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Hydrogen Fuelled Electricity Generation

Hydrogen Fuelled Electricity Generation

U.SURESH KUMAR*

* Professor/HOD in electrical electronics department

MOHAMED SATHAK ENGINEERING COLLEGE,KILAKKARAI,T.N,INDIA

E mail uskrk@sify.com

Summary 

          This paper describes their tasks and the current state of development of fuel cell and some of practical applications also explained why I have taken to this topic because,Hydrogen is being promoted as the perfect environmentally friendly fuel of the future.

Introduction

         It will still be available when fossil fuels are exhausted  It is the earth's tenth most abundant element and is the most abundant element in the universe It is generated from water and returns to water when it is burnt. It is available in vast quantities from the World's oceans.

           What many "Hydrogen economists" don't make clear is - Where will the energy come from to extract the hydrogen from the water?

              Hydrogen is an energy carrier, not an energy source, so the energy it delivers would ultimately have to be provided by a conventional power plant. 

Fuel Cells

1. The fuel cell was invented in 1839 by Welsh lawyer Sir William Robert. It takes in Hydrogen and Oxygen from the air and puts out electricity, heat, and water. It doesn't use fossil fuels and it doesn't produce greenhouse gases and so it should be the ideal solution to providing distributed or portable electrical power. Despite its obvious advantages it was not until the 1950s in response to the needs of the US space programmer that practical devices were developed. Even today, although there are many variants of fuel cells working in development labs throughout the world and small  scale deployment of demonstration units in some countries, there is still no volume production. What is holding back the commercialization of fuel cells? The following diagram shows the key system components for providing AC or DC power(see figure 1 )   

                         But this diagram only tells part of the story. Though the basic principle is quite simple, converting this into a practical product involves many engineering challenges and up to now the solutions proposed have not been cost effective. Fuel cells are an expensive way of providing electrical energy. The prize of cheap, clean, renewable energy is still unclaimed but engineers are getting ever closer to winning it.

How Fuel Cells Work: Fuel cells don't store energy like batteries. They only provide electrical energy while the active chemicals are supplied to the electrodes. The process is described in more detail in the two examples below.  

Proton Exchange Membrane (PEM) Fuel Cell The most common fuel cells use Hydrogen as the fuel and Oxygen from the air as the oxidant. The basic reaction can be illustrated by the Proton Exchange Membrane (PEM) fuel cell. (Also called the Polymer Electrolyte Membrane fuel cell.) The overall equation for the reaction is

2H2 + O2 ? 2H2O

The equation for the reactions at the individual electrodes are shown where they take place on the diagram below.fig2 

• The Electrical Energy The electron flow between the anode and the cathode caused by the chemical reactions in the cell represents the conventional electrical current flowing in the opposite direction. This electrical current is available to do work in the external circuit.  
• Catalysts Catalysts are needed to increase the rate of oxidation at the anode and the rate of reduction at the cathode. In this way they allow the chemical reaction to take place at a lower temperature. Alternatively to avoid the cost of expensive catalysts, some fuel cells are designed to work at elevated temperatures.The platinum catalyst used in PEM and some other cells is very expensive and extremely sensitive to poisoning by even small amounts of Carbon Monoxide making it necessary to employ an additional filtering processes in the system to eliminate potential contaminants.

                          The working of the direct Methanol fuel cell is similar to the PEM fuel cell shown in the above diagram.The electrolyte is a polymer and the charge carriers are the hydrogen ions. Liquid Methanol (CH3OH) is fed into the anode of the cell where it is oxidized in the presence of water generating Carbon Dioxide (CO2). The cathode chemistry is the same as in the PEM cell with the Oxygen combining with the Hydrogen ions and electrons from the external circuit to produce water. The reactions are as follows:

Anode Reaction:

CH3OH + H2O ? CO2 + 6H+ + 6e--

Cathode Reaction:

3/2 O2 + 6H+ + 6e-- ? 3H2O

Overall Cell Reaction:

