Air Conditioning System Sizing for Pure Electric Vehicle

Transcription

1 EVS28 KINTEX, Korea, May 3-6, 2015 Air Conditioning System Sizing for Pure Electric Vehicle Bongha Song 1, Jiwon Kwon 1, Yongsuk Kim 1 1 General Motors Korea Company, Cheongcheon-Dong, Bupyeong-Gu, Incheon, Korea , bongha.song@gm.com Abstract Emission standards have grown increasingly stricter, consequently triggering greater interest in issues surrounding environmental pollution. The customer needs for green car without toxic exhaust gas has also become natural and it is represented by pure EV (Electric Vehicle). However, the driving range of EV suffers from A/C (Air Conditioning) for occupant comfort especially in very hot or cold weather. Therefore, the sizing of A/C system is more important than the case of conventional internal combustion engine vehicle. Specifically, EV consumes electric power of battery pack in order to control the comfortability of passenger compartment due to not only the absence of engine heat rejection for interior heating, but no working power of compressor from engine for interior cooling. Therefore, energy consumption for cabin cooling/heating must be optimized for the energy efficiency of EV. In this paper, we concentrate on the refrigerating system sizing because indoor temperature in hot weather is more sensitive to passenger s thermal comfort. Keywords: Electric Vehicle, Refrigeration System, e-thermal, Electric Compressor, Driving Range 1 Introduction In recent, car industry has been faced with the higher request from market for a vehicle that is showing better fuel economy and eco-friendly features since the needs of this vehicle has been strongly motivated by high oil prices and stringent emission standards. In general, EV is using 70~85% of the electric power charged in the battery while the combustion energy of fossil fuel is mostly disappeared in the way of heat dissipation. Hence, EV is superior to internal combustion engine vehicle with regards to the energy efficiency itself. In addition, not only is EV known to be the representative for practical eco-friendly vehicle not due to producing carbon dioxide as the biggest cause of global warming, but also the customer s interest for green car without toxic exhaust gas is gradually on the rise. Therefore, almost of global car manufacturers are vehemently competing to predominate the market of EV through intensive research and development for future, and the Korean Government has also announced various pan-governmental policies to vitalize the effective distribution of EV [1]. However, the maximum vehicle speed for EV is lower than conventional engine vehicle, and also available driving range per a single charging is one of key parameters that are affecting the marketability of EV. The improvement of these weaknesses can significantly activate current EV market. Specifically, A/C system to cool down or warm up the cabin for thermal comfort influences directly the decrease of driving range, and it s important to make a proper sizing of refrigerating and heating systems in EV [2]. Since A/C system for EV uses battery power to operate the interior cooling and heating systems, operating time and mode for A/C system impact considerably on an EVS28 International Electric Vehicle Symposium and Exhibition 1

2 available driving range and especially in a severe operating mode for a long time. After all, driving range could be drastically damaged by the operation of A/C system. Recent researches tell us that it could be decreased by 50% of driving range when turning on A/C system in EV. Thus, A/C system is very important for EV development and it is imperative to design an HVAC system optimized for less electric power consumption [3]. And also the car has been becoming more to a special life space than just a mode of transportation. Consequently, passengers use most frequently A/C system for comfort cabin, and request better refrigerating and heating performances. Specifically, heating system for EV, it forms the highest battery power consumption rate, and many researchers are under study to develop an appropriate one for EV. Refrigerating system needs less electric power than heating system but it s very sensitive to passenger s satisfaction from interior thermal comfort. t only ride quality but also comfortable driving atmosphere could not easily be ignored by each car manufacturer due to the customer s needs for indoor fresh. In this research, we perform several studies to lay out a proper refrigerating system to secure maximum driving range per a single charge. 2 CAE Tool and BC (Boundary Condition) 2.1 CAE Tool In this study, we uses e-thermal which is developed to determine the overall performance of thermal subsystem in a vehicle such as powertrain cooling (PTC) and HVAC systems. It s built on the commercial solver for thermal and fluid, SINDA/FLUINT, which is generalized tool for simulating complex thermal/fluid systems such as those found in the automotive, electronics, petrochemical, turbo-machinery, and aerospace industries. SINDA/FLUINT is a comprehensive finite difference, lumped (circuit or network analogy) tool for heat transfer design and fluid flow modeling [4]. In addition to a number of system models written in SINDA/FLUINT, e-thermal also includes a database to store the information of the performance data from suppliers for thermal fluid devices such as heat exchangers, compressor, water pump, etc. It has been fully integrated into the thermal system design process for production in GM. We can summarize the characteristics of e- Thermal as follows; Heat exchanger sizing tool Heat transfer models of engine coolant, engine oil, transmission oil, CAC circuits Models for HVAC (refrigerant & ) systems and front end flow HVAC/PTC component database Used for component, sub-system or full vehicle analysis The interior cooling/heating models in e-thermal are represented by detailed transient, 1-D/pseudo 2-D, thermal-hydraulic models that simulate all important physics of a refrigeration/heating system. It solves for flow and energy conservation to predict pressure, temperature, heat transfer throughout the system. e-thermal is possible to model a refrigerating system that is composed of the combination of each component such as compressor (mechanical or electrical), condenser (-cooled or water-cooled), evaporator, chiller (refrigerant or mixture of glycol/water), internal heat exchanger (IHX), and expansion devices (TXV - Thermostatic Expansion Valve). Pressure loss and heat transfer for plumbing system are modelled as well. The cooling system for EV battery is linked together to a refrigerating system not to be overheated when charging or driving. On the other hand, battery cooling system could be connected to a refrigeration system with a coolant system for the battery like the one in Chevy Volt [5]. Figure 1 is the schematic diagram for the refrigeration system sizing. Interior cooling system is assumed by following ideal vapor compression refrigerating cycle described in Figure 2. When determining the sizing of a refrigerating system, we first set the temperature passing through an evaporator. This is named as target temperature, and then other components are sized to meet this requirement through each step of Figure 1. Capacity m ( h heva out) (1) where, m h h eva out Massflowrat eof inlet Enthalpy of inlet Enthalpy of evaporat or outlet EVS28 International Electric Vehicle Symposium and Exhibition 2

