Influence of the DHW load profile on the fractional energy savings: A casestudy of a solar combi-system with TRNSYS simulations

In this paper the influence of domestic hot water (DHW) load profiles with a constant total yearly heat demand is studied for a solar combisystem. Whe reas so far simulation studies are usually done with a simplified DHW load profile (e.g. according to prEN 12977), a more realistic profile was genera ted on a 1-min time scale with statistical means. Assumptions about the dis tribution of the DHW consumption during the year, depending also on the wee kday, and the time of the day were made. To study the influence of the draw -off duration and flow rate as well as the daytime of DH-W consumption, TRN SYS simulations were carried out with a variety of simplified and fairly re alistic load profiles. In this case study the investigated solar system con sists of a storage tank with an internal thermosyphonally driven discharge unit. Despite a rather minor influence of the draw-off flow rate on the fra ctional energy savings for the given system applying simplified profiles: a fairly wide range of more than 2 percentage points of the fractional energ y savings was found for the system with some constructive improvements or f or applying more realistic DHW-profiles. It can be concluded that the influ ence of the DHW-load profile may not be disregarded, when combistores are c ompared. This is true especially for stores, for which duration and flow ra te of a DHW draw-off have a severe influence on the temperature stratificat ion in the storage tank. (C) 2001 Elsevier Science Ltd. All rights reserved .