If all the heat consumed by humankind since it appeared on the face of
the earth were to be extracted from the Earth's core in one second,
what would be the resulting reduction in mean temperature of the
Earth's core and what would be the impact on the environment of the
Earth's surface?
I suspect that the answer to the first question is "negligible", and
that the answer to the second question is "none". If these turn out to
be, in fact, the correct answers, a third question raises itself! Why
is humankind currently satisifing it's heat requirement by the fossil
fuel route and by the nuclear route?

---

Request for Question Clarification by richard-ga on 21 Sep 2003 05:25 PDT

Hello:

Can you tell me what you mean by "heat consumed."  I think you'll find
that, thermodynamically speaking, the sun's heat was and is the source
of nearly all energy on the earth (the exception being radioactive
decay of earthly substances).  And given the law of conservation of
energy, some of the sun's heat has been stored over the years (for
example when used by green plants and converted to the potential
energy of coal and petroleum) and later released (for example when we
drive our cars).

Would you accept as an answer a calculation of the BTUs of fossil and
nuclear fuels consumed by humankind in proportion to the solar energy
that the earth receives?

Zozer-ga,

Wow!  What a question.  I love “what would happen if....” stuff like
this.

Here goes my best shot at an answer.  I’ve tried to present it in
clear, discrete chunks, and hope I’ve succeeded.  But if anything
requires more explanation, just let me know through a Request for
Clarification.  I’m happy to keep working on this until it’s all
clear.

pafalafa-ga

---------------------


Professor Hoffert at New York University has estimated the total
number of people who have ever lived on the planet Earth is 33.8
billion:

www.nyu.edu/classes/adler/cosmos/How_Many.pdf
"HOW MANY PEOPLE EVER LIVED?"

For the sake of simplicity -- and to be on the safe side -- I’ll  up
the number to 50 billion.

----------------------

Now, how much energy have these 50 billion people consumed over time?

(Energy isn't really "consumed" of course, since the laws of physics
remind us that energy cannot be destroyed.  But when humans burn, say,
a gallon of gasoline, they are converting the energy in that fuel from
a very useful, concentrated form to a less useful, dispersed form of
energy).


In the United States, the most energy-intensive nation in the world,
the average person consumes 327 million BTU's per year:

http://www.ecoworld.com/Articles/May23_BTU_GNP.cfm
"Top BTU Consumption by Country"


Again, let's round up to a nice easy number to work with...say, 500
million BTU's per person per year.

-----------------------

So, if all 50 billion people who ever lived consumed energy at the
rate of a modern US citizen (a ridiculous notion to be sure, but one
which leaves little likelihood of underestimating total energy
consumption for all humanity for all time), then total energy
consumption for humankind would be in the neighborhood of:

://www.google.com/search?sourceid=navclient&ie=UTF-8&oe=UTF-8&q=50+billion+times+500+million+%3D

50 billion people x 500 million BTU's per person =

2.5 x 10 e19 BTUs 
 
(which is 2.5 times 10 to the 19th power...it's awkward using
exponential notation in ASCII format)

So...we've got a handle on the first part of your question -- all the
energy consumed by all humanity since the dawn of time.

---------------------------

How much energy is this?  According to the US Department of Energy:

http://www.oit.doe.gov/e3handbook/appenf.shtml

one trillion (10 e12) BTUs is the equivalent of the amount of energy
released by a sizable earthquake (magnitude 7.0 on the Richter scale).

So, 2.5 x 10 e19 BTUs is more or less the equivalent of 10 million
large earthquakes!

You can get a sense from this that the numbers are *not*
inconsequential from the point of view of the planet Earth.  Sucking
all that energy out of the core is the equivalent of millions of
earthquakes, and would most certainly have an impact of some sort.

----------------------------

By the way, we'll need to do some energy conversions in a bit.  As you
can see at:

://www.google.com/search?num=100&hl=en&lr=&ie=UTF-8&oe=UTF-8&q=watts+in+btus%2Fsecond&btnG=Google+Search

releasing one BTU in a second is equal to about 1,000 watts of power
(it's actually 1,055, but I'm very fond of rounding off...it
simplifies things greatly, and doesn't really affect our conclusions).

