Battery-electric vehicle (BEV) range loss from winter use of PTC (positive temperature coefficient) heating has become a source of major concern as the industry works to make the BEV a mainstream choice. A fleet test of electric delivery trucks is under way on European roads. Addressing this particular problem area with an economically viable solution was deemed a worthwhile effort, made by the Austrian Institute of Technology (AIT), which is jointly owned by the Austrian government and the Federation of Austrian Industries.
The AIT project, results of which were presented at the recent SAE Thermal Management Systems Symposium (TMSS), began by noting a 20% loss in range at -15°C (+5°F) from the heating system. AIT set a target of recovering 25% of that loss, primarily with the use of a ventilation module. Available reference data (at -10°C/+14°F) showed approximately 75% of the cabin heat losses are from ventilation, AIT said, with the remaining 25% from heated air transmission through the interior.
The AIT researchers, led by Daniele Basciotti, looked at all the currently considered alternatives, particularly the heat pump and radiant heating panels, which they eschewed as too expensive. They also rejected maximum use of the HVAC recirculation mode as likely to cause windshield fogging (as well as possible interior discomfort from a rise in interior humidity, and respiratory issues from carbon-dioxide buildup).
The ventilation module system offers several advantages. First, it is an air and heat exchange approach, in which the heated interior air goes into the module, where it preheats incoming fresh air, prior to discharging that interior air. So, with an efficient heat exchanger, much of the heat is not lost as the interior air is rejected to ambient. Further, because the heat from interior air is used to preheat incoming air, the ventilation system is bringing in a constant stream of fresh air, which prevents windshield fogging.
To satisfy the minimum requirements of ANSI/ASHRAE standard No. 62-2001, the ventilation module system would need an air exchange rate of just 30 m3/hr/person. The air exchange rate in the AIT ventilation project was maintained at approximately 210 m3/hr, well above that minimum.
As a heat recovery unit (HRU), the AIT team used a Recair counterflow heat exchanger, 300 mm (11.8 in) in height, with an “ideal” rating of 95% thermal efficiency, which it built into the HRV (heat recovery ventilation) module. Recair is a Dutch supplier specializing in this area. Ventilation modules have long been used to deliver preheated fresh air for tightly-insulated commercial and residential buildings, as part of energy-efficiency upgrades.
The entire assembly, called an HRV module, was built by the AIT researchers in its climate-controlled chamber, with assembly procedures that maximized air-sealing of the joints. An “ideal” system, assuming no duct losses and a heat exchange efficiency of 95%, would reduce heat demand by 64%, the AIT team claimed. But the “real world” design, although it obviously couldn’t perform at that level, still showed it was capable of reducing the heat load by 37%, which should significantly improve vehicle range in cold weather. The AIT group concluded there still were opportunities for measurable improvements in the thermal efficiency of the HRV module.
For the conceptual part of the project, the AIT researchers focused on a small fleet of MAN eTGN delivery trucks presently in urban evaluation, that have a rated range of 200 km (120 mi). The fleet test is co-sponsored by CNL, an Austrian environmental group focused on sustainable technologies.
Determining an actual vehicle range increase would require road-testing a specific truck in which the system was installed, of course, and would vary according to the ambient temperatures of operation and amount of range lost to PTC operation.
In addition to the ventilation module, the AIT group also noted that preconditioning the interior as part of the battery charging mode, and upgrading interior insulation to reduce heat loss, were available low-cost technologies to further extend range. But because of the limitations of the AIT project, without a specific vehicle, they also could not be quantified.Continue reading »