Numerical investigation of rock-pile based waste heat storage for remote communities in cold climates

•A rock-pile waste heat storage is proposed, and its corresponding numerical model is developed.•Both fluid flow and heat transfer are validated against experimental data.•Local Thermal Equilibrium and Non-Equilibrium approaches are compared.•Parametric studies are performed on thermo-physical properties.•Payback time analysis is performed, and the system is economically justified. Remote communities in arctic climates are solely dependent on diesel generators for continuous power supply due to their detached loci from national power gridlines or natural gas pipelines. Moreover, to get along with the harsh, long winters, these communities directly or indirectly depend on fossil-fuel based heating systems. Due to the lower efficiency of the conventional diesel engines, these generators discard a significant amount of heat through the exhaust. Occasionally, during the wintertime, this waste heat from the exhaust is used directly as a heat source after recovery. However, during summer, when the heating demand is lower or nil, this heat is commonly discarded, and its energy potential wasted. A rock-pile based seasonal thermal energy storage is a viable solution that can sustainably resolve this issue. This paper presents a validated numerical study that focuses on the coupling of a waste heat recovery system to a diesel exhaust stream offering a performance assessment of the proposed rock-pile waste heat storage system in a remote, off-grid community located in northern Canada. It examines the impacts of local thermal equilibrium versus non-equilibrium approaches as well as temperature-dependent properties, variation of air mass flow rate, particle size and finally the thermo-physical properties on the proposed model. The presented results show that air should be treated as an ideal gas and that the local thermal non-equilibrium approach should be used for its considerably higher accuracy. Techno-economic assessment of the system resulted in a relatively short payback period of less than six years.