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Energy-saving analysis of a hybrid power-driven heat pump system
Accepted Manuscript Research Paper Energy-saving analysis of a hybrid power-driven heat pump system Sheng Shang, Xianting Li, Wei Wu, Baolong Wang, Wenxing Shi PII: DOI: Reference:
S1359-4311(17)30420-9 http://dx.doi.org/10.1016/j.applthermaleng.2017.04.151 ATE 10300
To appear in:
Applied Thermal Engineering
Received Date: Accepted Date:
20 January 2017 29 April 2017
Please cite this article as: S. Shang, X. Li, W. Wu, B. Wang, W. Shi, Energy-saving analysis of a hybrid powerdriven heat pump system, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/j.applthermaleng. 2017.04.151
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Energy-saving analysis of a hybrid power-driven heat pump system Sheng Shang, Xianting Li*, Wei Wu, Baolong Wang Wenxing Shi Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, China
* Corresponding author: Prof. Xianting Li Department of Building Science, School of Architecture Tsinghua University Beijing 100084, P.R. China Tel: +86-10-62785860 Fax: +86-10-62773461 E-mail: [email protected]
-1-
Nomenclature
cp
specific heat of air, kJ/(kg·K)
cps
specific heat of the coolant medium, kJ/(kg·K)
E
primary gas consumption, Nm3
F
view factor
h
specific enthalpy, kJ/kg
L
building load, kW
LCV
low calorific value, kJ/Nm3
m
mass flow rate, kg/s
n
variety index
PER
primary energy ratio
Q
heat transfer rate, kW
Qe
primary energy consumption, kW·h
Tr
torque of the engine, N·m
T
temperature, °C
UA
heat transfer capacity, kW/K
W
work consumption of compressor, kW
x
distance between the condenser and the waste HX, m
Abbreviations COP
coefficient of performance
DISP
displacement volume, m3/min
ESR
energy saving ratio
PR
pressure ratio of the compressor
RPM
rotation speed per minute, r/min
Subscripts a
air
c
condenser
e
evaporator
EC
EHP for cooling
EH
EHP for heating
2
fg
flue gas
gas
natural gas
GC
GEHP system for cooling
GH
GEHP system for heating
HC
HPHP system for cooling
HH
HPHP system for heating
i
hourly system performance
j
cylinder jacket
ise
isentropic
output
output work
r
refrigerant
rad
radiation
rc
compressor inlet and outlet refrigerant
re
heat rejection medium
vol
volumetric
waste
heat rejection
Greeks
ΔTm
logical mean temperature difference, °C
ε
efficiency
ƞ
power generation efficiency of the gas-fired power plant, %
ρ
density, kg/m3
Abstract
A gas engine-driven heat pump (GEHP) is traditionally applied for both heating and cooling; however, the cooling performance may be worse than that of an electric heat pump (EHP) because of the waste heat rejection of the gas engine. To improve the system performance of the air source heat pump for the entire year running, a hybrid power-driven heat pump (HPHP) system is proposed, in which the heat pump is driven by a gas engine for heating and by electricity for cooling. The mathematical 3
model of the HPHP system is established, and the performance of the HPHP system is compared with those of GEHP and EHP. It is demonstrated that the primary energy ratio (PER) of the HPHP is 28.5%~51.2% higher than that of the GEHP for cooling and 15.8%~25.3% higher than that of the EHP for heating. Compared to the GEHP and the EHP, the energy saving ratio of the HPHP is 10.9% and 14.4% in Beijing, respectively, and the corresponding value is 18.5% and 7.3% in Shanghai, respectively. These results highlight the great application potential of the HPHP system for both heating and cooling.
Keywords
Gas engine-driven heat pump; Electric heat pump; Hybrid power; Building energy efficiency; Space heating.
1 Introduction
Building energy consumption in China has increased rapidly in recent years, accounting for over 0.8 billion tce (tons of coal equivalent) in 2012 [1]. The air source heat pump (ASHP) was proposed with the development of low-temperature heating and the research on the thermal comfort of novel terminals[2,3] to enhance the primary energy efficiency and to reduce the pollutant emission. The ASHP can be divided into the electric heat pump (EHP) and the gas engine-driven heat pump (GEHP) according to the driving power. As for the heating performance, the GEHP system will be higher than the EHP 4
system. The reason is that the heat recovery of the gas engine makes the heating performance of GEHP better than the EHP system even the power generation efficiency of the gas-fired power plant is higher. The current studies on the GEHP are mainly focused on the heating performance improvement and the control strategy[4]. Yang et al. investigated the heating performance and the control strategy using both simulation and experiments; the results showed the primary energy ratio (PER) can reach 1.7 in certain working conditions[5,6], and the partial load performance is even better[7]. Elgendy et al.
conducted an experimental study of the GEHP’s heating
performance; the results showed that the heating PER is 1.4 ~ 1.8 for different working conditions[8]. Lazzarin et al. [9] and Sanaye et al.[10] conducted the economic analysis on the GEHP system based on a case study; the results showed that the running cost was much lower than the gas-fired boiler. Brenn et al. compared the heating performance of the GEHP with that of the EHP[11]; the results showed the annual primary energy efficiency of the GEHP is approximately 27.9% higher than that of the EHP. It can be seen that the heating performance of the GEHP is much better than that of the EHP from the abovementioned research. However, the cooling performance of the GEHP may be worse than that of the EHP. The reasons are that the output work efficiency of the gas engine is lower than the power generation efficiency of the gas-fired power plant, besides, the condensation temperature of the GEHP system will be affected by the heat rejection of the gas engine. To ensure the GEHP operates well in summer, the waste heat of the gas engine should be rejected. There are mainly two methods to solve this problem:
5
the first one is to use the waste heat for generating domestic hot water[6,12], and the other method is using an additional heat exchanger which installed outside the condenser, where the additional heat exchanger shares the same fan with the condenser [13,14].The first method may not be a proper solution in a building in which the load of domestic hot water does not match the cooling load, and the engine efficiency is lower when the waste heat of gas engine is not well dispersed. Elgendy et al. also investigated the cooling performance of the GEHP and found that the cooling PER is 1.35 when the evaporator inlet temperature is 13°C [15]; however, the cooling PER includes the heat recovery of the engine. The cooling performance may be lower than 1.0 if the waste heat recovery from the gas engine is ignored. Regarding the second method, the coolant of the engine passes through the heat exchanger outside the condenser, which may result in the increase of condensation temperature due to the radiation from the heat exchanger to the condenser, so that the power consumption of the fans and the compressor may be higher; thus, the COP and the cooling PER will be affected. Liu et al. investigated the GEHP’s cooling performance with the evaporative condenser[16]; the results showed that both the COP and the cooling capacity are increasing compared to the traditional GEHP system due to the well heat rejection of the gas engine. In contrast, the EHP may have a higher cooling PER than that of the GEHP. Therefore, by combining the heating performance of the GEHP with the cooling performance of the EHP in a hybrid system, higher energy saving potential for operation over the entire year may be achieved. Referring to hybrid electric vehicles, some researchers proposed a hybrid gas
6
engine-driven heat pump system[17], for which batteries for electricity storage or driving source are used when the heating or cooling capacity is greater than the building demand. Besides, the heat pump can also be driven by the gas engine and electricity in parallel[18,19]. That’s a solution to maintain the gas engine at a higher efficiency in summer, but the efficiency of heat pump is still low in summer because of the lower gas engine efficiency compared with gas-fired power plant. In this work, to achieve higher energy saving potential for operation over the entire year by utilizing the advantages of GEHP and EHP system, a novel hybrid power-driven heat pump (HPHP) system is proposed, and the mathematical model of the system is established to analyze the heating and cooling performance. Based on the system performance, the performance comparison between the EHP and the GEHP and the energy saving ratio for the entire year running in Beijing and Shanghai are investigated.
2 Working Principles and Operation Mode of the HPHP
The configuration of the hybrid power-driven heat pump is shown in Fig. 1; the HPHP is mainly comprised of three parts. The first part is the refrigeration cycle, which is similar to that of the traditional electric heat pump. The second part is the driving source, which has an electromotor and a gas engine; both of them have a clutch to connect to the gear or belt pulley that drives the open-type compressor. The last part is the heat recovery system, which can recover the waste heat of gas engine; a three-way valve on the water supply pipe can control the different operation modes. 7
Compared to the traditional GEHP system, there is no extra waste heat exchanger for the heat rejection of the engine.
