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Alternative energy
Solar energy the foundations: (solar thermal)
By Dr. Luca CARGNINO
The solar energy is the renewable source of more abundant energy. Inside of the sun, to temperatures of some million centigrade degrees, thermonuclear reactions of fusion happen incessantly, giving off enormous quantity of energy under form of electromagnetic radiations. The irradiated energy is spreaded in the space, and after having crossed the atmosphere it arrives to the ground with a medium equal intensity - in function of the inclination of the sun on the horizon- to approximately 1.000W/mq (radiation to the ground, in conditions of serene day and sun to noon). This enormous energy flow that arrives on the earth is equal to approximately 15.000 times the world-wide energetic consumption. Of this energy only a part can be transformed in useful energy. The use of the solar energy is reduced by its limited concentration and by the diurnal and seasonal variations.
The possible current methods of exploitation of the technology are: the conversion in eventual heat with ulterior transformation in kinetic energy, electrical worker or chemistry; the direct conversion in electric power; the direct conversion in chemical energy. The solar energy conversion in heat can happen through plane panels (lowland temperature heat) or through concentration systems (medium-high temperature heat).
High temperatures (beyond 500C°) are obtained concentrating, with heliostats, the solar beams on boiler. The heliostats are mirrors that, thanks to a sophisticated system, follow the sun in its appearing motion around the earth. This complex system is today employed exclusively in Helios team power-stations
Medium temperatures (100 - 300C°) can be obtained thanks to a simpler system that uses mirrors of cylindrical parabolic that rotate around a single axis. Such temperatures allow the working of solar motors with a satisfactory yield; they are employed in order to set in action compressors, pumps for raising of the water and similar.
The lowland temperature heating (inferior to 100C°), comprises systems that take advantage of solar panels in order to heat a liquid or the air. The aim is to use the heat of the sun in order to produce warm water or to heat the buildings. This accumulated warm water in appropriate tanks, can be employed for all the sanitary uses of a domestic house, for the hotel requirements, to heat swimming pools, for the camping’s and industrial uses. The efficiency of the conversion in heat of the solar radiation is limited by some factors such as: the reflection of the solar radiation on the surface of the collector; the losses of heat for conversion and conduction from the collector; the radiation of heat from the collector.
The plan collectors are constituted by: an absorbent surface; a transparent cover; a thermal insulation on the back; elements for the transfer of the fluid. They operate in a temperature range between the 40 and 100 ºC and have a conversion efficiency of approximately 70%. There are several types of systems concentrators (linear, parabolic, spherical) and can have concentration factors from 2 to 1.000. A thermal solar system concurs to extend the bathing season without damaging the atmosphere. It is usable with all the chemical substances for swimming pools available on the market.
How does it work ?
The solar thermal technology allows transforming directly the energy associated to the solar radiation in thermal energy.
It takes advantage of the basic principles of the thermodynamics, and particularly the transmission of the heat from “a warm” body to a “cold” one:
the warm body is the sun that spread the energy in the surrounding space ,the cold body is the fluid that slides inside of the panel. Therefore we have a system that works without the use of any fuel.
The incident solar radiation is used in order to heat a fluid (water, air or solutions of miscellaneous specific heat) that can circulate in heat exchangers or directly in radiating piping’s and bodies placed in the rooms to heat or in order to make to evaporate the substances flown that are used in the refrigeration cycles.
For the applications on small scale (the residential ones), for economization and simplicity of management, the solar energy is directly collected on fixed panels, opportunely oriented. In these applications, the water is used like a heat-transfer fluid.
The yield of the solar panels is increased of 30% in the last decade, rendering several applications in the building, the tertiary and agriculture commercially competitive.
The more common application is the thermal solar collector, used for heating sanitary water. In a day, a square meter of solar collector can heat to 45 - 60 °C water amounts between the 40 and 300 litres, according to the efficiency that can vary - between 30% and 80% - according to the climatic conditions and of the kind of collector.
The systems can be classified based on the temperature of exercise of the heat-transfer fluid: systems to lowland temperature (BT), medium temperature (MT) and high temperature (AT).According to this classification, we can obtain systems that work in different way among them.
The lowland temperature technologies include the systems that use a solar collector (solar panels) in order to heat a liquid or the air. The aim is to pick up and to transfer solar energy in order to produce warm water or to heat the buildings. With the denomination “lowland temperature”, we refer to fluids that are heated under the 100 °C (very rarely can arrive to 120 °C).
The lowland temperature thermal solar systems are employed for:
- the production of sanitary warm water for rooms, receptive and sports structures , offices
- integration of the systems of heating of atmospheres
- the discovered and covered swimming pool heating
Types of panels in the market
solar panels are divided in two great groups: those in glass and those uncovered.
Uncovered solar panels: are generally used in the summery season: discovered swimming pool heating, warm water for the showers in the bathing establishments, the camping’s, the hotel etc. The technological characteristic that differentiates the glass panels from those uncovered is that in these last ones, without glass, the water passes directly inside of the tubes of the panel, where it is heated by the solar beams and is ready for being used.
The limit of these panels is that they work at least with an ambient temperature of 20° C, and that the maximum temperature of the water does not exceed 4o° C.For this reason their employment is ideal for seasonal employment. The uncovered panels normally are made in polypropylene, so they can support a pressure of the water till 6 atmospheres (but there are some in neoprene and PVC also). Their cost is remarkably lower than the glass panels
Glass solar panels: are those classics composed by the real panel and by water accumulating tank : together they constitute a complete solar system. The heat absorber, inserted into the panel ,is isolated from the temperature of the external air, through a moderated glass, the insulator and the posterior body of the panel. In this way the selective effect of the infra red glass is exploited at the best and that allows catching up the maximum temperature and the greater efficiency.
He glass is transparent to the sunlight, but it is opaque to the infrared rays. The beams of the sun catch up the inner part of the panel, and the heat is kept inside. In virtue of such characteristics, these panels produce warm water in every month of the year. For such reasons they are more expensive than those uncovered, but they are most diffuse in the traditional domestic field.
There are three types of glass collectors:
- Selective surface panels : in order to improve the yield of the panel, the absorber of the heat, in black colour, comes dealt with an infrared selective product that withholds the heat of the sun and reduces the reflection increasing the analogous actions of the upper glass. This treatment involves a more elevated cost of the system.
- Not selective panels: the surface of the absorber does not come dealt, however it is painted of black for being able to withhold the solar beams. They render approximately 10% less than the selective surface panels but also the cost is inferior. These panels are particularly indicated for the second houses, used in the summery months, and/or for the zones with abundant insulations.
- Panels with integrated tank: the absorber of the heat and the accumulate tank are found in the same block where the solar energy heats directly the accumulated water. Because the warm water goes up and that cold goes down, a convective motion is created inside the tank. This motion distributes the heat to all the water mass. These blocks are easy to transport and to install at a relatively low cost. They can't be used in locality where the winter is along and rigid as the winter yield is insufficient and because the water inside of the tank could freeze.
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