| Abstract |
This study aims to conduct a long-term (since 2016) performance evaluation of the doublet-type Aquifer Thermal Energy Storage (ATES) system located in Esker geological formation in Stockholm, Sweden. Furthermore, the ATES is connected to a ground source heat pump (GSHP). The average annual heating and cooling used from the ATES are 158 MWh and 243 MWh respectively during the first 3 annual storage cycles of operation. The licensed amount of water extraction and injection is 50 liters per second with an undisturbed groundwater temperature of 9.5 ◦C. Over the first three storage cycles, the average injection and extraction temperatures for the warm side are 13.3 ◦C and 12.1 ◦C, and for the cold side 7.6 ◦C and 10.5 ◦C. The average temperature differences across the main heat exchanger from the ATES side are 4.5 K during winter and 2.8 K during summer which is 4-5 degrees lower than the optimum value. The data analysis indicated annual energy and hydraulic imbalances, resulting in undesirable thermal breakthroughs between the warm and cold sides of the aquifer. This was mainly due to suboptimal operation of the building energy system which led to insufficient heat recovery from the warm side, and subsequently insufficient cold injection in the cold wells, despite the building heating demand and the available suitable temperatures in the ATES. The cause of the suboptimal operation is the oversizing of the heat pumps which were designed to be coupled to larger thermal loads as compared to the ones in the final system implementation. As a result, the heat pumps could not be operated during small-medium loads. The seasonal performance factor (SPF) of the GSHP-ATES ranges between 24-53 for free heating and cooling mode and between 4.5-6.5 when coupled with the heat pump. Additionally, the paper proposes an additional thermal KPI named heat exchanger efficiency balance (β_HEX) that connects and evaluates the optimum operational point of temperature differences from both the building and ATES perspective. In addition to ATES energy and hydraulic KPIs, β_HEX can contribute to providing a more complete picture of the ATES-building interaction performance as well as highlight if the losses in energy recovery from ATES are due to the subsurface processes or building energy system operation which has been proven to be critical for the optimum ATES performance. |