3 Condensate m ( MR MReva out) (2) where, m MR MR eva out Massflowrat eof inlet Moist urerat ioof inlet Moist urerat ioof evaporat or outlet Using Formula (1) and Formula (2), evaporator s capacity and condensate flow rate are calculated and these become basic inputs needed to get ambient to cool down to target temperature defined at evaporator outlet. Following is a brief description for using the refrigeration cycle tool included in e-thermal. We define the compressor speed, maximum, and minimum refrigerant pressures for operating a compressor selected from the e-thermal database. Next is to set up the ambient temperature coming into the condenser, the high and low end temperatures of refrigerant at super-heating and sub-cooling in respective. These are all information to execute the refrigeration cycle tool. When we complete whole process described above, the capacity or working power of each component (evaporator, condenser, and compressor), refrigerant temperature, etc. are calculated. The compressor speed can be controlled to meet the target temperature. Change evaporator inlet condition on % recirculation mode Is recirculation mode available? Identify for a given vehicle, - AC compressor - Zero body flow in vent mode - Air inlet and discharge temperature requirements Exercise the refrigeration cycle tool, - Select compressor and set its displacement - Set all pressure drops and refrigerant temperature rise to zero (Ideal cycle) - Air inlet and discharge temperature requirements Is condenser out refrigerant temperature meeting the requirements on each operating condition? Is target evaporator discharge temperature achieved? Output, - Evaporator discharge temperature - Evaporator refrigerant in/out temperature - Evaporator capacity - Condenser capacity - Compressor work Change condenser subcool Exercise this option only if change in compressor pulley ratio is feasible Increase compressor rpm? Figure 1: Sizing Process of a Refrigerating System [6] 2.2 BC (Boundary Condition) The vehicle operating condition to determine the sizing of a refrigerating system for EV is listed in Table 1. Vehicle speeds are represented by idle, 50 and 80 km/h, and considered ambient temperature for summer season. Ambient temperature and relative humidity on 50 and 80 km/h are assumed as 38 C and 40%, respectively. 28 C and 50% for idle condition. Table 1: Vehicle Driving Condition Vehicle Speed [km/h] Temperature [ C] Relative Humidity [%] Idle Figure 2: AC System Components and Ideal Vapor Compression Refrigeration Cycle [6] EVS28 International Electric Vehicle Symposium and Exhibition 3

