New batteries for renewable energy farms

21 August 2013

Interview with

Cullen Buie, MIT

At the moment, there isa big debate going on about the fact that the UK needs more energy, can't decide where to get it from. One of the options is solar panels and wind turbines which offer really low carbon ways to generate power, but they only work when the sun shines or the wind blows. To make sure we've got power 24 hours a day, we need a way to store the energy that they generate and until now, this has been really expensive. However, writing in the journal Nature Communications this week, scientists from the Massachusetts Institute of Technology think that they've found a new and really cheap form of battery based on bromine solution. Dominic Ford spoke to Cullen Buie all about his idea.

Cullen - So, the great thing about this battery is that it gets the performance of systems with a membrane, but it eliminates the cost of the membrane. So, this is the next we have to no membrane which is one of the most expensive components of the cell.

Dominic - So, what's this membrane doing in the battery?

Cullen - So, the membrane is in the battery to separate the reactants. So you have reactions happening either side of the system and so, you need to separate those reactions or else they would happen spontaneously and you wouldn't be able to get any useful energy from it. What we propose is a membraneless hydrogen bromine batteries.  So, batteries are typically characterised by their chemistry. We have this hydrogen bromine chemistry which actually isn't new. This chemistry has been around and known since maybe the '70s and people have been excited about it because of the cheap and abundant reacting materials, and the potential for high efficiency. The problem is that it's very difficult to design a membrane that will last for the thousand of cycles and that is low cost. So, it's been limited by largely the materials because this hydrogen bromine battery makes hydrobromic acid which is difficult to work with for some membrane materials. Conversely, maybe 10 years ago, a research group at Harvard invented these membraneless systems. So, they were the first to propose eliminating membranes altogether and using fluid mechanics in order to keep your reactants separate. The issue there was that they used chemistries that- they didn't produce a lot of power. So, it was interesting, but for the most part, the industry has disregarded these cells because they never produce enough power or energy in order to make them practically viable. So, what we've done is taken these two technologies which have kind of been sitting on the shelf effectively and put them together. And by putting them together, we get a hydrogen bromine system that now eliminates its issues with the membrane. And so, by eliminating the physical structure, we eliminate one entire component and all of its associated costs and complexity.

Dominic - Now, you're using bromine. Why is that so especially cheap?

Cullen - So, bromine is all over the world. It's in saltwater. So, any place where you have a large body of saltwater, you're effectively, as a by-product of harvesting other things from saltwater, you get a lot of bromine.

Dominic - I know you're invisaging this battery for use with renewable energy generation. Why is it so important with renewable energy farms to have these large batteries?

Cullen - You have no control over when the sun is shining. Demand varies throughout the day. So, if you could couple storage with these intermittent sources like solar and wind, you can store the energy when it's not being used and then sell it back to the grid when it is being used.

Dominic - When I think of new battery technologies, I think of cell phones and iPads, and so on. How do the needs of renewable energy farms compare to what you need for say, a cell phone?

Cullen - They're much more cost restrictive. So, the battery in your cell phone or in your laptop is probably at least 10 times more expensive than what you would like to see for something that's going to go grid scale.

Dominic - I guess I find that quite surprising actually because you think if you're building a huge wind farm say, you've got a lot of infrastructure there.  You're pouring concrete for these windmills. I would've thought the cost of the battery on the back of that would be relatively slight cost.

Cullen - Our prevailing battery technologies are still very expensive. They use a lot of precious metals. They use things that are difficult to contain or control. And so, when you talk about that large scale, it's difficult to make it scale and make it affordable.

Dominic - Is lifetime also an issue here? I mean, I'm thinking, if you're building something in the remote desert to harness solar power or you're building an offshore wind farm presumably you don't want to be replacing those batteries every few months off the back of that renewable energy farm.

Cullen - Yeah, you nailed it right on the head. Lifetime is absolutely an issue. You want thousands of cycles, maybe 10,000 cycles. So, you mentioned batteries before.  How long does your battery actually operate? Maybe you get a year or two and then after 2 years, you notice all of a sudden, it can't hold any kind of charge. That would be unacceptable for something like a grid scale application.

Dominic - And the other problem that we have with cell phone batteries is the memory effect where if you're only ever half discharging it, then it doesn't tend to hold its full charge any longer. That I guess is also something you've got to avoid in these batteries that are being charged when the sun shines and discharged when it's cloudy.

Cullen - Yeah, you'd like to minimise all those effects, so it's a very challenging problem. What we presented is just one more way and the novelty in this one is that we've eliminated one of the more costly components in the battery.

Dominic - How long do you think it would take to get this from the lab into the field? I mean, if you're saving a lot od money, if this is very cheap then presumably, the energy companies would be very keen to use this as soon as possible.

Cullen - I would say, 5 to 7 years. We're talking about developing a new battery, a new type of battery.  New batteries don't come out very often and part of the reason is, battery development is hard.

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