Cascade Hydropower Reservoirs Research Articles

Hydropower unit commitment with nonlinearity decoupled from mixed integer nonlinear problem

This paper formulates a nonlinear and discrete hydropower unit commitment (UC) problem, which is particular useful to coordinate with the higher level operation of cascaded hydropower reservoirs. Given the outflow process, this UC problem maximizes energy production during a planning horizon, to determine the unit operating zones and allocate the release among units in one hydroplant. This work shows how the constraints on the start-up number and up/down hours can be transformed into linear equations. The objective is equivalently interpreted to sequentially minimize spillages, maximize generation efficiency, and maximize energy production, which makes it possible to handle the nonlinearity in a low-dimensional space due to the lowest priority of the third sub-objective where the nonlinearity comes. The UC problem is skillfully decomposed into a zone commitment (ZC) that determines the optimal operating zones using the mixed integer linear programming, and one-stage sub-problems that allocate the outflow among units using the hill-climbing method. The case studies that deal with quarter-hourly hydropower unit commitments during a day show that the present model and procedure turn out to be efficient for a UC problem with up to four units, becoming volatile then on, but still acceptable for up to nine units.

Carbon Emission from Cascade Reservoirs: Spatial Heterogeneity and Mechanisms.

Carbon emission from reservoirs is considered to tarnish the green credentials of hydropower and has been extensively studied in single reservoirs. However, it remains unclear how carbon emission differs in cascade reservoirs and the mechanism behind the differences. In this study, carbon dioxide (CO2) and methane (CH4) emissions from cascade hydropower reservoirs were measured in the Lancang River, the Chinese section of the Mekong River. Our results demonstrate that carbon emissions from the river were increased by dam construction but exhibited spatial heterogeneity among cascade reservoirs. The first, most upstream, reservoir acted as the hotspot of CH4 and CO2 emissions, which were 13.1 and 1.7 times higher than those in downstream reservoirs, respectively. Similarly, the CH4/CO2 ratio of 0.023 in the first reservoir was higher than the others and made a greater contribution to the global warming effects of the cascade reservoirs. The sediment organic carbon in downstream reservoirs was negatively correlated with reservoir age (r2 = 0.993) and decreased at a rate of 0.389 mg g-1 yr-1, suggesting a potential decrease of carbon emission in the future. This study adds to our understanding of carbon emissions from cascade reservoirs and helps to screen effective strategies for future mitigation of the global warming effects from cascade hydropower systems.

An aggregation-decomposition bayesian stochastic optimization model for cascade hydropower reservoirs using medium-range precipitation forecasts

The forecast information is essential to improve the utilization efficiency of hydropower resources. To address the uncertainties of forecasting inflow, the Aggregation-Decomposition Bayesian Stochastic Dynamic Programming (AD-BSDP) model is presented in the present paper by using the 10-days precipitation value of the Quantitative Precipitation Forecasts from Global Forecast System (GFS-QPFs). The application in China’s Hun River cascade hydropower reservoirs shows that the GFS-QPFs are beneficial for hydropower generation and the performance of AD-BSDP is more efficiency and reliability than the others models.

Centralized versus Distributed Cooperative Operating Rules for Multiple Cascaded Hydropower Reservoirs

AbstractMultiple cascaded hydropower reservoir systems serving the same area need to cooperate to increase overall benefits and reliability. To meet system demand while balancing among cascaded systems, both centralized and distributed five segment energy available based operating rules, making power decisions for subsystems and the whole system, are proposed for cooperation across multiple cascaded hydropower reservoirs. Unlike existing methods, the cooperative operating rules are optimized to the global objective of maximizing the minimum system power production, with constraints on local cascade minimum energy generation to confine the possible local profit earnings or losses because of cooperation, making the operating rules more acceptable for cooperative operations among local cascade system agents. The operating rule optimization models are solved using a genetic algorithm. A case study for three cascaded systems in southwest China shows that distributed and centralized cooperative operating rules ...

