-cycle HGSAHP, with variations of 8.3 C, 1.0 C, 9.2 C, 2.5 C, 3.0 C and 0.8 C, respectively. During a long period of 10e20 years, a soil temperature variation within 3e4 C can be regarded as acceptable [25]. The single-effect GSAHP will lead to severe heat accumulation while the GSEHP will lead to severe cold accumulation. After being integrated with a cooling tower or an auxiliary boiler, all of the GSHP systems can maintain good thermal balance and stable soil temperature. 3.3. GHX outlet fluid temperature The soil temperature will affect the outlet fluid temperature of the ground heat exchanger, which finally determines the COP and capacity of GSHP units. The minimum outlet fluid temperature in winter and the maximum outlet fluid temperature in summer during a 20-year operation are illustrated in Fig. 9 and Fig. 10. In Beijing, the minimum outlet fluid temperature of single-effect GSAHP increases from 12.7 C to 27.7 C, and that of GAX-cycle GSAHP increases from 11.2 C to 21.1 C in winter. On the other hand, the corresponding maximum outlet fluid temperature in summer increases, respectively, from 34.6 C to 53.8 C and from 34.4 C to 43.8 C. Though the increase of outlet fluid temperature in winter is helpful in heating mode, the increase of outlet fluid temperature in summer will lead to bad performance and low reliability of the cooling supply. In Shenyang, the minimum outlet fluid temperature of GSEHP drops from 0.7 to 6.1, while that of single-effect GSAHP increases from 8.0 C to 16.8 C in winter. On the other hand, the corresponding maximum outlet fluid temperature in summer respectively drops from 26.8 C to 18.8 C, and increases from 29.8 C to 39.2 C. Though the decrease of outlet fluid temperature in summer is helpful in the cooling mode, the decrease of outlet fluid temperature in winter will cause bad performance and low reliability of heating supply. Therefore, it is favorable to keep a stable GHX outlet fluid temperature to maintain both high efficiency and high reliability in both heating and cooling modes during long-term operations. In terms of the thermal imbalance ratio, average soil temperature, and GHX outlet fluid temperature, it can be concluded that GSEHP, HGSEHP, single-effect HGSAHP, and GAX-cycle HGSAHP are suitable in regions like Beijing, whereas HGSEHP, single-effect HGSAHP, GAX-cycle GSAHP, and GAX-cycle HGSAHP are suitable in regions like Shenyang. 4. Comparisons between novel GSAHP and conventional GSEHP The above suitable GSHP systems with good thermal balance will be comparatively analyzed to investigate the primary energy efficiency, required borehole number, and technical economy. 4.1. Seasonal and annual primary energy efficiency
Shenyang
RGHX
RHP
RGHX
RHP
92.0% 24.7% 40.7%
100% 100% 100%
100% 45.3% 59.8%
56.9% 100% 100%
The average HPEE, CPEE, and APEE values of different GSHP systems over 20 years are shown in Fig. 11 and Fig. 12. In Beijing, the HPEE relationship is GAX-cycle HGSAHP > single-effect HGSAHP > GSEHP > HGSEHP, and the CPEE relationship is
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Table 5 IR of different GSHP systems over 20 years in Beijing. Year
GSEHP
HGSEHP
Single-effect GSAHP
Single-effect HGSAHP
GAX-cycle GSAHP
GAX-cycle HGSAHP
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
2.2% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3% 2.3%
4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.3% 4.4% 4.4% 4.4% 4.4% 4.4% 4.5%
78.4% 78.4% 78.4% 78.4% 78.4% 78.4% 78.4% 78.4% 78.4% 78.5% 78.5% 78.5% 78.6% 78.6% 78.6% 78.7% 78.7% 78.8% 78.8% 78.9%
18.7% 17.4% 16.4% 15.6% 15.1% 14.8% 14.5% 14.2% 14.0% 13.9% 13.7% 13.6% 13.5% 13.5% 13.4% 13.3% 13.3% 13.2% 13.2% 13.2%
59.7% 59.6% 59.6% 59.5% 59.5% 59.4% 59.4% 59.4% 59.4% 59.3% 59.3% 59.3% 59.3% 59.3% 59.3% 59.3% 59.3% 59.3% 59.3% 59.3%
7.0% 6.4% 5.8% 5.5% 5.3% 5.1% 4.9% 4.8% 4.7% 4.6% 4.5% 4.5% 4.4% 4.3% 4.3% 4.3% 4.2% 4.2% 4.2% 4.2%
Table 6 IR of different GSHP systems over 20 years in Shenyang. Year
GSEHP
HGSEHP
Single-effect GSAHP
Single-effect HGSAHP
GAX-cycle GSAHP
GAX-cycle HGSAHP
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
