HYBRID POWER SYSTEM

Modelling and Simulation of Fuel Cell/Solar Hybrid Power System

Modelling and Simulation of Fuel Cell/Solar Hybrid Power System

Introduction

After many technological advances Proton Exchange Membrane Fuel Cells (PEMFCs) technology has now reached test and demonstration phase. (Jacobson 2009: 46-48). Many aspects of such a hybrid energy system need to be investigated e.g. cost, ef?ciency, reliability, and dynamic response of the electrolyzer, fuel cell and hydrogen storage components. One important aspect of a wind fuel cell hybrid energy system that needs further investigation is design and simulation of the control system. A block diagram of the hybrid energy system is shown in Fig. 1.

The load can be supplied from the wind turbine and/or fuel cell. If the wind turbine is producing enough power, the load will be supplied entirely from wind energy. In case of low wind a share of power can be supplied from the fuel cell. If the output power from the wind turbine exceeds the demand, the excess power may be used to produce hydrogen for later use in the fuel cell. (Kuo 2009: 133-138).

Description of the system

The system consists of a Southwest Wind Power Inc. AIR 403 wind turbine, a Proton Exchange Membrane Fuel Cell (PEMFC) such as Hpower.com EPAC500, an electrolyzer (www.stuartenergy.com), a wind mast, a dump load, a personal computer acting as controller and data acquisition system. Fig. 2 shows the details of system interconnections. (Costamagna 2009: 253-69)

For the study and analysis of the system the following parameters of this 48 V hybrid energy system are recorded: (a) Wind speed; (b) wind turbine current; (c) fuel cell voltage; (d) fuel cell current; (e) fuel cell temperature; (f) fuel cell pressure; (g) fuel ?ow rate; (h) wind direction; and (j) load current(Agbossuo 2009: 168-72). The inbuilt wind turbine controller runs the turbine in variable speed mode while extracting maximum power. Fuel cell system consists of a PEM fuel cell stack and an electrolyzer. A fuel cell stack consists of 65 individual fuel cells connected in series. The output current can vary between 0 and 25 A. Fuel cell delivers the current difference between the load current and the wind turbine current. If the output voltage of the fuel cell stack drops below 46 V its controller switches on. A PC based PID type fuel cell controller adjusts the fuel and oxygen ?ow rates to maintain a constant stack output voltage. Controller action compensates the drop in the fuel cell stack voltage caused by the load current variations. If the wind turbine generates more current than required by the load then the excess current is diverted towards an electrolyzer. (Rajashekara2008: 203-208)The electrolyzer-produced hydrogen is stored in a tank for later use in the fuel cell stack.

System model

The main components of the system are wind turbine, fuel cell stack, electrolyzer and controller. A standard classical method of representing the system by a set of differential equations and PID controller by a transfer function is used. Wind turbine rotor diameter is ...