Fig. 1 Structure of the hybrid power-driven heat pump. For the cooling mode, the electromotor is used to drive the open-type compressor to generate chilled water; in addition, the gas engine does not operate, and the clutch of the gas engine is disconnected. The return water does not pass through the heat recovery system and only passes through the evaporator, which is user’s heat exchanger in Fig.1. The working principle is similar to that of the electric heat pump. Therefore, no heat rejection process of the gas engine occurs in summer, and the condensation temperature will not be impacted. For the heating mode, the driving source switches to the gas engine, and the electromotor is turned off. In this case, the four-way valve of the refrigerant cycle switches to heating mode. In the same time, the three-way valve switches to another direction to allow the return water to pass
8
through the condenser, engine cylinder and exhaust gas heat exchanger successively. In this process, the natural gas is fed into the gas engine and is transformed into flue gas via the combustion process. The flue gas passes through the waste heat exchanger and then is discharged into the environment. The HPHP system can avoid the disadvantage of the GEHP and the EHP because of its configuration and operation modes.
3 Methodology
To investigate the system performance of the HPHP and compare the system performances among the EHP, GEHP and HPHP, a mathematical model is established. For the heat exchangers, the same evaporator and the condenser are designed for the EHP and the GEHP. For the compressor, the open-type compressor is chosen for the GEHP and the HPHP system, and the hermetic compressor is chosen for the EHP system. The supply water temperature in winter is 45°C, and the chilled water temperature in summer is 7°C. Next, the building load information and the calculating method of energy consumption are introduced to evaluate the energy saving potential. The comparison is based on the primary energy consumption, and the electricity is converted to the equivalent natural gas consumption in the gas-fired power station.
9
3.1 Mathematical model 3.1.1 GEHP model (1) Gas engine
The output work, gas consumption and flue gas temperature of the engine are the items of concern in this study; therefore, some product catalogues are referenced to calculate the output work, the cylinder jacket heat generation and the temperature of the waste gas. The rated output work of the chosen engine is 15kW (the catalogue of the gas engine is TM-18.03.2009 in MiracleGen). The gas consumption and the output work are affected by the engine rotation speed and load ratio for a given product catalogue. The proportion of cylinder jacket heat generation to the gas consumption is given by a model from TRNSYS (shown in Fig. 2). The temperature of flue gas is obtained using the fitting formula from Zhang et al.[20]. The equation is shown as Eq. 1, and the parameters are shown in Table 1. The mass flow rate of the flue gas is introduced in the thermal properties of the flue gas. Tfg c1 c2 RPM c3 RPM 2 c4 Tr c5 Tr 2 c6 RPM *Tr c7 RPM Tr 2 c8 RPM 2 Tr c9 RPM 2 Tr
2
(1)
Table 1 Parameters of flue gas temperature formula in Eq. 1. c1 c2 c3 c4 c5 -8 276.8 0.004527 -1.2010 0.3176 -1.1110-3 c6 c7 c8 c9 -5 -5 -9 -7.0710 2.36710 4.63910 -1.510-12
10
Fig. 2 Output work efficiency and waste heat of the cylinder of the engine model[21]. (2) Refrigerant cycle
The open-type compressor is chosen for the GEHP system, for which the refrigerant mass flow rate and work consumption should be calculated. To perform these calculations, the efficiency model is chosen, and the equations of volumetric efficiency and isentropic efficiency are given as follows[22]:
vol 1 0.04 PR
(2)
ise 0.9 0.0467 PR
(3)
The theoretical displacement of the compressor is 1.79×10(-4)cc/rev, and the compressor is referred to the 4G.2Y-K in BITZER’s product catalogue. And the mass flow rate of refrigerant is calculated using the following equation:
mr vol DISP RPM / 60
(4)
The work consumption is calculated according to the enthalpy difference between the inlet and the outlet of the compressor as well as the isentropic efficiency: W mr hrc ,out hrc ,in / ise
(5)
Next, the gas consumption is calculated according to the work consumption and the output work efficiency of the gas engine: 11
Qgas W / output
(6)
For the heat exchanger, the outdoor heat exchanger is air-cooled, and the user’s heat exchanger is the plate heat exchanger. In this work, the lump parameter is chosen to establish the refrigerant cycle so that the heat exchange capacity UA is given according to the heat transfer demands. The detail information of the heat exchanger is shown in Table 2. The heat transfer and energy balance equations are (7) to (9). Q ma (ha ,in ha ,out )
(7)
Q mr (hr ,in hr ,out )
(8)
Q UA Tm
(9)
Table 2 Detailed information of the heat exchangers in the mathematical model. Heating mode Heat exchanger
Cooling mode UA (kW/K)
LMTD (°C)
5.14
8
4.9
8.6
5.5
6.4
7.9
0.18
/
33
/
/
0.06
2.5
115
/
/
UA
Heat exchange
LMTD
(kW/K)
area (m2)
(°C)
Evaporator
6.4
160
Condenser
8
Cylinder jacket Waste heat exchanger
For the expansion valve, the expansion process is isentropic, and the superheat and subcooling degrees are set to 5°C.
(3) Thermal property of the flue gas
According to the gas flow rate and the excess air coefficient (which is taken as 1.2[23]), the specific heat, dew-point temperature and mass flow of the exhausted flue gas can be calculated, and the condensing heat of vaporization in the waste gas is considered. When the temperature of the exhausted flue gas is lower than its
12
dew-point, the vapor will condense and release the latent heat. The dependences of the flue gas temperature on the humidity ratio and on the enthalpy is shown in Fig. 3.
