The Influence of External DHW Modules on the Yearly Energy Consumption of Solar Combisystems
Abstract
The presence of external domestic hot water modules (DHWM) on the market is growing, amplified by stronger sanitary rules and an increasing popularity of large ‘solar’ storage tanks. In previous studies it has been shown that DHWM provide a potential for energy savings due to the large capacity of flat plate heat exchangers and thus the possibility of a good stratification in the storage tank. These results have been obtained numerically by modeling the parts and the control strategy of the investigated DHWM. In detailed measurements at SPF it has been shown that some modules have difficulties with the regulation of small flow rates, which result in temperature fluctuations and/or an increased DHW temperature. These effects lead to an increased primary return flow temperature, which has a negative influence on the system performance. A series of commercially available DHWM from six different manufacturers were tested extensively. Thereby three different influences on the yearly energy consumption were investigated separately; the electrical energy consumption, the additional heat losses of the module and additional system losses caused by high back flow temperatures. This paper focuses on the influence of the backflow temperature on the annual system performance based on measured values provided by a lookup table. The performance of the control mechanism was found to be most important for the energy saving potential of the tested modules. Based on simulated parameter variations a simplified method for the judgment of real DHWM was elaborated. This method allows predicting the influence of a DHWM to a reference solar combisystem by measuring the backflow temperature at only a few characteristic operation points.