Modelling the interference of thermally unbalanced Aquifer Thermal Energy Storage systems

Estelle Petitclerc (GSB) & Alain Dassargues (Liege University), 22 September 2021

In the pilot-area of Brussels, GSB focused on gathering data from two Aquifer Thermal Energy Storage (ATES) operating in adjacent buildings at Tour & Taxis (Figure 1) that use the same aquifer made up of mixed sandy and silty sublayers. Based on temperature and operational data obtained, a numerical model was built using FEFLOW® in collaboration with the Liege University and Artesia to simulate groundwater flow and heat transport in a confined aquifer in Brussels. The model was calibrated for groundwater flow and partially for heat transport.

Several scenarios were considered to determine if the two ATES systems were interfering. The results (Figure 2) showed that a significant imbalance between the injection of warm and cold water in the first installed ATES system (operating since 2014) led to the occurrence of a heat plume spreading more and more over the years. This plume eventually reached the cold wells of the same installation. The temperature, therefore, increased in warm and cold wells and the efficiency of the building’s cooling system decreased. When the second ATES system began to operate in 2017, the simulated results showed that, even if the heat plumes of the two systems had come into contact, the influence of the second system on the first one was negligible, during the first two years of joint operation. For a longer modeled period, simulated results pointed out that the joint operation of the two ATES systems was not adapted to balance, in the long term, the quantity of warm and cold water injected in the aquifer. The groundwater temperature would rise inexorably in the warm and cold wells of both systems. The heat plumes would spread more and more over the years at the expense of the efficiency of both systems, especially concerning building’s cooling with stored cold groundwater.

The renewability and efficiency of ATES systems could be threatened by inadequate conceptual and operational conditions. Therefore, the heating and cooling requirements of the building and the associated groundwater conditions should be well-identified before designing the project. The illustrated case study in this work is particularly instructive showing how one thermally unbalanced ATES system could partly jeopardize shallow groundwater conditions in an urban area where other systems could suffer from the induced thermal pollution limiting both efficiency and durability. The management of both ATES systems should therefore be changed quickly to rebalance the heat and cold storage. This change must be designed to limit interferences between wells of the same system and/or of the neighbor system and to improve efficiency in the short and long terms. This is especially true for Building n°1 since the ATES system in Building n°2 has been in operation for less than three years and no imbalance in heat and cold storage has been observed yet. To balance the amount of heat stored in the aquifer, less warm water and more cold water should be injected and the use of the system for heating should be maximized. Methods exist to balance the amount of heat and cold injected in the subsurface such as direct compensation, compensation from a heat pump, or night ventilation. In the same order of ideas, some measures will be taken soon about solar protections in Building n°1. These latter should reduce the solar energy gains in the building during warm days and therefore reduce the cooling needs. Indeed, the reliability and accuracy of the model results can be improved if new distributed temperature measurements are added as historical data for a more detailed and long calibration of the model. This is also what is started and ongoing with the recent installation of temperature sensors in 8 wells in this specific studied zone.

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