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Performance evaluation of R1234yf heat pump system for an electric vehicle in cold climate
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Performance evaluation of R1234yf heat pump system for an electric vehicle in cold climate Wanyong Li , Rui Liu , Yusheng Liu , Dandong Wang , Junye Shi , Jiangping Chen PII: DOI: Reference:
S0140-7007(20)30074-8 https://doi.org/10.1016/j.ijrefrig.2020.02.021 JIJR 4680
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International Journal of Refrigeration
Received date: Revised date: Accepted date:
26 September 2019 11 February 2020 13 February 2020
Please cite this article as: Wanyong Li , Rui Liu , Yusheng Liu , Dandong Wang , Junye Shi , Jiangping Chen , Performance evaluation of R1234yf heat pump system for an electric vehicle in cold climate, International Journal of Refrigeration (2020), doi: https://doi.org/10.1016/j.ijrefrig.2020.02.021
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Performance evaluation of R1234yf heat pump system for an electric vehicle in cold climate Wanyong Lia, Rui Liua, Yusheng Liua, Dandong Wanga, Junye Shia,b, Jiangping Chena,b,* a
Institute of Refrigeration and Cryogenics, Shanghai Jiaotong University, Shanghai, China b
Shanghai High Efficient Cooling System Research Center, Shanghai, China
* Corresponding author: [email protected] Tel. + (86) 21 34206775 Abstract To improve the efficiency of mobile air conditioning and to meet the need of protecting the environment, performance characteristics of the R1234yf heat pump were tested under various conditions in a cold climate. R1234yf is considered a promising replacement for R134a because of its low global warming potential (GWP). Many studies have examined the cooling performance of R1234yf, but little attention has been focused on its heating performance in cold climate. In this work, the system performance of R1234yf heat pump was analyzed and compared with that of R134a under different influencing factors such as charge, outdoor temperature, indoor temperature, outdoor wind speed, indoor air mass flow rate, compressor speed, inner condenser width, and economized vapor injection (EVI). The results obtained are presented in detail from the view of experimental analysis and comparison. Keywords: R1234yf; R134a; Heat pump; Economized vapor injection; Electric vehicle; Cold climate Nomenclature GWP global warming potential ODP
ozone depletion potential
EVI
economized vapor injection
SAE
Society of Automotive Engineers
PTC
Positive Temperature Coefficient heater
COP
coefficient of performance
PID
proportion-integral-differential
OHX outdoor heat exchanger Q
heating capacity (W)
W
compressor work (W)
T
temperature (℃)
DB
dry bulb temperature (℃) 1
WB
wet bulb temperature (℃)
M
motor
B
blower
EXV
electric expansion valve
h
enthalpy (kJ·kg-1)
q
mass flow rate (kg·h-1)
c
specific heat (kJ· (kg·K)-1)
Ф
diameter (mm)
Subscripts m
mass
1. Introduction The development of new energy vehicles is a significant trend in the global automobile industry and represents an important opportunity to promote the sustainable development of the world economy and society. Regulation (EU) No 517/2014 prohibited the use of refrigerants with GWP over 150 in mobile air conditioning systems. The Kyoto Protocol (UNFC, 1997) also defined the obligations of both developed and developing countries to cumulatively reduce their greenhouse gas emissions to 7% below 1990 levels for three major greenhouse gases, including carbon dioxide. The refrigerant R134a used in most automotive air conditioners currently in mass production must be replaced due to its GWP (global warming potential), which may be as high as 1430 (Leck, 2009), far higher than the EU standard. Therefore, a substitute product is required. There are several potential replacements, such as CO2, R152a, and propane. Previous studies have studied the use of R152a (Ghodbane, 1999) and CO2 (Lorentzen and Pettersen, 1993) to replace R134a in mobile air conditioning systems. R152a must use a secondary loop for flammability, requiring extra components such as pumps, pipes, a tank, and a radiator. CO2 is a natural refrigerant, but its working pressure is almost 8-10 times higher than that of R134a, which means all the components must be redesigned. Cecchinato et al. (2005) and Minetto et al. (2016) found that CO2's larger density increased the refrigerant mass rate and then boosted the cooling or heating capacity under low temperature conditions for the CO2 system. Wang (2017) found that when both the indoor and outdoor temperatures were −20℃, the CO2 heat pump showed a coefficient of performance (COP) of 3.1 and a heating capacity of 3.6 kW. For security reasons, a secondary loop heat pump was needed which reduced COP by 19%. New technologies such as electrocaloric cooling technologies have also been gradually developed in the past decade. Although it is a new type of 0 GWP 2
refrigeration technology, its heating capacity and COP are still low, which cannot meet the need of automobiles (Junye 2019). Cichong (2018) compared the heating performance of R134a, R1234yf, CO2 system, and propane system at -10℃ and found that the heating performance of R290 was about 80% higher than that of R134a and R1234yf, and 12% higher than CO2. R1234yf is a new refrigerant for automobile air conditioning, with GWP of only 4 and ozone depletion potential (ODP) of 0 (Leck, 2009). The physical properties, operating characteristics, and working performance of R1234yf are very similar to those of R134a (Li, 2019); so application of this refrigerant does not require redesign or changes in many components, facilitating its adoption by the automobile industry. The Society of Automotive Engineers (SAE) defined the safety level of R1234yf as A2L (SAE, 2010), non-toxic and weakly flammable, supporting its global use as a replacement refrigerant in mobile air conditioning systems. In cold weather, heating in traditional automobiles is provided by waste heat of engine cooling water, but electric vehicles do not have an engine that can meet the heating demand of the cabin (Zhang Z, 2018). The Positive Temperature Coefficient heater (PTC) for air conditioning of electric vehicles will cause drive range attenuation of 50-60%, which greatly hinders the application and promotion of electric vehicles (Eric, 2014). To improve the energy efficiency and range of air conditioning systems for electric vehicles, heat pump technology has been developed. Choi et al. (2017) found that for electric vehicles, the economized vapor injection (EVI) technology was very necessary under low temperature environment and under the same air outlet temperature (36 ℃), it improved the system heating capacity by 76% (from 1351W to 2387W). Zhang et al. (2016) tested the R134a vehicle heat pump system at low temperature, and found that the heating performance could be increased by 44.1% at -10℃ by use of vapor injection technology. Wang et al. (2018) analyzed the superheat degree for different evaporation temperatures (-20~10℃) and different condensation temperatures (40-60℃). The results showed coefficient of performance (COP) and heating capacity values of the R134a system were 1.39-4.06% and 0.53-4.08% higher, respectively, than those of the R1234yf system. Zhao et al. (2012) studied the performance of an automobile air conditioning system using a micro-channel parallel flow condenser together with R134a and R1234yf. The results showed that the charge of R1234yf was reduced by about 5% compared to that of R134a and the cooling capacity and COP of the system using R1234yf were about 12.4% and 9% lower respectively, due to differences in the thermophysical properties. Navarro (2013) found that systems using R1234yf had COP values that were about 19% lower than those obtained using R134a, with only minor differences at higher 3
condensing temperatures. Qi (2013) concluded that R134a displayed better heat transfer and flow performance than those of R1234yf in a laminated plate evaporator due to thermophysical and transportation properties under both low and high refrigerant pressure at the expansion valve inlet. The main thermophysical properties of R1234yf are shown in Table.1 and compared with those of R134a. The pressure and temperature conditions of R1234yf are very similar to those of R134a. The NIST REFPROP 8.0 (Lemmon et al., 2002) calculation was applied and the conditions of these two refrigerants circulation are very similar, which is a requirement for R1234yf to replace R134a (Zhao, 2012). Fig. 1 also shows the vapor density of R1234yf and R134a under different evaporator temperatures. Under the same displacement of the compressor, higher vapor density of the refrigerant means larger mass flow rate. The density of R1234yf is greater than that of R134a by 9.1% at 60℃ and by 27.1% at -20℃. The lower the temperature, the greater the ratio of difference in the vapor density of R1234yf and R134a. Table 1. Main thermophysical properties of R1234yf and R134a Chemical formula Molecular mass GWP Critical temperature /℃ Critical pressure /kPa Boiling point /℃
R1234yf CF3CF=CH2 114.04
R134a C2H2F4 102.03
4 94.7
1430 101.1
3382 -29.4
4059 -26.1
Fig. 1 Saturation pressure and Vapor density of R1234yf vs. R134a