So if all the energy ever consumed by human beings -- all 2.5 x 10 e19
BTUs of it -- were somehow consumed in one second, this would be the
energy equivalent of:


(2.5 x 10 e19) x 1000 = 2.5 x 10 e22 watts

That's 25 billion trillion watts.  That's a lot of wattage!


It's also an extreme upper limit, since I assumed everyone in history
used energy at the rate of a modern US energy-hog, which is certainly
not the case.  Suppose instead, that average energy consumption is
only one one-hundredth of modern consumption rates.  Then the global
historical total would be 2.5 x 10 e20 watts, or 250 million trillion
watts.


---------------------------

Now, how does that compare to the energy output of the earth's core?



The latest theories about heat generation in the earth's core are
summarized here:

http://www.evworld.com/databases/shownews.cfm?pageid=news110602-07

----
"Giant Nuclear Reactor May Run Earth's Magnetic Field"

"Herndon and Hollenbach theorize a five-mile-wide ball of uranium has
been operating as a nuclear reactor for about 4.5 billion years. Its
output is an awesome 4 million megawatts. Much of the energy it
produces is heat, and that might be what powers the mechanism that
produces the geomagnetic field, Herndon said."

----

Note that not everyone agrees with this theory, but for our purposes,
that's not important.  The heat output of the core -- 4 million
megawatts -- is something that scientists are pretty comfortable with,
even if they don't agree on how that heat is being generated.

Now, a megawatt is a million watts, and 4 million megawatts is 4 x 10
e12 watts.  A big number, to be sure.  But fairly puny next to our
total energy consumption figure for all humanity for all time.

Let me summarize:

--Energy consumed by all humans over history:  on the order of (10
e20) to (10 e22) watts depending on assumptions

--Energy produced by the earth's core:  on the order of (10 e12) watts

-------------------------


Bottom line:  Human beings have "consumed" a LOT more energy over
hundreds of thousands of years than the earth's core can produce in
one second.  So if all that energy were somehow extracted from the
core in a period of one second, the core would be sucked dry of all
its heat energy, and then some.


-------------------------

You asked what would happen to the earth's environment under these
circumstances, and the only honest answer I can give you is:  who the
heck knows?

First of all, it seems it would be impossible to actually extract so
much energy so quickly, since total human consumption (compressed into
a one second time frame) exceeds  the energy output of the earth's
core for one second of time.

Shutting down the earth's core could have a huge effect.  The earth's
magnetic field could collapse, possibly catastrophically, acting like
an electromagnetic pulse that could destroy all modern electronics.

Geodynamics could be affected in unpredictable ways, as the thermal
forces that drive plate tectonics were disrupted.  Would this mean an
end to earthquakes and volcanoes?  Maybe.  Or maybe it would somehow
trigger massive tectonic activity.  I just don't know, and I don't
think anyone could really say with any assurance.

On the other hand, cooling the earth's core might result
in...absolutely nothing!.  The tremendous pressures and extreme
conditions at the core might be enough to simply restart whatever
processes were halted by your imagined cooling, and the impact at the
surface of the earth might be minimal.

I suspect, though, that the impact would be dramatic, although in ways
that are hard to imagine.  Time is an extremely important factor in
considering energy flow.  An amount of energy released slowly over the
course of a year might hardly be noticed.  But compress all that
energy release into a single moment in time and we call it an
explosion!

Similarly, by compressing all humanity's energy consumption into a
single second of time, you are asking to imagine a tremendous
"negative explosion" -- a very rapid removal of energy from the
earth's core.  It's hard to know what the impact would be, but when
energy is released or withdrawn that quickly, something's bound to
happen.

---------------------------

All this speculation might be neither here nor there.  The last part
of your question -- if I may paraphrase it -- seemed to ask:  Why are
we even using fossil fuels and the like, when we have this enormous
untapped energy source that is the earth's core?

