In this paper we present a Comsol Multiphysics simulation model of a reduced size ULISSE mock-up. The mock-up is immersed in a container, playing the role of the lake. Both are filled with water at the room temperature. Then starts a cycle imitating the change of seasons. Hot water at 33.5 0 °C is injected by a pump in the ULISSE mock-up chasing the initial water. After a period of rest, the water is pumped back. The recovered energy is calculated. Real-time measurements are made by temperature sensors and water flow sensors allowing comparisons with the simulation results.