4
In low temperature environments, the evaporation temperature of the system may reach -25℃ or even lower. Under this condition, R1234yf has two advantages over R134a: a lower evaporation pressure and a higher vapor density. Fig.1 compares the saturation pressure curves of R1234yf and R134a. The presented data shows a higher saturation pressure of R1234yf than that of R134a at temperature below 41.3℃. If the evaporation pressure of the heat pump system is lower than the atmospheric pressure, the air may enter the system and cause irreversible damage. When the evaporation pressure is equal to the atmospheric pressure, the saturation temperature of R134a is 26.3℃ and that of R1234yf is -29.8℃. Therefore, in extreme conditions, the R1234yf system can withstand lower evaporation temperatures than R134a, which can make the system more reliable. Additionally, under the same environment and temperature, R1234yf can match to higher compressor speeds than the R134a system. Jarall (2012) studied the cooling performance of R1234yf system and R134a system from the theoretical level, and found that the pressure ratio, COP and refrigeration performance of R1234yf were lower. Lee Y (2012) found that the drop-in test result of the R1234yf system was 0.8-2.7% lower than the R134a system, the cooling capacity was reduced by 4%, and the discharge temperature was 6.4-6.7 ℃ lower. Kashif (2017) built a system model that could evaluate the potential of R1234yf and R1234yfze(E) to replace R134a for the residential heat pump water heaters, and found that R1234yf and R1234ze(E) could be substituted for R134a with comparable performance and no big change to the original system. Zilio (2011) found that the cooling performance of R1234yf was better than R134a by increasing the heat transfer area of the condenser and evaporator. Feng and Hrnjak (2018) tested the performance of R1234yf and R134a vehicle heat pumps in a low temperature environment. The results showed that the heating capacity of the R1234yf system was 28% higher than that of R134a at -20℃ ambient temperature because the R1234yf system could match the compressor operation at higher speeds. Many researchers have made in-depth investigations on the cooling performance of R1234yf, and also extensively researched the cooling and heating performance of R134a; however, the lowtemperature heating performance of R1234yf lacks the attention of researchers, especially below 10℃. This paper will conduct experimental research on the low-temperature heating performance of ground R1234yf, analyze the effects of various factors on R1234yf, and systematically display the heating characteristics and rules of R1234yf at low temperatures . 2. Experimental set-up
5
i) 2.1 Test apparatus The system was installed in two climatic chambers as shown in Fig. 2. The indoor condenser was installed in the indoor chamber and the other parts of the system were installed in the outdoor chamber. The climatic chambers allowed the control of the air temperature and humidity with a proportion-integral-differential (PID) controller. The velocity of the outdoor heat exchanger (OHX) was controlled by an axial flow fan. The wind tunnels were used to regulate air mass flow rate and measure the inlet and outlet air dry bulb (DB) and wet bulb (WB) temperatures. The rotation speed of the electric compressor was controlled using a controller provided by Shanghai Shiyu Co., Ltd. Refrigerant side pressure and temperature sensors were positioned at the inlet and outlet of each component. The mass flow rate of refrigerant was measured using Coriolis mass flow meter. The types and precision of each sensor were listed in Table. 2. The wind tunnel inlet temperature (t1), outlet temperature (t2), and the air mass flow rate (qm) were measured and the heating capacity was calculated. Compressor input power (W) was regulated by the power supply current (I) and voltage (U). Based on the uncertainty calculation solution proposed by Moffat (1998), the measurement uncertainty of the heating capacity Q and COP are ±2.2% and ±2.4%. Q= qmc(t2- t1)
(1)
W=IU
(2)
COP=Q/W
(3)
Table 2. Sensor type and precision Measurement parameter
Range
Precision
K-type thermocouple Piezoresisitive pressure Coriolis sensor mass flow
-50-200
±0.1
0-4000 0-200
±5 ±0.5
0-100
±0.25
Current /A
Coriolis meter mass flow Ammeter meter
0-25
±0.01
Voltage /V
Voltmeter
0-400
±0.01
Nozzle pressure difference/Pa
Yokogava air pressure
-500-500
±0.75
Temperature /℃ Pressure /kPa -1
Mass flow rate 1/(kg·h ) Mass flow rate 2 /(kg·h-1)
Sensor type
meter
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Fig. 2 Schematic diagram of the test apparatus Data for the system components are listed in Table 3. The OHX and the evaporator used in this test rig are aluminum micro-channel parallel flow heat exchangers. We used four inner condensers with different widths (189/207/225/240mm) and three electronic expansion valves (EXVs) in the system. The EXVs are driven by stepping motors and their opening can be regulated from 50 to 480 pulse. They also include the heating EXV and the EVI EXV. The inner diameters of the two electronic expansion valves are very different. The heating electronic expansion valve can be fully opened during cooling mode to ensure the smooth passage of the refrigerant without increasing the resistance. The refrigerant mass flow rate of the EVI is relatively small (less than 50kg·h-1), so EVI EXV with a smaller inner diameter (0.65 mm) was used. Electronic expansion valves have different control strategies. Due to the gas-liquid separator , the heating EXV cannot control the superheat degree of the compressor suction. Its control target is to adjust the subcooling degree of the refrigerant at the outlet of the inner condenser to 10 ℃. The target of the EVI EXV is to control the superheat degree of the refrigerant entering the compressor's inject port to be 2 ℃ (Qin 2015). A separator of 1000ml is installed at the inlet of the compressor to serve as a system refrigerant buffer. The pictures of main components are shown in Fig. 3. Table 3. System Component Information Components
Information 27cc/r. 1000-8500rpm.
Compressor scroll compressor, 350V Outside Heat
microchannel parallel flow heat exchanger,
Exchanger
660×500×16(mm)
7
microchannel parallel flow heat exchanger, Inner Condenser
Case 1:189×125×27(mm) Case 2:207×125×27(mm) Case 3:225×125×27(mm) Case 4:240×125×27(mm) microchannel parallel flow heat exchanger,
Evaporator 232×239.5×38(mm） Economizer
150×80×80(mm)，8 plates
EXV for heating
Ф9.2(mm)
EXV for vapor Injection
Ф0.65(mm)
EXV for cooling
Ф5.5(mm)
Fig. 3 Main components
ii) 2.2 Test methods In this research, the effects of different influencing factors on the heat pump system performance were studied experimentally, including charge, outdoor temperature, indoor temperature, outdoor wind speed, indoor air flow rate, compressor speed, inner condenser width, and EVI. Table 4 shows the complete test conditions. The optimal charge for the heat pump system was determined first under Test 1, in which the ambient temperatures of the indoor and outdoor environments were relatively high, resulting a high refrigerant pressure and further led to a high 8
refrigerant density, which met the system requirement for use of the largest amount of refrigerant. Tests 2-5 were designed to investigate the influence of outdoor wind speed on system performance. The determination of wind speed is based on the driving conditions of the car. From 1.5 to 4.5m·s-1, the corresponding driving conditions of the car are from idle to 80 km h-1. Tests 6-36 were designed to investigate the influence of the outdoor temperature, indoor temperature, outdoor wind speed, indoor air flow rate, compressor speed, and inner condenser width on the system performance. The maximum indoor temperature of the car was set to 20℃ and the minimum outdoor temperature was set to -20℃. We also analyzed the effect of economized vapor injection (EVI) on the system performance under different compressor speed with Tests 37-42. Table 4. Test conditions Outdoor Temperature
No.
(℃)
OHX wind
Compressor speed
speed (m·s-1)
(rpm)
4.5
Indoor Temperature (
Inner condenser
EVI
·h-1)
5500
20
330
Case1
N
5500
7
330
Case1
N
330
Case1
N
330
Case1
N
Case1
N
7
2-5
7
6-9
7/0/-10/-20
4.5
5500
20
10-14
-10
4.5
5500
-10/-1/7/13/20
5
(kg
℃)
1
1.5/2.5/3.5/4.
Indoor air mass flow rate
330/370/410/45
15-18
-20
4.5
5500
20
19-21
-20
4.5
4000/5500/7000
20
330
Case1
N
22-25
-20
4.5
4000
20
330
Case1/2/3/4
N
26-29
-20
4.5
5500
20
330
Case1/2/3/4
N
30-33
-20
4.5
7000
20
330
Case1/2/3/4
N
34-36
-10
4.5
4000/5500/7000
20
330
Case1
N
37-39
-10
4.5
4000/5500/7000
20
330
Case1
Y
40-42
-20
4.5
4000/5500/7000
20
330
Case1
Y
0
3. Experimental Results and Discussion 3.1 The effect of charge The optimal charge for the system was determined first. The system was operated under Test 1 in Table 4. The starting charge point was set to 1200 g and then refrigerant was charged into the system in a 75-100 g increment until reaching suitable heating capacity. Unlike traditional air conditioning systems, the refrigerant of the heat pump system is stored in the gas-liquid separator in front of the compressor, resulting in almost zero suction superheat degree.