Good question.

The answer is, we do use it...when we can.  But for the most part, the
energy at the earth's core is inaccessible to us, so we have to look
to other sources for our energy needs.

Let's return to our friends from MadSci again:


MadSci Network: Earth Sciences 

http://www.madsci.org/posts/archives/oct98/905866177.Es.r.html


where they have compared the amount of energy that the surface of the
earth receives from the sun, to that which reaches the surface from
the earth's core.  Their conclusion:

----

"Given the closeness of the numbers (±32 watts < 10%), the Earth
radiates little or no more energy than it receives from the Sun. In
other words, there is no significant energy received at the Earth's
surface from any interior source. (Contrast this with Jupiter, which
radiates about 10 times the energy it receives from the Sun."

"This answers your second question: the energy which drives ocean
currents comes from the Sun, not from the Earth's interior. One has to
go more than 100 miles down (the boundary between the Earth's crust
and upper mantle) before reaching a point where interior heat is the
primary driving force for physical processes."

----

In a nutshell, we don't much "feel" the heat energy from the earth's
core here at the surface of the earth.  We need to look elsewhere for
our energy sources (and if you're thinking "why not just drill down to
the core?", forget it...our drilling technology has never gotten us
anywhere near the earth's core).

-----------------------------

The exceptions to this general rule are the geothermal
hotspots...places at the surface of the earth where heat from the core
is transported up to the surface through unusual processes in the
earth's mantle layer.

A good overall explanation of geothermal heat can be found here:

http://geothermal.marin.org/pwrheat.html

It's especially worth noting the following:

----
HOW DOES GEOTHERMAL HEAT GET UP TO EARTH'S SURFACE?

The heat from the earth's core continuously flows outward. It
transfers (conducts) to the surrounding layer of rock, the mantle.
When temperatures and pressures become high enough, some mantle rock
melts, becoming magma. Then, because it is lighter (less dense) than
the surrounding rock, the magma rises (convects), moving slowly up
toward the earth's crust, carrying the heat from below.

Sometimes the hot magma reaches all the way to the surface, where we
know it as lava. But most often the magma remains below earth's crust,
heating nearby rock and water (rainwater that has seeped deep into the
earth) - sometimes as hot as 700 degrees F. Some of this hot
geothermal water travels back up through faults and cracks and reaches
the earth's surface as hot springs or geysers, but most of it stays
deep underground, trapped in cracks and porous rock. This natural
collection of hot water is called a geothermal reservoir.

----

HOW MUCH ELECTRICITY IS FROM GEOTHERMAL ENERGY?

Since the first geothermally-generated electricity in the world was
produced at Larderello, Italy, in 1904 the use of geothermal energy
for electricity has grown worldwide to about 7,000 megawatts in
twenty-one countries around the world. The United States alone
produces 2700 megawatts of electricity from geothermal energy,
electricity comparable to burning sixty million barrels of oil each
year.

---- 

WHAT ARE SOME NON-ELECTRIC WAYS WE CAN USE GEOTHERMAL ENERGY?

Geothermal water is used around the world, even when it is not hot
enough to generate electricity. Anytime geothermal water or heat are
used directly, less electricity is used. Using geothermal water
'directly' conserves energy and replaces the use of polluting energy
resources with clean ones. The main non-electric ways we use
geothermal energy are DIRECT USES and GEOTHERMAL HEAT PUMPS.

 
---------------------------


Well, zozer-ga, I hope the material here answers your very interesting
question.

I've certainly thrown a lot of numbers and calculations at you, and
presented a good deal of technical information without really knowing
whether it's too much of an oversimplification for your needs.

As I said earlier, if anything here is unclear, or needs elaboration,
just post a Request for Clarification to let me know.  I'll be happy
to provide more details for anything on this topic that is of interest
to you.

pafalafa-ga







search strategy:  

population "all the people who ever lived"

"energy consumption" "per capita" global average

watts in btus/second

geothermal energy 

"earth's core" "energy output"

---

Request for Answer Clarification by zozer-ga on 25 Sep 2003 09:41 PDT

I am delighted by the responses to my rhetorical – even whimsical –
questions, particularly those of Pafalafa-ga.  I appreciate the offer
of clarification, which I hereby accept.

