Optimized integration of storage tanks in heat pump systems and adapted control strategies

•For the first time an analytical model of an integrated storage tank in a heat pump system is given.•Fundamental results for this system are analytically derived.•Comparing the analytical results with real data show a remarkable accordance.•An outlook for improved control strategies is indicated.

Storage tanks are widely used in heat pump driven heating systems. In this application, storage tanks can: reduce the number of times the heat pump compressor starts; adapt the capacity of heat generation to heat demand; which can offer the possibility to use low rate electricity. This last point will assume increasing importance as smart grids become more prevalent.Floss and Dietrich (Optimierte integration von Pufferspeichern in hydraulische Systeme, in: Proceedings of the International Congress e-nova 2008, Pinkafeld, Austria, 2008, pp. 129–136, ISBN 978-3-9500919-8-4) points out that due to different effects such as mixing or heat conduction inside the tank, the supply temperature at the consumer is lower than the outlet temperature of the heat pump. The average (mass flow weighted) of this temperature drop was named drop of mean temperature (DoMT). It varies between 0.2 °C and 10 °C, depending on the hydraulic integration inside the storage tank, and can reduce the efficiency of the system up to 30%.This paper discusses options to reduce the DoMT, thereby raising the efficiency of the heat pump system through an optimized integration of storage tanks, and a variation of the mass flow rate in the hydraulic system. These results are based on theoretical and practical investigations undertaken at the Institute of Building Energy Systems, University of Applied Sciences, Biberach, Germany.