<p>Generally, batteries produce less current at lower ambient temperatures due to slowed chemical reactions and therefore discharge more quickly in a struggle to meet power demands. Thus, in colder climates, the cells in the battery of an electric vehicle are subject to environment-related performance degradation leading to a decrease in effective range. Active battery temperature regulation is often implemented in battery electric vehicles (BEVs) to mitigate the detrimental effects of extreme ambient temperatures on battery state of health and effective nominal capacity. However, ambient temperatures also impact driver comfort leading to added auxiliary power demands to regulate the cabin temperature. This work focuses on evaluating the increased auxiliary power demand from vehicle heating, ventilation, and air condition (HVAC) systems in cold climates. If the energy required to regulate cabin temperature could be accurately estimated as a function of temperature, this would allow researchers to better isolate the effects of ambient temperature fluctuations on BEV range specifically from temperature-related battery performance degradation. To accomplish this task, a lumped-capacitance model for heating a medium-duty delivery vehicle was used to estimate HVAC energy consumption. Using practical driving data collected over multiple weeks from a single instrumented delivery vehicle, the model was tuned and validated. A MATLAB script was developed to incorporate the tuned thermodynamic model to simulated vehicle performance for duty cycles with various temperatures near those for which data was available to tune the model. The heat balance method (HBM) was used for estimating the net heat transfer into and out of the vehicle cabin, using a series of algebraic equations that could be solved by numerical methods. Heating loads from solar irradiation, driver metabolic heat generation, and HVAC operation were balanced against the cooling ambient load caused by the temperature gradient between the heated cabin and the vehicle surroundings. This simple implementation solved the need for a high fidelity, low-order, computationally cheap, and control-oriented system thermal model. Cabin and ambient temperatures and vehicle speed served as the models’ major inputs. Vehicle thermodynamic properties, geometry, solar irradiation data, and cabin and ambient temperature data were used to tune lumped thermal resistances. The data was gathered from delivery vehicles in the state of Minnesota, between the months of February and March of 2020. While the vehicle data was collected from a gasoline-powered vehicle, the computed heat transfer from the engine to the vehicle cabin was determined to be negligible. Under the assumption that the BEV battery is similarly insulated from the cabin, the developed model also provides accurate HVAC energy load estimates for similar BEVs. So, the model was finally used to assess expected BEV range impacts from HVAC loads in cold climates for a similar BEV model with a 100 kWh battery and an average efficiency of 0.84 kWh/mi. The results confirm the significance of HVAC energy requirements for BEVs in cold weather environments independent of expected battery performance degradation and effective range loss associated with batteries operating in lower temperature environments. When HVAC energy demands were evaluated for different average ambient temperatures using the practical driving duty cycles, a roughly linear trend for HVAC energy expenditure per mile was found. With additional data collection, the proposed method could be used to construct a more complete curve to estimate energy expenditure as a function of ambient temperature for a simplified empirical model to use in BEV range prediction. Due to its simplicity and effectiveness, the proposed method shows promise for use in future studies on BEV performance in different climates.</p>

Cold Weather Impacts on Electric Vehicle Performance

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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