A P-graph model for multi-period optimization of isolated energy systems

Reliable isolated energy systems are necessary for supplying energy to remote areas where grid extension is not feasible. It may also be required to harness renewable energy to reduce the use of conventional fuel that involves potentially high transportation cost and carbon emissions. Polygeneration systems are suitable for distributed energy supply in remote areas with the advantages of compactness and operational flexibility. Also, the simultaneous production of multiple utilities and products provides the opportunity for process integration, hence increased fuel efficiency and reduced carbon emissions. The challenge is to identify the optimal design of the processes such that the system remains in operation regardless of anticipated changes in raw material supply and product demand during multi-period operations. Graph theoretic models in the form of a P-graph (process graph) have previously been developed for synthesizing single period polygeneration systems. This work aims to develop a P-graph model to handle multi-period operations of isolated energy systems. The resulting network is thus more robust since it is able to operate amidst changes in the availability of raw material supply and/or variations in product demand. In addition, the P-graph model is also capable of generating near-optimal solutions which provide insights into other intangible parameters that may be significant to decision-makers. A case study will be presented to demonstrate the proposed approach.