Solar glass delivers a small boost in EV range

  • 03-May-2013 04:24 EDT

Effects of A/C and heat on Mitsubishi EV range, calculated by NREL.

Automakers have to sell a lot of electric vehicles (EVs) to earn the Corporate Average Fuel Economy (CAFE) credits they need to sell gasoline-powered full-size cars in the years to come. However, to be able to sell more EVs, they must deal with the passenger cabin climate control issue that is causing precipitous drops in range in hot and cold weather.

At the same time, the U.S. Department of Energy has an "EV Everywhere" program to try to enable nationwide use, and its National Renewable Energy Laboratory (NREL) is backing that program with a series of research projects, NREL Senior Engineer John Rugh told an SAE 2013 World Congress session in mid-April.

The NREL project list includes improvements in HVAC equipment, preconditioning of the passenger compartment, and energy load reduction and management. But it's not skipping any possibility, and NREL's Rugh pointed out both the magnitude of the problem and an example of one small step forward: use of solar glass.

Mitsubishi i-MiEV tests 

EVs use electric compressors for air-conditioning and PTC (positive temperature coefficient) resistance heaters for cabin warmth in cold weather. So it's not surprising the rated 100-mi (160-km) range is achieved strictly in the moderate ambients of spring and fall. Rugh noted that a Mitsubishi study of its i-MiEV showed large drops in the EV range from A/C-on at 35°C (95°F) and heater-on at 0°C (32°F).

From Mitsubishi data, he calculated that even in ECO mode, the A/C-on range drops 34% (to 66 mi/106 km) and in Max Cool, about 46%, leaving a range of 54 mi (86 km). Because the resistance heaters are even less efficient than A/C, the drop in ECO heating mode is 46%. With the heat turned on high, range drops 68% (to just 32 mi/51 km).

If the car is in a long, solar-induced hot soak in summer, or not preconditioned on a cold winter day, so the driver gets in and sets A/C or heat to the maximum, those numbers establish the peak to the challenge the EV industry faces.

Solar glass can be part of a package that reduces the hot soak effect, along with passive and/or active cabin ventilation and solar reflective paint. So for initial cooldown, the range-conscious driver might be able to select ECO mode while maintaining comfort levels, instead of Max Cool, which can make a big difference in range. The project objective: quantify an equivalent effect on A/C capacity from lowering the vehicle interior soak temperatures with the solar glass.

The temperature reductions on the car interior from the use of solar glass were not insignificant. The NREL test used two identical 2011 Toyota Corollas—white with a black/gray interior—and developed a baseline with multiple days of 1-h temperature readings from 12:30-1:30 p.m. with the OE windshield, made with standard PVB (polyvinyl butral) interlayer. PVB is used to make laminated safety glass, and its use in windshields is mandated by safety regulations. The cars were parked side by side, with the windshields facing straight south, at the NREL laboratory in Golden, CO. Measurements were taken in a time period where day to day the environmental conditions were virtually the same, so any inherent differences between the two cars could be determined.

Solar control added to one windshield

The windshield in one vehicle was then replaced with an Eastman Chemical Saflex S Series solar control PVB interlayer, which has enhanced infrared-absorbing properties, can block over 99% of the UVA/UVB wavelengths, and also provides acoustic damping benefits. Once more, the measurements were taken the same time, day after day over four days of essentially identical weather (including wind).

The most impressive temperature drops were on the steering wheel (5 to 5.9°C/9 to 10.6°F), because that's where the driver's bare hands go and are likely to touch first. The dashboard showed the second largest drop—3.1 to 4°C (5.6 to 7.2°F). Because the solar windshield was retarding cabin heat buildup, its temperature went up—as much as 2.2°C (4°F).

The data was plugged into a modeling program to produce capacity figures for 35°C (95°F) at 60% relative humidity, typical of conditions that drivers experience and cause them to use A/C.

Using RadTherm software from ThermoAnalytics, NREL calculated the solar glass windshield could permit a 1.2°C (2.1°F) increase in A/C outlet duct temperature while maintaining the same cabin thermal performance. NREL also determined that the effective reduction in required cooling capacity was 4%.

The windshield alone can't compensate for the large A/C load, but it did indicate an increase of up to 1.0 mi (1.6 km) in EV city range and 0.6 mi (1.0 km) in highway. Every bit helps.

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