This is the first part of the series on the science of car window films. Read Part 2 of this series.
On a warm day, the cabin temperature inside of our parked cars can quickly reach scorching levels. A study by Levinson and associates in 2011 measured that the cabin air temperature in a black car can increase by nearly 35 degrees Celsius in just an hour. The interior air temperature increases because the heat from the Sun is able to penetrate into the car cabin but yet becomes trapped inside it. To understand why this happens requires us to delve deeper into the topic of solar radiation.
Solar radiation is the energy that is emitted from the Sun in the form of photons. Each photon carries a fixed amount of energy which determines the amount that photon vibrates. The distance moved by the photon during one of its vibrations is known as its wavelength.
Photons emitted from the Sun have a wide range of energies and wavelengths, and this whole range is known as the Sun’s electromagnetic spectrum. Almost all of the energy that are emitted from the Sun are in the wavelengths between 150 to 4000 nanometers (nm). But upon reaching Earth, wavelengths greater than 2500 nm are absorbed by our atmosphere’s water vapor and carbon dioxide, whereas wavelengths lesser than 290 nm are absorbed high in the atmosphere by nitrogen and oxygen. Thus, the solar radiation finally reaching us at the Earth’s surface usually has wavelengths between 290 to 2500 nm.
The energy the photon carries is inversely related to its wavelength. This means the longer the wavelength, the lesser the energy each photon carries. In contrast, the shorter the wavelength, the photons carry higher amount of energy. This is why shortwaved X-rays and gamma rays are dangerous because they carry high amounts of energy. In contrast, longwaved radio and mobile phone signals are safe because they carry much lesser energy.
When solar radiation hits the car glass windows, one part of the radiation is reflected off the glass, another absorbed by the glass, and the last is transmitted through the glass. Reflected solar radiation does not contribute to warming, but absorbed solar radiation does: by heating up the glass and warming the adjacent air (both inside and outside the car cabin). It is however the transmitted solar radiation that contributes to the bulk of warming. Once inside, the transmitted radiation will be absorbed by the various parts of the car cabin such as the dashboard, steering wheel, carpets, and car seats. As more incoming radiation is absorbed, these car interior surfaces will become increasingly warmer and, in turn, increasingly warm the interior ambient air.
It is important to note that all objects with temperature will emit radiation. The Sun, for instance, is very hot, so it emits relatively shortwaved radiation which contains high amounts of energy. Cooler objects, in contrast, would emit longwaved radiation because of the lower energy levels.
Consequently, these car interior surfaces, like any objects, would also emit radiation but, compared to that coming from the Sun, the radiation emitted from these interior surfaces carry much lower energy levels and thus have longer wavelengths. And because the outgoing radiation have lesser energy, they are unable to exit the car cabin. The outgoing radiation become trapped. More heat enters than exits the car, so heat builds up inside the car.
As mentioned earlier, the range of electromagnetic spectrum from the Sun that finally reaches Earth is between 290 to 2500 nm. Three important groups of wavelengths exist in this range: ultraviolet (UV; 290 and 400 nm), visible light (VL; 400-700 nm), and infrared (IR; 700 to 2500 nm). The proportion of these three wavelength groups vary, but typically, UV accounts 3% of total solar radiation, VL 43%, and IR 54%. Because of its high energy levels (thus, shorter wavelengths), UV is dangerous to health (e.g., causes sunburn, skin cancer, and eye damage) and can degrade object surfaces. VL is part of the electromagnetic spectrum which we can detect with our eyes; thus, this spectrum is visible to us. Both UV and IR, however, are not visible to us. But it is these three wavelength groups, UV, VL, and IR, that collectively contribute to warming our car cabin.
The car interior does not warm uniformly. Some parts are much warmer than others. In a typical hot day in Malaysia, the dashboard can reach a scorching 80 degrees Celsius, the front windshield 70 degrees Celsius, and the interior ambient air 50 degrees Celsius.
Several methods can be used to cool the car cabin. Solar-powered air ventilators can be installed at the car windows that can draw out the hot air in the car cabin and replace it with the cooler external air. Studies by Rugh and associates in 2007 showed that these ventilators reduced the maximum temperature at the windshield and the dashboard by a maximum of 2.3 and 8.3 degrees Celsius, respectively. Likewise, Flores and associates in 2008 showed that the air ventilators reduced the ambient car cabin air by a maximum of 7.4 degrees Celsius. However, these air ventilators typically suffer from having low air flow rates due to low-powered fans.
Instead of using mechanical air ventilators, leaving an opening (gap) in the car windows could also cool the car cabin by helping to circulate the flow of warm and cool air between the car cabin and the exterior. Kayiem and associates in 2010 showed that by leaving a 20-mm gap in the front side windows helped to reduce the front air temperature in the car cabin by as much as 10 degrees Celsius. However, there was little cooling effect when only the rear side windows were winded down to leave the same 20 mm gap. This difference in cooling effect between the front and rear side windows could be due to the higher temperature difference between the front ambient air and the exterior air than that between the rear ambient air and the exterior air. Recall the dashboard temperature can reach as high as 80 degrees Celsius, and this is a source of much heat exchange in the front car cabin area as compared to the cabin rear.
Nevertheless, use of mechanical air ventilators or leaving gaps in the car side windows can pose a security risk as well as encourage entry of more dust into our cars.