Fig. 3 Relationships between flue gas temperature and humidity ratio and enthalpy. (4) Radiation model in summer
In summer, an extra heat exchanger outside the condenser is required to reject the engine waste heat. The radiation from the waste heat exchanger to the condenser is considered (as shown in Fig. 4), and each of the heat exchangers is considered as a square plate. When the heat transfer resistance of the heat exchanger surface is ignored, the radiation heat transfer between the waste heat exchanger and condenser can be calculated by Qrad
A F z (Tre 4 Tcond 4 ) 1000
(10)
The temperature of the condenser is the condensation temperature. For the waste heat exchanger, the temperature is taken as the average of the inlet and outlet temperatures of the waste heat exchanger: Tre
Tre,in Tre,out 2
13
(11)
The length of the side is 1.8 m, and the distance between the two exchangers is 0.05 m; thus, the view factor F can be calculated as[24]: 1 2 (1 x 2 )2 12 2 2 F 2 (ln( ) 2 x(1 x ) tan 1 2 x 1 2x
x 1 2 2
) 2 x tan 1 x
(12)
(1 x )
Fig. 4 Configuration of the waste heat exchanger and the heat transfer process. The radiation heat transfer from the waste heat exchanger to the condenser will cause the condensation temperature to rise. The air first passes through the condenser to remove the heat generation, and then removes the engine waste heat. Using the heat transfer equations and the thermal balance equations, the impact of the radiation heat can be evaluated. The PER of the GEHP in winter is defined as follows: PERGH
Qc Q j Q Qgas
f g
(13)
In summer, the PER is defined as follows: PERGC
Qe Qgas
(14)
3.1.2 EHP model The heat exchanger is the same as the GEHP model, whereas the compressor’s 14
efficiency model is different. The volumetric efficiency and isentropic efficiency are calculated using the following equations[25]: 1
vol 0.94 0.085( PR n 1)
ise 0.9 0.0467 PR
(15) (16)
The volumetric efficiency of the compressor εvol is used to calculate the refrigerant mass flow rate, and the work consumption is calculated according to the isentropic efficiency. The equations are the same as Eq. 4 and Eq. 5. The detailed information of heat exchangers is shown in Table 2, and the evaporator and condenser for the GEHP and the EHP are the same. The system performance of the EHP is converted into the equivalent primary energy ratio using the COP and the power generation efficiency of the gas-fired power plant ƞ (Eq. 17). The average gas-fired power generation efficiency is taken as 0.45[26]. The heating and cooling PER are shown as Eq. 17 and 18. PEREH COPEH
(17)
PEREC COPEC
(18)
3.1.3 HPHP model
The HPHP system will operate in the GEHP mode for heating; thus, the GEHP model is used for heating, Eqs. 1~9 are used, and the supply water will pass successively through the condenser, cylinder jacket and waste heat exchanger. When the HPHP system is operated in cooling mode, the mathematical model is similar to that of the EHP model with the same heat exchangers, except the compressor 15
efficiency model is different. The compressor model is based on Eq. 2 and Eq. 3, which is different from the compressor model of the EHP model. The heating performance PERHH is the same as Eq. 13. Regarding the cooling performance PERHC, the equation is as follows: PERHC COPHC
(19)
3.1.4 Model validation The EHP model has been already validated in Yan’s book [25]. Thus, in this paper, the heating performance of the GEHP model is validated according to some experimental data[8]. Elgendy built an air-to-water GEHP platform and tested different inlet temperatures of the condenser under the condition that the ambient temperature is 11.9°C and the engine speed is 1300 rpm. The PER validation with Elgendy’s experimental data is shown in Table 3. The maximum relative error is found to be within 5%, which shows the model of GEHP is accurate enough. Table 3 PER Validation of the GEHP using Elgendy’s experimental data. Condenser inlet temperature (°C)
PER Relative error Experiment
16
Simulation
32
1.85
1.95
5%
33
1.83
1.9
4%
34
1.81
1.86
3%
35
1.8
1.82
1%
36
1.76
1.78
1%
37
1.73
1.74
1%
38
1.7
1.71
1%
39
1.69
1.68
-1%
40
1.67
1.65
-1%
41
1.6
1.62
1%
3.2 Building load and energy consumption 3.2.1 Building load
Beijing and Shanghai are chosen to investigate the system performance and the energy saving ratio. An office building model is established using DeST[26] in Beijing and Shanghai, and the building is 7 floors with the total area of 1050m2. The building load information is shown in Fig. 5. The red part denotes the heating load, and the blue part represents the cooling load. The peak heating load in Beijing is 80 kW, and the cooling load is 75 kW. In addition, the peak heating and cooling load information for Shanghai is 60 kW and 80 kW, respectively. Information of the building envelope and the peak building load is shown in Table 4. Table 4 Building load and envelope information in Beijing and Shanghai. Location
wall 2
windows
Heating season
Cooling season
2.2
1st Nov ~ 31st Mar
6st May ~ 24th Sep
2.5
27th Nov ~ 22nd Mar
1st May ~ 15th Oct
(W/(m ·K))
(W/(m2·K))
Beijing
0.3
Shanghai
0.5
17
Beijing
Shanghai
Fig. 5 Building load information of Beijing and Shanghai. 3.2.2 Calculating method of energy consumption
To compare the system performance and energy saving ratio, three systems, i.e., EHP, GEHP and HPHP, are separately operated in the same building both in Beijing and Shanghai. The hourly performance for each system can be obtained using the hourly ambient temperature. Thus, the energy consumption of each system can be calculated using the hourly building load. The hourly and year-round energy consumption Qei and Qe are calculated using the following equations: Qei
Li PERi
8760
Li
i 1
PERi
Qe
(20)
(21)
The HPHP will run in GEHP mode in winter and in EHP mode in summer. To calculate the energy saving ratio of the HPHP, the energy consumption of the GEHP will be chosen as the comparison baseline in summer, and the equivalent gas consumption of EHP is the baseline for winter. The gas consumption is calculated using Eq. 22:
18
E
Qe 3600 LCV
(22)
After calculating the hourly energy consumption and integrating the values for the whole year, the energy saving ratio of the HPHP can be calculated by ESR
E EHPHP E
(23)
4 Results and Discussion
4.1 System performance comparison
For the heating performance, the parameters of the refrigerant cycle of GEHP and EHP when the ambient temperature is 0°C are shown in Table 5. The supply water temperature is set to 45°C for both GEHP and EHP system. For the GEHP system, the return water is heated from 39.3°C to 43.5°C in the condenser, then to 45°C by the gas engine heat recovery process. As for the EHP system, the return water’s temperature is heated from 40.7°C to 45°C only by the condenser. So the condensation temperature of the GEHP system is 1.6°C lower than that of EHP due to the heat recovery of the gas engine. Therefore, the COP of the GEHP system is larger than that of EHP system. Besides, the heating capacity of GEHP system is 27.2% higher than that of EHP system due to the heat recovery of the gas engine. Table 5 The parameters of the refrigerant cycle of GEHP and EHP of heating mode on typical working condition condenser work evaporation condensation heating inlet water consumption of temperature temperature capacity temperature the compressor (°C) (°C) (kW) (°C) (kW)
19
GEHP
-7.6
39.3
46.1
47.49
11.70
EHP
-7.4
40.7
47.7
36.31
11.58
A heating performance comparison of the different systems is shown in Fig. 6. The heating PER of the GHEP and the EHP is higher when the ambient temperature is increasing, and the GEHP has the highest heating PER, which is approximately 0.15 higher than that of the EHP because of the heat recovery of the engine. The efficiency of the gas-fired boiler and the coal-fired boiler is given as 0.9 and 0.7[28], respectively. The comparison is based on the condition in which the ambient temperature is greater than -18°C because the locations where the ASHP can be put into application have temperatures mainly in this temperature range. Fig. 7 shows the cooling performance comparison among GEHP, EHP and HPHP. From Fig. 7, the EHP’s PER of cooling is much higher than that of the GEHP if we take the gas-fired power generation efficiency as 45% [27], and the PER is lower when the ambient temperature is higher because the condensation temperature is higher. In addition, the condensation temperature of the GEHP is higher than that of the EHP because of the waste heat rejection of the gas engine; thus, the PER of the GEHP is the lowest. Compared to the HPHP, the cooling PER of the EHP system is higher than that of the HPHP because of the different types of compressors used. The volumetric efficiency of the hermetic compressor is higher than that of the open-type compressor; in addition, the transmission loss is less than that of the HPHP. The PER of both the EHP and the HPHP is found to be much greater than that of the GEHP, and the difference between the PER of the EHP and the HPHP is small.
20
Fig. 6 Heating performance comparison among the different systems.
Fig. 7 Cooling performance comparison among the GEHP, EHP and HPHP. The condensation temperature of the GEHP and the EHP under the same condition is shown in Fig. 7. The GEHP’s condensation temperature is found to be approximately 0.5°C higher than EHP because of the radiation heat transfer of the engine waste heat; therefore, the COP will be slightly lower than that of the EHP. When the ambient temperature is 35°C, the average temperature of the heat rejection medium is 65.5°C, and the condensation temperature is approximately 47.8°C. In addition, the radiation heat transfer rate from the waste heat exchanger to the condenser is 0.65kW, and the heat transfer rate of the condenser is 50.49kW.
21
Compared to the GEHP system, the condensation temperature of EHP system is 0.5°C lower, and the cooling performance is even better with higher power generation efficiency. The detailed information is shown in Table 6. Table 6 Heat transfer process of the GEHP for a typical working condition in summer.
GEHP EHP
Inlet and outlet
Inlet and outlet
temperature of
temperature of
Tc
Qrad
Qc
waste HX
air
(°C)
(kW)
(kW)
(°C)
(°C)
71.4~59.5
35~44.3
47.8
0.65
50.49
35~42.7
47.3
Heat rejection (kW) 12.0
50.51
4.2 Energy consumption analysis in summer
The natural gas consumption in summer is shown in Fig. 8. From Fig. 8, the monthly gas consumption of GEHP is the highest, both in Beijing and in Shanghai, mainly because the cooling PER of EHP and HPHP is much higher than that of the GEHP. The GEHP’s gas consumption for July is 29.3% and 29.6% higher than that of the HPHP in Beijing and Shanghai, respectively. And the corresponding value compared to the EHP system is 29.6% and 32.1% respectively. The reason for this result is that the average temperature of Beijing in summer is lower than that of Shanghai; thus, the average cooling PER in Beijing is higher than that in Shanghai (shown in Fig. 9). However, the gas consumption in Shanghai is larger than that in Beijing because the cooling load of the building in Shanghai is larger.
