Difference between panels for electricity & hot water?
Solar thermal panels
Solar thermal panels are for hot water. These panels mounted on a roof convert the light that penetrates the panel's glass pane (short wave radiation) into heat. The radiation hits an absorber plate which is covered with a special black-paint that absorbs the available energy, with maximum efficiency. The absorber plate in flat-plate panels is much larger (whole panel) than evacuated tubes (thinner strips). A thermostat in the panels measures the temperature and sends a signal to the pump to circulate the heated water in the panels, when the optimum temperature is reached. The heat energy in the plate gets transferred to the cylinder by the pump through a coil that is located in the bottom of the cylinder. the pump stops automatically when the panel has once again cooled down. This cylinder acts like a giant battery storing the sun's available energy in a specially-constructed, insulated dual-coil tank.
PV (Photovoltaic) solar panels [for electricity]
Photo = light and Voltaic = electricity. The discovery of the PV effect is generally credited to Becquerel. When experimenting with a “wet cell” battery in the 1830’s, he discovered that voltage increased when the battery’s silver plates were exposed to sunlight. It was not for another century or so , in the 1950s, that PV really evolved with research on semi-conductors and a process known as doping.
Semiconductors are devices that are in most modern electronic devices such as computers and stereos. They are non-metallic materials, such as germanium and silicon, whose electrical characteristics lie between those of conductors (which offer little resistance to the flow of electric current) and insulators (which almost completely block the flow of current). Bell Labs, New Jersey, USA also produced the transistor around the 1950s. This transistor was made of semiconductors (usually silicon) which were “doped” with tiny amounts of selected impurities such as boron or phosphorus. As a result of these discoveries, doped silicon devices that were much more efficient than previous PV cells began to emerge. In 1958, solar cells were used to power a small radio transmitter in a US space satellite. This was the first successful demonstration of the use of PV as a power source, which really brought PV into the mainstream.
Now PV surrounds many of us everyday from PV solar calculators to PV parking meters. Yet the seductive, “solar revolution” idea of the last three decades that one day we will all use free electricity from the sun is probably a bit misleading. On a bright, sunny day the sun shines approximately 1000 watts of energy per square metre of the planet’s surface. If we could collect all of that energy, we could easily power our homes and offices for free. However, at present, PV solar cells capture about 15% or less of that energy. The reason is that visible light is only part of the electromagnetic spectrum. This spectrum is made up a range of different wavelengths and therefore different energy levels. Some energy passes through the PV cell as if it wasn’t there and if a photon of light has more energy than required, then that extra energy is lost. These two phenomena alone can account for a loss of around 70% of the radiation energy that falls on a PV cell.
How does PV work?
Put simply, PV cells consist of a junction between two thin layers of different semiconducting materials, known respectively as “p” (positive type) and “n” (negative type) semiconductors. These two dissimilar semiconductors create what is known as a “p-n junction”. By joining the two different types of semiconductor an electric field is created. What happens when light falls is that the positive side of the p-n junction becomes more negatively charged than normal and the negative side becomes more positively charged than normal. Or to phrase it; differently; when photons of light of a suitable wavelength fall within the p-n junction, they can transfer their energy to some of the electrons in the material and “promote” them to a higher energy level.
The Japanese PV industry is now the world’s largest. Japan has undertaken a major investment in both research and manufacturing, taking advantage of a market with 50% PV installation subsidies from the government. In 1998, US President Clinton announced a “one million solar roofs” programme, aimed at installing one million solar roofs, including solar thermal and PV in the US. Germany has its ambitious 100,000 roofs programme.