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Things to know before buying cells for battery storage applications, with DNV 

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DNV’s Jason Goodhand tells Energy-Storage.news Premium about the insights learned from testing dozens of cells for this year’s Battery Scorecard report.  

Published in April, DNV’s Battery Scorecard aims to give anyone in the industry interested in buying batteries for energy storage systems a heads-up of the things they need to think about. 

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Jason Goodhand leads the engineering, advisory, certification and testing group’s energy storage practice, with around 80 subject matter experts working across five global regions, modelling, testing, and carrying out due diligence on storage equipment and projects.  

As reported by Energy-Storage.news when the Battery Scorecard was published, the questions battery buyers should ask cover everything from technology readiness and bankability to degradation profiles and lifetime expectations and, of course, safety.  

DNV has extensively researched different cell chemistries and form factors and how, even when these stay the same, cells from different vendors or even different batches can produce results that vary more than expected.  

So, while the results of the latest scorecard tests are anonymised and don’t give specific data on named vendors’ cells, the report is designed to be a valuable resource that arms buyers with the right knowledge of how to compare batteries.  

“You could get the OEMs themselves to provide you with some information about the battery, but it may not be an apples-to-apples comparison,” Goodhand says. 

“So, you don’t necessarily know what you’re getting, or they may just give you a warranty curve that says, ‘Don’t worry exactly how our battery operates; we’ll just cover you if it falls below this level.’” (Following that advice might lead to undervaluing a cells performance in your financial model.) 

DNV’s research included testing cells under various conditions, including different C-rates, whereas an OEM may only provide a single curve at, say, C/2, whereas if, in practice, the battery will charge or discharge at 1Cto provide frequency regulation, the operator has no visibility into how it will behave.  

“By testing a bunch of different characteristics, you can create a bit of a fingerprint of a particular cell. While we don’t name the individual cells on our graphs, one of the things that I think is really powerful is to show the spread,” Goodhand says. 

“We do see that there’s quite a bit of a difference between cells.” 

CATL’s non-degradation play  

CATL’s recent launch of Tener, a containerised BESS solution, caused some industry waves for two major reasons: the first being its high energy density, packing 6.25MWh of storage capacity into a 20-ft container footprint, and the manufacturer’s claim its batteries would not experience degradation over the first five years of operation.  

Asked to comment, Jason Goodhand offers a couple of theories that he says may explain how CATL is able to make the ‘non-degradation’ claim. 

“If you look at an individual cell, it’s pretty likely that it is going to degrade over time. We have actually seen increasing capacity early in the cell life and that just has to do with the break-in process during the first few hundred cycles. 

“But the long-term trend is that they’re always going to decrease in capacity over time. Now, some of the reason that you lose capacity is that lithium gets lost in solution, or moves into a layer called the solid electrolyte interface (SEI).” 

However, if a battery was pre-lithiated, putting more lithium in there than the cell needs, CATL could “sort of skip over some of that lithium loss to the SEI or the electrolyte,” Goodhand says.  

“So, as lithium is lost, it doesn’t matter. There’s still enough lithium for the other active materials that are in the battery to hit its rated capacity.” 

That’s at the cell level. At system level, there could be some operational strategy in place, where changing the battery management system (BMS) settings through the initial life of the cells could ensure the batteries are underutilised over those first five years.  

This type of strategy could involve opening up the battery voltage as cells degrade, to match the guaranteed State of Health. Goodhand notes however that CATL did not mention any operating strategy and instead focused on the SEI and electrolyte in information shared about Tener, which he says he is excited to hear more about.  

Addressing degradation  

For project owners and developers, there are a couple of strategies to address the degradation issue.  

One of those is to “simply overbuild the system for what it needs to do at the outset,” Goodhand says, but this may not be optimal, particularly in terms of Capex.  

“Instead, you can plan an augmentation schedule. You have to look at how much money you’re going to make or how you will financially optimise that asset. You also potentially have firm requirements or constraints like a contract, or some sort of performance capability that allows you to bid in for certain merchant services.” 

Weighing up all of those considerations, the buyer must then take into account the cost of purchasing the batteries upfront, the expected cost of batteries when the time to augment comes around, and other factors including project discount rate and any incentives that may be available at the outset that might not be available later. 

Degradation testing of the type DNV carried out for the Scorecard is among the best ways to figure out the degradation curve, Goodhand claims, using software to model a degradation forecast based on how the battery behaves when undergoing duty cycles.  

Application  

Of course, how your battery is cycled depends on which applications it will be performing. A BESS asset could be performing one application or set of applications for the length of a services contract, for say, three years when it enters commercial operation.  

After that contract ends, the system will be doing something else for the remaining 15 years or more.  

In other instances the BESS might be performing different applications on weekdays versus weekends, or during different seasons.  

As long as the testing team is provided with an 8760 profile (hour-by-hour analysis for a full year) or sometimes less hourly data than that, for the battery system’s applications, DNV can examine the C-rate and State of Charge (SOC) when modeling the battery to provide estimates of its degradation curve.  

An interesting fact about batteries for EV versus BESS is that it is often considered stationary energy storage is far less demanding of the battery cells than electric transport. This is because EVs will be stopping and starting frequently, and Goodhand says it’s a fairly logical assumption energy storage batteries might face less wear and tear, but not entirely true.  

“One of the things that happens when you put a battery on load for several hours at full output [as with some BESS applications]: it develops a lot of heat internally, which changes the resistance and starts to create different side reactions,” Goodhand says. 

“You can only accelerate a car for a short period of time, you don’t hit those maximum output periods for very long in a car, you reach a cruising speed. They tend to cool off quite a bit easier, so actually the BESS can be much worse than the EVs. 

“With the exception of some service vehicles like delivery trucks or Ubers, vehicles are also not used for very long hours every day. Whereas a BESS asset won’t get good ROI in most cases without being used as much as possible. I suppose quick chargers may change this on the charging side.”

One industry trend that Goodhand says DNV has observed recently is an increase in fire testing at rack level, or even full-scale burn testing. System integrators Fluence and Wartsila are among the companies to have publicly announced burn tests on complete BESS containers.  

It’s obviously a “very expensive test” he says, given it consumes larger equipment, but it speaks to a growing tendency for authorities having jurisdiction (AHJs) wanting more info on how batteries can burn.  

Energy-Storage.news’ publisher Solar Media will host the 2nd Energy Storage Summit Asia, 9-10 July 2024 in Singapore. The event will help give clarity on this nascent, yet quickly growing market, bringing together a community of credible independent generators, policymakers, banks, funds, off-takers and technology providers. For more information, go to the website.

12 November 2024
San Diego, USA
As we see more storage projects become operational, the big question is “how to maximize your assets?” This event will prepare the industry for the road ahead, looking at the core fundamentals of asset management, understanding operational challenges, along with the latest optimization and software development. You can expect to meet and network with all the key industry players from major US asset owners, operators, RTOs and ISOs, optimizers, software and analytics providers, technical consultancies, O&M technology providers and more.

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