22
Beijing
Shanghai
Fig. 8 Monthly gas consumption of the GEHP and the EHP in Beijing and Shanghai in summer.
Beijing
Shanghai
Fig. 9 Hourly cooling PER of the GEHP and the EHP in Beijing and Shanghai in summer. 4.3 Energy consumption analysis in winter
The gas consumption of each month in winter is shown in Fig. 10. The gas consumption of GEHP and HPHP is found to be the same because the HPHP system operates in the GEHP mode in winter and the components are the same. The monthly gas consumption of GEHP/HPHP is lower than the EHP, both in Beijing and in Shanghai, mainly because the heating PER of the GEHP/HPHP is considerably higher than that of the EHP because of the heat recovery of the engine. In addition, the GEHP/HPHP’s gas consumption of January is 25% and 23% lower than that of the
23
EHP in Beijing and Shanghai, respectively. The results match with that of Brenn et al.[11], in which the energy saving ratio of GEHP is about 27.9% compared to EHP. The hourly heating PER is shown in Fig. 11, and the heating PER in Shanghai is larger because the average temperature is higher.
Beijing
Shanghai
Fig. 10 Monthly gas consumption of the GEHP and the EHP in Beijing and Shanghai in winter.
Beijing
Shanghai
Fig. 11 Hourly heating PER of the GEHP and the EHP in Beijing and Shanghai in winter. 4.4 Year-round energy saving potential
The year-round gas consumptions of Beijing and Shanghai are shown in Fig. 12. The primary energy consumption of the HPHP is the lowest, both in Beijing and in Shanghai because the HPHP runs almost exclusively in the most energy-saving mode,
24
both in winter and in summer. Although the cooling performance of the HPHP system in summer is worse than that of the EHP, the heating PER of the HPHP is much higher. In Beijing, the energy saving ratio of the HPHP is 10.9% and 14.4% relative to the GEHP and the EHP, respectively, and the corresponding results in Shanghai are 18.5 and 7.3% compared to GHEP and EHP, respectively. The energy consumption of the EHP is 5.1% higher than that of the GEHP in Beijing because the heating load is much higher than the cooling load and the GEHP can save more energy than the EHP. However, in Shanghai, the result is opposite: the GEHP consumes 14.4% more energy than the EHP. The building load feature will affect the energy saving ratio of different systems, and the GEHP is more suitable for the locations where the heating load is slightly higher. The HPHP system will consume less energy than the GEHP and the EHP during the year-round operation; thus, the HPHP system can be used to replace the EHP and the GEHP.
Fig. 12 Energy consumptions of different equipment in Beijing and in Shanghai.
25
5 Conclusions
To improve the system performance of the air source heat pump for the operation over the entire year, a hybrid power-driven heat pump was proposed to make full use of the advantages of the heating performance of the GEHP and the cooling performance of the EHP. The mathematical model was established to calculate the system cooling and heating performance. Next, the energy saving potential in Beijing and Shanghai for heating season, cooling season and the whole year running was investigated. The main conclusions from the above-described analysis are as follows: 1. When the ambient temperature is 0°C, the heating PER of the HPHP can reach up to 1.6, and the cooling PER is 1.55 when the temperature is 35°C. 2. The HPHP has 28.5%~51.2% higher cooling PER than the GEHP and 15.8%~25.3% higher heating PER than the EHP, and the condensation temperature of the HPHP in summer is about 0.5°C lower than that of the GEHP; 3. For the year-round operation, the energy saving ratio of the HPHP is 10.9% and 14.4% compared to the GEHP and the EHP in Beijing, respectively; the corresponding result in Shanghai is 18.5 and 7.3%, respectively.
Acknowledgments
The authors gratefully acknowledge the support of the Innovative Research Groups of the National Natural Science Foundation of China (grant number 51521005).
26
References [1] Tsinghua University Building Energy Saving Research Center, Annual Report on China Building Energy Efficiency, China Architecture and Building Press, Beijing, 2014 (in Chinese) [2] Xia Y, Zhang X S. Experimental research on a double-layer radiant floor system with phase change material under heating mode[J]. Applied Thermal Engineering, 2015, 96:600-606. [3] Zhao M, Zhu T, Wang C, et al. Numerical simulation on the thermal performance of hydraulic floor heating system with phase change materials[J]. Applied Thermal Engineering, 2016, 93:900-907. [4] Hepbasli A, Erbay Z, Icier F, et al. A review of gas engine driven heat pumps (GEHPs) for residential and industrial applications[J]. Renewable & Sustainable Energy Reviews, 2009, 13(1):85-99. [5] Wang M, Yang Z, Su X, et al. Simulation and experimental research of engine rotary speed for gas engine heat pump based on expert control[J]. Energy & Buildings, 2013, 64(5):95–102. [6] Yang Z, Cheng H, Wu X, et al. Research on improving energy efficiency and the annual distributing structure in electricity and gas consumption by extending use of GEHP[J]. Energy Policy, 2011, 39(9):5192-5202As the access to this document is restricted, you may want to look for a different version under "Related research" (further below) orfor a different version of it. [7] Zhang W, Wang T, Zheng S, et al. Experimental Study of the Gas Engine Driven Heat Pump with Engine Heat Recovery[J]. Mathematical Problems in Engineering, 2015, 2015:1-10. [8] Elgendy E, Schmidt J. Optimum utilization of recovered heat of a gas engine heat pump used for water heating at low air temperature[J]. Energy & Buildings, 2014, 80:375–383. [9 Lazzarin R, Noro M. District heating and gas engine heat pump: Economic analysis based on a case study[J]. Applied Thermal Engineering, 2006, 26(2-3):193-199. [10] Sanaye S, Meybodi M A, Chahartaghi M. Modeling and economic analysis of gas engine heat pumps for residential and commercial buildings in various climate regions of Iran[J]. Energy & Buildings, 2010, 42(7):1129-1138. [11] Brenn J, Soltic P, Bach C. Comparison of natural gas driven heat pumps and electrically driven heat pumps with conventional systems for building heating purposes[J]. Energy & Buildings, 2010, 42(6):904-908. [12] Zhang X, Yang Z, Wu X, et al. Evaluation method of gas engine-driven heat pump water heater under the working condition of summer[J]. Energy & Buildings, 2014, 77(7):440-444. [13] Zhang R R, Lu X S, Li S Z, et al. Analysis on the heating performance of a gas engine driven air to water heat pump based on a steady-state model[J]. Energy Conversion & Management, 2005, 46(s 11–12):1714–1730. [14] Zhang R R, Lu X S, Li S Z, et al. Analysis on the heating performance of a gas engine driven air to water heat pump based on a steady-state model[J]. Energy Conversion & Management, 2005, 46(s 11–12):1714–1730. [15] Elgendy E, Schmidt J. Experimental study of gas engine driven air to water heat pump in cooling mode[J]. Energy, 2010, 35(6):2461-2467. [16] Liu H, Zhou Q, Zhao H. Experimental study on cooling performance and energy saving of gas engine-driven heat pump system with evaporative condenser[J]. Energy Conversion & Management, 2016, 123:200-208. [17] Wan X, Cai L, Yan J, et al. Power management strategy for a parallel hybrid-power gas engine 27
heat pump system[J]. Applied Thermal Engineering, 2017, 110:234-243. [18] Ji W, Cai L, Men Q, et al. Research for Hybridization Degree and Logic Threshold Control Strategy of the Hybrid Power Gas Engine Heat Pump[J]. Procedia Engineering, 2015, 121:984-991. [19] Wan X, Cai L, Yan J, et al. Power management strategy for a parallel hybrid-power gas engine heat pump system[J]. Applied Thermal Engineering, 2017, 110:234-243. [20] Zhang R R, Shu-Ze L I, Xue-Sheng L U, et al. Experiment and Regressive Analysis on the Waste Heat Characteristic of Natural Gas Engine[J]. Journal of Shanghai Jiaotong University, 2005, 32(2):596-597. [21] TessLibs 17 in TRNSYS, Type 907:Internal Combustion Engine / Generator set [22] Hwang Y. Potential energy benefits of integrated refrigeration system with microturbine and absorption chiller[J]. International Journal of Refrigeration, 2004, 27(8):816-829. [23] Sanaye S, Chahartaghi M, Asgari H. Dynamic modeling of Gas Engine driven Heat Pump system in cooling mode[J]. Energy, 2013, 55(1):195-208. [24] Bergman T L, Lavine A, Incropera F P, et al. Fundamentals of heat and mass transfer[M]// Fundamentals of heat and mass transfer /. Wiley, 1985:494-497. [25] Yan Q. Refrigeration Technology for Air Conditioning[M]. China building industry press, 2010.(in Chinese) [26] Zhu D D, Yan D, Li Z. Modelling and applications of annual energy-using simulation module of separated heat pipe heat exchanger[J]. Energy & Buildings, 2013, 57(57):26-33. [27] Breeze P. Chapter 3 – Gas-Fired Power Generation Technology[J]. Gas-Turbine Power Generation, 2016:21-29. [28] Li X, Wu W, Zhang X, et al. Energy saving potential of low temperature hot water system based on air source absorption heat pump[J]. Applied Thermal Engineering, 2012, 48(26):317-324.
