Retrofit application of heat pumps with dual heat source
Heat pumps (HP) are the predominant heat generator in many scenarios. Market shares in the larger capacity range, though, are still limited, in particular in existing buildings. A major restriction for larger HP capacities are suitable heat sources (HS), especially for the boiler replacement in existing buildings, where limitations of the HS by the built environment can limit installations of HP. An option to overcome these limitations is the integration of multiple HS, in particular, if synergies among the HS can be used. The project “RenoSource” investigates the integration of the two most common heat sources used in Switzerland, outdoor air and ground probes regarding design, control and performance by system simulations and field monitoring, completed by an analysis of the cost and economy of the integration. Two strategies for the integration have been evaluated: a peak load coverage by the ground probes and outdoor air as base load and a regeneration of the ground probes with outdoor air. Simulation results from the preceding project HP-Source confirm that for both strategies, source integration can overcome the limitations, as the ground probe field can be notably downsized. Thereby, the additional cost of the second heat source can be refunded or even overcompensated, i.e. by the synergies between the sources, and both performance and economic benefits result. Consequently, the simulation results imply, that individual heat sources can be both technically and economically inefficient compared to multi-source designs for the larger capacity range. A dual-source system was implemented in a pilot project of two multi-family houses with a heating capacity of 170 kW. This paper presents results from the evaluation of the first year of operation of the pilot project, focusing on control strategies, measured system efficiency and the regeneration of the ground. Results of the first winter period confirm that with a notably smaller design of a peak load ground probe field and air source as base load, the heat demand can be covered. Additionally, the ground source can be regenerated by the air heat exchangers.