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Refrigerant level in gasliquid separator
Fig. 4 R134a system characteristics under different charge
Refrigerant level in gasliquid separator
Fig. 5 R1234yf system characteristics under different charge According to Fig. 4 and Fig. 5, the characteristics of the R134a system were very similar to those of R1234yf system. Taking R1234yf (Fig. 5) as an example, when the charge was relatively low (less than 2000g), the compressor discharge pressure and suction pressure were gradually rising. Under these conditions, the subcooling degree was almost 0℃ which means that gas phase refrigerant passed through the expansion valve. This caused a relatively low refrigerant mass flow rate, thus the system heating capacity was very low. The superheat degree gradually decreased from more than 30 10
℃ to almost 0℃ . When the compressor suction superheat was 0℃, the liquid refrigerant gradually increased through the transparent casing of the gas-liquid separator (Fig. 3) . With the increase of the refrigerant, the suction pressure became stable at 0.25Mpa, the discharge pressure reached 2.3MPa, and the performance was gradually improved. When the system charge reached 2000g, the system heating capacity was stable at around 3300W. Liquid refrigerant would be filled with gasliquid separator when the charge was 2700g and liquid phase refrigerant will enter the compressor, causing a dramatic decrease in heating capacity and an obvious drop in discharge pressure. Dan (2019) reported a similar phenomenon for a mobile heat pump system. Therefore, the suitable charge is 2000-2700 g for the R1234yf system. For the R134a system, the optimal charge ranges between 1900-2800 g, with an optimal heating capacity of 3660 W. The optimal charge of the system can be determined by the heating capacity, pressure, and degree of subcooling and superheating. 3.2 The effect of outdoor wind speed The speed of an electric vehicle changes greatly during running. During idling of the vehicle, only the condensing fan causes air to pass through the outdoor heat exchanger, and this wind speed is about 1.5 m·s-1. When the vehicle speed is increased to 80 km·h-1, the wind blowing through the outdoor heat exchanger can reach 4.5m·s-1 after the wind passes through the air intake grille. When we changed outdoor wind speeds according to Tests 2-5 (Table 4) , the heating capacities of R134a system were 3419W, 3563W, 3596W, and 3660W respectively, which was improved by 1.4~4% with the increase of wind speed by 1 m·s-1. For R1234yf system under the same conditions, the heating capacities were 3238W, 3281W, 3301W, and 3336W, which was improved by 0.6~1.3% with the increase of wind speed by 1 m·s-1. The performance of each system was slightly different under different outdoor wind speed. The R1234yf lifting ratio was slightly smaller than that of R134a. Fig. 6 showed the outlet temperature and pressure drop of the outdoor heat exchanger. The outdoor heat exchanger of R1234yf has an evaporation temperature that was 1.3 ℃ higher than that of R134a, resulting a larger mass flow rate for R1234yf, since the density of R1234yf was bigger than that of R134a. Additionally, the pressure drop increased from 87 kPa to 107 kPa for R1234yf and from 70 kPa to 80 kPa for R134a respectively, with the increase of outdoor wind speed.
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Fig. 6 Outlet temperature and Pressure drop of OHX under various wind speeds 3.3 The effect of outdoor temperature When the outdoor temperature decreased from 7℃ to -20 ℃, the evaporation temperature decreased, then the refrigerant density decreased, so the mass flow rate of both refrigerants declined a lot. Fig. 7 showed the effect of outdoor temperature on the refrigerant mass flow rate and heating capacity under Tests 6-9 (Table 4) . The mass flow rate of R1234yf decreased from 110.18 kg·h-1 to 42.2 kg·h-1, that of R134a decreased from 94.52 kg·h-1 to 36.07 kg·h-1. Overall, the mass flow rate of R1234yf was 15.1-21.1% larger than that of R134a. The decreased mass flow rate also directly caused a rapid decay in the system heating capacity, which decreased from 3300 W to 1521 W, by 57.0% for R134a and from 3374 W to 1622 W, by 53.9% for R1234yf. The average rate of decline for R134a was 79.7W·℃-1 and the rate for R1234yf was 65 W·℃-1. The gap between the heating capacity of R134a and R1234yf decreased when the outdoor temperature decreased, from the initial value 12% to only 4%. The calculation supported our objective to replace R134a with R1234yf in the heat pump system for an electric vehicle in cold climate. Even though there is still a gap between the heating capacity of R1234yf and R134a, but we can change other parameters to improve the heating capacity of R1234yf to make it reach that of R134a or even higher, which will be discussed in the following sections. Fig. 8 presented the P-h diagram of R134a and R1234yf under different outdoor temperatures. The only assumption in the system was that the throttling process is considered an isentropic process. As shown in the Fig. 8, when the outdoor temperature decreased, the evaporation temperature gradually decreased. But the difference between the outdoor temperature and the evaporation temperature was maintained at 5-10 ℃ over the range tested. The P-h diagram also showed that the decrease of the outdoor temperature caused a 12
decrease in the indoor condensing pressure, although the indoor temperature and air volume remained unchanged.
Fig. 7 Mass flow rate and Heating capacity under different outdoor temperature
Fig. 8 P-h diagrams of R134a and R1234yf for different outdoor ambient temperatures 3.4 The effect of indoor temperature The change of indoor temperature will greatly change the condensation pressure. Fig. 9 shows the indoor temperature effect on system performance under Tests 10-14, Table 4. Indoor temperature had little effect on the evaporation pressure in the heat pump system. Under these five conditions, the condensation pressure increased with indoor temperature as shown in Fig. 9, but the evaporation pressure kept almost stable around 0.15 MPa. So the compression ratios calculated were 3.2, 5.9, 6.3, 7.0, and 7.4 (for R134a) and 3.0, 5.6, 6.0, 6.4, and 6.8 (for R1234yf). The pressure ratio of R134a is a little bit higher than that of R1234yf. Cuevas (2009) and Sun (2010) reported that maximum isentropic efficiency of the process at a pressure ratio of 4.0. When the pressure ratio is smaller or bigger than the peak value, over-compression or insufficient compression could occur, which would decrease the isentropic efficiency. The COP also decreased rapidly because the power 13
consumption of the compressor was directly related to the pressure ratio. Therefore, the heating capacity should be optimal when the ratio of the condensing pressure to the evaporation pressure is closer to the optimum pressure ratio of the compressor design. According to Fig. 9, the maximum heating capacity of both R134a and R1234yf occurred at the indoor temperature of around -1℃. But the COP of both R1234yf and R134a systems decreased a lot with the increase of indoor temperature.
Fig. 9 The effect of indoor temperature on the system performance 3.5 The effect of indoor air mass flow rate The increase in indoor air mass flow rate can also enhance the heat exchange between the refrigerant and the air in the inner condenser. Fig. 10 shows the effect of indoor air mass flow rate on the system heating capacity and COP under Tests 15-18, Table 4. The heating capacities were 1621W, 1750W, 1800W, and 1820W for R134a and 1520W, 1687W, 1780W, and 1815W for R1234yf respectively, when the indoor air mass flow rates were 330, 370, 410 and 450 kg·h-1. As the air mass flow rate increased, the heating capacity gradually improved and also the COP increased. When the air mass flow reached 450 kg·h-1, the heating capacity and COP of R1234yf were very close to those of R134a, which were 1820W, 1.90 and 1830W, 1.93 for R134a and R1234yf respectively. It demonstrated the feasibility of replacing R134a with R1234yf for an electric vehicle heat pump system in cold climate. Also when the air mass flow rate reached 370 kg·h-1, the heating capacity and COP of R1234yf exceeded those of R134a at 330 kg·h-1. As can be seen from Fig. 10, for the R1234yf heat pump system, the increase in air mass flow rate by 10% will make the heating capacity and COP exceed those of R134a at previous unchanged condition. But excessive air mass flow also will cause excessive noise in a car, which adversely affects passengers' comfort.
