Renato C. Zambon

Optimization of Large-Scale Hydrothermal System Operation

This paper presents the development of a mathematical model to optimize the management and operation of the Brazilian hydrothermal system. The system consists of a large set of individual hydropower plants and a set of aggregated thermal plants. The energy generated in the system is interconnected by a transmission network so it can be transmitted to centers of consumption throughout the country. The optimization model offered is capable of handling different types of constraints, such as interbasin water transfers, water supply for various purposes, and environmental requirements. Its overall objective is to produce energy to meet the country’s demand at a minimum cost. Called HIDROTERM, the model integrates a database with basic hydrological and technical information to run the optimization model, and provides an interface to manage the input and output data. The optimization model uses the General Algebraic Modeling System (GAMS) package and can invoke different linear as well as nonlinear programming ...

Scenario tree reduction in stochastic programming with recourse for hydropower operations

A stochastic programming with recourse model requires the consequences of recourse actions be modeled for all possible realizations of the stochastic variables. Continuous stochastic variables are approximated by scenario trees. This paper evaluates the impact of scenario tree reduction on model performance for hydropower operations and suggests procedures to determine the optimal level of scenario tree reduction. We first establish a stochastic programming model for the optimal operation of a cascaded system of reservoirs for hydropower production. We then use the neural gas method to generate scenario trees and employ a Monte Carlo method to systematically reduce the scenario trees. We conduct in-sample and out-of-sample tests to evaluate the impact of scenario tree reduction on the objective function of the hydropower optimization model. We then apply a statistical hypothesis test to determine the significance of the impact due to scenario tree reduction. We develop a stochastic programming with recourse model and apply it to real-time operation for hydropower production to determine the loss in solution accuracy due to scenario tree reduction. We apply the proposed methodology to the Qingjiang cascade system of reservoirs in China. The results show: (1) The neural gas method preserves the mean value of the original streamflow series but introduces bias to variance, cross variance and lag-one co-variance due to information loss when the original tree is systematically reduced; (2) Reducing the scenario number by as much as 40% results in insignificant change in the objective function and solution quality, but significantly reduces computational demand. This article is protected by copyright. All rights reserved.

Trade-Off Analysis among Multiple Water Uses in a Hydropower System: Case of São Francisco River Basin, Brazil

AbstractThis paper evaluates the impacts of multiple water uses on the operation of a system of existing reservoirs that originally were designed for hydropower generation. The research reported in this paper utilizes a nonlinear optimization model, developed to optimize hydropower production for a system of interconnected reservoirs. This model was modified to include water withdrawals for consumptive uses. The formulated multiobjective optimization problem is solved by employing the weighting method. The formulation considers water allocation policy for consumptive uses and individual hydropower plants. By parametrically varying the weighting coefficients, the tradeoff relationship (Pareto front) between hydropower generation and water demand for consumptive uses was traced out. The modified model is applied to the Sao Francisco River Basin in Brazil, where there are several hydropower plants in operation with a total installed capacity of 10.7 GW. The river basin is an important agricultural land that ...

Hydro and Wind Power Complementarity and Scenarization in Brazil

This paper presents a study on the temporal correlation between the hydrological regime and the behavior of winds in the main regions of Brazil. Currently, the wind power participation in Brazilian energy is still relatively very small, with 1.3% of the total installed capacity of the system. The operation planning of the power system uses a deterministic forecast of wind power production based on expected average values and only the stochasticity of the inflows are considered. In a relatively near future, participation of wind generation in the country tends to be much more significant. The impacts of winds seasonality in the operation of the electrical system can be addressed by generating scenarios that preserve the spatial and temporal correlation between the sources that might be applied to improve existing models for planning system operation such as HIDROTERM, SolverSIN and NEWAVE. The proposed methodology studies the energy complementarity between wind and hydro sources, measuring the intensity of seasonal and annual correlation in the major country subsystems. The scenarios can be obtained through autoregressive multivariate models applied to the time series of wind and inflows of the major subsystems of the country. The product of this work is intended to provide input data considering wind stochasticity into decision support systems used for planning the operation of the Brazilian National Interconnected Power System.

