Optimized configuration of a seawater pumped-storage plant with a multiple variable speed reversible pump-turbines

The increasing quantities of non-programmable renewable energy plants are causing several problems for the management of power grids. In this context, the pumped hydro plants can play a vital role in the security and management of the electrical system. Seawater Pumped Hydro Storage (SWPHS), where the upper reservoir is located on the top of a steep shoreline and the lower reservoir is the sea, is more and more being considered as a viable alternative for hydro energy storage, especially for islands and coastal areas with significant density of non dispatchable renewable energy generation. In comparison with a traditional pumping station, it is highlighted the actual advantage, considering the low environmental impact of the works and the great extension of sea coasts with important level gradients in the vicinity. Near the Foxi Murdegu municipality in the Sardinia Island (Italy), a SWPHS plant location was identified, assuming the construction of an upper reservoir with a capacity of 1,200,000 m3, a head of about 350 m and an installed capacity of near 170 MW. One aspect addressed in the present work regards the enhancement of the efficiency of the storage system and its flexibility, concerning especially the management of low amounts of energy during the states where the plant needs to work at partial loads. A new accurate model was developed, considering configurations with one, two and three reversible variable speed pump-turbines. The model includes the operation in the Day-Ahead Market, in the more remunerative Balanced Market, sharing energy also with the nearby wind farms. The new energy model is able to consider also the technological constraints of the machines, i.e. the time necessary for starting from stop, or the change of rotation sense shifting from pumping to turbine mode and vice versa. Considering the high-risk financial investment, but at the same time the significant value of the indirect benefits to the electrical system, a package of incentive mechanisms through "certificates for pumping" has been proposed, with the partial redistribution of such welfares. The certification required to have a negative NPV zero financial risk, resulted 1.7€cents per kWh exchanged with the network: a value quite acceptable and lower than the indirect benefit to the entire Italian electricity system.