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Figure and Table captions Fig. 1 Structure of the hybrid power-driven heat pump. Fig. 2 Output work efficiency and waste heat of the cylinder of the engine model. Fig. 3 Relationships between the flue gas temperature and the humidity ratio and the enthalpy. Fig. 4 Configuration of the waste heat exchanger and the heat transfer process. Fig. 5 Building load information of Beijing and Shanghai. Fig. 6 Heating performance comparison among the different systems. Fig. 7 Cooling performance comparison among the GEHP, EHP and HPHP. Fig. 8 Monthly gas consumption of the GEHP and the EHP in Beijing and Shanghai in summer. Fig. 9 Hourly cooling PER of the GEHP and the EHP in Beijing and Shanghai in summer. Fig. 10 Monthly gas consumption of the GEHP and the EHP in Beijing and Shanghai in winter. Fig. 11 Hourly heating PER of the GEHP and the EHP in Beijing and Shanghai in winter. Fig. 12 Comparison of the energy consumptions of different equipment in Beijing and in Shanghai. Table 1 Parameters of flue gas temperature formula in Eq. 1. Table 2 Detailed information of the heat exchangers in the mathematical model. Table 3 PER Validation of the GEHP using Elgendy’s experimental data. Table 4 Building load and envelope information in Beijing and Shanghai. Table 5 The parameters of the refrigerant cycle of GEHP and EHP of heating mode on typical working condition Table 6 Heat transfer process of the GEHP for a typical working condition in summer.
29
Highlights
A hybrid power-driven heat pump system is proposed for heating and cooling.
Gas engine and electromotor are combined in the heat pump system.
No engine heat rejection leads to better cooling performance of hybrid system.
Hybrid system has less energy consumption for the whole year operation.
Hybrid heat pump system has great application potential in different regions.
30
S1359-4311(17)30420-9 http://dx.doi.org/10.1016/j.applthermaleng.2017.04.151 ATE 10300
To appear in:
Applied Thermal Engineering
Received Date: Accepted Date:
20 January 2017 29 April 2017
Please cite this article as: S. Shang, X. Li, W. Wu, B. Wang, W. Shi, Energy-saving analysis of a hybrid powerdriven heat pump system, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/j.applthermaleng. 2017.04.151
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Manuscript prepared for Applied Thermal Engineering
Energy-saving analysis of a hybrid power-driven heat pump system Sheng Shang, Xianting Li*, Wei Wu, Baolong Wang Wenxing Shi Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, China
* Corresponding author: Prof. Xianting Li Department of Building Science, School of Architecture Tsinghua University Beijing 100084, P.R. China Tel: +86-10-62785860 Fax: +86-10-62773461 E-mail: [email protected]
-1-
Nomenclature
cp
specific heat of air, kJ/(kg·K)
cps
specific heat of the coolant medium, kJ/(kg·K)
E
primary gas consumption, Nm3
F
view factor
h
specific enthalpy, kJ/kg
L
building load, kW
LCV
low calorific value, kJ/Nm3
m
mass flow rate, kg/s
n
variety index
PER
primary energy ratio
Q
heat transfer rate, kW
Qe
primary energy consumption, kW·h
Tr
torque of the engine, N·m
T
temperature, °C
UA
heat transfer capacity, kW/K
W
work consumption of compressor, kW
x
distance between the condenser and the waste HX, m
Abbreviations COP
coefficient of performance
DISP
displacement volume, m3/min
ESR
energy saving ratio
PR
pressure ratio of the compressor
RPM
rotation speed per minute, r/min
Subscripts a
air
c
condenser
e
evaporator
EC
EHP for cooling
EH
EHP for heating
2
fg
flue gas
gas
natural gas
GC
GEHP system for cooling
GH
GEHP system for heating
HC
HPHP system for cooling
HH
HPHP system for heating
i
hourly system performance
j
cylinder jacket
ise
isentropic
output
output work
r
refrigerant
rad
radiation
rc
compressor inlet and outlet refrigerant
re
heat rejection medium
vol
volumetric
waste
heat rejection
Greeks
ΔTm
logical mean temperature difference, °C
ε
efficiency
ƞ
power generation efficiency of the gas-fired power plant, %
ρ
density, kg/m3
Abstract
A gas engine-driven heat pump (GEHP) is traditionally applied for both heating and cooling; however, the cooling performance may be worse than that of an electric heat pump (EHP) because of the waste heat rejection of the gas engine. To improve the system performance of the air source heat pump for the entire year running, a hybrid power-driven heat pump (HPHP) system is proposed, in which the heat pump is driven by a gas engine for heating and by electricity for cooling. The mathematical 3
model of the HPHP system is established, and the performance of the HPHP system is compared with those of GEHP and EHP. It is demonstrated that the primary energy ratio (PER) of the HPHP is 28.5%~51.2% higher than that of the GEHP for cooling and 15.8%~25.3% higher than that of the EHP for heating. Compared to the GEHP and the EHP, the energy saving ratio of the HPHP is 10.9% and 14.4% in Beijing, respectively, and the corresponding value is 18.5% and 7.3% in Shanghai, respectively. These results highlight the great application potential of the HPHP system for both heating and cooling.
Keywords
Gas engine-driven heat pump; Electric heat pump; Hybrid power; Building energy efficiency; Space heating.
1 Introduction
Building energy consumption in China has increased rapidly in recent years, accounting for over 0.8 billion tce (tons of coal equivalent) in 2012 [1]. The air source heat pump (ASHP) was proposed with the development of low-temperature heating and the research on the thermal comfort of novel terminals[2,3] to enhance the primary energy efficiency and to reduce the pollutant emission. The ASHP can be divided into the electric heat pump (EHP) and the gas engine-driven heat pump (GEHP) according to the driving power. As for the heating performance, the GEHP system will be higher than the EHP 4
system. The reason is that the heat recovery of the gas engine makes the heating performance of GEHP better than the EHP system even the power generation efficiency of the gas-fired power plant is higher. The current studies on the GEHP are mainly focused on the heating performance improvement and the control strategy[4]. Yang et al. investigated the heating performance and the control strategy using both simulation and experiments; the results showed the primary energy ratio (PER) can reach 1.7 in certain working conditions[5,6], and the partial load performance is even better[7]. Elgendy et al.
