Electricity from Heat

 
Figures from Geothermal Education Officehttp://www.geothermal.marin.org/http://www.geothermal.marin.org/GEOpresentationshapeimage_4_link_0

What is Energy?: In physics, energy is the ability to do work. For most of us, that means the ability to cause something to move. Similarly, once something is moving, it takes work to make it stop because it now has kinetic energy (the energy of motion). Anyone who has been hit by a ball knows that the ball had energy (and gave part of it to you when you stopped it). Most people familiar with a tea-kettle or steam engine understand that adding heat is one way to make something move.


So three sources of energy become immediately apparent: 1) kinetic energy, for instance where a person exerts force to move an object a distance; 2) potential energy, such as elastic energy stored in a stretched spring or rubber band, or gravitational energy stored in objects at that can fall from a height; 3) thermal energy, in the form of heat, for instance when we boil tea (or coffee depending on how tire we are). All three of these added up constitute the mechanical energy in a system.


In these cases, energy is being transfered from a source to a sink. So the question really becomes how do we obtain energy to use and get it concentrated enough to be useful.



What is heat and how do we use it?: Heat production is one possible step towards getting an object moving so that it has kinetic energy. In order to understand how this is accomplished we need to know what heat is and what temperature measures. Heat, sometimes called thermal energy, is the motion of atoms and molecules, either vibrating or moving around so that they bump into each other. Temperature is a measurement of the average hotness or coldness of an object; a measure of how fast, on average, the atoms and molecules that comprise an object are vibrating or moving.


In a solid, the atoms have a fairly rigid structure and cannot move around freely because they are bonded together. When a solid gets hotter, those atoms tend to vibrate faster. If they vibrate fast enough, they can break free of their bonds and change how they are organized. If the solid gets hot enough it can even melt and become a liquid or ablate and become a gas.


In liquids and gases, the atoms are fairly free to move around. When a liquid or a gas gets hotter, atoms are moving faster and bumping together with more force. This tends to push them further apart from each other. If they are confined to the same volume the pressure increases. The simplest form of this relationship between temperature (T), pressure (P), and volume (V) in gases is expressed by the Ideal Gas Law as:  P*V = n * R * T; where n refers to the number of atoms in moles and R is Avagadro’s Constant (6.02214199 x 104 mol-1).


Thus, for most solids, and for liquids and gases, adding heat means either increasing pressure or volume and is referred to as thermal expansion.


A fluid under high pressure will tend to flow to a region of low pressure much like air escaping out of an inflated balloon.



So how is heat transferred?: There are many ways to transfer heat. Light (radiation) and sound waves are some examples. Slamming two objects together also creates heat by transferring the motion of the two objects to that of that atoms that comprise them, maybe reorganizing them or ripping them apart. For geologists studying geothermal systems the two most important mechanisms for transferring heat are conduction and advection. In conduction, individual atoms are bumping up against each other and transferring kinetic energy, causing the atoms to speed up or slow down. The heat moves even if the rock does not. In advection, a material that has heat is moving from one place to another and takes the heat with it. Examples of advecting fluids include water circulating through the pores of rock or magma rising from great depth to form a volcano.

HOW IS HEAT USED IN AN TURBINE TO CONVERT KINETIC ENERGY TO ELECTRICITY?


    1. In engines such as internal combustion engines or steam engines, the flow of the “working” fluid results from the thermal expansion and contraction of fluids that create differences in pressure..

    2. Thus, heat is used to cause high pressure in a volume of fluid, when this high pressured volume is connected to a low pressure volume (an outlet) the fluid flows from high to low pressure.

    3. The flow of a “working” fluid represents Kinetic Energy and spins the coil of copper wire by pushing on a series of fan blades.

    4. The potential created by passing a coil by a magnet leads to an electric potential we measure as a voltage. Like a difference in temperature or pressure between two regions, a difference in electrical potential can cause a flow of electrons from one region to another.

    5. This same process can be reversed to generate work from electricity.

    6. Some alternate methods employing the same concept of a working fluid include flow due to gravitational potential energy: hydro-electric and tidal. Wind as a source of energy works in a similar way, but in this case it is a combination of thermal expansion, gravity, and the principal of buoyancy that drives flow of the air in our atmosphere.

    7. (Solar energy, from Photovoltaics works in a fundamentally different way).

There are three basic types of turbines used to generate electricity (the flow of electrons) from geothermal heat.

    1. Dry Steam

    2. Flash Steam

    3. Binary Cycle


In all of these turbines, heat is extracted from the earth by bringing a fluid (either steam or water) from great depth where it is hot to the surface. During this transport the fluid retains most of its heat (it has a high temperature).


All of these systems depend on a temperature difference between the incoming hot fluid and the cooled output fluid. The difference of 1. vs. 2 &3 is that there is no phase change from liquid to water, which involves a very large volume change or the generation of great pressure.


The difference between 2 and 3 is in the working fluid: In both cases heat is extracted from the Earth, but in the Binary Turbine, that heat is transferred to another fluid through a heat exchanger (think of a radiator in a car). The reason for transferring the heat is that some fluids expand much more than water for a given increase in temperature, so for the same amount of heat they can produce more work.


Another consideration is how cooling is accomplished. One of the critical elements of these systems is that there is a temperature difference between the water entering the turbine and the water or steam leaving the turbine. This is necessary to have relatively high pressure at the inlet and low pressure at the outlet that drives flow and spins the turbine. That can mean that these kinds of turbines can produce more electricity when the outlet is cooler, so cooling the outlet by exposure to air (as in a car) is more efficient in the cold winter than the hot summer. Similarly, cooling by a fluid like water that can absorb a lot of heat and sweep it away increases the amount of electricity produced.