4 3 Component Sizing of Refrigera ting System Vehicle HVAC system consists of A/C control module, passenger compartment, and handling systems. In the past, HVAC system was designed to satisfy a peak demand from customers like the rapid cool-down of cabin after hot soaking. However, the needs of proper A/C system sizing is known to be very important to improve extremely the energy efficiency of EV. In this section, the plausible sizing of each component consisting refrigerating system for EV is performed, and finally evaluate the feasibility of a refrigerating system sized for next generation EV from GM from driving distance impact perspectives. evaporator capacity and condensate flow rate are calculated as well. In the case of idle, 3.8 kw and 0.6 g/s are the evaporator capacity and condensate flow rate, respectively. On 50 and 80 km/h of vehicle speed, 6.8 kw and 1.2 g/s for evaporator capacity and condensate flow rate, respectively. Using this information, the sizing of compressor and condenser is performed in next section. 3.1 Evaporator Sizing First, the temperature in cabin is defined by the temperature feeling comfort and the temperature through the evaporator is assumed to be increased considering the reality until it comes to passenger s breath area. Figure 3 shows the procedure to determine the evaporator load [6]. Figure 4: Psychrometric Chart for 28 Ambient Temperature and 50% Relative Humidity on Idle - Identify zero body flow - Air inlet and evaporator discharge temperature for module - Run the Psychrometrics tool in e-thermal - Obtain enthalpy and moisture ratio at inlet and outlet conditions - Compute evaporator load - Compute condensation generated Figure 3: Process to Determine Evaporator Load For evaporator sizing, enthalpy and moisture ratio on those conditions listed in Table 1 can be calculated using psychrometric chart provided by e-thermal. Figure 4 and Figure 5 show the psychrometric chart at each ambient condition. Enthalpy and moisture ratio for the ambient condition on idle are 58.9 kj/kg and 12.1 g/kg, respectively. In the same way, 83.0 kj/kg and 17.4 g/kg are enthalpy and moisture ratio for the ambient condition on the vehicle speed of 50 and 80 km/h. Enthalpy and moisture ratio on the required condition for interior cooling are 29.3 kj/kg and 7.6 g/kg, respectively. Using the enthalpy and moisture ratio calculated, Figure 5: Psychrometric Chart for 38 C Ambient Temperature and 40% Relative Humidity on 50/80 km/h Vehicle Speed 3.2 Compressor Sizing Compressor speed and working power are determined by the evaporator capacity calculated in the previous section. Compressor is identified with the one used in the production EV, its capacity is 36 cc, and 7000 rpm for maximum EVS28 International Electric Vehicle Symposium and Exhibition 4

5 speed. Refrigerant is R134a and low end of compressor pressure is limited as 205 kpa not to freeze the evaporator core. According to the study results, compressor s pressure ratio is known to be in between 7 and 9 due to the balance for both volumetric efficiency and isentropic efficiency [7]. In general, high end of compressor pressure is set as 1500 kpa to 1800 kpa. Revolution speed and working power for compressor are calculated to meet the target temperature at the evaporator outlet using the refrigeration cycle tool in e-thermal. Compressor s speed and working power for idle condition are 4190 rpm and 1.8 kw, respectively. In the case of 50 and 80 km/h, 6190 rpm and 2.3 kw on each. Table 2: Result of Compressor Sizing on each Driving Condition Vehicle Speed [km/h] Idle 50 & 80 Evaporator Capacity [kw] Compressor Speed [rpm] Compressor Working Power [kw] Table 3: Result of Condenser Sizing on the Highest Condenser Load Refrigerant Flow Rate [g/s] 54.7 Condenser Capacity [kw] 9.7 Refrigerant Temperature at Condenser Inlet [ C] Refrigerant Temperature at Condenser Sub-Cooling [ C] Refrigerant Low End Pressure [kpa, Gauge] Evaporator Capacity [kw] 6.81 end pressure of refrigerant is 1800 kpa and compressor s maximum speed is 7000 rpm. Condenser capacity is sized to meet the targeted sub-cooled refrigerant temperature. The amount of flow for condenser cooling is calculated using the refrigerant temperature and refrigerant flow rate determined by the highest condenser load. Analysis results are summarized in Table 3. Airflow rate needed for condenser cooling is 15.6 m 3 /min. 3.4 Driving Range Impact on Refrigerat ing System This is crucial step to evaluate that the refrigerating system fitted to EV provides the desired performance to meet passenger s comfort in cabin. Using electric compressor instead of mechanical one is the biggest feature of refrigerating system for EV. The power of this electric compressor is supplied from the battery charged and so it shows the significant impact on the driving distance. In this study, it is necessary to check the variation of driving range according to the operation of compressor sized for the refrigerating system used in EV. For this purpose, we evaluate the SOC (State of Charge) of battery system using the e-thermal model built for EV, and CAE analysis is performed for the driving mode simulating vehicle condition during hot summer season. This vehicle operating mode includes soaking, idle, and several vehicle speed conditions such as 50, 80, and 110 km/h and also mixed with two different HVAC modes, OSA (Out Side Air) and recirculation. Overall, this mode is for EV to run 125 km for 220 minutes. Compressor Working Power [kw] Condenser Sizing Condenser capacity is determined by considering the idle condition requiring the maximum revolution speed of compressor. This is the worst condition with the highest condenser load. Based on the information from compressor sizing, high Figure 6: Variation of Battery SOC on Compressor s Operation EVS28 International Electric Vehicle Symposium and Exhibition 5

