Integrated Modeling to Predict Occupant Thermal Comfort

The two primary functions of a vehicle climate control system are safety through de-icing and de-fogging windows, and occupant thermal comfort. However, vehicle air-conditioning systems can significantly impact fuel economy and tailpipe emissions of conventional and hybrid electric vehicles (HEVs) and reduce electric vehicle (EV) range. In order to meet the new U. S. Supplemental Federal Test Procedure (SFTP), as well as growing concern about vehicle fuel economy, automotive engineers are being challenged to evaluate a multitude of new opportunities for reducing the impact of vehicle air-conditioning systems on fuel economy and tailpipe emissions. Because there isn’t enough time to fabricate and test each system, a good modeling approach is essential. However, many models are required to evaluate solar spectral data, glazing spectral properties, cabin temperature and velocity fields, occupant thermal comfort, and vehicle fuel economy and tailpipe emissions. The focus of this paper is to describe an approach used at the U.S. Department of Energy’s National Renewable Energy Laboratory to evaluate the largest climate control load, air conditioning, by integrating diverse models. 1.0 Introduction The mission of the U.S. Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) is to lead the United States toward a sustainable energy future by developing renewable energy technologies, improving energy efficiency, advancing related science and engineering, and facilitating commercialization. To support this mission, NREL’s Cool Car Project works with the automotive industry to reduce the fuel used for vehicle climate control by 50% in the short-term and 75% in the long-term while maintaining or improving the occupants’ thermal comfort and safety. This paper focuses on the largest vehicle auxiliary load – air conditioning. The power necessary to operate a vehicle air-conditioning compressor can be greater than the engine power required to move a mid-sized vehicle at a constant speed of 56 km/h (35 mph). The air-conditioning load can decrease the fuel economy of a conventional vehicle by 10-20%, a mild HEV by up to 35%, and 3L/100-km vehicle by 50%. The United States could save over $6 billion annually if all the light-duty vehicles in the country achieved a modest 0.4-km/L (1-mpg) increase in fuel economy. It is challenging to reduce the climate control loads in a vehicle without adversely affecting occupant thermal comfort. Occupant thermal comfort modeling is essential to ascertain the acceptability of advanced, energyefficient thermal comfort systems. Modeling has certain limitations and assumptions, however it can provide a relative comparison between system configurations. A benefit of modeling is to evaluate and select systems prior to fabrication and testing, therefore, there is a great need to rapidly evaluate advanced thermal comfort system designs through modeling. The models involved are inter-disciplinary, including expertise in thermal/fluids, statistics, meteorology, optics and materials, human physiology and psychology, and vehicle systems, leading to creative thinking and innovation. 2.0 Background In 1998, gasoline use in the United States was about 473 billion liters (125 billion gallons) for on-road use, including gasoline-fueled commercial trucks. Also in 1998, there were about 203.6 million cars and light-duty trucks on the U.S. roads using an average of 2316 liters (612 gallons) of gasoline per vehicle annually. Given certain assumptions about automobile use and air-conditioning use, about 235 liters (62 gallons) of gasoline are required annually for operating the air-conditioning system. An additional 12.7 liters (3.4 gallons) per vehicle are used to carry the additional weight of the air-conditioning system leading to about 40 billion liters (10.6 billion gallons) of gasoline annually in the United States for operating vehicle air conditioning. Until recently, little has motivated U.S. auto makers to find ways to reduce the impact of air conditioning on fuel economy and emissions. But a new emissions regulation, the Supplemental Federal Test Procedure (SFTP), includes operating the air conditioning during part of the emissions testing procedure. The SFTP for vehicles with gross vehicle weight under 2720 kg (6000 lb.) applies to 25% of model year (MY) 2001 vehicles, 50% of MY2002 vehicles, 80% of MY2003 vehicles, and 100% of MY2004 vehicles. Although the SFTP is not used to measure fuel economy, reducing the weight of a mid-sized vehicle’s air-conditioning system by 9.1 kg (20 lb.) results in about a 0.04 km/L (0.1 mpg) increase in fuel economy on the current combined city/highway test. The Clean Air Vehicle Technology Center has measured the effect of the air-conditioning system on fuel economy and tailpipe emissions for a variety of vehicles. Table 1 compares seven vehicles (’95 Voyager, ’97 Taurus, ’95 Civic, ’95 F-150, ’97 Camry, ’96 Camaro, and ’95 Skylark) with the air-conditioning system on and with the air-conditioning system off over the SC03 drive cycle. Table 1. Measured Impacts of Air-Conditioning System Operation Increase with Air Conditioning On