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Hydrogen is the most plentiful element not only on Earth but also in the universe, accounting for 90 percent of the universe by weight. However, it is not commonly found in its pure form, since it readily combines with other elements and is most commonly found in combination with oxygen in water, and in organic matter including living plants, petroleum, coal, natural gas and other hydrocarbon compounds. The great attraction of hydrogen is that, once isolated, it is a clean burning fuel that produces neither carbon dioxide (a greenhouse gas) nor toxic emissions and can be used for electricity production, transportation, and other energy needs.
A hydrogen economy, in which this one gas provides is the source of all energy needs, is often touted as the long-term solution to the environmental and security problems associated with fossil fuels. However, before hydrogen can be used as fuel on a global scale we must establish cost effective means of:
Producing large volumes of the gas cleanly
Currently hydrogen is prepared by electrolysis or high temperature reforming of coal or hydrocarbons. Many of the processes can create substantial pollution. For hydrogen to be pollution free, the means of preparation must also be pollution free.
Storing and distributing the gas safely.
Hydrogen is the lightest element and its low density complicates the storage and distribution issue, as does its hydrogen's wide explosive range and extremely low ignition energy.
It has a high energy content per weight (nearly 3 times as much as gasoline), but its energy density is low under atmospheric conditions. The volumetric energy density can be increased by storing the hydrogen under elevated pressure or storing it at extremely low temperatures as a liquid and it can also be adsorbed into metal hydrides. Hydrogen is highly flammable; it only takes a small amount of energy to ignite it and make it burn. It also has a wide flammability range, meaning it can burn when its concentration in air is between 4 and 74 percent by volume. Lastly, it burns with a pale-blue, almost-invisible flame, making hydrogen fires difficult to see.
Hydrogen can be used in modified existing equipment such as reciprocating engines, turbines and boilers with significant improvement in emissions performance. Since the fuel contains no carbon, its combustion results in no carbon monoxide or carbon dioxide although it can result in some nitrogen oxides under certain conditions. However this attraction is insufficient to compensate for the cost of production and distribution. Most of the hydrogen produced today is consumed on site, such as at an oil refinery, and is not sold on the market. For large-scale production, hydrogen costs $0.32/lb if it is consumed on site. When hydrogen is sold on the market, the cost of liquefying the hydrogen and transporting it to the user must be added to the production cost. This can increase the selling price to $1.00-1.40/lb for delivered liquid hydrogen. Some users who require relatively small amounts of very pure hydrogen (such as the electronics industry) may use electrolyzers to produce high-purity hydrogen at their facilities. The cost of this hydrogen, which depends on the cost of the electricity used to split the water, is typically $1.00-$2.00/lb. The ultimate goals are to produce cost-effective hydrogen from renewable energy sources and to make it readily available for widespread use as a clean energy carrier and fuel. To achieve this, scientists must develop advanced technologies to safely produce, store, transport, use, and detect hydrogen.
However the attractions of the hydrogen economy are enough to stimulate interest in finding solutions to all these problems.
Imagine an economy in which sunlight is used to form hydrogen and oxygen from water and the safely stored elements are then transported to where they are needed and recombined to form water while at the same time producing power cleanly! Each of these steps can be done now! It only remains to resolve safety and cost issues.
The cleanest way of using hydrogen and oxygen to produce power is by the use of fuel cells. The technology is over 150 years old since the first fuel cell was demonstrated by Sir William Grove in 1839. Grove used porous platinum electrodes and sulfuric acid as the electrolyte bath. William White Jaques later substituted phosphoric acid in the electrolyte bath and was the person who coined the term 'fuel cell.' Significant fuel cell research was done in Germany during the 1920's that laid the groundwork subsequent development of carbonate cycle and solid oxide fuel cells. Since the 1960s, NASA has been using alkaline fuel cells to provide onboard electrical power for spacecraft.
Fuel cells are electro-chemical devices that operate at a high level of efficiency with little noise or air pollution. There are many potential applications for them, including electricity generation in stationary applications and provision of motor force for a new generation of transportation vehicles. All fuel cells operate on the same principle, in that they convert chemical energy directly into electricity and heat, rather than oxidize (burn) a fuel. In most, but not all fuel cells, the source of the fuel's chemical energy is hydrogen. In some cases, the fuel may need to be processed, or reformed before it can be used in the fuel cell.
Source: US DOE, Office of Energy Efficiency and Renewable Energy
Fuel cells consist of an electrolyte material that is sandwiched in between two thin electrodes (porous anode and cathode). The input fuel passes over the anode and oxygen passes over the cathode where they are dissociated catalytically into ions and electrons. The electrons go through an external electrical circuit to provide power while the ions move through the electrolyte toward the oppositely charged electrode. At the electrode, ions combine to create by-products, primarily water.
