An investigation into the optimal operation of a complex heat pump for the complete thermal management of an electric vehicle in cold climates

Electric vehicles (EV) suffer two inconveniences at low temperatures; poor lithium ion battery performance and a cabin heating demand which must be satisfied by the battery and cannot be provided using waste heat from a internal combustion engine. Both of these issues combine to give a 40% to 70% reduction in range at −20◦C. This leads to EV users having to potentially choose between range and comfort. New technologies and strategies are being proposed to reduce the impact of cabin heating and battery performance reduction on range. Primarily, heat pumps are becoming the likely candidate for future thermal management in electric vehicles. A heat pump is agnostic to its heat source, leading to many potential heat recovery opportunities around the vehicle. Heat pumps have been criticised for poor response times, leading to the addition of thermal storage devices to reduce dependence on positive thermal coefficient (PTC) heaters. Researchers have explored some of the new possible architectures which a heat pump allows; however, with many architectures possible a systematic comparison of all possibilities needs to be made. A novel implementation of a cost function which can be varied in weighting to prioritise range or comfort, coupled with conventional optimisation techniques, is used to control the trade off between range and comfort. A simulation environment is used to identify the optimal sizing of the thermal battery, systematically identify and compare all possible vehicle architectures, and produce an optimal heating trajectory for the electric battery. The results showed an average range extension of 22% compared to the baseline could be achieved by replacing the PTC heater with an optimised thermal battery, while simultaneously improving comfort by 28% over the temperature range of −15◦C to 15◦C. For the first time, a systematic and exhaustive comparison of potential combinations of thermal interactions on a heat pump system is performed. The result of this comparison showed that at −5 ◦C, selecting the correct operational mode enabled the range to be tunable within a 10% window according to cost function priority, corresponding to a 18% variation in comfort. Finally, optimising the heating trajectory of the battery for different cost function priorities created a range window between 116.9km and 140.4km, with the comfort improving by 17% when reducing the range from 140.4km to 116.9km. If an electric vehicle has multiple heat sources and sinks it is possible to select operational modes and control component interactions with the heat pump so that the trade off between range and comfort can be optimally controlled. This methodology may be repeated for different electric vehicles, using a different set of potential heat sources for the heat sink, and may also be repurposed to consider high temperature environments, thus contributing a systematic approach to addressing thermal management challenges in electric vehicles. The implementation of this methodology on a real vehicle would allow for the maximum thermal comfort to be delivered while ensuring there is enough range to meet the required duty cycle. This could potentially increase the uptake and acceptance of electric vehicles in regions where inhabitants experience cold winters.