The what and why of long-duration energy storage

Governments across the world are shifting their electrical grids away from polluting fossil fuels towards wind, solar and other forms of renewable energy. Powering your electrical grid with say 50% renewable energy is not too hard—Canada, Portugal, Spain, Germany, and many other countries are already doing that. But reaching a 100% emission-free electrical grid is difficult.

One of the difficulties relates to ensuring there is enough power when the sun isn’t shining and the wind isn’t blowing. Today, when it’s not sunny or windy, grid operators simply compensate by “ramping up” fossil fuel powered plants—essentially pressing the gas pedal to generate more power just like you would in your car. In the electrical grids of tomorrow however, we’d like to find non-polluting ways to provide that “gas pedal.”

Using big batteries to store renewable energy is part of the solution. Companies are already selling grid-scale batteries that can be recharged during the day when the sun is shining and then discharged in the evening after the sun has set. Batteries are great at providing powerful “bursts” of electricity but these bursts typically last no more than 4 hours, not nearly long enough to power a city for entire days or weeks of cloudy windless weather.

This is why companies and researchers are working on long-duration energy storage (LDES), a type of storage that, unlike your conventional battery, can discharge power over longer durations, typically anywhere from 8 hours to several weeks.

LDES can take many shapes and forms. Hydrostor, a Toronto-based company, has commercialized a type of LDES that works a bit like an underwater balloon. They ‘inflate the balloon’ when electricity is cheap by pumping compressed air into a large underwater cavern, and they ‘deflate the balloon’ when electricity is needed by releasing the entrapped air through a turbine. E-Zinc, another Toronto-based company, is developing batteries that use zinc instead of lithium which they say is a cost-effective way to store the amounts of energy needed for LDES. Others are thinking of converting electricity into hydrogen fuel, storing the fuel, and then converting it back to electricity when the electrical grid needs it most.

What did we do?

Our paper focuses on understanding how LDES technologies like these could impact our electrical grid and things like electricity prices, while also understanding under what circumstances these LDES technologies might be the most useful. Importantly, our study was technology agnostic. Rather than restricting ourselves to specific types of LDES, we modelled storage generally which allowed us to identify the LDES design parameters (e.g. the hours of discharge) that would be most relevant for different electrical grids.

To do this, we ran a mathematical optimization model that calculates the cheapest way to build and operate a 100% emission-free electrical grid under a set of assumptions. We then repeated the calculations but under a different set of assumptions—changing for example the cost of LDES or the cost of building new transmission lines. By looking at how LDES was used under each of the scenarios, we discovered when and how LDES technologies can benefit our electrical grids.

What did we discover?

Our paper found that several factors could cause LDES technologies to play a much larger role in our electrical grids than we might have previously thought.

For starters, if droughts were to intensify—as we are starting to see in many places in the world—then hydropower generation would diminish and LDES would often be the most cost-effective way to fill the gap.

Similarly, LDES could be the most cost effective way to provide energy if building, expanding, or relying on transmission lines were to become particularly difficult. There are reasons to believe this might be the case: wildfires are increasingly forcing utilities to temporarily turn off their transmission lines and building new lines is far from easy since it requires negotiations with land owners and neighbouring governments. From the perspective of electric utilities and system operators, it was also important to quantify how much the need for storage is underestimated if we overly simplify the topology of the transmission network, which can be common practice during capacity expansion planning.

Most exciting to us is that our research shows that government investment in LDES technologies, such as through the use of energy storage mandates, would lower the cost of electricity and reduce the occurrence of large spikes in electricity prices. This makes intuitive sense: LDES helps ensure that electricity prices don’t skyrocket when the sun suddenly stops shining or the wind stops blowing.

Of course, there’s no free lunch and government investments whether in LDES technologies or something else would ultimately come from taxpayers’ money. Still, investments in our electrical grids often end up being well worth their costs. The Ontario government built the Niagara Falls hydropower generation stations in the early 1900s which resulted in a boom of energy-intensive manufacturing jobs for the region. More recently, California implemented a first-in-nation energy storage mandate that has had multiplicative effects: solidifying the market for storage investors and suppliers, encouraging R&D, and producing operational benefits for the grid as more renewables are integrated. Several US states have since followed California’s lead. Investments in LDES could similarly produce stable and predictable electricity prices that reduce financial risks for industry while enabling low electricity prices for households.