There are several different kinds of fuel cell, the characteristics of which are summarized in the table below:
Source: Renewable Energy Policy Project
Source: Renewable Energy Policy Project
Alkaline Fuel Cell (AFC)
Alkaline fuel cells operate on compressed hydrogen and oxygen and generally use an aqueous solution of potassium hydroxide as the electrolyte. Because they produce potable water in addition to electricity, they are a logical choice for spacecraft and NASA selected them for the Space Shuttle fleet, as well as the 1960's Apollo program, mainly because of power generating efficiencies that approach 70 percent. A major drawback, however, is that alkali cells need very pure hydrogen or an undesirable chemical reaction forms a solid carbonate that interferes with chemical reactions inside the cell. Since most methods of generating hydrogen from other fuels produce some carbon dioxide, this need for pure hydrogen has slowed work on alkaline fuel cells in recent years. Another drawback is the need for large amounts of a costly platinum catalyst. However, several companies are examining ways to reduce costs and improve the cells' versatility for transport applications.
Phosphoric Acid Fuel Cell (PAFC)
PAFCs have been under development for more than 20 years and is the most mature fuel cell technology in terms of system development and commercialization. The electrolyte, is contained in a teflon bonded silicon carbide matrix and the matrix pore structure preferentially retains the acid through capillary action. Some acid may be lost, entrained in the fuel or oxidant streams, and addition of acid may be required after many hours of operation. Platinum catalyzed, porous carbon electrodes are used on both the fuel (anode) and oxidant (cathode) sides of the electrolyte. One issue for phosphoric acid fuel cells is that if the source of its hydrogen fuel is reformed gasoline, sulfur must be removed from the fuel entering the cell or it will damage the electrode catalyst.
Molten Carbonate Fuel Cell (MCFC)
MCFCs use a molten carbonate salt mixture as the electrolyte, which is suspended in a ceramic matrix. The anode is a nickel-chromium alloy, and the cathode is a lithium-doped nickel oxide. High-temperature molten carbonate fuel cells can extract hydrogen from a variety of fuels using either an internal or external reformer. They are also less prone to carbon monoxide 'poisoning' than lower temperature fuel cells, which makes coal-based fuels more attractive for this type of fuel cell. Demonstration units have produced up to 2 MW and designs exist for units of 50 to 100 MW capacity. Two major difficulties with molten carbonate technology put it at a disadvantage compared to solid oxide cells. One is the complexity of working with a liquid electrolyte rather than a solid. The other stems from the chemical reaction inside a molten carbonate cell. Carbonate ions from the electrolyte are used up in the reactions at the anode, making it necessary to compensate by injecting carbon dioxide at the cathode. In addition, the electrolyte used in molten carbonate fuel cells is highly corrosive, limiting some of it potential applications.
Solid Oxide Fuel Cell (SOFC)
SOFCs use a ceramic, solid-phase electrolyte which reduces corrosion considerations and eliminates the electrolyte management problems associated with the liquid electrolyte fuel cells. The solid oxide fuel cell is based upon the use of a solid ceramic as the electrolyte. The preferred electrolyte material is dense yttria-stabilized zirconia. It is therefore a solid state device that shares certain properties and fabrication techniques with semi-conductor devices. The anode is a porous nickel/zirconia cermet while the cathode is magnesium-doped lanthanum manganate. In development cells and small stacks, the solid oxide fuel cell has demonstrated 0.6V/cell at about 232 A/ft2. Lifetimes in excess of 30,000 hours for single cells have been demonstrated as have a number of heat/cool cycles. CO does not act as a poison and can be used directly as a fuel. The solid oxide fuel cell is also the most tolerant of any fuel cell type to sulfur and can tolerant several orders of magnitude more sulfur than other fuel cells. Because of its high operating temperature, the SOFC cell requires a significant start-up time. Since SOFCs utilize both hydrogen and carbon monoxide fuel inside the cell, they can readily operate on hydrocarbon fuels such as coal gas, gasoline, diesel fuel, jet fuel, alcohol, and natural gas. The efficiency of the solid oxide fuel cell used in CHP applications will be higher than the polymer electrolyte fuel cells for two major reasons. The first reason is that the hydrocarbon fuel is reformed into hydrogen and carbon monoxide fuel largely inside the cell. This results in some of the high temperature waste heat being recycled back into the fuel. The second reason is that air compression is not required. Especially on smaller systems, this results in a higher amount of net electricity being produced and quieter operation. Because of the high temperatures that the solid oxide fuel cell must run, they may not be practical for sizes much below 1,000 watts or when portable applications are involved.
Solid Oxide-Hybrid Fuel Cell Power Systems
A recent development in high temperature stationary fuel cell power plants is the coupling of a microturbine generator with a high-pressure, natural gas-fueled SOFC. High pressure waste heat from solid oxide fuel cell is routed into a microturbine, generating 10% or more additional power than if the exhaust gas energy had not been recaptured. In a recent test by Siemens-Westinghouse, the output of a 200 kW solid oxide fuel cell was boosted to 220 kW through use of a microturbine hybrid configuration. A new configuration using higher gas pressures and a 50 kW gas turbine is expected to boost output to 250 kW. These systems are to 55-60% efficient in converting the energy in natural gas into power, better than the current 50% efficiency of natural gas turbines. According to Siemens-Westinghouse, hybrid solid oxide fuel cells may have the potential to reach 70% efficiency as hybrid technology improves.