2CH3OH + 3O2 ? CO2 + 4H2O

 Like PEM fuel cells DMFCs work at low operating temperatures in the range from about 50ºC to 120ºC but they have a relatively low efficiency and power density. Output power using current technology is limited to about 1.5 kW which enough to power most consumer goods but insufficient for automotive applications which require much higher power. Nevertheless the ability to use liquid fuel coupled with the elimination of the reformer make these fuel cells very attractive 

Balance of Plant (BOP) The fuel cell stack alone can not generate electricity. Practical systems need sub-systems to supply the fuel and to provide the necessary control over the processes involved in the energy conversion. The essential ancillary equipment , the so called "balance of plant", can be just as expensive and complex as the fuel cell stack itself. Some of this equipment is outlined in the following list; 

• Fuel Supply or Storage

The largest item is the reformer (See below) which provides local generation of the Hydrogen fuel. The reformer itself must have storage capacity for the reformat fuel used in the process. If Hydrogen generation is not part of the system, there must be some form of storage to carry the Hydrogen fuel to be consumed by the fuel cell. This requires expensive high pressure tanks or cryogenic storage tanks (See also below)

• Pumps, Compressors and Expanders Pumps are needed to pump the reactant air through the stack and to provide forced cooling. Higher power systems require compressors to handle the higher airflow rates. Expanders are needed to reduce the high pressure of the stored Hydrogen to the required input pressure at the stack.
• Filters  Filters are needed to remove any contaminants from the fuel supplies which could poison the catalysts or damage the cells reducing their power production and ultimately causing their shut down. Particular offenders are Carbon Monoxide, resulting from incomplete reactions in the reformer, which affects the platinum catalysts and Sulphur found in reformats derived from fossil fuels, such as coal, oil, and natural gas, which contaminates the Hydrogen gas and in turn attacks and degrades the anodes.
• Thermal Management High power systems use forced cooling with fluid coolants to remove the heat. This requires fluid pumps and a radiator/heat exchanger to expel the heat.The system also requires heaters to bring the stack temperature up to its operating point on start up.An overall thermal management system is required to balance the heat flows to keep the temperature of the stack at its optimum operating point
• Water Management The conductivity of the electrolyte in the cell is proportional to the water content and it must be kept moist to remain conductive. The airflow and the heat generation in the cell tend to work against this. Consequently the air supplied to the cell must be humidified to stop electrolyte drying out and this requires a humidifier. Cold temperature operation in freezing conditions also brings problems due to the formation of ice crystals which can damage the electrolyte or membrane. The system must incorporate a method of purging the water or alternative anti-freeze controls.Another pump may be required to remove surplus water from the cathode.
• Electrical Power Management Though some fuel cells may be required to provide a steady operating current and voltage, most systems must be responsive to variable demands. This means that the system should provide for a variable output current and as a consequence, all the fuel, air and water flows must be varied accordingly. At the same time the heat dissipation will change and the temperature must be maintained within its designed operating range. The same will apply to the reformer if this is part of the systemThe fuel cell system output voltage is fixed but the application may require a different voltage or, in the case of most distributed power generators, an alternating current output. In these cases DC/DC converters or AC inverters may be an integral part of the system.
• Electric Motors Motors of different sizes are required to drive the pumps and compressors.
• Sensors Sensors are required to monitor temperatures, pressures, fluid and gas flows as well as electrical currents and voltages.
• Battery The fuel cell does not start to deliver electrical energy until it approaches its operating point. During start up, batteries are required to power all the electronic control systems, as well as the pumps, compressors and heaters needed to get the stack up to its operating point.The battery also provides an independent stable voltage to power the system electronics.Because of the slow dynamic performance of the fuel cell, the battery may also be required to provide a temporary power boost when the fuel cell is subject to a sudden demand.
• Safety Systems Safety systems must provide fail safe operation, protecting the system from out of tolerance conditions and abuse and shutting it down if necessary.
• Control SystemThe system could not function without comprehensive electronic control systems to manage all the sub-systems listed above. 

