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

Abstract

The assessment of cost, environmental impact, and overall performance of hybrid heating systems in residential buildings under dynamic, real-world operating conditions is not well established yet, although simulation results of individual heating systems and electricity technologies are frequently reported in literature. This study focuses on evaluating the performance of a system that combines District Heating (DH) with locally installed Heat Pumps (HPs) in a multi-apartment building in Gothenburg, Sweden. The performed evaluation is deemed important in terms of enhancing the overall energy efficiency of the building, as well as for lowering operational costs and greenhouse gas emissions, and for enabling seamless integration of renewable energy sources (such as photovoltaics) within such building of this kind. The reference building is equipped with two HPs, an electric heater, a photovoltaic array, DH supply and hot water storage tanks. Here, a dynamic model was developed in TRNSYS to calculate the performance of the hybrid system of the building. So as to increase model accuracy, and minimize computational time at the same time, several time-step sizes were evaluated. The model calculates, every minute, the thermal demand, the electrical demand of the heat sources, and the excess electrical energy of photovoltaics under three control strategies: (A) district heating priority, (B) heat pump priority, and (C) exclusive use of district heating. The environmental impact was assessed by use of the online tool VERIFY, which is able to perform global warming impact assessments by capitalizing on well-known Life Cycle Analysis (LCA) methodologies. According to the model, the contribution of each heat source under the three control strategies is: A) (55.4 % district heating) + (38.4 % heat pumps) + (6.2 % heater), B) (23.2 % district heating) + (76.8 % heat pumps) and C) (100 % district heating). Utilization of photovoltaics reduces the HP consumption by 13.5 % or 14.8 % depending on the implemented strategy. Finally, the order of the annual cost is: B) 2,627 €, A) 4,348 € and C) 6,312 €. Overall, the use of HPs as the primary heat source in multi-apartment buildings is determined be the most beneficial solution in terms of operating costs and environmental impact.