Proton Exchange Membrane (PEM)
The PEM Fuel Cell offers an order of magnitude higher power density than any other fuel cell system, with the exception of the advanced aerospace alkaline fuel cell, which has comparable performance. The proton exchange membrane can operate on reformed hydrocarbon fuels, with pretreatment, and on air. The use of a solid polymer electrolyte eliminates the corrosion and safety concerns associated with liquid electrolyte fuel cells. The anode and cathode are prepared by applying a small amount of platinum black to one surface of a thin sheet of porous, graphitized paper which has previously been wet-proofed with Teflon. Platinum requirements are currently 0.60 oz/kW. Improvements in proton exchange membrane performance can reasonably be expected to reduce platinum requirements to 0.035 oz/kW or about $2/kW. Its low operating temperature provides instant start-up and requires no thermal shielding to protect personnel. About 50% of maximum power is available immediately at room temperature. Full operating power is available within about 3 minutes under normal conditions. Recent advances in performance and design offer the possibility of lower cost than any other fuel cell system.
Regenerative Proton Exchange Membrane-based fuel cells
Properly designed, a PEM fuel cell can be run in reverse, acting as an electrolyzer. This dual-function system is known as a reversible or unitized regenerative fuel cell (URFC). A regenerative fuel cell uses water and electrical energy as inputs, electrolyzes the water, and emits hydrogen and oxygen as outputs. These units are currently in the prototype stage, with novel applications such as creating hydrogen during the day with solar electric power, then using the hydrogen fuel at night to power a hybrid solar/hydrogen fuel cell high-altitude unmanned reconnaissance airplane. The URFC is an excellent energy source in situations where weight is a concern because it is lighter than a separate electrolyzer and generator system. In 1995, the regenerative fuel cell, coupled with lightweight hydrogen storage, had by far the highest energy density of any chemical battery--about 450 watt-hours per kilogram.
Direct Methanol Fuel Cells (DMFC)
DMFCs are similar to the proton exchange membrane cells in that they both use a polymer membrane as the electrolyte. However, in the DMFC, the anode catalyst itself draws the hydrogen from the liquid methanol, eliminating the need for a fuel reformer. DMFCs are being considered for a number of applications, including transport, portable power including cellular phones and laptop computers, auxiliary power for instrumentation and vehicles, and as a battery replacement for combat personnel and for battlefield applications.
Direct Carbon Fuel Cells
This type of fuel cell is based on a process called direct carbon conversion, developed at Lawrence Livermore National Laboratory, in which carbon particles are joined in an electrochemical process with oxygen molecules to produce CO2 and electricity. The carbon fuel can come from any type of hydrocarbon, including coal, lignite, natural gas, petroleum, petroleum, coke, and biomass. Because it is carbon, and not hydrogen, that fuels this cell, hydrogen is released as a byproduct of the cell reaction and could potentially be captured for use in a separate hydrogen-powered fuel cell. The technology uses aggregates of extremely fine carbon particles, from 10 to 1,000 nanometers in diameter, distributed in a mixture of molten lithium, sodium, or potassium carbonate at 750-850°C.16 Total cell efficiencies are projected to be 70-80%, with power generation in the 1 kW/m2 range, sufficient for practical applications. The carbon fuel particles can be produced through pyrolysis of hydrocarbons, a thermal decomposition method well-known as the source of carbon black for tires, ink, and other applications in manufacturing industries. While the concept has been successfully demonstrated with a 3 W cell, this technology is still in the experimental phase of development. Because this is a high-temperature cell, it would be best suited for stationary applications, particularly in combination with CHP utilizing the waste heat energy.
The 2002 employment figures for the US fuel cell industry have been estimated (Fuel Cells at the Crossroads, 2002) at approximately 4,500 to 5,500. Of these,
In the study referred to above, three scenarios were studied for the North American fuel cell market development through 2021: a base case, a high capitalization case, and a low capitalization case. The base case reflected current expectations for market development; the high capitalization case, a more optimistic outlook for investment with market development accelerating 2 to 3 years in the transportation sector, 1 to 2 years in the stationary sector and 1 year in the portable sector; the low capitalization case reflected a more pessimistic outlook with delays in the market development of 5 to 7 years in the transportation sector, 2 to 3 years in the stationary sector, and 1 to 2 years in the portable sector.
The study found that as many as 189,000 jobs may be created by 2021 as a result of the fuel cell industry. Of these, roughly 75,000 would be directly associated with the industry and the remaining 113,000 would be indirectly associated with the industry. The estimate for indirect job creation was based upon a study conducted by Price Waterhouse in Canada, which found that applying a multiplier of 2.5 to direct fuel cell employment derives a reasonable estimate of total job creation for the Canadian fuel cell industry. The results are presented in the following Figures.
Source: Fuel Cells at the Crossroads
Source: Fuel Cells at the Crossroads
References and Useful Links:
California Hydrogen Business Council
California Fuel Cell Partnership
Fuel Cell Today
How stuff works
National Fuel Cell Research Center
US DOE Fuel Cell Technologies Program/a>
US DOE, National Hydrogen Roadmap, November 2002
US Fuel Cell and Hydrogen Energy Association
Fuel Cell Europe