Electrical Output

• Voltage Fuel cells typically generate about 0.6 Volts to 0.9 Volts DC per cell.Due to the internal impedance and losses within the cell, the output voltage falls as the current is increased. Multiple cells in a stack must be used to provide higher voltages. 
• Current and Power The current output from a single cell is directly proportional to the area of the electrodes. As with batteries the effective area of the electrodes and hence their potential current carrying capability can be increased without increasing their physical size by making the surface porous and using materials with very fine particle size.Typical power outputs are about 1 Watt /cm2 of electrode plates. 
• Dynamic Response PEM fuel cells operate at relatively low temperatures of around 80°C (176°F) which allow reasonably fast warm-up times (currently 10 to 20 seconds) compared with high temperature fuel cells which take as much as 30 minutes to reach their operating temperature. This is particularly important for automotive applications which require quick start-ups. 
• Efficiency Because the energy conversion in fuel cells is accomplished in a single direct conversion process, much higher efficiencies are possible than with conventional electricity generation by means of steam turbines which involve three energy conversion processes. As noted above, the output voltage of a fuel cell falls as the current drawn from it increases. The net effect of this is that the efficiency also drops as the power drawn from the cell increases so that the efficiency is almost proportional to the output voltage. The typical operating efficiency of a fuel cell running at 0.7 Volts is about 50%. This means that 50% of the energy content of the hydrogen input is converted into electrical energy; while the remaining 50% will be dissipated as heat or lost through incomplete oxidation within the cells.The waste heat from the fuel cell electricity generating process can be used in combined heat and power CHP) applications to provide local heating and thus improve the overall energy utilization efficiency of the Hydrogen fuel. This is particularly attractive for high temperature fuel cell systems. Fuel Cell Variants

A range of fuel cell designs using variants of the basic chemistry has been developed to meet different design or operating criteria such as less expensive construction, more efficient fuel utilisation, faster start-ups or the use of more convenient or less expensive fuels. Higher power outputs can be achieved by operating at high temperatures, by using catalysts to accelerate the fuel cell chemical reaction and by using electrodes with a greater surface area. Lower operating temperatures can be obtained by using more expensive catalysts.  

The main variants are as follows:

• PEM Proton Exchange Membrane Fuel Cells follow the basic design described above. They have a good combination of efficiency, power output and low operating temperature make it the cell of choice for automotive applications. Though the maximum working temperature of most designs is 100°C to avoid damage to the fragile membrane, some products have been designed to work at temperatures up to 120°C.
• AFC Alkaline Fuel Cells use aqueous electrolytes of potassium hydroxide. They were some of the earliest practical cells and were used in the Apollo space programme, generating drinking water as well as electrical power. Although they are inexpensive compared with PEM cells, operating efficiencies of 60% are possible. Unfortunately they have a low power output and the catalyst is prone to poisoning from Carbon Dioxide in the atmosphere.  
• PAFC Phosphoric Acid electrolyte Fuel Cells run at a high temperatures of around 220°C delivering high power of a MegaWatt or more but a with relatively low efficiency of around 35%. The consequence of poor conversion efficiency is high heat generation in the fuel cell stack. Because of the high working temperature the efficiency losses can be mitigated by using the waste heat in combined heat and power (CHP) applications. 
• MCFCMolten Carbonate Fuel Cells run at even higher temperatures of 650°C to 1000°C. Their unique chemistry needs Carbon Dioxide from the air a part of the process. Efficiencies achieved are 45% or more and power outputs of over 1 MegaWatt are typical in grid supply applications. Because of their high working temperature they can operate directly with hydrocarbon gases which are reformed within the cell and do not need a separate Hydrogen supply. The high temperature also means that less expensive catalysts are needed, but the molten electrolyte imposes special requirements on containment and anti corrosion measures.  
• SOFC Solid Oxide Fuel Cells also operate in the same or higher temperatures as the molten carbonate cells with the same fuel and catalyst advantages. The ceramic electrolyte which can run as hot as 800 degrees Celsius has the advantage that the electrolyte stays solid. They can deliver powers of several Megawatts but at a lower efficiency of around 35%.  

