This isn’t the case with the national energy grid. Whatever energy is generated, it goes directly into the copper wires of the transmission network. This is fine most of the time, because you can run more or fewer generators, and they can be made to work at higher or lower efficiency to meet immediate needs.
The real problem comes when we try to be green. Some fuel for generators can be used when needed, but other sources of energy have to be used when they are available. For example, it doesn’t matter whether you burn coal today or ten years from now, you still get the energy from it.
But think about wind power. If you don’t generate electricity from the wind as it blows, then you can’t go back later and use it – it’s gone with the wind. Same with solar energy, if you don’t harvest today’s sunshine, you can’t come back tomorrow and find it. Sure, you can use tomorrow’s sunshine, as long as it’s sunny – but not everyday is sunny.
As more and more electricity is generated from green sources, we need to harvest as much of it as we can when we can. This means that we need to be able to store energy in some form. This large-scale energy storage is the focus of much current research and even more construction.
If we can’t store electricity as electricity, it means we have to convert (transduce) it to some form of potential energy. Research and engineering is showing that we can do this in several ways. Let’s look at a few:
Compressed air storage – One source of potential energy is air under pressure. Of course, it would have to be a whole bunch of air, like an abandoned mine volume of air - or one speech from a politician. Several of these large-scale energy storage mechanisms have been set up in Europe, using mines or caverns.
Newer proposals seek to use pipelines to store the compressed air.
Under pressure, the air can remain as a source potential energy for an undetermined time. When the grid needs more electricity, the pressurized air is allowed to escape, passing across turbine blades and turning a generator. Basically, it’s electricity to wind to electricity.
Compressed air storage is about 45% efficient. If you use the heat created by compressing the air (pushing the molecules together creates friction and heat) to heat the air when it expands (usually it cools greatly, like when you spray off your computer keyboard with that can of air and the long red straw), you can increase the efficiency to about 70%. That ain’t bad.
Hydrogen – Hydrogen gas is a very dense fuel source, meaning that you get a lot of energy for the amount of fuel you use. However, you first have to produce the hydrogen. One way is to split water, just like plants do during photosynthesis. While plants use the power of the sun to split water, we can use electricity – this is called power to gas generation. Gas generated is usually hydrogen from water, but methane can also be produced from carbon dioxide plus water.
The hydrogen gas produced is then stored, similar to the compressed air storage described above. For more efficiency in storage, the hydrogen can be cooled and pressurized to be stored as a liquid. When electricity needs to be generated, the gas can be burned to heat water for conventional turbine generators, or it could be put through large fuel cells, as we discussed two weeks ago.
Caverns and mines can be utilized for storage, but Germany uses mostly hydrogen pipelines for storage, and has done so for many years. In fact, German hydrogen storage is some 5000x greater then their pumped water storage capability. I’d worry about explosions. Remember the Hindenburg - that was hydrogen gas.
Gordon Butte project will build a pair of 1.3 billion gallon (6.4 billion liter) reservoirs, one atop the butte and one 1025 ft (312 m) below, at the bottom of the butte. The U.S. and China have many of these facilities, the largest of which is located on the Virginia, West Virginia border (Bath County, 3 GW).
When there is excess energy in the grid, it will automatically be used to power pumps that will move water from the lower reservoir to the upper. This mass of water, positioned in the top reservoir is a powerful source of potential energy.
During peak usage hours, the water is allowed to fall to the lower reservoir, through a turbine that then powers a generator. At this point, the system acts exactly like a typical hydroelectric plant. These mechanisms are very efficient, returning 75-85% of the energy invested in them. The problems: you need sufficient space at two nearby locations, but at very different elevations, and two, reservoirs are very expensive to dig. I wonder if drought would be a problem.
The real advantage to pumped water storage over other large-scale storage methods is the timing. Pumping or generating can begin within just 5-7 minutes of declared need, while compressed air storage facilities take more than 30 minutes to ramp up.
V2H (vehicle to home) system also allows you to draw energy from the car battery in case your house power lines are down.
A study from 2011 used mathematics (eww!) to estimate the viability of electric vehicles as a large scale energy storage mechanism. In general, two things will have to happen. One, 10 million or more people (in U.S.) need to own these cars. And two, they have to be able to plug in their cars at work. Only with work and home charging will enough cars be plugged in at any one time so that a grid need will be met, either by pumping more energy into the car batteries or taking a bit of energy from each car.
There are several other methods – large, rechargeable batteries are starting to be used. Painesville, OH has a 1 MW vanadium battery in use, as well as large flywheels, or thermal storage. You can investigate these yourself and figure out how best to make green energy pay off in the long run..
Contributed by Mark E. Lasbury, MS, MSEd, PhD