Transcript Batteries and Fuel Cells

Batteries and Fuel Cells
Portable Electric Energy
The Battery
• A cell consists of two
electrodes of different
metals immersed in a
weak acid
• Multiple cells can be
stacked in series to make a
battery
• The positive terminal is
called the anode and the
negative terminal the
cathode
Connecting Batteries in Series
• Batteries connected
end to end will have a
voltage equal to the
total voltage of the
individual batteries
• Disposable dry cell
batteries have a typical
voltage of 1.5 V
1.5 V
3V
+
+
+
Amp-hours
• The total energy contained within a battery can be described
using Amp-hours
• Example: A battery that can provide 4 A-hrs can generate 4 A
for 1 hour, 2 A for 2 hrs., etc.
• Example: A 12 volt car battery can provide 60 A-hrs. of
energy. How many joules is this?
• Solution: If it drew 60 A of current at 12 V, that would be (60
A) x (12 V) = 720 W. And 1 hr = 3600 s, so (720 W) x (3600
s) = 2.59 x 106 J
• In other words: Energy (J) = (Amp-hours) x (Volts) x (3600)
How a Battery (Cell) Works
• Both electrodes slowly
dissolve in the acid
• At the anode, electrons are
used in chemical reactions
as the metal dissolves
• At the cathode, electrons are
absorbed into the electrode
as the metal dissolves
• The net result is a buildup of
electrons at the cathode
Disposable and Rechargeable
Batteries
• A rechargeable battery can be connected to
an electric current so that dissolved metals
reform on the electrodes
• Examples: lead acid, nickel cadmium,
lithium, etc.
• The chemical reactions that power a
disposable battery cannot be reversed
• Examples: alkaline dry cells, etc.
The Lead Acid Battery
• Two electrodes, one of
lead, the other of lead
dioxide (PbO2) immersed
in sulfuric acid
• Lead ions (Pb++)
dissolve, leaving two
electrons behind
• Two electrons flow
through the circuit and
are used to help lead
dioxide dissolve
Disposable Batteries
• A typical disposable battery
contains a carbon (graphite)
and a zinc electrode
• The electrolyte is a paste of
ammonium chloride
• Disposable batteries may
leak if too much of the zinc
can is dissolved
How Disposable Batteries Work
• Both electrodes generate
electrons when they dissolve
• The cathode generates more
than the anode
• To remove electrons from the
anode, positive ions “plate” or
stick to the anode
• A membrane separates the A+
and B+ ions
• Eventually, positive ions
accumulate near the cathode
and are depleted near the anode
Electric Vehicles
• Electric vehicles use
electric motors powered
by rechargeable batteries
• Both Honda and GM
(among others)
manufacture electric
vehicles
• Electric vehicles have
been around as long as
gas powered cars!
The GM EV-1
How and Electric Vehicle Works
• An electric motor replaces the gas
engine
– Electric motors are extremely
reliable
– During braking, electric motors
can act as generators and
recharge the batteries
• A rechargeable battery pack in the
trunk provides electric power
– Battery packs are heavy and
costly
– They must be replaced
eventually
Electric Motor
Battery Pack
Why Nobody Buys Electric Cars
“The battery challenge is vast. Even with our most
advanced experimental power pack, operating costs in
1998 would be unacceptable to the vast majority of drivers.
Essentially, it’s like asking the customer to buy a car with a
$15,000 gas tank--a $15,000 gas tank that holds the range
equivalent to 3 gallons of gasoline; a 3 gallon tank that
takes 8 hours to refill, compared to a few minutes at a selfservice gas station.”
D. Wilkie, 1994
Comparing Gasoline and
Batteries
Fuel Type
Gasoline
Energy Density Range (miles)
(kJ/kg)
48,000
350+
Lead Acid
110-180
Batteries
NiCad Batteries 200
70-90
Lithium
Batteries
270
540
110-120
Pros and Cons of Electric Cars
• Pros
– Pollution occurs at the
power plant where it
can be more easily
contained
– Lower operating
expenses (repairs,
refueling) than gas
engines
• Cons
– Limited range
– Batteries must be
replaced frequently
– Very expensive
– Same total amount of
pollution (when
including the power
plant that generates
electricity)
Fuel Cells
• In a fuel cell hydrogen is
“burned” by mixing with
oxygen in such a way that
it creates a voltage across
two electrodes
• Only water is produced as
a by-product
• Hydrogen can be fed in
directly or as part of larger
molecules such as methane
(natural gas)
How They Work
• Hydrogen molecules
give up their electrons
to the first electrode
• Electrons pass through
the circuit to the
second electrode
• Electrons are returned
to the molecules when
hydrogen and oxygen
combine to make water
Comments on Fuel Cells
• Fuel cells have been around for 100 years
• Fuel cells can be made to burn other
molecules, such as methane, propane, etc.
• Hydrogen can be extracted from gasoline
before being fed into a fuel cell
• Fuel cells cannot store energy, so they must
be used in conjunction with a storage
battery
Fuel Cells in Electric Cars
• Prototype cars have
been developed that
generate electricity
using fuel cells
• Pros
– Very little pollution
• Cons
– Expensive
– Hydrogen gas is
explosive
Fuel Cells to Replace Batteries
• Fuel cells can be used
to power a laptop
• Micro fuel cells have
been developed that
are small enough to fit
into a cell phone
• Fuel cells weigh less
and last much longer
than rechargeable
batteries
Why are Fuel Cells so
Uncommon?
• Methods are being developed to store
hydrogen in a porous material rather than as
compressed gas
• Fuel cells require expensive catalysts
• How do you pump compressed hydrogen at
a self-service gas station?
• What happens to the hydrogen tank in an
accident?
Flywheels?
• A flywheel (i.e. a heavy
disk) spinning in a
vacuum can store a large
amount of energy
• Electrical energy can be
extracted (and stored)
using magnetic fields
• What happens to a disk
spinning at 100,000 rmp
when you hit a bump?