Analysis of a hybrid heating system with TRNSYS: district heating, heat pumps and photovoltaics in a multi-apartment building

Hybrid heating systems that combine district heating from the network and locally installed heat pump, can offer higher efficiency and lower cost compared to standalone systems. Renewable energy sources, such as photovoltaics, can be connected to the hybrid system and enhance the flexibility of managing energy use. For the performance assessment of such systems, the real-time prediction of the energy supply and demand is crucial. It is common to simulate various heating and electricity technologies in buildings, yet comprehensive performance assessments of flexible systems in residential buildings remain limited. In this work a dynamic model with sophisticated control strategy of a multi-apartment building in Sweden with two heat pumps, electric heater, photovoltaic array, connection to district heating network and hot water storage tanks, was developed in TRNSYS. To increase the accuracy of the model results, but at the same time limit the computational time, several timestep sizes were tested. The model calculates every minute the thermal demand, the electric demand of the heat sources and the excess electric energy of the photovoltaics. The model results are used to calculate the energy demand and supply of the main building devices, as well as the operating cost and environmental impact of the heat sources in three control strategies with variable hierarchy: A) district heating priority over heat pumps, B) heat pumps priority and C) operation with district heating exclusively. The environmental impact was assessed with the online tool VERIFY which is able to perform global warming impact assessments by capitalizing on the well-known life cycle analysis methodologies. Moreover, the model calculates the time periods when the excess photovoltaic electricity can feed the heat pumps load and thus, the reduction of the grid electricity consumed by the heat pumps. According to the model, the thermal demand is almost the same for all three control strategies, while the supply is divided as follows: control strategy A) 55.4% district heating / 38.4% heat pumps / 6.2% heater, control strategy B) 23.2% district heating / 76.8% heat pumps and control strategy C) 100% district heating. Utilization of photovoltaics reduces the heat pumps consumption by 14.8% and 13.5% for district heating and heat pumps priority respectively. Finally considering the cost, operation with heat pumps priority results to the lowest cost at 2,627 €/year, it follows district heating priority at 4,348 €/year and exclusively district heating at 6,312 €/year. The costs are further reduced with photovoltaic excess energy exploitation.