Modelling of waste heat recovery of a biomass combustion plant through ground source heat pumps-development of an efficient numerical framework

Development of a reliable and convenient dynamic modelling approach for ground source heat pumps remains as an important unresolved issue. As a remedy, in this work a novel, computationally-efficient modelling framework is developed and rigorously validated. This is based upon an implicit computational modelling approach of the ground together with an empirical modelling of heat and fluid flow inside U-tube ground heat exchangers and waste heat calculations. The coupled governing equations are solved simultaneously and the influences of parameters on the performance of the whole system are evaluated. The outcomes of the developed framework are, first, favorably compared against two different existing cases in the literature. Subsequently, the underground storage and recovery process of the waste heat through flue gases generated by a biomass combustion plant are modelled numerically. This reveals the history of temperature distributions in the ground under different configurations of the system. The results show that for a biomass combustion plant generating flue gases at 485.9 K as waste heat with the mass flow rate of 0.773 kg/s, the extracted heat from the ground is increase by 7.6%, 14.4% and 23.7% per unit length of the borehole corresponding to 40 degrees C, 50 degrees C and 60 degrees C storage temperatures. It is further shown that the proposed storage system can recover a significant fraction of the thermal energy otherwise wasted to the atmosphere. Hence, it practically offers a sizable reduction in greenhouse gas emissions.