Two newspaper articles are out talking about storage of renewable energy. Both articles fail to notice that the US grid already runs on about 20% stored renewable energy through hydroelectric power. About 24 GW of that capacity can run backwards rather than just throttle so we already have quite a lot of what we might need. And, the articles don't notice that distributed renewable power is not very intermittent. The wind is always blowing somewhere and clouds rarely cover all of a continent. The trick is to shuttle the power from where it is produced to where it is needed. If you have enough capacity to meet the peak use, then you don't really care about storing the extra power you don't need when you are using less, you just find something fun and interesting to do with it. Remember, renewable energy is extravagant. Think of the amazing fecundity and diversity of a rain forest. It is about prosperity not scarcity.
But, before we get to the point where we produce more energy than we use most of the time, methods of storage can help to retire fossil energy plants more quickly. So, lets just list the kinds of storage that are covered in the articles and on the Real Energy blog so we know a few of the options. We'll organize it in the types of energy physicists like to use.
Hot or cold, thermal storage adds a certain amount of extra time to use the energy. In some cases like the high thermal mass house, you are just avoiding using energy that you don't really need. The daily fluctuations of external temperature are not important with good insulation and a high heat capacity. In one article ice is used to shift electricity use from day time to night time and also save on over-all use while in the another, molten salts are used to keep solar energy for use at night. You can see how these might work together.
Batteries have the potential for large scale storage and are mentioned in both articles. The anticipated sizes run up to 6 MWh. The batteries mention in the article are not exactly flow batteries which are also used together with wind farms and run up to 12 MWh. The blog also looked at using ammonia as a chemical storage method and producing hydrogen for later use is also a chemical method though it experiences high thermal loses. Aluminum can also be used for chemical storage and used to produce hydrogen on demand.
Here we have two choices, potential energy or kinetic energy. Both articles mention gas pressure storage, essentially a form of of potential energy similar to damming a river. The size of the facility mentioned is about 100 MW and presumably can run for a day or two. About half the energy comes from compressed air and half from natural gas. One article mentions flywheels which store kinetic energy. In this case the flywheel stores 18 MWs or 5 kWh. One can reduce the tensile strength requirements for a flywheel and increase its capacity by usinging a magenetic track. Then the strength requirements are compressive and much simpler.
Capacitors are used to store power when very large currents pulses are needed as for example in inertial confinement fusion. These capacitors store about 3 kWh. Super capacitors are less bulky and are being developed for transportation applications.
Superconducting Magnetic Energy Storage is used in some applications with capacities moving toward 20 MWh.
For very high energy density, excited nuclear states might be used. This is actually a new listing, but not very practical just now.
The complaint in the articles is that power storage adds cost to the the electric power distribution system. But, pretty clearly, the decreasing cost of renewable energy is making storage more attractive to utilities. Thermal storage in solar plants that work with thermal energy anyway is a natural extension to their capabilities. Similarly, those that work using chemical energy are designed to store energy from the beginning. It is clear that flywheel and magnetic storage are already being used for power conditioning. Very shortly, the cost of renewable power will drop well below the cost of other sources. For wind, it is already the cheapest way to produce power in many places. As it turns out, once we're ready to chase the skeletons our of the fossil energy closet, we'll be able to put in a great new closet organizer with slots for all kinds of storage that will make the dumping of the fossils all the more rapid. Energy storage is not an Achilles' heel for renewable energy, but rather a stepping stone to full deployment. Daniel Arvizu should know better.