Flat Plate Collector
The flat plate thermal solar collector has a good cost / effectiveness ratio in moderate climates and adapts correctly to a large number of applications (hot water heating, pool heating, heating support, preheating industrial fluids, etc.).
We can distinguish two basic types of flat plate 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 collector can be stratified with greater volume of water circulation by the branches of the interior of the collector and obtain a thermal jump of approximately 10ºC with a good performance.
- The serial configuration consists of a single continuous circuit with a small volume of circulating water and an upper thermal jump, with a good performance.
Operation of the flat solar collector
The operation of a sensor is very basic. In fact, any body exposed to the Sol receives an energy flow that heats it and, therefore, causes the temperature to rise.
A body at a given temperature emits energy around it, in the form of radiation, and this depends directly on the temperature difference between body temperature and ambient temperature.
If we cool the sensor by passing a fluid through the interior, this heat is used, so that a 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 collector to the outside environment. In this case, the working temperature is always lower than the equilibrium temperature.
If we want to get good performance, we have to work the collectors at the lowest possible temperature, as long as it is sufficient for the intended use.
The energy used is removed from the 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 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 sun's rays.
The greenhouse effect
This 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 radiation that can not come out is reflected inward again. A part of this energy heats the glass and the crystal sends it in and out.
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 third of the physical principles that intervene in the operation of the collectors is the isolation of the assembly from the outside, usually formed by an internal lining of the container box.
Components of a flat solar collector
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 100ºC.
2. A selective treatment, based on electrochemical deposits or paints with metal 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.
It has the function of isolating the sensor of the external environmental conditions - although it lets the solar radiation pass - that cause the greenhouse effect. Normally it is formed by a single sheet of tempered (resistant) glass with low iron content (very transparent) approximately 4 mm thick.
This element, as it happens in the rest of applications, fulfills the function of avoiding the losses of heat of the inside of the collector -specifically of the absorber- towards the outside and is normally formed by plates of synthetic foams (polyurethane, cyanurates, fiber of glass, etc.) located on the sides and on the back of the sensor.
The housing 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 housing will have condensate drain holes in the lower part.
Other types of solar thermal collectors
Flat collectors without cover
As the name suggests, the 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. In addition, they are resistant to corrosion and allow direct heating configurations 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 they have very good optical behavior but, on the other hand, they quickly lose their performance when the ambient temperature is below the temperature of work or with a high wind speed.
Therefore, this type of collectors are only advisable in low temperature applications in which 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 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.
Last review: May 30, 2018