E. Streicher, W. Heidemann, H. Muller-Steinhagen

University of Stuttgart, Institute for Thermodynamics and Thermal Engineering (ITW) Pfaffenwaldring 6, D-70550 Stuttgart Tel.: 0711 /685-3536, Fax: 0711 /685-3503 email: streicher@itw. uni-stuttgart. de

An important number for the assessment of thermal solar systems regarding environmental aspects is the energy payback time. This is the period, the system has to be in operation in order to save the amount of primary energy that has been spent for production, operation and maintenance of the system.

The present paper outlines the methodology for determination of the energy payback time of thermal solar systems. It is explained how factors like pump operating hours or fractional energy savings influence the energy payback time. This will be demonstrated by calculating the energy payback time for a typical solar domestic hot water system (SDHW system).

Solar heating systems for combined domestic hot water preparation and space heating, so-called solar combisystems, are more complex in their structure than SDHW systems. As solar space heating can be realized with different system concepts, a uniform methodology is necessary for comparison of different solar combisystems. This uniform methodology is presented in the second part of this paper.