In the late 1900s, a new type of electrochemical power device emerged that is believed by some sci- entists and engineers to hold promise as an alternative energy source: the fuel cell.
Hydrogen Fuel
The most talked-about fuel cell during the early years of research and development became known as the hydrogen fuel cell.As its name implies, it derives electricity from hydrogen. The hydrogen combines with oxygen (that is, it oxidizes) to form energy and water. There is no pollution, and there are no toxic by-products. When a hydrogen fuel cell “runs out of juice,” all that is needed is a new supply of hydrogen, because its oxygen is derived from the atmosphere.
Instead of combusting, the hydrogen in a fuel cell oxidizes in a more controlled fashion, and at a much lower temperature. There are several schemes for making this happen. The proton exchange membrane(PEM) fuel cellis one of the most widely used. A PEM hydrogen fuel cell generates ap- proximately 0.7 V of dc. In order to obtain higher voltages, individual cells are connected in series. A series-connected set of fuel cells is technically a battery, but the term used more often is stack.
Fuel-cell stacks are available in various sizes. A stack about the size and weight of an airline suit-
7-7 Construction of a silicon photovoltaic cell.
case filled with books can power a subcompact electric car. Smaller cells, called micro fuel cells, can provide dc to run devices that have historically operated from conventional cells and batteries. These include portable radios, lanterns, and notebook computers.
Other Fuels
Hydrogen is not the only chemical that can be used to make a fuel cell. Almost anything that will combine with oxygen to form energy has been considered.
Methanol, a form of alcohol, has the advantage of being easier to transport and store than hy- drogen, because it exists as a liquid at room temperature. Propaneis another chemical that has been used for powering fuel cells. This is the substance that is stored in liquid form in tanks for barbecue grills and some rural home heating systems. Methane, also known as natural gas, has been used as well.
Some scientists and engineers object to the use of these fuels because they, especially propane and methane, closely resemble fuels that are already commonplace, and on which society has devel- oped the sort of dependence that purists would like to get away from. In addition, they are derived from so-called fossil fuelsources, the supplies of which, however great they might be today, are nev- ertheless finite.
A Promising Technology
As of this writing (2006), fuel cells have not yet replaced conventional electrochemical cells and bat- teries. Cost is the main reason. Hydrogen is the most abundant and simplest chemical element in the universe, and it does not produce any toxic by-products. This would at first seem to make it the ideal choice for use in fuel cells. But storage and transport of hydrogen has proven to be difficult and expensive. This is especially true for fuel cells and stacks intended for systems that aren’t fixed to per- manent pipelines.
An interesting scenario, suggested by one of my physics teachers all the way back in the 1970s, is the piping of hydrogen gas through the lines designed to carry methane. Some modification of ex- isting lines would be required in order to safely handle hydrogen, which escapes through small cracks and openings more easily than methane. But hydrogen, if obtained at reasonable cost and in abundance, could be used to power large fuel-cell stacks in common households and businesses. The dc from such a stack could be converted to utility ac by power inverters similar to those used with PV energy systems. The entire home power system would be about the size of a gas furnace.
Quiz
Refer to the text in this chapter if necessary. A good score is 18 correct. Answers are in the back of the book.
1. The chemical energy in a battery or cell (a) is a form of kinetic energy.
(b) cannot be replenished once it is gone.
(c) changes to electrical energy when the cell is used. (d) is caused by electric current.
2. A cell that cannot be recharged is known as (a) a dry cell.
(b) a wet cell. (c) a primary cell. (d) secondary cell.
3. A Weston cell is generally used (a) as a current reference source. (b) as a voltage reference source. (c) as a power reference source. (d) as a fuel cell.
4. The voltage produced by a battery of multiple cells connected in series is (a) less than the voltage produced by a cell of the same composition. (b) the same as the voltage produced by a cell of the same composition. (c) more than the voltage produced by a cell of the same composition. (d) always a whole-number multiple of 1.018 V.
5. A direct short-circuit of a large battery can cause (a) an increase in its voltage.
(b) no harm other than a rapid discharge of its energy. (c) the current to drop to zero.
(d) a physical rupture or explosion.
6. Suppose a cell of 1.5 V delivers 100 mA for 7 hours and 20 minutes, and then it is replaced. How much energy is supplied during this time?
(a) 0.49 Wh (b) 1.1 Wh (c) 7.33 Wh (d) 733 mWh
7. Suppose a 12-V automotive battery is rated at 36 Ah. If a 100-W, 12-V bulb is connected across this battery, approximately how long will the bulb stay aglow, assuming the battery has been fully charged?
(a) 4 hours and 20 minutes (b) 432 hours
(c) 3.6 hours (d) 21.6 minutes 8. Alkaline cells
(a) are cheaper than zinc-carbon cells.
(b) generally work better in radios than zinc-carbon cells. (c) have higher voltages than zinc-carbon cells.
9. The energy in a cell or battery depends mainly on (a) its physical size.
(b) the current drawn from it. (c) its voltage.
(d) all of the above.
10. In which of the following devices would a lantern battery most likely be found? (a) A heart pacemaker
(b) An electronic calculator (c) An LCD wall clock (d) A two-way portable radio
11. In which of the following devices would a transistor battery be the best power choice? (a) A heart pacemaker
(b) An electronic calculator (c) An LCD wall clock (d) A two-way portable radio
12. For which of the following applications would you choose a lithium battery? (a) A microcomputer memory backup
(b) A two-way portable radio
(c) A stand-alone solar-electric system (d) A rechargeable lantern
13. Where would you most likely find a lead-acid battery? (a) In a portable audio CD player
(b) In an uninterruptible power supply (c) In an LCD wall clock
(d) In a flashlight
14. A cell or battery that maintains a constant current-delivering capability almost until it dies is said to have
(a) a large ampere-hour rating. (b) excellent energy capacity. (c) a flat discharge curve.
(d) good energy storage capacity per unit volume. 15. Where might you find a nickel-based battery?
(a) In a satellite
(b) In a portable cassette player (c) In a handheld radio transceiver (d) More than one of the above
16. A disadvantage of mercury cells and batteries is the fact that (a) they don’t last as long as other types.
(b) they have a flat discharge curve.
(c) mercury is destructive to the environment. (d) they need to be recharged often.
17. Which kind of battery should never be used until it dies? (a) Silver-oxide
(b) Lead-acid (c) Nickel-based (d) Mercury
18. The useful current that is delivered by a solar panel can be increased by (a) connecting capacitors in parallel with the solar cells.
(b) connecting resistors in series with the solar cells. (c) connecting two or more groups of solar cells in parallel. (d) connecting resistors in parallel with the solar cells. 19. An interactive solar power system
(a) allows a homeowner to sell power to the electric company. (b) lets the batteries recharge at night.
(c) powers lights, but not electronic devices.
(d) is totally independent from the electric company.
20. An advantage of methanol over hydrogen for use in fuel cells is the fact that (a) methanol is the most abundant element in the universe.
(b) methanol is not flammable.
(c) methanol is a solid at room temperature. (d) methanol is easier to transport and store.
ELECTRIC AND MAGNETIC PHENOMENA INTERACT.MAGNETISM WAS MENTIONED BRIEFLY NEAR THE
end of Chap. 2. Here, we’ll look at it more closely.