Modelling Of The Thermal Interactions Of Underground Railways With Nearby Vertical Ground Heat Exchangers In An Urban Environment

Abstract

Ground source heat pumps (GSHPs) can provide an efficient way of heating and cooling buildings due to their high operating efficiencies. The implementation of these systems in urban environments could have further benefits. In such locations ground source heat is potentially available from alternative sources such as underground railways (URs). The potential benefits for using the waste heat generated by URs with localised GSHPs are established in this thesis. This was achieved through investigations of UR-GSHP interactions. The research detailed here was mainly conducted through Finite Element (FE) numerical modelling and analysis. First a preliminary two-dimensional (2D) FE model was developed. This model was highly simplified to enable rapid analysis of the systems. The model was used to establish key parameters and phenomena for more detailed additional research. Since the operation of the URs and GSHP involves complex, transient, three-dimensional (3D) transport phenomena and extreme geometrical aspect ratios, 3D numerical models of URs and vertical ground heat exchangers (GHEs) were independently developed and validated. These individual models were then built into the same modelling environment for their combined analysis. Initial investigations with the combined 3D model showed that interactions occur between URs and localised GSHPs. In order to investigate the effect of specific parameter variations on the earlier established UR-GSHP interactions, a parametric analysis was conducted. The analysis included two sets of studies. The first group of studies considered different geometrical arrangements of the systems, and the second group investigated the effect of altered operational characteristics options on the interactions. Overall the results suggested that the performance of a GSHP can be significantly improved if the GHE array is installed near to the UR tunnel. It was shown that the improvement on the GHEs average heat extraction rate due to the heat load from the UR tunnel can be high as ~ 40%, depending on the size and shape of the GHE array and its proximity to the UR tunnel(s). It was also concluded that if the design aim is to enhance the heat extraction rates of urban GSHP systems, constructing the GHEs as close as possible to the UR tunnel would be essential. The results gathered from the parametric analysis were used to develop a formula. This formula is one of the key contributions to knowledge from this research. The formula developed allows approximating the GHEs’ heat extraction improvements due to the nearby tunnel(s) heat load(s). The formula makes use of a single variable named as interaction proximity. This variable was found to be one of the key parameters impacting on UR-GSHP interactions. At the end of the thesis, conclusions are drawn concerning the thermal interactions of URs with nearby vertical GHEs and the numerical modelling of such systems. Recommendations for further research in this field are also suggested.