Cogeneration for power and desalination could not only improve the economic benefit of the power plant, but also afford the high quality water to solve the freshwater shortage. Considering the demand of power and water, a detailed mathematical model of the cogeneration system targeting the minimum total annual cost (TAC), which includes the power plant, multistage flash (MSF) and reverse osmosis (RO), is proposed and described as a mixed integer nonlinear programming (MINLP) problem. The modified genetic algorithm (MGA) with mixed coding is put forward to solve the model developed by us. A case study, which is supposed to supply 250 MW of power and 12,000 m(3)/h of water for Huangdao District of Qingdao City, is analyzed in order to illustrate the model capabilities. The results show that the operation pattern of the cogeneration system could be varied in terms of the water demand. When the water demand is lower than 8000 m3/h, the combination of power plant associated with MSF is adopted and the condensing-extraction steam turbine is selected. When the water demand of water is higher than 8000 m(3)/h, the tri-combination of power plant, MSF and RO is the optimal choice, in which back pressure steam turbine is selected. (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved.