In electric mobility, there is often a conflict of objectives between thermal comfort and range. With an innovative thermal management concept, AVL qpunkt aims to counteract this. This can be achieved by a combination of using surface heating elements, alternative refrigerants for heat pump systems and utilisation of advanced control systems to achieve a reasonable driving range extension, while assuring a comfortable cabin climate.
SUCCESS OF ELECTRIC MOBILITY
According to statistics and market surveys, a significant increase in market penetration of electric vehicles (EVs) has been identified. In Austria, 353,320 new vehicles were registered in 2017, out of which 7,154 were electric, marking a 74 % increase of new registered EVs in comparison to 2016 . Similar trends within the automotive market can be observed not only in Europe, but also worldwide.
The success of electric mobility is determined, among other things, by one decisive factor: the driving range. The range of EVs can be enhanced by increasing the battery capacity and the energy efficiency of the entire vehicle. Air-conditioning and heating of the interior are considerable power consumers and significantly influence the EVs range, especially at extremely low or high ambient temperatures. In (1), a range reduction caused by the air-conditioning system at extreme ambient condition is shown. In this test, the power consumption of a full electric car was measured while driving the adapted Worldwide Harmonised Light Vehicles Test Procedure (WLTP) driving cycle.
Driving in the city at low average speeds, the electric power needed for cabin heating/ cooling is often in the same range as power needed for vehicle propulsion. As opposed to internal combustion engine driven vehicle, where the engine waste heat is used for cabin heating, in EVs the cabin is heated by an electrical heater or a heat pump powered by the battery. Furthermore, the heating and cooling strategy of the high-voltage battery for electric vehicles in combination with the interior air-conditioning must be presented in all operating points and driving conditions.
Strategies to reduce this range penalty through smart Heating, Ventilation and Air Conditioning (HVAC) concepts are discussed here. Development trends such as the introduction of heat pumps, change from purely convective heating to close-fitting conditioning based on combined radiation and convection of the interior will be addressed.
By installing a reversible refrigeration system used as a heat pump for heating in the winter and for cooling in summer, a significant reduction in energy consumption and thus an increase in driving range can be achieved. A heat pump is an efficient way to increase heat from the environment or heat from a lower temperature level to a usable cabin-heating temperature level.
In a concept shown in (2), a Micro AC cycle is shown. The operating modes of the Micro AC cycle are externally changed by switching the cooling circuits between heating, cooling and dehumidifying. The reason for the Micro AC cycle in contrast to an internally switchable refrigeration cycle is the reduction of number of refrigerant components and the amount of refrigerant. The various heat sources and heat sinks are integrated into the concept via coolant-driven secondary circuits. As heat sources, the ambient air as well as component waste heat can be used. The battery of the tested vehicle is air-cooled. Therefore, both the vehicle cabin and the powertrain can efficiently be conditioned.
In addition to most energy-efficient operation, an emphasis is placed on thermal passenger comfort. Here, both in the simulation and in the vehicle test, the user-specific comfort requirements as a function of driver/ passenger age and health are addressed in the air-conditioning operation. Infrared heating surfaces, (3), provide improvement in transient heating behaviour while reducing the air temperature at the same perceived temperature, (4).
Especially with radiation heating surfaces, the comfort rating is of even greater importance. Either cabin simulations or special measurements are carried out and these are evaluated by means of special scales. Comfort ratings are very often found using global comfort rating indices, such as the Ashrae 55 scale  or the derived Predictive Mean Vote (PMV) scale . The evaluation criteria were derived from the correlation of comfort, temperature, humidity, gender and duration of exposure in a subject study.
The global comfort indices have a systematic disadvantage when local complaints occur. For example, if the passenger has a warm head and cold feet, the global thermal comfort value could be neutral because both effects cancel each other out. For this reason, a local comfort rating is required, especially in the extremely inhomogeneous environment of a vehicle compartment.
The ISO standard DIN EN ISO 14505-2  defines equivalent temperatures and comfort indices of 16 body parts for summer and winter. Suppose passengers sit in an air-conditioned environment and wear light summer clothing, which equates to a clothing factor of 0.6 for the summer fall. Based on the comfort tables for the summer and winter cases shown in (5), evaluations are carried out using suitable models in the 3D CFD simulation.
In order to develop thermal management efficiently, a detailed analysis of the existing vehicle is required. For this purpose, extensive overall vehicle and component surveys are carried out, for example of the cabin, heat exchangers, the blower in the air-conditioning unit, the air-conditioning compressor and the existing cooling circuits.
The aim of the concept development is to design a refrigerant circuit based on propane (R290) as refrigerant for the efficient conditioning of the passenger compartment and drive train. Furthermore, by developing a suitable safety concept to meet the special requirements of using a refrigerant classified as flammable, it is ensured that no increased risk of personal injury can occur. In cabin conditioning at the same time, the sense of comfort of the occupants compared to the base vehicle must be maintained. In this context, an efficient regulatory strategy is being developed to achieve the two competing goals of reducing energy consumption and maintaining comfort.