System Cost/kW

     Care must be exercised in comparing costs since some estimates may be for the fuel cell stack alone while others may include all the balance of plant costs which could double the cost. 

 Large systems providing distributed power generation are significantly more expensive than small systems used in automotive applications. Currently, costs are around $650/kW.The Solid State Energy Conversion Alliance (SECA) formed by the US Department of Energy to promote the development of environmentally friendly solid oxide fuel cells (SOFC) has a cost target for a solid-state fuel cell module of no more than $400/kW. At this price, fuel cells would compete with gas turbine and diesel generators.Automotive ICE power plants currently cost about $25-35 / kW. A fuel cell system needs to cost less than $50 / kW for the technology to be competitive. Currently costs are around $70/kW.

                The US Freedom CAR project has set cost targets for PEM fuel cells at $45/kW by 2010 and $30/kW by 2015. 

Fuel Costs

The real cost of the energy supplied by fuel cells depends very much on the cost of the Hydrogen it consumes and this in turn depends on how the Hydrogen was produced.Until recently, steam reformation of natural gas was the cheapest way of producing Hydrogen but production costs have risen with the cost of the fuel. Currently, assuming the cost of natural gas is about $10per M Btu (Million Btu) the bulk cost of Hydrogen at the production plant will be about $5/Kg. The cost of pressurizing the gas and distribution it to refueling stations will add to this amount. Generating Hydrogen by electrolysis from wind farm electricity is now the cheapest way of producing the gas. Currently the retail price of pressurized hydrogen from an unsubsidized supplier is about $100/kg plus cylinder rental. 

Practical Fuel Cell System Applications  

1.Combined Heat and Power (CHP)

The chemical reaction taking place in a fuel cell is an exothermic catalytic oxidation. The excess heat generated in high temperature fuel cells such as SOFC, PAFC and MCFC can be captured and used to heat water in a combined heat and power (CHP) application giving overall system efficiencies of 80% or more. 

CHP is an ideal way of utilizing waste heat from less efficient fuel cell electricity generators fig 3.

           2.Automotive Applications Hydrogen powered internal combustion engines can already be found in emission free, traction (automotive) applications. The earliest examples were built in Germany by Rudolf Err en in the 1920s.Automotive engines can also be designed for multi-fuel use with the ability to use liquefied petroleum gas (LPG) or other fuels as well as Hydrogen. This could be an attractive option for early adopters of Hydrogen technology providing peace of mind on long journeys until a well developed network of Hydrogen dispensing stations has been installed.

         3. Electrical Power GenerationHydrogen powered internal combustion engines can also be used with rotary generators to generate electricity as shown in the following diagram: fig 4

 Though this is perfectly viable, small, stand  alone Hydrogen powered electricity generators are more likely to use fuel cells

Conclusion

1.               We have explained only few application of fuel cell. Also this one of the our ideas If we will generating the electricity   based upon the fuel cell application we have to be saved the our environment from co2 emission and also free from pollution

Reference

1.Tomorrow's Energy: Hydrogen, Fuel      Cells, and the Prospects for a Cleaner Planet (Hardcover) by peter hobffman (Author), tom harkin (Author)

 

1. 2.Fuel Processing: for Fuel Cells   BY GUNTHER KOLB (Institut für Microtechnik Mainz      GmbH, Germany)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

About the Author

i have qualified in master degree in applied elelctronics after ug with elelctrical and electronics engineeringand also i have experienceing in 10 yrs in acadamic and also 2yrs experienced in industrial and i have one of the members in question setting in various university like anna university, sathiyabama etc

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