Optimization of energy extraction for vertical closed-loop geothermal systems considering groundwater flow

Abstract

A combined simulation–optimization procedure is presented to regulate the operation of borehole heat exchangers (BHEs) in a multiple BHE field when groundwater flow exists. Such fields are of increasing interest for large-scale geothermal heating energy supply of buildings, but so far strategic adjustment of energy extraction rates (loads) of the individual BHEs has not been considered in practice. Groundwater flow means an additional advective energy supply, which is advantageous but also complicates proper BHE adjustment. In the presented procedure, the field is simulated by temporally and spatially superimposed moving line source equations. The optimization goal is formulated in an objective function to minimize the thermal impact in the ground, to avoid extreme temperature anomalies, and by this, ultimately improve heat pump performance. For a given seasonal energy demand and total operation time, linear programming efficiently delivers optimized BHE operation patterns. For an examined square lattice of 25 BHEs, the optimized radial load patterns characteristic for conduction dominated conditions change to patterns that are oriented at the groundwater flow when advection dominates. Through this, optimization always levels the temperature distribution in the ground. Also, in comparison to routine practice, mean BHE outlet temperatures can be increased. For the small study case, numerical simulation reveals that already more than 1K can be achieved, given a seasonal energy demand oriented at common conditions in central Europe. However, for a fixed energy demand, advective heat supply towards the BHEs increases with groundwater flow velocity and thus mitigates the benefits from optimization.