Signing the cheapest PPA is like conquering the Iron Throne: Before you even have time to enjoy the accomplishment, someone's on their way to take it from you.

In May, NextEra Energy Resources secured a 4.5-cent per kilowatt-hour solar-plus-storage PPA with Tucson Electric Power. That's low for just solar, but the 100 megawatts of generation will come backed by 30 megawatts of 4-hour-duration energystorage The system is set to run by the end of 2019.

Now, a usurper approaches. Not from the well-heeled realm of lithium-ion, where the cash flows and the revenues stack, but from the rugged wilds where the flow batteries bide their time -- watching, waiting.

Armed with a potent concoction of zinc and iron, ViZn Energy Systems has vowed to replicate the TEP deal with its own technology at 4 cents per kilowatt-hour, 20 percent below the current reigning king. That's actually a conservative estimate, the company says.

This challenge remains hypothetical -- ViZn has not yet signed a deal at this scale. But it's more than a thought experiment, said Mike Grunow, vice president of marketing.

"We wanted to illustrate from a total cost of ownership perspective how the ViZn adder to the TEP PPA is less than the lithium-ion adder to the TEP PPA," he told me.

Lithium-ion has grown up in stationary storage markets that favor power applications, like PJM's frequency regulation market and California's behind-the-meter demand-charge management play. Now, though, the market has evolved to storage-firmed renewables plants with long-term PPAs.

This year has seen a few record-breakers already. Nearly all the major solar developers are actively chasing such deals, so more surprises are sure to come.

After 20 years of service, some lithium-ion frailties become apparent. Those batteries need a suite of temperature control equipment to stay cool in the heat of the Arizona desert; that parasitic load cuts into the overall cost-effectiveness of the project. The less volatile zinc-iron flow chemistry doesn't need such thermal controls.

Lithium-ion cells, at this stage in the technology's evolution, have a more limited life expectancy than solar PV modules. Some models predict a need for 50 to 85 percent replenishment between years seven and 10 of a battery system's operating life, Grunow said.

That incurs a capital cost for the new battery components, but also an operational cost associated with calibrating the new cells to work alongside a system that will be close to a decade out of date.

"The fact that they degrade 3 to 5 percent per year and only last seven to 10 years is a big deal when you're looking at projects that will be around for 20 years," Grunow said.

ViZn, on the other hand, offers a "bumper to bumper" warranty for 20 years, including two cycles a day and a 95 percent power capacity guarantee, he said.

The 4 cent per kilowatt-hour quote does not factor in additional revenue that the batteries could generate from a second daily charge cycle overnight to provide capacity or grid services for the morning peak (that can happen after the federal Investment Tax Credit sunsets, taking away the incentive to charge mostly or entirely from solar power). That means the cost adder for storage could be even lower.

The core argument is that when a developer uses lithium-ion for a 20-year project, they bear costs that get passed along to the offtaker, preventing an even lower PPA price. ViZn has proposed that flow technology can already avoid some of those costs and come in lower on the total cost of ownership.

Now, a little expectation-setting.

ViZn is still a young startup with just five commercial-scale systems deployed for a few years of run time. The company has made $25 million in bookings so far this year and is "well on our path to deploying a bunch of these systems globally," according to Grunow. That's a far cry from the scale of 30-megawatt monster projects, which are equivalent to the size of the largest lithium-ion system installed today. (Tesla is currently building a 100-megawatt project.)

The challenge for this and other flow battery contenders is to convince large, conservative customers that a young upstart with a new chemistry can fulfill its promises of 20-year performance without degradation, and will be around long enough to keep that promise. ViZn has enlisted bigger and better-known partners for controls and hardware, EPC services and financing to help make that case.

The end of NextEra's low-PPA reign has not yet been written, and lithium-ion technology ruled a mighty 96.5 percent of U.S. energy storage deployments last quarter. Somewhere, out on the heath, long-duration batteries are sharpening their swords. The 20-year PPA market may be the battlefield where their rebellion erupts.