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Chemistry 1002 Chapter 13Energy |
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Overview
Energy consumption on large scale measured in "quads"
(quadrillions of BTU's of energy). One quad of energy is 1 X
1015 BTU of energy; 1 BTU energy released
when red tip of wooden match ignites. Boil about 0.5 cc water.
World consumption ('95): 362 quads/yr.
U.S.: 88 quads/yr (24% of world consumption; Problem
29). World population: 5.86 billion. U.S. population: 268 million
(4.57% of world population).
We use 6.7 times as much energy per person as the rest
of the world does.
ENERGY LAYERING
What is world energy supply accessible with current
technology and at current consumption rate? (Source: Energy Information
Administration online info at http://www.eia.doe.gov/index.html)
Petroleum: 5900 quads (16 yr.)
Gas: 4900 quads (13 yr.)
235U: 4400 quads (12 yr.)
<<<41 years of status quo energy>>>
Easy coal: 29,000 quads (80 yr.)
Fossil dregs (shale/coal dregs): 41,000 quads (112
yrs.)
238U (breeder): 600,000 quad (1,600
yr)
Grand Total: 1800 yrs with available technology, at
current consumption rate. Requires us to perfect breeder reactors
& work with plutonium.
D fusion (future): 8 billion yr supply.
Now let's do a little reality check.
Energy usage worldwide has been growing steadily at
a rate of 1.6% per year over the last ten years (ever heard of
"compound interest" involving money?).
Assuming 1.8% growth per year for the foreseeable future
(mildly pessimistic but not unrealistic estimate which simplifies
calculations), world energy consumption rate will double every
40 years. Remember the story of the kid who made a deal with
his father to give him an allowance which doubled every week starting
at 1 cent per week? (Kid was making over $20 Trillion/week at
one year mark.)
This known as exponential growth. Everything in nature
(ie. bacteria) does it as long as food is available (energy in
case of mankind).
Revised energy layer estimates:
Oil/gas: 18 years from today.
Switch to coal: 54 years from today.
Deplete fossil fuels and 235U:
87 years.
Kick in breeder reactors: 200 yrs. tot.
Take home message: Unless we start producing major
amounts of power from renewable resources (ie. solar) we have
200 years to get fusion working.
How long will it take at 1.8% growth to use up half
of all deuterium on earth? Until about 3035 (almost exactly one
millennium).
What's next after fusion? Anybody's guess but it's
out there somewhere. We may have to find it and develop it by
the fourth millennium to survive.
Current Day Energy Issues
ELECTRICITY
Electricity is a "secondary" energy source.
Generated by consuming "primary" energy sources (fossil
fuels, nuclear, solar, etc.) and then consumed in turn.
With secondary, tertiary, etc. sources of energy an
important issue is efficiency. How many joules of electrical
energy do you get when you consume one joule of primary energy
source (ie. fossil fuel)?
Answer: generating electricity about 30% efficient.
If you heat your house with electricity you could get about three
times as much bang for the buck by burning fossil fuel yourself
as by paying electric company to burn the stuff for you.
Why is generation of secondary energy source inefficient?
Typically whenever energy is consumed to do some kind of work
(or produce a secondary form of energy) much or most of the primary
source of energy is lost in the form of heat which escapes out
into the world at large. Even if you burn gas to heat a house,
eventually the heat you make escapes through walls of your house.
Problem 3 asks: Why does combustion of 100 units of
fossil fuel produce only 33 units of electricity? Where does
the heat loss occur?
Answer: There three different ways in which heat lost
into world at large during generation & transmission of electricity
made by burning fossil fuel.
1. Combustion heat loss. Fuel Æ
heat Æ steam Æ
power. (Lost steps 2,3.)
2. Friction heat loss. Moving parts in generators,
etc.
3. Electrical resistance heat loss. Electricity heats
up power lines.
NUCLEAR POWER
US currently produces a little over 7 quads per yr
of energy from fission of 235U. This is about
9% of total US energy production/consumption. Nearly all of this
energy used to make electricity. About 18% to 19% of total electricity
made in US produced via nuclear fission (Problem 16).
Reason nuclear energy so attractive is that enormous
amounts of energy can be produced with very small quantities of
material. Nuclear fission of one mole of 235U
atoms produces 25 million times as much energy as combustion of
one mole of methane (natural gas) molecules, and about 3 million
times as much energy as burning a mole of gasoline molecules (Prob.
5).
Fairer comparison probably weight basis. 235U
fission releases about 1 million times much energy as combustion
of equal weight of any fossil fuel.
What about the down side of nuclear power?
Currently in US we only use about 0.7% of energy readily
available (current technology) in uranium in nuclear reactors.
Uranium has two isotopes (same number of protons but different
number of neutrons). The lighter isotope, 235U,
makes up only 0.7% of natural uranium, but is more convenient
to use as fuel. To use the other isotope, 238U
(99.3% of natural uranium), need to first convert it into plutonium
("breeder" reactor technology). This involves more
danger than use of 235U for two reasons:
1. Breeder reactors currently more dangerous to run than 235U
reactors; more danger of accident.
