Simple Solutions For Cooling Down Hot Cars

by Greg Rucks, via Rocky Mountain Institute

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.

Greg Rucks is a Consultant for Transportation and Industry at the Rocky Mountain Institute. This piece was originally published at RMI’s Outlet and was reprinted with permission.

18 Responses to Simple Solutions For Cooling Down Hot Cars

  1. Dano says:

    Two things left out:

    o Excessive heat allows fuel to vaporize and if the system has a leak, smog precursors are released to the atmosphere.

    o Trees cast shade and lower ambient air temperatures, ameliorating conditions and improving performance.



  2. Pete H. says:

    When there is no rain in the forecast I always leave my windows down a 1/2 in. or so. Enough to vent hot air, but not enough to allow someone to easily break into the car. It’s made hot summer days much more bearable.

  3. catman306 says:

    Vehicles that are painted white are much cooler in the hot sun and therefore use less A/C (and fuel).

    Next time buy a white car or truck. I’ve got one of each, here in Georgia, and neither has A/C.

  4. Chris says:

    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.

    I believe this is perpetuating a false myth. The reason greenhouses get hot is not because the glass in opaque to infrared, but because of the lack of convection keeps the hot air trapped near the surface. R.W. Wood in 1909 built two greenhouses, one made of glass, another made of salt (transparent to infrared). The temps in both were identical.

    Now of course CO2 and H2O are greenhouse gases which are opaque to infrared and cause a greenhouse effect by slowing the rate at which the IR can escape not by convection.

  5. Joan Savage says:

    For me, an attraction of automobile AC is that it dehumidifies, affecting comfort.

    How much energy does a car dehumidifier use, if operated separately from AC?

    Could I save on gas if I kept my cabin air at lower R.H., while letting air temperature stay higher?

  6. Joan Savage says:

    I don’t have the pleasure of owning an EV, but obviously a similar question applies to EVs for how much charge it takes to operate a dehumidifier as compared to an AC.

  7. Paul Klinkman says:

    Cars need their greenhouse effect in winter.

    A simple white sheet deployed on the windshield, perhaps a stretch cloth with four hooks that latch onto four spots on the edges of the windshield, would reflect heat back out quickly. Deployment of the sheet could be automated.

    People might want a fan system that starts exhausting excess heat as soon as the car is electronically unlocked.

    Today’s smart cars should slowly exhaust the absolute worst of the heat, keeping internal temperatures down to, say, 105 degrees, all day if necessary. The amount of energy needed to take the top off of the temperature with intermittent fan use is far less than the amount of energy needed to cool the passenger compartment. Leave this feature off or on depending on when the driver expects to get back.

  8. SecularAnimist says:

    I have a 20 year old Kia that has no air conditioning, and gets 35 MPG in stop-and-go city driving, and over 50 MPG on the highway. Unfortunately, it is hard to find 2012 model cars that don’t have air conditioners factory-installed. And even if you never use the AC, it still adds a lot of unnecessary weight to the car, which as the article notes, reduces the fuel efficiency (or in the case of EVs the range) of the car.

    A while back I saw a gadget with small fans powered by a miniature solar panel, that mounted in the top of a car’s partly-rolled-down side window. You would stick the thing in a window facing the sun, and the fans would draw fresh air through the car while it was parked. Seems like a similar solar-powered ventilation system would be pretty easy to build in.

  9. Yes, indeed, Lawrence Berkeley National Lab has done excellent R&D on cool cars, based on high-albedo (heat-reflecting) paint finishes, notably white. See the recent research findings in the report: Levinson, R., H. Pan, G. Ban-Weiss, P. Rosado, R. Paolini, and H. Akbari, “Potential benefits of solar reflective car shells: Cooler cabins, fuel savings and emission reductions”, Applied Energy, 07/2011, available at:
    and also see ongoing research at:, and the 112-page (12 Mb) final research report funded by Calif Energy Commission, Cool‐Colored Cars to Reduce Air‐ Conditioning Energy Use and Reduce CO2 Emission, Jan 2011

  10. John Hollenberg says:

    My Nissan Leaf uses 1/4 to 1/2 kwh for my drive home in the summer. Since it takes about 9 kwh for the round trip, this means that the penalty for the trip is at most 5-6% compared to not using the AC. Other Leaf owners report similar high efficiency for AC use. Heating is a completely different story for the Leaf–it uses a lot of energy. A heat pump will hopefully be used in future models. Note that the link the author uses is from 2010, before the Leaf was even released, and is a worst case scenario most won’t encounter.

  11. ozajh says:

    In 1973 I commuted from home to university classes on a vehicle which cooled down EXTREMELY rapidly once moving on the road, easily kept up with non-freeway traffic, and got me about 100 miles per US gallon.

    It was a Honda C90 scooter.

  12. AFAYOL says:

    In France, I installed 5m2 of solar panels on my EV:

    1) it powers me 5000km to more than 10000km yearly depending of driving mode.

    2) during hot sunny hours the 5m2 PV also offer shade and cooling!

    However for winter we need to insulate better EVs (including double glasses windows?)

  13. lizardo says:

    Hopefully newer cars will be better designed.

    Since most of us are going to have to deal with the type of vehicle we have now, here are my strategies. Beside the obvious ones of parking in shade, cracking windows

    a) carry an old sheet in vehicle or trunk. Park facing the sun’s path (heading south). stretch sheet over front windshield and secure ends in both driver and passenger doors. (Caution: can’t do this when it might rain as the sheet will wick rain into the vehicle).

    I got this (same) idea from a sun-shade using friend who thought that the sun hitting the glass would heat up the car, interior sun-shade or no.

    I felt that an exterior-fixed sunshade would get pinched, and my ratty sheet is locked in place and not very desirable. Even as those pandering ads say “especially these days.”

    b) I’ve noticed that the metal vents heat up in the sun so that when starting a hot car the cold air runs hot. If you run the AC for just a short while it will chill those bits so that the cold air (only, not AC) is adequate breeze effect on driver (and passenger). For a longer trip this can mean using AC only part of the time. I am not able to tell if this increases my mpg.

    I’m seriously considering cutting a few little trees (aak) to create more shade parking near my house too. (Not as illogical as it sounds.)

  14. lizardo says:

    Well, a couple of “doh” amendments to my comment.

    Firstly, well new cars are probably not going to be better designed without a lot of pushback. Time for those who can to link to this comment, tweet away.

    Secondly, I meant to say I didn’t know if my on/off AC use when it’s in the high 90s, decreased my mpg, not increased.

  15. catman306 says:

    Strickly my opinion: To use an auto A/C as a dehumidifier, let the A/C run but after the car is cool enough set the A/C to the highest temperature so the compressor doesn’t run often, just the fans.

  16. Robert In New Orleans says:

    Ten years from now, the number of dark colored cars on the road will decline as manufacturers struggle to sell them while summer temperatures soar, ditto dark interiors.

  17. Ric Merritt says:

    That figure about loss of MPG to AC usage must be some kind of dire worst case. In my Prius, the effect of AC usage on mileage is barely detectable. Maybe a couple percent?

  18. J4zonian says:

    My 1984 Trek 710 (200,000+ miles and counting) does the same thing, although I usually feel warmer when I finish than when I started. Don’t mind it at all; just like cutting wood to burn warms you twice, riding a bike saves money in oh, about a dozen ways, including no membership at the gym.