I used the expression “heat consumed by human kind” advisedly.  While
I fully appreciate that heat is not “consumed” in so far as it is
actually converted, nevertheless, for all practical intents and
purposes it is consumed by humankind, in the same way as, for example,
food is consumed by humankind.  Food is, like heat, not actually
consumed, but converted – paradoxically – into heat, tissue and
another substance.

In light of the incisive observation by Racecar-ga re: “average
life-span” and his (or her) lucid analogy of a bank account, I will
shift my questions slightly away from dramatic / hyperbole and towards
practical relevance to humankind’s current situation.

My questions now read:

1. If all the heat used by humankind since 1904 had been extracted
from the earth’s upper mantle and crust as and when it was required by
humankind, what would have been, and what would be, the magnitude of
the adverse effect on the earth’s surface environment and atmospheric
environment.


2. Why do experts dismiss, out of hand, humankind’s ability to tunnel
down to the heat?  Quotes:
“We do use it….when we can” 
“The energy at the earth’s core is inaccessible to us”
“One has to go more than 100 miles down (the boundary between the
earth’s crust and upper mantle) before reaching a point where interior
heat is the primary driving force for physical processes”
“Forget it…our drilling technology has never gotten us anywhere near
the earth’s core” “The answer to the third question is that the
earth’s core is far less accessible than the surface of the moon”
“Knowing the middle of the earth is hot is not the same thing as being
able to extract energy from it”.

I, on the other hand, do not see such tunneling as a problem. For
example, I heard somewhere that a 55 mile tunnel was constructed near
Chicago – to carry water, I believe – in under 3 years, in the mid
nineteen-thirties.  This feat, and many such others, were carried out
without the benefit of the powerful hydraulic excavation machinery
which we all now take completely for granted.  It was done by the
strength of men’s back and the sweat of men’s brows.  If the effort of
those men, with their picks, shovels, spades and wheelbarrows had been
continued from then until now, a tunnel would have been created of
1,246 miles in length. Imagine if all the tunneling and mining effort
of humankind had been concentrated in tunneling to satisfy humankind’s
heat requirement.  I would submit that we could have dug not just
thousands of miles of “heat-tunnels”, but hundreds of thousands of
miles of “heat-tunnels”.  Imagine if this effort (and our
technological advances) were to have been added to the sorely
misguided effort which we have expended to satisfy our heat
requirement by the fossil and nuclear routes.  Completely fulfilling
our heat requirement by an available method which would have
negligible adverse environmental impact, as opposed to the devastating
environmental impact we have suffered (and will increasingly suffer)
on foot of our blind, obsessive addiction to fossil fuels.

“But it would get hotter and hotter the deeper we went” I hear you
say.
“Good!” say I, “The hotter the better!”

I consider that an unmanned, water-cooled tunneling machine which
would function at temperatures of 400 decrees centigrade to 500
degrees centigrade could easily be designed, built and operated by
humankind.  The super-heated coolant would drive steam turbines to
generate electricity.  Ordinary sea-brine could be electrolysed into
hydrogen and oxygen.  Internal combustion engines will run just as
well (and probably better) on liquid hydrogen fuel as they will on
liquid fossil fuel and everything else can run, as the street lighting
in Larderello in 1904, on electricity.

Instead of the potent mixture of toxins which currently exit the
exhausts of our internal combustion and jet engines (and our power
station chimney stacks), all which would exit our exhausts in my
proposed scenario is that which one gets when one burns hydrogen in
air – steam / water – which as we all know, is completely
non-pollutant of our atmospheric environment.

This second question is actually a question with a series of
observations and opinions attached.  These are opinions which I have
held since my teens in high school and I have become increasingly
convinced with each passing decade, of the correctness of my
perspective and the arrant folly of humankind’s perspective.

