Cool Solutions for Hot Cars
On this first day of summer, many car owners are likely to experience the following scenario: enter your car to leave work for the day and the temperature is sweltering—much hotter than outside. The ignition, steering wheel, and seat surface are almost too hot to touch. You roll down your windows or turn on the air conditioner (or both) to get some air moving to quickly mitigate the sauna-like conditions.
Cars are a classic case of the greenhouse effect: visible light is absorbed by the various surfaces within the vehicle. As those surfaces re-emit that energy as heat, glass—opaque to the long-wavelength radiation associated with infrared heat energy—traps it inside.
This is more than just a nuisance on hot days. Of the oil consumed by U.S. passenger vehicles, 5.5 percent is used for air conditioning. For today’s average internal-combustion-driven vehicle, air conditioner use results in up to a 26 percent reduction in mpg. For an electric vehicle, this translates to a 36 percent reduction in range.
Vehicle air conditioning systems are sized to handle worst-case temperatures such as the hottest of hot summer days, with interior temperatures well in excess of 110 F. Stringent human comfort standards require that the temperature be brought within a comfortable range—typically around 70 F—within minutes. A rarely-encountered, temporary condition thus determines the permanent capacity, size, weight, and cost of the evaporator, coolant, fans, ducts, and compressor that make up the air conditioning system. Given their mostly unused excess capacity, these systems then operate at suboptimal efficiency in moderate conditions—the majority of the time.
By using approaches that harness an understanding of heat transfer, human physiology and psychology, and advanced technology, thermal comfort (not to mention fuel economy and EV range) can be improved in tomorrow’s vehicles—while making a substantial step toward U.S. oil independence.
Finding the Leverage Points
Imagine the possibilities if our cars took lessons from nature. For example, a parked car’s cabin temperature could be maintained closer to the outside temperature by passively drawing in outside air. Termites make use of this technique in Africa and Australia, inducing passive convection airflow to cool the interior of their mounds by up to 20 F.
A 2007 study conducted by the National Renewable Energy Lab (NREL) indicates that strategically-placed vents that induce natural convection airflow could do something similar for vehicles, reducing interior cabin temperature on hot days by 11 F, and allowing a 25 percent reduction in air conditioning compressor power and a significantly downsized air conditioning system. This zero-energy approach provides nearly equivalent benefit to forcing outside air into the cabin by running the ventilation fans at medium power while parked.
If worst-case maximum cabin air temperatures could be reduced, drivers would not only save on fuel costs associated with blasting their air conditioners, but would further benefit from the increased space and reduced weight and cost of a downsized (but equally capable) air conditioning system. Best of all, drivers would experience increased comfort due to experiencing cooler temperatures upon entering the vehicle, and have to worry less about leaving their groceries or dog in the car on a moderately warm day.
Expanding the Problem
Passively cooling the car’s interior to reduce heat loads enables a smaller system to provide the same comfort, but passive techniques can’t get us all the way there. Vehicles still need some type of active system to provide ventilation and deliver thermal comfort to the passenger. But is the current approach of blowing air from the dashboard the best way to do it?
No, suggests research from NREL and the Center for the Built Environment at University of California, Berkeley. There are several alternative techniques that could actually improve comfort with reduced energy consumption relative to today’s approaches.
Humans maintain thermal comfort by achieving a heat balance. If the heat produced by the body’s metabolic processes is not dissipated at a constant rate, heat builds up in the body and we feel hot. If heat is lost faster than it is produced metabolically, we feel cold. Our own thermoregulation system has built-in means of heating us up (shivering to increase our metabolic rate), or cooling us down (sweating to provide evaporative cooling). So long as the produced and incoming heat is balanced with the outgoing heat—barring any extreme or asymmetric local temperatures from one part of the body to the next—we feel comfortable.
In a car, there are several ways to achieve this balance. Blowing air from the dash is one of them, but it turns out to be among the least efficient. Because the passenger is in constant contact with the seat, cooling via direct contact becomes a reliable means of efficiently dissipating heat.
Ventilated seats are not a new concept in the auto industry, but electric vehicle manufacturers are taking renewed notice of the important contribution they can make to improve range if their efficiency is considered within the thermal comfort system as a whole. The seat can also provide a delivery platform for something UC Berkeley calls “task ambient cooling” whereby small fans deliver close-proximity air flow to the face and neck, among the parts of the body that influence thermal comfort the most. This air flow can furthermore be delivered via “transient” means, providing small bursts of cooling on an intermittent basis: an approach that elevates the thermal comfort response of the passenger relative to the “steady state” airflow approach typical of today’s vehicles.
RMI looks for high-leverage, high-impact ways to reduce U.S. oil dependence and move our country toward a petroleum-free U.S. transportation system by 2050. Tackling the problem of thermal comfort in vehicles could dramatically reduce oil consumption in the near term, while enhancing the value proposition of EVs by lending customers greater range, savings, security, and comfort.