Lecture 3 Review

Moving on from Solar Thermal energy, Lecture 3 covered Solar Photovoltaic. Using this technology we can convert solar energy directly to electricity. Many of us have this available on calculators, however solar cells have far higher capabilities, and the Photovoltaic energy represents one of the most promising means of generating electricity without contributing to global warming. The technology has only been seriously considered since the mid 1970's, and large advances have been made since then, to the point where an efficiency of 36% has been achieved (Spectrolab).

To understand how a solar cell works, you need a some background information on semi-conductors. Solar cells are made from silicon mainly, but by adding small amounts of group 3/5 elements (doping) the electrical properties of the silicon are changed so that they become able to conduct electricity. This is due to the shrinking of the forbidden gap which brings the conduction and valence bands closer together. Energy given to electrons in the valence band then allows them to jump to the conduction band. If this is combined with the use of a junction, the electrons will flow, generating a current.

There are two types of silicon used in solar cells, crystalline and amorphous. The more expensive crystalline silicon has higher efficiencies and is stable, where as the amorphous silicon is cheaper, but has lower efficiencies and degrades to some extent over the first two years of use. Cells are arranged in systems called arrays, which are made up of separate solar panels. These systems are connected to other equipments to make the solar electricity usable. This involves converting it from DC to AC, and sometimes storing it, for use during overcast times, or at night.

There are 3 types of solar system. The stand alone kind is for use in remote locations, and generally is used for simple tasks such as water pumping, where it would be difficult to get power from an electrical grid. The hybrid system involved combining solar panels another form of power generation (e.g. wind or diesel generator) so that the maximum solar energy can be extracted depending on the conditions. The load receives a constant supply from the combination of generators. The final option is a grid tied system, which allows any electricity not used at the generation site to be sold back to the grid, thereby reducing the owners electricity bill.

Finally the economics of solar power were covered. Currently it is a very expensive means of energy production (40 cents/kiloWatt hour). So it only makes economic sense in certain locations. The best locations are the remote ones, i.e. more than 0.5 miles from the grid. At these locations it would save you money to use solar energy, as a pose to installing a connection to the grid, provided that the site received enough solar energy.