Decarbonization of district heating and cooling: Role of seasonal thermal energy storage and heat pumps
The decarbonization of district heating (DH) systems requires scalable solutions capable of integrating renewable heat sources while reducing reliance on peak boilers. Seasonal thermal energy storage (sTES) and large-scale heat pumps (HPs) are among the most promising technologies to tackle this challenge, particularly in solar-assisted district heating (S-DH) networks where fluctuating solar heat availability requires flexible thermal management. This study evaluates the combined integration of a large-scale sTES and a water-towater HP into a representative Austrian DH system, focusing on optimal sTES sizing and the impact of HP placement on system efficiency, solar fraction, reduction on CO2-emissions and operating cost reduction. A dynamic simulation model of S-DH is developed, incorporating detailed sTES and HP components and a top-level control strategy for operational dispatch. Two sTES technologies; tank (TTES) and pit (PTES), are examined ranging from 210,000 m3 up to 360,000 m3. Three alternative HP integration configurations are compared: (i) HP coupled directly to sTES, (ii) HP upgrading solar heat, and (iii) HP installed on the DH return line. Findings indicate that TTES provides the best overall performance, with an optimal volume of 330,000 m3 and efficiency of around 90%, while PTES exhibits substantially higher thermal losses and lower efficiencies despite slightly higher solar fractions. Outcomes pinpoint the DH-return configuration with the strongest technical performance, achieving the highest SCOP values and enabling the largest transition from gas-based production. Yet, the direct sTES–HP coupling shows the highest sTES efficiency-around 3% points higher than the other configurations-due to deeper discharge and lower operating temperatures. The DH-integrated HP configuration resulted in the greatest annual OPEX savings, exceeding 35% at high HP capacities. Hence, detailed design and layout of HP and sTES integration significantly enhances renewable utilization, system efficiency, and cost-effectiveness, providing a clear pathway for next-generation low-carbon district heating systems.