Counteraction of buoyancy flow in high temperature aquifer thermal energy storage systems by applying multiple partially penetrating wells

Abstract

The thermal recovery efficiency of High Temperature Aquifer Thermal Energy Storage (HT-ATES) systems can be limited due to the effect of buoyancy flow of the injected hot water. This thesis has researched the application of a Multiple Partially Penetrating Wells (MPPWs) as a well design method to counteract the effect of buoyancy flow and improve the performance of HT-ATES systems (>60°C). A MPPW is a well with more than one screen that allows injection and extraction of water at different depths in the aquifer.This method to counteract the effect of buoyancy flow was tested through numerical modelling with SEAWATv4. The modelled HT-ATES systems were running for four recovery cycles each including injection-storage-extraction-rest phases. The thermal recovery efficiency was determined over these cycles for 7 different scenarios and four different cases: a) A regular HT-ATES system with a fully penetrating screen, b) a regular HT-ATES system where buoyancy flow is neglected, c) an HT-ATES system with a MPPW with two screens, d) an HT-ATES system with four screens. The latter case was tested for three different control approaches based on data from four different locations in the aquifer.For the reference scenario where 90°C water was injected, a regular HT-ATES system had a thermal recovery efficiency of 0.61. With the application of MPPWs for both two or four screens this was 0.81 in the fourth recovery cycle, which approaches the case without buoyancy which had a thermal recovery efficiency of 0.88. The application of two or four screens did not show significant difference in thermal recovery efficiency after the first recovery cycle.A sensitivity analysis showed that the absolute increase in thermal recovery efficiency of an HT-ATES system with a MPPW compared to a regular HT-ATES is higher for larger buoyancy flow (i.e, high injection temperature and high (vertical) hydraulic conductivity), smaller injection volume and larger aquifer thickness. An applicability analysis showed that application of MPPWs is beneficial if the buoyancy flow (which is defined as the vertical hydraulic conductivity times the density ratio of the ambient groundwater and injected water) is greater than 0.1 meters per day.