Optimising power generation efficiency for head-sensitive cascaded reservoirs in a competitive electricity market S.J.P.S. Mariano a , J.P.S. Catala ˜o a, * , V.M.F. Mendes b , L.A.F.M. Ferreira c a Department of Electromechanical Engineering, University of Beira Interior, R. Fonte do Lameiro, 6201-001 Covilha, Portugal b Department of Electrical Engineering and Automation, Instituto Superior de Engenharia de Lisboa, R. Conselheiro Emı ´dio Navarro, 1950-062 Lisbon, Portugal c Department of Electrical Engineering and Computers, Instituto Superior Te ´cnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal Received 9 June 2006; received in revised form 7 June 2007; accepted 9 June 2007 Abstract This paper is on the problem of short-term hydro scheduling (STHS), particularly concerning head-sensitive reservoirs under com- petitive environment. We propose a novel method, based on nonlinear programming (NLP), for optimising power generation efficiency. This method considers hydroelectric power generation as a nonlinear function of water discharge and of the head. The main contribution of this paper is that the maximum water discharge, thus giving the maximum power generation, is also considered as head-dependent in order to obtain more realistic and feasible results. The proposed method has been applied successfully to solve a case study based on one of the Portuguese cascaded hydro systems, providing a higher profit at an acceptable computation time in comparison with classical opti- misation methods based on linear programming (LP) that ignore head dependence. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Short-term hydro scheduling (STHS); Electricity market; Head dependence; Nonlinear programming (NLP) 1. Introduction The satisfaction of the demand for electric energy has been mainly achieved with hydro resources and thermal resources. Hydro resources particularly run-of-the river resources are considered to provide a clean and environ- mentally friendly energy option, while thermal resources particularly fossil fuel-based resources are considered to provide an environmentally aggressive energy option, but nevertheless still in nowadays a necessary option. Hence, promoting efficiency improvements in the exploitation of the hydro resources is increasingly important, reducing the reliance on fossil fuels and decreasing greenhouse emis- sions which are major contributors to climate change. The hydro scheduling problem is usually divided into different time horizons: • Medium and long-term hydro scheduling, which encircle a time horizon of one or more years, discretised in weekly or monthly intervals. Stochastic models are used [1]. • Short-term hydro scheduling (STHS), which encircles a time horizon of one day to one week, usually discretised in hourly intervals. Deterministic models are used. Where stochastic quantities are included, such as hydro inflows or energy prices, the corresponding forecasts are used [2,3]. In a regulated environment, the main goal of the hydro scheduling problem is the minimisation of the deviation between total hydroelectric generation and electric energy demand, accomplishing the reservoir storage conditions at the beginning and at the end of the scheduling time hori- 0142-0615/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijepes.2007.06.017 * Corresponding author. Tel.: +351 275 329759; fax: +351 275 329972. E-mail addresses: sm@ubi.pt (S.J.P.S. Mariano), catalao@ubi.pt (J.P.S. Catala ˜o), vfmendes@isel.pt (V.M.F. Mendes), lmf@ist.utl.pt (L.A.F.M. Ferreira). www.elsevier.com/locate/ijepes Available online at www.sciencedirect.com Electrical Power and Energy Systems 30 (2008) 125–133