Why should we ban “CdTe” Solar Cells?

A review on applications of Silicon Solar Cells instead of CdTe to meet the world’s energy demand

The world craves for energy. Current world’s energy supply depends largely on fossil fuels, solar and nuclear accounting for about twenty per cent of the energy budget. Depletion of fossil fuels and greenhouse emissions in their use, compels the world to seek alternatives. Only options available are solar and nuclear.

Solar energy is abundant, the total amount of energy in sunlight, reaching the earth in one year exceeds the whole reserve of fossil and nuclear fuels. Hydro, wind and firewood are indirect solar derived energy sources, in contrast to direct conversion of solar radiation into heat or electricity generation via solar cells.

With the advent of the “Silicon Solar Cell” countries all over the world have diverted much effort in obtaining a significant portion of their energy requirement from the sun. However, as sunlight is so diluted, power generation equivalent to fossil fuel power plant or a nuclear reactor could be obtained only by covering a comparatively very large area with solar cells.

Although the silicon solar cell production, determined as cost per watt has steadily gone down, solar electricity is still more expensive compared fossil fuel. Yet, as a long term investment and a non-polluting source, the advantages are tremendous, prompting worldwide research, geared towards development of cheaper and more efficient solar cells.

To date the only other type of solar cell sometimes used for commercial power generation is one based on a material termed “cadmium telluride (CdTe)” These cells are generally less efficient compared to the silicon cells, but the cost of pro- duction is somewhat lower. They also have the advantage, as the cells are based on thin films can be built into flexible substrates.

However, the virulent toxicity of cadmium, which is also been implicated in the Sri Lanka kidney disease has been questioned and debated. Some have expressed concern regarding ground water cadmium contamination from large scale CdTe solar farms. Currently, large solar energy harvesting systems based on CdTe solar cells are installed in desert areas, where any seepage of toxic cadmium is minimal.

Although CdTe by itself remain chemically stable, the very act of producing the electric current may leach some cadmium in the presence of moisture. Even more severe potential hazard is the processing of cadmium ores to synthesize cadmium telluride. Many countries restrict or ban cadmium processing in their soil and any country opting for this category of solar cells in the long run will be compelled to process cadmium in their own soil.

Another reservation for CdTe is disposal of ill-functioning modules. Some studies have concluded the toxicity issue CdTe can be resolved by proper precautionary measures, here the question is discipline needed to impose and maintain these measures, especially in the case of roof-top modules. Furthermore, the rarity of tellurium expected to escalate the cost CdTe technology. At present over 90% of solar photovoltaics is silicon based. If the efficiency of CdTe is further increased and the toxicity of cadmium emanations from cells during use and processing is more convincingly demonstrated to be negligible, CdTe might gain market.

As thin film cells are cheaper and have other practical advantages provided they use non-toxic materials, worldwide effort in this direction continues on basis of new innovative ideas. In this context it heartening to note that some key concepts related to new types of solar cells , now vigorously researched in leading laboratories had originated at the Institute of Fundamental Studies, Kandy, Sri Lanka about a decade prior to 2008 and not continued with same vigor and innovativeness thereafter.

The maximum efficiency a solar cell can approach is about sixty percent. Much less efficient solar cells are promising if the production is cheap and environmentally benign. Ongoing research points to avenues of achieving both these goals. These concepts takes decades to mature and reach the level of a practically viable invention.

Obviously, Sri Lanka or any other country cannot await such research developments, to implement their solar energy programs. They need resort to current state-of–art technology immediately. The silicon solar cell is time tested, environmentally friendly and is the most widely used solar cell. The pure quartz needed for extraction of silicon amply available in Sri Lanka is exported without value addition. The first step in utilizing this resource for economic gains should be expansion of silicon solar cell based power supplementation. Article By, Dr. R.M. Gamini Rajapakse Senior Professor of Chemistry, Department of Chemistry, Faculty of Science, University of Peradeniya.