In the development of the heat pump concept, a new refrigerant circuit using the environmentally friendly refrigerant R290 is designed. The refrigerant circuit is very compact in order to achieve a refrigerant charge <150 g. R290 is a non-toxic Hydrocarbon (HC) and has an ozone depletion potential of zero. Its direct contribution to the greenhouse effect is very low with a GWP100a value of 3. For comparison, R134a has a value of 1430. In (6), selected characteristics of the refrigerants R134a, R1234yf and R290 are listed.
Furthermore, systems operated with R290 are characterised by the improved low-pressure level, especially at low ambient temperatures of -10 to -20 °C. The thus enlarged operating range of the heat pump has a favourable effect on the achievable range. However, HC refrigerants are flammable and therefore subject to international safety regulations and laws (for example, explosion protection guidelines, safety regulations). EN 378 , for example, limits the allowable refrigerant charge for comfort air-conditioners or heat pumps, depending on the security group, installation area and type of installation (direct or indirect system). For refrigerant charges of less than 150 g, systems with flammable refrigerants can be installed in a single occupancy area without any particular restrictions. Special standards dealing with the use of HC refrigerants in car air-conditioning systems are currently not known.
The distribution of the heat energy is performed via coolant-carrying secondary circuits. As a result, the refrigerant-carrying components run decoupled from the cabin, so that the risk can be excluded by the influx of the flammable refrigerant into the vehicle cabin in this way, or a complete encapsulation of the refrigeration system, (2) (blue coloured), is possible. In the first development step, the base vehicle was benchmarked to determine the necessary heating and cooling capacity and the various heat flows in the powertrain and the entire vehicle under different environmental conditions and driving cycles.
Another part of the benchmarking involves the analysis of the space with the aim of developing an ideal packaging solution that will enable the construction of a demonstrator vehicle at the end of the project. The heat pump concept covers the entire development of the refrigeration circuit and the cooling circuits and thus includes the entire thermal management. In order to achieve efficient operation with a high Energy Efficiency Ratio (EER) in cooling mode or a high Coefficient of Performance (CoP) in heating mode, individual components have to be redesigned.
Based on extensive measurements of the vehicle and certain components, a virtual development concept was created and COPs and EERs were determined. By means of modelling, appropriate operating strategies and control strategies of the system can be derived and the concept can be virtually optimised before it is integrated into the vehicle. Through virtual development, development times and hence development costs are reduced.
ENERGY SAVING POTENTIAL
Due to the low heat loss of the powertrain of an electric vehicle at a low temperature level, the heat cannot be recycled directly, but it is indirectly used as an additional heat source for the heat pump. As a refrigerant for such a heat pump, various fluids with different fluid properties are available. Propane delivers a high heating power even at low temperatures down to -20 °C, good environmental compatibility compared to conventional refrigerants and operates at comparable operating pressures. By additionally improving thermal insulation of the heat carrying lines, the high pressure level in the refrigerant circuit can be reduced at the same target temperature. Therefore, the refrigerant cycle must be placed as close to the air-conditioner as possible.
Long pipe lengths from the front of the vehicle to the refrigerant cycle are not a disadvantage, since they indirectly increase the heat exchanger area of the front heat exchanger (Front HX), (2). From the measurements of the base vehicle with high voltage auxiliary heater, the required electrical energy for cabin heating was determined, as well as the respective reduction of the range per driving cycle. When integrating a heat pump with propane refrigeration cycle, due to the COPs detected, the electrical power required for the heater (for example at -20 °C) is drastically reduced. This effect naturally has a positive effect on the range of the car, (7).
The development process of a thermal management concept using a heat pump for a specific vehicle application was presented. The concept combines different technologies such as infrared heating, a heat pump with a natural refrigerant (R290) and the application of thermal storage systems with the aim of maintaining the range with the same comfort despite extreme weather conditions.
The Quiet project was funded under the European Union’s Horizon 2020 research and innovation programme. The content of this publication was developed mainly through development cooperation with the consortium partners and does not necessarily reflect the views of the European Commission or its institutions.
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DIPL.-ING. (FH) PETER DRAGE is Engineering Center Manager at AVL qpunkt GmbH in Graz (Austria).
DIPL.-ING. MARKUS HINTEREGGER is Project Leader Advanced Development at AVL qpunkt GmbH in Graz (Austria).
DIPL.-ING. GERALD ZOTTER heads the Department Thermal Management/Simulation at AVL qpunkt GmbH in Graz (Austria).
ING. MARIJAN ŠIMEK, M. A. is Key Account Manager at AVL qpunkt Deutschland GmbH in Munich (Germany).