Abstract
Off-grid systems are systems that are not interconnected with the grid and that congruently operate in islanded mode permanently. In order to operate the off-grid system efficiently, an energy management system (EMS) is needed. The EMS is responsible for optimizing the operation of the off-grid system with the objective of maximizing the use of renewable energy resources (RES) and minimizing the start up costs of conventional generators reducing thereby operation costs at maximum. This paper presents a rule-based EMS for an off-grid system including electrical and thermal elements. A rule-based EMS is very simple to implement, robust, and it could be also used as a back-up in case of failures of EMSs implemented by means of a unit commitment model. Computational effort and complexity are also reduced. The rules to drive the EMS have a heuristic nature and they are actually derived from the results of a detailed unit commitment model. Electrical and thermal elements are simultaneously dispatched for each dispatch interval through different, sequentially executed rules. Rules related to the thermal cooling demand are executed first due to small storage capacity of the cold water tank, followed by rules related to the net electrical demand and thermal heating demand. The rule-based EMS considers constraints with respect to the thermal and electrical balance as well as element-related constraints. Security-related constraints play a minor role in the studied off-grid system and they are not considered. The derived rules have been successfully applied to the off-grid system under study. It will be shown that reasonable dispatch decisions can be obtained by the rules contemplating a very limited prediction horizon of the demands.
