Classification of Solar Thermal Energy Systems

For many years now, solar thermal technologies have been used for domestic hot water, and in colder climates, for space heating etc. Solar thermal energy systems convert solar radiation into usable heat. However, like any other field, different technologies have evolved with time, which has led to different classification of Solar Thermal Energy Systems.

Classification based on Concentration:

When exposed to the sun, the ‘collector’ heats up a thermal transfer fluid (either water with or without glycol for frost protection or thermo-oil). These collectors can be two types either Non-Concentrating or Concentrating.

Non-Concentrating:

• They do not concentrate solar radiation and their solar radiation area is typically equal to the solar radiation absorbing area.
• Example: Flat plate collectors, evacuated tube Collectors

Concentrating:

• They work on the principle of reflecting and concentrating direct solar radiation at its focus (a point or line), thereby using the concentrated solar radiation as a high temperature thermal energy source to produce heat.
• The solar radiation received on receiver area is concentrated onto a smaller absorber area, which defines the efficiency of concentration.
• Example:  Compound parabolic concentrator (cpc), parabolic trough, fresnel concentrator, array reflectors (heliostats)

Now the Concentrating Solar Collectors can also be divided into two major types. These are:

Imaging Type:

• Imaging concentrators produce an image of the sun by reflecting it on the receiver.
• Because of high efficiency, they are used to achieve the highest temperatures possible through solar thermal energy systems.
• Example: Parabolic imaging concentrators.
• The imaging concentrators can again be divided into single axis (fresnel and parabolic trough collectors ) and double-axis (concentrating dish collectors).

Non-Imaging Type:

• Non-imaging concentrators are called “non-imaging” because they do not produce any optical image of the source.
• Their concentration efficiency is less compared to Imaging type.
• Example: Compound parabolic concentrator (cpc).

Classification based on Achievable Temperature

On the basis of achievable temperature (of heating water or other standard fluids), the solar thermal energy systems can be classified into three categories:

Low Temperature Systems (less than 150 DegreeC)

• Flat plate collectors & evacuated tube collectors.
• Compound parabolic collectors.

Medium Temperature Systems (more than 150 DegreeC & less than 400 DegreeC)

• Parabolic trough collectors,
• Linear Fresnel collectors

Medium Temperature Systems (more than 400 DegreeC)

• Large Parabolic trough collectors
• Linear Fresnel collectors