Basically, there are two collector types (in terms of their structure): flat collector and tube collector. There are some differences between them, according to their sizes, efficiency, installation and their already mentioned structure. Below we present a shortcut which helps to understand their specification better.
1. Flat collector
This collector type consists of an absorber, thermal insulation, glass plate, pipes with working fluid and its cover. Solar radiation energy penetrates solar glass and falls onto the plate of absorber. Absorbed heat is transported by wires with heat transfer fluid to heat exchanger in solar tray. All these mentioned elements are placed in sealed housing, covered with thermal insulation.
We can also distinguish vacuum flat collectors. The difference is that inside collector is a vacuum. Its also has glass secured with brackets, preventing from it deflection. Vacuum transfers by radiation very well, limits conduction heat lost and almost completely excluded convection lost. Housing of vacuum flat collector also has full aeration protection.
2. Tube collector
Tube collector absorber contains closed on both ends vacuum glass pipes of 65-100 mm diameter. Vacuum play a role of insulation. Collector is made from 6-30 pipes in a row, connected manifold pipe. We can distinguish two types of tube collectors: direct heat fluid transfer collectors and heat pipe collectors.
In first type heat fluid flows directly through vacuum pipes. In the middle of collector housing is placed coaxial heat exchanger pipe. Inside it flows fluid, transporting heat to absorber, which convey heat through heat pipe exchanger form heat transfer fluid. Than it comes back by external exchange heat pipe.
In second type in the heat pipe is placed inside absorber. It is filled with liquid, which evaporates in low temperature. When solar radiation affect a collector, absorber leads heat into heat pipe and then liquid evaporates. Above capacitor steam gives it heat through heat exchanger transfer fluid, then it is condense. Condensate fluid flows down through heat pipe and then process starts again.
Which collector is the best?
There is no easy answer to this question. It all depends from standard of evaluation and collector manufacture brand. Here you got some features comparasing of this collector types in our opinion (first number in order means highest position):
1 - flat collector
2 - tubes collector
summary of general benefits
To close our summary, we would like to add few important points:
1) vacuum flat collectors have the lowest heat losses ratio; their got constant heat efficiency during their “lives spam”; it is possible to change residual gas (air) for krypton, to increase their efficiency; dust do not get into collector and there is no corrosion due to condensation of moisture; incorrect insulation may cause, that air will get into vacuum and then recreation of vacuum will be needed
2) in tube collectors even small defects on production stage can cause improper work of a collector and decreased efficiency; it is necessary to mention, that majority of this collectors are being produced in China
3) heat pipes collectors need to have minimum 20 degree slope, which allows to proper liquid functioning
4) standard flat collectors are best choice for climate conditions with frequent storms, hails, etc.