Long-Duration Energy Storage
Renewable wind and solar energy have taken off in the last decade mostly because their respective manufacturing and deployment costs have been plummeting faster than expected. As a result, it has become much less expensive to build large solar and wind farms to feed significant power into nearby grids, even in remote areas of the planet. But as I have talked about in a previous article the biggest challenge with these technologies is that they are by nature, intermittent and thus, not wholly reliable in terms of power generation.
So when the sun is not shining or the wind is not blowing, the grids require some backup power generation - otherwise electricity will not be available. We could continue using natural gas or worse, coal-fired power plants to back up the solar and wind production gaps, but these are much less attractive as they will only emit more carbon-based greenhouse gases.
The Energy Transition requires energy storage
The world is moving to transition away from burning more fossil fuels so to best utilize these cleaner power alternatives like (intermittent) solar and wind, we need to pair them with energy storage.
Today, the most popular method for storing energy is to use lithium-ion (or li-on) batteries. These are familiar to us of course, as the smaller versions have been powering our phones and laptops forever.
Some companies have been building larger versions that are capable of storing more energy for longer periods of time. For example, the Goal Zero and Jackery brands each provide portable “power stations” that are handy in an emergency. When adequately charged, these devices can sit in your closet or garage for months and be ready to go when the lights go out in your house, say during a bad storm.
These portable power stations are just larger lithium-ion batteries in convenient packages.
We need Energy Storage at grid scale
The challenge here is that it is difficult to scale up current li-on batteries to provide full power, at megawatt scale, over an extended period of time. Specifically, when we want to discharge stored power for more than 4 hours continuously, as in the case of a city-wide blackout, it can become prohibitively expensive to use lithium-ion battery technology.
What is considered “long duration”?
A California group recently attempted to define the parameters for a long-duration storage solution. At a high level, a suitable plant must be able to provide a constant flow of at least 50 megawatts of electricity over an 8-hour period.
To come back to solar and wind, the problems are bigger than a few tranquil mornings or overcast afternoons. We have to recognize much larger patterns of seasonal variation in the natural world. Instead of thinking about on and off days, consider measuring periods of months. For example, in some areas of the world, strong Winter winds will slow down significantly as Spring approaches. In other regions, long Summer days will naturally recede as we head into Fall. This affects how much renewable power will be available year round so we need to consider being able to store massive amounts of energy across entire seasons.
We are getting better and predicting the availability of sunshine but it is still quite tricky to plan around the variability of wind.
Viable business models are elusive
Research and development into long-duration energy storage projects have struggled and investments in this space have routinely yielded poor returns. There is a broad range of experiments, that have shown early success “in the lab”, but there are no obvious winners and advocates are struggling to find the right cost model to support a viable business.
I will review the prevailing options, some are more proven and hold more promise than others.
Pumped hydro
Pumped hydro (PHS) is one good/proven example and by some estimates, still provides 95 percent of the storage for the U.S. power grid. Stations like the one I visited in Georgetown, Colorado are relatively inexpensive to run once they are operational but it is difficult to scale pumped hydro storage. Besides the obvious physical requirements around available water sources and sufficient elevation to drive the whole gravity-based operation, it has become much harder in recent years to obtain permits for projects like these. Still, new projects are in the works in Montana, California and Virginia. [need links from below]
Nevertheless, a handful of such projects are inching forward. The 400-megawatt Gordon Butte project in Montana has permits and financial backing; the 1,300-megawatt Eagle Mountain in California has a federal license to construct and backing from NextEra Energy. And utility Dominion Energy is working on an 800-megawatt, 10-hour duration system in southwestern Virginia.
“The big exception to technology risk is pumped hydro, which has been used at scale for decades. Those projects grapple instead with high capital expense and environmental concerns.”
An alternative gravity-based approach
Automated cranes stack and unstack massive towers of concrete blocks to store energy. Read more here.
The company Energy Vault has iterated on the pumped hydro approach and has created a working system that employs robotic cranes that can stack and unstack thousands of large concrete blocks. Relying on automated software instead of water, the system still uses the Earth’s gravity to lower the heavy blocks when power is needed and the resulting kinetic energy can be used to create electricity.
The touted benefits of Energy Vault’s system is that it can be deployed in many more places as it does not require access to water or elevated land. This “stationary storage technology” company has already raised more than $100M in investment and claims to have deployed early working versions of its offering.
Other contenders
The field really spreads out here to include all kinds of natural and technological combinations. It should be noted that some methods described below require excess electricity up front to prime the storage system, just like pumping water back up the hill or stack the concrete blocks. But constraints like this may be balanced by creating solutions that, unlike li-ion batteries, don’t degrade over time, making them much cheaper to operate.
Compressed Air - Caverns and similar cavities in the earth have been used for decades to store compressed air (CAES) which can then be released later to turn turbines that create electricity. Because it can be difficult to find the ideal underground locations, companies have also used water to help maintain appropriate pressure for the air.
Liquid Air - Highview Power is a U.K. company that has spent more than 15 years developing its liquid-air energy storage (LAES) technology, a process for cooling down and compressing air in above-ground tanks, and has already built large-scale plants for the purposes of storing energy for the grid.
Flow Batteries - There seems to be a collective fingers-crossed hope out there for an alternative to the lithium-ion battery. Relatively new companies including ESS, Form Energy and Avalon Batteries (now Invinity Energy Systems) are exploring flow batteries that use different elements like iron and vanadium to circulate liquid electrolytes to charge and discharge electrons.
Green Hydrogen - Hydrogen in all its colors (I wrote about the rainbow here) is another contender for long-duration energy storage. Green hydrogen in particular refers to the use of clean, renewable power to drive the electrolysis process for creating hydrogen.
Hot-rock Thermal Storage? - And the list goes on. I was intrigued to stumble on this article about how Siemens Gamesa is turning to using hot rocks to store energy. More on that approach and the pilot that is underway here.
Still another fascinating idea that I intend to explore more in future posts is the all-for-one type of proposal that we can tap into the aggregated battery power of hundreds or thousands of idle electric vehicles that might be conveniently pooled in a parking lot or garage during the work day. When electric utility prices spike during the day, downtown office buildings could switch away from the standard grid to this local, alternative energy source by connecting these otherwise underutilized car batteries. I do believe there will be some creative options in the vehicle-to-grid (V2G) space in the not too distant future.
Conclusion
The more I read and listen to the experts talk about the future of renewable energy, the more I’m hearing about combining cleaner energy sources with energy storage and specifically longer and longer-term energy storage. Consumers may, over time become more familiar with in-home batteries like Tesla’s Powerwall to better weather or avoid entirely any unexpected electric outages. But we will need to go bigger, much bigger to create utility-scale energy storage and push forward more rapidly on clean but intermittent renewable sources like solar and wind.