CBS Newsletter
Fall 1994
pg. 8

Smart Thermal Skins for Vehicles

With a modest effort, many of the energy-efficient technologies developed for buildings can be transferred to the transportation sector. The goal of vehicle thermal management research at LBL is to save the energy equivalent of one to two billion gallons of gasoline per year, and improve the marketability of next-generation vehicles using advanced solar control glazings and insulating shell components to reduce accessory loads. Spectrally selective and electrochromic window glass and lightweight insulating materials improve the fuel efficiency of conventional and hybrid vehicles and extend the range of electric vehicles by reducing the need for air conditioning and heating, and by allowing the downsizing of equipment. At the same time, thermal comfort is greatly improved, safety is enhanced by reduced glare and heat, degradation of interior surfaces is slowed, and the weight and cost of the vehicle are reduced because of downsized heating and cooling equipment.

Benefits of Thermal Skins

The Problem

For several decades, the trend in automotive design has been toward larger and more steeply sloped windows. While these designs improve aerodynamics, they also increase the amount of solar heat gain through the glass. On a hot summer day the interior air temperature of a car parked in the sun will exceed 65 degrees C (150 degrees F). Interior surface temperatures will exceed 90 degrees C (200 degrees F). Automakers in Detroit have traditionally solved the cooling problem with the use of large inefficient air conditioners, typically of the same size that would be used to cool an average house. In terms of fuel penalty, manufacturers have no incentive to downsize equipment because air conditioning does not enter the fuel efficiency equation used to determine compliance with Corporate Average Fuel Economy standards. There has been no incentive to move towards more efficient air conditioners because they are more expensive. For fuel-efficient next-generation vehicles, minimizing the power drawn by heating and cooling equipment is imperative because it consumes a larger fraction of the fuel as the drive system becomes more efficient. It is also necessary to minimize loads in electric vehicles because heating and air-conditioning can reduce the effective range by as much as 40%. Although automotive air conditioning efficiency data are surprisingly difficult to come by, efficiency can almost certainly be improved.

EV range for different heating options

Advanced Technology Solutions

The problem of creating a comfortable thermal environment without adding excessive weight, space, or power requirements to the vehicle can be solved through aggressive thermal management using advanced window glazings, insulations, and fans. Photovoltaic powered ventilating fans are particularly well-suited to automotive applications and would substantially reduce interior temperatures for cars parked in the sun. Winter heat losses, a particularly serious problem for electric vehicles because they have no waste engine heat, can be reduced using body insulation, low-emissivity coatings on the glass, and thin double-glazed windows. Double-glazed windows are already being used in expensive European sedans to reduce the noise level in the passenger compartment. New insulation technologies, developed originally for appliances and buildings, are also promising for transportation applications. LBL recently patented one such advanced insulation, gas-filled panels (CBS News, Winter 1993, p. 9), that is particularly well suited to automobiles and aircraft because it is lightweight and inexpensive. Gas-filled panels are CFC/HCFC free and are up to four times more insulating than conventional fiberglass and twice as insulating as CFC foams.

Spectral properties of an ideal solar control glass.

The single largest climate control problem in most vehicles is reducing the solar heat gain through the window glazing. Fortunately, several promising glazing technologies are available or under development, largely because of the work to improve windows in buildings. Solar radiation incident on a window is either transmitted, reflected, or absorbed. The thick blue line in the figure above shows the spectral properties of an ideal spectrally selective glazing, which transmits most of the visible wavelengths of the solar spectrum while reflecting the invisible ultraviolet and infrared wavelengths.

Some green and blue tinted glasses can have a sharp spectral response as described above, although they absorb rather than reflect the solar infrared. The main advantages of tinted glasses are that they have the same high durability as clear glass, and cost only slightly more. However, the energy and visual performance of absorbing glazings is not as good as that of the best reflective products. Spectrally selective window glazings block solar infrared, but are nearly clear and have the high visible transmission required by law for windshields. Silver-based thin films most closely approach the ideal behavior illustrated in the figure. For the back side and rear windows, the transmission band can be lowered to reduce solar heat gain even further.

The ultimate solar control product will be a glazing whose properties can be controlled dynamically by applying a small control voltage. Electrochromic films work like a thin film battery, allowing active optical response to changing environmental conditions. Electrochromic devices change optical properties according to an applied current, so they can be linked directly to the vehicle HVAC system. They require very low power, and since they only require power to switch, they are stable in any given state, and won't suddenly go dark if there is a power failure. Although still in the developmental stage, electrochromic films show great promise if the cost and performance targets can be met.

Instrumented thermal mannequin used to study human comfort.

To assess the effectiveness of solar control glazings and insulation in improving thermal comfort, field and laboratory tests are planned in collaboration with the Thermal Comfort Laboratory at the University of California, Berkeley. The laboratory has high quality, specialized instrumentation, including a segmented thermal mannequin that approximates a full-scale human sensor. It can make measurements of thermal stratification, radiant asymmetry, local jets of high-speed air flow, and the convective plume formed around the body by its own heat. The mannequin will be used in conjunction with infrared thermography, temperature measurements, and thermal analysis models.

—Deborah Hopkins

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Deborah Hopkins
Building Technologies Program
(510) 486-4922


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