I challenge you to show me that I am mistaken.

---

Clarification of Answer by pafalafa-ga on 26 Sep 2003 09:21 PDT

Hello again, zozer-ga,

This has been quite a challenge.  I’ve learned a good deal in the
process, and find that I still have a lot to learn, yet, before I
could possibly give anything approaching a complete answer to your
complex questions.  But thanks so much for providing the opportunity. 
It has been fun.

Here’s my best efforts at responses to your follow-up questions, with
an important caveat: I don’t know a heck of a lot about drilling
technologies!  What I’ve offered below is a combination of my meager
knowledge on this topic, along with some speculation and plain old
common sense...take it all with a few grains of salt.


--------------------



1. If all the heat used by humankind since 1904 had been extracted
from the earth’s upper mantle and crust as and when it was required by
humankind, what would have been, and what would be, the magnitude of
the adverse effect on the earth’s surface environment and atmospheric
environment.

The impact in terms of the actual heat balance of the earth would be
negligible.  As I mentioned in my original answer, TIME is a very
important factor.  Removing all our heat needs at one moment -- which
was the scenario of your original question -- is the equivalent of an
enormous inverse explosion, with a lot of disruptive potential.

But removing energy on an as-needed basis is another matter entirely. 
As energy-hungry as humanity is, we are still a relatively small force
next to the energy-generating capacity of the entire planet.  Sucking
up some heat from the planet’s interior would not appreciably affect
the internal temperature or overall energy supply of the planet.

HOWEVER, the actual physical process whereby the heat was extracted is
another story entirely.  I’m not sure how humankind actually goes
about the extraction process in your scenario.

You mentioned “hundreds of thousands of miles of heat tunnels” as one
possibility.  Such tunneling itself has environmental impacts,
including (1) the energy needed to dig the tunnels in the first place
(2) the enormous material needs to create such tunnels, which are not
simply holes in the earth, but need to be lined, butressed, monitored,
and otherwise supported by complex infrastructures (3) the enormous
amount of waste material associated with tunneling activities which,
like mining, is not without its own environmental problems.

Whatever method was used to extract the energy from the earth’s
interior, there would be environmental consequences in terms of air,
water and land pollution.  How it would compare to our current state
of pollution is difficult to say.
 
 
2. Why do experts dismiss, out of hand, humankind’s ability to tunnel
down to the heat?

The analogy I see often used about the current state of our deep-earth
drilling technology is this: if the earth were an apple, the outer
crust would be the skin.  Our drilling technologies, so far, haven’t
even managed to pierce the skin of the apple, much less penetrate into
the interior of the earth.

The biggest problems with tunneling are heat and pressure.  Building a
HORIZONTAL tunnel over a long distance (say, from England to France)
is a fairly manageable challenge, because the heat and pressure do not
increase with the length of the tunnel.

But for a VERTICAL tunnel penetrating ever-deeper into the earth, the
internal forces at work rather quickly become overwhelming.  As
drilling technologies go deeper and deeper, more and more of the
borehole has to be taken up with thicker and thicker, immensely strong
support tubing to keep the hole from collapsing in on itself.  The
deepest holes ever drilled -- which have only managed to penetrate a
few miles into the earth -- have rather narrow bore sizes through
which actual materials or energy can flow.

Although the initial hole may be fairly large -- several feet in
diameter -- by the time all the support casing  material is put in
place, the remaining bore size is only nine or ten inches across. 
When you have a hole that is a hand-span in size, it’s difficult to
transfer much from the depths to the surface, whether you’re
transferring solid materials, water or energy.

Another problem with vertical drilling is that lifting material out of
the hole is simply harder when one is lifting it up, against gravity,
then when the lifting is largely in a horizontal direction, as is the
case with a traffic-type tunnel.  Partly for this reason, the deepest
tunnels are generally drilled out in the ocean, from deep sea drilling
vessels, or from drilling platforms.  By taking advantage of the fact
that the earth’s crust is simply thinner under the oceans than under
the land -- that is, there is less material to hoist up -- drilling
into the earth’s deeper regions is deemed more practical out at sea.

