There are a number of alternative energies that one might
consider using in western Pennsylvania. These include
geothermal, wind, and solar power. How and what to use
these energies for are some relevant questions. Regardless
of the type of alternative energy being discussed remember
that any time you transform energy from one form to another
it comes at a cost.
Place a thermometer outside your window and you
will see that the temperature of the air outside changes in a
pronounced way from day to night, from one day to the next, and from
one season to the next, but if you bury that same thermometer a few
feet down in the dirt you will find that the temperature fluctuations
become significantly muted. (*** readings from the field station and
link to field station weather station ***).
Temperature variation with depth--an interesting aside
Long cold snaps (such as Ice Ages) can cause larger variations in the
temperature but they are still muted compared to the changes in
temperature we see from day to day in the air. The temperature does,
however, change with the depth of the hole you dig for your
thermometer. The deeper you dig down into the earth the hotter it
gets (the center of the earth is molten iron). This heat energy comes
from a variety of sources, radiactive decay and left over heat from
when the earth was formed are the two most important sources.
Away from the edges of
techtonic plates the rate at which the ground warms with depth (i.e.,
the geothermal gradient (***link to wikipidea***)) is about 20-30
^oC/km (60-90^oF/mile). Practically speaking the ground temperature
in Lawrence county PA at a depth of 30m (100ft) is 11^oC (52^oF).
This means that
for every 100ft you drill down the temperature rises 1^oF and 4.5miles
down the rock is 100^oC/212^oF, hot enough for water to boil.
There are local variations in the geothermal gradient, with a hot spot
in Clarion and Venago counties. This is likely to be caused by a
local radioactivity hot spot. A detailed analysis is available
Harvesting the energy
While the fact that earth gets hot the deeper you go is interesting,
it is unrealistic to think that you would be able to pump hot water up
from 3 miles down in the earth. The cost and effort rule that out.
What the earth provides is a way to make an unlimited supply 11^oC
(52^oF) water. A loop of pipe burried a modest distance underground
with cold (compared to the ground) flowing through it will be heated
by the ground. When paired with a heat pump the heat extracted from
the ground can be concentrated for heating the inside of a house.
A heat pump is basically a refrigerator, but in this case the
equipment is used to move heat into the box (cold coils in fluid
that passes out into the ground loop, heat deposited through the hot
coils in to the
house) instead of out of the box (cold coils in the refrigerator hot
coils outside the refrigerator).
The difficult/expensive part of wind power is usually reaching the wind. The
wind is always blowing up high and so to reach the reliable wind source you
need a tall tower. To keep the tower from falling over in high winds you need
a big block of concrete to anchor the tower in. Once these are in place the
turbine can collect the energy.
Harvesting the energy
To collect the wind power you need a propeller on an electric motor with
permanent magnets (run a current through a wire in a magnetic field under the
right conditions and there will be a force on the wire, move a wire through a
magnetic field under the same conditions and a current will be forced to flow.)
There are tricks to getting more energy out (a 3 blade propeller works better
than the old fashion windmill wheels because each propeller blade is an airfoil
[a wing is another example of an airfoil] and so it is not just pushed but
also lifted which actually allows the blade to collect energy from more of the
air.) There are also tricks for keeping the turbine/blade combination from
tearing itself to pieces when the wind gets too strong.
We have an array of photovoltaic panels on campus that convert the sun's light
into electricity. This works because the panels are essentially diodes with
very large surface area. They work light Light Emitting Diodes (LEDs) that
are being run backwards.
LEDs give off light because of what amounts to an
electron waterfall in the material.
All diodes have regions of two types of semiconductor that are arranged so that
it is easy (energetically) for the electrons to fall from one material into the
other, but hard for the electrons to go the opposite way. When the electrons
fall from one material into the other the energy from the fall is given off
in the form of light. In the solar panels the light gives the electrons the
boost they need to go backwards, from the bottom of the fall to the top. The
electrons that have been pumped by the light into the higher energy material
pile up and can be used just like a the electrons from a battery.
This electricity is then converted (using an inverter) from
its DC form to
an AC from that is carefully shaped
(frequency, voltage, and phase) so that it can be pushed
into the electrical grid. On sunny days this works so well that the set-up
produces more power than we need at the field station and the electrical meter
runs backwards. The rest of the time electricity flows in from the power
company. The details of how we made this work are available
To see how much electricity the inverter has pushed out recently see
this graph of its output.