Use of floating PV plants for coordinated operation with hydropower plants: Case study of the hydroelectric plants of the São Francisco River basin

Highlights

• The use of floating PV plants in coordinated operation with hydroelectric plants.

• Procedure for sizing floating PV plants for coordinated operation with hydroelectric plants.

• The Brazilian region case-study on the hydroelectric plants of the São Francisco River basin.

• The average energy gain was 76%, and the capacity factor increased by 17.3% on average.

Abstract

In recent years, the Brazilian electricity sector has seen a considerable reduction in hydroelectric production and an increase in dependence on the complementation of thermoelectric power plants to meet the energy demand. This issue has led to an increase in greenhouse gas emissions, which has intensified climate change and modified rainfall regimes in several regions of the country, as well as increased the cost of energy. The use of floating PV plants in coordinated operation with hydroelectric plants can establish a mutual compensation between these sources and replace a large portion of the energy that comes from thermal sources, thereby reducing the dependence on thermoelectric energy for hydropower complementation. Thus, this paper presents a procedure for technically and economically sizing floating PV plants for coordinated operation with hydroelectric plants. A case study focused on the hydroelectric plants of the São Francisco River basin, where there has been intense droughts and increased dependence on thermoelectric energy for hydropower complementation. The results of the optimized design show that a PV panel tilt of approximately 3° can generate energy at the lowest cost (from R$298.00/MWh to R$312.00/MWh, depending on the geographical location of the FLOATING PV platform on the reservoir). From an energy perspective, the average energy gain generated by the hydroelectric plant after adding the floating PV generation was 76%, whereas the capacity factor increased by 17.3% on average. In terms of equivalent inflow, the PV source has a seasonal profile that compliments the natural inflow of the river. Overall, the proposed coordinated operation could replace much of the thermoelectric generation in Brazil.

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.