Heat source shifting in buildings supplied by district heating and exhaust air heat pump

The heat supply for Swedish multi-family residential buildings is becoming more complex, and today it is fairly common to combine district heating with a second heat source. The most common heat source to combine with district heating in Sweden is an exhaust air heat pump. On average, the exhaust air heat pump covers 31% of the yearly heat load and is given full load priority. There is a missed potential in cost and CO2 savings when one heat source is given full load priority, since marginal production costs and CO2 emissions constantly vary in both the electrical grid and the district heating system. The aim of this study is to evaluate how buildings with several heat sources should be operated using hourly energy prices. Hourly heat and electricity prices for Gothenburg have been established for two years based on the marginal costs of heat and electricity generation. These prices have been used to evaluate the most common combinations of heat sources. Results show that the most common combination with an exhaust air heat pump with full load priority does not lower costs compared to the reference case with only district heating. However, having a control system that allows heat source shifting and gives load priority to the heat source with the lowest cost each hour can greatly reduce the heating cost, and systems with larger heat pumps show even greater potential for heat source shifting.