WHAT IS TURBINE?  FARADAY’S LAW OF INDUCTION

An electric current is generated in a conductor (think copper wire, etc.) moved through a magnetic field. Spinning a coil past magnets thus produces a continuous alternating current. A generator, or alternator, uses this principle to generate electric potential and that drives a flux of electrons.


So all we need to do is figure out how to make the turbine spin without have to stand there turning the crank ourselves.

EXAMPLES OF TURBINE SYSTEMS IN GEOTHERMAL POWER PLANTS

WHAT IS ENERGY AND WHERE DOES IT COME FROM?

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First we need to understand something more basic: What is electricity?

We need to start by understanding the structure of atoms. Atoms are composed of three distinct parts: protons and neutrons that form a massive nucleus at the center of the atom, and electrons that orbit the nucleus. Protons and electrons are attracted to each other by having opposite electrical charges. Protons have a positive charge whereas electrons have a negative charge (neutrons have a neutral charge as their name suggests). The closer protons and electrons are together, the stronger the force attracting the these two particles.

Heat

Source

Heat and Work both provide a means of transferring energy; thus they can be used to add energy to a system that we can harness. Electricity, particularly alternating current, is another means of efficiently transferring energy over large distances.

For a synopsis defining electricity and how it is generated in power plants, please visit:
The Energy Kid’s Page developed by the National Energy Education Development Project


OK, so the turbine we described above generates and alternating current and is called an alternator. That means that the flow of electrons is constantly reversing, flowing in one direction and then back. Generally, this reversal happens 60 times per second by convention in the United States (it is just a function of how the alternator is set up. What is important here is that the electrons keep moving creating a flow that we call electricity and therefore generates a magnetic field that can exert force and do work. As far as light bulbs are concerned, the flow of electrons, in whatever direction, is all that is needed.


For a synopsis defining alternating current (AC) and its advantages please see:

How electricity works?: Direct versus Alternating current

Wikipedia

Education Champions: Alternating Current (AC) Electricity

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Figures from Energy Kid’s Pagehttp://www.eia.doe.gov/kids/energyfacts/sources/electricity.html
Figure from Energy Kid’s Pagehttp://www.eia.doe.gov/kids/energyfacts/sources/electricity.html

Heating from below causes a special type of advection called convection.

Thus heating a fluid, or causing it to change to a more disordered phase, such as water to steam, creates pressure that is used to spin a turbine. In combustion engines that power cars and coal-fired power plants, heat is provided by burning a fuel. In nuclear power plants, the heat comes from radioactive decay, the fission, of atoms. In geothermal systems, the enormous natural heat of the earth circumvents these steps.


This brings us to how we can use the Earth’s heat to generate work and then electricity.


So, how do we go from heat to kinetic energy to electricity?

Electricity is really just the flow of electrons, which happens all of the time in nature. When electrons move, or even just orbit around the atomic nucleus, they generate a magnetic field. This is a key issue because that magnetism exerts force that we can use to do work. Magnetic force can also be used to force electrons to move, creating a flow of electrons. In bar magnets, the electrons are arranged so that they all spin the same direction and the force of each spinning electron works together to create a larger net magnetic force. (If the electrons spun randomly their net forces would cancel out.)

A simple example of this in a natural system is the static electricity we might generate walking along a carpet or sliding down a plastic slide. That static electricity charges our hair with electrons, all of which have negative charges which repel each other causing our hair to rise. That charge can be dissipated by completing a circuit (say touching someone and giving them a shock) that allows the electrons to flow. Electrons will only flow and generate electricity when there is a complete circuit or path to flow along and a difference in electron density, called potential, that drives the flow of the electrons.


How do we use heat to generate electricity?: Well... we use a turbine made up of magnetics and materials that are good at conducting electrons.

So, heat “flows” from regions of high heat to regions of low heat. This makes sense if we remember that the atoms in the hot material is moving rapidly and bumping into other atoms, whereas the cold material is moving slowly.


Also, because adding or removing heat can cause a material to expand or contract while still retaining the same mass, the density of the material changes. It turns out Archimedes learned more than 2000 years ago that relative density determines whether an object floats or sinks in a fluid. If the object is less dense than the fluid it floats; conversely, if the object is denser than the fluid it sinks. We call this behavior buoyancy. That means that adding heat from below can cause water deep in the Earth to expand and rise up buoyantly. As it rises the warm fluid rises, displaces cooler denser water above that subsequently sinks to take its place. This circulation of fluid due to heating from below is called convection.



Where does all of this energy come from?:

Temperature in the earth increases with depth until it reaches a maximum temperature greater than 5000°C. We live just on the very surface of this vast reservoir of heat and do not have direct access to it. However, there is enough heat in the shallow crust of the Earth to provide a viable energy source. We drill a short way to it, much like we drill a water well, but to depths more similar to wells drilled for petroleum extraction. For geothermal engineers, pumping steam or water from deep in the Earth provides access to this source of heat. Similarly, pushing water to depth so that it heats up and then pumping it back to the surface provides a continuing source of heat. So we are simply transferring this energy to places we can use it.


These are mechanisms of transferring energy to someplace we can use it. What we know from physics is that energy cannot be created or destroyed, it is constant, and this is called the Conservation of Energy and the First Law of Thermodynamics:

    ΔU = Q - W

    U = Internal energy

    Q = Heat

    W = Work

What experiments in physics have revealed is that the energy in a “system” (read object, location, machine, etc.) is the combination of heat and work. Since energy is constant, then heat can produce work or work can produce heat, but it is merely a transfer of energy from one form to another. This is the concept inherent in a steam engine or internal combustion engine.

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© Nicholas C. Davatzes

Last Updated: 2008/12