If you've ever worked with renewable energy, you know that storage is a major stumbling block. Unlike with a coal-fired or natural gas power plant, you can't adjust hydropower energy, wind energy and solar energy production based on grid needs.

Producing enough energy for a given area probably means overproducing when grid demand is low. You're also likely underproducing when grid demand is high, meaning your green energy source still relies on fossil fuels to keep the grid powered.

Batteries are available, and battery tech has come a long way in the past few years. While suitable for a home solar system, however, these batteries are expensive and not practical for long-term or grid-level storage.

Energy storage startups have found a new way to store energy. They're pumping it underground. This is how green energy startups are using a new kind of pumped hydropower to store energy.

How Pumped Hydropower Stores Energy Underground

The technology works by effectively converting excess energy created by wind turbines and other green energy sources into potential energy that a company can later draw from.

In times of low energy demand, the system pulls excess energy from the grid to pump water up a hill from a lower reservoir to a higher reservoir. When energy production slows down, the water in the upper reservoir is released, flows downhill and spins a turbine, producing energy.

The tech isn't new — pumped hydropower accounts for about 95 percent of all stored energy projects in the U.S. Almost all existing hydropower projects are old, however — many were built in the 1970s and '80s. High costs, the limited utility of the projects in conventional energy applications and the difficulty of securing enough land has made new construction impractical.

Some startups behind the revival of pumped hydropower are taking a new, less land-intensive approach. Instead of pumping the water into an upper reservoir, they're pumping it underground. When energy demand is low, this system pumps water into an underground well, where it's stored under pressure. When energy demand exceeds the capacity of green sources, the well water is released, rushes back to the surface and spins a turbine.

The resurgence of the tech is part of a turn towards more eco-friendly construction and energy — like the increased popularity of water-filled cofferdams, which have a lower environmental impact than more conventional earth-filled cofferdams. The approach is a new one and hasn't been tested at scale yet. The first attempts at the project aim to store an estimated 10 hours of energy.

New Pumped Hydropower Projects Given the Go-Ahead

The rise of the underground hydropower approach has also been accompanied by renewed interest in traditional hydropower storage. One pumped hydropower company, Phoenix-based Pumped Hydro Storage, LLC, recently applied for a preliminary permit for a project that would create pumped hydropower storage along the Colorado River.

There are some concerns, however, about the potential environmental impacts the energy storage strategy may have. The proposed Colorado River project is an open-loop hydropower storage project, meaning water is pumped out of a natural body of water, not a reservoir, and released from the upper reservoir back into that body of water.

American Rivers, a nonprofit environmental protection charity, has already intervened in the project's approval process, citing potential environmental damage to the Colorado River.

Closed-loop, underground hydropower storage projects may not create the same environmental hazards, especially if they take advantage of existing underground structures like abandoned mineshafts and natural caverns to create reservoirs. However, it's likely that any hydropower storage project will have some environmental impact. This impact will need to be weighed against the utility the project provides.

Pumped Hydropower May Help Solve Green Energy's Storage Problem

The demand for renewable energy is likely to continue growing at a rapid pace in the coming years. Green energy companies will need to find some way to manage energy storage requirements.

Renewable energy systems are expanding rapidly, but they remain limited by the difficulty of scaling or storing the energy they produce. Stored hydropower is one potential solution that's picking up speed. However, even the most eco-friendly and least land-intensive version of the tech may come with an environmental cost. Energy professionals should keep these costs in mind when considering the technology.