Empirical Validation and Numerical Predictions of an Industrial Borehole Thermal Energy Storage System

Abstract

To generate performance predictions of borehole thermal energy storage (BTES) systems for both seasonal and short-term storage of industrial excess heat, e.g., from high to low production hours, models are needed that can handle the short-term effects. In this study, the first and largest industrial BTES in Sweden, applying intermittent heat injection and extraction down to half-day intervals, was modelled in the IDA ICE 4.8 environment and compared to three years of measured storage performance. The model was then used in a parametric study to investigate the change in performance of the storage from e.g., borehole spacing and storage supply flow characteristics at heat injection. For the three-year comparison, predicted and measured values for total injected and extracted energy differed by less than 1% and 3%, respectively and the mean relative difference for the storage temperatures was 4%, showing that the performance of large-scale BTES with intermittent heat injection and extraction can be predicted with high accuracy. At the actual temperature of the supply flow during heat injection, 40 °C, heat extraction would not exceed approximately 100 MWh/year for any investigated borehole spacing, 1–8 m. However, when the temperature of the supply flow was increased to 60–80 °C, 1400–3100 MWh/year, also dependent on the flow rate, could be extracted at the spacing yielding the highest heat extraction, which in all cases was 3–4 m.

Borehole Thermal Energy Storage Systems for Storage of Industrial Excess Heat : Performance Evaluation and Modelling

“…For the distance between ground and groundwater level, the borehole is separated from the surrounding rock by a steel pipe. Because of this and due to the fact that the pipe system ends at a depth of 146 m, the length of a borehole heat exchanger is 144 m. [49].…”

“…The criterion for annual steady state by considering one temperature point is illustrated in Figure 11, in which case annual steady state is considered reached in year six. Reprinted from Nilsson and Rohdin [49].…”

“…One-dimensional steady-state heat conduction through the layer covering the borehole field, from the position of GT4 to the atmosphere, gives a storage heat loss of 255 MWh and 225 MWh respectively for modelled and measured temperatures for the three-year period, corresponding to about 3% of the total injected energy. [49].…”

“…The curves will continue to flatten out as a spacing is approached at which the heat transfer process around a borehole is negligibly affected by that of any other borehole. Reprinted from Nilsson and Rohdin [49].…”

“…The results shown in Figure 19 support, nonetheless, the concept of Başer et al [24] that a BTES system installed in unsaturated ground formations may achieve higher performances than a BTES system installed in saturated ground formations, because of the lower ground thermal conductivity of the former. Reprinted from Nilsson and Rohdin [49].…”