Energy supply which is secure, reliable, affordable, clean is fundamental to global economic growth and human development and presents huge challenges for us. The world is becoming more global, where new technologies brings faster innovations and demand changes in our behavioural responses, the task of predicting the future becomes harder. In future, electrical power generation requires mix technologies which includes renewable energy sources. Traditional mineral energy sources such as fossil fuels will be depleted in a few years and will result in serious problems on economic growth. Hence, renewable energy sources play a vital role in power generation. The main advantage in renewable energy is an indigenous resource available in considerable quantities to all developing nations and capable in principle, of having a significant local, regional or national economic impact and requires less maintenance when compared with traditional energy sources.
Fuel cells are environmentally sound renewable energy sources which operates at efficiencies greater than traditional energy production methods.
Fuel cell is a device which converts chemical energy into electrical energy. There are two electrodes in fuel cell, one positive and one negative, called anode and cathode respectively It consists of an electrolyte, which carries charged particles from one electrode to the other, and a catalyst, which helps to speed the reactions at the electrodes. The basic fuel used is hydrogen. One main advantage of fuel cell is that they generate electricity with very little pollution and produce harmless byproduct, namely water.
PEMFC has less operating temperature compared to molten-carbonate, solid-oxide and phosphoric acid FCs. More expensive fuel cells are alkaline and direct methanol. The PEMFC has less operational temperature ranging from sixty to eighty degree Celsius and high energy density. Because of low operating temperature, PEMFC warms up in short period of time which helps in generation of electricity.
Polarisation Characteristics
The polarisation characteristic of fuel cell is a curve which is obtained by plotting cell voltage versus current density. The primary sources from which the losses occurred are ‘ohmic, activation and concentration losses.’
The voltage falls slowly for high temperature fuel cells (Eg.SOFC) whereas the voltage falls rapidly for low temperature fuel cells (Eg.PEMFC). The theoretical maximum voltage of fuel cell is called open circuit voltage. Because of the irreversible losses in the system, the actual potential of a cell decreases. Any voltage drop from this maximum value is termed as overvoltage. This overvoltage has to be minimised to operate fuel cell as efficiently as possible.
Power Electronics Interface Requirements
Currently, the standard output voltage rating for fuel cells has not been established. The output voltage of present fuel cell stack modules will be in the range 24–150 VDC. However, for large number of fuel cell applications, it is necessary to interface power electronics. This interface should:
• control voltage of fuel cell
• convert the fuel cell output to get appropriate type and magnitude
• give a high power factor (grid applications)
• reduce the harmonics
• operate efficiently under all conditions.
Power Converters
The device which is used for converting electrical energy is called power converters.
Broadly, the power converters are classified on the basis of whether the input and output are alternating current (AC) or direct current (DC).
• DC conversion
a. DC-DC conversion
b. DC-AC conversion
• AC conversion
a. AC-DC conversion
b. AC-AC conversion
Application of DC Converters in Fuel Cell Systems
The limitations of fuel cells such as low voltages, low current densities, and unstable power production, has made to use the DC converter as a most important component in fuel cell systems for portable or standalone applications.
Single-stage topologies
Researchers have developed various topologies for single-stage conversion either using a DC-DC converter or a DC-AC inverter to fulfill the fuel cell operational requirements. By using a single-stage topology component counts can be reduced and it is simple to control. Many studies have claimed that desired regulated output voltage level and an input current ripple below 2 per cent of the nominal input current for low power fuel cells can be maintained by using DC-DC boost converter. For low-voltage fuel cell applications boost converter topology is more suitable out of Buck and Boost topologies.
Multi stage topology
Two types of power conditioning combinations such as DC- DC converters or an AC inverter are realised as multistage topology. In this topology, fuel cell voltage or current will be first converted to desired value by using DC-DC converter. This DC-DC converter may consist boost, buck-boost, and so on to get desired voltage or current becoming input variable. In this topology second step is DC voltage from DC-DC converter inverted to AC voltage.
Recent development in DC-DC converters
1. By using an auxiliary network along with the boost diode, boost inductor and boost switch, and boost diode the efficient soft-switched boost converter can be increased as shown in Figure 5. By changing the pulse-width of the main switch, the output voltage of this converter can be regulated.
2. A high step- up voltage conversion ratio is achieved by using a new zero-voltage switching DC-DC converter for renewable energy conversion systems based on a boost converter and a voltage-doubler configuration with a coupled inductor as shown in Figure 6. A new soft-switched pulse-width-modulated (PWM) quadratic boost converter has the efficiency which is equal to 92.3 per cent.
Conclusion
Increase in global energy consumption and demand owing to human being quest for top active standards. The major source of energy is from fossil fuels. Due to these fuels, global warming problems are increasing which will lead to life hazards like hurricanes, tornadoes and floods etc. So, as to overcome from these problems, renewable energy sources such as proton exchange membrane fuel cell (PEMFC) are proposed which uses hydrogen as a fuel. By using power converters performance of fuel cell can be improved. Power electronics have created revolution in the concept of power control for power conversion. In recent technology power electronics have secured a significant position and used in large variety of high-power products such as power supplies, vehicle propulsion systems, HVDC systems, heat control, light control and motor control.
M Malathi
Assistant Professor, Department of Electrical and Electronics Engineering, NIE Institute of Technology (NIEIT), Mysore.
Dr Usha Surendra
Professor, Department of Electrical and Electronics Engineering, Christ University, Kengari Campus, Bangalore.
