Types of Fuel Cells

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Fuel cells are categorized by the type of electrolyte they use: a chemical that conducts charged ions from one electrode to another. In fact, the type of electrolyte determines the kind of chemical reactions that occur in the cell, the type of catalysts required, the temperature range in which the cell operates,

Electric Gircuit (40%-60% Efficiency)

Electric Gircuit (40%-60% Efficiency)

Proton Exchage Membrane

Figure 7.7. Continued

Figure 7.7. Continued and the fuel required. A number of fuel cells are under development, each with its specific advantages, limitations and potential applications.

Currently six systems are being pursued. In addition to PEM, there are

Direct methanol fuel cells, powered by methanol mixed with steam and fed directly to the anode. Methanol is easier to transport and supply, but it is only now beginning to be studied.

Alkaline fuel cells, which use potassium hydroxide in water as the electrolyte. This was the earliest of the fuel cells and was used by NASA since the 1960s for its space program. But it is too easily degraded by carbon dioxide, and is extremely expensive, which means that it is unlikely to be used commercially.

Phosphoric acid fuel cells, which use liquid phosphoric acid as the electrolyte. This cell does not provide sufficient current or power to drive automobiles, and its longer warmup time makes it doubly unsuitable and expensive. It is currently used for small, stationary power generation.

Molten carbonate fuel cells - which are being developed for use in natural gas and coal- based power plants and for military applications. These high--emperature fuel cells operate at 650°C (1200°F) using a molten carbonate/salt mixture as their electrolyte. Although these are highly corrosive, breaking down cell components relatively rapidly, they appear to be highly efficient.

Regenerative fuel cells, which employ electricity from solar power to perform the electrolysis of water that yields the hydrogen and oxygen needed. This fuel cell is being developed by NASA for its special applications.

As Table 7.4 indicates, each of these fuel cells has its unique advantages, applications, and limitations. Of one thing we can be certain—several will become realities and take their place supplying a range of electrical applications. At the moment, PEM and hydrogen is the frontrunner.

With hydrogen projected as the fuel of the future, several questions arise: Where will it come from, will there be a plentiful supply, is it safe, and can it be stored?

Hydrogen is the most abundant element in the universe, but it is not a free element. All hydrogen is bound to chemicals in plants and animals, and is found in fossil fuels—coal, oil, and natural gas, and, of course, bound to oxygen as water. Consequently there are a number of straightforward ways to obtain it, including producing it directly from water via electrolysis and solar and nuclear thermochemical splitting of water by extremely high temperatures. Most of the hydrogen used in the United States currently is produced by steam reforming of natural gas. Thermochemical conversion processes can produce hydrogen from hydrocarbons and biomass-derived gases. Clearly, there is no foreseeable lack of hydrogen. In fact, it could be said to be limitless. The only concerns are which are the most economically sound ways of producing abundant supplies. The long-term goal is to obtain hydrogen from renewable sources such as nuclear power—of which a modified Pebble Bed reactor looks promising, and is being tested by the U.S. Department of Energy. Wind and solar power are also being pursued as additional means of procuring adequate supplies. However, and interestingly enough, producing hydrogen from biomass, for example, also produces large amounts of unwanted carbon monoxide. With researchers in hot pursuit of solutions, a team of chemists at the University of Wisconsin (Madison) recently found that using a gold catalyst reactor removes the carbon monoxide. Because the conventional procedure employs high-temperature steam to react with carbon monoxide, forming CO2, this new gold catalyst procedure should obviate the need for steam, bringing production prices down, and increasing the safety of the process [37].

