Flow batteries: more cost effective for residential and utility-scale installations?


Before joining Imergy, Watkins served as chief executive of Bridgelux, Inc., a leading developer and manufacturer of solid state lighting technologies and solutions. Before that, Watkins served as CEO of Seagate Technology, the world’s leading disc drive and storage solutions company, from 2004 to 2009. During his tenure at Seagate, Watkins was named Forbes Magazine 2006 CEO of the year.

pv magazine: How much cheaper are Imergy vanadium-based batteries to comparable lithium and lead-acid batteries for use in conjunction with either residential rooftop solar arrays or large utility-scale PV installations?

Bill Watkins: For anything that needs long-duration storage, the flow battery will win every time, simply because it can provide very short duration cycles, for the stability – rapid cloud cover type of things – and it can do very long duration, whereas the lithium ion and lead acid can’t do long duration very well. They’re extremely expensive when duration gets longer than an hour to 90 minutes and they need to be largely oversized to avoid deep discharge, so you’ve got a clear winner from the point of view of long-duration. The only issue is space. If it’s a residential environment, and the footprint is really small, then lithium ion has an advantage, but then it can’t do long-duration. So long-duration lithium is going to be much bigger than short-duration lithium, and long-duration lead acid is bigger than a flow battery. For example in Germany they’re selling small residential systems for about €11,000 for a 2.5 kW/10 kWh system, and I think those are lithium. Imergy’s flow batteries are about 50% more cost effective for both residential and utility-scale installations.

Lithium has won a lot of projects for short, quick duration and it may be fine for that. In order to do long duration with lithium you have to continue to oversize it. The other thing which none of the lithium manufacturers talk about is what we call mid-balance – you want to be able to charge and discharge, which means you have to be below full. And if you hold a lithium ion battery below full for long periods it just dies. So you don’t want to operate lithium in that mode at all, whereas a flow battery can operate in that middle band without any impact for any number of cycles.

How much are Imergy batteries at the moment?

The cost of an Imergy battery depends on several factors – the power size and the discharge duration are typical examples of the factors driving that cost. As I noted previously, our cost structure is substantially below the pricing for an equivalent capability delivered by lithium batteries. Because an Imergy battery is very long-lived, we can deliver the lowest levelized cost of stored energy (LCOSE) of any competitor in the market. Just as the LCOE for PV is much higher in Germany than in the U.S. Southwest due to sunshine availability, the LCOSE for storage depends on factors ranging from ambient air temperature, depth of discharge, initial costs, O&M, equipment replacement costs, and discharge duration, to name a few. We have analyzed many use case comparisons to know that in case after case, Imergy can deliver a lower LCOSE than any other battery in the market. You add to that LCOSE the benign environmental characteristics of the Imergy battery and you have a clear winner.

How do Imergy batteries compare in physical size with lithium and lead-acid batteries of similar capacity?

In general, the initial footprint of a flow battery is larger than the footprint of a lithium or lead-acid battery. However, on a usable capacity basis over 10 years, we would be of similar size to lithium and smaller than lead acid.

A lead acid battery can be discharged only 50%, so as an example, if the lead acid battery is 1 kWh and you need 1 kWh of power, you’ll need two lead acid batteries to garner a full 1 kWh capacity. A single 1 kWh flow battery will deliver the full 1 kWh capacity because you can repeatedly discharge a flow battery to zero and recharge it fully. So now you have two lead acid batteries to get the same capacity as one flow battery. A single flow battery will last more than 20 years. But your lead acid battery (and you’ve got two of them to garner 100% of your required 1 kWh capacity) will need replacing every three years or sooner, and you’ve got to double the number/size to get to six years, and then double again to get to nine years, so you’re at eight times the size of the initial, single lead acid battery to get the same kWh capacity and reliability as with one flow battery, which is now much smaller by comparison. The same model can be used to compare lithium and flow batteries, but lithium starts off very small, so in the end, on a usable capacity basis, a flow battery is of similar size to lithium, and smaller than lead acid over a 10-year period.

For batteries to become cost-competitive with fossil fuel, it’s estimated that they will need to cost around $100 a kilowatt-hour. Do you see Imergy batteries reaching that threshold soon and are those estimates realistic?

We see a metric such as $/kWh used a lot, while, in fact, it’s not a good metric at all. The important question is, what is the lifetime of the battery and what are its cost and performance attributes over that lifetime, not just the initial unit purchase cost. The initial unit purchase cost does not equate to levelized cost of energy (LCOE). Batteries with a low initial cost and a high LCOSE tend to focus on the cost/kWh metric, much to their detriment with sophisticated customers.

Second, a battery can provide so much more in raw capability than traditional fossil fuel generation. For example, a battery provides optionality, it provides capability to easily shift load, and it provides the ability to instantaneously absorb excess generation as well as instantaneously deliver electricity. In addition, an Imergy battery is environmentally benign and can be considered a renewable resource when charged with wind or PV. Given these attributes, comparisons to fossil fuel are not very relevant.

To what extent does the solar PV market play a role in Imergy’s overall business?

It's a very important part of the market. The Germans like the self-consumption model, generating, storing, and consuming energy yourself, without giving it back to the grid. It creates grid independence. And it reduces a significant amount of system losses.

Read the entire interview with Imergy CEO Bill Watkins in the forthcoming issue of pv magazine, due out on October 1.

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