Alternative Energies

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.

Geothermal energy

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 here.

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).

Wind energy

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.

Solar Power

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 here. To see how much electricity the inverter has pushed out recently see this graph of its output.