conducted an experimental study of the GEHP’s heating
performance; the results showed that the heating PER is 1.4 ~ 1.8 for different working conditions[8]. Lazzarin et al. [9] and Sanaye et al.[10] conducted the economic analysis on the GEHP system based on a case study; the results showed that the running cost was much lower than the gas-fired boiler. Brenn et al. compared the heating performance of the GEHP with that of the EHP[11]; the results showed the annual primary energy efficiency of the GEHP is approximately 27.9% higher than that of the EHP. It can be seen that the heating performance of the GEHP is much better than that of the EHP from the abovementioned research. However, the cooling performance of the GEHP may be worse than that of the EHP. The reasons are that the output work efficiency of the gas engine is lower than the power generation efficiency of the gas-fired power plant, besides, the condensation temperature of the GEHP system will be affected by the heat rejection of the gas engine. To ensure the GEHP operates well in summer, the waste heat of the gas engine should be rejected. There are mainly two methods to solve this problem:
5
the first one is to use the waste heat for generating domestic hot water[6,12], and the other method is using an additional heat exchanger which installed outside the condenser, where the additional heat exchanger shares the same fan with the condenser [13,14].The first method may not be a proper solution in a building in which the load of domestic hot water does not match the cooling load, and the engine efficiency is lower when the waste heat of gas engine is not well dispersed. Elgendy et al. also investigated the cooling performance of the GEHP and found that the cooling PER is 1.35 when the evaporator inlet temperature is 13°C [15]; however, the cooling PER includes the heat recovery of the engine. The cooling performance may be lower than 1.0 if the waste heat recovery from the gas engine is ignored. Regarding the second method, the coolant of the engine passes through the heat exchanger outside the condenser, which may result in the increase of condensation temperature due to the radiation from the heat exchanger to the condenser, so that the power consumption of the fans and the compressor may be higher; thus, the COP and the cooling PER will be affected. Liu et al. investigated the GEHP’s cooling performance with the evaporative condenser[16]; the results showed that both the COP and the cooling capacity are increasing compared to the traditional GEHP system due to the well heat rejection of the gas engine. In contrast, the EHP may have a higher cooling PER than that of the GEHP. Therefore, by combining the heating performance of the GEHP with the cooling performance of the EHP in a hybrid system, higher energy saving potential for operation over the entire year may be achieved. Referring to hybrid electric vehicles, some researchers proposed a hybrid gas
6
engine-driven heat pump system[17], for which batteries for electricity storage or driving source are used when the heating or cooling capacity is greater than the building demand. Besides, the heat pump can also be driven by the gas engine and electricity in parallel[18,19]. That’s a solution to maintain the gas engine at a higher efficiency in summer, but the efficiency of heat pump is still low in summer because of the lower gas engine efficiency compared with gas-fired power plant. In this work, to achieve higher energy saving potential for operation over the entire year by utilizing the advantages of GEHP and EHP system, a novel hybrid power-driven heat pump (HPHP) system is proposed, and the mathematical model of the system is established to analyze the heating and cooling performance. Based on the system performance, the performance comparison between the EHP and the GEHP and the energy saving ratio for the entire year running in Beijing and Shanghai are investigated.
2 Working Principles and Operation Mode of the HPHP
The configuration of the hybrid power-driven heat pump is shown in Fig. 1; the HPHP is mainly comprised of three parts. The first part is the refrigeration cycle, which is similar to that of the traditional electric heat pump. The second part is the driving source, which has an electromotor and a gas engine; both of them have a clutch to connect to the gear or belt pulley that drives the open-type compressor. The last part is the heat recovery system, which can recover the waste heat of gas engine; a three-way valve on the water supply pipe can control the different operation modes. 7
Compared to the traditional GEHP system, there is no extra waste heat exchanger for the heat rejection of the engine.
Fig. 1 Structure of the hybrid power-driven heat pump. For the cooling mode, the electromotor is used to drive the open-type compressor to generate chilled water; in addition, the gas engine does not operate, and the clutch of the gas engine is disconnected. The return water does not pass through the heat recovery system and only passes through the evaporator, which is user’s heat exchanger in Fig.1. The working principle is similar to that of the electric heat pump. Therefore, no heat rejection process of the gas engine occurs in summer, and the condensation temperature will not be impacted. For the heating mode, the driving source switches to the gas engine, and the electromotor is turned off. In this case, the four-way valve of the refrigerant cycle switches to heating mode. In the same time, the three-way valve switches to another direction to allow the return water to pass
8
through the condenser, engine cylinder and exhaust gas heat exchanger successively. In this process, the natural gas is fed into the gas engine and is transformed into flue gas via the combustion process. The flue gas passes through the waste heat exchanger and then is discharged into the environment. The HPHP system can avoid the disadvantage of the GEHP and the EHP because of its configuration and operation modes.
3 Methodology
To investigate the system performance of the HPHP and compare the system performances among the EHP, GEHP and HPHP, a mathematical model is established. For the heat exchangers, the same evaporator and the condenser are designed for the EHP and the GEHP. For the compressor, the open-type compressor is chosen for the GEHP and the HPHP system, and the hermetic compressor is chosen for the EHP system. The supply water temperature in winter is 45°C, and the chilled water temperature in summer is 7°C. Next, the building load information and the calculating method of energy consumption are introduced to evaluate the energy saving potential. The comparison is based on the primary energy consumption, and the electricity is converted to the equivalent natural gas consumption in the gas-fired power station.
9
3.1 Mathematical model 3.1.1 GEHP model (1) Gas engine
The output work, gas consumption and flue gas temperature of the engine are the items of concern in this study; therefore, some product catalogues are referenced to calculate the output work, the cylinder jacket heat generation and the temperature of the waste gas. The rated output work of the chosen engine is 15kW (the catalogue of the gas engine is TM-18.03.2009 in MiracleGen). The gas consumption and the output work are affected by the engine rotation speed and load ratio for a given product catalogue. The proportion of cylinder jacket heat generation to the gas consumption is given by a model from TRNSYS (shown in Fig. 2). The temperature of flue gas is obtained using the fitting formula from Zhang et al.[20]. The equation is shown as Eq. 1, and the parameters are shown in Table 1. The mass flow rate of the flue gas is introduced in the thermal properties of the flue gas. Tfg c1 c2 RPM c3 RPM 2 c4 Tr c5 Tr 2 c6 RPM *Tr c7 RPM Tr 2 c8 RPM 2 Tr c9 RPM 2 Tr
2
(1)
Table 1 Parameters of flue gas temperature formula in Eq. 1. c1 c2 c3 c4 c5 -8 276.8 0.004527 -1.2010 0.3176 -1.1110-3 c6 c7 c8 c9 -5 -5 -9 -7.0710 2.36710 4.63910 -1.510-12
10
Fig. 2 Output work efficiency and waste heat of the cylinder of the engine model[21]. (2) Refrigerant cycle
The open-type compressor is chosen for the GEHP system, for which the refrigerant mass flow rate and work consumption should be calculated. To perform these calculations, the efficiency model is chosen, and the equations of volumetric efficiency and isentropic efficiency are given as follows[22]:
vol 1 0.04 PR
(2)
ise 0.9 0.0467 PR
(3)
The theoretical displacement of the compressor is 1.79×10(-4)cc/rev, and the compressor is referred to the 4G.2Y-K in BITZER’s product catalogue. And the mass flow rate of refrigerant is calculated using the following equation:
mr vol DISP RPM / 60
(4)
The work consumption is calculated according to the enthalpy difference between the inlet and the outlet of the compressor as well as the isentropic efficiency: W mr hrc ,out hrc ,in / ise
(5)
Next, the gas consumption is calculated according to the work consumption and the output work efficiency of the gas engine: 11
Qgas W / output
(6)
For the heat exchanger, the outdoor heat exchanger is air-cooled, and the user’s heat exchanger is the plate heat exchanger. In this work, the lump parameter is chosen to establish the refrigerant cycle so that the heat exchange capacity UA is given according to the heat transfer demands. The detail information of the heat exchanger is shown in Table 2. The heat transfer and energy balance equations are (7) to (9). Q ma (ha ,in ha ,out )
(7)
Q mr (hr ,in hr ,out )
(8)
Q UA Tm
(9)
Table 2 Detailed information of the heat exchangers in the mathematical model. Heating mode Heat exchanger
Cooling mode UA (kW/K)
LMTD (°C)
5.14
8
4.9
8.6
5.5
6.4
7.9
0.18
/
33
/
/
0.06
2.5
115
/
/
UA
Heat exchange
LMTD
(kW/K)
area (m2)
(°C)
Evaporator
6.4
160
Condenser
8
Cylinder jacket Waste heat exchanger
For the expansion valve, the expansion process is isentropic, and the superheat and subcooling degrees are set to 5°C.
(3) Thermal property of the flue gas
According to the gas flow rate and the excess air coefficient (which is taken as 1.2[23]), the specific heat, dew-point temperature and mass flow of the exhausted flue gas can be calculated, and the condensing heat of vaporization in the waste gas is considered. When the temperature of the exhausted flue gas is lower than its
12
dew-point, the vapor will condense and release the latent heat. The dependences of the flue gas temperature on the humidity ratio and on the enthalpy is shown in Fig. 3.
