Low-Temperature Solar Thermal
Low thermal solar installations are considered those installations that provide useful heat at temperatures below 65ºC through solar energy.
Circulation of the 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 in this case an external contribution of electrical energy is needed.
Solar collectors capture solar radiation to transform it into heat energy or heat. There are different types of solar collectors, currently we know those with flat plates, those with vacuum tubes and absorber sensors without protection or isolation.
The solar collectors are composed of the following elements:
- Cover: It must be a transaprente 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 heat losses by convection and radiation and therefore must have a solar transmittance as high as possible.
- Air channel: It 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 convective losses and the high temperatures that can occur if it is too narrow.
- Absorbing plate: The absorbing plate is the element that absorbs solar energy and transmits it to the liquid that circulates in the pipes. The main characteristic of the absorbing plate is that it has to have a great solar absorption and a reduced thermal emission. To meet this requirement, combined materials are used to obtain the best absorption / emission ratio.
- Tubes or conduits: The tubes are touching (sometimes welded) the absorbent plate to maximize the exchange of heat energy by thermal conduction. The liquid that will heat and circulate the heat to the accumulation tank flows through the tubes.
- 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.
Applications of solar thermal energy
Solar thermal systems 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 (pausing, laundry, etc.) .)
Production of Sanitary Hot Water (DHW)
The main application of solar thermal energy is the production of Sanitary Hot Water (DHW) for the domestic and service sector. Domestic hot water is used at a temperature of 45 ° C, a temperature that can easily be reached with flat solar collectors that can reach an average temperature of 80 ° C. It is considered that the annual coverage percentage of the ACS 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 can be a complement to the heating system, especially for systems that use delivery water at less than 60 ° C.
For heating with solar input, the system that works best is the radiant floor (pipe circuit through the floor), since the temperature of the fluid circulating through this circuit is about 45 ° C, easily achievable by sensors solar Another system used is fan-coil or fan heaters.
Pool water heating
Another of the extended applications is the heating of swimming pool water. The use of collectors can allow the energetic support in swimming pools to the outside, lengthening the bathing period, while in facilities for winter use, in the times of little solar radiation, they will be able to supply a small part of support to the conventional installation. In addition, it is necessary to consider that the Regulation of Thermal Installations in Buildings (RITE) prohibits the heating of uncovered pools with conventional energy sources.
Air conditioning by means of absorption machines
One of the fields of maximum development of solar thermal installations that will be seen in a short period of years will be vacuum collectors or high performance planes that produce ACS, heating in winter and, through absorption machines, produce cold in summer.
The use of solar thermal energy for all these systems together is the best way to take advantage of the installation, because the use only for ACS and heating produces some surplus in summer, causing overheating in the installation that is necessary to avoid using 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 plane flat solar collector is formed by a metallic plate that is heated with its exposure to the Sun (absorber); This plate is black so that it does not reflect the sun's rays. Normally the plate is placed in a box with a glass cover. Inside the box, water is circulated through a coil or tube circuit so that heat is transmitted to the fluid. 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, this plate heats up and transmits the heat to the liquid that circulates through the tubes.
For pool water heating applications, non-glazed collectors can be used. These are formed simply by a large number of tiny tubes of metal or plastic arranged in a coil through which the water circulates. They do not need a box or glass cover, for this reason the temperature increase is low, around 30 ° C. 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.
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.
Last review: April 24, 2015