Buildings as thermal energy storage – Pilot test and large-scale implementation for district heating systems

Licentiate thesis, 2015

Heat loads in a district heating system can exhibit significant variations within a single day, which sets problematic conditions for efficient heat generation. Short-term thermal energy storage (TES) can decrease these daily variations and make the conditions for generating heat more favorable. This study presents the results from a pilot test in which the short-term TES capacity is tested for five multifamily residential buildings in Gothenburg, Sweden. By periodically over-heating and under-heating the buildings, causing small variations in the indoor temperature, their thermal inertia is utilized for short-term TES. The signal from the outdoor temperature sensors in the test buildings was adjusted in different cycles over a total of 52 weeks. The heat loads and indoor temperatures were measured during the test to find their correlation with the control signal. Based on the results from the pilot test, a large-scale implementation of short-term TES in buildings is simulated for the district heating system in the city of Gothenburg, Sweden. It is shown that heavy buildings with a structural core of concrete can tolerate relatively large variations in heat delivery while still maintaining a good indoor climate. Storing 0.1 kWh/m2floor area of heat will very rarely cause variations in indoor temperatures greater than ±0.5°C in such buildings. A short-term TES in 20% of the heated floor area in Gothenburg’s district heating system would decrease the daily heat load variation by 50%, hence reducing the need for (often fossil-fueled) peak heat generation and the number of starts and stops of heat-generation units. This could be a cost-efficient solution that can increase system efficiency and reduce the environmental impact from district heating systems.