The efficiency of solar collectors depends on the temperature of the working medium.
The efficiency of flat plate collectors decreases with increasing temperature of the working medium, therefore, more than one should not be connected in series.
The number of series-connected Collectors depends on this, what temperature of the medium we want to obtain, what flux of solar energy is available in the area and the size of the collectors.
According to American research (Duffie, John, Beckman: „Solar energy therma processes” 1974), The efficiency of the collectors to reach 38 ° C of the working medium increased sharply, then, up to a temperature of 50 ° C, it increased more slowly, and from 53 ° C it dropped. So, the connection diagrams of the collectors are as follows: serial, parallel and parallel-serial.
There are also collectors with open water flow. Corrugated sheet collectors are mounted on the sloping roof, covered with glass, where the water is distributed through a perforated tube placed on top of the collector, and after flowing through open channels, it is collected in the lower insulated reservoir. This type of solution is more often used to heat water in swimming pools.