Seawater pumped hydro plant with a variable speed reversible pump turbine: a case study for the Italian Islands

In response to the objectives of the EU, the increase in electricity production from non-programmable renewable sources, such as photovoltaic and wind power, has led to a renewed interest in large storage energy systems, otherwise difficult to integrate into the national electricity grid, because of its critical aspects. The Pumped Hydro Plants are still the most efficient large energy storage systems, in order to facilitate electric load balancing, frequency stability of the network and energy reserve creation. In particular, Seawater Pumped Hydro Plants, with a lower reservoir formed by the sea, are a potentially very convenient way for hydropower storage, especially in the islands and in coastal areas with a significant density of production from non-programmable renewable sources. The advantages of this type of plant, compared with a conventional type, are represented by the fact that it is only necessary to build the upper reservoir, with a lower environmental impact and a substantial reduction of the construction costs. Italy, which has a considerable coastal development in mountainous areas near the sea, is a favorable territory to the realization of this kind of systems. A Seawater Pumped Storage plant site was identified near the Foxi Murdegu village in Sardinia (Italy), assuming the construction of an upper reservoir with a storage capacity of 1,200,000 m3 and a head of 350 m, equipped with a pump/turbine machinery of 130 MW in the generation phase and 173 MW in the pumping phase. Simulating the Day-Ahead Market (DAM) energy exchange during the project life, it was possible to conclude that the plant would not be sustainable from the economic point of view, despite the construction cost per unit power is significantly lower compared with the other storage technologies. The purpose of the present research is to estimate the additional economical components of the Foxi Murdegu plant from different points of view: the direct revenue of the pumped hydro system through the energy exchange with the Balancing Market (BM) and the optimal use of reversible variable speed machines; the surrounding wind farms income increases through the imbalance costs reduction; the beneficial impact on the electrical system from the point of view of the dispatching costs reduction and the possibility of integrating larger quantities of energy produced from renewable sources. The analysis revealed that the total direct compensation would increase substantially thanks to the energy exchange with the Balancing Market. However, there remain critical issues regarding the financial sustainability, mainly due to the high degree of uncertainty in annual revenues, which depends strongly on the future development trend of the electricity system and market. The benefits to the Italian electricity system resulting from the Foxi Murdegu completion, would be relevant: unbalanced energy reduction of the surrounding wind farms, CO2 emission reduction of coal and Social Welfare increase due to the dispatching costs reduction, etc. In this regard, a new financial simulation was performed, assuming the full transfer of the indirect benefits for the reduction of CO2 emissions, which would ensure greater sustainability, with almost no financial risks.