Análise de critérios para definição da vazão remanescente em usinas hidrelétricas.

Abstract

Worldwide, the demand for energy has been increasing. In Brazil, with the economy warming up in the last years, the demand for energy supply further increased. The Brazilian power generating facilities are heavily based on hydroelectric generation, which represents 77,6% of the total generated electricity. Among the environmental impacts caused by the construction of dams and reservoirs are the changes on downstream discharges, known as the remaining discharge. This discharge should ensure the appropriate conditions to the survival and perpetuation of ecosystems, linking human and wild life needs of each region. In this respect, two case studies are taken as object of analysis: (1) a small hydroelectric power plant Santa Gabriela, located in Correntes river, in the limits of Mato Grosso and Mato Grosso do Sul States; (2) the hydropower plant Batalha, in São Marcos river, in the limits of Minas Gerais and Goiás States. Although it s been observed that this issue is widely discussed by technical and academic communities, no legal framework or an explicit definition, or criteria, or limit values has been identified for the establishment of the remaining discharge in hydroelectric power plant projects. Generally, values have been defined based on criteria adopted in state s water use rights acts that define minimum discharge parameters based on time series frequency analysis such as Q90%, Q95% or Q7, 10. These methodologies imply on a single value discharge throughout the year, which does not reflect the variability of natural hydrological regime. Current studies lead to an, so called, ecological hydrogram , which would represent discharges variability during drought and flood periods. The methodology of analysis in which the discussion presented in this master dissertation was based, involved the comparison of values obtained by different water use rights criteria adopted in some states like Q90%, Q95% and Q7, 10 and, methods quoted in the literature to define the ecological discharge (Tennant, Texas, Base Flow and Wetted Perimeter), as well as National Water Agency s (ANA) declarations related to the Water Availability Reserve Statement (WARS) which specifies remaining dischargesflow for the reservoir filling and operating phases. It was observed that the application of the different methods in the two case studies resulted in significantly different values for the definition of the remaining discharge. However, it s worth to note that the publication of the Guidelines for Water Availability Study for Hydroelectric Power Plants (ANA, 2009) that aims the standardization of documents to obtain WARS and granting the use of water for energy generation purposes in federal jurisdiction represents an advance in these matters. By that, the developer may propose and negotiate water demands for wild life needs with the involved authorities, which should be discussed in the initial technical meeting that will count on the National Electrical Energy Agency (ANEEL), ANA, the related environmental agency, the developer and the organization/agency responsible for the environmental studies.