The energy of the hot water poured by the customer is defined as:

Ethw(i) = mthw(i) • CP • (Tthw — Tcw) (1 1)

Where thw stands for tap hot water and cw is cold water. The quantity of hot water poured from the tank during one time step can be higher or lower than the mass of water contained in one element. Never the less, the next relationship for the energy of an element n stands.

Where a is the number of elements providing the hot water for the pouring, 1 or higher. If n+a is larger then the total number of elements, N, than cold water is added to the bottom elements.

3.2 Model validation

To valid our numerical model a comparison is done with a commercial solar thermal software TSol and with experimental data. The computation time step p is one hour. Identical systems are compared in both cases located in two cities: Paris and Grenoble. The present model over estimates the performance of the system with 1.4% in Paris and 1.3% in Grenoble when comparing with TSol, and with less than 3% in both cities when comparing with experimental data.

4. Results and comments