Sensing to Solve EV Thermal Challenges
The complexities of electric-vehicle HVAC require new sensing solutions. An expert at TDK explains.
A common criticism of electric vehicles (EVs) is that extremes of heat and cold adversely affect their performance, particularly range. OEMs have been aware of the issue and have innovated and iterated technologies to solve it. EVs increasingly are being engineered with sophisticated heating, cooling and ventilation (HVAC) systems based on some of the most advanced and robust sensors and actuators available.
These systems mitigate against the effects of thermal extremes that, admittedly, still challenge IC-engine vehicles on an infrequent basis. But there are tradeoffs. The ICE vehicle’s HVAC is a relatively simple affair involving air cooling, a coolant (water ethylene glycol, or WEG), heaters, and heat exchangers (including air conditioners). HVAC systems for EVs by necessity are far more complex. Understanding the details is important to designing an effective system.
EV HVAC differences
An EV HVAC system has multiple temperature-sensitive subsystems and components, all with different optimal temperature ranges that only sometimes overlap. Specifically:
- Magnetic elements in the motor must be kept below 80° C (176° F) to keep them from demagnetizing.
- Motor inverter and charging electronics must be kept at less than 120° C (248° F)
- Lithium-ion batteries must operate between 10° C and 40° C (50° F and 104° F).
- A hydrogen fuel cell may need to run somewhere between 100° C and 200° C (212° F and 392° F, depending on the fuel-cell stack technology)
- Passengers in any vehicle tend to be most comfortable within a few degrees of 20° C (68° F).
EV HVAC complexity derives from having to satisfy all requirements, often simultaneously. The complexity begins with the three distinct heating/cooling loops that are part of every EV. The battery system, power system and the passenger space each have a dedicated loop. Handling so many systems that operate in different temperature ranges requires careful management. For example, EV manufacturers had to devise a means of handling both power electronics (operable to 120° C) and the batteries (operable to 40° C).
EV engineers divide what would have been a single thermal loop covering both the power electronics and the battery into two channels to take advantage of the existence of the vehicle’s air-conditioning unit.
In one channel of the thermal loop, WEG cycles through power electronics and radiator at some temperature between 40° C and 100° C, while in the other channel of the loop, the WEG is chilled below 40° C by an air-conditioning system. This is done with a chiller that transfers the heat from the battery loop WEG to the air-conditioning refrigerant.
Regulating EV battery temp
An EV battery can have thousands of battery cells. Each cell has to be individually monitored for temperature and voltage. Voltage sensing is necessary in large part because batteries operate better when the load across all the constituent cells is balanced as evenly as possible. This data is fed into each EV’s battery management system (BMS). For temperature sensing, in hot weather the battery needs to be cooled to avoid battery degradation that begins to accelerate at higher than 40° C. That involves using coolants, as described above, as well as vents and fans.
During cold weather, the battery needs to be heated. When Li-ion batteries get too cold, range is compromised. Note, too, that the battery must be kept warm even when the vehicle is not being driven. This is why EV makers have been equipping batteries with dedicated heaters. Keeping the batteries in the optimal temperature range has to be done anyway and it helps maintain range when the weather grows cold – a feature that EV critics frequently miss.
The process of heating and cooling the multiple EV subsystems so that they remain in their optimal temperature ranges involves more heat pumps, water pumps, three-way valves, expansion valves and other equipment than found in ICE vehicles. All of these elements must be monitored and controlled. This requires temperature sensors, pressure-temperature sensors mounted in various positions and pressure sensors for the WEG refrigerant.
Many of these sensors need protection from the elements and so must be mounted in appropriate packaging. They all need to be positioned and attached with care, with an awareness that motor vehicles are perpetually subject to shock and vibration on the road. TDK, for example, provides many of the required actuators (HVC 4xyzF) and sensors (HAL 39xy) to extract the maximum range from a battery at all times.
Effective EV HVAC = safety
Precise measurement is necessary not just for the efficient operation of the vehicle. It is also an issue of safety.
Most EVs will rely at least partially on taking an AC source as an input and converting this into DC to charge the battery. The amount of power through both AC and DC charge points often is in excess of 100 kW.
Generating heat as a by-product is inevitable. If not managed, the accumulated heat could become a potential safety hazard. That makes temperature sensing even more important at all critical points throughout the EV power system.
The IEC demands that charging plugs include a temperature sensor to ensure safe operation. TDK supplies high-voltage-resistant negative temperature coefficient (NTC) sensors in these hotspots, as they feature a high operating temperature and high electrical insulation.
The complex HVAC required by EVs depends on leading-edge sensors, actuators and controllers to assure that the vehicles operate at peak performance – including when it’s cold outside.
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