Installation of thermal solar energy

Low Temperature Solar Thermal Energy

Low temperature solar thermal energy

Low-temperaturesolar thermal energy installations are those solar thermal installations that provide useful heat at temperatures below 65 degrees Celsius through solar energy.


A low temperature solar thermal installation is made up of:

  • Solar collectors.

  • Two water circuits (primary circuit and secondary circuit).

  • Heat transfer fluid.

  • Heat exchanger.

  • Accumulator.

  • Glass of expansion.

  • Pipelines.

The circulation of the water inside the circuits can be obtained by means of a thermosyphon, taking advantage of the difference in density of the water at different temperatures or by means of a circulation pump. Although with a circulation pump an external supply of electrical energy is needed.

What Is Solar Thermal Energy Used For?

The use of low temperature solar thermal energy is used in many applications of which we highlight the following:

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 sectors. Domestic hot water is used at a temperature of 45 degrees Celsius. This temperature can be easily reached with flat solar collectors that can reach an average temperature of 80 degrees Celsius.

Low Temperature Heating

Solar heating systems are a complement to the traditional heating system, especially for systems that use make-up water below 60 ° C.

For heating with solar input, the system that works best is the underfloor heating system (circuit formed by a network 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.

Pool Water Heating

The use of solar collectors allows energy support in outdoor pools, lengthening the bathing period.

It must be considered that the legislation of some countries does not allow the heating of outdoor swimming pools using conventional energy sources, which are usually non-renewable energy sources.

In this type of installations, solar collectors without glass can be used. These solar panels are simply made up of a large number of tiny metal or plastic tubes arranged in a serpentine way through which the water circulates.

How Is Solar Radiation Capture Done?

The solar radiation collection system is made up of solar collectors connected to each other. Its mission is to capture solar energy to transform it into thermal energy, increasing the temperature of the fluid that circulates through the installation. 

The most widespread type of thermal collector is the flat solar collector that achieves temperature increases of 60 ° C at a low cost.

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 in color so that it does not reflect incident solar radiation. Normally the solar panel is placed in a box with a glass cover. Water is circulated inside the box through a coil or a circuit of tubes so that the heat is transmitted to the fluid through a thermodynamic process.

The effect that occurs is similar to that of a greenhouse, sunlight passes through the glass plate and heats the blackened plate. Glass is a "solar trap", as it lets the radiation from the Sun 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 circulating through the tubes.

There are also solar vacuum collectors on the market. They consist of metal tubes that cover the metal 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.

Solar thermal collectors are made up of the following elements:

  • Cover: The cover must be made of transparent material. The function of the cover is to minimize thermodynamic heat losses.

  • Air channel: The air channel is a space that separates the cover from the absorbent plate and may or may not be empty.

  • Absorbent plate: The absorbent plate is the element that absorbs solar energy and transmits it 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 low thermal emission.

  • Tubes or ducts: The tubes are touching the absorbent plate to maximize heat energy exchange by thermal conduction. The liquid circulates through the tubes that will be heated and will 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. For the best possible insulation, the insulating material should have a low thermal conductivity.


    Published: April 21, 2015
    Last review: November 27, 2021