Use of floating PV plants for coordinated operation with hydropower plants: Case study of the hydroelectric plants of the São Francisco River basin
Introduction
In Brazil, approximately 91% of the power generation is from hydropower plants (64.3%) and thermal plants (26.6%) [1]. The predominance of these two sources is due to the mode of operation of the Brazilian power and electrical system. Specifically, hydropower plants (with low emissions and costs) operate as a generation base [2], and thermal plants (with high emissions and costs) operate in a complementary state, thereby providing energy during the dry period and meeting the peak demand [3]. However, since 2012, the Annual Energy Balance [1] has exhibited a notable reduction in the contribution of hydroelectric plants and a gradual increase in the contribution of thermal power plants to the total energy supply. According to [4], low hydroelectric production can be linked to the recent climatic changes that have affected rainfall regimes in several regions of the country, mainly in the northeast. Prado et al. [5] noted that this trend is part of a vicious cycle of increased emissions, accelerated climate change, reduced hydropower production, increased dependence on thermal plants, and higher energy costs.
Thus, there is an evident need to investigate low-cost and clean energy sources that are capable of reducing the dependence on thermoelectric plants and complimenting hydropower. Among them, the use of solar energy could provide an important alternative from both an environmental perspective, due to low emissions [6], and a cost reduction perspective associated with future technological advancements [7]. However, the replacement of thermal power generation will require the construction of large centralized photovoltaic (PV) plants in the power system. This process can have adverse effects due to the typical fluctuations in the power output of these sources [8]. According to An et al. [9], the coordinated operation of a PV power plant and a hydroelectric plant (connected to the electric system through the same substation) can stabilize the PV output power and allow the introduction of the energy source at a large scale. Alternatively, the PV energy can supplement hydroelectric power generation in dry periods and can increase the ability to meet peak demands.
For hydro/PV coordinated operation to be possible, the PV power plant must be physically close to the hydropower plant so that both can be dispatched from the same substation [9] and that potential disturbances to frequency and speed regulators caused by the high variability in PV power generation in different geographical regions can be reduced [10]. This proximity requirement makes floating PV plants interesting options compared to land-based plants due to the possibility of occupying the large space that is available on the surface of the reservoir of the hydroelectric plant [11] rather than occupying surrounding areas that could be developed for other activities (recreation, tourism, etc.) [12] and that usually have unfavorable topography for the construction of large flat areas (on the order of km2) with PV panels.
This paper presents a procedure for technically and economically sizing floating PV plants for coordinated operation with hydroelectric plants. To consider the various losses associated with large photovoltaic systems, calculations were performed with the help of PVSyst® software. The case study focused on hydroelectric plants in the São Francisco River basin, the second most important basin in the country. This basin is mainly located in a region that is extremely vulnerable to intense droughts and that has experienced a corresponding increase in the dependence on thermoelectric energy to compliment hydropower production [13].
The paper is organized as follows. Section 2 presents a summary of the main projects using floating PV technology to demonstrate the technological variations and results of each project. Section 3 presents the simulation model used to calculate the energy output of floating PV plants and the methods used to determine the optimum tilt angle of the panels and to evaluate the energy benefits provided. Section 4 presents the results and discussion of tilt angle optimization, the levelized cost of energy (LCOE) value, and the energy gains associated with the coordinated operation proposed in this paper. Finally, Section 5 presents the conclusions of the study.
Section snippets
Floating PV projects
Trapani and Santafé [14] presented a timeline with several floating solar energy generation systems that were installed from 2007 to 2013 around the world considering facilities with fixed panels and tracking systems. The photovoltaic panels covered the surfaces of enclosed water bodies (reservoirs and lakes) mainly used for irrigation purposes. In floating PV plants constructed in Spain and Italy (at latitudes of approximately 40°), the tilt angle of the panels reached as high as 10°. The main
Materials
The case study focused on the hydroelectric plants in the São Francisco River basin. These plants are located between the southeast and northeast regions of Brazil along the 2863 km that is occupied by the São Francisco River. Table 1 presents the main data from the hydroelectric plants that were analyzed.
The PV panel that was used in the simulation was a generic 250 Wp (60 cells) panel composed of polycrystalline silicon with dimensions of 1650 × 992 mm. This panel and the associated
Simulation results
The simulation parameters defined in Section 3.1 were used in simulations executed with PVSyst® for a floating PV power plant in the Três Marias hydroelectric reservoir for different tilt angles (α), and the results are presented in Table 4.
The maximum shading limit angle (θ) that ensures the floating PV plant at Três Marias will not suffer losses caused by mutual shading in a period of 8–16 h is θ = 32°. The value of θ is controlled by the spacing between the rows of panels, which is called
Conclusions
Recent climate changes and intense drought have contributed to a decrease in hydroelectric production and an increase in the dependence on thermoelectric power plants to meet energy needs, especially in the northeast region of Brazil. In this sense, this study presents an alternative to complement the hydroelectric plants through coordinated operation with utility-scale PV floating plants. The addition of large PV plants to compensate for hydroelectric plants could reduce the variability and
Acknowledgements
The authors would like to thank the Brazilian National Council for Scientific and Technological Development (Conselho Nacional de Desenvolvimento Científico e Tecnológico, CNPq; in Portuguese) for granting a productivity in research scholarship to Prof. Regina Mambeli Barros (PQ2, Process number: 301986/2015-0) and Prof. Geraldo Lúcio Tiago Filho and to the Brazilian Coordination for the Improvement of Higher Education Personnel (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior,
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