Thermal management is of critical importance to range and performance, both directly in terms of keeping the battery pack within its optimum operating window (approx. 20-40 degrees Celsius), as well as indirectly via the rest of the powertrain and vehicle systems. Motors and inverters all need cooling to work efficiently, as do charging systems, while occupants must be kept comfortable, too. Inefficiencies in the thermal management of any of these elements will ultimately reduce the certified range, which is tested, in idealised conditions, without air conditioning.
Take the battery pack – extreme cold can slash range by up to 60 per cent, while high heat can deplete it by 30 per cent. Even modestly low temperatures have a significant effect. At -5 degrees Celsius range is typically reduced by around 20 per cent. Temperature extremes can also accelerate degradation of the battery cells, resulting in decreased performance/range, reduced residual values and costly warranties for manufacturers.
One then has the motors and inverters to consider. Permanent magnet motors are used in most premium EVs and generally rely on neodymium magnetic material, which needs to be kept below around 80 degrees Celsius to prevent demagnetisation issues. If cooling capacity is insufficient, extended high-speed running or hard acceleration will see the motors derated to prevent damage. The same consideration also applies to battery management systems (BMS) that will step in to prevent either cell damage or, in extreme circumstances, thermal runaway, which could lead to a battery fire.
All these systems require their own cooling circuits, and typically a separate positive temperature coefficient (PTC) heater. These can all cause inefficiencies and make it more challenging to recover or reuse heat. The situation is further compounded in hybrid applications where the internal combustion engine also needs thermal management. The solution is to design and manage the thermal systems to reuse and recover energy correctly to reduce waste. Reversible heat pump systems offer a one-stop-shop for both heating and cooling through one set of components and enables higher levels of energy capture/reuse. However, they are significantly more complex, bringing new challenges and minimal legacy data to base engineering judgement on.
As a result of these myriad of demands, manufacturers are looking to develop ever more complicated thermal management systems that require greater levels of development and optimisation than before. Take the example of coolant control valves: whereas in internal combustion engine vehicles these may have had two or perhaps four ports, suppliers are now offering multiple valves per vehicle, or valve units with eight or more ports to accommodate the fiendishly complex EV cooling solutions under development. If you then take a similar example but for refrigerant circuits, OEMs must use valves to close off parts of the system to improve efficiency, but in doing so lubrication for the compressor can be trapped in the closed part of the circuit. Manufacturers already experience failures, recalls and expensive warranty claims as a result of starving the compressor of lubrication – now the complexity is increasing, the risk only gets bigger.
Previously, the thermal system was a lower priority. Energy was readily available and so was not tightly controlled. Most development and validation work was conducted at vehicle level once prototypes were available. Now, with the demand to get vehicles to market sooner alongside high complexity systems, the approach is changing. Developing an entire thermal management system ‘off-vehicle’ and earlier in the development programme drastically reduces development costs, timescales and risks while ensuring a stable platform for baselining the system and optimising its performance – well in advance of prototype vehicle availability.
Virtual modelling and simulation is now commonplace and often seen as the saviour of the hour, reducing OEM time, risk and cost. However, what is often missed is the correlation data. This data ensures the virtual system performs as it should in the real world. Without this, the OEM is basing the whole architecture, sizing and controls strategy on data sheets and approximations.
With system-level physical validation these simulations can be given a high degree of confidence, reducing the need for costly reworks later in a vehicle’s development programme. Utilising the capabilities of its new Vehicle Thermal Energy Optimisation Suite, HORIBA MIRA is able to offer this physical validation, at system level, as part of its thermal engineering service. This covers everything from concept architectures through ‘Model-based design’ in the virtual world, component characterisation, system level development through to vehicle level climatic wind tunnel testing and hot/cold environmental testing.
For both new and legacy programmes, all aspects of thermal management are now critical to vehicle performance and are an area where OEMs and Tier 1s should be concentrating their efforts.
Ben Gale, Energy Efficiency and Thermal Systems Solutions Leader, HORIBA MIRA
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