Reducing range impact of EV cabin conditioning
Kia's Soul BEV is OE-equipped with features designed to reduce climate-control impact on driving range, including a heat pump. (Kia)

Studies seek to reduce range impact of EV cabin conditioning

If the battery-electric vehicle (BEV) is to enter the affordable mainstream, it has to overcome the challenge of 50-60% loss of range from the impact of cabin heating, ventilation and air-conditioning (HVAC): cabin heating in the winter and A/C cooling in summer inflict an enormous draw on precious battery capacity.

Countless strategies have been advanced and innovative systems installed in development vehicles, but they raise complexity and cost issues. Two presentations at the recent SAE WCX 2018 conference focused on simpler alternatives that showed cost-effective results.

One approach is to add thermal storage capacity to help maintain range otherwise lost to cabin heating. The other is to reduce climate-control loads in warm and cold weather—which could provide faster interior “time-to-comfort” while enabling use of smaller battery packs or extend range with existing packs.  

Enhancing thermal storage

When cost and vehicle size aren’t major factors, a larger battery pack is one answer to maintaining desired driving range without diminished cabin comfort; fitting a heat-pump system is another. Toyota’s current Prius Prime plug-in hybrid-electric vehicle (PHEV), for example, has a sophisticated heat pump system that works down to -10° C (14° F), by incorporating a liquid-gas separator circuit. Typically, automotive heat pumps work down to only about 0° C (32° F) because the mass-flow rate of refrigerants slows as ambient temperature drops. The Prime installation is serving as a technology test bed, as the car’s gasoline engine is capable of providing cabin heat even when battery pack capacity is depleted.

Hanon Systems, working with the Hyundai-Kia America Technical Center and the Department of Energy’s NREL (National Renewable Energy Laboratory), selected the 2015 Kia Soul BEV for its project. The Soul EV is typical of the 75-100 mi (120-160 km) range of lower-cost BEVs marketed for commuter use and local driving—and which encounter significant range-reduction issues in cold winter temperatures and summer heat.

In his WCX presentation, Hanon Technical Fellow Dr. John J. Meyer pointed out that in addition to a conventional PTC heater, the Soul BEV seemingly already has “it all” for maximum range:  it is OE-equipped with a heat pump, climate-controlled seats, a heated steering wheel, cabin pre-conditioning (heating or cooling while plugged in), partial-recirculation A/C, dual-zone and driver-only HVAC, Eco-mode BEV operation and an air-cooled battery pack. Thus Meyer’s research project focused on adding thermal storage.

Meyer noted that the Soul EV’s heat pump use can recover 30-40% of the heat-related range loss that still could leave as little as about 60%, or 60 miles (96 km) of range for a 100-mile BEV. But the heat pump may not be the best use of investment, as the heat pump circuit adds cost, complexity, packaging issues and a large increase in refrigerant charge. As a result, the handful of U.S.-market vehicles with heat pumps use R-134a as the refrigerant, obviously influenced by the fact it’s less than 10% of the price of R-1234yf.

A heat pump circuit with its additional heat exchangers, complex flow paths and tubing may result in nearly double the required amount of refrigerant. The Soul EV has 33 oz (900 g), versus 19 oz (550 g) without a heat pump. Moreover, the use of R-134a in new vehicles is scheduled to end no later than 2021, assuming the applicable Environmental Protection Agency (EPA) regulation legally holds up.

It would seem that aside from the cost of a heat pump system and need for extra refrigerant, the heat pump should have the greatest climate-related effect on BEV range. But that “conventional wisdom” does not necessarily hold true for the typical drive distance of the commuter/local-driving cycle, the Hanon/Kia-Hyundai/NREL study showed.

Hanon modified the Soul BEV by adding two valve-connectible glycol coolant circuits, each with a heat exchanger and total fluid capacity of 2 l (0.5 gal). One heat exchanger contains a 1500-watt electric heater that also produces source heat for a cabin pre-conditioning heater core.  During vehicle operation, that electric feed is turned off and the heating system uses the stored heat and additionally gleans waste heat from the motor/electronics circuit—a heat source that continues to be available when stored heat is depleted.   

The typically short BEV drive, Dr. Meyer said, was 40 min and 50 km (30 mi) and three simulated trips on the EPA UDDS (city cycle) were run in the wind tunnel. The cabin had been  pre-conditioned during battery-pack charging and the HVAC control was set to 22° C (72° F).  The wind-tunnel test temperatures were 5° C (41° F), -5° C (23° F) and -18° C (0° F).

In all three tests, the heat pump alone used more energy than the PTC heater with thermal storage. The testing indicated that the heat pump comes out ahead only when combined with thermal storage. And for short-range operation in a price-sensitive BEV class, the thermal storage system with the PTC heater seems to be the best combination of efficiency with cost-effectiveness. For long-range operation, the heat pump seems likely to come out ahead, although the data is yet to be developed.

Reducing thermal load

The climate-control system thermal-load reduction project at NREL, explained researcher Jason Lustbader, seeks to produce an average 20% increase in electric drive range during summer heat and winter cold. That’s a broader mandate than just the issue created by loss of all-electric range on the 75-100 mi small BEVs addressed by the Soul BEV research.

For this study, the vehicles chosen were a pair of identical plug-in hybrids (PHEVs). In the first (completed) phase of the project, the possible technologies were modeled and their apparent value noted. The second phase consisted of installing them on a Hyundai Sonata PHEV and performing real-world testing on the combined EPA city/highway drive cycles.

Two Sonata PHEVs were used: one unmodified, the second with a special thermal-reduction package: Pittsburgh Plate Glass supplied an electrically-heated windshield, solar reflective glass and solar reflective paint; Gentherm provided heated surfaces around the driver, door glass demisters and climate-controlled seats. The special seats, glass and paint were found to impact both heating and cooling.

Winter testing was performed in Fairbanks, Alaska in late February, 2017 (high ambient was -7° C (20° F). Summer testing was done in Mohave, CA with ambient temperatures between 38-50° C (99-122° F).

The Fairbanks, AL testing of the windshields was an objective measurement. Electric heating cleared the ice in just six minutes, using 0.1 kWh of electricity. The conventional defroster took 19 minutes and required an electrical equivalent of 2.6 kWh.

The comfort measurements were based on passenger-reported sensations. The researchers noted that the driver and passenger could turn down the set points for heat and A/C because their comfort level was achieved with lower energy inputs. In the cold-weather testing, comfort was reached in 15 minutes with the modified Sonata, 29 minutes without the thermal-loading improvements. 

The hot-weather testing started with both vehicles making an all-electric run around the oval at the Hyundai America Technical Center proving grounds in Mohave, CA. With the thermal package, BEV range increased 11.4% and A/C energy usage was cut by 23.7% compared with the unmodified vehicle.

Other testing, including climate-chamber tests, showed even greater improvements and faster time-to-comfort; NREL will be projecting range improvement data across the United States, using simulation and regional climate data it has accumulated from earlier testing and research.  Like the Hanon Systems work on which it cooperated, NREL believes it demonstrates there are cost-effective climate-control strategies that will enable the smaller-battery-capacity, shorter-range BEVs to better meet consumers’ everyday driving needs. Continue reading »
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