Clear Science!

Good catch, adriantumble. Of all the energy that starts on the left, 58% of it ends up as “rejected energy” on the right. That’s huge! What does this mean? Does it mean we did a bad job and wasted a bunch of energy? Well, not necessarily. Laws of the universe (the second law of thermodynamics) say there is a limit to how much of the energy on the left we can actually use. Some of it must be rejected.

adriantumble:

A great infographic. I found it interesting that transportation consumed the most energy, even more than residential and commercial combined. Not sure what “rejected energy” is on the right, but if it’s produced but unused energy, that’s pretty stunning too.

clearscience:

When talking about global energy use, a common unit to discuss is the quad. This is short for one quadrillion BTUs. In SI units that is 1.055 x 10^18 joules or 1.055 exajoules.

We’ll use the United States as an example (because data for the U.S. are easy to get). The U.S. uses 94.6 quads per year. The sources of this energy are shown on the left. 40.4% of this is used to generate electricity. The rest is used directly. The final uses are shown on the right: residential, commercial, industrial, and transportation.

You notice that petroleum is used almost entirely as a transportation fuel. If the U.S. could convert to an electric vehicle base, the flow of energy to transportation could come from the “electricity generation” box, which involves renewable sources like solar and wind. These renewables are a small fraction, but if they could be made cheaper, their fraction would increase.

Since entropy and disorder must increase with time, entropy is the scientific quantity that gives time a direction. We just talked about how entropy makes heat flow from hot to cold. Imagine: if time went backwards, it would pretty much mean that heat flowed from cold to hot instead.

(from Saturday Morning Breakfast Cereal) 

The Kelvin-Planck statement is one way of saying the second law of thermodynamics. A thought experiment can prove it. Take a power plant just like we drew before, but assume the heat rejected to the river (lost!) has been reduced to 0. Great, right? Well, no: that results in what is called a perpetual motion machine, which cannot exist. Because now only one reservoir is involved!

To complete the thought experiment, say we take all the energy (A) we make with the power plant and use it to power a refrigerator between the hot and cold reservoirs. That is drawn above on the left side. You see that the energy A isn’t really doing anything but leaving the hot reservoir and then returning to it, so we can just subtract it out, giving us the picture on the right side.

This proves it, because now we have energy (heat) flowing from cold to hot all by itself! Which is not allowed! We still have the refrigerator drawn there, but nothing is powering it. This is the equivalent of heat flowing from ice water to a warm room. We all know heat flows the other way: from hot things to cold.

The scientific way to draw the Rankine cycle, or power plant, is like this. Heat goes from a hot place (the boiler) and ends up in a cold place (the cold river, for example). Some energy gets drawn off, and this is what we turn into electricity. It is hard to overstate this: civilization as we know it happened because of this concept and ones like it.

But pretend you’re in charge of this plant. The electricity you get out is A reduced by B. Your job is to minimize B. Soon you think: can I make it 0?

The second law of thermodynamics, which deals with entropy and the increase in disorder, has no one specific definition. Instead there are several scientific laws that appear to be different, but actually are saying exactly the same thing. One is the Kelvin-Planck statement. (This is the same Planck as the Planck constant, BTW!)

It says a device (power plant) cannot exchange heat with only one reservoir and make work. What?? Well, electricity is work. And a reservoir is a place of constant temperature. The hot boiler and cold river are reservoirs: that’s what they mean here. (We drew them as red and blue clouds.)

Coming back to power generation, let’s consider a typical power plant, which is shown in cartoon form above. Steam (water) continually flows clockwise in a pipe, going through:

1. a boiler where heat is added
2. a turbine, which the steam turns
3. a cold place, like a river, where heat is rejected
4. a pump, which starts it over again

This scheme is called the Rankine cycle. In a coal plant, the heat in is made by burning coal. If it’s nuclear plant, a nuclear reaction makes this heat. Work out is the electricity you get (by turning a generator with the turbine). The first law of thermodynamics says that HEAT IN + WORK IN = HEAT OUT + WORK OUT. Easy!

We’re going to talk about entropy for a while. You may have heard of entropy: it is often called a measure of the “disorder.” The second law of thermodynamics says that the entropy of an isolated system must increase with time. The universe is assumed to be an isolated system, and therefore the “disorder” of the universe is increasing.

When entropy was first discovered by Rudolf Clausius in 1865, the main application for such a finding wasn’t abstract, philosophical musing about the fate of the universe. Rather it was the question of machines and power generation by mankind, which are directly related to entropy in a practical way.

Entropy is related to temperature. (Its units are joules per kelvin (J/K), meaning that it has similar units as energy, but is always multiplied by temperature.) In the example of a glass of ice water in a room, the ice, water, and room air all have some value of entropy, and this value is different at different temperatures (it is a “function” of temperature).

Everyone knows this: put a glass of ice water in a room, and the ice will melt, and the water will eventually become the same temperature as the room. Obvious, right? We know that the ice water doesn’t get colder, warming up the air. But why doesn’t that happen? It’s entropy and the fact that it must increase overall (overall meaning the ice plus water plus glass plus room air).

This is a figure from an actual scientific paper recently in the Journal of Organic Chemistry. It’s whimsical, but does illustrate a fundamental concept:

1. H is enthalpy. Enthalpy is a fancy name for energy.
2. S is entropy.
3. G is “free energy,” which is the combination of energy and entropy.

G = H - TS (which you can work out from the numbers there)(T is temp, BTW). Everything that happens in the world happens because it makes free energy go down. Enthalpy and entropy are competing effects: they want to go down and up respectively. It’s like they’re two gnomes that make everything happen.

Next week, we’ll talk about entropy, in the most practical way we can. 

tocrofl:

http://pubs.acs.org/doi/abs/10.1021/jo902075z

Gnomes control EVERYTHING in science

(via Marty)

(via freshphotons)