While this may work fine for scientists extracting deep-earth cores to
study, it works less well if your intent is energy transfer, since
once the energy is bought to the surface, it still has to somehow find
it’s way to land-based civilization.

--------------------

All these are obstacles to the development of deep-earth heat
extraction technology.  But none of them are necessarily overwhelming
obstacles.

Why then, hasn’t this opportunity been more actively pursued as an
energy-source?

Proponents of alternative energy sources always ask this
question...why isn’t society embracing solar energy, renewable
biomass, wind power, tidal power, geothermal energy?  And the answer
is always a bit elusive.

The major argument against alternative energy resources is one of
simple economics.  Other sources of energy just can’t compete against
the relatively cheap energy provided by petrochemical mainstays...oil,
natural gas, and coal.  And besides, some would say, we’re finding
plenty of new sources of petrochemical fuel, and we’re  learning to
burn these fuels with far less of an environmental impact than was the
case decades ago, so there’s no compelling need to look for
alternatives.

Hold on, say the proponents of alternative energy.  Petrochemicals are
“economical” only because society has vastly subsidized the whole
power-generation infrastructure...from subsidized mining and drilling,
to government support of pipelines, transmission lines, railways,
roads and other infrastructure needed to transport fuels and energy. 
Offer the same degree of subsidy to alternative sources, and they
could compete just as easily.

And to think that petrochemicals will ever burn “cleanly” is
delusional.  Even if the overtly toxic components of the emissions are
removed, you’re still left with unbelievable tonnage of carbon
dioxide, which is accelerating the rate of global climate change
around the planet.

-------------

So...who do you want to believe?  And if alternative energy is your
cup of tea, *which* alternative source makes the most sense?  There’s
plenty of solar energy reaching the earth to meet all our energy
needs.  There’s also an abundant supply under the earth, if we can
only find a practical way to tap it.

I haven’t definitively answered any of these questions, to be sure. 
But I hope I’ve managed to shed some interesting light on the issues,
just the same.

The door is still open to you (or to anyone else) who wants to comment
here.  If you need more information....just ask.  I’m at your service.

pafalafa-ga

---

Request for Answer Clarification by zozer-ga on 01 Oct 2003 12:32 PDT

Dear Paralafa,

I really appreciate the thoroughness of the research which is
obviously going into your answers to my questions.  Our discourse may
well prove to be the “straw which broke the camel’s back” in the
matter of humankind’s mistaken pursuit of the fossil fuel and nuclear
fuel solution.  Each of these pursuits is surely doomed to failure,
the implications of each pursuit, for the environment in which we have
to live, being too awful to contemplate.

Perhaps our little joust could be the starting point of a proper and
sound evaluation as between the various means available to us for
using to satisfy our heat requirement.

I am delighted with your confirmation that satisfying humankind’s heat
requirement from earth-heat on an on-going basis would have a
negligible effect on our planets temperatures.

You worry about, “The energy needed to dig the tunnels in the first
place”
What is the ratio of the ‘energy-needed’ to mine coal to the
‘energy-recovered’? 10 to 1? 20 to 1?

What is the ‘energy-needed’/‘energy-recovered’ ratio to mine oil? 10
to 1? 20 to 1?

Surely consideration of this ‘energy-needed’ requirement to the
exclusion of other elements of humankind’s effort to mine coal or oil
is a mistake.  Obviously what we require for a proper
evaluation/comparison is a model which attributes a weighing to each
of the negative elements associated with the particular method of
extracting the  heat we need, be it fossil, nuclear or simply
direct-extraction.  The main elements of this model would probably be
:

1. ‘Energy-needed’/‘energy-recovered’ ratio
2. Environmental impact
3. Investment required (start-up)
4. Investment required (on-going)

I strongly suspect that when proper weightings are attributed to each
of the 4 headings, by far the major weight will be attributed to No.2
, “Environmental Impact” and that earth-heat will win the competition
by a long distance.

