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D.Ipsakis et al. systems for the production of electrical energy were studied extensively in the past where each subsystem was modelled and validated with experimental data [2,3]. However, these research activities dealt with the each subsystem and not with the integrated system. In this paper, three different energy <strong>management</strong> strategies were developed and used in order to study the behavior of the system during a typical four month period and mainly how the total hydrogen stored was affected by these strategies. Furthermore, the effect of key variables to the system was examined in order to gather all the information needed for future optimization studies and control analysis. 2. Description of the Stand-Alone Power System An application utilizing solar and wind energy with hydrogen production through water electrolysis, storage and utilisation in fuel cells is currently under development at Neo Olvio in Xanthi in the framework of a research project with the participation of Chemical Process Engineering Research Institute and Sunlight Systems S.A. The RES, consists of a PV array with an installed capacity of 5 kW p and three wind generators rated at 3kW p totally. Surplus energy is to be supplied to a PEM electrolyzer, rated at 4.2kW p after the demands of a 1kW constant load have been met. The hydrogen produced is to be stored in cylinders under high pressure. In case there is a lack of energy, a PEM fuel cell rated at 4kW p is to be used to provide power. Also, in order to account for short-term needs, a lead-acid accumulator with a total capacity of 3000Ah- 48Volt, is also used and charged by the renewable energy sources or the fuel cell depending on the availability of the renewable energy sources. A back up unit (diesel generator) could be used in order to cover the electrical needs that can’t be met by the system. Its potential use however, would be identified during the study of the system. Furthermore, power electronic converters are employed for power <strong>management</strong> and for the integration of the various subsystems. Fig. 1 represents a layout of the system developed. Fig. 1: Block diagram of the proposed Stand-Alone Power System 3. Simulation of the stand-alone power system during a typical four-month period The design and analysis of such a complicated system requires the existence of detailed mathematical models for each subsystem. Due to the large number of equations that such a system comprises, mathematical model will be given where is possible. The theoretical study and simulation of the stand-alone power system was performed by using the MATLAB ® simulation program tool.
<strong>Energy</strong> <strong>management</strong> on a stand-alone power system for the production of electrical energy with hydrogen long term storage 3.1. Renewable energy system (RES) The output power from the PV-array is given by: P = V pv ⋅ I ⋅η pv pv conv (1) where P pv denotes the output power from the photovoltaic array in Watt, I pv the operation current in A, V pv the operation voltage in Volt and η conv the efficiency of the DC /DC converter (~90-95%) [2]. In a similar way, the output power of the wind turbine is given by the following equation [4]: ρ ⋅ Α w 3 P = c λ , β ) ⋅ v (2) m p ( wind 2 where P m denotes the mechanical output power of the wind turbine in Watt, c p the performance coefficient of the turbine, ρ the air density in kg/m 3 , Α w the turbine swept area in m 2 , v wind the wind speed in m/s, λ the tip speed ratio, and β the blade pitch angle in deg. The relationship for c p is based on the characteristics of the turbine [4]. From the above equations, the output power of each subsystem of the renewable energy system was calculated and the results are shown at figures 2a and 2b. Output Power, Watt 5000 a) 4000 3000 2000 1000 Output Power, Watt 1800 1600 1400 1200 1000 800 600 400 200 b) 0 0 500 1000 1500 2000 2500 3000 Time, h 0 0 500 1000 1500 2000 2500 3000 Time, h Figure 2: a) Output power from the photovoltaic system during a typical four month period b) Output power from the wind generators during a typical four month period 3.2. Operation strategies for the stand-alone power system The output power from the RES, P res , has been calculated as the sum of the output power from the photovoltaic system and the wind generators. The power demand for the load, P load , is constant throughout the year at 1kW. Therefore, the shortage or surplus power is calculated as: P = P RES − P load (3) Based on the above equations and with the developed energy <strong>management</strong> algorithms all the subsystems were studied simultaneously. Two limits for the state-of-charge (SOC) were used: The minimum limit, SOC min (84%), where energy should be supplied
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