Inside the solar collectors without concentration we find the flat plate solar collectors.
They were the most used, but have lost ground in favor of vacuum tube.
Plate Solar Collector: Overview
In the flat plate solar collectors, the collector is located in a rectangular box (housing), whose usual dimensions range between 80 and 120 cm wide, 150 and 200 cm high, and 5 and 10 cm thick ( although there are larger models).
The face exposed to the sun is covered by a very thin glass, while the five remaining faces are opaque and thermally insulated.
Inside the box, on the side that is exposed to the sun, a metal plate (absorber) is placed.
This plate is attached or welded to a series of ducts through which a heat carrier (usually water, glycol, or a mixture of both) flows.
A selective surface treatment is applied to said plate to increase its heat absorption, or it is simply painted black.
What is a flat plate solar collector?
A flat plate solar collector, are simple devices to heat water, has a large flat surface (absorbent) to maximize exposure to the sun, and has small tubes attached to it.
The fluid circulates through the copper tubes, collecting the heat delivered by the absorber. The sides and bottom of the collector are well insulated, and the glass at the top completes the insulation
How does a flat plate collector work?
Flat solar collectors work by taking advantage of the greenhouse effect (the same principle that can be experienced when entering a car parked in the sun in summer).
After passing through the glass (transparent for wavelengths between 0.3 μm and 3 μm) the radiation reaches the surface of the absorber, which is heated and emits radiation with a wavelength between 4.5 μm 7.2 μm, for which the glass is opaque.
Approximately half of this last radiation diffuses to the outside, losing itself; but the other half returns inwards and thus contributes to further heating the surface of the absorber.
When passing through the box, the heat transfer fluid heats up and increases its temperature at the expense of the absorber, which will decrease.
Then the heat transfer fluid transports that thermal energy wherever it is desired.
How is a flat plate collector built?
The flat plate solar collector consists of 4 main elements :
1) Transparent cover: must possess the necessary qualities (coefficients of transmission and adequate thermal conductivity) to cause the greenhouse effect and reduce losses; ensure the tightness of the collector to water and air, in conjunction with the housing and the joints; Do not keep the dirt adhered to the outer surface so the rain can easily slide off.
2) Absorber: receives solar radiation and converts it into heat that is transmitted to the heat transfer fluid.
The forms are diverse: metal plates separated by a few millimeters, a metal plate with welded tubes or sausages or two metal plates with a circuit inside.
The face exposed to the sun must capture the greatest amount of radiation so it is usually painted black or endow it with a selective surface (very absorbent to radiation and low emitted).
Paints are cheaper than selective surfaces and have a better overall thermal behavior at temperatures close to the ambient temperature, but they are spoiled by the continued action of ultraviolet radiation and with temperature variations between day and night.
The selective surfaces have in general a better behavior and are obtained by superposition of several layers (metal and metal compounds) or special surface treatments.
The most modern manufacturing technique is laser welding.
3) Insulation: it is used to reduce the thermal losses in the rear part of the absorber that must be of low thermal conductivity. The materials can be glass wool, rock wool, cork, polyethylene or polyurethane.
4) Housing: usually made of aluminum or stainless steel, it protects and supports the elements of the collector, also allowing anchoring and securing the collector to the assembly structure. It must withstand temperature changes (dilatations) without losing the tightness and must resist corrosion.