Flexible Sector Coupling (FSC) of Electrical and Thermal Sectors via Thermal Energy Storage (TES) : A Case Study on Oskarshamn Energi

Sammanfattning: The integration of distributed energy sources and systems is of high relevance for the transition towards a more sustainable energy system. Taking into consideration the  amount of emissions produced by the heating sector, which account for at least half of the energy demand in buildings, district heating systems have the potential to play a key role in the transition, by enabling the integration of various energy sources and provide flexible energy services to the grid. The objective of this research is to evaluate the potential for flexible sector coupling between the electrical and heating sectors, utilizing thermal energy storage alongside various heat generation units, including heat pumps and a combined heat and power (CHP) unit. To examine this concept, we used a district heating facility located in Oskarshamn, Sweden, as our case study. At present, the production mix at this facility comprises various production units- that utilize mainly biomass as their fuel source, including wood pellets, wood chips, and occasionally, bio-oil. Extensive research was conducted to review the existing literature and gain a comprehensive understanding of the technologies and concepts associated with FSC. This thorough examination allowed for a comprehensive overview of the current state-of-the-art in FSC. As the main contributions of this work, two numerical models respectively for production and dispatch optimization were developed and simulated complementary, concerning the thermal and electrical system of the studied plant. A dispatch model was developed with the aim of analyzing the operating behaviour of the system, identifying the available energy sources and optimizing their hourly dispatch. Subsequently, utilizing the open-access tool for capacity and investment optimization (OSeMOSYS), various scenarios were examined to evaluate the potential of thermal energy storage (TES), where a water tank was found to be the most cost-effective solution, and heat pump integration in enhancing the plant performance and providing flexibility. The study was divided into two distinct time periods. The first period focused solely on hourly dispatch optimization until 2035. In the second period, the analysis extended to include investment optimization, followed by the subsequent dispatch optimization until 2050, hence, using both tools. To effectively compare and assess the different scenarios, several key performance indicators (KPIs) were chosen, including the levelized cost of energy (LCOE), capital expenditure (CAPEX), generation costs, and emissions. These scenarios were designed to account for variations in crucial variables such as electricity prices, the plant’s self-consumption potential, and the capital cost of storage. By considering the aforementioned factors, a comprehensive analysis was conducted to determine the optimal approach for maximizing performance and cost-effectiveness. It is important to mention that the electrical self-consumption within the plant was considered as one of the potential improvements. While the potential for electrical self-consumption was mainly studied on a shorter-term, the variability in the capital cost of the TES system was better considered on the long-term investment analysis. From the different simulations, the cases where self-consumption is included result in smaller operating costs, as producing electricity via the CHP unit of the plant is cheaper on average than the prices offered by the local distribution company. The obtained capacities for TES and the heat pump vary among the studied scenarios. Higher electricity prices favor investments in alternative fuel boilers like wood chips or wood pellets, while lower electricity prices result in increased TES capacities and higher heat pump production. The capital cost of storage also determines the capacity chosen for the storage water tank, sometimes investing a bit more to gain efficiency and reduce operational costs. Throughout the project, various sustainability aspects have been addressed. These encompass environmental responsibilities, with a focus on reducing CO2 emissions, enhancing social equity by implementing a more efficient heating system within the municipality, and assessing the economic viability of these initiatives. In conclusion, the study provides evidence and showcases the viability of implementing FSC in Oskarshamn’s power plant, as results from the different scenarios commonly show that FSC could bring down the total costs, as well as the amount of CO2 emissions on a long-term basis. Based on the findings, additional recommendations are proposed to optimize the plant’s performance and leverage the potential of this innovative approach. The proposed recommendations include increasing the time resolution in the model simulations to improve result accuracy and exploring different scenarios, which may involve considering various electricity or fuel price predictions, among other factors.