The objective of this study was to design and test envelope, heating, and ventilating systems, which will reduce the reliance on fossil-fuel-based heating technologies in electric vehicles operating in cold climates. To accomplish this, passive solar design and test methods were adapted from buildings to vehicles. Two electric shuttle bus designs were tested over the past two heating seasons in New England. The first bus, a US Electricar 22-foot shuttle bus, is the first battery-powered bus in New England and is owned by the University of Massachusetts Lowell as a shuttle bus both on campus and at Logan Airport. The second bus is an Advanced Vehicle Systems (AVS) 22-foot shuttle bus, owned by the Greater Portland Transportation District, Portland, ME.
The buses were tested using a coheating test which involves providing a known amount of energy to the interior space while measuring the temperature gradient between interior and exterior and measuring infiltration (air changes per hour) using an inert gas chromatograph. These tests were performed under different circumstances including various external factors such as different times of day, and weather conditions, and operation factors such as speed, and door openings per hour.
The modifications and innovations that were tested include installing double and triple glazed windows and doors, insulation in the ceiling, walls, floor, and battery chambers, air-to-air heat exchangers, waste heat recovery systems, infiltration sealing materials, and a means of thermally separating the driver compartment from the passenger compartment. The glazing changes consisted of adding storm windows of 1/8" transportation grade Lexan (with reasonably good transmittance) to the interior surface of each window frame in the passenger compartment. The added insulation included fiberglass in the body panels of the ceiling and walls, rigid foam board under floors, front dash board surfaces, and on all battery box surfaces, and spray-on expanding foam insulation on the front and bottom of the AVS bus. The air to air heat exchangers that were installed are capable of conserving 80% of the energy contained in the exiting cabin air of the bus which is designed to hold up to 22 people. The waste heat recovery systems involved drawing ventilation air from compartments warmed with motor and controller waste heat. The infiltration sealing measures included filling gaps in the body envelope with expanding foam insulation and adding gaskets to the window and door seals. The thermal separation of the driver compartment was accomplished with a transparent sheet of plastic hung behind the driver across the width of the bus.
Among the results of analysis on models developed from the test data are the following conclusions. In order to attain the goal of retaining the classification of zero emission vehicle, which requires no fossil fuel use during periods with ambient temperatures above 40 F, and of using less than 10% of the electric energy storage capacity for heat, an electric bus operating in Boston will need insulation resulting in a total UA of less than 200 Btu/ft^2-h-F, including infiltration losses. We have found that this goal can be met using effective air sealing, double glazed windows in the passenger compartment, insulation of R-7 in the roof, walls, and floor of the body, and heat recovery from the motor controllers and motor. More insulation earlier in the vehicle design stage is recommended for all electric vehicles.
The advantages of the design changes of this study include the maximization of passive solar space heating in electric vehicles in cold climates and minimization of fossil fuels use. Passive solar gain estimates were reported in a previous paper. Electric vehicles can become more viable and more environmentally beneficial by using the above well-established passive solar technologies.
Acknowledgement: The work on the AVS bus was undertaken jointly with Evermont through funding from DARPA through NAVC.
For more information contact John_Duffy@uml.edu
