NEW APPROACH TO FUEL CELL TECHNOLOGY

Fuel cell may be defined as “an electro chemical device for the continuous conversion of the free energy change in a chemical reaction to electrical energy”.

Fuel cell systems offers clean and efficient energy production and are currently under intensive development by several manufacturers for both stationary and mobile applications. Fuel cells found its first application during the NASA Apollo moon landing program in the late 1960s. It was a logical choice as an energy source. It had no moving parts, was compact in size, and was founded with an unlimited budget. Powered simply by hydrogen gas, the fuel cell produced dc power, pure water as its exhaust, and heat. The dc supply charged the spacecraft batteries. The water was used by the crew and the heat generated by the fuel cell was rejected to the void of space. At that time a brief study was conducted as to how fuel cells could be used to power homes in the same manner as spacecrafts. The study was very short lived but the dream of powering automobiles and houses using fuel cells was born in earnest.

Fuel cells, until recently a curiosity largely confined to the space program, are emerging as a valuable clean and efficient generator of electricity. A number of companies are developing fuel cells for use in stationary applications. Most of the current applications for fuel cells utilize natural gas as a fuel. In certain states, such as New York and Connecticut, fuel cells operating on natural are recognized by the states as a renewable energy source. Recently, however, fuel cells, mostly phosphoric acid, have been shown to operate well on renewable biogas fuels, such as anaerobic digester gas (ADG) produced at wastewater treatment plants as well as landfill gas (LFG) and gas produced at beer breweries.

2. FUEL CELL

       A fuel cell is an electrochemical cell. The electricity is generated through the reaction, triggered in the presence of an electrolyte, between the fuel (on the anode side) and an oxidant (on the cathode side). The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.

Fuel cells are different from conventional electrochemical cell batteries in that they consume reactant from an external source, which must be replenished– a thermodynamically open system. By contrast, batteries store electrical energy chemically and hence represent a thermodynamically closed system.

Many combinations of fuels and oxidants are possible. A hydrogen fuel cell uses hydrogen as its fuel and oxygen (usually from air) as its oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include chlorine and chlorine dioxide.

2.1 Design:

A fuel cell works by catalysis, separating the component electrons and protons of the reactant fuel, and forcing the electrons to travel through a circuit, hence converting them to electrical power. The catalyst typically comprises a platinum group metal or alloy. Another catalytic process puts the electrons back in, combining them with the protons and oxidant to form waste products (typically simple compounds like water and carbon dioxide).

A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load. Voltage decreases as current increases, due to several factors:

• Activation loss
• Ohmic loss (voltage drop due to resistance of the cell components and interconnects)
• Mass transport loss (depletion of reactants at catalyst sites under high loads, causing rapid loss of voltage).

To deliver the desired amount of energy, the fuel cells can be combined in series and parallel circuits, where series yields higher voltage, and parallel allows a higher current to be supplied. Such a design is called a fuel cell stack. Further, the cell surface area can be increased, to allow stronger current from each cell.

2.2 Why Fuel Cells ?

Fuel cells are the perfect melding of benefits from energy sources. In taking the easy refueling and continuous operation potential from internal combustion engines and the highly efficient and quiet operation of batteries, fuel cells seem like the ideal energy alternative. They lack the need for recharging that batteries have and also the pollution creation that plagues both batteries (further up the power line from electric power plants) and combustion engines.

Like combustion engines, fuel cells operate using fuel from tanks that can be easily refueled. As long as fuel is available, the cell can run continuously. The major advantages that fuel cells hold over internal combustion engines however are the high efficiency of operation and the lack of harmful pollutants. Below is a projection made by Mercedes-Benz on the efficiency of compact cars powered by fuel cells versus other energy sources:

2.3 Categories of Fuel Cell Types:

A single fuel cell emits only minimal electrical energy. In order to make a fuel cell produce more electrical energy, a definite number of fuel cells must be stacked together in a series. Researchers have also successfully developed a fuel cell system by incorporating a ‘fuel reformer.’ Within the fuel reformer, it is possible to extract hydrogen from almost any hydrocarbon source, whether natural gas, methane or gasoline for generating electrical energy.