Planning and Operation of Large-Scale Water Distribution System with Hedging Rules

One of the most critical problems that all megacities (cities with more than 10 million people) face today is the shortage of water supply. Finding a solution to this problem presents a great challenge to hydrologists, urban planners and environmentalists. This paper describes the development of an optimization model for planning and operation of a large-scale water supply distribution system. An important characteristic of the model is that it considers the hedging rules during a dry period when water supply is insufficient to meet the planned demand and water shortage occurs. The water distribution system is formulated as a network flow model in terms of nodes and links and solved by the GAMS language, which has access to several linear and nonlinear algorithms. A user friendly interface is developed to facilitate the manipulation of a large amount of data and to generate graphs and tables for analysis for decision makers. The developed methodology has been implemented for the Sao Paulo Metropolitan Area Water Distribution System that supplies water to 18 million people. Some preliminary results obtained show good performance of the model.

Brazilian Hydrothermal System Operation: Interconnected Large System or Isolated Subsystems?

Hydrothermal systems are formed by hydropower plants and different types of thermal power plants, i.e., plants that use different types of fuel. Thermal power plants are very versatile to meet energy demand; therefore, they can be dispatched at any time, with the exception of certain plants, such as nuclear, that are operated on a demand basis. Hydropower plants are also versatile, but exhibit a serious operational constraint, which is water availability. It is clear, therefore, the importance of planning the operation, especially in large hydrothermal systems. In countries where a high percentage of the energy consumed is produced by hydropower, there is a need to determine the degree of connection of the hydro system to increase its effectiveness and efficiency. A large interconnected hydro system can be better managed considering the hydrological diversity occurring in different watersheds. On the other hand, isolated subsystems of smaller sizes, may possibly be able to operate with greater safety. This paper discusses the issue of the energy gain of a hydrothermal system in view of its size. The case study is the national interconnected hydrothermal system in Brazil. The analysis employs an optimization model, HIDROTERM, which has been developed for the optimal planning and operation of the Brazilian hydrothermal system. A comparative analysis is made comparing the Brazilian system operated in a fully interconnected scheme and with some isolated subsystems operating independently. The results can be used to determine the optimal configuration of the hydrothermal system.

The São Francisco River Water Transfer System: An Optimization Model for Planning Operation

Brazil’s poorest region, located in the country’s northeast, suffers mainly because of its dry climate and limited water resources. The demand for water and irrigation is very high, and shifts in Brazilian water resources policy can minimize drought social impacts. The most important source of water supply is the Sao Francisco river basin. Many years ago a water transfer system was designed in order to address the planned irrigation demand in the dry area. At that time the main conflict was between irrigation demand and the hydroelectrical impacts on the Sao Francisco hydropower system. The project changed its basic objectives and today the main water uses are irrigation and water supply in many cities located in the dry areas. Today the quantity of water planned to be transferred from the Sao Francisco river to the semi-arid area of the northeast region known as the “Drought Polygon” is small compared to the originally proposed amount, so the impacts on hydropower production are minimized. Despite this the project is still very controversial due mainly to its negative environmental impacts related to water quality in the Sao Francisco river. In order to minimize these impacts it is necessary to develop a highly efficient model to plan the operation of the system. The SISAGUA model developed by Barros et al. (2005) is a very convenient tool to accomplish this goal. This paper presents the application of SISAGUA to the optimal operation of the Sao Francisco river water transfer system and some very interesting results related to environmental impacts.

