Vehicle thermal management, once almost exclusively about safely rejecting powertrain heat, continues to evolve to dealing with two extremes in operation of battery electric vehicles (BEVs) – accumulating every possible BTU for cabin heating in winter and cooling battery packs in high-ambient, high-load operation while preparing for a future with ultra-fast recharging.
SAE’s recent Thermal Management Systems Symposium (TMSS) covered significant new developments in this widely evolving aspect of vehicle development. Climate control systems, primarily heating in winter but also air-conditioning (A/C) in warmer months, have been a major focus for BEVs because conventional heaters (PTC and resistance) and A/C operation smack down vehicle range by an overall average of about 33%.
Enhanced seat heating and cooling
General Motors and Gentherm, a Tier 1 supplier of heated/cooled seats, introduced the “Microclimate Comfort System,” based on a Gentherm concept named ClimateSense, originally evaluated on a BMW I3, a BEV. It’s a dual-zone system for the driver and front passenger. The objective: reduce EV range loss from cold-weather cabin heating by enhancing and tuning seat-heating operation, adding body-warming surfaces and limiting cabin HVAC operation.
ClimateSense researchers also evaluated using the A/C and seat-cooling system to reduce A/C cooling load in hot weather. Testing on a Bolt EV at the GM Milford, Mich. proving grounds delivered impressive results: a 50% reduction in heating load at -7°C (19°F) ambients, with minimized HVAC operation (69% reduction without HVAC). ClimateSense with limited HVAC showed a 30% reduction in cooling load during an AC17, the industry test used to validate Environmental Protection Agency (EPA) credits for Corporate Average Fuel Economy (CAFE). The AC17 test is run at varying temperatures, averaging 77° F (25°C).
The winter testing produced an estimated 50-mile (80-km) improvement in driving range, based on a 260-mile (418-km) range with no HVAC at moderate ambients. With standard PTC heating at -7°C, the range would drop 40.5% to 160 miles (257 km), according to Gentherm’s Vladimer Jovovic, director of advanced engineering. ClimateSense, he said, held the range at 210 mi (338 km). Using ClimateSense in the AC17 tests reduced the Bolt’s range just 11.2%, versus a 16% range reduction without.
Thermal-load savings were based on maintaining, within 1°C, the “EHT” (equivalent homogeneous temperature), a new rating in passenger comfort. The EHT metric refers to how the passenger feels in the cabin, roughly comparable to the wind-chill effect, versus measured temperature; EHT is a single number used for controlling the system, with heating effect from:
- Interior radiation (warmed surfaces, including consoles, doors and knee bolsters)
- Convection (from seat and neck warmers). Neck warmers provide a quick sensation of comfort because of the number of blood vessels close to the skin surface
- Conduction (from seat heaters, heated steering wheel, console and door armrests)
- Ambient temperature and cabin humidity
Cooling is achieved primarily by the seat’s solid-state A/C (thermo-electric cooling) with limited HVAC operation. The EHT system uses ASHRAE values for clothing, which assume warm clothing (coat, long pants, etc.) in winter, light clothing in summer. A motorist would feel comfortable at 68°F (20°C) HVAC setting in winter, but likely raise that setting in summer, according to Jeffrey Bozeman, a GM HVAC engineer.
Testing was performed with Thermetrics manikins, particularly “STAN” (Seat Test Analytic Network), a manikin with no body as such, but which covers the back of the torso, ending at the knees. It’s instrumented with 46 convection and 46 radiation sensors and with addition of weights can adjust seat compression. Thermetrics line of five automotive specialty manikins also includes an automotive HVAC manikin, a carbon epoxy body form that simulates a 50th-percentile Western or Asian male, with sensors measuring air velocity, temperature, radiant heat flux and relative humidity.
Quick-charging and heat-pump challenges
Cooling power electronics and the battery pack is a prominent PHEV/BEV hot-weather issue. Lithium-ion batteries live longest when they’re maintained at 15-35°C (59-95°F); cooling challenges may develop in high-ambient/high-load operations, also during high-rate battery charging or discharging. Dr. Cedric Rouaud, thermal systems project director for Ricardo U.K., said the industry presently is at about 100-kW peak charging rates and an average of 65 kW, but is looking to move to 300-350 kW. This would, he said, increase heat rejection requirements by 150-175% and possibly 200%. Improved cell-cooling needs will soar, he said, adding that high charge rates will mean not only will the cells require more cooling, but even charge cables – both on and off the vehicle, may have to be cooled.
Air cooling is the original method, used by Toyota for the Prius hybrid and then by Nissan for the Leaf BEV. But today, BEV and PHEV battery packs require liquid cooling, typically water-glycol solutions flowing through circuits in the packs. Many also have a shunt circuit through a “chiller” heat exchanger (HEx) with the A/C. Also available is a pure refrigerant circuit, incorporating an evaporator as a “base” for the pack, positioned just under the cells and tied into the vehicle A/C.
Immersion cooling – cells immersed partly in a dielectric fluid – are in use to cool computer servers and likely will see automotive use. The fluid transfers heat from the cells and circulates it through a chiller and radiator for cooling, returning it to the battery pack. For ultra-fast charging, Dr. Rouaud said, Ricardo proposes a liquid-cooled condenser in conjunction with the A/C condenser, low-temperature radiator (used for power electronics) and phase-change material surrounding the cells if needed. Ricardo also is working on a heat pipe system (Tesla already has patented a design) and thermo-electric cooling. Ultra-fast charging is a special challenge because the batteries are likely depleted and hot from vehicle operation.
In his symposium review of battery-cooling technologies, Ted Miller, Ford’s manager of electrification subsystems and power supply research, said at present the heat pipe is low in efficiency, while immersion cooling needs cost reduction. The chiller widely used in battery-cooling systems led Sanhua Holdings to propose a “smart” design, integrating it with the company’s electronic expansion valve, which combines a refrigerant pressure/temperature sensor with duty-cycle-controlled valve. Edwin Stanke, R&D scientist who delivered Sanhua’s presentation, noted the circuit is simplified and a single local interconnect network (LIN) connection to the system ECU allows exchange of digital signals from the sensor and for vehicle speed, coolant temperature, compressor rpm and OBD data.
Heat pumps typically merely reverse refrigerant flow to switch from cooling to heating. Mahle/Fiat Chrysler’s approach, applied to a 2015 Fiat 500e, is a secondary-loop design – an ultra-compact refrigerant loop, all underhood, so it uses much less refrigerant. The condenser and chiller in the refrigerant loop also include water-antifreeze coolant heat-exchange loops. One goes through the condenser to produce heated coolant, which runs to the under-dash heater core. Another coolant loop goes through the chiller to deliver cold coolant for the battery pack; it also runs to another under-dash HEx for cabin cooling. As described at the symposium, it improved BEV range 13% at -10°C (14°F).Continue reading »