3.1. General remarks
In general solar energy can be used the better the higher the temperature level is. Even if it can be used exceptionally at low temperatures, e. g. for swimming pool heating, for water heating resp. preheating, for preheating for room heating support or for increase of the back-flow temperature, these processes could be much more efficient at higher collector temperatures. This is to be explained in the following.
3.2. High collector temperatures increase the efficiency factor of the installation:
The conventional backup heating can only be avoided efficiently and permanently if the solar outlet temperature is permanent considerably higher than the relevant desired temperature. As the efficiency factors of boiler and installation are especially bad out of the heating season, every single switching of the boiler avoided by solar heat saves many times the amount of energy that the solar collector system feeds into the tank. If e. g. a boiler working with oil with an efficiency factor of 90 % and an internal water volume of 60 l has to heat this water from 20 °C to 70 °C before it can heat 150 l hot water in the tank by 5 K, only 16 % of the consumed fossil fuel is used for water heating. Has the solar collector system, however, heated the hot water tank sufficiently and the boiler does not need to start, it saves with every solar kilowatt-hour 6.25 kilowatt-hours of fossil energy.
In addition to its annual solar energy harvest the solar collector system helps to save an amount of energy by avoiding boiler and installation losses that can be as large or even larger than the pure solar energy harvest. Low temperature collectors can never have this synergetic effect, not even in summer, because they need delta-T-controllers which usually prevent high temperatures.
Permanently high collector temperatures are coupled with big temperature differences what decreases the volume flow demand. Thus electrical pump energy and pump working time is saved.