Based on the electrical energy generated and the types of chemicals used as the fuel and the oxidant, fuel cells can be categorized into the following basic types:

·         Alkali fuel cells (power capactiy:10 KW to 100 KW);

·         Direct methanol fuel cell (power capacity: 100 KW to 1MW),

·         Reformed methanol fuel cell (power: 5W to 100 KW):

·         Direct ethanol fuel cell (power capacity: up to 140mW/ cm²);

·         Molten carbonate fuel cells (MCFC) [power capacity: 100 MW],

·         Proton exchange membrane fuel cells (PEM) [Power capacity:100W to 500 KW]

·         Solid oxide fuel cells (SOFC)[Power capacity: Up to 100 MW].

·         Phosphoric acid fuel cells (PAFC) [power capacity: up to 10 MW],

A fuel cell can be used in electric vehicles, power plants and smart phones as well as for various portable charging applications. Worldwide, scientists are exploring ways to make fuel cell vehicles economically viable by 2020.

2.4 Fuel Cell Efficiency:

The efficiency of a fuel cell is dependent on the amount of power drawn from it. Drawing more power means drawing more current, which increases the losses in the fuel cell. As a general rule, the more power (current) drawn, the lower the efficiency. Most losses manifest themselves as a voltage drop in the cell, so the efficiency of a cell is almost proportional to its voltage. For this reason, it is common to show graphs of voltage versus current (so-called polarization curves) for fuel cells. A typical cell running at 0.7 V has an efficiency of about 50%, meaning that 50% of the energy content of the hydrogen is converted into electrical energy; the remaining 50% will be converted into heat. (Depending on the fuel cell system design, some fuel might leave the system unreacted, constituting an additional loss.)

For a hydrogen cell operating at standard conditions with no reactant leaks, the efficiency is equal to the cell voltage divided by 1.48 V, based on the enthalpy, or heating value, of the reaction. For the same cell, the second law efficiency is equal to cell voltage divided by 1.23 V. (This voltage varies with fuel used, and quality and temperature of the cell.) The difference between these numbers represents the difference between the reaction's enthalpy and Gibbs free energy. This difference always appears as heat, along with any losses in electrical conversion efficiency.

Fuel cells do not operate on a thermal cycle. As such, they are not constrained, as combustion engines are, in the same way by thermodynamic limits, such as Carnot cycle efficiency.At times this is misrepresented by saying that fuel cells are exempt from the laws of thermodynamics, because most people think of thermodynamics in terms of combustion processes (enthalpy of formation). The laws of thermodynamics also hold for chemical processes (Gibbs free energy) like fuel cells, but the maximum theoretical efficiency is higher (83% efficient at 298K  in the case of hydrogen/oxygen reaction) than the Otto cycle thermal efficiency (60% for compression ratio of 10 and specific heat ratio of 1.4). Comparing limits imposed by thermodynamics is not a good predictor of practically achievable efficiencies. Also, if propulsion is the goal, electrical output of the fuel cell has to still be converted into mechanical power with the corresponding inefficiency. In reference to the exemption claim, the correct claim is that the "limitations imposed by the second law of thermodynamics on the operation of fuel cells are much less severe than the limitations imposed on conventional energy conversion systems". Consequently, they can have very high efficiencies in converting chemical energy to electrical energy, especially when they are operated at low power density, and using pure hydrogen and oxygen as reactants.

It should be underlined that fuel cell (especially high temperature) can be used as a heat source in conventional heat engine (gas turbine system). In this case the ultra high efficiency is predicted (above 70%).

