Optimal operation of integrated power and thermal systems for flexibility improvement based on evaluation and utilization of heat storage in district heating systems

Heat storage in district heating system is promising for renewable energy accommodation because it effectively improves system flexibility. However, its proper evaluation remains unclear, preventing its full utilization. This work distinguishes heat storage mechanisms of heating network (HN) and buildings rigorously by combining a unified multi-energy flow model and energy conservation analysis. Heat storage of HN is attributed to heat migration processes inside, while that of buildings originates from heat capacities of envelopes. On this basis, water flow rate and supply water temperature in primary HN (PHN) and set values of indoor air temperature are optimized to improve the system flexibility, namely the wind power accommodation. Different combinations of the three optimizations are also studied to improve system flexibility to the greatest extent. Results show that optimizing water flow rates and supply water temperature in PHN and optimizing water flow rates in PHN and set values of indoor air temperature yield 14.3% and 15.3% additional increase of wind power accommodation, respectively, compared with their ideal superposition. On the contrary, optimizing supply water temperature in PHN and set values of indoor air temperature gives a 30.2% reduction of wind power accommodation increase compared with the sum of two standalone optimizations.