Clear Science!

We mentioned storing electricity in a battery. A battery stores energy electrochemically, meaning that electrons are “stored” in a chemical with high energy. Any chemistry that involves electrons as reactants and products is electrochemistry. A fuel cell is similar to a battery: both have two electrodes (anode and cathode) and an electrolyte.

There is more than one way to distinguish a battery and a fuel cell:

1. Batteries store their chemicals inside the battery, while fuel cells are fed from outside. 
2. Fuel cells often involve catalysis, while batteries may not.
3. Fuel cells operate continuously. Batteries operate in a batch manner, i.e. charge/discharge.

A current (simplified) view of the electrical grid shows that the demand for electricity and the amount of electricity generated must essentially match. This is done by ramping power plants up and down as people change how much electricity they want.

Solar power and wind power are not easy to regulate. But we want to be able to make them a large part of the grid, since they’re green. One way to do this is add electrical storage to the grid, which is like being able to put electricity in a box and save it for later. During periods of low demand you fill up the storage and then use it during high demand. A charged battery is an example of “stored” electricity.

Humanity uses about 15 TW of power. (That’s 15,000,000,000,000 watts. TW means terawatt, which is a trillion watts.) By comparison 120,000 TW of sunlight falls on the Earth. Ideally, this means one hour of sunlight could power us for one year. Practically, the top end of what we could collect would be around 600 TW, which is still a huge number. In other words, solar power could solve a lot of problems.

Solar cells like those you have probably seen work by using sunlight to make electrons move in a circuit, which is electricity. The most common design is made of layers of n- and p-type semiconductors. Light separates an electron and a hole, and the semiconductor layers make them go different directions to recombine. You cleverly make the electron go through a circuit and you get electricity.

The challenges to widespread use of solar cells are cost and intermittency. This is a good problem for scientists and engineers.

We’re going to address the question of why making energy generation more environmentally sustainable is an important but difficult problem. Take solar power. One very big problem with solar power one may not realize is that it is intermittent and unpredictable.

Of course you know the sun shines in the daytime and goes down at night. If you plot power generated versus time of day, this will produce a smooth parabola (arc or rainbow shape). When clouds go over, however, it causes sudden dips. Your solar array might even shut off. 

You may wonder, even though the Fukushima reactors were immediately shut down during the Sendai earthquake, why are they having a possible meltdown several days later? The figure shown above is from the website AllThingsNuclear. This plot shows the gallons per minute of cooling water boiled by a reactor of the Fukushimi type as a function of time since shutdown.

Even after control rods are inserted and a reactor is shut down, neutrons will still be produced for some time in the fuel rods. Over time the reaction will slow down and stop, but it is not instant on/off. Engineers use fairly complicated mathematics called control systems to regulate things like this that have time lags between a control input (“turn off”) and a system response (it actually turning off).

Even a week after shutdown, the reactor needs to boil off 60 gallons of cooling water per minute to stay at a steady temperature. This is all planned for. The problem in Japan is that the fossil fuel generators meant to keep cooling water flowing in an emergency failed.

In light of the emergencies at the Fukushima I and II power plants in Japan, we’re going to talk about nuclear power plants for a while. Nuclear power is not super-complicated: there is nuclear fuel, for example either UOX or MOX pellets (uranium oxide or mixed uranium and plutonium oxides in this case), which are packed into zircaloy ceramic rods. The job of the rods is to get hot. This is just like in a fossil fuel power plant, when it’s the coal or oil’s job to get hot.

The fuel rods are surrounded by water, which gets hot, boils, and carries the heat away. In the picture above, the red line is the heat generation profile in the rods: they generate the most heat at their center. Think of the heat as a thing that is generated there, diffuses through the solid rod to the outside, transfers to the cooling water, and exits with the water as it boils and becomes steam.

If there is an interruption in this heat transfer path, then heat will begin to build up. Think of it as a heat traffic jam. If the heat begins to collect in one spot, the temperature there will rise. The zircaloy and nuclear fuel pellets are ceramics and have extremely high melting points. However, if you get a big enough heat traffic jam, they will eventually melt. That’s called a meltdown.

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.

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.