Experimental and numerical investigation of the impact of operating conditions on thermocline storage performance

Abstract Thermocline storage performance is studied using a numerical model with three phases (fluid, solid, wall) and one dimension, which is then validated by comparison with experimental results. The impact of the interstitial fluid velocity on storage performance is presented and numerical simulations show the existence of an optimal velocity of 4 ⋅ 10-4 m.s-1 that maximises the storage utilisation rate (80.6 %) for ideal charges between 293°C and 393°C. This optimal velocity remains identical when the temperature level of the storage is shifted down and slightly increases to 4.8 ⋅ 10-4 m.s-1 when the temperature difference is decreased by half (343°C-393°C). A numerical sensitivity analysis is presented on the impact of heat losses, of thermal diffusion and of the convective heat transfer between the fluid and the solid phases, providing a physical interpretation of the location of the optimum depending on operating conditions. Experimentally, the impact of fluid velocity is too moderate to observe an optimal velocity, especially because of non-ideal inlet temperature conditions, but a deterioration of storage performance is observed at the lowest and highest velocities, with respectively -2.8 % and -3.8 % compared to the maximal utilisation rate. This moderate influence of both fluid velocity and temperature shows that thermocline storage presents good robustness of its performance to variations in operating conditions.