Air Conditioning System Sizing for Pure Electric Vehicle

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 air temperature in hot weather is more sensitive to passenger’s thermal comfort.


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 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.Not 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 air.In this research, we perform several studies to lay out a proper refrigerating system to secure maximum driving range per a single charge.

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 & air) systems and front end air 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 (air-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 air temperature passing through an evaporator.This is named as target air temperature, and then other components are sized to meet this requirement through each step of Figure Using Formula (1) and Formula (2), evaporator's capacity and condensate flow rate are calculated and these become basic inputs needed to get ambient air to cool down to target air 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 air 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 air temperature.

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.

Component Sizing of Refrigera ting System
Vehicle HVAC system consists of A/C control module, passenger compartment, and air 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 Sizing
First, the air temperature in cabin is defined by the temperature feeling comfort and the air 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].
-  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 air condition for interior cooling are 29.3 kJ/kg and 7.6 g/kg, respectively.Using the enthalpy and moisture ratio calculated, 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.

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 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 air 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.

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 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 airflow 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.

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.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.Figure 7 shows the interior cooling performance called as the evaporator outlet air 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 air 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.

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 air condition on each driving condition listed in Table 1.
(2) Compressor`s revolution speed and working power were determined to meet the required air 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 airflow 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.

Figure 1 :Figure 2 :
Figure 1: Sizing Process of a Refrigerating System [6] 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 air inlet and outlet conditions -Compute evaporator load -Compute condensation generated

Figure 3 :
Figure 3: Process to Determine Evaporator Load

Figure 6 :
Figure 6: Variation of Battery SOC on Compressor's Operation

Figure 7 :
Figure 7: Variation of Evaporator Outlet Air Temperature on Compressor Working Power

Table 1 :
Vehicle Driving Condition

Table 2 :
Result of Compressor Sizing on each Driving Condition

Table 3 :
Result of Condenser Sizing on the Highest Condenser Load