انت هنا الان : شبكة جامعة بابل > موقع الكلية > نظام التعليم الالكتروني > مشاهدة المحاضرة

خلايا الوقود والبطاريات

As early as 1839, William Grove discovered the basic operating principle

of fuel cells by reversing water electrolysis to generate electricity from

hydrogen and oxygen. The principle that he discovered remains

unchanged today.

A fuel cell is an electrochemical “device” that continuously converts

chemical energy into electric energy (and some heat) for as long as fuel

and oxidant are supplied.

Fuel cells therefore bear similarities both to batteries, with which they

share the electrochemical nature of the power generation process, and to

engines which — unlike batteries — will work continuously consuming a

fuel of some sort. Here is where the analogies stop, though. Unlike

engines or batteries, a fuel cell does not need recharging, it operates

quietly and efficiently, and — when hydrogen is used as fuel — it

generates only power and drinking water. Thus, it is a so-called zero

emission engine. The thermodynamics of the electrochemical power

generation process are analyzed, where fuel cells are compared to thermal

engines. Thermodynamically, the most striking difference is that thermal

engines are limited by the Carnot efficiency while fuel cells are not.

In the transportation sector, fuel cells are probably the most serious

contenders to compete with internal combustion engines (ICEs). They are

highly efficient because they are electrochemical rather than thermal

engines. Hence, they can help to reduce the consumption of primary

energy and the emission of CO2. What makes fuel cells most attractive

for transport applications is the fact that they emit zero or ultralow

emissions. And this is what mainly inspired automotive companies and

other fuel cell developers in the 1980s and 1990s to start developing fuelcell-

powered cars and buses. Leading developers realized that although

the introduction of the three-way catalytic converter had been a

milestone, keeping up the pace in cleaning up car emissions further was

going to be very tough indeed. After legislation such as California’s

Zero Emission Mandate was passed, people initially saw battery-powered

vehicles as the only solution to the problem of building zero emission

vehicles. However, the storage capacity of batteries has turned out to be

unacceptable for practical use because customers ask for the same drive

range that they are accustomed to with internal combustion engines. In

addition, the battery solution is unsatisfactory for another reason:

With battery-powered cars the location where air pollution is generated is

merely shifted back to the electric power plant that provides the

electricity for charging. Once this was understood, people began to see

fuel cells as the only viable technical solution to the problem of carrelated

pollution.

Unfortunately, public perception of fuel cells subsequently became

blurred, and all sorts of miracles were expected from this fledgling new

motor. It was supposed to make us entirely independent of fossil fuels

(since “it only needs hydrogen”), and undoubtedly many still believe that

fuel-cell-powered cars will run on a tank full of water.

When the first fuel-cell-powered buses rolled out of the labs of Ballard

Power Systems, it soon became clear that buses would make the fastest

entry into the market because the hydrogen storage problem already had

been solved. The prospects of fuel-cell-powered vehicles are fully

discussed in Chapter 10; the fueling issue, particularly for cars.

Clearly, the automotive market is by far the largest potential market for

fuel cells. When developers started doing their first cost calculations, they

realized they were in for steep competition against improved internal

combustion engines, hybrid cars, and other possible contenders. The main

competitors of fuel cell powered cars.

A whole family of fuel cells now exists that can be characterized by the

electrolyte used — and by a related acronym as. All of these fuel cells

function in the same basic way. At the anode, a fuel (usually hydrogen) is

oxidized into electrons and protons, and at the cathode, oxygen is reduced

to oxide species. Depending on the electrolyte, either protons or oxide

ions are transported through the ion-conducting but electronically

insulating electrolyte to combine with oxide or protons to generate water

and electric power.

the fuel cells that are currently undergoing active development.

Phosphoric acid fuel cells

(PAFCs) operate at temperatures of 200°C, using molten H3PO4 as an

electrolyte. The PAFC has been developed mainly for the medium-scale

power generation market, and 200 kW demonstration units have

now clocked up many thousands of hours of operation. However, in

comparison with the two low temperature fuel cells, alkaline and proton

exchange membrane fuel cells (AFCs, PEMFCs), PAFCs achieve

only moderate current densities.

The alkaline fuel cell, AFC, has one of the longest histories of all fuel cell

types, as it was first developed as a working system by fuel cell pioneer

F.T. Bacon since the 1930s. This technology was further developed for

the Apollo space program and was key in getting people to the moon. The

AFC suffers from one major problem in that the strongly alkaline

electrolytes used (NaOH, KOH) adsorb CO2, which eventually reduces

electrolyte conductivity. This means that impure H2 containing CO2

(reformate) cannot be used as a fuel, and air has to “scrubbed” free of

CO2 prior to use as an oxidant in an AFC. Therefore, the AFC has so far

only conquered niche markets, for example space applications (the

electric power on board the space shuttle still comes from AFCs).

Some commercial attempts has been made to change this. Most notably,

ZETEK/ZEVCO started in the mid-1990s to reexamine the AFC

technology developed by ELENCO, a Belgian fuel cell developer that

had previously gone into bankruptcy. A number of ZETEK’s activities

attracted extensive publicity.

In the late 1990s, ZETEK presented a so-called fuel-cell-powered London

taxi. Little is known about the technology of the engine in this vehicle.

However, the AFC employed had a power range of only 5 kW, which

means it cannot be the main source of power and merely served as a

range extender to some onboard battery. Other recent activities based on

AFC technology include the construction of trucks (by ZEVCO) and

boats (eating GmbH). A big advantage of the AFC is that it can be

produced rather cheaply.

This may help this technology penetrate the highly specialized market for

indoor propulsion systems, such as airport carrier vehicles, and possibly a

number of segments in the portable sector.