Power storage may be the big story in renewable energy in the next few years, thanks to a proliferation of battery chemistries. Lithium-ion varieties, however, may be overshadowing other chemistries, such as flow batteries and the always evolving lead-acid. Consider, for example, Axion Power International’s (www.axionpower.com) lead-carbon, or PbC battery. The company calls it an advanced lead-acid design because it replaced the negative lead plate with an activated carbon plate giving the battery unusual characteristics, such as super-capacitor qualities. In addition, the PbC design uses the same caustic plate as a lead-acid battery along with components, casing, and electrolytes, and it’s recyclable.
“The failure mechanism in a conventional lead-acid battery is the negative plate,” says Axion CEO Tom Granville. “So we have taken the lead completely out of the negative plate and only use it in the positive plate.” The result is a battery that lasts three to four times longer than a conventional design.
An inherent and more useful property of the PbC battery is the activated carbon plate’s tendency to control charge rate. When charging a string without a battery management system, the batteries equalize without intervention. The battery with a low charge has a low internal resistance, so it accepts the most charge; batteries with a higher internal charge have a higher resistance, thus resisting the charge. Even when not charging, the string voltage supports one another. Additionally, a typical lead-acid battery close to 100% could be over charged, causing it to emit gas, dry out, lose design function and eventually expire.
With reference to discharges, Granville says some lead-acid battery manufacturers talk of 100% discharge, but they really mean 80% because the last 20% is unusable. Typically, the PbC battery operates at about 80% state of charge.
“We have taken the battery to zero charge and back again for more than 2,500 cycles. By comparison, the website for a top of the line lead-acid battery mentions 550 cycles,” he says.
Consider the recharge rate for comparisons to other batteries. The Axion design recharges about twice as fast, and in some cases, four times faster than other battery chemistries – including lithium-ion. In addition the PbC charge acceptance far exceeds that of its lead-acid competitors, especially over time. In the first couple of months on the job, a lead-acid battery has good charge acceptance characteristics, but these characteristics are lost as lead crystals begin to form on the negative plate. That is, as lead-acid batteries age, their ability to accept charge – in partial state of charge applications – significantly decreases, especially when compared to a PbC battery.
“So six months out, the PbC design charges four times faster, and nine months out, about 10 times faster than conventional lead-acid batteries,” says Granville.
Lithium batteries’ also take longer to recharge than PbC because of the heat factor created in recharging. So in applications such as frequency regulation, the need for fast continual recharging will create heat and reduce the battery’s ability to fully function. Of course, the heat build up also affects Lithium batterys’ life expectancy.
Cost comparisons with other batteries depend, of course, on the application. With regard to cost, the PbC design is approximately one and a half to two times that of an advanced lead-acid battery, but Granville says Axion’s battery will last four times as long.
“For a few comparisons with Lithium chemistry, if we are talking about Li-phosphate, we are approximately 30% cheaper. If the comparison is to Lithium titanate, then we are about one-third the cost,” he clarifies.
The whole point of power storage is to stabilize the grid by regulating frequency. For this task Axion builds the PowerCube, a scalable megawatt storage system in a custom shipping container.
“The idea is to keep the line frequency in balance, keep a level sine wave,” says Granville. “The further out of synch line frequency gets, the more power it takes to bring the system back into balance. For instance, if 1 MW out, it might take 2 MW to bring it back in, if 2 MW out, it might take 6 to bring it back in, and so on. So it behooves the utility to correct the frequency and bring it back as soon as possible.”
Demand response programs allow for the PowerCube to go online within milliseconds of receiving a power request from the utility through the Connection Service Provider (CSP).
“Power requests are almost continuous. The utility provides payment in one hour increments, even if it only needs the power for a minute. So we sort of hopscotch from one request to the next,” Granville adds.
Utilities have moved toward a pay-for-performance model, meaning the faster a power provider can respond to them, the more it gets paid. Granville says the Axion system responds in 55 ms.
“When that utility started the program, it was taking about 15 minutes to respond to a power request. That was further trimmed to three minutes, and that kicked out some batteries that could not respond quickly enough. Our score, on a scale of 0 to 100 with 70 as passing, has averaged over 94 since we began participating in 2011,” he continues.
In the frequency regulation market, the battery does not discharge much more than 30 to 35%. In this application, Granville says the chemistry is capable of hundreds of thousands of such cycles. What’s more, there is huge demand this winter for frequency regulation. The PowerCube on the company’s site has been tied into the PJM net since November 2011, allowing the company to participate in the regulation task every day. PJM pays Axion to accept an electrical charge from them and to in turn provide it in a demand response application.
To assist with other applications, the company developed a math model to simulate them. “We worked with the CSP to establish a minimum bid,” Granville explains. “We bid in at the rate of say, 5 ¢/kWh minimum, and if the utility accepts, (or if it is paying a higher rate, that higher rate number pertains) that is what we are reimbursed. The PowerCube lets us become a spot market provider for them. If you can believe it, for a short period in January, they paid more than $2,800/kWh because power demand was so great. That figure, however, is by far the largest amount paid in the last 2.5 years.”
Granville adds that the company’s system model is well established. It suggests a PowerCube owner would be paid a gross figure of about $250,000 a year per MW (the CSP charges a percent for their part in the transaction).
“The good news for solar now is that if an installation includes power storage, the developer gets the 30% tax credit on the whole system and 30% on the battery storage component as well. Utilizing our model, that can provide a return of investment in 4 to 4.5 years,” Granville says. WPE
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