Validation and optimization of a solar district heating system with large scale heat storage

Solar district heating systems with large-scale heat storage can contribute to a sustainable future. Currently, there are few mature systems and a lack of suitable tools, hindering large-scale applications. High-precision system models and efficient control strategies are necessary for designing optimal plants. This study developed a TRNSYS model of a large solar heating plant with components like a solar collector field, pit heat storage, heat pump, boilers, piping, pumps, and a MATLAB control system. The model is validated using a solar district heating system in Dronninglund, Denmark, showing good agreement with the measurement, with a difference in the solar thermal production of only 0.3% and an average temperature difference in the pit storage of 1 K. The model of the absorption chiller is improved for wide heat pump simulation, yielding a difference of approx. 5% compared to the measured coefficient of performance. The predicted solar fraction is 0.51, differing by 1.5% from the measurement. The study also proposed two new control strategies: adaptive low-temperature operation strategy and full-time heat pump operation strategy. These optimized strategies are compared with the original control strategy. Results show that the low-temperature and full-time optimization strategies improve the heat pump efficiency by 20%, the collector efficiency by 10%, and the solar utilization by 15%. The economic and environmental performance of the solar heating plant will also be enhanced with a maximum of 27% lower heat costs and 32 kg/MWh lower carbon emissions compared to the original system. In contrast, without the absorption heat pump, the solar heat production decreases by 10%. The system does not meet carbon neutrality if natural gas is used as the auxiliary heat source. This study aims to provide new simulation models and control strategies, which could be utilized for design of optimal large-scale solar thermal applications.