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Fig. 10 Heating capacity and COP under different air mass flow rates 3.6 The effect of compressor speed The speed of an electric compressor can be changed and can reach 12000 rpm. Fig.11 shows the performance of heat pump systems with R134a and R1234yf at different compressor speeds under Tests 19-21, Table 4. At speeds of 4000, 5500, and 7000 rpm, the R1234yf and R134a heating capacities were 1154W, 1521W, and 1682W and 1280W, 1622W, and 1771W respectively. The heating capacity of R1234yf was 9.93%, 6.23%, and 6.5% lower than that of R134a. When we increased the compressor speed of R1234yf system while did not change the compressor speed of R134a system and then compared their heating capacities, we found the gap of system heating capacity between these two refrigerants decreased. As the speed was increased by 10% for R1234yf ( 4400, 6050 and 7700 rpm), the heating performance of was 5.56%, 2.5%, and 2.2% respectively lower than that of R134a (4000, 5500 and 7000 rpm). But the results proved that increasing compressor speed by 10% for R1234yf did not achieve the same heating capacity as that of R134a working at previous unchanged compressor speed. Fig. 11 also shows the COP at different speeds. The COP of R1234yf is lower than that of R134a by 6.5% to 9.5% at compressor speed from 4000 to 7700 rpm, and the decrease of COP is obvious with the increase of speed. Even though increasing compressor speed by 10% for R1234yf did not make the heating capacity of R1234yf equal to that of R134a working at previous unchanged compressor speed, but we observed another beneficial result that the compressor discharge temperature of the R1234yf system was 2.5-3.5℃ lower than that of the R134a system at the same speed, which protected the compressor.
15
Fig. 11 Heating capacity and COP under different compressor speeds 3.7 The effect of inner condenser The inner condenser of an electric vehicle is placed in the car to warm the air. In this section, we tested four inner condensers under Tests 22-33, Table 4. They differ only in width, with other features exactly the same: Case 1: 189 × 125 × 27 (mm), Case 2: 207 × 125 × 27 (mm), Case 3: 225 × 125 × 27 (mm), and Case 4: 240 × 125 × 27 (mm). It can be seen from Fig. 12 that for the same inner condenser (Case 1), the heating capacity of R1234yf was lower than that of R134a. When the compressor speeds were 4000, 5500, and 7000 rpm, it was 1154W, 1521W, and 1597W for R1234yf, and it was 1280W, 1622W, and 1771W for R134a. But the heating capacity of R1234yf with Case 2 was comparable to that of R134a using Case 1, the gap of difference was within only ±2%. When the width of the inner condenser was increased by 10% for R1234yf but was kept unchanged for R134a, the heating capacity of R1234yf and R134a were substantially the same. However, the heating capacity of R1234yf with Case 4 was slightly improved by 1.9-2.3% compared with the one using Case 3, suggesting that Case 3 has fully utilized the heating potential of the system. The inner condenser width over 225 mm will have little influence on the heat pump system performance improvement. What’s more, as shown in Fig. 12, the COP of R1234yf with Case 1 was 5.5-8.5% lower than that of R134a with Case 1, but the COP of R1234yf with Case 2 was 4.1- 6.5% higher than that of R134a using Case 1. The results show that increasing the inner condenser width within a proper range can improve the heating capacity and COP of the R1234yf system to make them equal to or even higher than those of the R134a system working under previous inner condenser width. 16
Fig. 12 Heating capacity and COP under different inner condenser width 3.8 Economized vapor injection technology Several studies have shown that vapor injection (VI) can be used to improve the heating capacity at extremely cold temperatures. There are two types of VI, flash tank vapor injection and economized vapor injection (Xu X, 2011). Wang (2009) found that the flash tank system may have better performance than the economized system, but this conclusion was not widely accepted. Wang (2008) and Ma and Zhao (2008) concluded that the heating capacity and COP of the flash tank system were higher than those of the economizer system. Zhang (2016) demonstrated that use of an air-source heat pump with the EVI technique could improve the thermal performance by 4–6% compared to that without EVI. EVI technology was applied here in this research, and the cycle schematic diagram is shown in Fig. 13. In the EVI system, Refrigerant is divided into two ways after coming from the condenser, one way flows through the expansion valve and the other flows directly into the economizer. Inside the economizer, vapor refrigerant is injected into the compressor and subcooled liquid flows into the outside heat exchanger. Fig. 14 and Fig. 15 show the heating capacity and COP at -10℃ and -20℃ for Tests 19-21 and 34-42, Table 4. The heating performance of the R1234yf heat pump system with EVI technology has improved by 27.4% and 19.9% on average at -10 ℃ and -20℃, respectively, compared to systems without EVI technology; for R134a, the average heating performance was improved by 27.1% and 17.1% at -10℃and -20℃, respectively. The heating capacities of systems without EVI for R1234yf and R134a at -20℃were 1153W, 1521W, 1573W and 1280W, 1560W, 1771W respectively at compressor speed of 4000, 5500, 7000 rpm. But after implementing EVI technology, the heating capacities were increased to 1391W, 1622W, 2085W 17
and 1457W, 1720W, 2251W respectively with an improvement of 20.6%, 6.6%, and 32.5% for R1234yf and 13.8%, 10.3%, and 27.1% for R134a. The heating capacities of R1234yf and R134a were greatly improved at high compressor speeds. The COP of both R134a and R1234yf decreased with the increasing of compressor speed. But the COP of R134a was higher than that of R1234yf. When comparing the system performance of R1234yf with EVI and R134a without EVI, we found that R1234yf with EVI had a higher heating capacity. Higher ratio was 8.6%, 3.9%, and 17.7% respectively at the compressor speed of 4000, 5500 and 7000rpm.
Fig. 13 Schematic diagram of the EVI heat pump system
Fig. 14 EVI heating capacity
18
Fig. 15 EVI heating COP
4. Conclusion This research tested and compared the heating performance of R134a and R1234yf in a heat pump system for an electric vehicle in cold climate under 42 steady-state tests with the goal to evaluate whether the heating performance of the R1234yf can reach that of R134a or even higher to help people replace R134a with R1234yf for the consideration of relieving the global warming effect. The effect of charge, compressor speed, outdoor wind speed, outdoor temperature, indoor flow rate, indoor temperature, inner condenser width, and economized vapor injection were analyzed. We first decided that the optimal charge for R134a was 2050-2800g, and that for R1234yf was 20002700g. The outdoor wind speed had little effect on the heating performance, the improvement was within 5% when the wind speed was increased from 1.5 m s-1 to 4.5 m s-1. The R1234yf system is more suitable for OHX with a smaller pressure drop. When outdoor temperature decreased from 7 to -20℃, the heating capacity decreased by more than 50% and the gap of heating capacity between those two refrigerants also decreased from 12.3% to only 4.1%. As the indoor temperature decreased, the heating capacity first increased and then became smaller. The maximum heating capacity of both R134a and R1234yf occurred at the indoor temperature of around -1℃. But the COP of both R1234yf and R134a systems increased from around 1.5 to 2.5 with the decrease of indoor temperature from 20 to -10℃. The increase in indoor air flow rate would increase the heating performance of both refrigerants. When the air mass flow reached 450 kg h-1, the heating 19
capacity and COP of R1234yf were very close to those of R134a. Also when the air mass flow rate reached 370kg h-1, the heating capacity and COP of R1234yf exceeded those of R134a at 330kg h-1. An increase in air mass flow rate by 10% for the R1234yf heat pump system will make the heating capacity and COP of R1234yf exceed those of R134a. This result demonstrates the feasibility of replacing R134a with R1234yf for an electric vehicle heat pump system in cold climate. The 10% compressor speed increase did not allow R1234yf to obtain the same heating capacity as that of R134a, but the discharge temperature of R1234yf was 2.5-3.5℃ lower than that of R134a under the same compressor speeds, which was beneficial to protect the compressor. Changing the width of the inner condenser by 10% allowed R1234yf to obtain the same heating capacity of R134a with a higher COP, which also proves that it is viable to replace R134a with R1234yf, but condenser width had little influence when beyond 225 mm. When comparing the system performance of R1234yf with EVI and R134a without EVI, we found that R1234yf with EVI had a higher heating capacity. Higher ratio was 8.6%, 3.9%, and 17.7% respectively at the compressor speed of 4000, 5500 and 7000rpm. EVI technology is also a promising method to help replace R134a with R1234yf to relieve the global warming effect. By using EVI technology, increasing the inner condenser's width or increasing the air volume , the heating performance and COP of R1234yf could exceed those of R134a.