44.4% 43.8% 43.4% 43.0% 42.7% 42.3% 42.0% 41.7% 41.5% 41.3% 41.0% 40.8% 40.7% 40.5% 40.3% 40.2% 40.1% 40.0% 39.8% 39.7%
3.5% 3.4% 3.4% 3.4% 3.4% 3.4% 3.4% 3.3% 3.3% 3.3% 3.3% 3.3% 3.3% 3.3% 3.3% 3.3% 3.2% 3.2% 3.2% 3.2%
59.6% 59.5% 59.4% 59.4% 59.3% 59.3% 59.3% 59.2% 59.2% 59.2% 59.2% 59.2% 59.2% 59.1% 59.1% 59.1% 59.1% 59.1% 59.1% 59.1%
15.2% 14.6% 14.0% 13.9% 13.5% 13.3% 13.0% 13.0% 12.9% 12.7% 12.7% 12.6% 12.5% 12.5% 12.5% 12.5% 12.4% 12.3% 12.2% 12.1%
25.4% 25.3% 25.2% 25.1% 25.0% 24.9% 24.9% 24.8% 24.7% 24.7% 24.7% 24.6% 24.6% 24.6% 24.5% 24.5% 24.5% 24.4% 24.4% 24.4%
15.9% 15.7% 15.7% 15.8% 15.9% 15.8% 15.8% 15.8% 15.7% 15.8% 15.7% 15.8% 15.7% 15.8% 15.7% 15.9% 15.9% 16.0% 16.0% 16.0%
Single-effect GSAHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP GSEHP HGSEHP
Annual average soil temperature ( C)
30 28 26
Single-effect GSAHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP GSEHP HGSEHP
22 20 Annual average soil temperature ( C)
32
24 22 20 18 16 14 12
18 16 14 12 10 8 6 4 2
0
5
10
15
20
Year Fig. 7. Soil temperatures of different GSHP systems over 20 years in Beijing.
0
5
10
15
20
Year Fig. 8. Soil temperatures of different GSHP systems over 20 years in Shenyang.
32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2
56
Single-effect GSAHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP GSEHP HGSEHP
331
Single-effect GSAHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP GSEHP HGSEHP
54 52 50 GHX outlet temperature ( C)
GHX outlet temperature ( C)
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334
48 46 44 42 40 38 36 34 32 30
0
5
10
15
20
0
5
Year
10
15
20
15
20
Year
(a) heating mode
(b) cooling mode
24 22 20 18 16 14 12 10 8 6 4 2 0 -2 -4 -6 -8
44
Single-effect GSAHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP GSEHP HGSEHP
Single-effect GSAHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP GSEHP HGSEHP
42 40 38 GHX outlet temperature ( C)
GHX outlet temperature ( C)
Fig. 9. GHX outlet temperatures of different GSHP systems over 20 years in Beijing.
36 34 32 30 28 26 24 22 20 18
0
5
10
15
20
0
5
Year
10 Year
(a) heating mode
(b) cooling mode
Fig. 10. GHX outlet temperatures of different GSHP systems over 20 years in Shenyang.
winter; on the other hand, GAX-cycle HGSAHP needs the operation of the cooling tower and the cooling pump, which leads to higher electricity consumption and lower CPEE in summer. In any case, owing to greater advantages of GSAHP in heating mode and a
1.8 1.6
HPEE CPEE APEE
1.4 Primary energy efficiency
GSEHP > HGSEHP > GAX-cycle HGSAHP > single-effect HGSAHP, and finally, the APEE relationship is GSEHP > HGSEHP > GAX-cycle HGSAHP > single-effect HGSAHP. Though the APEE of HGSEHP is a little lower than that of GSEHP, HGSEHP requires less boreholes with the aid of a cooling tower, which will be presented later on. The GSAHP systems are advantageous in heating mode and inferior to GSEHP systems in cooling mode, and the APEE of GAX-cycle HGSAHP is very close to that of HGSEHP in regions like Beijing. As for Shenyang, with a much higher heating load and lower cooling load, the advantages of GSAHP systems in heating mode will be much more favorable to the annual performance. The HPEE relationship is GAX-cycle HGSAHP > GAX-cycle HGSAHP > singleeffect HGSAHP > HGSEHP, and the CPEE relationship is HGSEHP > GAX-cycle GSAHP > GAX-cycle HGSAHP > single-effect HGSAHP, and finally, the APEE relationship is GAX-cycle HGSAHP > GAX-cycle HGSAHP > HGSEHP > single-effect HGSAHP. The efficiency difference between GAX-cycle GSAHP and GAX-cycle HGSAHP is comprehensively caused by heat pump COP and water pump consumption. Compared with GAX-cycle GSAHP, GAX-cycle HGSAHP has lower heating COP and higher cooling COP due to lower soil temperature (Fig. 8). However, GAX-cycle HGSAHP requires fewer boreholes, and thus a smaller fluid flow rate, so the water pump consumption is smaller and the HPEE is higher in
1.2 1.0 0.8 0.6 0.4 0.2 0.0
GSEHP
HGSEH P
Single-e GAX-cy ffect H GSAHP cle HGSAHP
Fig. 11. Average primary energy efficiency of different GSHP systems in Beijing.
