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.

There’s a dinner party tonight so the Clear Science staff is testing the kitchen equipment. Have a good weekend, Clear Scientists!

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. 

Don’t worry, atomgoren! The Clear Science staff are all fine. Things have just been busy. We’re going to resume on a schedule a little slower. By the way, Clear Science is over a year old now.

Thanks whimsicalday! The Clear Science staff has been very busy with science. We’ll be posting again shortly.

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.

Let’s consider a nuclear reaction with uranium-235 as the fuel. Inside the fuel rods, a neutron with the appropriate energy collides with a uranium-235 atom and is incorporated into this atom’s nucleus. The uranium atom now has an extra neutron and becomes uranium-236. However, uranium-236 is unstable and immediately decays to two smaller atoms—the fission products. Many different fission products are made, such as cesium-133, iodine-135, etc. Wikipedia has a nice entry explaining the fission product yield for uranium-235.

Breaking atomic bonds also releases energy in the form of heat. The purpose of a nuclear power plant is to capture this heat and turn it into electricity. This is analogous to a fossil fuel power plant, where chemical bonds are broken to release heat.

When the uranium-236 decays, extra neutrons (and some other things) are also released. These are called prompt neutrons because they come directly from the fission reaction. (They’re produced promptly.) These neutrons collide with more uranium-235 and the reaction continues. Fission products can also sit around for a while and then decay to produce neutrons, and these are called delayed neutrons. If neutrons are being produced, the fission reaction will continue, and the rate of reaction will be a function of the number of neutrons being produced.