Solar thermal

Here the heat from rays of sunlight is used directly. The two most frequent applications are the heating of water in homes and heating buildings.

Radiant heat is recovered thanks to a heat transfer fluid (water + antigel or air), which is heated while being circulated through an absorber placed under glass. This glass improves the performance of the system because it allows rays of sunlight to pass through it, but traps inside a portion of the heat that is emitted by the absorber (see diagram). The heat is then distributed throughout the area to be warmed, or is transmitted to a reservoir of sterile water, via the fluid that has been heated. Thus there is no transformation of the heat into electricity, at least not in standard home heating systems. This is a very localized system that is appropriate for heating individual homes. Four square meters’ worth of thermal solar panels are sufficient for the hot water needs of a family of four people. But 10 to 20 panels are required to heat a small house, and even then supplementary heating methods would be necessary during periods during which the climate is not favorable.

Photovoltaic solar energy

Solar radiation in this system is directly transformed into electricity by monocrystalline silicon cells. The light that strikes the semi-conductive material (silicon) has the effect of producing and transferring positive and negative electric charges, thus generating an electric current.

This is a very old technique that has proven its usefulness, because there is a complete absence of mechanical or fluid movement, and the life span of such a system can easily exceed 20 years. Further, the energy budget is favorable because a photo voltaic module pays off the energy that was required for its manufacture after one to four years of exposure to the sun. This technique thus is quite appropriate for use in the sunny countries of the South, where there are no large electric grids, and where the demand for electricity is small and made on an individual basis for the most part (requirement of several kWh per day). One square foot of photovoltaic surface is sufficient to light a lamp, play a radio or a CD player, or a small black and white television. Ten square meters take care of the electricity needs of a house. The difficulties begin when it becomes necessary to supply a whole community with electricity, including a number of larger machines. That is why performance needs to be improved, and cost driven down. Besides the research with silicon modules, there are other possibilities that attempt to do without silicon, replacing this by polymers (organic photovoltaic cells). The advantage of this would be twofold: the material for the cells would be cheaper and easier to manufacture, and above all more adaptable. It’s hard to imagine roof tiles made of silicon, but plastic roof tiles are already in use.

In addition, photovoltaic solar energy encounters another difficulty: at night, production stops – despite the fact that the need is greater at night (at least this is true for home-based activities). Research thus is also concentrating on storage of electric power from photovoltaic sources.

Thermodynamic solar energy

This is undoubtedly the earliest form of solar energy ever used (it was known in Antiquity): one concentrates the sun’s rays on a single point in order to obtain very high temperatures. Today, heating plants of this type use a large number of collectors, often parabolic in shape and capable of being aimed at the sun, that concentrate rays of sunlight and focus them on a single boiler. This allows heat transfer fluids to be heated (usually oils or dissolved salts) up to temperatures ranging from 250 to 1000°C. In turn, these fluids heat water vapor that causes turbines to turn, thus producing electricity as in classic generating plants.