Declaration of Interest Statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
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Ziqi Z , Wanyong L , Junye S , et al. A Study on Electric Vehicle Heat Pump Systems in Cold Climates[J]. Energies, 2016, 9(11):881-. ZOU Huiming, HUANG Guangyan, SHAO Shuangquan, et al. Experimental study on heating performance of an R1234yf heat pump system for electric cars[J]. Energy Procedia, 2017, 142:1015-1021.
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Performance evaluation of R1234yf heat pump system for an electric vehicle in cold climate Wanyong Li , Rui Liu , Yusheng Liu , Dandong Wang , Junye Shi , Jiangping Chen PII: DOI: Reference:
S0140-7007(20)30074-8 https://doi.org/10.1016/j.ijrefrig.2020.02.021 JIJR 4680
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
26 September 2019 11 February 2020 13 February 2020
Please cite this article as: Wanyong Li , Rui Liu , Yusheng Liu , Dandong Wang , Junye Shi , Jiangping Chen , Performance evaluation of R1234yf heat pump system for an electric vehicle in cold climate, International Journal of Refrigeration (2020), doi: https://doi.org/10.1016/j.ijrefrig.2020.02.021
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Performance evaluation of R1234yf heat pump system for an electric vehicle in cold climate Wanyong Lia, Rui Liua, Yusheng Liua, Dandong Wanga, Junye Shia,b, Jiangping Chena,b,* a
Institute of Refrigeration and Cryogenics, Shanghai Jiaotong University, Shanghai, China b
Shanghai High Efficient Cooling System Research Center, Shanghai, China
* Corresponding author: [email protected] Tel. + (86) 21 34206775 Abstract To improve the efficiency of mobile air conditioning and to meet the need of protecting the environment, performance characteristics of the R1234yf heat pump were tested under various conditions in a cold climate. R1234yf is considered a promising replacement for R134a because of its low global warming potential (GWP). Many studies have examined the cooling performance of R1234yf, but little attention has been focused on its heating performance in cold climate. In this work, the system performance of R1234yf heat pump was analyzed and compared with that of R134a under different influencing factors such as charge, outdoor temperature, indoor temperature, outdoor wind speed, indoor air mass flow rate, compressor speed, inner condenser width, and economized vapor injection (EVI). The results obtained are presented in detail from the view of experimental analysis and comparison. Keywords: R1234yf; R134a; Heat pump; Economized vapor injection; Electric vehicle; Cold climate Nomenclature GWP global warming potential ODP
ozone depletion potential
EVI
economized vapor injection
SAE
Society of Automotive Engineers
PTC
Positive Temperature Coefficient heater
COP
coefficient of performance
PID
proportion-integral-differential
OHX outdoor heat exchanger Q
heating capacity (W)
W
compressor work (W)
T
temperature (℃)
DB
dry bulb temperature (℃) 1
WB
wet bulb temperature (℃)
M
motor
B
blower
EXV
electric expansion valve
h
enthalpy (kJ·kg-1)
q
mass flow rate (kg·h-1)
c
specific heat (kJ· (kg·K)-1)
Ф
diameter (mm)
Subscripts m
mass
1. Introduction The development of new energy vehicles is a significant trend in the global automobile industry and represents an important opportunity to promote the sustainable development of the world economy and society. Regulation (EU) No 517/2014 prohibited the use of refrigerants with GWP over 150 in mobile air conditioning systems. The Kyoto Protocol (UNFC, 1997) also defined the obligations of both developed and developing countries to cumulatively reduce their greenhouse gas emissions to 7% below 1990 levels for three major greenhouse gases, including carbon dioxide. The refrigerant R134a used in most automotive air conditioners currently in mass production must be replaced due to its GWP (global warming potential), which may be as high as 1430 (Leck, 2009), far higher than the EU standard. Therefore, a substitute product is required. There are several potential replacements, such as CO2, R152a, and propane. Previous studies have studied the use of R152a (Ghodbane, 1999) and CO2 (Lorentzen and Pettersen, 1993) to replace R134a in mobile air conditioning systems. R152a must use a secondary loop for flammability, requiring extra components such as pumps, pipes, a tank, and a radiator. CO2 is a natural refrigerant, but its working pressure is almost 8-10 times higher than that of R134a, which means all the components must be redesigned. Cecchinato et al. (2005) and Minetto et al. (2016) found that CO2's larger density increased the refrigerant mass rate and then boosted the cooling or heating capacity under low temperature conditions for the CO2 system. Wang (2017) found that when both the indoor and outdoor temperatures were −20℃, the CO2 heat pump showed a coefficient of performance (COP) of 3.1 and a heating capacity of 3.6 kW. For security reasons, a secondary loop heat pump was needed which reduced COP by 19%. New technologies such as electrocaloric cooling technologies have also been gradually developed in the past decade. Although it is a new type of 0 GWP 2
refrigeration technology, its heating capacity and COP are still low, which cannot meet the need of automobiles (Junye 2019). Cichong (2018) compared the heating performance of R134a, R1234yf, CO2 system, and propane system at -10℃ and found that the heating performance of R290 was about 80% higher than that of R134a and R1234yf, and 12% higher than CO2. R1234yf is a new refrigerant for automobile air conditioning, with GWP of only 4 and ozone depletion potential (ODP) of 0 (Leck, 2009). The physical properties, operating characteristics, and working performance of R1234yf are very similar to those of R134a (Li, 2019); so application of this refrigerant does not require redesign or changes in many components, facilitating its adoption by the automobile industry. The Society of Automotive Engineers (SAE) defined the safety level of R1234yf as A2L (SAE, 2010), non-toxic and weakly flammable, supporting its global use as a replacement refrigerant in mobile air conditioning systems. In cold weather, heating in traditional automobiles is provided by waste heat of engine cooling water, but electric vehicles do not have an engine that can meet the heating demand of the cabin (Zhang Z, 2018). The Positive Temperature Coefficient heater (PTC) for air conditioning of electric vehicles will cause drive range attenuation of 50-60%, which greatly hinders the application and promotion of electric vehicles (Eric, 2014). To improve the energy efficiency and range of air conditioning systems for electric vehicles, heat pump technology has been developed. Choi et al. (2017) found that for electric vehicles, the economized vapor injection (EVI) technology was very necessary under low temperature environment and under the same air outlet temperature (36 ℃), it improved the system heating capacity by 76% (from 1351W to 2387W). Zhang et al. (2016) tested the R134a vehicle heat pump system at low temperature, and found that the heating performance could be increased by 44.1% at -10℃ by use of vapor injection technology. Wang et al. (2018) analyzed the superheat degree for different evaporation temperatures (-20~10℃) and different condensation temperatures (40-60℃). The results showed coefficient of performance (COP) and heating capacity values of the R134a system were 1.39-4.06% and 0.53-4.08% higher, respectively, than those of the R1234yf system. Zhao et al. (2012) studied the performance of an automobile air conditioning system using a micro-channel parallel flow condenser together with R134a and R1234yf. The results showed that the charge of R1234yf was reduced by about 5% compared to that of R134a and the cooling capacity and COP of the system using R1234yf were about 12.4% and 9% lower respectively, due to differences in the thermophysical properties. Navarro (2013) found that systems using R1234yf had COP values that were about 19% lower than those obtained using R134a, with only minor differences at higher 3
condensing temperatures. Qi (2013) concluded that R134a displayed better heat transfer and flow performance than those of R1234yf in a laminated plate evaporator due to thermophysical and transportation properties under both low and high refrigerant pressure at the expansion valve inlet. The main thermophysical properties of R1234yf are shown in Table.1 and compared with those of R134a. The pressure and temperature conditions of R1234yf are very similar to those of R134a. The NIST REFPROP 8.0 (Lemmon et al., 2002) calculation was applied and the conditions of these two refrigerants circulation are very similar, which is a requirement for R1234yf to replace R134a (Zhao, 2012). Fig. 1 also shows the vapor density of R1234yf and R134a under different evaporator temperatures. Under the same displacement of the compressor, higher vapor density of the refrigerant means larger mass flow rate. The density of R1234yf is greater than that of R134a by 9.1% at 60℃ and by 27.1% at -20℃. The lower the temperature, the greater the ratio of difference in the vapor density of R1234yf and R134a. Table 1. Main thermophysical properties of R1234yf and R134a Chemical formula Molecular mass GWP Critical temperature /℃ Critical pressure /kPa Boiling point /℃
