Sources of Energy

1
Sources of Energy

• A favorite form of energy is electricity
• Where does electricity come from?
• Even though electricity is a very useful form of
  energy, there are very few direct sources of
  electrical energy on earth. (One example is a
  lightning storm.)
• Electricity is really a secondary energy source,
  which we get by converting another type of energy
  into it.
• The original source of energy can be
• Nuclear
• Wind
• Sun
• Hydrodynamic
• Chemical energy

2
Current Energy System

• Whats wrong with our current energy system?
• The current world energy consumption is 13 TW, or
  13 trillion watts.
• This number is HUGE. 3000 Niagara Falls worth of
  energy.
• Most (85) of that energy is converted from
  chemical energy.
• Most of the chemical energy is coming from the
  burning of fossil fuels oil, gas, and coal.
• Burning fossil fuels generates carbon dioxide,
  CO2.
• Lets examine a gallon of gasoline.
• Each gallon of gasoline generates over 1000
  gallons of CO2 gas at atmospheric pressure.
  Thats more than 17 pounds of CO2.
• So every 100 gallons burned creates nearly TON
  (2000 lbs) of CO2.

3
Renewable sources currently make up a small
percentage of US energy
4
Carbon Dioxide Emissions

• Why is CO2 a problem?
• All of the fossil fuels that we are burning lead
  directly to carbon dioxide.
• Most of this carbon dioxide is being poured
  directly into the atmosphere, where it adds to
  the existing CO2 levels.
• The CO2 concentrations in the earths atmosphere
  have already risen by over 25 in the past
  century.
• CO2 is a greenhouse gas. Increasing its
  concentration in the earths atmosphere leads to
  a warming of the earth.
• The effect is already being observed, in higher
  air temperatures, receding glaciers, increase in
  wildfires, rising sea levels

5
Sustainable and Renewable Energy

• Solutions
• To ward off significant climate change, changes
  will need to be made in how we get our energy.
• Interest in sustainable, renewable, and clean
  sources of energy.
• Sustainable energy one that is not
  substantially depleted by continued use, does not
  cause significant pollutant emissions or other
  environmental problems, doesnt cause substantial
  health hazards or social injustices (from Boyle)
• Renewable energy energy obtained from the
  continuous or repetitive currents of energy
  recurring in the natural environment (Twidell and
  Weir, 1986)
• energy flows which are replenished at the same
  rate as they are used (Sorensen, 2000)
• energy generated from natural resources
  (Wikipedia)

6
Sustainable Energy

• Some ideas that are being pursued include
• Wind
• Solar cells
• Solar thermal
• Biofuels
• Energy from the Ocean in the form of waves or
  tides
• Geothermal energy (e.g. Iceland)
• Clean fuels
• Some forms of fuel dont produce as much CO2.
  The gold standard in a clean fuel is hydrogen
  (H2). When hydrogen is burned, it produces no
  CO2 at all, only water. One of the ways of
  extracting this chemical energy from hydrogen is
  to react it with oxygen in a fuel cell.

7
Transportable Energy

• Transportable energy
• In addition to solutions like solar cells, or
  wind turbines, we need a way to store energy, and
  to move it around with us.
• We need portability for many applications (e.g.
  driving a car)
• We also need energy on demand (so we can have it
  even in the dark).
• Thats why fuels are so desirablethey are a
  transportable, storable form of energy.
• One way to store energy is in the form of
  hydrogen. Remember that hydrogen is considered
  the cleanest of the clean fuels because when it
  reacts with oxygen, the only product formed is
  water.

8
Fuel Cells

• Getting electrical energy from chemical energy
• We could just put hydrogen and oxygen together in
  a reactor, effectively burning the
  hydrogen, to get energy out.
• A more efficient way of doing this is to use a
  fuel cell.
• A fuel cell directly converts chemical energy
  (that from reacting H2 with O2) into electrical
  energy.
• It does this by only letting the oxygen contact
  the hydrogen in a very controlled fashion.
• A fuel cell is designed like a sandwich
• Lets delve further into fuel cells

hydrogen flame
9
Fuel Cells

• Fuel cells are devices that convert chemical
  energy into electrical energy
• Efficiencies are potentially higher than if using
  the fuels in an engine
• Current efficiencies are 40-60
• Fuel cells are similar to batteries, but with
  replenishable materials (fuel)
• Under consideration for both large scale power
  generation and small scale portable applications
  (e.g. laptop and cell phone power)

Converts fuel into electrical energy
Stores energy through an electrochemical system
similarities
Unlike a battery, a fuel cell is not consumed
when it produces electricity
Unlike a combustion engine, a fuel cell directly
converts chemical energy into electrical, without
going via heat and mechanical energy
differences
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Pros and Cons of Fuel Cells

• Advantages
• Clean and green
• Higher potential efficiencies
• No moving parts
• Lower particulate emissions
• Silent, mechanically robust
• Scaleable, transportable

• Disadvantages
• Expensive
• Fuel availability
• Power/energy density issues (for portable
  applications)

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Fuel Cell Basics What is a Fuel Cell?

