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Energy:
In many respects, energy is analogous to mass. It is conserved in systems, it can be moved by processes similar to advection and diffusion of mass, and it is very important for understanding many environmental issues. The amount of time that we have in this class allows for only a cursory look at energy, but I will try to hit some high points. First, a few definitions:
Energy: work done by a certain force (gravitational, electric, magnetic, force of inertia, etc.). It can also be defined as Force*Distance. It has many different forms depending on the particular forces involved, but can be lumped into two main types of energy: kinetic and potential.
Kinetic energy: energy that is realized, energy that a body possesses as a result of its motion. The basic equation for kinetic energy is KE=1/2mv², where KE is kinetic energy, m is mass, and v is velocity.
Potential energy: energy that is "captured" or "stored" in an object. Gravitational potential energy is PE=mgh, where PE is potential energy, m is mass, g is acceleration due to gravity, and h is the height of the object. Electrical potential energy is referred to as voltage. Chemical potential energy includes energy that might be released in a chemical reaction, including changing the state of a material from gas to liquid or from liquid to solid.
Units of energy:
- 1 Joule (1 J) is equivalent to 1 N-m and 1 kg-m²/s²
- 1 calorie is equivalent to 4.186 J, while 1 kilocalorie (i.e., 1 food calorie) is equivalent to 1000 calories
- 1 Watt (1 W) = 1 J/s
- 1 kiloWatt-hour (1 kWh) = 1000 Watts * 1 hour; this can be converted to Joules by converting 1 hour into seconds
Energy consumption: Humans use many different types of energy, but fundamentally we use only two types of energy in our homes: heat and electricity. In a simple sense, the basic challenges for providing humans with the energy that they need (or perhaps just want) are to convert a particular energy resource into either usable heat or electricity and to deliver that energy to place it is consumed and at the time it is consumed. This brings up three basic rules of energy supply:
1. Energy is "lost" in the conversion from one energy form (e.g., solar, hydro, etc.) into a usable form (e.g., electricity). The efficiency of a system is the ratio of energy produced at the end versus the initial energy. For example, solar panels range in efficiency from ~5-35%, meaning that 65-95% of the solar energy reaching a solar panel is converted to energy other than electricity. That energy might be thermal (heating the solar panel), it might be chemical (reactions on the surface of the solar panel), or it might simply be reflected off the solar panel's surface.
2. The need to transport the energy from where it is produced to where it is consumed is the basic reason why we have an electrical grid. On a local scale, this is why we have electrical wiring and heating vents in our houses. Heat is much harder to transport far distances, so we generally produce heat in our houses by burning oil, natural gas, coal, or wood, or by converting electricity into heat.
3. The need to have electricity and heat when they are needed means that you need a way to be flexible in your energy production (we use thermostats to regulate heat production in our houses) and/or ways to store energy for when you need it (two large problems with solar and wind power, since you have little control over the timing of production).
Example 1: How much electricity could we generate by burning all of the junk mail received by the US population each year?
(electricity)=(mass of junkmail per household)*(# of households)*(thermal energy produced per mass of paper burned)*(efficiency of converting thermal energy to electrical energy)
A blogger online claims that he has received 170 lbs of junk mail from the beginning of the year through Oct. 3. We can prorate this for the entire year and convert to kg, which would give us 100 kg of junk mail for the entire year (a few pounds per week, which is pretty true for my house). Let's assume that this is representative for all US households, although we could certainly be more accurate if we did a broader survey. There are approximately 100 million US households as recorded in the US census. Experimental results show that burning 1 kg of paper produces 2x107 J in thermal energy. The energy efficiency of converting this to electricity varies widely, but a good average number is 30%. Putting this all together, the junk mail would produce 2x109 W of electricity, almost twice what the Limerick Nuclear Power Plant produces (1.2 gigaWatts, enough energy for 2 million households).
Example 2: What would the temperatures of planets be without greenhouse gases?
(Temperature of planet)=(Temperature of Sun)*(1-albedo)¼*[(radius of Sun)/(2*radius of planet's orbit)]½
Here are the data for the 8 planets and Pluto:
|
Planet |
Distance (km) |
Albedo |
Actual temperature (K) |
Predicted temperature (K) |
|
Mercury |
58,605,000 |
0.119 |
452 |
434 |
|
Venus |
108,895,000 |
0.75 |
755 |
232 |
|
Earth |
150,295,000 |
0.306 |
288 |
254 |
|
Mars |
228,635,000 |
0.25 |
210 |
210 |
|
Jupiter |
779,025,000 |
0.343 |
152 |
110 |
|
Saturn |
1,430,095,000 |
0.342 |
148 |
81 |
|
Uranus |
2,871,685,000 |
0.3 |
80 |
58 |
|
Neptune |
4,504,695,000 |
0.29 |
100 |
47 |
|
Pluto |
5,913,520,000 |
0.5 |
33 |
37 |
Two basic observations: 1) Almost all of the planets are warmer than the prediction because their greenhouse gases keep them warm. The magnitude of their greenhouse effect can be estimated by looking at the difference between the predicted and actual temperatures. Venus has the most extreme greenhouse effect, with what is often referred to as a "run-away greenhouse" because the oceans have boiled off into the atmosphere and there is now no mechanism by which to decrease the greenhouse gas concentrations. Earth has enough of a greenhouse effect to bring the average temperature of the planet into a range that is livable. 2) Mars has essentially no atmosphere, because the components of greenhouse gases (mainly carbon) are locked in the Martian lithosphere (the rocks) without processes to return those elements to the atmosphere. The surface of Mars indicates that there was liquid water at the planet's surface early in its history. There is much research effort now to understand how this was possible, and whether there is still liquid water in the subsurface, because liquid water is thought to be a prerequisite for having life on the planet.
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