In northern England, the seasons are the rhythm that gives all my memories a cozy home. Long walks along the Bridgewater Canal on hot summer days, hockey matches in the autumn drizzle, driving back from Polish Christmas Eve dinner in a frosty chill, the excitement of spring days getting longer—the variety was part of the joy of it all. One of the hardest things about living in California was the absence of that rhythm; it felt as though time wasn’t moving, and that was intensely disconcerting. I continue to feel the seasons very strongly today. I like to be able to identify my place in the yearly cycle by the cues that still mark it, even in a modern society: animals, the air, the plants, and the sky. And the foundation of all those riches is the bit of physics that keeps things spinning unless something stops them.
Spin has a direction, the axis around which everything is rotating. We imagine the axis of the Earth as a line that goes from the South Pole to the North Pole, sticking out slightly, pointing away into space. But because it’s been clonked by solar system debris in the past (especially the huge collision that made the Moon), the spinning top that is the Earth doesn’t point straight up relative to the rest of the solar system. Imagine looking down on the solar system, with the Sun in the middle and the planets circling around on a flat plane. The axis of the Earth points slightly off to the left. And now that it’s spinning about that tilted axis, it has to stay spinning about that same axis. So when the Earth is on the left of the Sun as we’re looking at it, the north end of the axis is pointing away from the Sun, out into space. But six months later, when the Earth is on the right of the sun, the north end of the axis is still pointing to the left—which is now toward the sun. The spin axis of the Earth does not change direction at it goes around the Sun—there’s nothing pushing on it, so it must keep going as it did before. But that means that the North Pole gets more or less sunshine, depending on where the Earth is on its orbit. This is where our seasonal cycle comes from.§§ We have a day/night cycle because the Earth keeps spinning, and a seasonal cycle because the axis of that spin is tilted.??
Spinning is part of our lives in lots of ways. But there’s one device in particular that relies on spin, and it’s one we might all be seeing more of in the future: a flywheel. Anything that’s spinning has extra energy because of its spin. So if a rotating object will keep spinning indefinitely, that also means it can act as a store of energy. If you can get the energy back as you slow the spin down, you’ve effectively got a mechanical battery. This is what a flywheel is, and it isn’t new; they’ve been around for centuries. But a new wave of flywheels is about to arrive in our society, a set of very efficient modern devices that could help solve a really thorny problem.
One of the biggest challenges for any energy grid is matching up supply and demand on very short timescales. If everyone cooks dinner at about the same time, energy use across the country will rise for an hour or so and then fall. Ideally, someone monitoring the system would allow energy into the grid as it’s needed, to match that spike. But that’s a problem if the energy is coming from a coal-fired power station that takes hours to start and stop. And you may not even be in control of the rate or timing of energy generation. One of the difficulties with many renewable energy sources is that you can’t dictate when they’re generating energy—it’s easy to make hay (or store energy) when the sun shines, but what if that’s not when you need it?
Surely, you might say, all you need is a battery to store the extra energy until you can use it? But electric batteries aren’t really up to the job. They’re expensive to manufacture, they’re often based on relatively rare metals, they have a limited number of charge/discharge cycles, and there are limits to how quickly they can store and release energy. In response, over the past few years, along have come some prototype flywheel projects. And it looks as though this technology may offer a workable solution, at least some of the time. A flywheel is a heavy spinning disk or cylinder, with bearings that are as frictionless as possible. Once it’s spinning, it will keep spinning. And since there is energy associated with rotation, that spin can store energy. You use any excess energy in the grid to get your flywheel spinning, and it will just keep spinning, holding on to the energy. Then, when you want energy back, you slow the wheel down by converting the energy into electricity. There’s no limit to the number of times you can charge and discharge the flywheels, and they can release their energy very quickly. You only lose about 10 percent of the energy you had to start with, and very little maintenance is needed. Even better, you can make them to suit your needs: a small one to go with the solar panels on your roof, or a vast bank of them to moderate spikes in the whole energy grid. Small portable flywheels are even being tried out on hybrid buses, storing energy as the bus brakes and supplying it back to the wheels when the bus needs to speed up again. Flywheels are appealing because they’re based on a beautifully simple idea—the conservation of angular momentum. Eggs and spinning tops and stirred tea all follow the same principle. But it takes efficient modern technology to turn it into a practical solution. It’s still early days for the new incarnation of this technology, but you may well be seeing spinning flywheels around much more often in the future.
* I should emphasize that I did enjoy it, mostly, and I would definitely recommend the experience. As long as you’re a confident cyclist to start with, it’s a great way to appreciate rotational physics in a very visceral way.
? I’m sure that the pizza lovers out there will all have their own strong opinions on what makes the best pizza dough and how to shape it. I can vouch from personal experience that the pizza produced by this restaurant was superb. But don’t write me letters if you disagree with the chef’s conclusions!
? Unless it’s a drop of liquid so small that surface tension can do the job. But a droplet has to be very tiny for that to be enough.
§ Bitter about this, ten years on? No, no . . . what would possibly make you think that?
? Sputnik had an elliptical orbit, so its height above the surface ranged from 139 miles to 590 miles. That gives you a gravitational pull of between 93% and 76% of the value at the Earth’s surface.
# For the sake of domestic harmony, it is probably better not to butter this particular experiment. If you insist on replicating the real situation, at least put some newspaper on the floor where it’s going to land, or whatever it is that does that job in a paperless society. Protecting surfaces is one function of a newspaper that a fancy tablet computer is never going to replace.
** You may be wondering why the boot in the trebuchet can stop rotating and zooms off in a straight line when it is released, but the toast must keep spinning. The difference is that the toast is held together as a single object by internal forces, and as long as it’s a single object, it has a fixed amount of angular momentum that must be conserved. If any part of it was released from the rest (maybe a crumb fell off one side), then that piece would travel in a straight line.