The success of transitioning to sustainable energy supplies in the United States relies to a large degree on our ability to store energy produced by intermittent energy sources such as solar, wind and biomass. We have plenty sunlight and wind to go around. Conversion of biomass to a liquid or gaseous fuel is a convenient method for storing energy, but photosynthesis is quite inefficient compared to other ways of capturing solar energy. Also any biomass to energy scheme will involve burning something which always has some negative health consequences.
The future could be powered by electricity from solar and wind exclusively but how will we store the electricity for use when the sun isn’t shining or the wind isn’t blowing? Batteries are an obvious way of storing energy but are impractical for storing energy on the scale of an electric utility.Batteries for powering transportation are in use now and will expand greatly in the future.
Most electric cars today use Lithium ion batteries. They have the best energy to volume and energy to weight ratios referred to as energy density. The problem is that even the best batteries pale in comparison to the energy density of gasoline. Liquid fossil fuels like diesel and gasoline are very energy dense and can produce 50 times as much energy as a Lithium ion battery of equal weight or volume. With current technology the Nissan Leaf, an all electric vehicle, has a range of under one hundred miles. Batteries being developed now can increase the energy density by five to ten fold, giving electric cars a range of several hundred miles.
Storing energy for the electrical grid can accommodate a wider range of methods. One of the simplest ways of storing energy is to pump water up a hill.All you need are two reservoirs, one higher than the other. When energy is available it is used to pump the water to the upper reservoir. When energy is needed, the water is released, causing the turbines to reverse direction and generate rather than consume energy. The only limitation is space and geographic relief.
Another utility scale energy storage method being examined is compressed air. Just as pumping water up a hill stores energy, so does compressing a gas. In the latter part of the nineteenth century, several European cities used compressed air for energy storage. Rather than convert the energy in the compressed air to electricity, it was piped and metered to do mechanical work. Everything from motors for heavy industry to sewing machines ran on the compressed air. The major limitation of compressed air storage is the necessity of a large underground reservoir to hold the compressed air. Wind turbines in the midwest will in the future store energy with compressed air.
Flywheels are another method to store energy. An electric motor spins up the flywheel, later when energy is needed the motion is used to power a generator. The big advantage of flywheel storage is that it can be done anywhere. No need for a big hole in the ground or pairs of reservoirs at different altitudes.
One of the best methods to pair production and storage of energy is solar thermal. Simply heat a fluid with sunlight. When electrical energy is needed use the heat to power a generator. Power towers have a collection of mirrors pointed at the top of a tower. A fluid is circulated through the heated zone, then sent to a storage site for later extraction of energy.
All these techniques involve converting one form of energy to another, but can ultimately be used to generate electricity even when the wind isn’t blowing and sun isn’t shining.