energy technology

Ten types of energy storage

A transition to 100% renewable energy is not just a matter of closing down coal power stations and building wind turbines instead. They function differently on the grid, with the main difference being the variability of renewable energy. Wind and solar power depend on the weather, something we don’t have any control over. The times when power is available aren’t necessarily the times when it is most needed. I experience this most days of the year, when we start cooking the evening meal just as the sun drops off the solar panels.

Somewhat bizarrely, I still regularly see sceptics of renewable energy point out variability as if nobody’s thought of it. And of course they have. The National Grid has already incorporated huge amounts of renewable electricity into the grid, and we don’t notice it because it works.

To get to 100% though, we’ll need to be able to store power to ensure an even supply. We don’t want to have to fire up gas to make up the difference, and that means storage. The form of storage that comes to mind first is generally batteries, but that’s just one of the many options. Here are some of them:

1. Batteries: Lithium ion batteries, already available for domestic and grid scale use, are the tip of the iceberg. More sustainable and long term types of battery already exist, such as phase change, flow, and salt batteries. Some of these, such as Aquion’s saltwater battery, have been slightly ahead of their time and haven’t found a market yet, but they will. Electric cars also have batteries in them, and in future a vast network of plugged in cars could be used for grid balancing.

2. Hydrogen: Because it is easy to make through electrolysis, spare renewable energy can be used to make hydrogen. This can go into the gas grid or stored to use later, and it’s already being done in several places, including the Orkneys. With large scale storage, such as in salt caverns, this is one of the few ways to bank a large amount of energy from season to season. Countries in the northern hemisphere could use hydrogen to store summer solar power for heating in the winter.

3. Gravity: the main form of energy storage on the grid in Britain at the moment is pumped hydro, which is gravity powered. Water is pumped uphill into a reservoir when power is cheap, and then released when needed. You can visit a power plant under a mountain at Dinorwig. Gravity is a whole category though, and would include falling weights in mine shafts that I described recently, or hauling retired railway cars up hills and letting the roll back down.

4. Air storage: Compressed air can be used for energy storage, at large or small scale – pressurising the air and then releasing it to generate power. A related technology that’s been in the news recently is the world’s largest air battery, just given the green light in Manchester. That’s a cryogenic battery that will hold the air as a super-chilled liquid.

5. Flywheels: an old idea with new applications, flywheels have been used since the industrial revolution to accumulate power and release it. They’re used more for balancing loads than mass storage, but New York has a plant with 200 spinning wheels. They are cheaper than lithium batteries, and the newest ones spin with magnetic levitation instead of ball bearings, so they can be very low maintenance.

6. Water pressure: It was theoretical for a long time, but the world now has several examples of energy storage underwater. Some of them overlap with air pressure, such as the Canadian firm Hydrostor. Others, such as the StenSea project in Germany, pump water into concrete spheres and use the pressure to generate electricity when it is needed.

7. Smart appliances: Taking a holistic view of the energy system, appliances can play a role in storing power and balancing demand. New forms of fridge, such as Sure Chill, can hold their temperature for a long time. Smart fridges would be able to run while solar power was available and then switch off. Night storage heaters can do the same thing with heat.

8. Buildings: Speaking of storing heat, buildings themselves can be considered a form of storage, according to a report out of Italy last month. A well insulated building can hold warmth for days, making it possible to plan ahead and heat a building when renewable energy is available. This is a pretty novel idea, but it’s exactly the kind of system thinking that a 100% renewable energy transition will need.

9. Heat: also relatively new to the market are phase change batteries that store surplus electricity as heat, ready to be used for hot water or heating later. This is done with a phase change chemical, which basically lasts forever and has no moving parts. I looked at getting a Sunamp unit installed in my house, but couldn’t physically fit it anywhere. In future, they’ll be fitted to new homes as standard, instead of a hot water tank.

10. Molten salt: Finally, ‘solar power doesn’t work at night’ is the kind of lazy snark that one still hears from time to time from sceptics, and that’s why molten salt is important, because then you can run your solar power plant at night. Using molten salt in concentrated solar power plants makes them operational 24 hours a day, and it’s a big reason why the cost of CSP has fallen dramatically in recent years, after it looked as if the sector would never come to anything.


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