3. PHOSPHORIC ACID FUEL CELLS (PAFC)

3.1 Phosphoric Acid Fuel Cell History

Experimenters have used acids as electrolytes since the time of William Grove's first gas battery in 1842 – he used sulfuric acid. But phosphoric acid, a poor conductor of electricity, was not as attractive, and PAFCs were slower to develop than other types of fuel cells. In 1961, G. V. Elmore and H. A. Tanner revealed new promise in phosphoric acid electrolytes in their paper "Intermediate Temperature Fuel Cells." They described their experiments using an electrolyte that was 35 percent phosphoric acid and 65 percent silica powder pasted into a Teflon gasket. "Unlike sulfuric," they noted, "phosphoric acid is not reduced electrochemically under cell operating conditions." Also, their PAFC ran on air, rather than pure oxygen. "An acid cell was operated for six months at a current density of 90 [milliamps per square centimeter] and 0.25 v. with no apparent deterioration."

Experiments with sulfuric acid electrolytes were underway in 1963 at both the California Research Corporation and the Surface Processes Research and Development Corporation. PAFCs are almost absent, however, from the papers in George J. Young's two volume compilation of a fuel cell symposia held in 1959 and 1961. In the late 1960s and 1970s, major advances in electrode materials and lingering problems with other types of fuel cells spurred new interest in PAFCs.

3.2 Phosphoric Acid Fuel Cell Technology

Phosphoric acid fuel cells (PAFC) operate at temperatures around 150 to 200 C (about 300 to 400 degrees F). As the name suggests, PAFCs use phosphoric acid as the electrolyte. Positively charged hydrogen ions migrate through the electrolyte from the anode to the cathode. Electrons generated at the anode travel through an external circuit, providing electric power along the way, and return to the cathode. There the electrons, hydrogen ions and oxygen form water, which is expelled from the cell. A platinum catalyst at the electrodes speeds the reactions.

The formation of carbon monoxide (CO) around electrodes can "poison" a fuel cell. One advantage of PAFC cells is that at 200 degrees C they tolerate a CO concentration of about 1.5 percent. Another advantage is that concentrated phosphoric acid electrolyte can operate above the boiling point of water, a limitation on other acid electrolytes that require water for conductivity. The acid requires, however, that other components in the cell resist corrosion.

Hydrogen for the fuel cell is extracted from a hydrocarbon fuel in an external reformer. If the hydrocarbon fuel is gasoline, sulfur must be removed or it will damage the electrode catalyst. Efficiencies of PAFCs average 40 to 50 percent, but this can rise to about 80 percent if the waste heat is reused in a cogeneration system. PAFCs of up to 200 kw capacity are in commercial operation, and units of 11 MW capacity have been tested.

4. WATER TREATMENT FACILITIES

Anaerobic decomposition involves microorganisms that derive energy from metabolizing organic materials to decompose organic waste at WWTFs. In the absence of oxygen the byproducts of their metabolism are carbon dioxide (CO₂) and methane (CH₄) plus trace quantities of other components, such as hydrogen sulfide (H₂S) and organic halides (mostly chlorides). ADG is primarily a mixture of these gasses (60% methane and 40% carbon dioxide). A simplified diagram of the Wastewater Treatment process is shown in Figure.

ADG is generally collected and either used as fuel in boilers to keep anaerobic digesters war, flared off, or, in some cases, used in internal combustion engines to produce electricity. At many WWTFs, ADG is being utilized in efficiently, or not at all. For example, many facilities are located in temperate climates in which the requirement for heat in summer is minimal.

If ADG is released un combusted, it significantly contributes to the greenhouse effect. This occurs principally through emission of methane, which traps at least 10 times as much heat as carbon dioxide. For this reason and for odor control, excess ADG is typically flared (burned) in flame towers, a process that eliminates methane emission. However, flaring is only a partial solution, since ADG combustion generates photo reactive ozone precursors, such as nitrogen oxides and volatile organic components. This designation necessitates installation of control and monitoring technologies, which can be very costly. Fuel cells provide the most effective solution to these problems. They efficiently generate premium quality electricity and much needed thermal energy, while consuming ADG and emitting negligible amount of regulated pollutions. In addition, they permit significant reductions in carbon dioxide emissions compared to flaring. As a result, WWTFs are primary candidates for clean distributed generation and for win-win partnerships between the WWTF operators and utilities.