To the question as to whether hydrogen is a safe fuel, the ready response must be that when handled properly, hydrogen is a safe fuel, and like all fuels, must be treated with respect. Hydrogen can be stored in metal or chemical hydrides, or as a gas or liquid in pressurized tanks at 5000 pounds per square inch (lb/in2; psi). With automobiles, in the event of a collision, the chance of a

TABLE 7.4. Comparison of Fuel Cell Technologies

Operating

Fuel Cell Type Electrolyte Temperature Applications Advantages Disadvantages

TABLE 7.4. Comparison of Fuel Cell Technologies

Operating

Fuel Cell Type Electrolyte Temperature Applications Advantages Disadvantages

Polymer

Solid organic polymer

60-100°C

Electric utility

Solid electrolyte reduces

Low temperature

electrolyte

polyperfluorosulfonic

(140-212°F)

Portable power

corrosion and management

requires expensive

membrane

acid

Transportation

problems

catalysts

(PEM)

Low temperature

High sensitivity to fuel

Quick startup

impurities

Alkaline

Aqueous solution of

90-100°C

Military

Cathode reaction faster in

Expensive removal of

(AFC)

potassium hydroxide

(194-212°F)

Space

alkaline electrolyte, so high

C02 from fuel and

soaked in a matrix

performance

airstreams required

Phosphoric

Liquid phosphoric acid

175-200°C

Electric utility

Up to 85% efficiency in

Requires platinum

acid

soaked in a matrix

(347-392°F)

Transportation

cogeneration of electricity

catalyst

(PAFC)

and heat

Low current and

Can use impure H2 as fuel

power

Large size/weight

Molten

Liquid solution of

600-1000°C

Electric utility

High efficiency

High temperature

carbonate

lithium, sodium, and/or

(1112-1832°F)

Fuel flexibility

enhances corrosion

(MCFC)

potassium carbonates.

Can use a variety of catalysts

and breakdown of

soaked in a matrix

cell components

Solid oxide

Solid zirconium oxide

600-1000°C

Electric utility

High efficiency

High temperature

(SOFC)

to which a small

(1112-1832°F)

Fuel flexibility

enhances breakdown

amount of yttria is

Can use a variety of catalysts

of cell components

added

Solid electrolyte reduces

corrosion and management

problems

Low temperature

Quick startup

fuel leak is less likely than that from a gasoline tank, due to the structure of the storage tanks. Additionally, hydrogen is lighter than air, which would allow it to quickly dissipate should a leak occur. Also, it is far safer than gasoline, which ignites rapidly with a spark. Hydrogen does not produce smoke when burning. Inhalation is the primary cause of death in gasoline fuel fires and explosions. Moreover, a hydrogen fire radiates little heat, and hydrogen is nontoxic and nonpoisonous. As for storage and distribution, as hydrogen demand grows, economies of scale will likely make centralized hydrogen production more cost- effective than will distributed production. But that will require a cost-effective means of transportation and delivery. Currently, hydrogen is transported by road via cylinders, tube trailers, cryogenic tankers, and pipeline. Increased delivery will require high-pressure compressors for gaseous hydrogen and liquefactors for cryogenic hydrogen—both have significant costs and inefficiencies associated with them, which must be reduced before hydrogen can become competitive and widely available.

Delivering bulk hydrogen to hydrogen (gas) stations is one thing; use as a fuel for fuel cells in motor vehicles is quite another. A "fuel cell automobile" is in electric vehicle that uses hydrogen as fuel, rather than a battery, to produce electricity. The fuel cell utilizes a catalyst to perform a chemical process (no moving parts) that, as we have seen, combines hydrogen and oxygen to produce electricity. Hydrogen gas can be safely stored within a car or truck in a pressurized tank, and is supplied to the fuel cell at less pressure than gasoline is to a fuel-injected engine. Fuel cell vehicles are twice as efficient as gasoline-driven vehicles and produce zero tailpipe emissions—other than water. In a direct hydrogen fuel cell vehicle, hydrogen is piped to the fuel cells, where it combines with oxygen from an air compressor. An electrode coated with a catalyst splits the hydrogen into electrons and protons. The movement of the electrons generates electricity, which is sent to a traction inverter module that converts the electric energy into mechanical energy that turns the wheels. This is the flow plan that drives the Ford Focus FCV and P2000. However, the challenge facing the marketing of mass-produced cars is to provide hydrogen to the stack module. Two alternatives appear possible: direct, onboard storage of hydrogen, and/or onboard reformation of hydrogen from liquid fuels such a methanol or gasoline. Direct hydrogen provides the greatest environmental benefit, and the simplest fuel cell system, but requires the development of an extensive hydrogen infrastructure that would go head-to-head with existing gas stations owned by the oil companies, who are not ready to pass into history. Onboard reformation of hydrogen would use a liquid fuel option with an established fueling infrastructure, but that requires additional onboard complex chemical fuel processing. Both require additional development to determine final cost estimates. Furthermore, safety is of primary concern. Several safety systems are available. The Ford Focus incorporates four hydrogen sensors: two in the trunk, one under the hood, and another in the passenger section. If a leak is detected, the car simply shuts down. In addition, eight small fans continually vent the car while driving, filling at a hydrogen gas station, and when the fuel door is opened.

