Can Tesla Compete with This Flow Battery?
Tom Lombardo posted on March 23, 2015 |
Can Li-ion batteries outperform flow batteries in non-mobile applications?

Imergy Power Systems, maker of vanadium flow batteries, recently introduced its newest product: the EPS250, a battery system capable of delivering up to 250 kW with a one megawatt-hour capacity. The storage unit is designed for backup power, peaker plant replacement, microgrid storage, peak shaving, and storage for renewable energy applications.

Imergy’s press release came on the heels of Tesla’s announcement that they would be producing lithium-ion batteries for the home solar/storage market. I spoke with Herve Mazzocco, Imergy’s VP of Solutions, and Tim Hennessey, President of Imergy, to see how their product compared to Li-ion batteries.

Inside the Imergy EPS250 (Image courtesy of Imergy Power Systems)

What’s Inside

The EPS250 comes packaged as a pair of stacked 40 foot shipping containers. The lower half contains the electrolyte tanks and the upper container houses the stacks (the cells where the electrochemical reactions take place) and the control electronics. The unit includes a bidirectional multimode inverter that’s capable of switching between stand-alone and grid-tied operation. The inverter provides reactive power for loads that have a low power factor. Communication hardware provides remote monitoring and control, as well as smart grid capabilities.

Cost Comparison

Imergy’s flow batteries have a much lower levelized cost of energy (LCOE), due to their longer life spans. According to Hennessey, a flow battery’s LCOE over 20 years is 70% lower than that of its Li-ion counterpart. Flow batteries use an electrolyte that has a near-infinite number of charge-recharge cycles, compared to a few thousand cycles for Li-ion. A flow battery’s pumps may need to be replaced every 7-10 years, but that’s a small part of the overall system cost.


Flow batteries can operate at any depth of discharge (DoD) with no degradation of lifespan. Li-ion batteries, on the other hand, have their lives shortened significantly when the DoD goes beyond 50%. Hennessey said that the flow batteries are slightly less efficient when deep-cycled, but it doesn’t affect the overall lifespan. Under normal operating conditions, Imergy’s flow batteries have a 75% round-trip efficiency, compared to 85% from Li-ion batteries. Increasing a flow battery’s capacity is as simple as increasing the volume of electrolyte, so they are somewhat easier to scale up - just use bigger tanks and more electrolyte.

Is There Room for Both?

Since Tesla’s announcement that they would be entering the behind-the-meter storage market occurred just a few days before I interviewed Herve Mazzocco, I asked him what he would say to [Tesla CEO] Elon Musk. Herve paused and responded, “Good luck.” Then he clarified by adding, “Tesla’s technology is useful and has its place. You won’t be seeing flow batteries in cell phones, laptop computers, or electric cars, so there’s still a good market for Li-ion. But for stationary energy storage, flow batteries are safer and provide a longer life cycle.”

With Li-ion batteries being used in so many devices, it’s almost certain that prices will continue to drop while the technology improves. Will Li-ion reach a point where it can outperform flow batteries in non-mobile applications? Stay tuned…

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