R1234yf CF3CF=CH2 114.04
R134a C2H2F4 102.03
4 94.7
1430 101.1
3382 -29.4
4059 -26.1
Fig. 1 Saturation pressure and Vapor density of R1234yf vs. R134a
4
In low temperature environments, the evaporation temperature of the system may reach -25℃ or even lower. Under this condition, R1234yf has two advantages over R134a: a lower evaporation pressure and a higher vapor density. Fig.1 compares the saturation pressure curves of R1234yf and R134a. The presented data shows a higher saturation pressure of R1234yf than that of R134a at temperature below 41.3℃. If the evaporation pressure of the heat pump system is lower than the atmospheric pressure, the air may enter the system and cause irreversible damage. When the evaporation pressure is equal to the atmospheric pressure, the saturation temperature of R134a is 26.3℃ and that of R1234yf is -29.8℃. Therefore, in extreme conditions, the R1234yf system can withstand lower evaporation temperatures than R134a, which can make the system more reliable. Additionally, under the same environment and temperature, R1234yf can match to higher compressor speeds than the R134a system. Jarall (2012) studied the cooling performance of R1234yf system and R134a system from the theoretical level, and found that the pressure ratio, COP and refrigeration performance of R1234yf were lower. Lee Y (2012) found that the drop-in test result of the R1234yf system was 0.8-2.7% lower than the R134a system, the cooling capacity was reduced by 4%, and the discharge temperature was 6.4-6.7 ℃ lower. Kashif (2017) built a system model that could evaluate the potential of R1234yf and R1234yfze(E) to replace R134a for the residential heat pump water heaters, and found that R1234yf and R1234ze(E) could be substituted for R134a with comparable performance and no big change to the original system. Zilio (2011) found that the cooling performance of R1234yf was better than R134a by increasing the heat transfer area of the condenser and evaporator. Feng and Hrnjak (2018) tested the performance of R1234yf and R134a vehicle heat pumps in a low temperature environment. The results showed that the heating capacity of the R1234yf system was 28% higher than that of R134a at -20℃ ambient temperature because the R1234yf system could match the compressor operation at higher speeds. Many researchers have made in-depth investigations on the cooling performance of R1234yf, and also extensively researched the cooling and heating performance of R134a; however, the lowtemperature heating performance of R1234yf lacks the attention of researchers, especially below 10℃. This paper will conduct experimental research on the low-temperature heating performance of ground R1234yf, analyze the effects of various factors on R1234yf, and systematically display the heating characteristics and rules of R1234yf at low temperatures . 2. Experimental set-up
5
i) 2.1 Test apparatus The system was installed in two climatic chambers as shown in Fig. 2. The indoor condenser was installed in the indoor chamber and the other parts of the system were installed in the outdoor chamber. The climatic chambers allowed the control of the air temperature and humidity with a proportion-integral-differential (PID) controller. The velocity of the outdoor heat exchanger (OHX) was controlled by an axial flow fan. The wind tunnels were used to regulate air mass flow rate and measure the inlet and outlet air dry bulb (DB) and wet bulb (WB) temperatures. The rotation speed of the electric compressor was controlled using a controller provided by Shanghai Shiyu Co., Ltd. Refrigerant side pressure and temperature sensors were positioned at the inlet and outlet of each component. The mass flow rate of refrigerant was measured using Coriolis mass flow meter. The types and precision of each sensor were listed in Table. 2. The wind tunnel inlet temperature (t1), outlet temperature (t2), and the air mass flow rate (qm) were measured and the heating capacity was calculated. Compressor input power (W) was regulated by the power supply current (I) and voltage (U). Based on the uncertainty calculation solution proposed by Moffat (1998), the measurement uncertainty of the heating capacity Q and COP are ±2.2% and ±2.4%. Q= qmc(t2- t1)
(1)
W=IU
(2)
COP=Q/W
(3)
Table 2. Sensor type and precision Measurement parameter
Range
Precision
K-type thermocouple Piezoresisitive pressure Coriolis sensor mass flow
-50-200
±0.1
0-4000 0-200
±5 ±0.5
0-100
±0.25
Current /A
Coriolis meter mass flow Ammeter meter
0-25
±0.01
Voltage /V
Voltmeter
0-400
±0.01
Nozzle pressure difference/Pa
Yokogava air pressure
-500-500
±0.75
Temperature /℃ Pressure /kPa -1
Mass flow rate 1/(kg·h ) Mass flow rate 2 /(kg·h-1)
Sensor type
meter
6
Fig. 2 Schematic diagram of the test apparatus Data for the system components are listed in Table 3. The OHX and the evaporator used in this test rig are aluminum micro-channel parallel flow heat exchangers. We used four inner condensers with different widths (189/207/225/240mm) and three electronic expansion valves (EXVs) in the system. The EXVs are driven by stepping motors and their opening can be regulated from 50 to 480 pulse. They also include the heating EXV and the EVI EXV. The inner diameters of the two electronic expansion valves are very different. The heating electronic expansion valve can be fully opened during cooling mode to ensure the smooth passage of the refrigerant without increasing the resistance. The refrigerant mass flow rate of the EVI is relatively small (less than 50kg·h-1), so EVI EXV with a smaller inner diameter (0.65 mm) was used. Electronic expansion valves have different control strategies. Due to the gas-liquid separator , the heating EXV cannot control the superheat degree of the compressor suction. Its control target is to adjust the subcooling degree of the refrigerant at the outlet of the inner condenser to 10 ℃. The target of the EVI EXV is to control the superheat degree of the refrigerant entering the compressor's inject port to be 2 ℃ (Qin 2015). A separator of 1000ml is installed at the inlet of the compressor to serve as a system refrigerant buffer. The pictures of main components are shown in Fig. 3. Table 3. System Component Information Components
Information 27cc/r. 1000-8500rpm.
Compressor scroll compressor, 350V Outside Heat
microchannel parallel flow heat exchanger,
Exchanger
660×500×16(mm)
7
microchannel parallel flow heat exchanger, Inner Condenser
Case 1:189×125×27(mm) Case 2:207×125×27(mm) Case 3:225×125×27(mm) Case 4:240×125×27(mm) microchannel parallel flow heat exchanger,
Evaporator 232×239.5×38(mm） Economizer
150×80×80(mm)，8 plates
EXV for heating
Ф9.2(mm)
EXV for vapor Injection
Ф0.65(mm)
EXV for cooling
Ф5.5(mm)
Fig. 3 Main components
ii) 2.2 Test methods In this research, the effects of different influencing factors on the heat pump system performance were studied experimentally, including charge, outdoor temperature, indoor temperature, outdoor wind speed, indoor air flow rate, compressor speed, inner condenser width, and EVI. Table 4 shows the complete test conditions. The optimal charge for the heat pump system was determined first under Test 1, in which the ambient temperatures of the indoor and outdoor environments were relatively high, resulting a high refrigerant pressure and further led to a high 8
refrigerant density, which met the system requirement for use of the largest amount of refrigerant. Tests 2-5 were designed to investigate the influence of outdoor wind speed on system performance. The determination of wind speed is based on the driving conditions of the car. From 1.5 to 4.5m·s-1, the corresponding driving conditions of the car are from idle to 80 km h-1. Tests 6-36 were designed to investigate the influence of the outdoor temperature, indoor temperature, outdoor wind speed, indoor air flow rate, compressor speed, and inner condenser width on the system performance. The maximum indoor temperature of the car was set to 20℃ and the minimum outdoor temperature was set to -20℃. We also analyzed the effect of economized vapor injection (EVI) on the system performance under different compressor speed with Tests 37-42. Table 4. Test conditions Outdoor Temperature
No.
(℃)
OHX wind
Compressor speed
speed (m·s-1)
(rpm)
4.5
Indoor Temperature (
Inner condenser
EVI
·h-1)
5500
20
330
Case1
N
5500
7
330
Case1
N
330
Case1
N
330
Case1
N
Case1
N
7
2-5
7
6-9
7/0/-10/-20
4.5
5500
20
10-14
-10
4.5
5500
-10/-1/7/13/20
5
(kg
℃)
1
1.5/2.5/3.5/4.
Indoor air mass flow rate
330/370/410/45
15-18
-20
4.5
5500
20
19-21
-20
4.5
4000/5500/7000
20
330
Case1
N
22-25
-20
4.5
4000
20
330
Case1/2/3/4
N
26-29
-20
4.5
5500
20
330
Case1/2/3/4
N
30-33
-20
4.5
7000
20
330
Case1/2/3/4
N
34-36
-10
4.5
4000/5500/7000
20
330
Case1
N
37-39
-10
4.5
4000/5500/7000
20
330
Case1
Y
40-42
-20
4.5
4000/5500/7000
20
330
Case1
Y
0
3. Experimental Results and Discussion 3.1 The effect of charge The optimal charge for the system was determined first. The system was operated under Test 1 in Table 4. The starting charge point was set to 1200 g and then refrigerant was charged into the system in a 75-100 g increment until reaching suitable heating capacity. Unlike traditional air conditioning systems, the refrigerant of the heat pump system is stored in the gas-liquid separator in front of the compressor, resulting in almost zero suction superheat degree.
