Foundation wall heat exchanger model and validation study

Abstract

Making use of foundation substructural elements as ground heat exchangers is an attractive option for larger non-residential buildings. An alternative to Energy Piles is to use wall substructures - so called diaphragm or screen walls - with embedded pipes that are partly below ground and partly exposed to basement spaces. This paper will describe the development of a model of such a heat exchanger that uses a weighting factor approach known as Dynamic Thermal Networks (DTN). This approach allows for detailed representation of the wall section geometry and multiple boundary conditions. In this case thermal boundary conditions are applied at surfaces representing the adjacent ground and the semiexposed basement wall surface in addition to the pipe surface. The weighting factors for the model have been derived using a parametric numerical model that has been developed using the OpenFOAM library. Validation of the model has been carried out using data from an extended series of thermal response test (TRT) measurements at a full-scale diaphragm wall heat exchanger in Barcelona. In this paper, development of the model using the DTN approach will be briefly described along with the parametric numerical modelling approach used to derive the weighting factor data. Validation test procedures will be presented along with comparisons between the predicted and measured fluid temperatures and heat transfer rates. Given some uncertainty in the experimental thermal properties, the model was able to predict the dynamics of thermal response over a range of operating conditions with reasonable accuracy and using very modest computational resources.