The Raccoon Mountain Pumped-Storage Plant is, in effect, a huge battery. (Tennessee Valley Authority)
George Harvey
In recent news on electricity storage, one issue that has come up repeatedly is how long the supply will last. For example, lithium-ion batteries are sometimes said to be impractical for storing electricity for more than a few hours. This is confusing, because the batteries will actually store electricity for weeks or months. What is really meant by the “few hours” is not actually how long the batteries will store electricity. What is meant is the amount of time the batteries will be able to deliver electricity if they are discharging at full power.
It is an issue that people who have designed or built small systems dependent on batteries, such as those found in homes, should be able to understand. Part of the design is figuring out the balance between the length of time the backup battery will have to deliver electricity versus the cost of the battery.
For a small system a decision on this issue will require other things to be factored in. For a person with a stove that burns cord wood, in a very well-insulated home, with water fed by gravity from a spring, a minimal system could be small. If the heating system requires electricity, insulation is imperfect, and the pump draws a heavy load, the backup system would need to be scaled quite differently.
The battery systems for utilities have similar issues, though on a very different scale. Utilities have been installing huge batteries lately, far larger than anything that might have been envisioned only a few years ago. The amount of new grid-scale battery additions in the United States has been growing at an average rate of over 23% per year for the last several years, and this is expected to continue, according to the global consulting house Frost & Sullivan, as reported in Solar Industry ().
Part of what is driving the market is a decline in battery costs. An article at Energy & Environmental Science explained the extent of the decline. It said the “real price of lithium-ion cells, scaled by their energy capacity, has declined by about 97% since their commercial introduction in 1991 ().”
All this means much bigger batteries are practical. But the falling cost of batteries is not the only thing going on. The number of new technologies that are being developed is truly astonishing. Flow batteries, liquid-air batteries, super-capacitors, and many more are in development.
Some of the research and development work is just intended to make batteries cost less. Other projects, however, are specifically intended to extend the amount of time a battery can deliver at full power. An article on this, “Big Storage,” appeared at Bloomberg Green. It talked of batteries that could deliver full power for long times, for example a system that could deliver one megawatt at full power for 150 hours ().
An article at CleanTechnica shows why renewable power and batteries are so compelling. “[The] California Energy Storage Alliance estimates the value of storage-backed solar at 2.5 cents/kWh! (kWh is kilowatt-hour) ()!”
That price, 2.5¢/kWh, is significantly lower than the best price we might expect from fossil fuels. In Lazard’s Levelized Cost of Energy and Levelized Cost of Storage – 2020, the lowest price for electricity from fossil fuels, combined cycle natural gas from fully depreciated plants, is 2.8¢/kWh. But we are comparing new solar plus batteries with old, paid down natural gas. If we want electricity at the lowest cost, and we have a fully paid down natural gas plant, the least-cost option would often be to cut down the natural gas plant and replace it with solar panels and batteries ().
If our conclusion is that it might be the time to tear down natural gas plants in general, then we can take comfort in the fact that some utilities are already doing just that. Natural gas load-following and peaking plants are being torn down already to be replaced by batteries. What was once California’s largest natural gas plant, for example, is being replaced with what will be one of the world’s biggest batteries ().
The problem of how long batteries can deliver power is also being addressed in a number of other ways. Research and development of new technologies is just one of them. Demand response using smart meters is another. Yet another is building out transmission lines. Diversifying resources is another. Adding home systems that are available to utilities and bi-directional car chargers is another. And the list goes on.
One more thing to consider is the combination of technologies available. The sun is not always shining, but it often is shining wherever the wind is not blowing. And though the wind may not be blowing in some specific place, for places on the grid it is always blowing somewhere. Any time it rains, the hydro reservoirs fill up. And there are other technologies for generating power.
Long-term battery storage may be one key to having a 100% renewable grid. But we are getting ever closer to having it.
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