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Active solar systems are systems that collect solar radiation, and then transfer it in the form of heat to water, air, or other fluid. The technology applied is quite simple and there are many possibilities for its application in low temperature thermal applications. The most widespread application of these systems is the production of hot water, known to all solar water heaters. A typical hot water system consists of flat solar panels, a heat storage tank and pipelines. Solar radiation is absorbed by the collector and the collected heat is transferred to the storage tank. Flat solar panels are usually mounted on the roof of the building, with a south-facing direction of 30 ° -60 ° to the horizon, to maximize the amount of radiation collected annually. Active solar systems consist of two main parts:

  • The collection section (the solar collectors, the sun's absorption surface).
  • The storage section (the water storage tank) which usually has an electrical resistance with a thermostat, so that hot water can be produced in periods of low or zero sunlight.

Active solar systems are divided into two types depending on the circulating circuit of the heated medium:

  1. Open Circuit: Directly heats the water used (the heated medium is the water we will use).
  2. Closed Circuit: Indirect heating of used water (the heated medium circulates in a special circuit that heats the water we will use without mixing it through a heat exchanger).

Operation of solar panels:

  1. The solar collector is positioned south.
  2. Solar radiation hits the black, usually metallic, flat surface of the solar collector, which absorbs the radiation and heats it.

Above the absorbent surface is a transparent cover, usually of glass or plastic, that allows the sun's rays to pass through but prevents heat from escaping. The tubes through which the fluid circulates are in contact with the absorbent surface resulting in heat transfer to the fluid.

The simplest and most widespread solar power system is the solar water heater. Its principle of operation is simple and is based on the fact that the water heated in the collector expands and becomes lighter than the lower water temperature of the tank. This difference in water density results in the natural circulation of the collector medium and the transfer of hot water to the storage tank. At the same time the cold water of the tank is pushed towards the collector. A prerequisite for the natural circulation of water is to place the tank at a point higher than the collectors. If lower, the water circulates with the help of proper automation.

Another application that is spreading in the European market is the combination of hot water production and space heating with active solar systems. The use of these systems is considered both technically and cost-effective if combined with proper design / construction of the building (good insulation, exploitation of passive solar benefits, etc.) and user cooperation. Active solar systems can also be used for space cooling, with the appropriate technologies (absorption cooling machines). Apart from home use, which is the most widespread today, active solar systems can be used wherever low temperature heat is needed (such as space heating / cooling, water desalination systems, pool water heating, etc.).

The technology of central solar systems is considered as a very cost-effective solution for the production of hot water and is widely used in homes, hotels, restaurants, industries, etc.


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