A solar collector is a structure or device that converts energy from the sun into heat. Formally, solar collectors are: a steel barrel painted black on a dacha shower; a thin-walled hose rolled around the garden; exposed to the sun plastic PET bottles filled with water. Even a simple bucket of water basking in the sun can be considered a solar collector. We will not delve into the classification of such self-made ultra-economical options for using solar energy. To form a complete picture, we will only mention such types of water heaters open collectors, which have neither glazing nor a special coating. Most often, these are black rectangular plastic containers filled with water.
Flat solar collectors.
The main element of a flat solar collector is an absorber – a metal plate with a special absorbing coating and a flow pipe soldered onto it. The absorber is enclosed in a special case, in which the front wall is transparent (through it the solar radiation penetrates into the collector. And the back insulates with a mineral wool plate or a layer of other insulation.
The internal pipeline through which the coolant circulates can be located on the absorber in different ways. There are 2 main types of arrangement: “meander” and “harp”.
To increase the efficiency of the collector, a special selective coating can be applied to the absorber. The presence of a selective coating significantly increases the productivity of a flat collector. But at the same time increases its cost.
To reduce heat loss in the cold season. The flat collector body is made as tight as possible. Thus, the thermal insulation of the absorber is achieved. By a layer of air or inert gas from the side of the transparent front wall. And a layer of insulation from the side of the rear wall.
Flat collectors are more efficient in the warm season, however, in winter, their efficiency significantly is reducing due to rather high heat losses.
There is also another type of flat solar collector – a vacuum flat collector. In a flat vacuum collector. The thermal insulation of the absorber from the environment achieves not due to a layer of thermal insulation. But due to the creation of a deep vacuum inside the box, which prevents heat loss. Such collectors have the highest productivity among flat collectors, however, they are more difficult to install and operate, and, what is important, very expensive.
The indisputable advantages of flat solar collectors are their low price and high efficiency in the warm season. The disadvantages include low productivity in winter, as well as the relative inconvenience of their installation on hard-to-reach roofs. The flat collector is a one-piece non-separable structure, which is why it should lift and install on the roof entirely.
Tubular vacuum solar water heaters
It is much easier to install a tubular vacuum collector on a sloped roof. The manifold tube consists of a frame or fittings, a heat exchanger body (so-called manifold), and glass vacuum tubes. Thus, the installation of the tubular collector can be carried out in several stages. First, we install the frame, fix the manifold on the frame, and then insert the vacuum tubes into the manifold one at a time.
Tubular vacuum collectors are also more efficient than flat collectors in the cold season, as the vacuum provides minimal heat loss.
Directly heated coaxial vacuum tubes
In coaxial vacuum tubes of direct heating. The heat carrier (usual water) heat by direct contact with the inner heat-absorbing wall to increase heat absorption. The glass wall covers from the inside with a special selective coating.
Direct heating collectors on coaxial tubes are simple and cheap, but they have a number of disadvantages:
- Due to the direct contact of the coolant with the tube, the collector cannot work under high pressure.
- The collector works only during the warm season.
Coaxial vacuum tubes with a heat-pipe system
Heat exchanger through a special socket.
The heat-pipe coaxial vacuum tubes are highly efficient and easy to install. In addition, in such a collector, you can easily replace not only one vacuum tube. But even a separate heat-pipe heat exchanger in case of failure. Moreover, it is not at all necessary to dismantle the collector from the roof. All replacements can be carried out locally with minimal time and effort.
Collectors with heat-pipe vacuum pipes are affordable. Highly reliable and can be used without restrictions in high-pressure solar thermal systems.
Coaxial vacuum tubes with a U-type system
Another type of solar tubular collector is a coaxial tube collector, in which a U-shaped flow-through heat exchanger is located.
With high efficiency, U-type pipes have one serious drawback – they are one piece with the manifold. And must be assembled as a whole. At the same time, this design excludes the replacement of single tubes in case of failure. Also, the disadvantages of collectors with U-type pipes, in comparison with heat-pipe pipes․ Are their higher hydraulic resistance. Increased requirements for the coolant and, importantly, a higher price.
Feather vacuum tubes
The most efficient tubular manifolds are the Feather tubular manifolds. Feather tubes consist of a heat-pipe system mounted օn a selectively coat flat heat absorber that is placed under vacuum, in other words, it is a kind of hybrid of heat-pipe pipes and vacuum flat collectors.
While high efficiency, fountain tubes are more expensive and more difficult to install. In addition, if such a tube fails, it will have to be replaced entirely.