Optimal Design and Operation of Hydraulically Coupled Hydropower Reservoirs System

This paper presents an optimization formulation for reliability-based optimal design and operation of a hydraulically coupled cascade hydropower reservoirs system, where the upstream power plant's tailwater is affected by the downstream reservoir's water level. The problem is formulated as a mixed integer nonlinear program (MINLP) with the objective function of maximization of the system's firm energy production while controlling the reliability level of hydroenergy production. Particle swarm optimization (PSO) is used as the optimization algorithm to solve the MINLP. The performance of the proposed methodology is tested through its application in the Karoon2-Karoon3 cascade reservoirs system in Iran as a real-world case study. Karoon3 Dam has already been constructed, whereas Karoon2 hydropower system is being designed. The results are used for capacity optimization of the Karoon2's reservoir and powerplant as well as optimal operation of the whole cascade system.

Daily Generation Scheduling of Cascade Hydro Plants Considering Peak Shaving Constraints

AbstractThis paper considers the operation of hydro plants, tackles the problem of daily hydro-generation scheduling (DHGS), and obtains the optimal hourly operation of cascade hydropower reservoirs. To meet the practical operational demands of power grid, an improved benefit-maximization model in which the peak shaving demands are taken as constraints is proposed for DHGS, and the joint peak load regulation (JPLR) method is applied to the proposed model. The model can enhance the power-generation efficiency and reduce the water spillage significantly under the condition of peak load regulation. Meanwhile, a hybrid method that combines discrete differential dynamic programming with progressive optimality algorithm is proposed to solve the JPLR problem. The complicated constraints can be handled by the proposed algorithm effectively. Moreover, the hydro-unit-commitment problem is solved by a rapid searching algorithm based on equal incremental principle and empirical methods. With this unit-commitment stra...

Identifying Explicit Formulation of Operating Rules for Multi-Reservoir Systems Using Genetic Programming

Operating rules have been widely used to handle the inflows uncertainty for reservoir long-term operations. Such rules are often expressed in implicit formulations not easily used by other operators and/or reservoirs directly. This study presented genetic programming (GP) to derive the explicit nonlinear formulation of operating rules for multi-reservoir systems. Steps in the proposed method include: (1) determining the optimal operation trajectory of the multi-reservoir system using the dynamic programming to solve a deterministic long-term operation model, (2) selecting the input variables of operating rules using GP based on the optimal operation trajectory, (3) identifying the formulation of operating rules using GP again to fit the optimal operation trajectory, (4) refining the key parameters of operating rules using the parameterization-simulation-optimization method. The method was applied to multi-reservoir system in China that includes the Three Gorges cascade hydropower reservoirs (Three Gorges and Gezhouba reservoirs) and the Qing River cascade hydropower reservoirs (Shuibuya, Geheyan and Gaobazhou reservoirs). The inflow and storage energy terms were selected as input variables for total output of the aggregated reservoir and for decomposition. It was shown that power energy term could more effectively reflect the operating rules than water quantity for the hydropower systems; the derived operating rules were easier to implement for practical use and more efficient and reliable than the conventional operating rule curves and artificial neural network (ANN) rules, increasing both average annual hydropower generation and generation assurance rate, indicating that the proposed GP formulation had potential for improving the operating rules of multi-reservoir system.

Chance-Constrained Optimal Hedging Rules for Cascaded Hydropower Reservoirs

AbstractA multiobjective model with chance constraints is proposed to obtain hedging rules for hydropower reservoirs based on initial reservoir operating rules derived using stochastic dynamic programming. In the model, hedging rules are evaluated through simulations and the maximizations of minimum powers in simulation horizon of hydropower reservoirs are taken as objectives. The maximum-minimum objectives are substituted by aggregated functions. A genetic algorithm is used to solve the model, with a fuzzy optimal method evaluating the individuals. A case study shows that the proposed approach is feasible and efficient to produce a series of hedging rules with different reliability levels, and valuable to balance the magnitudes of generation below firm power and the chances of such events.