The PC25C phosphoric acid fuel cell was modified to operate on ADG. This involved modifications to the cell stack assembly, reformer, thermal management system, piping valves, controls, etc. ADG differs from pipeline natural gas in the following ways:

Ø  ADG contains trace quantity of sulfur compounds, typically in the form of hydrogen sulfide and organic compounds, which contain chlorine. Both of these species can react with the catalysts in the reformer system, resulting in deactivation of the catalysts.

Ø  ADG typically contains 60% methane, while natural gas contains in excess of 95%. This lower methane content of ADG results in a higher volumetric flow of gas, which can increase system pressure drops.

These differences require modification of the PC25C, originally designed to operate on natural gas only. These modifications were principally:

Ø  Mechanical components, such as piping and valves, in the reactive gas supply system were modified/ enlarged to accommodate the larger volume flow rates resulting from the use of diluted methane fuel. This modification helped reduce system pressure drops.

Ø  An external gas compressor skid was added to raise the inlet pressure of the ADG to compensate in part for the increased pressure drops of the diluted fuel.

Ø  An external gas processing unit (GPU) was added to remove the hydrogen sulfide contained in the ADG stream. This GPU consists of a specially treated charcoal, which converts the hydrogen sulfide into elemental sulfur and water. The sulfur is absorbed on the charcoal, which is then removed on a periodic basis; the water evaporates into the ADG stream; and the purified gas is fed to the fuel cell.

Ø  A halide absorber was added internally to the PC25C to remove these compounds (mostly chlorides).

Ø  Fuel-to-air ratios over the entire operating range were adjusted within the wider-than-usual boundaries to compensate for broader-than-anticipated methane concentration variations in ADG.

Ø  Additional drains were installed in the facility fuel line to remove large amounts of entrained water periodically blocking ADG supply to the GPU.

Ø  A blower was installed for lower-than-anticipated ADG pressure.

4.1 Fuel cells at Wastewater Treatment Plants:

Wastewater treatment plants that utilize the anaerobic digestion process produce a gas mixture of about 60% methane (CH₄) and 40% carbon dioxide (CO₂), plus ppm levels of hydrogen sulfide and, in some cases, organic halides (mostly chlorides). This gas mixture, called ADG, may be utilized in a fuel cell to produce power and heat. However, the sulfur and halide compounds must be removed to prevent deactivation of certain key components ion the fuel cell. An ADG fuel cell system is shown in the figure

The GPU accepts ADG directly from the anaerobic digesters and delivers a pretreated gas to the modified power module. The GPU, developed by UTC Fuel Cells in cooperation with the United States Environmental Protection Agency, consists of a demister to remove any entrained water and two beds of specially treated charcoal, which convert the hydrogen sulfide (H₂S) into elemental sulfur and water by reacting with air, which is fed separately to the unit. The unit utilizes nonregenerable potassium hydroxide-impregnated redundant activated carbon beds to remove hydrogen sulfides from ADG. The unit is sized to process ADG flows of up to 4,800 scf/hr. The two carbon beds are capable of operating for about six months with ADG containing 200 ppm of H₂S. Each bed contains approximately 1,200 lbs. of carbon. The GPU contains sampling ports so that the H₂S content may be monitored to determine when the beds need to be changed. The sulfur is absorbed on the charcoal, which is then removed on a periodic basis; the water evaporates into the ADG stream; and the sulfur-free gas is fed to the fuel cell. The unit is designed such that the charcoal in one bed may be removed and replaced with fresh charcoal while the second bed is used to continue to purify the ADG. The chemical reaction that takes place in the bed is

H₂S (gas) + O₂ (gas) = H₂O (gas) + S (solid sulfur).

After the ADG exists the GPU, it consists of methane, carbon dioxide, and very low levels of organic halides and water. The methane can be used as a fuel in the power plant; the carbon dioxide merely acts as an inert gas in the system and, therefore, need not be removed inside the fuel cell prior to reaching those components that they can affect. To achieve this removal, a halide adsorption bed is added to the fuel processing stream inside the fuel cell power plant, where it is incorporated into the reactant supply system. Prior to entering this bed, the organic halide compounds are converted, inside the power plant, into inorganic halide compounds. These compounds are absorbed onto the halide bed.

