Geothermal Energy Production

        Geothermal power plants generate electrical energy to power homes and businesses using heat from deep within the Earth. The process generally begins by drilling deep into the bowels of the Earth to reach a point within the Earth where the crust is thin and heat from the molten center of the earth may get nearer the surface.  In the FORGE project we are drilling an injection well and a production well, and fracturing the rock between them to allow fluids to flow between the two wells.

Water is pumped deep below the ground through the injection well into the hot ground. The temperatures here are well above the boiling point of water (over 150 °C). There the water may then boil into steam. Often in geothermal processes the water is at such high pressure that it remains a liquid above a temperature of 100 °C, but later becomes steam as it approaches the surface and pressures are dropped.  In both processes the pressurized fluid (liquid or gas) is forced up the production well.  

In the geothermal plant, this high-pressure steam is used to turn a turbine to generate the desired electrical power for homes and businesses.  After the turbine the steam has lost pressure and temperature and is sent to a condenser to be converted back into liquid water, which can complete the cycle and be sent down the injection well again.  

One of the key reasons geothermal energy is important is because the process is renewable and the water used can be recycled. Further, geothermal can meet our minimum energy demands reliably, not being interrupted by factors like weather or night and day cycles.

Turbines

        The process of actually generating the electrical energy is performed by a spinning turbine, which is a common tool in power generation from hydroelectric to nuclear to coal-fired power plants.  Turbines spin as the fluid flows through the blades which rotates an electromagnetic shaft attached to the turbine.  An electrical generator produces electrical energy from this rotating, electromagnetic shaft, according to Faraday’s Law.  Within the generator, a small electrical current is generated from rotating the shaft inside coils of wire; however, with the large number of coils of wire, many small electrical currents add together to form one large electrical current.  This final large current is what is used to power our homes and businesses.

Above you can see the blower used in this module that we’ve disassembled. The four wire coils are held still. As air enters through the blower opening, the turbine blades, which contains magnets, catch the flowing air and are made to spin around the coils. This movement of the magnetic field induces a current in the wires which can be used to, say, turn on a light.

Power

Electrical power can be expressed in terms of voltage and resistance.  According to Joule’s First Law, power can be expressed by the following:

Where P is power, I is electric current, and R is resistance. To obtain power in terms of measured voltage across a know resistance, R, we must also use Ohm’s Law:

Solving for I and substituting into Joule’s First Law we obtain an equation for power in teams of what is known in this module, voltage and resistance:

Using the voltage, V,  value across the 1 kΩ resistor, R, the amount of power generated by the person blowing into the turbine can be determined.  For example, if the voltmeter read 2.0 V, we could calculate the power generated to be 4 milliwatts by:

Light Color

        Not every photon of light takes the same amount of energy to produce. The energy of a photon is a function of its wavelength according to:

where E is the photon’s energy, h is the Plank constant (6.626e-34 J⋅s), c is the speed of light (2.998e8 m/s), and λ is the wavelength of light.  As energy is a function of one over the wavelength, as the wavelength goes up the energy of the photon decreases.

Some different colored lights on different fans are more difficult to light up than other colored lights.  While it is not a direct path between the equation above and the force from the lungs needed to light up a light, they are related and this module allows for the students to make a very tactile connection to photonic energy.

The red light corresponds to photons with a relative large wavelength (≈ 650 nm) within the visible spectrum while a blue light corresponds to photons with a relatively small wavelength (≈ 450 nm) .  This means that the red light corresponds to a lower energy level, and to achieve the same light brightness less blowing is needed than with the blue light.

This difference can also be seen in the requirements to light each LED. Red and yellow LEDs generally take around 2 V to power, while blue and green LEDs generally take 3 V. Going back to the electric power equation, we can see that more volts are also a sign of more power spent (making some simplifying assumptions).