Over the past decade, prices for solar panels and wind farms have reached all-time lows. As the saying goes though, the wind isn’t always blowing and the sun isn’t always shining.

The question of how to “firm” renewables — that is, ensuring there’s always enough saved energy no matter the time of day or weather — is one of the biggest challenges in the industry. We need a good way to store energy for later. The go-to option right now is lithium ion batteries.

But, though lithium ion is dropping in price, experts say it will remain too expensive for most grid-scale applications. Right now, lithium ion batteries just can’t store more than four hours worth of energy at a price point that would make sense. Plus, they pose a fire risk and their ability to hold a charge fades over time.

Companies are experimenting with a variety of different solutions, including flow batteries, thermal batteries, and gravity-based systems. If any of these player can crack the code to long-duration energy storage, renewables like wind and solar will finally be able to compete with fossil fuels.