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Refrigerant level in gasliquid separator
Fig. 4 R134a system characteristics under different charge
Refrigerant level in gasliquid separator
Fig. 5 R1234yf system characteristics under different charge According to Fig. 4 and Fig. 5, the characteristics of the R134a system were very similar to those of R1234yf system. Taking R1234yf (Fig. 5) as an example, when the charge was relatively low (less than 2000g), the compressor discharge pressure and suction pressure were gradually rising. Under these conditions, the subcooling degree was almost 0℃ which means that gas phase refrigerant passed through the expansion valve. This caused a relatively low refrigerant mass flow rate, thus the system heating capacity was very low. The superheat degree gradually decreased from more than 30 10
℃ to almost 0℃ . When the compressor suction superheat was 0℃, the liquid refrigerant gradually increased through the transparent casing of the gas-liquid separator (Fig. 3) . With the increase of the refrigerant, the suction pressure became stable at 0.25Mpa, the discharge pressure reached 2.3MPa, and the performance was gradually improved. When the system charge reached 2000g, the system heating capacity was stable at around 3300W. Liquid refrigerant would be filled with gasliquid separator when the charge was 2700g and liquid phase refrigerant will enter the compressor, causing a dramatic decrease in heating capacity and an obvious drop in discharge pressure. Dan (2019) reported a similar phenomenon for a mobile heat pump system. Therefore, the suitable charge is 2000-2700 g for the R1234yf system. For the R134a system, the optimal charge ranges between 1900-2800 g, with an optimal heating capacity of 3660 W. The optimal charge of the system can be determined by the heating capacity, pressure, and degree of subcooling and superheating. 3.2 The effect of outdoor wind speed The speed of an electric vehicle changes greatly during running. During idling of the vehicle, only the condensing fan causes air to pass through the outdoor heat exchanger, and this wind speed is about 1.5 m·s-1. When the vehicle speed is increased to 80 km·h-1, the wind blowing through the outdoor heat exchanger can reach 4.5m·s-1 after the wind passes through the air intake grille. When we changed outdoor wind speeds according to Tests 2-5 (Table 4) , the heating capacities of R134a system were 3419W, 3563W, 3596W, and 3660W respectively, which was improved by 1.4~4% with the increase of wind speed by 1 m·s-1. For R1234yf system under the same conditions, the heating capacities were 3238W, 3281W, 3301W, and 3336W, which was improved by 0.6~1.3% with the increase of wind speed by 1 m·s-1. The performance of each system was slightly different under different outdoor wind speed. The R1234yf lifting ratio was slightly smaller than that of R134a. Fig. 6 showed the outlet temperature and pressure drop of the outdoor heat exchanger. The outdoor heat exchanger of R1234yf has an evaporation temperature that was 1.3 ℃ higher than that of R134a, resulting a larger mass flow rate for R1234yf, since the density of R1234yf was bigger than that of R134a. Additionally, the pressure drop increased from 87 kPa to 107 kPa for R1234yf and from 70 kPa to 80 kPa for R134a respectively, with the increase of outdoor wind speed.
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Fig. 6 Outlet temperature and Pressure drop of OHX under various wind speeds 3.3 The effect of outdoor temperature When the outdoor temperature decreased from 7℃ to -20 ℃, the evaporation temperature decreased, then the refrigerant density decreased, so the mass flow rate of both refrigerants declined a lot. Fig. 7 showed the effect of outdoor temperature on the refrigerant mass flow rate and heating capacity under Tests 6-9 (Table 4) . The mass flow rate of R1234yf decreased from 110.18 kg·h-1 to 42.2 kg·h-1, that of R134a decreased from 94.52 kg·h-1 to 36.07 kg·h-1. Overall, the mass flow rate of R1234yf was 15.1-21.1% larger than that of R134a. The decreased mass flow rate also directly caused a rapid decay in the system heating capacity, which decreased from 3300 W to 1521 W, by 57.0% for R134a and from 3374 W to 1622 W, by 53.9% for R1234yf. The average rate of decline for R134a was 79.7W·℃-1 and the rate for R1234yf was 65 W·℃-1. The gap between the heating capacity of R134a and R1234yf decreased when the outdoor temperature decreased, from the initial value 12% to only 4%. The calculation supported our objective to replace R134a with R1234yf in the heat pump system for an electric vehicle in cold climate. Even though there is still a gap between the heating capacity of R1234yf and R134a, but we can change other parameters to improve the heating capacity of R1234yf to make it reach that of R134a or even higher, which will be discussed in the following sections. Fig. 8 presented the P-h diagram of R134a and R1234yf under different outdoor temperatures. The only assumption in the system was that the throttling process is considered an isentropic process. As shown in the Fig. 8, when the outdoor temperature decreased, the evaporation temperature gradually decreased. But the difference between the outdoor temperature and the evaporation temperature was maintained at 5-10 ℃ over the range tested. The P-h diagram also showed that the decrease of the outdoor temperature caused a 12
decrease in the indoor condensing pressure, although the indoor temperature and air volume remained unchanged.
Fig. 7 Mass flow rate and Heating capacity under different outdoor temperature
Fig. 8 P-h diagrams of R134a and R1234yf for different outdoor ambient temperatures 3.4 The effect of indoor temperature The change of indoor temperature will greatly change the condensation pressure. Fig. 9 shows the indoor temperature effect on system performance under Tests 10-14, Table 4. Indoor temperature had little effect on the evaporation pressure in the heat pump system. Under these five conditions, the condensation pressure increased with indoor temperature as shown in Fig. 9, but the evaporation pressure kept almost stable around 0.15 MPa. So the compression ratios calculated were 3.2, 5.9, 6.3, 7.0, and 7.4 (for R134a) and 3.0, 5.6, 6.0, 6.4, and 6.8 (for R1234yf). The pressure ratio of R134a is a little bit higher than that of R1234yf. Cuevas (2009) and Sun (2010) reported that maximum isentropic efficiency of the process at a pressure ratio of 4.0. When the pressure ratio is smaller or bigger than the peak value, over-compression or insufficient compression could occur, which would decrease the isentropic efficiency. The COP also decreased rapidly because the power 13
consumption of the compressor was directly related to the pressure ratio. Therefore, the heating capacity should be optimal when the ratio of the condensing pressure to the evaporation pressure is closer to the optimum pressure ratio of the compressor design. According to Fig. 9, the maximum heating capacity of both R134a and R1234yf occurred at the indoor temperature of around -1℃. But the COP of both R1234yf and R134a systems decreased a lot with the increase of indoor temperature.
Fig. 9 The effect of indoor temperature on the system performance 3.5 The effect of indoor air mass flow rate The increase in indoor air mass flow rate can also enhance the heat exchange between the refrigerant and the air in the inner condenser. Fig. 10 shows the effect of indoor air mass flow rate on the system heating capacity and COP under Tests 15-18, Table 4. The heating capacities were 1621W, 1750W, 1800W, and 1820W for R134a and 1520W, 1687W, 1780W, and 1815W for R1234yf respectively, when the indoor air mass flow rates were 330, 370, 410 and 450 kg·h-1. As the air mass flow rate increased, the heating capacity gradually improved and also the COP increased. When the air mass flow reached 450 kg·h-1, the heating capacity and COP of R1234yf were very close to those of R134a, which were 1820W, 1.90 and 1830W, 1.93 for R134a and R1234yf respectively. It demonstrated the feasibility of replacing R134a with R1234yf for an electric vehicle heat pump system in cold climate. Also when the air mass flow rate reached 370 kg·h-1, the heating capacity and COP of R1234yf exceeded those of R134a at 330 kg·h-1. As can be seen from Fig. 10, for the R1234yf heat pump system, the increase in air mass flow rate by 10% will make the heating capacity and COP exceed those of R134a at previous unchanged condition. But excessive air mass flow also will cause excessive noise in a car, which adversely affects passengers' comfort.
