Low temperature thermal solar energy is used in applications that require temperatures between 100ºC and 250ºC. From 80 degrees Celsius flat collectors have practically no performance anymore and other collection systems have to be used.
Low temperature thermal energy systems are mainly used for applications that demand thermal energy, that is, heat between 125ºC and 400ºC. To reach higher temperatures, it is necessary to concentrate solar radiation.
- Vacuum solar collectors
- Concentration solar collectors
Vacuum Solar Collectors
Vacuum solar collectors are a set of vacuum tubes that take advantage of thermal solar energy.
This type of solar collector is made up of linear collectors housed in vacuum glass tubes. The solar panel has a comb structure, with a mast that conducts the heat transfer fluid, and a series of spiked tubes where solar radiation is captured.
It is an intermediate solution between flat and medium temperature collectors, there are vacuum collectors that allow temperatures of up to 120ºC to be reached. They usually use a collection surface made up of a series of finned tubes, covered with a selective surface and circulating. the fluid-heat-carrier inside (photovoltaic solar energy).
Advantages of This Type of Collectors
The main advantage of solar vacuum collectors over flat collectors is their better thermal insulation.
In flat collectors there are thermodynamic losses due to convection, while in tubes, being isolated under vacuum, these losses are reduced to values around 5%. Compared to flat solar panels, these losses represent up to 35% less.
The reduction of heat loss allows a significant increase in performance.
Types of Vacuum Tube Solar Collectors
Currently, there are two types of vacuum tube solar thermal collectors, with quite different techniques in terms of heat transmission from the vacuum tube to the primary circuit.
- Vacuum solar collectors with direct fluid.
- Vacuum solar collectors heat pipe.
Concentration Solar Collectors
Concentration solar collectors are a type of solar collector that thanks to their shape concentrates the solar radiation received on a surface at one point.
To reach higher temperatures, it is necessary to concentrate the solar radiation.
What Does This Technology Consist Of?
The concentrating solar collector consists of a parabolic trough mirror that reflects all the solar radiation received on a glass tube located along the focal line of the mirror. Inside this tube is the absorbent surface in contact with the fluid-heat-carrier. This arrangement of the absorber and the heat-carrier fluid is intended to reduce losses by convection.
Optical procedures with lens devices are enormously expensive. The most developed concentrators at present are those of reflection by means of mirrors and which for this temperature range correspond to linear concentrators with a parabolic-reflective cylinder surface.
Types of Concentration Sensors
Concentration collectors can be classified in different ways:
Depending on the concentration ratio (the ratio of the incident radiation density to that used by the absorber) they can be:
- Low concentration.
- Medium concentration.
- High concentration.
Depending on the dimensions it has it can be:
According to the orientation:
Depending on the reflective surface they can be classified into:
- Parabolic collectors
- Paraboloid collectors
- Cylindrical-parabolic collectors
- Spherical sensors