Rarely has such a crucial enterprise for the future of human civilization led to such little commercial success.

Long-duration energy storage holds great potential for a world in which wind and solar power dominate new power plant additions and gradually overtake other sources of electricity. Wind and solar only produce at certain times, so they need a complementary technology to help fill the gaps. And the lithium-ion batteries that supply 99 percent of new storage capacity today get very expensive if you try to stretch them out over many hours.

The problem is, no clear winner has emerged to play that long-duration role. Here at Greentech Media, we’ve spent years covering the contenders, which range from quixotic defiers of the laws of physics to understated, scientifically minded strivers. The makeup of this roster has fluctuated to the rhythm of bankruptcies and new investments.

Plenty of options technically “work.” The question is, do they work with an acceptable price point and development cycle, and can the businesses providing them stay afloat long enough to actually prove that? That last step has been hard for companies to fulfill, insofar as in previous years there were practically no places to actually sell this stuff.

That’s finally starting to change, thanks to two connected trends. First, wind and solar are now competing very effectively for capacity additions in the U.S. and other developed countries. The proliferation of these resources creates its own push for long-duration storage in places with high concentrations of wind and solar farms. A particularly appealing early market is in remote or island grids, where renewables-plus-storage already outcompete imported diesel fuel on price.

Second, spurred by this success, many utility companies, states and nations are upping their targets for clean energy. Once a jurisdiction officially commits to 100 percent carbon-free power, it has to start thinking in earnest about how to replace the gas plants that currently provide the flexible counterpart to renewables’ ups and downs. These policies typically give prime billing to the clean energy sources, but they just as well could be considered market-creation tools for the long-duration storage asset class.

In light of these developments, we've rounded up the surviving long-duration contenders in one convenient place, in no particular order. Criteria for selection include: plausible technology; recent investment; market traction (graded on a generous long-duration curve); and, for companies, not being bankrupt. Bookmark this page and come back in a decade to see how we did.

1. Pumped hydro

Midcentury modern design is hot again, so why not midcentury storage technology? This gravity-based concept physically moves water from a low to a high reservoir, from which the water descends, when needed, to generate electricity. This dates from way before lithium-ion’s heyday and still provides some 95 percent of U.S. grid storage, according to the U.S. Department of Energy.

Once built, these systems boast a very low cost of storage, and they hold truly massive amounts of energy compared to even the world’s biggest battery. The problem is that it’s extremely difficult to build new pumped-hydro storage plants, due to the permitting implications of large water-based infrastructure and recent difficulty in executing massive construction projects in general.

The new school of pumped hydro focuses on isolated reservoirs that don’t disrupt river ecosystems; this simplifies permitting, but projects still face a decade-long development timeline and billion-dollar price tags.

Nevertheless, a handful of such projects are inching forward. The 400-megawatt Gordon Butte project in Montana has permits and financial backing; the 1,300-megawatt Eagle Mountain in California has a federal license to construct and backing from NextEra Energy. And utility Dominion Energy is working on an 800-megawatt, 10-hour duration system in southwestern Virginia.

The rise of renewables has forced a new look at this old technology. Now it’s up to the pumped hydro industry to deliver.

2. Stacked blocks

What if, instead of using batteries or pumping water, you stored surplus power by automating a six-armed robotic crane to stack thousands of purpose-built, 35-metric-ton monoliths into a Babel-like tower and drop them down again when you needed to release the power?

That’s the provocative question asked by startup Energy Vault, a spinoff of serial software entrepreneur Bill Gross’ Idealab incubator.

If slick slide decks decided technological arms races, this company would rule the sector. The idea arose from iterating on pumped hydro’s gravity storage, but adapting it for greater geographic diversity and to avoid the limitations described above. Gross threw in technological advancements that had already matured in other industries — machine vision, concrete fabrication, cranes — and came up with a wholly original grid storage concoction.

That vision landed the biggest-ever investment in a stationary storage technology startup: $110 million from SoftBank last summer. And Tata Power signed up for an early 35-megawatt-hour installation due in 2019, signaling interest from serious customers.

If that system is now up and running, Energy Vault hasn’t said so. Establishing that it works in commercial practice is crucial for all new technologies in this space. And in an industry where slight deviations from standard operating practices can spook customers and financiers, Energy Vault has a lot of trust-building to do.

3. Liquid air

Highview Power doesn’t consider itself a startup anymore. After 15 years of refining its technology, this U.K.-based company has moved from running pilots to developing large-scale plants.

The company’s mechanism cools down air and stores it in pressurized above-ground tanks. The compression equipment and power generators come from established supply chains in mature industries. The technological innovation here is using them for grid storage.

Highview’s leaders realize they need to self-finance early projects to show the market that they work. They raised $46 million from Sumitomo Heavy Industries in February to do just that.

4. Underground compressed air

Humans have stored power in underground caverns for decades: A plant in Huntorf, Germany dates from 1978, and Alabama's McIntosh plant opened in 1991. But those pioneering projects failed to kick off a trend. Several startups tried and failed to improve on that technique.

The basic concept is to use excess electricity to pump compressed air into a suitable underground formation that acts like a giant storage tank. Releasing the pressurized air allows the plant to re-generate electricity when needed.

But not everybody has ideally structured salt domes in their own backyard. Canadian company Hydrostor took a different approach: pumping compressed air into purpose-built caves or existing ones, such as abandoned mine shafts, and using water to maintain pressure. The water keeps things at a constant pressure and allows for the use of smaller cavities than typically used in traditional techniques. The goal is to liberate compressed-air storage from the geological constraints that held it back while minimizing technology risk thanks to equipment borrowed from other industries.

Hydrostor has a commercial system operating in Canada and is wrapping up a demonstration site in Australia. Last fall it raised another $37 million to develop a pipeline of increasingly ambitious plants.

5. Flow batteries

Flow batteries have been considered promising for as long as anyone’s thought about long-duration storage, but that hasn’t given them many advantages in the marketplace.

The archetypal flow battery company is either insolvent or still aspiring to its first substantial commercial deployment. But a lot of flow battery scientists swear by the technology, which circulates liquid electrolytes to charge or discharge electrons via redox reaction.

The company ESS is still working on building its first utility-scale project, but it raised another $30 million in November on the strength of its iron flow chemistry. It prides itself on using a cheap and abundant material for its active ingredient. That draws a contrast with the vanadium flow acolytes that hitched their wagons to a mineral that took a wild ride in the commodity markets in recent years.

Avalon Batteries, arguably the most successful vanadium flow maker in terms of number of systems deployed, found a way around the materials cost challenge. It created an arrangement for renting vanadium from mining companies, which would like to see a new market for their product. By mass-producing turnkey systems in a factory, Avalon has shipped 160 flow batteries, escaping the dead-end little leagues of hand-built pilots. These systems are not long-duration per se, but they compete with lithium-ion batteries on cycle life in high-throughput applications.

On the strength of that showing, Avalon will soon take over U.K. flow company RedT, which innovated commercially but suffered from chronic stock price declines and looming capital requirements.

That deal brought in new investment and marks an unusually positive development for the sector by showing that flow battery M&A activity can take place outside of the bankruptcy-induced fire sale.