A standard PC25C power module reactant supply system is sized for natural gas with a nominal heating value of 980 to 1,200 BTU/scf (HHV). The modifications required to operate on ADG with nominal heating values of 500 to 700 BTU/scf consists primarily of resizing inlet fuel valves and piping to reduce pressure drop and increase fuel flow capacity. Power module controller settings are tuned to maintain the appropriate level of process fuel, steam, and burner air when running on ADG. Additionally, ADG software modifications are implemented, and a separate natural-gas piping to the reformer start-up burner is provided. This separate piping will supply natural gas during start-up of the fuel cell.

The purified ADG from the GPU flows to the fuel processor, which consists of  a metal vessel containing catalyst. In this vessel the methane in the ADG reacts with steam produced by the fuel cell stack to produce a stream consisting mostly of hydrogen and carbon dioxide; the CO₂ contained in the ADG does not react but passes through as an inert diluent. The hydrogen production reaction is

CH₄ (gas) + 2H₂ (gas) = 4H₂ (gas) + CO₂ (gas).

The hydrogen is fed to the fuel cell stack where it reacts electrochemically with air to produce power, water vapor, and heat. A portion of the product water vapor is condensed into a liquid, vaporized by cooling the fuel cell stack, and then used in the fuel processor to react with the methane. Any hydrogen not utilized in the fuel cell stack (<5%) is combusted to provide the heat required by the fuel processor. The product water and carbon dioxide are exhausted to the ambient air. Any fuel cell heat not used to boil water for the fuel processor is available for use in the WWT process.

The GPU includes the gas analysis unit consisting of a sample pump, regulator, and H­₂S detector cell. The H₂S sensor detects any hydrogen sulfide in the gas entering the fuel cell, and it provides a signal to the fuel cell controller to initiate an alarm or a safe shutdown before damage can occur.The fuel cell stack dc is converted to 480 Vac using a static inverter.

5. APPLICATIONS OF FUEL CELLS

The concept of using fuel cells powered by hydrogen gas provides the ideal solution to pollution in our world. Fuel cells have been developed that can be used for virtually any application requiring electrical power or mechanical energy. They are now classified into three basic categories: portable devices, transportation applications, and stationary power.

Portable devices:

For portable applications such as laptop computers, cameras, and cell phones, direct methanol fuel cells (DMFCs) shoe the most promise as future replacements for batteries. Potentially, they can store over ten times as much energy as a lithium battery, which would translate to longer operating times. Instead of being recharged from “plug-in” ac adapters, these units would get their “charge” from small disposable cartridges of fuel plugged into the device offering total independence from a wall plug for energy. For the DMFCs they biggest hurdles are size and weight versus batteries and the amount of heat radiated. These devices are low efficiency; thus, more heat is produced in the energy conversion process. Technology advancements to address these issues are underway and fuel cells may emerge as the energy source of choice for many portable power applications.

Transportation Applications:

One of the largest efforts in fuel cell development involves the proton exchange membrane (PEM) type fuel cells for use in automobiles. Many demonstration programs have been accomplished and real promise exists for significant levels of automobile production using PEM fuel cells in hybrid vehicles. A key challenge in any fuel cell program is volume of units built. Economy of scale will drastically lower the cost, say proponents, and lower cost will drive demand.

Stationary power:

Stationary power application of fuel cells represents the biggest opportunity for hydrogen to truly impact the world’s environment. At its ultimate stage of deployment, hydrogen fed fuel cells could produce all of the energy needs of an average residence and eliminate the need for many of the world’s fossil-fuel power plants. Currently, molten carbonate fuel cell (MCFC) systems appear to be the leading contender for this application. They have been built and demonstrated at 200 kW levels and are powering dozens of demonstration sites ranging from post offices to personal homes. Many of these sites use existing natural gas lines as the energy source. Natural gas is “reformed” onsite to produce the hydrogen to power the fuel cell. This reforming process does consume electricity and is a key issue in designing cost-effective distributed energy devices that can serve as onsite commercial and residential power plants.