• Electrochemical energy conversion device
• directly converts chemical energy to electrical
  energy
• fuel can be H2 or hydrocarbon (e.g. methanol)
• The combustion reaction is split into two
  electrochemical half reactions

Fuel cell
O2
H2O
H2
Electricity
H2½ O2 ? H2O
12
Fuel Cells

• Some fuel cell reactions
• These are basically combustion reactions.
• As with batteries, the idea is to harness the
  electrons from the redox reaction to produce
  electrical energy.
• Fuel cells contain (1) a thin membrane that
• conducts ions, (2) an anode and (3) a cathode
• Both the anode and the cathode need to be
• catalytically active or contain added catalyst
• in order to break up the H2 (or hydrocarbon)
• and O2.

Hydrogen
H2½ O2 ? H2O CH3OH (3/2)O2 ? CO2 2H2O
Methanol
Figure from F. Prinz
13
Oxidation and Reduction Reactions

• We are interested in a class of reactions that
  involve electron transfer at the atomic scale.
  These are called Redox reactions
• The overall chemical reaction is broken up into
  two electrochemical half reactions
• Oxidation Electrons are lost from a species
• Reduction Electrons are gained by a species

H2 ? 2 H 2 e-
examples
Zn ? Zn2 2 e-
½ O2 2 H 2 e- ? H2O
examples
2 e- Cu2 ? Cu
14
Oxidation and Reduction Reactions

• In an electrochemical device (such as a fuel cell
  or battery), the electrochemical half reactions
  take place at electrodes.
• The electrode is conductive, i.e. it needs to
  conduct charge.
• Anode the electrode where oxidation takes place
• Cathode the electrode where reduction takes
  place
• Whether the anode and cathode are positively or
  negatively charged depends on the type of device.
  
• For a galvanic cell (produces electricity), the
  anode is negative
• For an electrolytic cell (consumes electricity),
  the anode is positive

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Schematic of a Fuel Cell
Air in
Fuel in
Flow structure
Porous electrode
Anode
Electrolyte
Cathode

• The steps in the fuel cell process are
• Deliver reactant (transport)
• Electrochemical reaction at both anode and
  cathode (requires catalyst too)
• Movement of ions through the electrolyte
  movement of electrons through the external
  circuit
• Remove product (transport)

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Fuel Cells
A fuel cell is just a battery with replenishable
electrode materials
17
Membranes
Properties desired for membrane
electrolyte High ionic conductivity (minimizes
resistive losses) Low electronic conductivity
(minimizes current losses) Chemical stability
in both oxidizing (anode) and reducing (cathode)
environments) Low fuel crossover Mechanical
strength and manufacturability Categories
liquid, solid, polymeric
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State-of-Art of Fuel Cells
 
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The Proton Exchange Membrane (PEMFC)

• The membrane must conduct protons (hydrogen ions,
  H) but not electrons (otherwise would short
  circuit)
• Most common membrane for PEM fuel cells is Nafion
  (Dupont), a polytetrafluoroethylene (Teflon) with
  sulfonic acid (SO3-H) functional groups
• Fixed charge sites (SO3-) act as temporary
  centers where the moving ions can be accepted or
  released. H ions move by detaching from from
  sulfonic acid sites and forming hydronium
  complexes (H3O) with water
• Nafion relies on liquid water humidification of
  the membrane to transport protons
• Therefore, water management (humidification)
  systems are necessary.
• Temperatures must be kept below 80-90oC so wont
  dry out.

Nafion
20
Phosphoric acid fuel cell (PAFC)

• First commercial fuel cell type
• Liquid H3PO4 electrolyte in SiC matrix
• Operated at 150-200oC expelled water used as
  steam for space and water heating
• Used for stationary applications with a combined
  heat and power efficiency of about 80
  electrical power efficiency alone is 40
• PAFCs dominate the on-site stationary fuel cell
  market 200 kW and 300 kW plants

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The Solid Oxide Fuel Cell (SOFC)

• Advantages
• Solid electrolyte
• Doesnt need humidification
• Fuel flexibility (H2 and simple hydrocarbon)
• Non-precious metal catalyst (at high T,
  perovskites are used as catalyst )
• Relatively high power density

YSZ (yttria-stabilized zirconia) cubic fluorite
structure
http//www.doitpoms.ac.uk/tlplib/fuel-cells/sofc_e
lectrolyte.php
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Y3 substitutes for Zr4 ions Creates oxygen
vacancies! For every 2 Y3 ions substituting
for Zr4 ions there is a O2- vacancy created
(charge neutrality)
Oxygen and vacancy exchange

• Membrane conductivity is proportional to the
  concentration of O2- vacancies
• But too much Y doping leads to vacancy-vacancy
  interactions which decreases mobility
• Maximum conductivity occurs at about 8 doping

23
Current Density vs Voltage Polarization Curve
The losses in voltage from the ideal output
voltage are referred to as polarizations
24
Fuel Cells
Honda FCX Clarity zero-emissions fuel cell
vehicle (shown with Jamie Lee Curtis) Vehicle
uses a PEM fuel cell stack Will these compete
with electric cars?