6 Figure 6 shows the real time SOC of battery when we turn on and off the refrigerating system in EV. Analysis results represent that approximately 13% battery power is consumed when turning on the refrigerating system. This amount of power consumption can be converted to 42 km in driving distance. temperature in cabin as increasing the amount of compressor working power. In the case of EV refrigerating system, the interior cooling performance will be improved by the increase of compressor working power, but it will hurt directly the efficient consumption of electric battery power. Figure 8 shows the available driving distance which is considering the amount of electric power used for compressor operation. We can know that driving distance is decreased by 3 km as increasing each 20% of compressor working power. Therefore, this led that the working power of compressor in the refrigerating system has a negative impact on the driving range per a single charge. Figure 7: Variation of Evaporator Outlet Air Temperature on Compressor Working Power Figure 8: Available Driving Distance on Compressor Working Power Used Figure 7 shows the interior cooling performance called as the evaporator outlet temperature depending on compressor working power. For the clear comparison, we define the working power of the compressor sized for this study as 100%, and then we have two different study cases as 20% decrease / 20% increase in compressor working power. It results in lower 4 Conclusions In this study, the capacity of each component of EV refrigerating system is determined by e- Thermal, which is the GM in-house tool for deciding the overall performance of thermal subsystem in the vehicle. The interior cooling system is laid out and applied to the next generation EV from GM. In addition, we evaluate the driving distance impact on the sizing of refrigerating system. Conclusions are summarized as follows. (1) Evaporator capacity and condensate flow rate were calculated by the enthalpy and moisture ratio for the ambient condition on each driving condition listed in Table 1. (2) Compressor`s revolution speed and working power were determined to meet the required temperature at the evaporator outlet on each driving condition listed in Table 1. (3) Sizing of condenser for EV refrigerating system was performed by considering the condenser capacity, refrigerant temperature, and flow rate needed for condenser cooling. (4) Interior cooling performance can be improved by the increase of compressor working power, but it hurts the driving range on a single charge. From this study, the driving range could be decreased by 3 km on each 20% increase of compressor working power. References [1] Sang-Kyu Hwang, Review on the Strategies of Recharging Infrastructures for Promoting Electric Vehicles, The Korea Transport Institute, EVS28 International Electric Vehicle Symposium and Exhibition 6

7 [2] Sangwon Jung and Changwon Lee, Development of PWM Intelligent-control High-voltage PTC Heater for Eco-friendly EV, Auto Journal Vol.33, 12, [3] M D Oh, Trends in Development of Air Conditioning System for Electric Vehicles, KSAE Vol.16,.1, [4] Todd M.Tumas et al., e-thermal: A Vehicle- Level HVAC/PTC Simulation Tool, SAE [5] Tao Ye, A Multidisciplinary Numerical Modeling Tool Integrating CFD and Thermal System Simulation for Automotive HVAC System Design, SAE [6] Mukund S. Wankhede, Air Conditioning System Sizing Simulation Guidelines, GM VSAS Procedure, 2012 [7] S. Lawson and H. Millet, Rating Technique for Reciprocating Refrigeration Compressors, International Compressor Engineering Conference, Authors Dr. Bongha Song received his Ph.D. degree in Mechanical Engineering from the University of Ajou, Suwon, Korea, in He currently is working for General Motors Korea Company as an engineering group manager for Aero & Thermal CAE team in Vehicle Engineering Center. Dr. Yongsuk Kim received his Ph.D. degree in Industrial & Information Systems Engineering from the Seoul National University of Science & Technology, Seoul, Korea, in He currently is working for General Motors Korea Company as a Director for CAE Integration Division in Vehicle Engineering Center. Jiwon Kwon received his master degree from the University of Soongsil, Seoul, Korea, in He currently is working for General Motors Korea Company as an engineer for Thermal CAE in Vehicle Engineering Center. EVS28 International Electric Vehicle Symposium and Exhibition 7