BEV Heat Pumps Recovering Cost Investments
They add 50 percent to climate-system outlay, but improved designs indicate costs can be recovered, for Jags or an electric school bus.
Can a heat pump-equipped battery electric vehicle (BEV) overcome major energy losses for cabin heating comfort in cold ambient conditions –and at reasonable system cost? These questions were addressed at the 2018 SAE Thermal Management Systems Symposium, and although definitive answers may not all be production ready, promising work has been done.
The industry recognizes as unacceptable the BEV battery pack range losses of up to 40%+ from winter operation with PTC (positive temperature coefficient) heating, which is the conventional approach. Testing by Hanon Systems in 2017 showed simple, coolant-circulating waste heat-gleaning circuits from power electronics, electric motor and battery pack would produce sufficient thermal storage during a commute-length drive (50 km/30 mi) in a pre-conditioned cabin (as part of a battery pack charge).
The Hanon research indicated this approach is far more cost-effective and efficient than a heat pump. However, as BEVs with greater range enter the market, the heat pump comes back into play. It also is being considered for such other uses as school busses, now typically diesel-powered, but being evaluated for electrification.
Jaguar’s new 2019 I-Pace AWD model (top, winter testing in Arjeplog, Sweden) features a 90 kW·h battery pack and up-to-240-mi/384 km rated range. It’s a premium-market vehicle with more flexibility in its cost/pricing structure than a Nissan Leaf, for example. The electric Jag also is expected to deliver near its rated range in very cold weather. And although it includes a heat pump, there is much more to the system.
In his SAE TMSS presentation, Nilabza Dutta, a Jaguar Land Rover (JLR) thermal-management engineer, cited “30% to 50%” as the extra cost of the I-Pace’s HVAC system. However, he said, there would be added cost of 150-450% for covering the range shortfall of using PTC by increasing the size of the battery pack. There would be an additional cost over that of the battery pack: recharging it from the grid, which Dutta said was in the range of 80-300%.
The Jaguar approach, therefore, was “do whatever is necessary” within the alternative cost framework. First, the HVAC air distribution is three-zone: driver only, driver and a front passenger and both front and rear. Next, there is a heat-gleaning coolant circuit for the electronics. It is integrated with a vapor-injection HVAC heat pump system for maximum heating season performance. It does not cover the regenerative braking – a “one-pedal” control system that is based off the accelerator and similar to those used on various BEVs.
The vapor-injection heat-pump design, introduced on motor vehicles last year by Denso on the Prius Prime plug-in hybrid, incorporates a liquid-gas separator. This approach, adapted from commercial heat pump systems, injects the hot vapor into the compressor to produce system heat at low ambient temperatures. In this arrangement the heat pump, which normally is ineffective at 0°C/32°F because of the slowdown in refrigerant mass flow in low temperatures, remains operational.
Toyota cites 14° F/-10°C as the lower end for the Prime. The I-Pace system, with its integrated heat gleaning, can pull up to 4.5 kW, Dutta said. At 0°C (32° F), it can draw 2.5 kW from the electronics/motor system with a collection loss of 0.3 kW, for a net gain of 2.2; and 1.9 kW with a loss of 0.3, for a total of 3.8 kW, he told the TMSS session. Even at -8°C (18°F), the system can pull a total of 3.5 kW, he added.
The effect of cabin preconditioning, as demonstrated by the Hanon research, is enormous even for the Jaguar I-Pace. Without it, the heat pump saves only 17%; with it, the saving jumps to 83%. And even with the heat pump turned off, the preconditioned car saves 49%. Range extension on the I-Pace as equipped is an impressive 30-45 km (18-27 mi), in operation from minus 10 C to 0 C (14-32 F).
Additionally, the I-Pace has an over-the-air software update feature for the battery control module, to optimize range. It works with any Alexa voice communication device, and at this time also can update the infotainment system.
School bus heat pump project
Schools typically are closed for vacation during the hottest weather months, so bus cabin comfort typically focuses on heat in cold weather, with under-seat and driver/stairwell units. There are geographical exceptions, however, and A/C is used in perhaps 25% of school busses, according to a TMSS presentation by Shawn Vehr of Emerson Climate Technologies.
But the need for long refrigerant lines to the rear of the vehicle has been a deterrent to A/C installations. Although diesel-powered school busses are predominant, work is underway to develop electrically powered busses. Emerson’s proposal is for an all-electric bus using two electrically powered self-contained heat pump assemblies with vapor injection. This would permit year-round use.
According to Vehr, off-vehicle testing indicated a severe loss with PTC heating alone for an electric bus with a 600-volt, 440 kW·h battery pack, with a projected range of 305 mi/488 km diminishing to 177 mi/283 km. With a vapor-injected heat pump system, however, the range would be 230-257 mi/368-411 km, he said.
The heat pump was demonstrated to be capable of pulling cabin temperature from a cold soak at 0°F up to 68°F (-18°C to 20°C). It also would meet the NTSB (National Traffic Safety Bureau) standard for hot soak pulldown to 80°F from 100°F (27°C to 38°C) in 30 minutes, though falling slightly short of high- performance cooling standards.
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