You also worry that “The biggest problems with tunnelling are heat and
pressure”.
As pointed out in my supplementary ‘question’, heat is not a problem,
it is the solution. When we encounter a heat ‘problem’, we will simply
transfer it to the surface by exactly the same method as we currently
use to transfer the much more ‘difficult-to-manage’ heat generated by
nuclear fission – water coolant.

I will be very difficult to convince that pressure is a problem.  As I
understand it pressure arising from depth is a phenomenon which occurs
in penetrating liquids, not in penetrating solids.  Given that
earth-heat transfers to the earth’s surface at many locations, of its
own volition, and that there is a 1 degree centigrade rise in
temperature for every 100 feet we penetrate into the earth’s crust, is
there not a compelling case to be made that we could probably satisfy
our heat requirement from the earth’s mantle, never mind its core?

Forget about drilling technologies. Think
tunnelling-technology/mining-technology.  Coal-miners were
tunnelling/mining literally miles deep over two centuries ago.

I am convinced that an unmanned, water-cooled tunnelling machine which
would function at temperatures of 400 degrees centigrade to 500
degrees centigrade could easily be designed, built and operated by
humankind.  The super-heated coolant would drive steam turbines to
generate electricity.  Ordinary sea-brine could be electrolysed into
hydrogen and oxygen.  Internal combustion engines will run just as
well (and probably better) on liquid hydrogen fuel as they will on
liquid fossil fuel and everything else can run, as the street lighting
in Larderello in 1904, on electricity.

Instead of the potent mixture of toxins which currently exit the
exhausts of our internal combustion and jet engines (and our power
station chimney stacks), all which would exit our exhausts in my
proposed scenario is that which one gets when one burns hydrogen in
air –  steam/water – which as we all know, is completely non-pollutant
of our atmospheric environment.

I do not believe that you have succeeded in showing me that I am
mistaken and I look forward with eager anticipation to your effort to
do so.

---

Clarification of Answer by pafalafa-ga on 01 Oct 2003 13:10 PDT

OK...I'll bite.  I hope I did an acceptable job of answering your
original question, which centered not only on heat balance, but on the
important question:  "Why is humankind currently satisyfing it's heat
requirement by the fossil fuel route and by the nuclear route?"

And I'm willing to go back and forth on this topic ("joust" as you put
it) as long as it continues to be valuable and adds some perspective
on the new issues you're raising.


But first...a question back at you.  

Granted, there's plenty of heat below the surface of the earth that
could be tapped for use here on the surface.  BUT...there are also
many other sources of energy waiting to be tapped -- solar energy,
wind energy, tidal energy, to name the major sources.  Given these
alternative energy sources available on the surface, can you convince
me that digging underground for heat energy is the most efficient
option?

Let me know your thoughts on that, and then let the joust begin!

paf

The calculation of total heat consumed by all people ever is missing
one factor: average lifespan.  pafalafa multiplies energy used per
person per year by number of people, but neglects the fact that people
live more than one year.  That would take her upper bound to something
like 2 e21 BTUs, or 2 e 24 Joules.  If we assume this energy was
extracted uniformly from the entire earth, which has a mass of about 6
e24 kg, and assume an average heat capacity of 1000 Joules/kg/K, we
get that the earth cools by about .0003 degrees, which is negligible. 
So I think the effect would not be noticible.  The answer to the third
qestion is that the earth's core is far less accessible than the
surface of the moon.  Yes there's lots of thermal energy there, but
there's a whole lot more in the sun and every other star.  Knowing the
middle of the earth is hot is not the same thing as being able to
extract energy from it.

racecar-ga,

Thanks for your observation.  

As for total BTUs -- you're absolutely right.  I should have added a
multiplier for lifespan.

Nevertheless, the answer would not be appreciably different.  Total
BTU's consumed by humanity would still be a very large number, and
when compressed down to a one-second extraction, would greatly exceed
the amount of heat generated in the earth's core.  Factoring in human
lifespan would make this effect even more pronounced.

