Exergy Optimisation for Cascaded Thermal Storage

Cascaded thermal storage, consisting of multiple Phase Change Materials (PCMs) with different melting temperatures, has been proposed to solve the problem of poor heat transfer caused by unavoidable decrease of temperature differences during heat exchange process. This paper conducts a theoretical study of the overall thermal performance for a cascaded thermal storage system. Both heat transfer rate and exergy efficiency are taken into account. The main findings are: the cascaded arrangement of PCMs enhances the heat transfer rate by up to 30%, whilst it does not always improve the exergy efficiency (-15 to +30%). Enhanced heat transfer and reduced exergy efficiency can both be attributed to the larger temperature differences caused by the cascaded arrangement. A new parameter hex (exergy transfer rate) has been proposed to measure the overall thermal performance. It is defined as the product of heat transfer rate and exergy efficiency, representing the transfer rate of the utilisable thermal energy. The simulation results indicate that the cascaded thermal storage has higher overall thermal performance than the single-staged storage despite of higher exergy efficiency loss. Nomenclature Bi Biot number, hd/λ (dimensionless) cp specific heat of HTF (kJ/kg) cs specific heat of PCMs (kJ/kg) d characteristic length (m) dA differential thermal area (m) dq differential heat flow (kJ) m 。 mass flow rate (kg/s) h system dimension (m) h heat transfer coefficient (W/m K) hs enthalpy of PCMs (kJ/kg) HL latent heat (kJ/kg) L system length (m) Nu Nusselt number, hd/λ(dimensionless) P pressure (Pa) Pr Prandtl number, Cpμ/λ (dimensionless) q overall heat exchange rate (W/m) Re Reynolds number, ud/ν (dimensionless) Rg ideal gas constant (kJ/kg K) s specific entropy (kJ/kg°C) T temperature (°C) T0 ambient temperature (°C) Tf0 inlet HTF temperature (°C) Innostock 2012 The 12 International Conference on Energy Storage u flow velocity (m/s) X anergy (kJ) Greek symbols λ thermal conductivity of HTF (W/m K) ρ density (kg/m) μ dynamic viscosity of HTF (Ns/m) ν kinetic viscosity of HTF (m/s) ηex exergy efficiency (%) Subscripts f HTF –heat transfer fluid m melting s PCMs