Electricity Storage 8 – So What Have We Learned?

I started this thread of posts to discover whether electricity storage is a major barrier to wider deployment of renewables. So, what have we learned about electricity storage over the last nine weeks?

Renewable energy has not created the need for storage. Power stations of all types drop off the grid regularly. In a planned way, or accidentally. Whether a grid is fossil fuel or renewable, you solve the problem in the same way.

It starts with geographic and technological diversity – different types of power generation, spread across many locations. To this you add storage, commercially used since 1907. The latest research says we can deliver sustainable and cost-effective 100% renewable grids across the world.

There are differences between storage for a renewable grid and a conventional grid. You need more storage with renewables and it needs to provide different services.

In the conventional grid, large lumps of spinning metal (the generators) help stabilise frequency and voltage. You don’t have that on a renewable grid, and fast responding electricity storage is needed to keep everything stable.

What are the options for electricity storage at grid scale?

We have plenty of different technologies already commercially available to support a renewable grid.

Six Fundamental Ways to Provide Electricity Storage

The oldest is pumped hydro, in use since early in the 20^th Century. You have two reservoirs at different heights. When spare electricity is available, you use it to pump from the lower to the higher reservoir. To recover the energy, you allow the water to fall back down by gravity through a generator turbine. Despite the high capital cost and the need for the right geography, pumped hydro accounts for 95% of the global electricity storage capacity.

Pumped hydro stores energy by lifting a mass of water against gravity. Other ideas are lifting weights up and down deep shafts, dragging heavy rail cars up an incline or building towers of large concrete blocks. These can use spare electricity to drive a winch to raise the weights, then letting them down again like a grandfather clock; the winches now as generators.

There are other mechanical approaches to storage, such as spinning flywheels or compressing air into an underground chamber. The flywheels can be used for fast response and grid stabilisation, and the compressed air systems for longer-term storage.

Heat is another way to store electricity. One interesting option is using phase changes, where a material changes from solid to liquid or liquid to gas. Liquid air storage is an example of liquid-gas, and pure silicon is an example of solid-liquid phase changes. You can also use temperature differentials between two heat stores. Drive a heat pump electrically to heat one store and cool the other. Then run the heat pump in reverse to regenerate the electricity.

It would be very handy if we could store electricity directly as electricity, avoiding all the complexity and losses of converting energy from one form to another. Fortunately, there are two commercialised ways of doing it; supercapacitors and superconducting magnets. These don’t hold a large amount of power but can discharge in milliseconds. A blazingly fast response that makes them useful for grid stability applications.

Batteries are the fastest growing storage solution. Lithium-ion batteries are well known, and because of their use in electric vehicles and continuous development have seen prices fall from $730/kWh in 2013 to $150/kWh in 2022. Other battery types exist. Redox batteries, also called flow batteries, have benefits for large scale static applications such as grid-level storage. The IEA expects battery storage to overtake pumped hydro somewhere between 2025 and 2030.

Finally, you can convert electricity into a chemical form such as hydrogen. The Power-to-X approach. Green hydrogen has many applications; including green steel, chemicals and sustainable fuels. The interesting challenge with green hydrogen for electricity storage is whether it will compete with other uses. Some applications of green hydrogen have no practical alternatives, whereas we have plenty of options for electricity storage.

Which technology will win?

Which technologies can we expect to dominate in the coming years? Not surprisingly, it depends on the application. A recent study from Imperial College London looked at nine different technologies across 12 storage applications. They looked at how the Levelized Cost of Storage (LCOS) would develop to 2050, and how that affected technology choice for different uses. LCOS is the discounted cost per unit of delivered electricity and considers all lifetime costs.

How Will Levelized Cost of Storage Affect Choice of Technology

A key chart from their paper shows the best choice of technology depending on the number of discharges a year and how long the storage has to sustain the discharge. Long discharge and infrequent use for long-term high-volume storage is top left of the chart. A lot of short intense discharges for grid stability are bottom right.

The crucial difference between 2020 and 2040 is Lithium-ion batteries providing more capacity across a wider range of applications, and hydrogen becoming the key tool for very long-term storage. Between them squeezing the scope for pumped hydro and compressed air storage. I am not yet convinced about green hydrogen, but the growth of batteries seems certain.

Summary of the summary

People will tell you that 100% renewable grids are impossible because you need storage to deal with intermittent generation. That is false. Just as with conventional grids, technological and geographic diversity boost resilience. And just like conventional grids, you need storage.

We have a range of storage technologies already commercially available, using six fundamentally different methods. On top of that, enormous investment is going into increasing the cost-effectiveness, capacity and flexibility of most of these technologies. Electricity storage is not a barrier to increasing the use of renewable electricity; no matter what anyone tells you!

Previous articles:

• Renewable Energy and Storage – the Options
• Electricity Storage Options 2 – Using Gravity
• Electricity Storage Options 3 – Springs and Tops
• Electricity Storage Options 4 – Turning up the Heat
• Electricity Storage Options 5 – Pure Electricity
• Electricity Storage Options 6 – Batteries
• Electricity Storage Options 7 – Power-to-X