Linear optimisation of the portfolio of generators in a district heating network

Abstract

This work explores to what extent renewable energy technologies in a district heating network could reduce air pollution and greenhouse gas emissions while maintaining minimal costs. The neighbourhood of Orlandovtsi in Sofia, Bulgaria, is used as a case study for this research. A linear programming optimisation was developed, to determine the optimal installed capacities and hourly outputs across five scenarios – two using traditional technologies, two with renewable technologies, and one combining all considered technologies. These scenarios were optimised for the energy prices in 2017 and 2022 to explore how the changing energy markets call for different installed capacities and their dispatch. Moreover, the optimal capacities of each scenario obtained under the 2017 prices, were dispatched under the energy prices observed between 2018 and 2022. In this way, the ability of each scenario to deliver heat at a consistent price was tested. Lastly, the two renewables scenarios were optimised for different supply temperatures to map the effects of supply temperature reductions on the levelized costs of heat. The integration of renewable technologies significantly enhanced all performance metrics. The best-performing scenario “All technologies” under the 2017 price optimisation reduced PM2.5-equivalent emissions by more than 90% and GHG emissions by 45% compared to the current status quo in the neighbourhood while consistently maintaining significantly lower prices than the current district heating network. Overall, the optimal portfolio of generators consists of an air source heat pump (ASHP) unit for baseload work, an electric boiler for making use of low electricity prices, a pit thermal energy storage (TES) to augment the flexibility of the ASHP and electric boiler and a natural gas technology to tap into an alternative energy source at times of high electricity prices. This natural gas technology could be either a combined heat and power plant (CHP) or a simple boiler. If coupled with TES, a CHP unit was found to be of great benefit when the system is confronted with high electricity prices, as it can produce surplus heat and electricity in moments of peak electricity demand. When the system was optimised for lower electricity prices, however, the CHP was replaced by a cheaper natural gas boiler. The supply temperature was found to have minor effect on the cost of the produced heat, although it does reduce the role of ASHP units, which makes the respective scenarios less resilient when faced with escalating electricity prices. The results of this study show unequivocally that the transition away from fossil fuels is needed beyond addressing climate change. On top of reduced greenhouse gas emissions, the integration of renewable energies holds promise to reduce the cost of energy for the final consumer while mitigating air pollution.