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Fig. 10 Heating capacity and COP under different air mass flow rates 3.6 The effect of compressor speed The speed of an electric compressor can be changed and can reach 12000 rpm. Fig.11 shows the performance of heat pump systems with R134a and R1234yf at different compressor speeds under Tests 19-21, Table 4. At speeds of 4000, 5500, and 7000 rpm, the R1234yf and R134a heating capacities were 1154W, 1521W, and 1682W and 1280W, 1622W, and 1771W respectively. The heating capacity of R1234yf was 9.93%, 6.23%, and 6.5% lower than that of R134a. When we increased the compressor speed of R1234yf system while did not change the compressor speed of R134a system and then compared their heating capacities, we found the gap of system heating capacity between these two refrigerants decreased. As the speed was increased by 10% for R1234yf ( 4400, 6050 and 7700 rpm), the heating performance of was 5.56%, 2.5%, and 2.2% respectively lower than that of R134a (4000, 5500 and 7000 rpm). But the results proved that increasing compressor speed by 10% for R1234yf did not achieve the same heating capacity as that of R134a working at previous unchanged compressor speed. Fig. 11 also shows the COP at different speeds. The COP of R1234yf is lower than that of R134a by 6.5% to 9.5% at compressor speed from 4000 to 7700 rpm, and the decrease of COP is obvious with the increase of speed. Even though increasing compressor speed by 10% for R1234yf did not make the heating capacity of R1234yf equal to that of R134a working at previous unchanged compressor speed, but we observed another beneficial result that the compressor discharge temperature of the R1234yf system was 2.5-3.5℃ lower than that of the R134a system at the same speed, which protected the compressor.
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Fig. 11 Heating capacity and COP under different compressor speeds 3.7 The effect of inner condenser The inner condenser of an electric vehicle is placed in the car to warm the air. In this section, we tested four inner condensers under Tests 22-33, Table 4. They differ only in width, with other features exactly the same: Case 1: 189 × 125 × 27 (mm), Case 2: 207 × 125 × 27 (mm), Case 3: 225 × 125 × 27 (mm), and Case 4: 240 × 125 × 27 (mm). It can be seen from Fig. 12 that for the same inner condenser (Case 1), the heating capacity of R1234yf was lower than that of R134a. When the compressor speeds were 4000, 5500, and 7000 rpm, it was 1154W, 1521W, and 1597W for R1234yf, and it was 1280W, 1622W, and 1771W for R134a. But the heating capacity of R1234yf with Case 2 was comparable to that of R134a using Case 1, the gap of difference was within only ±2%. When the width of the inner condenser was increased by 10% for R1234yf but was kept unchanged for R134a, the heating capacity of R1234yf and R134a were substantially the same. However, the heating capacity of R1234yf with Case 4 was slightly improved by 1.9-2.3% compared with the one using Case 3, suggesting that Case 3 has fully utilized the heating potential of the system. The inner condenser width over 225 mm will have little influence on the heat pump system performance improvement. What’s more, as shown in Fig. 12, the COP of R1234yf with Case 1 was 5.5-8.5% lower than that of R134a with Case 1, but the COP of R1234yf with Case 2 was 4.1- 6.5% higher than that of R134a using Case 1. The results show that increasing the inner condenser width within a proper range can improve the heating capacity and COP of the R1234yf system to make them equal to or even higher than those of the R134a system working under previous inner condenser width. 16
Fig. 12 Heating capacity and COP under different inner condenser width 3.8 Economized vapor injection technology Several studies have shown that vapor injection (VI) can be used to improve the heating capacity at extremely cold temperatures. There are two types of VI, flash tank vapor injection and economized vapor injection (Xu X, 2011). Wang (2009) found that the flash tank system may have better performance than the economized system, but this conclusion was not widely accepted. Wang (2008) and Ma and Zhao (2008) concluded that the heating capacity and COP of the flash tank system were higher than those of the economizer system. Zhang (2016) demonstrated that use of an air-source heat pump with the EVI technique could improve the thermal performance by 4–6% compared to that without EVI. EVI technology was applied here in this research, and the cycle schematic diagram is shown in Fig. 13. In the EVI system, Refrigerant is divided into two ways after coming from the condenser, one way flows through the expansion valve and the other flows directly into the economizer. Inside the economizer, vapor refrigerant is injected into the compressor and subcooled liquid flows into the outside heat exchanger. Fig. 14 and Fig. 15 show the heating capacity and COP at -10℃ and -20℃ for Tests 19-21 and 34-42, Table 4. The heating performance of the R1234yf heat pump system with EVI technology has improved by 27.4% and 19.9% on average at -10 ℃ and -20℃, respectively, compared to systems without EVI technology; for R134a, the average heating performance was improved by 27.1% and 17.1% at -10℃and -20℃, respectively. The heating capacities of systems without EVI for R1234yf and R134a at -20℃were 1153W, 1521W, 1573W and 1280W, 1560W, 1771W respectively at compressor speed of 4000, 5500, 7000 rpm. But after implementing EVI technology, the heating capacities were increased to 1391W, 1622W, 2085W 17
and 1457W, 1720W, 2251W respectively with an improvement of 20.6%, 6.6%, and 32.5% for R1234yf and 13.8%, 10.3%, and 27.1% for R134a. The heating capacities of R1234yf and R134a were greatly improved at high compressor speeds. The COP of both R134a and R1234yf decreased with the increasing of compressor speed. But the COP of R134a was higher than that of R1234yf. When comparing the system performance of R1234yf with EVI and R134a without EVI, we found that R1234yf with EVI had a higher heating capacity. Higher ratio was 8.6%, 3.9%, and 17.7% respectively at the compressor speed of 4000, 5500 and 7000rpm.
Fig. 13 Schematic diagram of the EVI heat pump system
Fig. 14 EVI heating capacity
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Fig. 15 EVI heating COP
4. Conclusion This research tested and compared the heating performance of R134a and R1234yf in a heat pump system for an electric vehicle in cold climate under 42 steady-state tests with the goal to evaluate whether the heating performance of the R1234yf can reach that of R134a or even higher to help people replace R134a with R1234yf for the consideration of relieving the global warming effect. The effect of charge, compressor speed, outdoor wind speed, outdoor temperature, indoor flow rate, indoor temperature, inner condenser width, and economized vapor injection were analyzed. We first decided that the optimal charge for R134a was 2050-2800g, and that for R1234yf was 20002700g. The outdoor wind speed had little effect on the heating performance, the improvement was within 5% when the wind speed was increased from 1.5 m s-1 to 4.5 m s-1. The R1234yf system is more suitable for OHX with a smaller pressure drop. When outdoor temperature decreased from 7 to -20℃, the heating capacity decreased by more than 50% and the gap of heating capacity between those two refrigerants also decreased from 12.3% to only 4.1%. As the indoor temperature decreased, the heating capacity first increased and then became smaller. The maximum heating capacity of both R134a and R1234yf occurred at the indoor temperature of around -1℃. But the COP of both R1234yf and R134a systems increased from around 1.5 to 2.5 with the decrease of indoor temperature from 20 to -10℃. The increase in indoor air flow rate would increase the heating performance of both refrigerants. When the air mass flow reached 450 kg h-1, the heating 19
capacity and COP of R1234yf were very close to those of R134a. Also when the air mass flow rate reached 370kg h-1, the heating capacity and COP of R1234yf exceeded those of R134a at 330kg h-1. An increase in air mass flow rate by 10% for the R1234yf heat pump system will make the heating capacity and COP of R1234yf exceed those of R134a. This result demonstrates the feasibility of replacing R134a with R1234yf for an electric vehicle heat pump system in cold climate. The 10% compressor speed increase did not allow R1234yf to obtain the same heating capacity as that of R134a, but the discharge temperature of R1234yf was 2.5-3.5℃ lower than that of R134a under the same compressor speeds, which was beneficial to protect the compressor. Changing the width of the inner condenser by 10% allowed R1234yf to obtain the same heating capacity of R134a with a higher COP, which also proves that it is viable to replace R134a with R1234yf, but condenser width had little influence when beyond 225 mm. When comparing the system performance of R1234yf with EVI and R134a without EVI, we found that R1234yf with EVI had a higher heating capacity. Higher ratio was 8.6%, 3.9%, and 17.7% respectively at the compressor speed of 4000, 5500 and 7000rpm. EVI technology is also a promising method to help replace R134a with R1234yf to relieve the global warming effect. By using EVI technology, increasing the inner condenser's width or increasing the air volume , the heating performance and COP of R1234yf could exceed those of R134a.
Declaration of Interest Statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
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