A numerical methodology and analysis of borehole thermal energy storage performance

Abstract

The heating and cooling industry occupies major energy demand across the European Union in the form of the residential, commercial, and industrial sectors. The share of renewable energy has grown from 11.7% in 2004 to 21.1% in 2018 (Eurostatistics, 2020) creating fluctuation for heat production. Underground thermal energy storage balances the mismatch between the availability and demand of heat by storing heat underground. Borehole Thermal Energy Storage (BTES) is the promising underground large-scale energy storage option due to its ease of construction, eco-friendly and cost-effective materials. BTES has a major edge on the integration of various renewable heat sources to conserve heat energy for longer periods. However, they are certain optimizing aspects that need to be resolved for each BTES system such as the design of geometry, heat extraction losses, geographical and hydrological conditions, optimal temperature drop between discharge and charge periods, storage surface area to depth ratio, optimal borehole spacing, and overall efficiency.

This work presents the numerical modeling and analysis of the BTES using monitoring data as the input in finite element analysis-based software COMSOL Multiphysics ®. In this 3D modeling, we are doing the numerical analysis of the hexagonal geometry of the seasonal BTES system for 5 years to analyze the effect of the operational parameters on the system. The cases of High-Temperature BTES system of 126 boreholes tested with varying geometrical and thermal properties to analyze their effect on heat losses.

The main objective is to reduce the heat losses by studying and evaluating the operational parameters and thereby providing the optimal efficiency without dropping the minimum temperatures in BTES. Results show that the BHE arrangement, mass flow rate, thermal conductivities of the soil, and rock, top insulation material are major factors in reducing the heat losses which can be optimized to improve efficiency. This work is carried out in collaboration with Heliostorage Oy, providing part of the initial parameters from existing pilot BTES projects in Finland.