Sequential long-term optimization of shallow geothermal systems under descriptive uncertainty and dynamic variation of heating demand

Abstract

Unmanaged heat extraction, as well as the adjacency of multiple borehole heat exchangers (BHEs) in a field, can lead to undesirable thermal conditions in the ground. The failure to properly control induced thermal anomalies is perceived as a severe risk to closed-loop geothermal systems, as the detrimental effects on the ground can substantially deteriorate performance or nullify the compatibility of an operating system with regulatory mandates. This paper presents a flexible framework for the combined simulation-optimization of BHE fields during the entire lifespan. The proposed method accounts for the uncertainties in subsurface characteristics and energy consumption in order to minimize the temperature change caused by the heat extraction during the operation. The descriptive uncertainty is introduced as a deviation of the monitored temperature from the simulated temperature change, whereas the variation of the energy demand appears as over- or under-consumption against the scheduled demand. The presented new sequential procedure, by updating the thermal conditions of the ground with temperature measurements, continuously executes the optimization during the operation period and enables the generation of revised load distributions. In this study, two fields with five and 26 BHEs are considered to demonstrate the performance of the proposed method. Sequential optimization outperforms single-step optimization by providing the basis for more strategic load-balancing patterns and yielding lower temperature anomalies of about 2.9 K and 8.9 K in each BHE configuration, respectively, over 15 operational years.