Investigation of the Waste Heat Recovery System of a Biomass Combustion Plant through Ground Source Heat Pumps

Abstract

Renewable energies are to respond to the challenges raised by the growing energy demands, consumption of fossil fuels and the resultant emission of greenhouse gases. Biomass is regarded as a very promising source of renewable energy for electricity and heat generation and transportation fuels in the future. However, in a biomass plant, large amounts of high temperature heat is wasted into the environment and one of the main goals of the current study is to present and investigate the beneficial use of the these waste heats through ground source heat pump systems. To analyze the thermal performance of the waste heat recovery system, 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 outcome of the developed framework is, the underground storage and recovery process of the waste heat through flue gases generated by a biomass combustion plant are modelled numerically. 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 , 50 and 60 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.