Finally, a way to tell how clean grid batteries actually are

A carbon-free grid needs batteries that can store solar and wind power for use when the sun isn’t shining and the wind isn’t blowing. But batteries don’t necessarily make power grids cleaner — not unless the power they charge up with is much lower-carbon than the power they displace.

Though this sounds obvious, it’s a tricky thing to measure in practice. But Quinbrook Infrastructure Partners, a clean-energy and battery developer, has made some progress.

Last month, it unveiled the results of a real-world test of an emissions measurement tool for grid batteries. That’s a vital step for energy storage projects looking to sell to customers bound by strict carbon-accounting standards — including data centers, green hydrogen producers and a growing roster of ​“24/7” clean power seekers.

These kinds of opportunities need suppliers to ​“credibly offer insights into where they’re getting their power,” said James Allan, senior director at Quinbrook. ​“Underpinning all that is being able to trace all these things on a credible, accepted standard.”

This sort of technology is critical to enabling true hour-to-hour accounting of grid emissions, a task that is itself the cornerstone of actually transitioning the power grid to 100 percent clean energy. In the course of that transition, we need a way to track how each new battery works within the system. And right now, there’s just no universally adopted way to certify that a kilowatt-hour of battery power is actually clean.

For Quinbrook, that standard comes from EnergyTag, a nonprofit backed by big clean-energy developers, buyers and certification bodies that’s spent the past four years figuring out how to track and certify clean power on an hour-by-hour basis. Earlier this month, EnergyTag released the second version of its Granular Certificate standard, which is already being used to track gigawatts’ worth of clean energy trades in Europe and the U.S.

The technology to implement the EnergyTag standard comes from Quintrace, the in-house platform Quinbrook launched last year to give customers ​“real-time visibility into their hourly carbon footprint.” Quintrace is one of a number of technology platforms now actively tracing the carbon emissions of power flowing from generators to end users across grids in the U.S. and Europe as demand for granular emissions tracking grows.

Quinbrook’s latest data shows how these standards and tracking technologies apply to a real-world operating environment — the Byrd Ranch battery project in Sweeny, Texas, about 60 miles south of Houston. The 50-megawatt battery, owned by developer Glidepath, is actively charging and discharging power to the Texas grid in response to signals from energy market operator ERCOT.

“We demonstrated you can trace the power through a battery on a sub-hourly basis,” Allan said. In other words, the project ​“proved this is possible now.”

How to track a battery’s clean-to-dirty power ratio 

To be clear, Byrd Ranch wasn’t always using clean power in its test. Instead, it tracked a certain portion of its power coming from the grid as if it was being sent directly to the battery from a hypothetical solar farm, with a typical solar project’s pattern of generation throughout the day, to ​“simulate renewable energy slices within the battery,” as Quinbrook’s white paper puts it.

That power was intermixed with everyday grid power, bearing hour-by-hour carbon emissions-intensity metrics provided by ERCOT. Quinbrook assumed that the battery used solar energy to charge whenever it was available from day to day and hour to hour, and used ​“slices” of grid power for the rest of its charging.

Keeping track of clean energy going into and coming out of a battery is complicated. The first trick is determining a commonly acceptable way to differentiate between ​“slices” of clean and dirty power; batteries don’t distinguish between electrons coming from solar panels and electrons coming from coal plants.

The time when batteries charge and discharge also complicates matters. Some batteries may charge up days before they’re called on to discharge. Other batteries may be charging and discharging multiple times per hour. That latter mode of operations applies to the Byrd Ranch battery, which primarily provides fast-responsive services to ERCOT that can be called on every 4 seconds and measured for payment in 15-minute increments.

In these cases, ​“batteries are jumping around a lot within an hour,” Allan said. Proving just how clean each of those increments of power discharges is, based on records of charging that may have come minutes beforehand, is ​“a nontrivial problem.”

Batteries have another challenge: their ​“round-trip efficiency” losses. Even the most efficient lithium-ion batteries lose about 10 percent of the power coming into them compared to the power they can discharge. These losses come mainly from energy that is dispersed in the process of converting grid alternating current to battery-ready direct current — and back again when the battery is discharged to send power back to the grid — as well as the power used to operate the battery and power conversion and control equipment itself. Batteries also slowly discharge when at rest, a form of loss called ​“self-discharge.”

