Medium temperaturesolar thermal energy is used in applications that require temperatures between 100 and 400 degrees Celsius. Solar thermal energy is a renewable energy source that converts solar energy into thermal energy.
In general, flat-plate collectors are used in domestic solar thermal energy systems. However, for temperatures above 80 degrees Celsius, flat collectors practically no longer have any performance, and it is necessary to resort to other solar concentration systems.
Medium temperature thermal energy collection systems are mainly used for industrial applications. In contrast, domestic systems are used to obtain domestic hot water, and high-temperature systems are used to produce steam and generate electricity.
The two types of medium temperature concentrators are:
1. Evacuated Solar Tube Collectors
Evacuated tube solar collectors are a set of linear tubes through which a heat transfer fluid circulates in the center. The space that remains between the conduit through which the fluid circulates and the outer layer of the tube is a vacuum. It prevents heat transmission by convection to reduce losses and obtain better use of solar energy.
The heat transfer fluid is kept in a closed circuit and transfers thermal energy through an external heat exchanger.
Vacuum collectors allow temperatures of up to 120º C to be reached.
Advantages of Evacuated Tube Collectors
The main advantage over flat collectors is their better thermal insulation.
In flat collectors, there are heat losses due to convection, while in tubes, being vacuum insulated, these losses are reduced to values around 5%. Compared to flat solar panels, these losses are up to 35% less.
1. Concentrating Solar Collectors
Concentrating solar collectors are a type of solar collector that, thanks to their shape, concentrates the solar radiation received on a surface at a point.
There are different types of concentrating collectors. However, they all work based on a set of mirrors that reflect all the solar radiation onto a point. Depending on the type, the target is a glass tube along the mirror's focal line.
At the focal point is the absorbent surface in contact with the heat transfer fluid. This arrangement between the absorber and the fluid aims to reduce convective losses.
Optical procedures with lens devices are enormously expensive. Therefore, the most developed concentrators at present are those with a reflection using mirrors which for this temperature range correspond to linear concentrators with a parabolic trough reflective surface.
