Batteries are known to be the most unsustainable and expensive part of system exploiting renewable energy.
So I am wondering, are there any alternatives to storing energy in batteries? Here I am especially interested in household power supply.
How Stuff Works lists a few:
- One of the oldest techniques people have used is the falling weight. You lift the weight to store the energy in it and then let the weight fall to extract the energy. Many grandfather clocks and cuckoo clocks use this technique. By running the string attached to the weights through a gear train, you can use a heavy weight and let it fall over a long period of time. (See How Pendulum Clocks Work.) This approach doesn't work very well in an electric car, but it has worked well in clocks for hundreds of years.
- Many power plants use the "falling weight" approach in the form of water. The water is pumped uphill to a lake at night when the power plant has excess capacity. During high-demand daytime periods, the water runs through a turbine on its way downhill to a lower lake.
- Another way to store energy is in some form of repeatable mechanical deformation. This is the idea behind a spring used in a wind-up clock or a rubber band used in a wind-up airplane. You store the energy by bending (deforming) the material in a spring, and the material releases the energy as it returns to its original shape. At the scale of a car, this technology has problems because of the weight of the spring, but at smaller scales (like a wristwatch) it works great. See also this page for an interesting example.
- Nature has been storing energy for a long time, and if you want to think about it in this way, gasoline is really a form of stored energy. Plants absorb sunlight and turn it into carbohydrates (see How Food Works for a discussion of carbohydrates). Over millions of years, these carbohydrates can turn into oil or coal. On a more human time scale, we burn wood (which is a carbohydrate) to release stored energy, or turn corn into alcohol and burn the alcohol.
- Another technique that nature uses to store energy is fat, which many of us are familiar with in a personal way. It is interesting to think about a car that somehow eats grass or some other carbohydrate and stores it as fat!
- You can take energy and split water into its hydrogen and oxygen atoms using electrolysis. By storing the hydrogen and oxygen in tanks, you can later create energy by burning it, or (more efficiently) by running it through a fuel cell (see How Fuel Cells Work).
- You can use the energy to spin up a flywheel and then later extract the energy by using the flywheel to run a generator. This patent has lots of information.
- You can store heat directly and later convert the heat to another form of energy like electricity. This page discusses some of NASA's experiments in this area.
- You can use compressed air to store energy. Toys like the Air Hog store energy in this way. Compressing gases like nitrogen enough produces liquid nitrogen, and this page talks about how you can use liquid nitrogen to power a car.
- One of the new technologies that may become available in the future involves antimatter. When you combine normal matter with antimatter, you get energy. You store the energy by creating the antimatter. This page talks about it a bit.
I find the compressed air or water an interesting idea. Is it possible to compress air or water during the day using photo-voltaic power and then generating power at night by decompressing air or water? Water especially can be interesting in combination with rainwater harvesting. But is that efficient? Are any of the above more efficient and cost effective than batteries?