Fig. 3 Relationships between flue gas temperature and humidity ratio and enthalpy. (4) Radiation model in summer
In summer, an extra heat exchanger outside the condenser is required to reject the engine waste heat. The radiation from the waste heat exchanger to the condenser is considered (as shown in Fig. 4), and each of the heat exchangers is considered as a square plate. When the heat transfer resistance of the heat exchanger surface is ignored, the radiation heat transfer between the waste heat exchanger and condenser can be calculated by Qrad
A F z (Tre 4 Tcond 4 ) 1000
(10)
The temperature of the condenser is the condensation temperature. For the waste heat exchanger, the temperature is taken as the average of the inlet and outlet temperatures of the waste heat exchanger: Tre
Tre,in Tre,out 2
13
(11)
The length of the side is 1.8 m, and the distance between the two exchangers is 0.05 m; thus, the view factor F can be calculated as[24]: 1 2 (1 x 2 )2 12 2 2 F 2 (ln( ) 2 x(1 x ) tan 1 2 x 1 2x
x 1 2 2
) 2 x tan 1 x
(12)
(1 x )
Fig. 4 Configuration of the waste heat exchanger and the heat transfer process. The radiation heat transfer from the waste heat exchanger to the condenser will cause the condensation temperature to rise. The air first passes through the condenser to remove the heat generation, and then removes the engine waste heat. Using the heat transfer equations and the thermal balance equations, the impact of the radiation heat can be evaluated. The PER of the GEHP in winter is defined as follows: PERGH
Qc Q j Q Qgas
f g
(13)
In summer, the PER is defined as follows: PERGC
Qe Qgas
(14)
3.1.2 EHP model The heat exchanger is the same as the GEHP model, whereas the compressor’s 14
efficiency model is different. The volumetric efficiency and isentropic efficiency are calculated using the following equations[25]: 1
vol 0.94 0.085( PR n 1)
ise 0.9 0.0467 PR
(15) (16)
The volumetric efficiency of the compressor εvol is used to calculate the refrigerant mass flow rate, and the work consumption is calculated according to the isentropic efficiency. The equations are the same as Eq. 4 and Eq. 5. The detailed information of heat exchangers is shown in Table 2, and the evaporator and condenser for the GEHP and the EHP are the same. The system performance of the EHP is converted into the equivalent primary energy ratio using the COP and the power generation efficiency of the gas-fired power plant ƞ (Eq. 17). The average gas-fired power generation efficiency is taken as 0.45[26]. The heating and cooling PER are shown as Eq. 17 and 18. PEREH COPEH
(17)
PEREC COPEC
(18)
3.1.3 HPHP model
The HPHP system will operate in the GEHP mode for heating; thus, the GEHP model is used for heating, Eqs. 1~9 are used, and the supply water will pass successively through the condenser, cylinder jacket and waste heat exchanger. When the HPHP system is operated in cooling mode, the mathematical model is similar to that of the EHP model with the same heat exchangers, except the compressor 15
efficiency model is different. The compressor model is based on Eq. 2 and Eq. 3, which is different from the compressor model of the EHP model. The heating performance PERHH is the same as Eq. 13. Regarding the cooling performance PERHC, the equation is as follows: PERHC COPHC
(19)
3.1.4 Model validation The EHP model has been already validated in Yan’s book [25]. Thus, in this paper, the heating performance of the GEHP model is validated according to some experimental data[8]. Elgendy built an air-to-water GEHP platform and tested different inlet temperatures of the condenser under the condition that the ambient temperature is 11.9°C and the engine speed is 1300 rpm. The PER validation with Elgendy’s experimental data is shown in Table 3. The maximum relative error is found to be within 5%, which shows the model of GEHP is accurate enough. Table 3 PER Validation of the GEHP using Elgendy’s experimental data. Condenser inlet temperature (°C)
PER Relative error Experiment
16
Simulation
32
1.85
1.95
5%
33
1.83
1.9
4%
34
1.81
1.86
3%
35
1.8
1.82
1%
36
1.76
1.78
1%
37
1.73
1.74
1%
38
1.7
1.71
1%
39
1.69
1.68
-1%
40
1.67
1.65
-1%
41
1.6
1.62
1%
3.2 Building load and energy consumption 3.2.1 Building load
Beijing and Shanghai are chosen to investigate the system performance and the energy saving ratio. An office building model is established using DeST[26] in Beijing and Shanghai, and the building is 7 floors with the total area of 1050m2. The building load information is shown in Fig. 5. The red part denotes the heating load, and the blue part represents the cooling load. The peak heating load in Beijing is 80 kW, and the cooling load is 75 kW. In addition, the peak heating and cooling load information for Shanghai is 60 kW and 80 kW, respectively. Information of the building envelope and the peak building load is shown in Table 4. Table 4 Building load and envelope information in Beijing and Shanghai. Location
wall 2
windows
Heating season
Cooling season
2.2
1st Nov ~ 31st Mar
6st May ~ 24th Sep
2.5
27th Nov ~ 22nd Mar
1st May ~ 15th Oct
(W/(m ·K))
(W/(m2·K))
Beijing
0.3
Shanghai
0.5
17
Beijing
Shanghai
Fig. 5 Building load information of Beijing and Shanghai. 3.2.2 Calculating method of energy consumption
To compare the system performance and energy saving ratio, three systems, i.e., EHP, GEHP and HPHP, are separately operated in the same building both in Beijing and Shanghai. The hourly performance for each system can be obtained using the hourly ambient temperature. Thus, the energy consumption of each system can be calculated using the hourly building load. The hourly and year-round energy consumption Qei and Qe are calculated using the following equations: Qei
Li PERi
8760
Li
i 1
PERi
Qe
(20)
(21)
The HPHP will run in GEHP mode in winter and in EHP mode in summer. To calculate the energy saving ratio of the HPHP, the energy consumption of the GEHP will be chosen as the comparison baseline in summer, and the equivalent gas consumption of EHP is the baseline for winter. The gas consumption is calculated using Eq. 22:
18
E
Qe 3600 LCV
(22)
After calculating the hourly energy consumption and integrating the values for the whole year, the energy saving ratio of the HPHP can be calculated by ESR
E EHPHP E
(23)
4 Results and Discussion
4.1 System performance comparison
For the heating performance, the parameters of the refrigerant cycle of GEHP and EHP when the ambient temperature is 0°C are shown in Table 5. The supply water temperature is set to 45°C for both GEHP and EHP system. For the GEHP system, the return water is heated from 39.3°C to 43.5°C in the condenser, then to 45°C by the gas engine heat recovery process. As for the EHP system, the return water’s temperature is heated from 40.7°C to 45°C only by the condenser. So the condensation temperature of the GEHP system is 1.6°C lower than that of EHP due to the heat recovery of the gas engine. Therefore, the COP of the GEHP system is larger than that of EHP system. Besides, the heating capacity of GEHP system is 27.2% higher than that of EHP system due to the heat recovery of the gas engine. Table 5 The parameters of the refrigerant cycle of GEHP and EHP of heating mode on typical working condition condenser work evaporation condensation heating inlet water consumption of temperature temperature capacity temperature the compressor (°C) (°C) (kW) (°C) (kW)
19
GEHP
-7.6
39.3
46.1
47.49
11.70
EHP
-7.4
40.7
47.7
36.31
11.58
A heating performance comparison of the different systems is shown in Fig. 6. The heating PER of the GHEP and the EHP is higher when the ambient temperature is increasing, and the GEHP has the highest heating PER, which is approximately 0.15 higher than that of the EHP because of the heat recovery of the engine. The efficiency of the gas-fired boiler and the coal-fired boiler is given as 0.9 and 0.7[28], respectively. The comparison is based on the condition in which the ambient temperature is greater than -18°C because the locations where the ASHP can be put into application have temperatures mainly in this temperature range. Fig. 7 shows the cooling performance comparison among GEHP, EHP and HPHP. From Fig. 7, the EHP’s PER of cooling is much higher than that of the GEHP if we take the gas-fired power generation efficiency as 45% [27], and the PER is lower when the ambient temperature is higher because the condensation temperature is higher. In addition, the condensation temperature of the GEHP is higher than that of the EHP because of the waste heat rejection of the gas engine; thus, the PER of the GEHP is the lowest. Compared to the HPHP, the cooling PER of the EHP system is higher than that of the HPHP because of the different types of compressors used. The volumetric efficiency of the hermetic compressor is higher than that of the open-type compressor; in addition, the transmission loss is less than that of the HPHP. The PER of both the EHP and the HPHP is found to be much greater than that of the GEHP, and the difference between the PER of the EHP and the HPHP is small.
