In a standard system the auxiliary electric heater run identically every day, no matter the season. During night, it brings the upper volume of the tank at the set-up temperature (here 70°C). This one is usually preset. Solar energy is recovered in the lower part of the tank, and, if the temperature rises above the set-up temperature in the upper volume, than the solar loop will heat up the entire tank. Due to a large variation of the solar radiation between winter and summer, the heat recuperation varies also. During winter the heat input of the solar loop is very poor and the tap hot water is produced mainly by the auxiliary heater. Contrary, during summer, the solar contribution to the tap hot water is important, even complete. Thus, the tank can contain an important thermal energy at a sufficiently high temperature to provide tap hot water without running the auxiliary electric heater.

Introducing a control device able to take into account the weather forecast for the next day, we try to diminish the electricity consumption during warm seasons keeping the customer’s comfort at a good level. For this, we are able to control the set-up temperature of the auxiliary heater depending on the solar radiation of the next day. We decided to introduce three temperature levels:

• If the maximum solar radiation of the next day is above 800 W/m2 the set-up temperature is Tsetup = 50°C, representing mostly the summer operation;

• If the maximum solar radiation of the next day is between 450 W/m2 and 800 W/m2 the set-up temperature is raised at Tsetup = 60°C, usually during intermediary seasons;

• And finally, for solar radiation lower than 450 W/m2 the set-up temperature is pushed up to the standard temperature applied during winter Tsetup = 70°C.

Next graphic presents the comparison between the standard system and the HWSH with weather forecast. The temperature variation at 10 levels[12] in the tank and the operation of the electric heater are shown for both cases. The tap hot water pouring, solar radiation and pump operation are alike for both cases.

One can observe that on April 1st the electrical heater change its behaviour due to the lower set-up temperature. This small electric economy is multiplied by an important number of days leading to a considerably annual economy. This one is presented in the next table computed for Paris and Grenoble.

One notice the annual electricity economy given by the weather forecast control of around 60 kWh in both cases, meaning 7% of the total electric energy consumption.

5. Conclusions and perspectives

The numerical model presented in this paper is able to simulate a HWSH with an advanced control system taking into account the weather forecast for the next day. Using a simple modification of the electrical heater algorithm we are able to cut down 7% of the annual electrical consumption and to increase with 3% the solar cover coefficient.

The electricity gain can be optimised by a fine setting of the two solar radiation limits used to determine the set-up temperature of the heater, depending on the system location and tap hot water pouring.

References

[1] F. A. Peuser, K-H. Remmers, M. Schnauss, (2003) Installations solaires thermiques. Conception et mis en oeuvre, Systemes Solaires, Paris.

[2] T. Prud’homme, D. Gillet, Energy and buildings, 33 (2001) 463-475.

[3] M. LeBreux, M. Lacroix, G. Lachiver, Energy and buildings, 38 (2006) 1149-1155.