Transcript Fuel cells

Fuel cells
• An electrochemical conversion device
• Chemical reactions cause electrons (current)
to flow
• Requires a fuel, an oxidant and an electrolyte (
a substance that contains free ions and acts as
a conductor)
• Typical type of fuel cell is called a proton
exchange membrane fuel cell (PEMFC)
Hydrogen Fuel Cells
• Clean-only emission is water
• Expensive to produce
• Highly efficient-in an automobile, efficiencies
of converting fuel energy to mechanical
energy of 60% could be achieved, almost
double the current efficiencies
• Hydrogen itself has issues as a fuel source
Issues with Hydrogen
• Abundant in nature, but not a freely available
fuel
• Must be unbound from compounds
• Currently obtained via steam reforming
– Steam and a nickel catalyst react, producing H
– Need steam at very high temperatures, 1600F
• In the future, H is anticipated to be produced
by the electrolysis of water, requiring large
amount of water and electricity
Electrolysis
• Pass an electrical current through water and
obtain H
• Pass a direct current from a battery or other DC
power supply through a cup of water (salt water
solution increases the reaction intensity making it
easier to observe).
• Using platinum electrodes, hydrogen gas will be
seen to bubble up at the cathode, and oxygen will
bubble at the anode.
• Choice of the electrode is critical, you do not
want a metal that will react with oxygen
Issues with Hydrogen
• Storage-occurs in gas form at room temperature,
hard to contain
• As a liquid, it can be stored, but needs
temperatures of -253 C.
– As a liquid, its energy density increases 1000 times
– In principle, could replace gasoline as a liquid fuel, but
not practical at this time
• One solution is to store it as a metallic hydride
(the negative ion of Hydrogen in a compound
with another element) at room T.
Issues with H
• Highly explosive
– Forms a volatile mixture with air
• A mixture of 4-75% of H in air is explosive,
compared with natural gas which is only
explosive in a range of 5-15% concentration in air
• Ignition energy is small, needing only 2 x 10-5 J
(basically a spark of static electricity can ignite H)
• Only good news is its low density means if there
is a H leak, it disperses quickly
Hydrogen
• Hindenburg disaster
• Hindenburg was a
German passenger airship
(zeppelins) built for
transatlantic air flight.
• Filled with Hydrogen
• Something caused
ignition of the Hydrogencause is debatable
• 36 fatalities out of 79
people onboard
Alchohol
• Use methanol or ethanol as a fuel
– Already gone over ethanol
• Methanol is already in use at Indy 500 race
– Proven that no significant loss of performance is
experienced (though they are in the process of switching
to ethanol)
• About ½ the energy content of gasoline
• Produces only CO2 and water
– Some nitrogen oxides produced in the engine
• Can be manufactured from re-newable sources
(biomass for example)
• Technologies exist now.
Disadvantages
• Very dangerous
– Burns with no visible flame-needs a colorant
added
– Fumes are toxic
• CO2 is a greenhouse gas
• Currently made mostly from natural gas-a
non-renewable fossil fuel
• Possibly more corrosive than ethanol to
engine parts
Use in liquid fuel cells
• Another use is as a input to a
liquid feed fuel cell
• In these cells, Methanol
replaces hydrogen
• Methanol has a much higher
energy density and is easier to
store than H
• Current methanol fuel cells
produce power too low for
vehicles, but can be used in
cell phones, laptops etc
• Advantage is that they store
lots of power in a small space,
which they over a long period
of time
Environmental effects of energy
production
• All of our energy producing mechanisms have
some effect on the environment
– Production of waste products pollutes air, water
and ground
– Disruptions to local ecosystems
• Our job is to understand and mitigate these
effects to the best of our ability
• Philosophy : If it hurts (the environment)
when you do that, don’t do that!
Air pollution
• If its in the air, its in your body
• Components of the Earth’s Atmosphere:
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Nitrogen
78.08%
Oxygen
20.95%
Argon
0.93%
Also small amounts of Neon, Helium, Krypton,& Hydrogen
• In addition, there are compounds whose
concentrations vary with height: water vapor, carbon
dioxide, methane, carbon monoxide, ozone, ammonia
• These are naturally occurring concentrations, any
additional influx or destruction of these compounds
via human beings alters the system.
Profile of the Earth’s atmosphere
Atmospheric profile
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Exosphere
– From 500–1,000 km (310–620 mi; 1,600,000–
3,300,000 ft) up to 10,000 km (6,200 mi;
33,000,000 ft), contain free-moving particles that
may migrate into and out of the magnetosphere
or the solar wind.
Exobase
–
Also known as the 'critical level', it is the lower
boundary of the exosphere.
Ionosphere
– The part of the atmosphere that is ionized by
solar radiation stretches from 50 to 1,000 km (31
to 620 mi; 160,000 to 3,300,000 ft) and typically
overlaps both the exosphere and the
thermosphere. It plays an important part in
atmospheric electricity and forms the inner edge
of the magnetosphere. Because of its charged
particles, it has practical importance because it
influences, for example, radio propagation on the
Earth. It is responsible for auroras.
Thermopause
–
The boundary above the thermosphere, it varies
in height from 500–1,000 km (310–620 mi;
1,600,000–3,300,000 ft).
Atmospheric profile
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Thermosphere
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From 80–85 km (50–53 mi; 260,000–280,000 ft)
to over 640 km (400 mi; 2,100,000 ft),
temperature increasing with height. Although the
temperature can rise to 1,500 °C (2,730 °F), a
person would not feel warm because of the
extremely low pressure. The International Space
Station orbits in this layer, between 320 and 380
km (200 and 240 mi).
Mesopause
–
The temperature minimum at the boundary
between the thermosphere and the mesosphere.
It is the coldest place on Earth, with a
temperature of −100 °C (−148.0 °F; 173.1 K).
Mesosphere
–
From the Greek word meaning middle. The
mesosphere extends from about 50 km (31 mi;
160,000 ft) to the range of 80–85 km (50–53 mi;
260,000–280,000 ft). Temperature decreases with
height, reaching −100 °C (−148.0 °F; 173.1 K) in
the upper mesosphere. This is also where most
meteors burn up when entering the atmosphere.
Atmospheric profile
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Stratopause The boundary between the mesosphere and the
stratosphere, typically 50 to 55
Stratosphere
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From the Latin word "stratus" meaning spreading
out. The stratosphere extends from the troposphere's
7–17 km (4.3–11 mi; 23,000–56,000 ft) range to about
51 km (32 mi; 170,000 ft). Temperature increases with
height. The stratosphere contains the ozone layer, the
part of the Earth's atmosphere which contains
relatively high concentrations of ozone. It is mainly
located in the lower portion of the stratosphere from
approximately 15–35 km above Earth's surface,
though the thickness varies seasonally and
geographically.
Ozone Layer
–
Though part of the Stratosphere, the ozone layer is
considered as a layer of the Earth's atmosphere in
itself because its physical and chemical composition is
far different from the Stratosphere. Ozone (O3) in the
Earth's stratosphere is created by ultraviolet light
striking oxygen molecules containing two oxygen
atoms (O2), splitting them into individual oxygen
atoms (atomic oxygen); the atomic oxygen then
combines with unbroken O2 to create O3. O3 is
unstable (although, in the stratosphere, long-lived)
and when ultraviolet light hits ozone it splits into a
molecule of O2 and an atom of atomic oxygen, a
continuing process called the ozone-oxygen cycle. This
occurs in the ozone layer, the region from about 10 to
50 km (33,000 to 160,000 ft) above Earth's surface.
About 90% of the ozone in our atmosphere is