This article is presented by:J. Rugh
R. Farrington

Vehicle Ancillary Load Reduction Project Close-Out Report

INTRODUCTION

The amount of fuel used for climate control in vehicles affects our nation’s energy security significantly as it lowers the fuel economy of the 230 million light-duty conventional vehicles in use in the United States today. Researchers at the National Renewable Energy Laboratory (NREL), a U.S. Department of Energy (DOE) national laboratory, have estimated that the United States consumes about 7 billion gallons of fuel per year just to air-condition light-duty vehicles. Therefore, the primary mission of NREL’s Vehicle Ancillary Load Reduction Task was to develop and evaluate technologies that reduce the amount of fuel used for automobile air-conditioning (A/C). This report summarizes the results of evaluations, conducted over the last 10 years, of technologies and techniques for reducing A/C fuel consumption.
NREL’s researchers used a variety of tools to research and develop innovative techniques and technologies that reduce the amount of fuel needed for a vehicle’s ancillary loads. Specifically, their efforts have led to the following:
Development and testing of ancillary load reduction technologies for reducing A/C fuel consumption while keeping occupants comfortable
Development of a passenger compartment cooling system using waste heat as an energy source.

In addition to its impact on the fuel economy of conventional vehicles, A/C can reduce the fuel economy of advanced vehicles by as much as 35%, which in turn can increase their fuel consumption as much as 50%. To address these issues, NREL worked closely with the automotive industry to develop techniques to reduce ancillary loads, such as climate control, in vehicles. We conducted research to improve vehicle efficiency and fuel economy by controlling the climate in the vehicle while keeping passengers comfortable. As part of this effort, we conducted research in integrated modeling, optimized techniques to deliver conditioned air to vehicle occupants, conducted thermophysiological modeling, and investigated waste-heat cooling and heating opportunities.
There is great potential to reduce A/C fuel consumption because A/C systems have traditionally been designed to maximize capacity, not efficiency. Therefore, among other modeling activities, we used an integrated vehicle thermal modeling process to estimate the potential reduction in A/C system size and fuel use resulting from the use of solar reflective glass and solar-powered parked-car ventilation (reducing capacity). Using a Cadillac STS as an example vehicle, we determined that the vehicle’s A/C cooling capacity of 5.7 kW could be reduced by 30% to 4.0 kW while maintaining a cooldown performance of 30 minutes. A vehicle simulation showed that reducing the A/C load by 30% decreased A/C fuel consumption by 26%.
Reducing Thermal Loads
When a vehicle is parked, typically 50% to 75% of the thermal energy entering the passenger compartment is from the solar energy transmitted and absorbed by window glazing. Reflecting the solar radiation incident on the vehicle’s glass is a critical step in making significant reductions in the thermal loads. Lower thermal loads make it possible to reduce the capacity of the A/C system. As part of the Improved Mobile Air Conditioning Cooperative Research Program (I-MAC), NREL tested a new type of solar-reflective glass that improved the reflection of the near-infrared (IR) portion of the solar spectrum on a 2005 Cadillac STS. The Sungate EP automotive glass allowed only 3% of the IR energy to be transmitted through the glass. Using this technology at all glazing locations reduced the average air temperature by 7.1°C (12.8°F), the seat temperature by 8.7°C (15.7°F), the windshield temperature by 19.3°C (24.7°F), and the instrument panel surface temperature by 14.6°C (26.3°F).
Another way to assess these data is in terms of maximum possible temperature reductions, in other words, the difference between a baseline vehicle’s average air temperature and ambient. Using solar-reflective glass in all locations reduced the average air temperature by 34% of the maximum possible, and the seat temperature by 35%. Using reflective shades and electrochromic switchable glazing are also effective techniques for reducing the solar energy entering the passenger compartment. We also found that solar-reflective coatings on exterior opaque surfaces and body insulation can reduce a vehicle’s interior temperatures, but to a lesser extent than solar-reflective glazing, shades, and parked-car ventilation can. Heat pipes were found to significantly reduce instrument panel (IP), windshield, and air temperatures.
Improving Ventilation
Because solar energy entering a vehicle heats the interior mass of the passenger compartment, which in turn heats the air, venting the warm air and pulling in cooler ambient air can reduce the temperature of the vehicle’s interior. Therefore, NREL tested various natural and forced ventilation techniques for parked cars on a 2000 Jeep Grand Cherokee. The data showed that using strategically located air inlets for natural convection induced flow can be nearly as effective as using forced convection ventilation with the heating, ventilating, and air-conditioning (HVAC) blower speed set to medium. With the sunroof open 6 cm and floor inlets, buoyancy induced flow reduced the cabin’s air temperature by 5.7ºC, a reduction of 38% of the maximum possible. In contrast, operating the HVAC blower on the medium setting reduced the average air temperature by 6.9ºC.
We also tested a solar-powered ventilation system for a parked car as part of I-MAC thermal soak testing on the Cadillac STS. The average air temperature was reduced 5.6ºC, and the seat temperatures were reduced 5º-6ºC when air was exhausted from the vehicle. These represented 26% and 21% of the maximum possible temperature reductions, respectively. Using interior window shades to block solar radiation can result in a warm layer of air between the window and shade. So, we investigated ventilation strategies that exhaust this warm air and prevent it from mixing with the air in the cabin.
Using Innovative Cooling Technologies
Conventional vehicles generate waste heat typically equivalent to 60% to 80% of the chemical energy in the fuel. Therefore, we investigated capturing a vehicle’s waste heat using thermoacoustics to power a cabin cooling system. We also developed a thermoelectric analysis tool to assess the feasibility and performance of thermoelectric waste heat recovery systems.
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