If other researchers or commenters have any perspectives to add on
this Q&A, I'd be interested to hear them, and I imagine zozer-ga would
appreciate the additional insights as well.

pafalafa-ga

Hey Pafa,

I agree that the earth's core does not generate anything like the
amount of power you get if you divide total energy used by humans by
one second.  That means that you can't continually take out this much
energy per second from the core adinfinitum.  However, if you just
take the energy out once, it doesn't matter over what period of time
you extract it--the end result is the same.

To make my point clear, take this excerpt:

Bottom line:  Human beings have "consumed" a LOT more energy over
hundreds of thousands of years than the earth's core can produce in
one second.  So if all that energy were somehow extracted from the
core in a period of one second, the core would be sucked dry of all
its heat energy, and then some.
 
Once again, I agree with the first sentence.  But the second does not
follow.  Removing more energy in one second than the core puts out per
second does not mean you have removed all the heat energy.  A good
analogy is a bank account.  Say you have $1,000,000 in the bank,
making 5% interest per year.  This means you make about 58 cents per
second in interest.  Now suppose you make a withdrawal of $100, and
suppose, for the sake of argument, it takes you one second to do so. 
The fact that you withdrew, in one second, far more money than the
per-second output certainly does not mean you have overdrawn your
account.  In fact, the difference in the size of the account is
negligible.

Hi again, zozoer-ga.

I just wanted to let you know I saw your latest posting.  I want to
spend a bit of time thinking it over, and doing a bit more research. 
Give me a day or two, and I'll get back to you with an update.

paf

An interesting question and a very impressive answer.  I must say
though, I'm still not convinced that the length of time over which
energy is hypothetically removed from the earth is of any consequence.
 We already calculated that for the amount of energy in question, the
earth would cool by less than a thousandth of a degree.  Granted,
extracting energy evenly from the entire earth is impossible.  But if
it did happen, I don't think it would have any noticible effects.

pafalafa, I am interested in knowing if you have found any sources
which show the mean temperature at given depths. Also, it should be
possible to calculate the pressure at different depths. If I were to
guess I would say its adaquately hot at 100 miles deep. What is the
density of the upper mantle? What would the pressure be like at that
depth? Using that number, you can calculate from there the thickness
of the steel tube required. I think that the steel thickness required
will be a quadratic relationship. So, at some point, the hotter
temperatures will no longer be reasonable to dig deeper for due to the
exponentially thicker steel. I think it is absolutely essential to
find that point, so you can figure out if it is really feasible or
not.

Lets say that the mean density of earth down to 100 miles is 4g/cc.
1 square inch = 6.4516 square centimeter
6.4516cc * 4g = 25.8g/in^3 rock density
25.8g/in^3 = .91oz/in^3
100 miles * 5280 ft/mi * 12 in/ft = 6,330,000 inches
6,330,000 inches * .91oz/inch^3 = 5,766,000 psi

So, using a pipe thickness calculator found at:
http://www.engineersedge.com/pipe_bust_calc.htm

I plugged in an 80kpsi tensile strength steel. Using a 20 foot thick
steel wall, and a very small margin of safety, you can get a 4-inch
bore at the 100 mile mark. I would want a shaft a lot wider than that
to generate power. The energy that you are going to get is going to be
small and the cost of a 100 mile long 20 foot thick pipe is going to
be extremely enormous. Even if the density is only 2g/cc that leaves
you with a 20 foot thick steel pipe encasing a 7 inch shaft.
Unfortunately it doesn't work out. However, there may be shallow spots
where you may only have to go a few miles down. Lets say you can find
a 20-mile depth to get a steady 500C. Then you would cut your pressure
by 4/5 to 1,153,000. In that case a 20 foot thick pipe would still
only give you a 20 inch bore. But the thing is, when you are dealing
with a pipe that heavy it could just break from its foundation and
sink down to the core.

If carbon nanotubes are ever a low-cost building material, it may give
the pipe just enough strength to be feasible.