One popular and economical heat rejection method is the use of reflective sunshades. Local studies by Al-Kayiem and associates in 2010 and Jasni and Nasir in 2012 showed that sunshades are highly effective to reduce dashboard and windshield temperatures by as much as 30 and 20 degrees Celsius, respectively. However, compared to window tints, sunshade has been shown to be surprisingly far more effective in cooling the dashboard. Window tinting, for instance, cooled the dashboard by an average of only 7 degrees Celsius (with a maximum reduction of 12 degrees Celsius). Nevertheless, given enough time, the car cabin temperature under the protection of a sunshade would eventually reach the same heat levels as that without a sunshade protection. A sunshade placed only in the front windshield also offered little cooling effect to the rear of the car cabin.
Research have shown that window tinting is overall the best heat rejection method. Jasni and Nasir in 2012 compared the car cabin temperatures protected with a window tint (85 and 65% IR rejection rates for windshield and other windows, respectively) and that with a sunshade (placed on all car windows). They found that window tinting was more effective than the sunshade in cooling the car cabin temperature (front and rear ambient air). There was instead little difference between the car cabin temperature with and without the sunshade protection. The sunshade only cooled the car cabin by an average of 2 degrees Celsius (with a maximum reduction of 6 degrees Celsius), whereas the tint cooled by an average of 5 degrees Celsius (with a maximum reduction of 8 degrees Celsius). Jasni and Nasir also found that air ventilators performed better than sunshade in rejecting heat but not as well as the window tint.
Having a cooler car cabin brings several more advantages other than just giving a better thermal comfort. A cooler car cabin reduces the workload of the car’s air-conditioner (AC). Rugh and associates in 2001 estimated that for every 1 degree Celsius reduction in the car cabin temperature of a Ford Explorer reduces the AC compressor power by 4.1%. Likewise, Levinson and associates in 2011 estimated that by reducing the car cabin temperature of a Honda Civic by an average of 5 to 6 degrees Celsius, the car’s AC workload is reduced; thereby, lowering fuel consumption by 0.21 L per 100 km and lowering emissions of greenhouse gases such as carbon dioxide by 4.9 g per km and nitrous oxides by 9.9 mg per km.
Lastly, window tints have another major advantage over some heat rejection methods: window tints operate at all times even when the vehicle is moving. In contrast, sunshades and air ventilators have to be manually setup and removed each time the car is parked or driven.
Recall that all three wavelength components of solar radiation (UV, VL, and IR) are responsible for warming the car cabin. An effective window film or tint is consequently one that filters out all or at least a large portion of the UV and IR components. The visible light, VL, component cannot be completely filtered out; otherwise, we would not be able to see through the car windows! This also means it is impossible to have a perfect 100% heat rejection window tint because this method would also require the complete removal of the essential VL component.
It is impossible to have a perfect 100% heat rejection window tint because this method would also require the complete removal of the visible light component.
Some car owners, however, deliberately choose window tints that have low visibility levels. Such dark tints or tints with high VL rejection rate are dangerous because these tints can cause road accidents. A 2004 report by Baldock and associates from the Centre of Automotive Safety Research argued against such tints because dark tints placed even on the side windows can make it harder for a driver to see other cars, cyclists, and pedestrians at night, particularly when the driver is turning, dealing with the glare of headlights, or stopping at intersections. Dark tints also prevent other road users to see into a car to assess the intentions of the driver or to see that if the driver has seen them.
Consequently, an effective window tint is one with a high rejection rate for IR and UV and at the same time allowing in a comfortable VL level. In Malaysia, the law states that VL rejection for car window tints installed on the windshield should be no more than 30% (i.e., 70% VL transmission or VLT).
The second part of this series will discuss about the make up of window films and how to choose the correct one for our cars.
- Al-Kayiem, H.H., Sidik, M.F.M. and Munusammy, R.A.L. 2010. Study on the thermal accumulation and distribution inside a parked car cabin. American Journal of Applied Sciences, 7: 784-789.
- Baldock, M.J.R., McLean, A.J. and Kloeden, C.N. 2004. Front side window tinting visual light transmittance requirements. CASR Report Series No. CASR002. Centre for Automotive Safety Research, The University of Adelaide, Australia.
- Flores, A., Parsy, P., Burnett, I. and Carrasco, A. 2008. SolarVent1530: Solar powered car ventilation system. Mechanical engineering capstone student design projects program.
- Jasni, M.A. and Nasir, F.M. 2012. Experimental comparison study of the passive methods in reducing car cabin interior temperature. In: International Conference on Mechanical, Automobile and Robotics Engineering (ICMAR’2012). Penang, Malaysia, pp. 229-233.
- Levinson, R., Pan, H., Ban-Weiss, G., Rosado, P., Paolini, R. and Akbari, H. 2011. Potential benefits of solar reflective car shells: Cooler cabins, fuel savings and emission reductions. Applied Energy, 88: 4343-4357.
- Rugh, J.P., Hendricks, T.J. and Koram, K. 2001. Effect of solar reflective glazing on Ford Explorer climate control, fuel economy, and emissions. Paper presented at International Body Engineering Conference & Exposition, October, Detroit, MI, USA.
- Rugh, J.P., Chaney, L. and Lustbader, J. 2007. Reduction in vehicle temperatures and fuel use from cabin ventilation, solar-reflective paint, and a new solar-reflective glazing. SAE World Congress, Detroit, Michigan, April 16-19, 2007.