The flat plate solar collector is a type of solar thermal panel whose objective is to transform solar power into thermal energy.
This type of thermal solar panel has a reasonable cost/effectiveness ratio in moderate climates and are well suited to a large number of thermal applications, such as:
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Space heating.
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It can be used to heat water for a pool.
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Support for solar water heating systems - domestic hot water (DHW).
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Preheating of industrial fluids.
How does a flat plate collector work?
The operation of a flat plate collector is based on heat transfer.
Solar radiation falls on the heat-absorbing plate of the collector. When incident solar radiation hits the absorbing plate surface, part of its energy is converted into heat. Consequently, the temperature of the solar collector increases.
If we pass a fluid inside the collector, the fluid temperature rises because part of the absorbing plate heat is transferred to the liquid according to the first and second laws of thermodynamics. However, the rest of the energy is still lost in radiation to the outside environment.
The fluid transports the heat from the collector to heat exchangers to supply the energy system.
To obtain good performance of this type of solar thermal collector and reduce energy consumption, we must work the collectors at the lowest possible temperature.
What advantage do glazed flat plate collectors have?
This type of collector has a transparent cover that traps the radiation that bounces off the absorbing plate and increases the overall efficiency. To achieve this, the transparent cover of the collector generates a greenhouse effect.
The greenhouse effect is generated in some transparent bodies, which usually are only traversed 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 glass.
Once passed through, the radiation finds the absorbing plate heated by solar radiation and emits radiation between 4.5 and 7.2 microns that cannot pass through the glass again.
This solar radiation that cannot go out is reflected inwards. So a part of this energy heats the glass, and the glass sends it in and out.
Types of flat plate solar collectors
We can distinguish two basic types of flat plate solar collectors, depending on the configuration of the absorbing plate and whether it is glazed or not:
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Parallel absorbing plate configuration; This type of collector can be used horizontally and vertically. The parallel design favors that the collector temperature can be stratified with a greater volume of water circulation through the branches inside the collector.
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Series absorbing plate configuration: it is composed of a single continuous circuit with a low volume of circulating water and a higher thermal jump, with good performance.
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Glazing - Generally used in domestic water heating systems or 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 infrared radiation and insulates the air gap above the absorbing plate to keep warm.
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Unglazed: this type of flat collector is much cheaper but less widespread. Depending on the outside temperature, it is generally used in swimming pool heating systems but can sometimes be used as a hot water heating system in hot countries. In temperate areas, the operating temperature is generally below 30 degrees Celsius.
Components of a flat plate collector
Optically efficient, low-cost, selective coatings are needed for flat plate collectors. A typical flat-plate collector is made up of the following elements:
1. Absorbing plate
The absorbing plate is the element that intercepts solar radiation inside the collector and is responsible for transforming solar energy into thermal energy.
The absorbing plate is usually formed by a metallic foil, usually copper (an excellent thermal conductor), that darkens. Dark, matte bodies capture solar radiation better than any other color.
There are two ways to darken the absorbing plate:
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First, a thin film of heat black paint resists high temperatures.
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Selective treatment is based on electrochemical depositions or paints with metallic oxides with high absorption of solar radiation and low heat emissivity.
The absorbing plate incorporates a grid of conduits through which the heat transfer fluid will circulate.
2. Glazing cover
The glazing cover has the function of isolating the solar collector from external environmental conditions and allowing solar radiation to pass through. This cover is what causes the greenhouse effect.
It is usually made up of a single sheet of solid glass approximately 4mm thick.
This part is only present in glazed flat plate solar collectors, designed to minimize heat loss.
3. Isolation
The insulation reduces heat losses from the inside of the collector - specifically from the absorbing plate - to the outside. It is usually made up of synthetic foam sheets (polyurethane, cyanides, fiberglass, etc.) located on the sides and the back of the solar panel.
4. Housing
The housing serves to house the rest of the components of the solar collector.
This closure is typically formed by an aluminum profile that guarantees a resistance of the assembly, even under extreme working conditions. In addition, the casing has perforations for draining condensates in the lower part.
Flat plate collectors without cover
Flat plate collectors without cover consist of the absorber element, generally formed by plastic material tubes, EPDM, rubber, or polypropylene.
This type of solar collector is 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 pool heating.
In contrast, the collectors without cover with synthetic absorbers tend to have very steep performance curves. With no glass having outstanding optical performance, they quickly lose their performance when the ambient temperature is below the working temperature or 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.