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W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334 1.8 1.6
100
HPEE CPEE APEE
80
1.4 1.2
Borehole number
Primary energy efficiency
96
1.0 0.8 0.6 0.4
60
54 43
40
34
20
0.2 0.0
HGSEH P
Fig. 12. Average primary energy efficiency of different GSHP systems in Shenyang.
bigger percentage of heating load in regions like Shenyang, the annual performance of GAX-cycle HGSAHP becomes the highest among all the GSHP systems, which is 17.5% higher than that of the conventional HGSEHP. 4.2. Required borehole numbers and occupied areas Although the HGSAHP is not absolutely the best selection in terms of APEE, its ability to reduce the required borehole number makes it much more preferable. The design borehole numbers of the aforementioned GSHP systems with good thermal balance are demonstrated in Fig. 13 and Fig. 14. Compared with the currently widely used HGSEHP, the single-effect HGSAHP can reduce the required borehole number by 52% in Beijing and 37% in Shenyang, while the GAX-cycle HGSAHP can reduce the required borehole number by 38% in Beijing and 20% in Shenyang. This is because HGSAHP has lower heating COP, it extracts less heat from the soil and it requires less boreholes for heat exchange. It should be noted that GAX-cycle GSAHP requires more boreholes, though it can keep the soil temperature variation within 3e4 C, so it is much better to use GAX-cycle HGSAHP integrated with an auxiliary cooling tower. Fewer boreholes means not only less borehole construction and investment, but also less occupied land, which is of great significance nowadays in the high-rise buildings in high-density cities. With a borehole spacing of 5 m, each borehole needs 25 m2 area; the total occupied areas required by different GSHP systems are calculated in Table 7. It is obvious that the occupied areas can be effectively reduced by single-effect HGSAHP and GAX-cycle HGSAHP.
100
Borehole number
80
74
60
0
G GAX-cy Single-e ffect H cle GSA AX-cycle HGS GSAHP AHP HP
56
40
HGSEH P
GAX-cy GAX-cy Single-e ffect H cle HGS GSAHP cle GSAHP AHP
Fig. 14. Required borehole number of different GSHP systems in Shenyang.
4.3. Economic analysis The initial investments and 20-year lifecycle costs of the balance-kept GSHP systems using different energy sources are calculated to further evaluate their applicability. The initial investments are presented in Table 8, which are estimated based on the selection of main equipment composing different GSHP systems. Compared with HGSEHP, HGSAHP reduces investment in boreholes and boiler, but increases investment in the GSHP unit, pump, cooling tower, and heat exchanger. Two factors lead to higher investment in an absorption-type GSHP unit: (1) higher unit price per heating capacity of GSAHP; (2) much lower ratio of cooling capacity to heating capacity (Fig. 3), which means that greatly redundant heating capacity has to be selected to guarantee the same cooling capacity. The combination of GSAHP and GSEHP can be further studied in the future to solve this problem. The 20-year lifecycle total costs of different GSHP systems are shown in Fig. 15 and Fig. 16. The cost in the 0th year stands for the initial investment of each system. In Beijing, the lifecycle cost is respectively 300.5 104CNY for GSEHP, 296.4 104CNY for HGSEHP, 300.7 104CNY for single-effect HGSAHP (coal), 469.0 104CNY for single-effect HGSAHP (gas), 236.5 104CNY for GAX-cycle HGSAHP (coal), and 345.8 104CNY for GAX-cycle HGSAHP (gas). As a result, GAX-cycle HGSAHP (coal) needs the lowest lifecycle cost among all the systems, while GSEHP, HGSEHP and single-effect HGSAHP (coal) are very close and rank at the second lowest level. The HGSAHP systems using gas as the energy source is relatively expensive due to the current high price of gas. In Shenyang, the lifecycle cost is respectively 311.7 104CNY for HGSEHP (coal), 374.1 104CNY for HGSEHP (gas), 279.9 104CNY for single-effect HGSAHP (coal), 454.7 104CNY for single-effect HGSAHP (gas), 295.3 104CNY for GAX-cycle GSAHP (coal), and 414.2 104CNY for GAX-cycle GSAHP (gas), 233.4 104CNY for GAX-cycle HGSAHP (coal), and 356.3 104CNY for GAX-cycle HGSAHP (gas). Consequently, GAX-cycle HGSAHP (coal) still requires the lowest lifecycle cost among all the systems. It is worth noting that lifecycle cost of GAX-cycle HGSAHP (gas) becomes
35 27
Table 7 Total occupied areas required by different GSHP systems.
