Techno-economic analysis for the conversion of utility-scale heat pump to a refrigerating machine for district cooling application : a case study at Norrenergi

Sammanfattning: Heat pumps with wastewater as a heat source are used at Solnaverket, Norrenergi to provide a base load district heating and some cooling services. Currently, the waste cooling produced from the heat pumps is needed to supply a growing demand for district cooling during hot summer days. At the same time, the heating demand is very low in summer. Part of the heating base load could instead be imported from Stockholm Exergi with very low production costs for district heating in the summer. While the heating cost decreases as a result, the supply of district cooling becomes a more expensive product for Norrenergi. Bromma sewage treatment plant will presumably shut down in 2027, which means that Norrenergi needs to replace the wastewater with for example lake water as heat source. This study investigates how the district cooling production could become more economical and resource efficient by converting a heat pump to a chilling machine for summer operation. The technical possibilities of a redesign have been investigated. This resulted in three alternatives; two of those focused on converting a heat pump to a refrigerating machine in the summer with the possibility to use lake water as a cooling source. The third alternative connected the evaporator directly to the district cooling network, but still required a heating load in order to operate. An operational model of how the district cooling network prioritizes which cooling equipment starts first was built to simulate the converted heat pump behavior for three representative historical cooling demands. The option of converting a heat pump to a refrigerating machine with an intermediate cycle connected to the condenser was considered as the best investment. The sensitivity analysis showed that the climate data is the factor that affects the results the most, followed by the electricity price. Before Bromma sewage plant shuts down, the annual savings are too low to recommend an investment. However, this alternative was profitable considering an increased cooling demand and the inclusion of a cooling buffer energy storage in the future. A scenario considering the whole system profit also showed to be profitable. After Bromma sewage plant shuts down, the alternatives must be compared with the investment in new chilling machines, which are expensive, and potentially replacing heating capacity. Therefore, an investment could be recommended for this case, but needs further investigation.