The circulation of water inside the circuits can be obtained by thermosiphon, taking advantage of the density difference of the water at different temperatures or by means of a circulation pump. Although with a circulation pump an external contribution of electrical energy is needed.
Applications of solar thermal energy
Low-temperature solar thermal energy is a form of renewable energy that can be applied in many areas in order to reduce dependence on other sources of non-renewable energy such as natural gas or coal combustion.
Solar thermal systems at low temperature are useful in applications such as heating swimming pools, for domestic use (hot water and heating), for industrial uses that require hot water but not at a higher temperature of 65ºC.
Production of Sanitary Hot Water (ACS)
The main application of solar thermal energy is the production of domestic hot water (DHW) for the domestic and service sector. Domestic hot water is used at a temperature of 45 degrees Celsius. At this temperature can be easily reached with flat solar collectors that can reach as average temperature 80 degrees Celsius.
It is considered that the coverage percentage of the annual hot water DHW is approximately 60%; this percentage is spoken of, and not superior, so that in the epoch of greater solar radiation not on energy. The energy provided by the collectors must be such that in the most favorable months it contributes 100%. The rest of the needs that are not provided by the collectors are obtained from an auxiliary system, which is usually diesel, gas or electric power. With this percentage of coverage the amortization periods are reduced.
Low temperature heating
Solar thermal energy at low temperature can be a complement to the heating system, especially for systems that use water supply at less than 60 ° C.
For heating with solar supply, the system that works best is the radiant floor (circuit of pipes through the floor), since the temperature of the fluid that circulates through this circuit is about 45 ° C, easily achievable by solar collectors .
Swimming pool water heating
Another widespread application of low temperature solar thermal energy is the heating of pool water. The use of collectors can allow energetic support in swimming pools to the outside, lengthening the bathing period, while, in installations for winter use, in times of low solar radiation, they will be able to supply a small part of support for the conventional installation.
It must be borne in mind that, in some countries, the current legislation does not allow the air conditioning of outdoor swimming pools using conventional energy sources that are usually non-renewable sources of energy.
For pool water heating applications, solar collectors without glass can be used. These solar panels are simply formed by a large number of tiny metal or plastic tubes arranged in coils through which the water circulates.
These collectors do not need a box or glass cover, for this reason the temperature increase is low, around 30 degrees Celsius. The heat losses are large which limits its application to other types of installations. Flexible pipes tolerate the passage of aggressive water, such as chlorinated pool water, but they can not withstand the mechanical stresses that occur when water and surface scratches freeze. They are more economical than flat solar collectors.
Air conditioning by absorption machines
One of the fields of maximum development of low-temperature solar thermal installations that will be seen in a short period of time will be vacuum collectors or high-performance planes that produce domestic hot water DHW, heating in winter and, by machines absorption, produce cold in the summer.
The use of low temperature solar thermal energy for all these systems together is the best way to take advantage of the installation, because the use only for domestic hot water ACS and heating produces some surplus in summer, causing overheating in the installation that is necessary to avoid by means of some system of the existing ones.
The applications of solar thermal energy also extend to the industrial sector: hot water and preheating of process water, heating, hot air and cooling.
Solar radiation collection systems
The solar radiation collection system consists of solar collectors connected to each other. Its mission is to capture solar energy to transform it into thermal energy, increasing the temperature of fluid circulating through the installation.
The most widespread type of thermal collector is the flat solar collector that achieves temperature increases of 60 ° C at a reduced cost. These collectors are suitable for the production of hot water for various applications: domestic hot water, underfloor heating, etc.
The flat solar collector is formed by a metal plate that is heated by solar radiation with its exposure to the Sun (absorber). This plate is black so that it does not reflect the incident solar radiation. Normally the solar panel is placed in a box with glass cover. Inside the box, water is circulated through a coil or tube circuit so that heat is transmitted to the fluid through a thermodynamic process. The effect that is produced is similar to that of a greenhouse, the light of the Sun goes through the glass plate and heats the blackened plate. The glass is a "solar trap", because it lets the Sun's radiation pass (short wave) but does not let out the thermal radiation emitted by the blackened plate (long wave) and as a consequence,
There are also on the market the solar vacuum collectors. They consist of metal tubes that cover the metallic tube that contains the working fluid, leaving between them a chamber that acts as an insulator. They have a very high yield, but their cost is also high.
The solar thermal collectors are composed of the following elements:
- Cover: The cover must be of a transparent material, usually it is glass, but there are certain plastics that can also be used with the advantage of being cheaper and more manageable. The function of the cover is to minimize the thermodynamic losses of heat by transmission by convection and radiation and therefore must have a solar transmittance as high as possible.
- Air channel: The air channel is a space that separates the cover of the absorbent plate and may be empty or not. To calculate its thickness, it is necessary to balance the convection losses and the high temperatures that can be produced if it is too narrow.
- Absorbent plate: The absorbent plate is the element that absorbs solar energy and transmits the heat to the liquid that circulates through the pipes. The main characteristic of the absorbent plate is that it has to have a high absorption of solar radiation and a reduced thermal emission. To meet this requirement, combined materials are used to obtain the best absorption / emission ratio.
- Pipes or ducts: The tubes are touching (sometimes welded) the absorber plate to maximize the exchange of heat energy by thermal conduction. The liquid that is heated will circulate through the tubes and transport the heat to the accumulation tank.
- Insulating layer: The purpose of the insulating layer is to cover the system to avoid and avoid thermal losses. In order for the insulation to be the best possible, the insulating material must have a low thermal conductivity.