2. If plutonium from breeder reactor stolen by terrorists,
they can easily make nuclear bombs with it.
Current world supply of 235U: 4400
quads (only 12 year supply; hardly seems worth additional problems
involved with nuclear energy).
Current world supply of the other isotope (238U):
600,000 quads. 1600 yr supply (current consumption rate) or 200
yr supply (exponential growth).
Three other problems with 235U
usage other than scarcity (Problem 17):
1. Plutonium problem. Current reactors unavoidably
convert some 238U in reactor fuel into plutonium.
2. Waste problem. Nuclear waste dangerous for 100,000's
of years, also it contains plutonium.
3. Reactor accident hazard. While conventional 235U
reactors can't undergo nuclear explosion, they can undergo core
meltdown followed by chemical explosion; very dirty; can cause
millions of cancer deaths.
What happens during a core meltdown (ie. Chernobyl
Unit 4 accident; see Problem 18)? If reactor gets out of control,
reaction generates heat so fast that uranium fuel (usually solid
U3O8) melts and forms
liquid puddle at bottom of reactor. This material hot enough
to melt way through metal reactor housing, releasing and turning
steam in reactor into hydrogen gas (explosive). If hydrogen gas
explodes off goes the roof of containment building and radioactive
junk blown miles into sky.
At TMI accident in US a "partial" core meltdown
occurred. U3O8 fuel melted,
but radioactive steam was released into atmosphere before hydrogen
could form and explode. Release of radioactive material occurred,
but was minimized.
After initial damage is done by a meltdown reaction
cools down spontaneously; U less than critical mass, and cannot
continue reaction without moderator.
NUCLEAR CHEMISTRY
Most stable isotope of most stable element on Periodic
Table is 56Fe (56 total nucleons in nucleus;
atomic number 26 protons for iron means 56 -26 = 30 neutrons in
this isotope). Any isotope of any element heavier than 56Fe
can in theory be broken down into lighter elements with release
of energy (nuclear "fission"). Any isotope of any element
lighter than 56Fe can in theory be combined
with other lighter isotopes to make larger more stable elements
(nuclear "fusion," Prob. 20).
Problem 22:
Superscripts total to same number both sides of arrow.
Same with subscripts. Subscripts are atomic numbers.
Fossil fuels will probably be exhausted during our
children's lifetimes. Three realistic options for dealing with
this:
1. Work on improving safety of breeder reactors so they won't
have nuclear explosion if they malfunction, and so plutonium-handling
security procedures terrorist-proof.
2. Attempt to create a workable fusion reactor (Tokomac research)
before fossil fuels run out.
3. Develop solar technology to fill in gap between rising fossil
fuel costs and nuclear technology shortfalls.
Prudent energy management requires that we do all three
of these things.
FUSION RESEARCH
Plutonium waste probably blessing in disguise. Neutron
source for making more plutonium from 238U
and tritium (fusion) out of deuterium in water.
Fusion rxn: 2H + 3H
Æ 4He + n
Solar Energy
The continental US gets 46,000 quads per year (Problem
28) worth of energy from the sun (almost 550 times current annual
usage) in spite of the fact that about half of the sunlight that
hits the earth's atmosphere gets reflected back into space (concept
of "albedo;" see Problem 26).
Obviously we can't cover the entire continental US
with solar collectors but we could pass laws requiring every new
house to be built with solar panels covering entire roof. Assuming
about 40 million houses in US averaging 2500 square ft of roof
space we could collect about 131 quads/yr from rooftops. By time
we can implement this we may be using this much energy per year
in US.
Problem is commercial solar panels are still only about
23% efficient. There is steady tech. improvement in this area.
Some research materials now at 40%.
There are now even a few major power plants in operation
generating electricity from solar panels. Buzz word "photovoltaics."
Acronym: "PV."
Best guess is that solar heating and PV will probably
not become the major source of energy worldwide the way fossil
fuels now are. Will probably provide the stopgap we need to develop
nuclear technology (eventually fusion).
PV does not really provide "free" energy.
It takes energy to make PV materials. Originally took more energy
to make PV materials than these materials paid back in electricity
over their lifetime (losing situation). Currently PV pays back
about 4X production energy costs (Source: Australian New Zealand
Solar Energy Society).
Thing to keep in mind here is that there is no real
"energy crisis." There's plenty of energy. "Crisis"
is a management crisis as with most issues.
Alternative Energy Sources
Although solar power will undoubtedly become the most
important of the "alternative" energy sources which
man uses to bridge the gap between chemical and nuclear technology,
some of these other sources of energy, already in use, will continue
to play a small role in this effort:
Geothermal
Wind
Biomass
Hydroflow (ie. hydroelectric dams)
Thermal ocean gradients
Garbage burning
Show picture of garbage-burning plant in downtown Nashville,
TN. Ash from burned garbage takes up only 10% as much landfill
space as regular garbage.
Comments?
Last Revised : Sunday, October 5, 1997
Copyright © 1997
Louisiana State University, Department of Chemistry.
All rights reserved.
http://www.chem.lsu.edu/lucid/courseinfo/chem1002/ch13.html
