The National Renewable Energy Laboratory (NREL) has initiated projects to investigate the benefits and design challenges of using heat pipe/two-phase flow technologies to provide sub-system cooling and thermal management in future advanced vehicles, hybrid electric vehicles, and heavy-duty vehicles. The projects focus on vehicle instrument panel (IP) cooling and passenger seat thermal management, but will also begin investigating engine cooling, electric motor cooling, battery cooling, and fuel cell cooling in the future. Simulated vehicle cabin testing conducted at NREL has recently studied the effect of heat pipe IP cooling. Experimental results have demonstrated IP surface temperature reductions of 20–30°C during maximum solar intensity environments of 525–800 W/m2 (typical of Golden, CO from January to April) compared to uncooled conditions. The heat pipe cooling effect in the IP also reduced windshield temperatures by 9–12°C compared to the non-cooled configuration in April 2001 testing. The cooler IP and cooler windshield would significantly improve driver/passenger thermal comfort in a typical vehicle cabin. In addition, IP cooling also appears to dramatically reduce cabin air temperature by 4–10°C during long-term vehicle thermal soak conditions in these tests. Current estimates indicate that cabin air temperature reductions of 4–6°C may reduce vehicle air conditioning compressor sizing requirements by 16–25%, and increase vehicle fuel economy by 0.8–1.2 miles per gallon (mpg) and 1.6–2.5 mpg in current conventional vehicles with base fuel economies of 20 mpg and 30 mpg, respectively. IP temperature and cabin air temperature reductions may very well be larger in higher solar intensity conditions typical of summer months and more southern latitudes.
Freeze/thaw tests were also conducted on the heat pipe systems in the IP cooling demonstration at NREL. All the heat pipes survived the freezing event, and all but one heat pipe demonstrated the ability to start up and operate normally in both an evaporator-led thaw and a condenser-led thaw process.