Abstract
Borehole thermal energy storage (BTES) systems facilitate the subsurface seasonal storage of thermal energy on district heating scales. These systems’ performances are strongly dependent on operational conditions like temperature levels or hydraulic circuitry. Preliminary numerical system simulations improve comprehension of the storage performance and its interdependencies with other system components, but require both accurate and computationally efficient models. This study presents a toolbox for the simulation of borehole thermal energy storage systems in Modelica. The storage model is divided into a borehole heat exchanger (BHE), a local, and a global sub-model. For each sub-model, different modeling approaches can be deployed. To assess the overall performance of the model, two studies are carried out: One compares the model results to those of 3D finite element method (FEM) models to investigate the model’s validity over a large range of parameters. In a second study, the accuracies of the implemented model variants are assessed by comparing their results to monitoring data from an existing BTES system. Both studies prove the validity of the modeling approaches under investigation. Although the differences in accuracy for the compared variants are small, the proper model choice can significantly reduce the computational effort.
Highlights
Borehole thermal energy storage (BTES) systems are suitable for large-scale storage of thermal energy in the subsurface over periods of several months, facilitating seasonal storage of, e.g., solar thermal energy or waste heat [1,2,3]
The comparison of FEFLOW and MoBTES should be regarded as validation of the new model by numerical means
The presented MoBTES model facilitates the deployment of different modeling approaches for its sub-models, allowing for an adaption to the numerical requirements of varying applications
Summary
Borehole thermal energy storage (BTES) systems are suitable for large-scale storage of thermal energy in the subsurface over periods of several months, facilitating seasonal storage of, e.g., solar thermal energy or waste heat [1,2,3]. The concept is principally based on storage of thermal energy in the subsurface, while the subsurface (i.e., soil or rock and pore water) serves as storage medium, the heat is injected and extracted with an array of borehole heat exchangers (BHE). Heat can be injected or extracted by circulating a heat carrier fluid through the BHEs. The temperature difference between the heat carrier fluid and the rock determines the direction of heat transfer. Operational conditions like the temperature and volume flow of the entering fluid have a strong impact on the storage performance. These conditions are defined by other components of the district heating system in which the BTES is embedded.
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