applied to the problem of unit sizing in a hybrid power system such that demand of residential area is met. Optimal sizing of two systems is considered. In the system No.1, the produced power is delivered to the load and the hydrogen produced by the reformer is stored in the tank. If the power produced by the wind turbine is more than the demand, the remainder of wind turbine's power is delivered to the electrolyzer to produce hydrogen, such that when the wind power cannot meet the demand, the fuel cell is fed by the stored hydrogen and produces enough power, together with the wind turbine's power. In the system No.2, the hydrogen produced by the reformer is delivered to the fuel cell directly. When the power produced by the wind turbine plus power produced by the fuel cell (fed by the reformer) is more than the demand, the remainder is delivered to the electrolyzer. In contrast, when the power produced by the wind turbine plus that produced by the fuel cell (fed by the reformer) is less than the demand, some more fuel cells are employed and they are fed by the stored hydrogen. Our aim is to minimize the costs of the system such that the demand is met. PSO algorithm is used for optimal sizing of system's components. Index Terms--wind turbine, fuel cell, reformer, hybrid power system, optimal sizing, particle swarm optimization I. NOMENCLATURE P conv wg - Power delivered from wind turbines to converter (kw) P el wg - Power delivered from wind turbines to electrolyzer (kw) P k el tan - Power delivered from electrolyzer to hydrogen tank (kw) P fc k - tan Power delivered from hydrogen tank to fuel cell (kw) P conv fc - Power delivered from fuel cell to converter (kw) P load conv - Power delivered from converter to load (kw) P fc ref - Power delivered from reformer to fuel cell (kw) P k comp tan - Power delivered from compressor to hydrogen tank (kw) S. M. Hakimi is with the Department of Electrical Engineering, K. N. Toosi University of Technology, Tehran-Iran (e-mail: sm_hakimi@yahoo.com). S.M.M.Tafreshi is with the Department of Electrical Engineering, K. N. Toosi University of Technology, Tehran-Iran (e-mail: tafreshi@eetd.kntu.ac.ir). M. R. Rajati is with the Department of Electrical Engineering, K. N. Toosi University of Technology, Tehran-Iran (e-mail: mohammadreza.rajati@gmail.com). P wt Power generated by wind turbines (kw) Pload Load power (kw) E k tan Stored energy in the hydrogen tank (kwh) fc , el , conv Efficiency of fuel cell,electrolyzer,converter NPCindex Net present cost (the index shows the corresponding component) ($) S Single-payment present worth factor R Life time of project (year) L Life time of each components (year) Ir Interest rate C * Optimal cost($) N index Optimal number(the index shows the corresponding component)