In simple terms, that means that every unit of power going into a battery yields nine-tenths or less of a unit of power coming out of it. Any system that tracks clean versus dirty power going into a battery must take these losses into account, as this graphic from Quinbrook’s white paper illustrates.

But the losses in any particular battery system ​“are estimates in most cases — they’re not hard numbers,” Allan said. Today, these figures are calculated based on metering how much energy goes into and out of a battery and combining that with a ​“state-of-charge” measurement — how full the battery is, basically.

But these state-of-charge measurements are notoriously imprecise and can be subject to ​“errors due to temperature fluctuations, voltage plateaus across the state of charge, sensor calibration, and other data measurement issues,” Quinbrook’s white paper notes. (Think of the odd and rapid changes in battery-charge status that can occur with smartphones, only multiplied by orders of magnitude.)

Finding ways to quantify and track these uncertain shifts in measured data from hour to hour is vital for any system that purports to accurately measure the emissions-intensity of a battery, Allan said. Without some method of catching erroneous measurements, ​“depending on how you carry that through the accounting process, you can have compounding errors,” he said — ​“and you can get an increasingly silly result.”

Why batteries need a way to track the carbon emissions of their power 

Quinbrook’s white paper gets into the details of how it managed battery measurement challenges through the course of its Byrd Ranch project: by using two of the options the EnergyTag standard offers.

The core goal of EnergyTag’s standards is to give power producers, buyers and battery operators a clear understanding of how they’re accounting for the variables involved in treating batteries as a ​“reservoir” for ​“time-shifted” energy, Katrien Verwimp, EnergyTag’s audit committee chair, said during a March webinar introducing the nonprofit group’s latest standards.

Batteries are commonly described in these terms — as a way to move clean energy from when it’s abundant to when it’s needed — but without precise measurement, it’s impossible to guarantee that a grid-connected battery is actually doing that.

Any system that asks power buyers to certify the emissions-intensity of their electricity needs to ​“have clarity on recording all these attributes, and also taking a track or keeping track of the losses” at the batteries, she said. EnergyTag’s recently released second-version standards offer ​“much further detail than what went into the first version,” she said — but more testing in real-world conditions is underway.

Most of today’s grid batteries charge when power is cheap and discharge when it’s expensive — and high and low power prices don’t necessarily correlate to high and low carbon emissions. Excess midday solar power can be cheap — but so can coal-fired power being generated in the wee hours of the night, when fewer people want to buy it.

These dynamics are in part why a 2015 study by professors Eric Hittinger at the Rochester Institute of Technology and Inês Azevedo at Carnegie Mellon University found that U.S. grid batteries were, on balance, more likely increasing rather than decreasing electricity-sector emissions.

This problem is only beginning to be tackled by governments and regulators. California was an early mover, issuing regulations in 2019 meant to ensure that batteries funded by a multibillion-dollar state incentive program would be operated in ways that lead to an overall reduction in grid emissions. But that program didn’t include standards for tracking charging and discharging as closely as the EnergyTag standard is taking on.

But a commonly accepted and verifiable standard is becoming increasingly important for battery operators, Allan said. Already, major clean-energy buyers like Google and Microsoft are demanding a way to include batteries in their 24/7 carbon-free energy procurements, which are part of a goal to power their data centers with round-the-clock carbon-free energy by 2030. Quinbrook got started on its Quintrace work ​“as part of our green data center strategy,” he said.

In the past few years, government mandates have been added to these corporate pressures, he added — notably, the strict eligibility requirements for U.S. and EU green hydrogen subsidies. Batteries will need standards like EnergyTag to play a role in providing some of the clean power that electrolyzers will need to meet strict hourly-matching rules under the 45V hydrogen tax credit regime proposed by the Biden administration and similar clean-energy rules for hydrogen producers in the European Union, for example.

Allan expects more and more industries and governments will adopt hourly clean-energy matching regimes as renewable energy grows to make up a majority of the electricity flowing across grids in the U.S., Europe and beyond. At that point, having the standards and measurements that allow battery operators to sign contracts and meet mandates to deliver carbon-free energy during hours when the sun isn’t shining or the wind isn’t blowing will become far more necessary and valuable.

“We’re solving difficult problems for green data centers — and this is equally applicable to green commodities,” Allan said. ​“If you’re doing green hydrogen — or green steel, or green chemicals — you have to be able to support your green credentials.”