Again, at this time, the major deterrent to mass marketing of a fuel cell vehicle is the lack of infrastructure. A recent study by DOE/Ford found that factory-built hydrogen refueling stations capable of supporting 100 cars could produce hydrogen that is cost-competitive with gasoline. These hydrogen refueling stations would produce hydrogen by steam reforming of natural gas, or via electrolysis of water, and would utilize existing natural gas and electric power infrastructures. An initial hydrogen infrastructure based on on-site natural gas reformers could account for 10-15% of all conventional filling stations in the country today. This small percentage would be sufficient to support mass production of direct hydrogen fuel cell cars. As is evident, nothing is simple. A great deal of planning and money and public confidence will be needed for transition to a hydrogen economy. Until these long-term solutions are sorted out, hybrid vehicles can (will) serve as a bridge to reduce emissions and ease dependence on fossil fuels.

What is a hybrid? Any vehicle is a hybrid when it combines two or more sources of power. Most hybrid cars run off of a rechargeable battery and gasoline. Essentially, it works this way. In Toyota's Prius, turning on the ignition activates the electric generator/starter, which cranks up the internal-combustion engine (ICE). When the ICE is warm, it automatically shuts off. The electric motor is now operating, and will remain in all-electric mode until 15 mph. If the car remains at low speed, it will effectively be an electric car with no gasoline being used, and no tailpipe emissions. Above 15 mph and into cruising speed, the gasoline engine powers the vehicle, and provides power to the battery for later use. All the while, the onboard computer is making decisions about when to go to gasoline and when to electric. When the car stops at a traffic light, both the gasoline engine and the electric motor shut off to save energy, but the battery continues to power auxiliary systems, such as air conditioning and dashboard panel. This technology can reduce emissions by 80%, and allows traveling for 400-500 miles on a tank of gasoline.

Evidently there are many ways to design a hybrid system; every car manufacturer has several designs in the works. In some hybrids, the batteries power electric motors, which drive all four wheels. In others, the engine may drive the rear wheels while the batteries power the electric motors running the front wheels. In yet others, the batteries or other electric source acts as auxiliary power. The Ford model "U" concept car is propelled by an ICE that's optimized to run on hydrogen fuel instead of gasoline. Because there are no carbon atoms in the fuel, combustion produces no hydrocarbons or CO2 emissions. It is entirely possible that with further refinement, many of these hybrids will have air leaving their tailpipes that is actually cleaner than the air entering the engine.

Of course, fuel cells are not limited to automobiles. Manhattan Scientifics, Inc., has developed a fuel-cell-powered mountain bike that uses hydrogen and air as fuel and emits only water vapor as a waste product. The Hydrocycle has a range of 40-60 miles on a flat surface, and can achieve speeds of up to 18 mph. Because a fuel cell stack powers its electric motor, it's extremely quiet—

with no moving parts, and does not need to be recharged, only refilled. Here again, the lack of hydrogen gas stations makes this more than an inconvenience. Nevertheless, the company sees major markets for the bike in Asian cities, where bicycles and gas-powered scooters make up a major portion of vehicle traffic, and would help reduce the severe air pollution of many Asian cities.

It remains only for the future, and not too far into it at that, for computers, cars, buses, home appliances, and industry to be powered by hydrogen fuel cells.

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Hybrid Cars The Whole Truth Revealed

Hybrid Cars The Whole Truth Revealed

Hybrid Cars! Man! Is that a HOT topic right now! There are some good reasons why hybrids are so hot. If you’ve pulled your present car or SUV or truck up next to a gas pumpand inserted the nozzle, you know exactly what I mean! I written this book to give you some basic information on some things<br />you may have been wondering about.

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