If we ever are able to get a really high percentage of our electric power from renewable sources, there is one piece that few people consider, but is needed to really make it work. The smart grid won't be able to reach its full potential without it.
The two biggest sources for renewable electric power are solar and wind. But the sun and the wind come and go at their own timetable, which might not match what our timetable is for using electricity. Clouds might come and go, wind speeds are usually intermittent. Even if some sites are pretty predictable, such as deserts with few cloudy days, or offshore wind that is more constant, the peaks might not always correspond exactly to load.
With current contributions of wind and solar to electricity generation, a smart grid is able to shift power to where it is needed and other power sources can be adjusted to demand. Most new electrical generation capacity being built in this country uses natural gas. Natural gas has a lot of advantages for generating electricity. Plants are relatively cheap to build, they produce less pollution than coal, and for the moment at least it is not too costly. But the biggest advantage in a grid with lots of wind and solar is that they can be turned on or off quickly, so that they can be used to adjust to fluctuations in renewable power. But natural gas is still a fossil fuel. The new methods to extract it are dangerous to the environment. It contributes to global warming. It will run out, sooner or later.
If renewable sources ever get to high percentages of total power, 50 percent or more, then we need some way to match supply and demand. We need ways to store a lot of electrical power.
Storing large amounts of electrical power will enable a smart grid to make full use of power not only from any place, but from any time. The better our energy storage, the easier it will be to do really large scale wind and solar.
There is a lot of activity in electric power storage. I will try to summarize some of the options.
Pumped water:
Use extra power to pump water to a higher elevation. When you need to produce power, let the water flow down, generating electricity as an artificial hydroelectric system. This technology is already in use in many places around the world. The Bath County Pumped Storage Station in Virginia is the largest in the US. These installations are fairly expensive to build, but the biggest problem is that you need to have a high reservoir with a lot of capacity to store much power, so it might be hard to build in places like Kansas.
Compressed air:
Pump air to pressurize a large chamber, then use the pressurized air to power generators. This method is being used some at utility scale, but because of the thermodynamics of compressing and expanding gases, current utility scale designs use some natural gas to heat the expanding air when power is being generated, making such plants a more efficient natural gas load leveling plant. Other designs are more difficult for large scale.
Batteries:
Batteries of course have long been the main means of small scale electric storage. There is some use by utilities to smooth out power fluctuations, on the order of 15 minutes. There are a lot of battery technologies with various advantages and disadvantages. Newer types such as flow batteries or sodium sulfur batteries may be useful for larger scale energy storage. To me the most interesting possibility is distributed storage among large number of consumers, which I discuss more below.
Hydrogen:
Use electric power to separate water molecules into hydrogen and oxygen. The hydrogen is stored somehow and burned to produce electricity when needed. To do this on a utility scale requires some sort of large cavern. Since hydrogen is a small molecule and reacts easily, storing it in natural caverns is hard. Cavities in salt deposits seem to be the best chance. If hydrogen ever is used to power vehicles, then a hydrogen economy could develop that would make large scale hydrogen production and storage an obvious thing to do, promoting it for large scale electricity load leveling also.
Flywheels:
Store energy in a large rotating flywheel. Very low friction is required, so vacuums and magnetic bearings make flywheels more efficient. So far flywheels have only been used for smoothing out short term fluctuations.
Superconducting magnetic energy:
Use superconductors to produce a large magnetic field. The field could store a large amount of energy, and the superconductor keeps the field stable until needed. There is some use of these for power smoothing. The biggest problem is that current superconductors only work at very low temperatures. If practical high temperature superconductors are ever developed, then this method might be developed for a larger scale.
Distributed storage:
If every house or building that has solar panels or a wind turbine had its own storage, the the combined storage of all of those places could add up to a large total. Buildings with renewable energy that can sell power back into the grid are already common. If these buildings also had some sort of electric power storage, they would become a large power reservoir. Off-the-grid homes already have battery storage systems that work with their electricity sources, so these sorts of systems can be added to any building. These storage systems could be used as back up power for a building so if there was a grid power outage they would still have some power. Probably these home storage systems would be some sort of battery.
An intriguing possibility is use of electric cars as power storage. If an electric car is not being used, it could conceivably be used to sell power back into the grid during high demand, then recharged at a lower rate during times of low demand. It could also be used as backup power during a grid outage. A plug-in hybrid such as the Chevy Volt could even use its engine as a backup electric generator during a power outage.
Such distributed storage will require a very smart grid indeed. But it will provide a lot of redundancy and resilience to the system.
Here is the Wikipedia article on grid energy storage, which is my main source here. If you have more time, energy, and expertise than I do to look into this subject, try this report, or check out the Electric Storage Association.