The popularity of electric vehicles is growing and there are few commentators who are not predicting an overwhelming move to electrically based vehicle drive train in the near future (10 to 20 year time frame). Continual advances in both battery and fuel cell technologies as well as other powertrain systems are bringing some excellent vehicles and powertrain systems onto the market.
These continual developments are helping to significantly reduce the environmental impact of vehicles and mobility. There are several key advantages to the electric drive train compared to the conventional internal combustion engine vehicle, if you are reading this white paper then chances are you are already familiar with and which have been covered extensively by other authors. However a brief summary of the key features being:
- Eliminate on vehicle exhaust emissions offering significant potential to improve urban air quality
- Greater flexibility on fuel source for example grid electricity from renewables for BEV and hydrogen fuel cells for FCEV
- Reduced noise and vibration
- Improved on vehicle efficiency with efficiencies > 80% being achievable in terms of converting energy to motion compared to <50% with an internal combustion engine
- Improved torque delivery capability of the electrical powertrain simplifies or eliminates the need for ratio changing gearboxes
There are some drawbacks that accompany these benefits for example; in an internal combustion engine vehicle there is an abundance of waste heat that can be used to heat the cabin of the vehicle, which is not the case on an electric vehicle. Whilst the main purpose of the vehicle is to provide mobility, keeping the occupants safe and comfortable is high up the list of priorities ! Current vehicle design does not take into account good cabin insulation from a thermal point of view in the same way as a building would, because the heat for heating the vehicle has in the past been freely available.
The much higher achievable efficiencies of the electric powertrain mean that less heat is rejected from the powertrain in operation. For example a typical family saloon car with a 100kW rated diesel or petrol engine will need to deal with a heat load of around 80kw being rejected to coolant and > 100kW through the exhaust. The equivalent electric vehicle may need to deal with less than 15kW of heat rejection under peak conditions and it may be believed that this would lead to a greatly simplified cooling system.
Unfortunately, this is not the case. At a recent meeting someone quoted that the heat flux from an IGBT semiconductor device (the current preferred switching device for high voltage electric and hybrid vehicle drives) is the same as the surface of the sun at around 400 W/cm2, I was initially quite sceptical about this claim, but on researching it the figures are correct  &  Due to this very high heat flux, smaller delta to ambient due to the lower electronic component operating temperatures and reduced system thermal mass the challenges of cooling this system, carrying away waste heat from key components and the requirement to provide energy to keep the occupants of the vehicle at a comfortable temperature and the challenge can be seen to be considerable.
The electrical and electronic devices are sensitive to temperature and can be damaged or severely degraded by operating at temperatures above their design limits and their operational lives will definitely be shortened. Hot electrical devices are also typically less efficient than cool ones, due to the increase in the resistivity of conductors and reduction in the field strength of magnetics as temperature rises.
Higher power density electronics modules and requirements for good component reliability and efficiency mean there is a requirement for minimum coolant flow rates through the system to prevent localised boiling of the coolant in the heat sink due to the very high heat fluxes involved, these flow rates will vary depending on the load the device is working at. There is also the issue of high heat fluxes inducing huge thermal stress as the device and its surroundings rapidly heat and cool as the device operates.
The small volume that heat is being removed from leads to complex designs for cold plates, which tend to have a high pressure drop and also very small fluid channels. This can lead to a coolant system which is hard to correctly fill and means that a relatively much more powerful pump is needed when compared to the heat rejection in the context of an ICE engine, of course more pump power leads to the possibility of higher parasitic loses on the system making it important to ensure that the design is highly optimised.
There is also typically a thermal management requirement for the electric vehicle battery. Heat is generated in the battery pack by the electrical current inflows and outflows as a function of current and the internal resistance of the battery cells and interconnections, during vehicle acceleration, deceleration and also charging. Like the motors and power electronics the EV and HEV battery is sensitive to operating temperature. Current battery technologies typically require an element of heating in low ambient temperatures.
Care should therefore also be taken not to induce thermal shocks in the system with inadequate control of the coolant flow. Thermal shock can be caused by excessive as well as insufficient coolant flows. A wildly oscillating system temperature will lead to premature electronic and energy storage system component failure.
Outside the box OtB the system typically comprises of a liquid cooling medium, an electric cooling pump such as the AVID WP29, heat exchangers such as AVID’s thermal management systems and electrically powered fans such as the AVID FiL-11. It is often a surprise to the vehicle designedr to discover that due to the low temperature gradients and other considerations that the required heat exchanger and fan arrangement for the EV powertrain can actually be larger than the conventional vehicle.
In order to minimise thermal stress in the system and to reduce parasitic power consumption the coolant pump and fan speed should be controlled by a system or vehicle level controller such as the AVID CANecu. The system control algorithm should feature multi variable control to account for the interdependency between the coolant flow delivered by the pump and flow of cooling air from the electric fans and maintaining a steady operating temperature of the motor and electronics modules. The key control variables being device and coolant temperature. There may also be additional cooling requirements from items such as braking resistors, transmission oil coolers and other devices on the vehicle to be considered.
In addition to thermal load and heat flux another key consideration is the design of the Internal (ItB) and External (OtB) heat exchangers themselves. Typically the ItB will use a serpentine pipe embedded in a cold plate or a cooling jacket around the electrical machine, or a pin-fin construction method. New motor designs with more targeted cooling of the stator and rotor are emerging, such as those from DHX. The coolant passageways in the system can be of a very small diameter when compared to a typical internal combustion engine (ICE). There is also the possibility that the motor, electronics and battery systems will be much more highly distributed around the vehicle than a conventional ICE vehicle. Meaning very long coolant pipe runs with little opportunity for a gain in system “hydraulic” head. Surface tension effects on the coolant in the small diameter pipes and passage ways as well as varying system pressure drops due to the fluid behaviour at higher flow rates can be a critical design consideration. Routing of the long pipework and bleeding the system to remove air pockets during routine maintenance can also be very problematic in these systems, specialist service tools to drain and fill the coolant system are often required.
For the passengers, early electric vehicles used PTC heating elements to provide additional heat for the passenger cabin. However, this was found to be very inefficient and had the impact of significantly depleting vehicle battery range in cold weather. For example, a PTC heater of between 3 and 5kW was required in a typical medium sized car, very minimal control for the heater was implemented. A battery pack for a Nissan Leaf for example is 24kWh  and the Tesla Model S is 60 to 90kWh depending on specification . The reduction in vehicle range for full heating could be as much as 30%, which considering a typical range of 100miles which is already at the lower end of consumer expectations is highly undesirable. The current state of the art cabin heating uses a heat pump system. This heat pump system is effectively like the traditional air conditioning system running in reverse, or the systems found in buildings and they have a Coefficient of Performance (CoP) of greater than 3 to heat the cabin. This means that a smaller reduction in range is achieved under full cabin heating load, but it will still be around 10% if full heating is required. Improved design of the passenger compartment to improve heat distribution, insulation and air exchange is a growing area.
In summary the challenge associated with thermal management in an EV and HEV is significant and no less demanding than for an ICE application. Parasitic loses are arguably more important for EV’s where the capacity and cost of battery vs vehicle range is a key performance metric that all are measured by. Therefore optimising the thermal management system for performance and efficiency is a highly critical task. AVID is the market leader in advanced thermal management system and electrified component supply for heavy duty and high performance hybrid and electric vehicles. Our products can be found in many of the leading hybrid and electric vehicles available on the market today. Contact AVID to find out how we can help in the design and manufacture of advanced thermal management systems for your EV and HEV powertrain program.