Evaluation of optimization operation models for cascaded hydropower reservoirs to utilize medium range forecasting inflow

This paper evaluates the performances of the models that incorporate forecasting inflow for cascaded hydropower reservoirs operation. These models are constructed separately on the concepts of explicit stochastic optimization (ESO) and implicit stochastic optimization (ISO) as well as parameterization-simulation-optimization (PSO). Firstly, the aggregation-disaggregation method is implemented in ESO models to reduce the complexity of stochastic dynamic programming (SDP). And the aggregate flow SDP (AF-SDP) and aggregation-disaggregation SDP (AD-SDP) are constructed respectively. Secondly, in ISO model, decision tree is the well-known and widespread algorithm. The algorithm C 5.0 is selected to extract the if-then-else rules for reservoir operation. Thirdly, based on the PSO model, the hedging rule curves (HRCs) are pre-defined by fusing the storage and inflow as state variable. The parameters of the HRCs are determined by using the simulation-optimization model. Finally, China’s Hun River cascade hydropower reservoirs system is taken as an example to illustrate the efficiency and reliability of the models. In addition, the values of quantitative precipitation forecasts of the global forecast system (10 days lead-time) are implemented to forecast the 10 days inflow.

Development of Cascade Hydropower Reservoirs Operating System Rule Using Refill and Deplete Ranking Orders

A new model is developed for a cascade of four hydropower reservoirs operation. The aim is to improve the total power generation from the system. Daily data of reservoir level, release and power generated which varies from 4-20 years are used for analysis. Long-term data of reservoir level and inflow are used to determine the critical period. The critical period is classified into four seasons; these are filling, depleting, upper and lower level operating season. Mathematical models are used to rank the refill and the deplete order of the reservoirs. A new rule models are presented using the results of refill and depletion ranks. Power generation using the developed model is compared to the long-term historical generated; and it is found that the new rule model boost the daily power production by 5.3% and the plant factor by 2%.

Multi-objective Ecological Operation Model of Cascade Hydropower Reservoirs

This paper, based on the analysis of the nature and characteristics of ecological operation, is to discuss the influence of cascade reservoirs operation on the reservoirs themselves and on the ecological environment of the downstream channels, and to further analyze and calculate the minimum ecological discharge that satisfy the demands of ecosystem health. Based on the previous study, a multi-objective ecological operation model will then be proposed and established, with the maximum hydroelectric benefit, the maxim water supply benefit and the minimum ecological environment water shortage as objectives, and the reservoir water balance equation, the reservoir water storage restrictions, the reservoir water supply and the water demand as constraints. Moreover, the non-dominated sorting genetic algorithm II (NSGA-II) will also be proposed as the solution technique. The practicability and effectiveness of the integrated model has been verified by case study.

Derivation of Aggregation-Based Joint Operating Rule Curves for Cascade Hydropower Reservoirs

Operating rule curves have been widely applied to reservoir operation, due to their ease of implementation. However, these curves are generally used for single reservoirs and have rarely been applied to cascade reservoirs. This study was conducted to derive joint operating rule curves for cascade hydropower reservoirs. Steps in the proposed methodology include: (1) determining the optimal release schedule using dynamic programming to solve a deterministic long-term operation model, (2) identifying the forms of operating rule curves suitable for cascade hydropower reservoirs based on the optimal release schedule, (3) constructing a simulation-based optimization model and then using the non-dominated sorting genetic algorithm-II (NSGA-II) to identify the key points of the operating rule curves, (4) testing and verifying the efficiency of the generated joint operating rule curves using synthetic inflow series. China’s Qing River cascade hydropower reservoirs (the Shuibuya, Geheyan and Gaobazhou reservoirs) were selected for a case study. When compared with the conventional operating rule curves, the annual power generation can be increased by 2.62% (from 7.27 to 7.46 billion kWh) using the observed inflow from 1951 to 2005, as well as by about 1.77% and 2.52% using the synthetic inflows generated from two alternative hydrologic simulation methods. Linear operating rules were also implemented to simulate coordinated operation of the Qing River cascade hydropower reservoirs. The joint operating rule curves were more efficient and reliable than conventional operating rule curves and linear operating rules, indicating that the proposed method can greatly improve hydropower generation and work stability.