The connection between heat pump and buffer storage was investigated for one exemplarily solar thermal system
revealing the following results:
x Number of zones: A two zone operation allows using different set temperatures for space heating and DHW
preparation. This leads to lower condenser temperatures and thus a better heat pump performance.
x Volumes: The optimum variants have a DHW volume of 100 l and a SH volume between 200 and 300 l. Dead
and separation zone are necessary but may be reduced to a small volume of e.g. 40 l each.
x Set temperature: In case of two heated zones, the set temperature of the DHW zone has only a small impact on
the system performance, while the set temperature in the SH zone should be variable according to the heating
curve. The design of the space heating components and the specification of the heating curve play an important
role to avoid unnecessary high condenser temperatures.
x Sensor position: The sensor position defines the volume which is kept on the set temperature. A high sensor
position reduces this volume and thus the electricity consumption and the heat pump cycles. A lower sensor
position allows a lower set temperature without reducing the comfort. The best combinations of sensor position
and set temperatures between 40 °C and 50 °C lead to almost identical total electricity consumption values. The
use of two sensors in each heated zone shows no advantage.
x Flow rate of heat pump: Both flow rates should be minimized with regard to the characteristics of the ground heat
exchanger in case of the evaporator and to the rise of the inlet temperature in case of the condenser side.
x Overall: The total electricity consumption may be reduced by 15 % with an optimal heat pump storage
connection compared to the base case with one heated zone.
This study reveals from simulations in one defined heating system important facts regarding the connection of
heat pump and storage tank. For a more general view, the study has to be extended to different systems and
boundary conditions. In addition, test rig experiments should be carried out to verify and extend the outcomes.