SolverSIN: A Practical Model for Large Hydrothermal System Analysis

Almost 90% of the electrical energy in Brazil is produced by hydropower plants, but in order to cover national demand and to ensure production during dry periods the system operates in conjunction with a set of thermal power plants. The Brazilian hydrothermal system is completely linked together and the main purpose of operation is to minimize the cost of energy production considering a long term horizon, generally five years. Therefore the complexity of the planning operation involves a large set of variables and many decisions have to be made in order to ensure maximum efficiency. Today the Brazilian National System Operator (ONS) runs a set of models to define the dispatch and the marginal cost of the energy in different parts of the country in a weekly time step. These models are very complex and are not practical to support basic information for the electrical agents in a short time. For this reason a very practical Decision Support System called SolverSIN was developed. This paper describes the SolverSIN model and shows its performance in dealing with a very complex hydrothermal systems analysis. The model was prepared to run with the Brazilian system considering its equivalent energy reservoirs scheme but its methodology can be adapted for any system. The main variables of the planning operation are estimated in a very short time and different situations can be explored to conduct sensitivity analysis of the main ones. Another important SolverSIN characteristic is its applicability to run different hydrological scenarios in order to evaluate the impacts on dispatches and energy costs. This is very important considering that the greater the safe thermal energy production of the system, the lower the electrical costs and the greater the protection to the environment due to minimization of oil consumption.

Water Supply Planning and Operation in the Metropolitan Region of São Paulo: Worst Drought in History, Conflicts, Response, and Resilience

The Metropolitan Region of Sao Paulo, with 20.8 million inhabitants, has one of the largest water supply systems in the world. In 2013-2014 the region suffered from the worst drought in its history. The regions water supply is derived from eight partially interlinked systems. Cantareira is the largest, formed by an inter-basin water transfer and a set of reservoirs, serving about nine million inhabitants. With water levels below the minimum active storage in the Cantareira system, floating pumps, built specially for drought, are used to pump water from the dead storage. Other mitigation measures in the region include operational changes, pressure reduction at night and discounts as water saving incentives. Though projects for capacity expansion or resilience improvement are under study, they may conflict with the priorities of neighboring basins. One alternative for improving the Cantareira system that does not increase intake and treatment capacities is the additional transfer of up to 5 m 3 /s of water from the Jaguari reservoir. Currently, this source of water is used for energy production and water supply to cities in the Paraiba Valley, and is the main source of water supply to the Metropolitan Region of Rio de Janeiro and its 12.1 million inhabitants. Different modeling techniques have been employed to optimize the operation of complex reservoir systems, particularly for dealing with droughts. SISAGUA is a MINLP optimization model that has been developed for the optimal planning and operation of water distribution systems with preemptive priorities: avoiding or minimizing shortages, optimizing storage distribution and minimizing operational costs. This paper presents an example of using the SISAGUA model as a decision support tool to evaluate the benefits and impacts of the proposed water transfer alternative.

Evolution of Storage Capacity in the Brazilian Hydropower System

In Brazil over the last 12 years, on average 90% of energy consumed has come from hydropower generation. The hydro system includes 150 medium and large hydropower plants. Among these 150 plants, 135 are in operation today and 15 are planned to be operational by the end of 2017. The Brazilian hydro system is completely interconnected. About half of the hydropower plants have large storage capacities to regulate flows. The system is linked with the power grids in Paraguay, Argentina, Venezuela, and Uruguay. Brazil also receives natural gas imports from Bolivia. Each month hydropower generation must be determined. This is a complex decision-making process. The objective is to optimize the entire system operation, which depends on the inflows, storages, forecasts of demands, fuel prices for thermal units, and expansion of the system over a given planning horizon, typically looking ahead five years. Environmental, technical, social, and economic constraints have made it increasingly difficult to project and build new reservoirs with large storage capacities. As a result, newly built hydropower plants are mostly the run-of-river type. To better understand the process, this paper presents a historical survey of the installed capacity of hydropower and storage capacity of the system since 1908 and extends to the planned expansion over the next four years, through 2017. The storage capacity is represented by the concept of maximum stored energy in each reservoir and subsystem. The paper also presents the history of operation over the last 12 years. Data from 2000 and projections through 2017 indicate a significant reduction of relative regulating capacity, represented by the ratio of maximum storage and installed capacity. This reduction has a direct impact on decisions regarding operation and expansion of the complementary thermal system.