Energy, Power, and Climate Change 8.1 Energy Degradation and Power Generation

1. FUELS: INDIRECT ENERGY SOURCES If you've ever camped out you've probably used a campfire in these two obvious ways: Energy, Power, and Climate Change8.1 Energy Degradation and Power Generation (a) For cooking (b) For warmth In both cases, it is the heat released during combustion (a chemical reaction) that is used. The reason the title of this slide has the word "indirect" in it is because if you want to burn wood or coal, and get useful work out of it, you have to somehow convert the heat to mechanical motion. In general, the device which converts this energy into mechanical motion is called an engine.

2. FUELS: INDIRECT ENERGY SOURCES Here is a simple means of converting heat to useable mechanical energy: A hot air balloon. Energy, Power, and Climate Change8.1 Energy Degradation and Power Generation One form of mechanical energy we could "extract" from the balloon is potential energy. By using it to lift a quantity of water up high, we could release the water in a stream, to turn a turbine. The turbine could be used directly to turn a winch, or it could be used indirectly to run a generator to produce electricity. Eventually, the water will run out and the balloon will have to descend to obtain more. In other words, the balloon "engine" must go through repeated cycles in order to keep producing mechanical energy. Getting the balloon back down is simple: Let it cool down by releasing its heat to the surrounding environment.

3. FYI: All the kinetic energy of the box is converted to heat via friction. Obviously we cannot convert that heat back into mechanical energy to make the box travel back to where it started: THE SECOND LAW OF THERMODYNAMICS The second law of thermodynamics states that although it is possible to convert mechanical energy completely into thermal energy, it is NOT possible to convert all heat energy into mechanical energy. Energy, Power, and Climate Change8.1 Energy Degradation and Power Generation More simply phrased we may say that "it is impossible to take heat from a hot object and use it all to do work without losing some of it to the environment." The hot air balloon example illustrates this law: During its descent, all of its heat energy is lost to the environment. FYI: It is also lost during its ascent! How?

4. FYI: During each stage of the cycle, energy is being lost to the environment in a form that cannot be recovered for useful work. FYI: We call this imperfect conversion of energy from one form into another ENERGY DEGRADATION. ENERGY DEGRADATION Suppose we are using wood to heat up the air to make the balloon rise. FYI: Think of energy degradation as a spreading out of our original energy source into ever wider surroundings. Energy, Power, and Climate Change8.1 Energy Degradation and Power Generation Think of the wood as a concentrated form of chemical energy. FYI: In terms of ENTROPY, you may view energy degradation as an increase in entropy (which is the tendency of the universe toward disorganization). We begin with a very compact and organized energy source (the wood) and end up with a lot of thermal energy spread out over larger areas. We then burn the wood, releasing heat to the environment. This has the effect of spreading out the energy into the balloon and the surrounding air. Some of this heat actually causes the balloon to rise, increasing the potential energy of the system. The rest is lost. The stream of water converts potential to kinetic energy (both forms are mechanical energy). The kinetic energy of the water is then transferred to the turbine blades, with much of it lost to the surroundings. The kinetic energy of the turbine can then be converted to electricity through use of a generator.

5. Chemical Energy 100 MJ Potential Energy 70 MJ Mechanical Energy 45 MJ Kinetic Energy 60 MJ SANKEY DIAGRAMS We can show energy degradation for the hot air balloon engine with a flow chart called a Sankey diagram. Energy, Power, and Climate Change8.1 Energy Degradation and Power Generation High Water Falling Water Wood Turbine Heat 15 J Heat 30 J Heat 10 J Note that each conversion has an efficiency associated with it, which can be calculated using Output Input 70 100 = 0.7 = 70% Efficiency = = Conversion from wood to potential energy is, for example...

6. TYPICAL EFFICIENCIES Here are some examples of efficiencies: Energy, Power, and Climate Change8.1 Energy Degradation and Power Generation Machine Efficiency (%) Steam Locomotive 5 to 10 Human Muscle 20 to 25 Automobile less than 25 Compressor 80 Electric Motor 70 to 95 FYI: You should be able to make Sankey diagrams for each of these machines. The lost energy is unusable heat, given up to the environment.

7. TYPICAL EFFICIENCIES You may recall from biology the "efficiency" of organisms - for example a caterpillar. Energy, Power, and Climate Change8.1 Energy Degradation and Power Generation This flow chart illustrates energy partitioning. If we look at the caterpillar's biomass, only 33 J/200 J = 16.5% of the total energy consumed by the caterpillar becomes caterpillar.

8. TYPICAL EFFICIENCIES In fact, the higher up in the food chain you go, the less of the original energy you have left: Energy, Power, and Climate Change8.1 Energy Degradation and Power Generation

9. Wire FYI: This EMF can be produced by moving the wire through a stationary magnetic field, or by moving a magnetic field past a stationary wire. FYI: The polarity of the EMF can be reversed if the relative motion is reversed (called ALTERNATING CURRENT). GENERATING ELECTRICITY Electricity is one of the most useable forms of energy we have because it is so easily transportable. FYI: Most of our electricity is produced using generators, which are in turn run by turbines, which are rotated by fluids such as water (dams), wind, or steam (coal fired or nuclear power plants). Energy, Power, and Climate Change8.1 Energy Degradation and Power Generation You may recall that moving electrons produce a magnetic field. It turns out that the process is symmetric: A moving magnetic field produces moving electrons (called an electromotive force or EMF). FYI: From the Sankey diagrams, we see that every energy conversion between the basic energy source and the useable form of energy results in energy degradation. Thus a coil of wire spinning in a magnetic field will produce an EMF (aka a voltage) that can be used to drive electrical devices. B-Field Electrons