Influence Of The DHW Load Profile On The Fractional Energy Savings: A Case Study Of A Solar Combi-System With TRNSYS Simulations

Jordan, U., Vajen, K.
July 2001

Solar Energy, vol 69-6, p. 197-208

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. Whereas so far simulation studies are usually done with a simplified DHW load profile (e.g. according to prEN 12977), a more realistic profile was generated on a 1-min time scale with statistical means. Assumptions about the distribution of the DHW consumption during the year, depending also on the weekday, 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 DHW consumption, TRNSYS simulations were carried out with a variety of simplified and fairly realistic load profiles. In this case study the investigated solar system consists of a storage tank with an internal thermosyphonally driven discharge unit. Despite a rather minor influence of the draw-off flow rate on the fractional energy savings for the given system applying simplified profiles: a fairly wide range of more than 2 percentage points of the fractional energy savings was found for the system with some constructive improvements or for applying more realistic DHW-profiles. It can be concluded that the influence of the DHW-load profile may not be disregarded, when combistores are compared. This is true especially for stores, for which duration and flow rate of a DHW draw-off have a severe influence on the temperature stratification in the storage tank.