Here we see the journey energy goes through on the US Energy Flowchart, from a fossil fuel to electricity.
A fossil fuel (such as octane) comes out of the ground, loaded with energy in its chemical bonds
Combustion releases this chemical energy as heat
The heat boils steam in a power plant’s boiler
The steam turns a turbine or works a piston, making electricity
The steam is condensed to water, which has to involve rejecting some heat
For this reason, you can’t recover all of the fossil fuel’s chemical energy as electricity. Some must be rejected. Engineers and scientists try to minimize this rejection.
A more thorough explanation of entropy and why heat must be rejected can be found here. 

Here we see the journey energy goes through on the US Energy Flowchart, from a fossil fuel to electricity.

  1. A fossil fuel (such as octane) comes out of the ground, loaded with energy in its chemical bonds
  2. Combustion releases this chemical energy as heat
  3. The heat boils steam in a power plant’s boiler
  4. The steam turns a turbine or works a piston, making electricity
  5. The steam is condensed to water, which has to involve rejecting some heat

For this reason, you can’t recover all of the fossil fuel’s chemical energy as electricity. Some must be rejected. Engineers and scientists try to minimize this rejection.

A more thorough explanation of entropy and why heat must be rejected can be found here

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.

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.

Scottish engineer James Watt made a grand leap in the history of energy generation in 1765, when he conceived the idea to add a separate chamber for the condensation of steam to water to occur in. (This was called the "condenser.") He was repairing a Newcomen engine and thought it was inefficient to repeatedly cool down and heat up the entire cylinder. The unit of power, the watt, is named after him.
Modern power generation still uses Watt’s concept, having a separate boiler and condenser. A turbine between them produces work, for the same reason as in a Newcomen engine.  

Scottish engineer James Watt made a grand leap in the history of energy generation in 1765, when he conceived the idea to add a separate chamber for the condensation of steam to water to occur in. (This was called the "condenser.") He was repairing a Newcomen engine and thought it was inefficient to repeatedly cool down and heat up the entire cylinder. The unit of power, the watt, is named after him.

Modern power generation still uses Watt’s concept, having a separate boiler and condenser. A turbine between them produces work, for the same reason as in a Newcomen engine.  

We said that Denis Papin built the first piston steam engine, which could do work by cooling a piston-cylinder filled with steam. The volume change and resulting vacuum caused by the condensing steam would force the piston to move.
Thomas Newcomen, who was English, used this concept to build the Newcomen steam engine in about 1710. This excellent animated gif borrowed from Wikipedia illustrates how the device worked. The cylinder was filled with steam, which was then condensed with the aid of a cold source.
Concepts like this were central to advancement of science and the development of modern life. Work is being done here, but the source of the work is not anything in motion. Rather, it is caused by heat. The work is the result of boiling and subsequent condensation. People began to realize the equivalence between heat and work, which was not obvious before. Today we recognize these are just different kinds of energy.

We said that Denis Papin built the first piston steam engine, which could do work by cooling a piston-cylinder filled with steam. The volume change and resulting vacuum caused by the condensing steam would force the piston to move.

Thomas Newcomen, who was English, used this concept to build the Newcomen steam engine in about 1710. This excellent animated gif borrowed from Wikipedia illustrates how the device worked. The cylinder was filled with steam, which was then condensed with the aid of a cold source.

Concepts like this were central to advancement of science and the development of modern life. Work is being done here, but the source of the work is not anything in motion. Rather, it is caused by heat. The work is the result of boiling and subsequent condensation. People began to realize the equivalence between heat and work, which was not obvious before. Today we recognize these are just different kinds of energy.

In 1690, French scientist and inventor Denis Papin built the first piston steam engine, which worked on the following concept:
A piston-cylinder is open to the atmosphere
Steam is allowed to fill the cylinder, displacing the air inside
The cylinder is closed, now filled only with steam
The device is cooled, causing the steam inside to condense as water, which has a far smaller volume than steam
When this condensation happens, a vacuum is created, and the piston head is sucked in. We discussed how strong a vacuum is. This movement of the piston can be used to do work. This work has no origin in motion: it wasn’t caused by a person or animal, the wind, or a flowing stream. All that was required was the heat to make steam, and coldness to cool it off.

In 1690, French scientist and inventor Denis Papin built the first piston steam engine, which worked on the following concept:

  1. A piston-cylinder is open to the atmosphere
  2. Steam is allowed to fill the cylinder, displacing the air inside
  3. The cylinder is closed, now filled only with steam
  4. The device is cooled, causing the steam inside to condense as water, which has a far smaller volume than steam

When this condensation happens, a vacuum is created, and the piston head is sucked in. We discussed how strong a vacuum is. This movement of the piston can be used to do work. This work has no origin in motion: it wasn’t caused by a person or animal, the wind, or a flowing stream. All that was required was the heat to make steam, and coldness to cool it off.

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 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 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.)

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:
a boiler where heat is added
a turbine, which the steam turns
a cold place, like a river, where heat is rejected
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!

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!

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).

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).