The law of solar thermal group that has absolutely nothing to do with its photovoltaic counterpart and is based on the greenhouse effect. But the general principle, although known to all may seem less evident if we start to go into details!
You may have noticed that dark-colored objects are warmer than bright colors when exposed to the sun for some time. Indeed, the less a body reflects light – that’s why it looks dark – the more it absorbs solar energy in the range of the visible spectrum. We must also know that a “black body,” a scientific name given to objects absorbing solar radiation, in our case, re-emits energy in the form of infrared radiation, and those, especially since it is dark. Thus a white surface warms much less and more slowly in the sun, but more slowly releases its energy later, at night, for example.
We, therefore, have every interest in using a black surface traversed by a copper coil where water circulates. But the fact that this dark surface loses more heat than a white surface is it embarrassing?
In fact, not at all, because that’s where our greenhouse effect comes in. If we place the black plate in an enclosure covered with regular glass, all exposed to the sun, the lens will let the visible ray, (This is the interest of a window!) But block the infrared rays that are re-emitted by our black body! The infrared will be reflected on the glass and will then be redirected to the plate, and therefore, the temperature of the plate and the enclosure will climb … Especially since it is sheltered from the wind.
All that remains is to communicate this heat generated to water via a copper pipe snaking in contact with the copper plate. Moreover, without this water to “cool” the panel, the dish could probably easily exceed 200 ° C during a heat wave!
Our solar panel works! It is based on the capture of solar radiation and not thermal conduction (except for the water pipe). Thus a solar panel will work almost at the same performance it is 15 ° C or 30 ° C outside if there is sun and its elevation (depending on the season) is the same!
The final prototype, deviating somewhat from the original plan I have drawn, to a surface of 1.75 m². Indeed 1m² would have been a better choice to efficiently perform a performance calculation and quickly evaluate the performance of the solar panel, but the aluminum panel that I found was 2m ²; I chose to exploit it to the maximum to make his purchase profitable (80 euros). The frame is made of joinery wood assembled with screws, but nothing prevents you from using other materials such as aluminum, composite or steel. A layer of rock wool or glass wool insulates the inside of the panel to limit heat loss. In my case, I used a “new generation” insulation material consisting of a thousand-sheet laminated aluminum (mylar) and Dacron that is easily found in any hardware store. Its thickness (2-3cm) is less compared to glass wool. It is stapled to the inside walls of the trunk.