Long term performance monitoring and KPIs’ evaluation of Large Scale ATES-GSPH system: Case study in Stockholm

Abstract

This study presents the performance evaluation of ATES-GSHP system based on long term monitoring since 2016. It consists of two office buildings with a combined net heated floor area of 18000 m2. ATES-GSHP system is in operation since 2016. The aim evaluate the performance as well as propose KPIs that take into account both the building HVAC system and ATES long term sustainable operation. The system is equipped with comprehensive instrumentation in HAVC system, heatpumps and aquifer including a unique and continuous installation of high resolution distributed temperature sensing using fiber optic cables throughout the aquifer.The system is connected to district heating and has two heat pumps with a total nominal cooling and heating capacity of 1.5 MW and 1.8MW. Allowable groundwater extraction and injection is 50 l/sec. with undisturbed groundwater temperature of 9.5 ◦C.  The monitoring period analyzed for the HVAC system is March 2019 - March 2020 (for ATES from March 2016- March 2020). For the year 2019/2020, the total heating load (including domestic hot water) and cooling load was 456 and 381 MWh respectively. The total average heating and cooling used from the ATES are 673 MWh and 743 MWh respectively during the first 3 annual storage cycles of operation. Over the first three storage cycles, the average injection and extraction temperatures in the warm and cold ATES sides range between 7.6◦C and 13.3◦C. The average temperature differences across the main heat exchanger for ATES are 4.5-2.8 K which is 4-5 degrees lower than the optimum value. The average thermal recovery efficiency over the first 3 storage cycles were 47 % and 60 % for warm and cold storages respectively. The seasonal performance factors SPF for the system ranged between 5-54 depending on the boundary levels 0, 1 and 2 according to Annex 52 boundary definition. Furthermore, it discusses possible improvements to be implemented regarding the system boundary definition and GSHP-ATES coupled operation. The data analysis indicated annual energy and hydraulic imbalances which results into undesirable thermal breakthrough between the warm and cold side of the aquifer. Despite having favorable conditions from aquifer point of view, 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 attributed to oversizing of the heat pumps. As a result, the heat pumps could not be operated during small-medium loads. Additionally, the limitations of currently used energy and thermal KPIs for ATES are discussed and additional thermal KPI named heat exchanger efficiency balance (βHEX) that connects and evaluate the optimum operational point of temperature differences from both the building and ATES prospective is proposed to contribute in providing more complete picture on the ATES-building interaction performance and highlights the losses in energy recovery from ATES are due to the subsurface processes or building energy system operation.