Carlos André Bulhões Mendes

Errors in river lengths derived from raster digital elevation models

Length of river reaches is one of the most important characteristics of stream networks when applying hydrological or environmental simulation models. A common method of obtaining estimates of river lengths is based on deriving flow directions, accumulated area and drainage lines from raster digital elevation models (DEM). This method leads to length estimates with variable accuracy, which depends on DEM horizontal resolution, flatness of terrain, DEM vertical accuracy, the algorithm used to obtain flow directions and the way by which distances are calculated over raster structures. We applied an automatic river length extraction method for eight river reaches in the River Uruguay Basin (206000km^2), in Southern Brazil, and compared its results to the lengths obtained from drainage vector lines digitalized over satellite images. Our results show that relative errors can be higher than 30% in flat regions with relatively low DEM resolution. Preprocessing of DEM by the method known as stream burning greatly improves results, reducing errors to the range 1.9-7.4%. Further improved estimates were obtained by applying optimized values for the length of orthogonal and diagonal steps called distance transforms, reducing the errors to the range -2.0-3.3%.

Modelling a hydropower plant with reservoir with the micropower optimisation model (HOMER)

Hydropower with water accumulation is an interesting option to consider in hybrid systems, because it helps dealing with the intermittence characteristics of renewable energy resources. The software HOMER (version Legacy) is extensively used in research works related to these systems, but it does not include a specific option for modelling hydro with reservoir. This paper describes a method for modelling a hydropower plant with reservoir with HOMER by adapting an existing procedure used for modelling pumped storage. An example with two scenarios in southern Brazil is presented for illustrating and validating the method explained in this paper. The results validate the method by showing a direct correspondence between an equivalent battery and the reservoir. The refill of the reservoir, its power output as a function of the flow rate and installed hydropower capacity are effectively simulated, indicating an adequate representation of a hydropower plant with reservoir is possible with HOMER.

PUMPED STORAGE HYDROPOWER IN BRAZIL AND THE WORLD: APPLICATION AND PERSPECTIVES

Environmental concerns and the increase of fossil fuel prices have influenced the development and expansion of renewable energy for electricity generation in Brazil and the world. Many of these technologies produce energy from unpredictable renewable sources, and this affects the operation of the electricity distribution grid, because the generation must match the demand at all times. The storage of surplus energy for using it during peak load periods can contribute to balance the load and regulate the frequency. Pumped storage hydropower plants, also known as pumped water storage or reversible hydropower is the most established technology for large scale electricity storage. This paper presents a literature review on electricity storage through pumped storage hydropower plants, and the application of this technology in the global and Brazilian scenarios. This document briefly presents and compares the available options for electricity storage, and it also describes the operating principles, environmental impacts, application, opportunities and challenges associated with reversible hydropower in the world. Finally, the development and perspectives of this technology in the Brazilian scenario are discussed.

Prioritizing small hydropower projects in scenarios with limited financial resources

Because of the maturity of its technology, small scale hydropower generation is one of the main energy sources to be considered to provide electricity to areas far from the national grid. Very often, because of the economic limitations of the communities that can be supplied with electricity, these projects are unappealing from the entrepreneurs point of view; however, they can have such a great positive impact on the life of these communities, with little impact on the environment, that they may well justify subsidies for their construction. This paper describes a mathematical model that combines multicriteria analysis and a knapsack problem formulation for optimizing the distribution of limited financial resources among several options available of hydropower projects with a stochastic behavior in their initial cost. The test data were obtained mainly from 14 prefeasibility studies of sites in rural Nicaragua with potential for small hydropower. Five scenarios are presented, assessing the impact of multicriteria and different subsidy conditions or economic efficiency requirements. The results indicate that the presented method is suitable to be used for an efficient distribution of limited monetary resources for funding small hydropower projects, and, therefore, can be a useful tool in decision making process. The proposed method can be adapted for application to projects based on other forms of renewable energy, and can also be adapted for prioritization of projects based on different energy resources.

Optimization Water Allocation Model to Natural Resources Exploration

Environmental impacts caused by the exploration of natural resources may cause damage to third parties, since the action of such agents on the environment may harm other economic activities, making them unfeasible in some cases. In this context, the present study aims to develop a methodology for planning the economic allocation of water among producers of renewable and non renewable resources, seeking optimum production levels, considering water quality parameters established by Brazilian laws. This methodology proposes an optimization model which maximizes the net present value (NPV) of natural resources exploration over a planned period of time. The paper presents an application of this methodology in a Brazilian river basin, where rice crops and coal mining coexist. Coal mining generates acid mine drainage, which in many cases is dumped into adjacent rivers without treatment. The cultivation of rice requires the catchment of large volumes of water. As a result, water quality standards are violated, because the catchment for irrigation reduces the flow in rivers, exacerbating the concentration of contaminants from mining. Different scenarios were analyzed, varying the type of effluent treatment adopted by the miners and the fraction of the water flow to be left in the rivers for other activities. The case base ignores water quality standards. Results for the most restrictive case indicated a 10% reduction of the NPV compared to the base case scenario and preserved 52% of the water flow to other activities.

SYSTEM DYNAMICS FOR SEQUENCING BATCH REACTORS MODELING

Wastewater treatment by sequencing batch reactors is a variation of the activated-sludge process, because they combine all of the treatment steps and processes into a single tank. This paper presents a model created in Vensim and based on system dynamics with the aim of simu- lating the filling and depletion of the tanks in a wastewater treatment system by sequencing batch reactors, considering a high variability of the inflow along the day. The system shown is comprised of two tanks, a bypass, and an optional influent equalization basin. Some examples are included to illustrate the capacity of the model to simulate different system configurations, something that is supported by the results of these examples. The model can be adapted to include characteristics of similar wastewater treatment systems, allowing the evaluation of different conditions depending on the characteristics of wastewater inflows and physical or chemical features of the treatment plant.