An integrated framework for overall assessment of underground pumped storage hydropower

Abstract

There is a worldwide growing claim for the elimination or, at least a drastic reduction, in the consumption of fossil fuels that could contributes by decreasing the greenhouse effect. In this compass, renewable sources, such as wind and solar photovoltaics, have grown exponentially in several countries. Even with the predominance of hydroelectric power plants, Brazil also presents a rapid growth of these new sources of electric energy production, largely thanks to subsidies offered by the Federal Government. However, the intermittency of these sources, which depend directly on variations in wind speed and solar radiation, is an undesirable characteristic that needs to be overcome through energy storage. Thanks to the large volume of storage available in hydraulic power plants reservoirs, Brazilian power system has been operated without other types of energy storage. Nevertheless, this storage volume is finite and due to socio-environmental barriers, the construction of new reservoirs through dams is unfeasible in most cases, which leads to the need for the development and use of other energy storage technologies. To face socio-environmental barriers, the construction of underground reservoirs or even, in a more optimized way, the use of deactivated mines or natural caverns to implement Underground Pumped Storage Hydropower – UPSH, might be a solution, instead of large battery banks, which also face socio-environmental barriers, in addition to economic unfeasibility. The main purpose of this Thesis is to draw the attention of the academic community, decision makers, sectoral planners, entrepreneurs and governments to study, understand and develop UPSH technology, through the demonstration of its technical and economic viability, considering that this technology has great potential in the world and, especially in Brazil where there is hydrological abundance and many deactivated deep underground mines. To achieve such an objective, a comprehensive review of the literature, legislation and regulations was carried out to present an integrated framework for general assessment of UPSH. This framework is composed of 5 phases: (i) prospecting, where the availability of resources is assessed, such as water availability, soil survey, demand for energy storage, among others; (ii) design, where the dimensioning of machines, underground reservoirs, penstocks, among others, is refined; (iii) business plan, where the business model is defined based on the size of the plant and its compliance with current regulations. At this stage, an economic feasibility analysis is carried out, for which revenue stacking, combinatorial auction, among other innovations for the economic regulation of Energy Storage Systems – ESS were proposed; (iv) implementation, where the actual project is put into practice; and (v) decommissioning, where the closure of activities is defined, as well as the destination of unusable facilities and the recovery of the degraded environment. The integrated framework for general assessment of UPSH was applied to two case studies, concluding by the technical and economic feasibility of a μUPSH with 500 kW power and 1500 kWh capacity, taking advantage of the potential of the lake on the campus of Federal University of Itajubá. The plant was classified as Distributed Generation – DG and inserted in the Electric Energy Compensation System – SCEE. Also, the technical and economic feasibility of an ultra-deep UPSH in Lake Mundaú, state of Alagoas, with 1000 m depth, 500 MW nominal power and 1 GWh capacity was attested. In this case, the economic regulation proposed in this Thesis was used. Also, the regulation for hydroelectric plants and underground mines was used, by similarity when applicable, in both case studies. It is concluded that UPSH technology is technically and economically viable and lacks adequate legislation and regulation to provide legal security to entrepreneurs interested in the technology. Further studies are needed to consolidate the regulatory proposal presented in this Thesis, as well as for comparisons with other technologies.