Wave energy is a renewable energy source having a highly exploitable potential in several locations worldwide. In the framework of energy transition, the exploitation of wave energy combined with end-of-life offshore stranded gas reservoirs may lead to two positive impacts: the stabilization of the energy supplied to the grid and a better penetration of renewable energy in areas where the grid is not able to compensate the fluctuations associated to renewable energy production. Moreover, in order to guarantee the dispatched energy schedule, wave energy needs to be coupled with back-up systems aimed at valley filling. In the present study, an innovative approach to the conceptual design of hybrid energy systems based on wave energy is developed, entailing an operation strategy that complies with the dispatching needs of grid-connected generation systems. The probability of correct dispatching that the producer assures to the Transmission System Operator is used as a parameter to optimize the design of a Gas to Power back-up system used for valley filling. The approach supports the preliminary design of offshore hybrid energy systems based on wave energy, starting from historical wave data up to the definition of an optimal back-up system valorizing residual reservoir fuels and its operation strategy. The proposed design is evaluated through a Multi-Criteria Decision Analysis, including the technological, economic, environmental and safety aspects, which allows the assessment of the overall sustainability performance of the hybrid system, considering the fluctuations associated to wave power generation during a typical operation period. The methodology was applied to two test-cases in different offshore operating theaters (North and Adriatic seas), in order to test its potentiality. The results highlighted that, in both sites, similar design choices are suggested for the hybrid system. However, the annual energy production resulted 6.5 times higher in the North Sea test-case. The low energy generation in the Adriatic Sea test site caused a levelized cost of energy of 3960 EUR/MWh, much higher than the value obtained for the North Sea case (610 EUR/MWh). In both cases, the gas turbine park impacts negatively on the cost of energy production, but is critical in meeting the design value of the probability of correct dispatching.