20
0
GSEHP
HGSEH P
GAX-cy Single-e ffect H GSAHP cle HGSAHP
Fig. 13. Required borehole number of different GSHP systems in Beijing.
System
Beijing
Shenyang
GSEHP HGSEHP Single-effect HGSAHP GAX-cycle GSAHP GAX-cycle HGSAHP
1850 m2 1400 m2 675 m2 e 875 m2
e 1350 m2 850 m2 2400 m2 1075 m2
W. Wu et al. / Applied Thermal Engineering 90 (2015) 322e334
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Table 8 Initial investments of different GSHP systems. Beijing (unit: 104CNY) Item GSHP unit Boiler Borehole Pump Cooling tower PHX Total
GSEHP 29.3 0.0 74.0 4.0 0.0 0.0 107.3
HGSEHP 29.3 0.0 56.0 3.9 0.3 0.3 89.7
Single-effect HGSAHP 90.1 0.0 27.0 14.0 3.8 3.6 138.4
GAX-cycle HGSAHP 80.4 0.0 35.0 9.0 2.1 2.1 128.5
HGSEHP 29.8 7.2 54.0 7.0 0.0 0.0 98.0
Single-effect HGSAHP 63.6 0.0 34.0 11.7 3.0 2.9 115.1
GAX-cycle GSAHP 64.0 0.0 96.0 7.0 0.0 0.0 167.0
GAX-cycle HGSAHP 64.0 0.0 43.0 5.3 1.4 1.4 115.0
Shenyang (unit: 104CNY) Item GSHP unit Boiler Borehole Pump Cooling tower PHX Total
Note: The current exchange rate is 1 CNY ¼ 0.1609 USD.
500
4
Total cost over 20 years ( 10 CNY)
450 400 350
system efficiency, reduce the required borehole number, and lessen the occupied area. Hourly simulations of the single-effect HGSAHP and GAX-cycle HGSAHP have been carried out and compared with other GSHP systems to investigate the advantages and disadvantages. The following conclusions can be made:
GSEHP HGSEHP Single-effect HGSAHP(coal) Single-effect HGSAHP(gas) GAX-cycle HGSAHP(coal) GAX-cycle HGSAHP(gas)
300 250 200 150 100 50 0
5
10
15
20
Year Fig. 15. Lifecycle cost of different GSHP systems in Beijing.
lower than that of conventional HGSEHP (gas) assisted by an auxiliary boiler. 5. Conclusions HGSAHP combining GSAHP with a cooling tower is proposed to obtain better thermal balance, keep stable soil temperature and 500
Total cost over 20 years ( 10 CNY)
450 400 350 300
References
250 200 150 100 50 5
Acknowledgements The authors gratefully acknowledge the support from the Natural Science Foundation for Distinguished Young Scholars of China (grant No. 51125030) and the National 12th Five-year Science and Technology Support Project of China (grant No. 2011BAJ03B09).
HGSEHP(coal) HGSEHP(gas) Single-effect HGSAHP(coal) Single-effect HGSAHP(gas) GAX-cycle GSAHP(coal) GAX-cycle GSAHP(gas) GAX-cycle HGSAHP(coal) GAX-cycle HGSAHP(gas)
0
(1) The thermal balance can be well kept by HGSAHP, with IR reduced from 60e80% to within 20%, and soil temperature variation staying within 3 C after 20 years' operation; (2) The HGSAHP are advantageous in heating mode and inferior to HGSEHP in cooling mode. The annual performance of GAX-cycle HGSAHP is very close to that of HGSEHP in Beijing, while being the highest, and 17.5% higher than that of HGSEHP in colder regions like Shenyang; (3) Compared with HGSEHP, single-effect HGSAHP can reduce the required borehole number by 37e52%, while the GAXcycle HGSAHP can reduce the required borehole number by 20e38%. The land areas occupied by the boreholes can also be effectively reduced; (4) The lifecycle cost of GAX-cycle HGSAHP (coal) is the lowest in both Beijing and Shenyang, while GAX-cycle HGSAHP (gas) becomes cheaper than conventional HGSEHP (gas) assisted by an auxiliary boiler.
10
15
20
Year Fig. 16. Lifecycle cost of different GSHP systems in Shenyang.
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