Modelling and optimisation of monolithic solid-to-gas heat exchangers

Abstract

Ultra-high temperature thermal energy storage (UHTS) is an attractive grid-scale energy storage technology that can be used to decarbonise the electricity grid, and mitigate the threat of climate change. In the UHTS system, heat from the solid thermal storage medium is added to a combined cycle gas turbine using a monolithic solid-to-gas heat exchanger. Monoliths are also found in chemical reactors and catalytic converters.

A novel numerical model is proposed for the steady-state thermal performance and pressure loss of monolithic solid-to-gas heat exchangers. Within the solid monolith, the finite element method is employed to solve the 3-dimensional heat conduction equation with a temperature-dependent thermal conductivity. The gas is modelled as compressible, with temperature-dependent properties. Within each duct, the flow is modelled as being 1-dimensional, and an adaptive step size is used.

Correlations for the Darcy friction factor and Nusselt number allow for accurate predictions of heat exchanger performance in both the laminar and turbulent regimes. Not only does the model determine the distribution of the mass flow rate between the ducts, it also automatically generates and meshes the monolith geometry from a set of parameters, which will allow for automatic optimisation of the geometry.

In the work herein, the model is validated with experimental data from the literature. It is then used to explore the effect that different heat exchanger parameters have on performance, by using geometric parameter sweeps. Two duct placement patterns are considered — a legged pattern in which the duct spacing is non-uniform, and a uniformly spaced hexagonal pattern. Reducing the duct diameter is found in general to increase the thermal performance of the heat exchanger. The legged pattern is able to give an integral heat transfer coefficient that is higher than that of the hexagonal pattern by a factor of 1.86, albeit with a pressure loss that is higher by a factor of 2.02.

A basic optimisation is performed for a heat exchanger in a UHTS system. A maximally spaced hexagonal pattern is found to give optimum performance. The optimum heat exchanger found herein has an effectiveness of 97.5 %, an integral heat transfer coefficient of 2416 W m-2 K-1, and a pressure loss of 1.18 %. This heat exchanger takes up 0.018 % of the UHTS storage medium by volume.