Evaluation of Heat Pump Operation in a Large Scale Solar Thermal System

In the north of Graz, Austria, a large scale solar thermal system (gross collector area: 3885 m²) was installed in 2009 primarily to supply solar heat to a district heating (DH) network especially in summer and to cover part of the heat demand of the on-site buildings. In order to reduce the auxiliary heat needed for the on-site buildings and to prolong the operation time of the solar thermal system, a heat pump, which uses the solar collector loop as exclusive heat source, was installed later on. This way solar heat that is available at temperatures too low for direct solar charging of the TES (thermal energy storage) can still be used. After having performed dynamic system simulations of the installation without the heat pump in TRNSYS to validate the simulation model, the heat pump was implemented into the simulation model. Applying different control strategies to the solar heat pump system an increase of the solar fraction for the heating period (October to March) from 20.5 % without the heat pump up to 33.0 % with heat pump could be reached in the simulations. The interaction between solar thermal system and heat pump in this special configuration turned out to be challenging in terms of control optimization; hydraulic decoupling of the two units is recommended. Based on onsite measurements the performance of the system in February 2013 is evaluated