Every kind of preheating has a negative effect on the total efficiency factor of the boiler or the power plant respectively. This is especially evident with condensing boiler technology because return-flow temperatures increased by solar energy reduce or block condensing boiler technology. However, also with every other back-up heating the total efficiency factor is decreased by preheating processes. The causes are e. g. higher standstill and operating losses as well as higher demand for pump energy and storage tanks.

4. Summarization, Conclusions

Tube collectors in general and especially CPC vacuum tube collectors with water as heat transfer medium have proved to have potential to simplify and improve solar-thermal systems so that flat plate collectors can be called outdated. In the following examples there is no reasonable alternative to this technology at the moment:

— all-year production of process temperatures over 60 °C beyond the tropics and subtropics

— avoidance of heat losses with oversized storage tanks without stagnation

— avoidance of installation damage due to unintended thermal stagnation

— hot start of the installation from stagnation for (the purpose of) permanent usability and so that the collector can be used as an additional storage tank

— operation without antifreeze

— in case the collector orientation differs essentially from the optimal cardinal point In general solar energy can be used the better the higher the temperature level is.