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Fig. 6 Heating performance comparison among the different systems.
Fig. 7 Cooling performance comparison among the GEHP, EHP and HPHP. The condensation temperature of the GEHP and the EHP under the same condition is shown in Fig. 7. The GEHP’s condensation temperature is found to be approximately 0.5°C higher than EHP because of the radiation heat transfer of the engine waste heat; therefore, the COP will be slightly lower than that of the EHP. When the ambient temperature is 35°C, the average temperature of the heat rejection medium is 65.5°C, and the condensation temperature is approximately 47.8°C. In addition, the radiation heat transfer rate from the waste heat exchanger to the condenser is 0.65kW, and the heat transfer rate of the condenser is 50.49kW.
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Compared to the GEHP system, the condensation temperature of EHP system is 0.5°C lower, and the cooling performance is even better with higher power generation efficiency. The detailed information is shown in Table 6. Table 6 Heat transfer process of the GEHP for a typical working condition in summer.
GEHP EHP
Inlet and outlet
Inlet and outlet
temperature of
temperature of
Tc
Qrad
Qc
waste HX
air
(°C)
(kW)
(kW)
(°C)
(°C)
71.4~59.5
35~44.3
47.8
0.65
50.49
35~42.7
47.3
Heat rejection (kW) 12.0
50.51
4.2 Energy consumption analysis in summer
The natural gas consumption in summer is shown in Fig. 8. From Fig. 8, the monthly gas consumption of GEHP is the highest, both in Beijing and in Shanghai, mainly because the cooling PER of EHP and HPHP is much higher than that of the GEHP. The GEHP’s gas consumption for July is 29.3% and 29.6% higher than that of the HPHP in Beijing and Shanghai, respectively. And the corresponding value compared to the EHP system is 29.6% and 32.1% respectively. The reason for this result is that the average temperature of Beijing in summer is lower than that of Shanghai; thus, the average cooling PER in Beijing is higher than that in Shanghai (shown in Fig. 9). However, the gas consumption in Shanghai is larger than that in Beijing because the cooling load of the building in Shanghai is larger.
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Beijing
Shanghai
Fig. 8 Monthly gas consumption of the GEHP and the EHP in Beijing and Shanghai in summer.
Beijing
Shanghai
Fig. 9 Hourly cooling PER of the GEHP and the EHP in Beijing and Shanghai in summer. 4.3 Energy consumption analysis in winter
The gas consumption of each month in winter is shown in Fig. 10. The gas consumption of GEHP and HPHP is found to be the same because the HPHP system operates in the GEHP mode in winter and the components are the same. The monthly gas consumption of GEHP/HPHP is lower than the EHP, both in Beijing and in Shanghai, mainly because the heating PER of the GEHP/HPHP is considerably higher than that of the EHP because of the heat recovery of the engine. In addition, the GEHP/HPHP’s gas consumption of January is 25% and 23% lower than that of the
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EHP in Beijing and Shanghai, respectively. The results match with that of Brenn et al.[11], in which the energy saving ratio of GEHP is about 27.9% compared to EHP. The hourly heating PER is shown in Fig. 11, and the heating PER in Shanghai is larger because the average temperature is higher.
Beijing
Shanghai
Fig. 10 Monthly gas consumption of the GEHP and the EHP in Beijing and Shanghai in winter.
Beijing
Shanghai
Fig. 11 Hourly heating PER of the GEHP and the EHP in Beijing and Shanghai in winter. 4.4 Year-round energy saving potential
The year-round gas consumptions of Beijing and Shanghai are shown in Fig. 12. The primary energy consumption of the HPHP is the lowest, both in Beijing and in Shanghai because the HPHP runs almost exclusively in the most energy-saving mode,
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both in winter and in summer. Although the cooling performance of the HPHP system in summer is worse than that of the EHP, the heating PER of the HPHP is much higher. In Beijing, the energy saving ratio of the HPHP is 10.9% and 14.4% relative to the GEHP and the EHP, respectively, and the corresponding results in Shanghai are 18.5 and 7.3% compared to GHEP and EHP, respectively. The energy consumption of the EHP is 5.1% higher than that of the GEHP in Beijing because the heating load is much higher than the cooling load and the GEHP can save more energy than the EHP. However, in Shanghai, the result is opposite: the GEHP consumes 14.4% more energy than the EHP. The building load feature will affect the energy saving ratio of different systems, and the GEHP is more suitable for the locations where the heating load is slightly higher. The HPHP system will consume less energy than the GEHP and the EHP during the year-round operation; thus, the HPHP system can be used to replace the EHP and the GEHP.
Fig. 12 Energy consumptions of different equipment in Beijing and in Shanghai.
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5 Conclusions
To improve the system performance of the air source heat pump for the operation over the entire year, a hybrid power-driven heat pump was proposed to make full use of the advantages of the heating performance of the GEHP and the cooling performance of the EHP. The mathematical model was established to calculate the system cooling and heating performance. Next, the energy saving potential in Beijing and Shanghai for heating season, cooling season and the whole year running was investigated. The main conclusions from the above-described analysis are as follows: 1. When the ambient temperature is 0°C, the heating PER of the HPHP can reach up to 1.6, and the cooling PER is 1.55 when the temperature is 35°C. 2. The HPHP has 28.5%~51.2% higher cooling PER than the GEHP and 15.8%~25.3% higher heating PER than the EHP, and the condensation temperature of the HPHP in summer is about 0.5°C lower than that of the GEHP; 3. For the year-round operation, the energy saving ratio of the HPHP is 10.9% and 14.4% compared to the GEHP and the EHP in Beijing, respectively; the corresponding result in Shanghai is 18.5 and 7.3%, respectively.
Acknowledgments
The authors gratefully acknowledge the support of the Innovative Research Groups of the National Natural Science Foundation of China (grant number 51521005).
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Figure and Table captions Fig. 1 Structure of the hybrid power-driven heat pump. Fig. 2 Output work efficiency and waste heat of the cylinder of the engine model. Fig. 3 Relationships between the flue gas temperature and the humidity ratio and the enthalpy. Fig. 4 Configuration of the waste heat exchanger and the heat transfer process. Fig. 5 Building load information of Beijing and Shanghai. Fig. 6 Heating performance comparison among the different systems. Fig. 7 Cooling performance comparison among the GEHP, EHP and HPHP. Fig. 8 Monthly gas consumption of the GEHP and the EHP in Beijing and Shanghai in summer. Fig. 9 Hourly cooling PER of the GEHP and the EHP in Beijing and Shanghai in summer. Fig. 10 Monthly gas consumption of the GEHP and the EHP in Beijing and Shanghai in winter. Fig. 11 Hourly heating PER of the GEHP and the EHP in Beijing and Shanghai in winter. Fig. 12 Comparison of the energy consumptions of different equipment in Beijing and in Shanghai. Table 1 Parameters of flue gas temperature formula in Eq. 1. Table 2 Detailed information of the heat exchangers in the mathematical model. Table 3 PER Validation of the GEHP using Elgendy’s experimental data. Table 4 Building load and envelope information in Beijing and Shanghai. Table 5 The parameters of the refrigerant cycle of GEHP and EHP of heating mode on typical working condition Table 6 Heat transfer process of the GEHP for a typical working condition in summer.
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Highlights
A hybrid power-driven heat pump system is proposed for heating and cooling.
Gas engine and electromotor are combined in the heat pump system.
No engine heat rejection leads to better cooling performance of hybrid system.
Hybrid system has less energy consumption for the whole year operation.
Hybrid heat pump system has great application potential in different regions.
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