The California Air Resources Board will require new cars sold in California to have windows that reflect or absorb heat-producing rays from the sun. Starting in 2012, windows must prevent 45 percent of the sun's total heat-producing energy from entering the car, and the windshield must reject at least 50 percent. In 2016, this increases to 60 percent.
The reason for this edict is to reduce global warming pollution and fuel consumption by reducing air conditioning loads. Other benefits include cooler interiors, and reduced time for the air conditioning to reach a comfortable temperature. It will also reduce upholstery fading and dashboard cracking. Possible solutions include adding chemicals to the glass to absorb the sun's energy and using laminated glass coated with invisible microscopic specks of reflective metal.
The new regulation allows alternative technologies to achieve an equivalent result, so why not use techniques, namely photovoltaic arrays, to generate electricity. Less heat needs to be absorbed or reflected when some of the sun’s energy is turned into electricity. The electric power can be used, for example, to power the air conditioning - further reducing fuel consumption.
As an example, New Energy Technologies’ SolarWindow™ uses transparent, organic solar cells that would be coated onto transparent glass. These are reportedly the smallest organic solar cells of their kind in the world. According to the company, SolarWindow™ can be installed virtually anywhere where there is either direct sunlight or artificial lighting, including fluorescent system lighting. The latter includes homes and commercial offices. Vehicle windows and windshields are another application, if the full windshield visibility required by the CARB can be met.
New Energy Technologies’ solar cells have the same electrical properties as silicon, but have considerably better capacity to ‘optically absorb’ photons from light to generate electricity. Transparency is achieved by use of conducting polymers. Each solar array is composed of a series of twenty ultra-small solar cells measuring less than a quarter of the size of a grain of rice each. The ultra-thin film is only 1/1000th the thickness of a human hair, or 1/10th of a micrometer. The organic solar cells are fabricated using environmentally-friendly hydrogen-carbon based materials.
The New Energy Technologies’ cells, mounted on a one-inch by one-inch substrate, have been tested against today’s popular photovoltaic materials. In every case, these solar cells outperformed all of the conventional materials tested under normal office lighting conditions, without the benefit of outside natural light from windows. They produced almost a 2-fold greater output power density compared to mono-crystalline silicon, over a 8-fold greater output power density over copper-indium-selenide, and more than a 10-fold improvement compared to flexible thin-film amorphous-silicon.
The Swiss Federal Institute of Technology in Lausanne has also developed a transparent solar cell. The transparent cell prototype now converts 10 percent of the energy in sunlight into electricity. Although this cannot yet match the efficiency of commercial silicon-based solar panels, it is estimated that these cells will cost only a tenth as much.
The transparent cells make use of a redox process, which is more like the natural photosynthesis process in plants. They rely on an electrolytic process between two panes of glass. The inside surface of the glass is coated with a titanium dioxide film and on top of it is a layer of photosensitized chemical. The photosensitized chemical harvests the sunlight, and the titanium dioxide film harvests electrons and transfers them to and from an external circuit. So far, the cells have been demonstrated in solar-powered watches, in which the glass of the watch powers the timepiece. One promising potential application is in windows.
This article was written by Bill Siuru, PhD, PE, a technical editor for Green Car Journal.