DFN: One of the problems Calpine had was that the electricity it produced had to be used, or plants had to be shutdown to stop producing, if there was no customer to buy / use the electricity
How large-scale energy storage works
There are three ways to store large amounts of electricity.
Right now, both in the United States and internationally, there is a huge effort underway to bring sources of renewable power online. Huge wind farms and solar arrays are being built all across the country. There is just one small problem — right now, these sources of power are unreliable.
On some days the wind blows briskly and there is lots of wind power available. Other days the wind may not blow at all. The sun shines during the day, but never at night. On cloudy days, a solar power plant produces far less than its full potential.
The problem is that people expect the power grid to work all the time. Our computers, refrigerators, air conditioners, TVs and so on are useless without electricity. With a coal-fired power plant or a nuclear power plant, reliability is easy. These plants run consistently, 24 hours a day, seven days a week. They are not dependent on variables like the weather or the hour of the day.
The problem with the unreliability of wind and solar power is that power companies can’t shut down their coal fired power plants. If the wind doesn’t blow for a day, or if it rains for a day or two on the solar arrays, there needs to be a reliable way to replace the lost electricity.
What this has done is put a new emphasis on power storage. Right now, the nation’s power grid really does not need to store power. All the electricity we use is generated on an as-needed basis, with plant managers adjusting the output of their power plants as the demand for power changes during the day. What wind and solar plants need is a way to store a day or two of electricity so they can release it evenly and reliably. A solar plant, for example, needs to be able to produce a consistent level of power reliably, sunshine or no.
Currently there are three ways to store large amounts of electricity. The oldest, and therefore the most common, is to use water. A power plant will use two bodies of water — for example, one lake that is uphill from another lake — with large pipes connecting the two lakes together. The water from the pipes runs through hydroelectric turbines. When the power plant is generating excess power, the turbines act like pumps and pump water uphill to the upper lake. When power is needed, the turbines reverse to act like generators and use the energy of the water falling from the upper lake to produce electricity.
This system has been in use for decades, it is well understood and extremely reliable. However, it does depend on topography. Solar plants are often built in large, very flat desert areas. Deserts do now lend themselves to this approach very well because there are no hills to house the upper lake, there is a scarcity of water to fill the lakes, and water tends to evaporate too quickly even if the lakes could be filled.
Solar plants therefore often use a second approach that involves molten salt. Many large-scale solar plants are designed to concentrate the sun’s rays to produce very high temperatures. Parabolic trough collectors concentrate sunlight onto a pipe. Solar towers use hundreds of huge mirrors that all focus their reflections on a single point on top of a tower. The heat is so great that it can melt salt. This molten salt is then used to heat water to create steam that drives a steam turbine.
One way to store power is to store the molten salt in large, insulated tanks. When the sun isn’t shining at night, the heat stored in the tanks can continue to boil water to create steam. Solar power turns form a daytime activity into a consistent 24-hour source of electricity.
Wind farms are often situated in places (for example in the ocean, or in the plains of North Dakota) where pumped storage doesn’t work. And wind power does not start with heat like solar energy often does. So there is a third technology that wind farms are starting to employ. Large, sealed underground caverns are filled with pressurized air. The electricity that the wind farm produces drives huge air compressors that fill the caverns. When the wind dies, the compressed air comes out of the caverns to drive electrical generators.
With these storage technologies in place, solar and wind farms can become reliable parts of the power grid.
Contact Marshall Brain at marshall.brainm
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