Installation of thermal solar energy

Solar power plant

Flat Solar Collector

Flat solar collector

The flat plate solar collector is a type of solar thermal panel whose objective is to transform solar radiation into thermal energy. This type of solar collector has a good cost / effectiveness ratio in moderate climates and adapts correctly to a large number of applications of solar thermal energy (heating of sanitary water, heating of swimming pools, support for heating, preheating industrial fluids , etc.).

We can distinguish two basic types of flat plate solar collectors, depending on the configuration of the absorber: the "grid type" parallel, in the vertical and horizontal versions and the "serpentine type" series. Basically, the difference between the two is:

  • The parallel configuration favors that the temperature of the solar collector with a greater volume of water circulation can be stratified by the branches of the inside of the collector and obtain a thermodynamic jump of approximately 10ºC with a good performance.
  • The series configuration consists of a single continuous circuit with a small volume of circulating water and an upper thermal jump, with a good performance.

Another way to classify flat solar collectors is whether they are glazed or unglazed:

The flat glazed solar collector is generally used in domestic water heating systems or in installations. The operating temperature is generally between 30 ° C and 60 ° C. It consists of an insulated box covered by glazing. Inside, there is an absorbent in which the heat transfer fluid circulates. The glazing blocks the infrared radiation and isolates the air space above the absorber to maintain heat.

The unglazed flat solar collector is much cheaper but less widespread. Depending on the outside temperature, it is usually used in pool heating systems, but can sometimes be used as a hot water heating system in warm countries. In temperate zones, the operating temperature is generally less than 30 degrees centigrade. It consists only of an absorbent in which the refrigerant circulates.

Performance of Flat Solar Collector 

The operation of a solar thermal collector for the use of solar thermal energy is very basic. In fact, any body exposed to solar radiation receives an energetic flow that heats it and, therefore, increases the temperature. This increase in temperature implies an increase in internal energy.

A body at a given temperature emits energy around it thanks to its thermodynamic properties, in the form of radiation, and this depends directly on the temperature difference between the body temperature and the ambient temperature.

If we cool the solar collector by passing a fluid through it, this heat is used, which means that part of the captured energy is transmitted to this fluid as useful energy. The rest of the energy is still lost in the form of radiation from the solar thermal collector to the outside environment. In this case, the working temperature is always lower than the equilibrium temperature.

If we want to obtain a good performance, we must work the collectors at the lowest possible temperature, as long as it is sufficient for the intended use.

The renewable energy used is removed from the solar collector by means of the heat transfer fluid, generally formed by a mixture of water with antifreeze and corrosion inhibitors.

Physical Principles of the Operation of the Flat Solar Collector 

The flat solar collector works from the application of the following physical principles:

The Black Body (the Absorber)

The incident solar radiation is partially absorbed by the bodies. The rest is reflected or passes through them.

The relationship between these effects depends on:

  • The nature of the body
  • The state of the surface.
  • The thickness of the body.
  • The type of radiation. The wavelength
  • The angle of incidence of the solar rays.

Dark and matt bodies capture better solar radiation than any other color; that is why the absorber of the solar collector is usually of dark colors, to take maximum advantage of the solar radiation.

The Greenhouse Effect

The greenhouse effect is generated in some transparent bodies, which are normally only crossed by radiation with a wavelength between 0.3 and 3 microns. Since most of the solar radiation is between 0.3 and 2.4 microns, sunlight can pass through a glass. Once crossed, the radiation finds the absorber, which is heated by solar radiation and emits radiation between 4.5 and 7.2 microns for which the glass is opaque.

This solar radiation that can not escape is reflected inward again. A part of this energy heats the glass and the crystal sends it inwards and outwards.

Some plastics (such as polycarbonate) have a similar behavior to glass (they let shortwave radiation from the Sun pass by and stop the longwave emissions coming from the absorber plate).

The Isolation

The third of the physical principles involved in the operation of solar collectors is the isolation of the assembly from the outside, usually formed by an internal lining of the container box. Good insulation improves the use of solar thermal energy.

Components of a Flat Solar Collector

The flat solar collector can be glazed or unglazed. The collector with glazed cover is the most used for sanitary hot water production facilities. This team consists of the following elements:


The absorber is the element that intercepts the solar radiation inside the collector and is responsible for transforming solar energy into thermal energy.

The absorber is usually formed by a metal foil, usually copper (good thermal conductor) that darkens basically with:

1. A thin film of black heat paint that resists working temperatures above 100 degrees Celsius.

2. A selective treatment, based on electrochemical depositions or paints with metallic oxides that have a high absorption of solar radiation (short wave) and a low heat emissivity (long wave).

The absorber incorporates a grid of pipes through which the heat transfer fluid will circulate.

Transparent Cover

The transparent cover has the function of isolating the solar collector from the external environmental conditions - although it allows solar radiation to pass - that cause the greenhouse effect. Normally it is formed by a single sheet of tempered glass (resistant) with low iron content (very transparent) of approximately 4 mm thickness.


The insulation is the element, as it happens in the rest of applications, fulfills the function of avoiding the losses of heat of the interior of the collector -specifically of the absorber- to the outside and is usually formed by plates of synthetic foams (polyurethane, cyanide, fiberglass, etc.) located on the sides and on the back of the solar panel.


The case of the flat solar collector has the function of housing the rest of the components. This closure is usually formed by an anodized aluminum profile that will guarantee a resistance of the assembly, even in extreme working conditions. Likewise, the casing will have condensate drainage holes in the lower part.

Flat Solar Collectors Without Cover

As its name suggests, the solar collectors without cover basically consist of the absorber element, generally formed by a set of tubes of plastic material, EPDM, rubber or polypropylene.

This type of solar collectors are very economical and easy to install since they usually have flexible configurations that allow them to be placed on any surface. They are also resistant to corrosion and allow direct heating configurations, such as in the case of pool heating.

In contrast, the collectors without cover with synthetic absorber tend to have very steep performance curves because, having no glass have very good optical performance but, instead, quickly lose their performance when the ambient temperature is below the working temperature or with a high wind speed.

For this reason, flat roofless collectors are only advisable in low temperature applications where the working temperature is close to the ambient temperature, for example, to extend the swimming season in open pools. A variable of this type of collectors without cover is the collectors of embedded and offset metallic plate, designed specifically for applications in closed circuit.

This variant presents a better thermal performance of the solar collector and allows it to be used to produce sanitary hot water or other low temperature applications. Another example of this type of collector available in our market, consists of a multifunctional cover that combines the qualities of a maintenance-free stainless steel cover with the efficiency of a selective surface solar collector.


Published: May 30, 2018
Last review: June 14, 2019