Control of a hybrid solar/electric thermal energy storage system

A controller for operating a hybrid thermal energy storage system (HTESS) is presented. The storage system accumulates solar energy during sunny days and releases it later at night or during cloudy days and, simultaneously, it stores electric energy during off-peak periods and releases it later during on-peak periods. The control of the system rests on an anticipatory strategy and on a regulation strategy. The anticipatory strategy is based on a fuzzy logic and feedforward controller (FLFFC) that can handle simultaneously the storage and retrieval of both electricity and solar energy. It takes into account the weather forecasts for solar radiation and outside air temperature, and optimizes the off and the on-peak periods for electrical heating. The regulation strategy depends on a PID controller which regulates the air flow from an electric fan in order to maintain the room temperature at the set point. Numerical simulations were conducted over one to three-month winter periods to test the response of the controller. Results indicate that the proposed control system is far superior to traditional control systems. It remains robust and reliable even for cases in which the weather forecasts are of poor reliability and accuracy (5-day horizon weather forecasts with reliability of 50%, −10 K temperature accurate and −50% solar radiation accurate). The performance of the HTESS as well as the thermal comfort of the room is maintained in all situations and at all times. Moreover, the electricity consumption for space heating is minimized and 95% of this electricity is consumed during off-peak hours.