Optimal operation and sizing of a set of cogenerations

International audienceA cogeneration system provides electrical (or mechanical) and useful thermal energ y from the same primary energ y source. When comparing efficiencies between cogeneration and separate production of electricity and heat, it appears that cogeneration allows considerable savings in fuel consumption. But, beside these important environmental issues, economical viability of a cogeneration project is still challenging for operators. Economical viability relies on many factors, among which how to operate each unit. In this study, we shall focus on the heat-match mode where the operator’s priority is to satisfy the local heat consumer demands. When these demands are satisfied (possibly by means of auxiliary boilers), the amount of excess electricity is exported to the grid, while the lack of electricity is bought from an electricity market. In this framework, we will consider both the question of optimal operation and optimal sizing for a set of cogenerations. First we shall consider an operator who operates over a given designed (technolog y, size, location) set of cogeneration units. The best operating mode minimizing the convex but non differential cost of operation for the overall set of cogeneration units is achieved through an optimization procedure. The optimum relies on the ordered sequence of marginal costs associated with the different operational modes for each unit. This merit order indicates that the optimal operation of the cogeneration power plant is achieved through an economic dispatch mode. Based on this merit order result, an optimal sizing method is addressed so as to minimize the total cost of the plants’ operational strateg y. The sizing problem is formulated in order to decide the nominal electrical power of the cogenerations. We can show that this formulation still leads to a convex problem