Today a fuel-cell based power system is a very expensive method of producing electricity when the initial costs are amortized into the electricity rate. Plus, the average operating reliability and life of the fuel cell is still an area of concern. All of these issues are being addressed by fuel cell developers and will continue to improve with time. One area of significant advancement is lower-cost power electronics that convert the dc output of the fuel cell to useable ac power. These converters make it easy to adapt the fuel cell power system to any country regardless of voltage or frequency allowing the production of truly “universal” power systems.

 Power:

Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, rural locations, and in certain military applications. A fuel cell system running on hydrogen can be compact and lightweight, and have no major moving parts. Because fuel cells have no moving parts and do not involve combustion, in ideal conditions they can achieve up to 99.9999% reliability. This equates to around one minute of down time in a two year period.

Since electrolyzer systems do not store fuel in themselves, but rather rely on external storage units, they can be successfully applied in large-scale energy storage, rural areas being one example. In this application, batteries would have to be largely oversized to meet the storage demand, but fuel cells only need a larger storage unit (typically cheaper than an electrochemical device).

One such pilot program is operating on Stuart Island in Washington State. There the Stuart Island Energy Initiative has built a complete, closed-loop system: Solar panels power an electrolyzer which makes hydrogen. The hydrogen is stored in a 500 gallon tank at 200 PSI, and runs a ReliOn fuel cell to provide full electric back-up to the off-the-grid residence.

Cogeneration:

Micro combined heat and power (MicroCHP) systems such as home fuel cells and cogeneration for office buildings and factories are in mass production phase. The system generates constant electric power (selling excess power back to the grid when it is not consumed), and at the same time produces hot air and water from the waste heat. MicroCHP is usually less than 5 kWe for a home fuel cell or small business. A lower fuel-to-electricity conversion efficiency is tolerated (typically 15-20%), because most of the energy not converted into electricity is utilized as heat. Some heat is lost with the exhaust gas just as in a normal furnace, so the combined heat and power efficiency is still lower than 100%, typically around 80%. In terms of exergy however, the process is inefficient, and one could do better by maximizing the electricity generated and then using the electricity to drive a heat pump. Phosphoric-acid fuel cells (PAFC) comprise the largest segment of existing CHP products worldwide and can provide combined efficiencies close to 90% (35-50% electric + remainder as thermal) Molten-carbonate fuel cells have also been installed in these applications, and solid-oxide fuel cell prototypes exist.

6.CONCLUSION

When will fuel cells take their rightful place among the prominent energy sources in the 21st century? The benefits of fuel cells are undeniable. The quiet, clean, and reliable nature of their operation and the efficiency with which energy is produced make fuel cells prime candidates for the soon-to-be vacancy atop the energy hierarchy left by fossil fuels. The Clean water for drinking, washing and industrial uses is a scarce resource in some parts of the world now a day, its availability in the future will be even more problematic. So, in addition to increasing the power production of the new fuel cell, the researchers are seeking ways to reduce production costs associated with materials and design configurations. The process may also offer solutions for creating more clean water for both developing and industrial nations. The research team expects to roll out an improved design in one to three years.

7. REFERENCE

Ø From IEEE editions

Ø  William H. Hayt, Jr.: “Engineering Electromagnetics”, McGraw-Hill Book Company.

Ø  J.B. Gupta: “Electric Power Systems”, S.K.Kataria & sons

Ø  S.O.Pillai: “Solid state physics”, New Age International Publications.

Ø  M.V. Deshpande: “Elements of Power System Design”, Wheeler Publishing Company.

Ø  http://www.britannica.com/bcom/eb/article/7/0,5716,108547+1+106048.html

Ø  C.L. Wadhwa: “Generation and Utilization of Electrical Energy”, New Age International Publications.