Investigation of system effects of return flow temperature in a district heating system containing waste heat sources
Examensarbete för masterexamen
n order to meet the climate change mitigation in Göteborg city, the district heating (DH) company is aiming to increase the share of recycled energy sources in the DH-system. This goal can be reached by maximizing the usage of currently available waste heat, both in the nearby industries in the city and internal waste heat at company. This master thesis is trying to investigate a concept to recycle the available low temperature heat in the return flow of the DH in order to use it in an industrial heat pump to lift the temperature in the supply flow of the DH. The effect of the heat pump on achieving a maximum temperature reduction of the DH return flow was studied in detail. Since the waste heat delivery in city of Göteborg to the DH-network occurs from different companies, the study tried to cover the entire waste heat provider conditions, mass-flow and temperature, The project comprises also an analysis of the effect of potential lower DH return temperature on the energy generation units of the DH-system. An economic analysis has been implemented, including the variability of electricity prices throughout the year, to analyze the impact of the various investigated concepts to waste heat prices. In one investigated scenario, the project introduces a new concept of heat exchanging between the waste heat supplier and DH-company in order to achieve a better usage of waste heat at lower temperature. In this context, it is discussed that the introduced concept can lead to better utilization of higher temperature waste heat for other purposes like carbon capture for e.g. refineries in Göteborg. The modeling results indicate that a temperature reduction of about 7,5 degrees can be achieved with a coefficient of performance (COP) of 3,87 for the current system of one refinery and the DH-company and this results in about 11 MW more extracted heat from the heat exchanger(HEX). Furthermore, a temperature reduction of 8,7 degrees can be achieved with a heat pump COP of 3,31 and additional mass-flow of DH-water of 82 kg/s (i.e., considering 15% more available heat at 10°C lower temperature) this would result in around 25 MW more extracted heat from the HEX. The result of economic analysis indicates that the concept of installing heat pump by DH-company will become cost-efficient for a cheaper waste-heat price. The introduced price calculation approach showed that for a waste heat price about 30% lower than current prices, the heat pump installation would be cost beneficial for the DH-company. However, the results show that this will not be highly motivated by waste heat providers for the cooling savings related with the reduced return temperature. Further, it is discussed in this paper that by deeper analysis of the economic aspects with a higher resolution (e.g. hourly variation of electricity prices, real waste heat and cooling prices) and by considering the environmental issues, the motivation for this project for both the DH-company and the waste heat providers can be adjusted in between the business parts.