Storing green energy

A lot of electric cars have “zero emissions” emblazoned on the side. But they’re only as green as the energy used to charge them. Renewable energy is becoming more available, but not necessarily “on tap” 24/7. For instance, the sun doesn’t shine at night! Kathryn Toghill, from the University of Lancaster, works on one potential solution to this problem, as she explained to Chris Smith and Katie Haylor...

Kathryn - Renewable electricity has been penetrating the market at a significant rate over the last few years. We use a third of it, it's this intermittent energy. So we need some kind of mediator between the energy generation and the use on the grid to actually balance that out. And that's where these batteries come in.

Chris - Is it as simple as the kinds of batteries one puts in a torch, you just have loads and loads of those, and you put them near your wind farm or you put them near your solar array, or even in your house, and you just basically use them like an energy sponge, they soak up any surplus when you don't need it? Is that basically the concept?

Kathryn - That is basically the concept, yes.

Chris - So what's not to like?

Kathryn - The cost of the system is the issue. So the problem with the batteries we've made for these portable technologies like for electric vehicles or for your phone, anything like that, is that they're designed for high power, high usage, they're designed to be lightweight, they're designed for a completely different profile of energy use. Whereas these stationary energy systems, they need to take on large amounts of energy, they need to scale up on a considerably higher scale than your portable systems, and they need to deliver that energy in a smoother way as well.

Chris - I suppose that a weight, when you're talking about a stationary battery that's just parked on a site or even in your utility room, is less of an issue than it is for an electric vehicle where the battery's got to be a certain weight, a minimum, and presumably also got to fit certain specifications because it's got to be a certain size and fit into a certain space in the car. You're less constrained with a domestic or a wind farm system.

Kathryn - Absolutely. Yes, so you've now got a completely different area of chemistry to be exploring because you're not so constrained anymore. The other issue is that what we really want from a stationary storage system is something that has longevity. So now lifetime really matters and the depth of your cycling really matters. So whereas earlier you were talking about your batteries dying very quickly in your phone or in your laptop - we can't have that happening in stationary storage. So that's where the alternatives to what we already have are coming in. That's where new chemistry is being designed.

Chris - Now when you say new chemistry; in other words, we're not just going to take the batteries that work in an electric car and just basically repackage them for domestic use? You could do that, but actually you're saying you could do it better.

Kathryn - Yeah, there'd be no advantage to that. First of all there's going to be high demand for lithium ion, so lithium is not that sustainable if every kind of technology is going to be using it; so where we can switch to a different chemistry, that's a better idea generally. The alternative chemistries we can be using will have a different design entirely as well. So we don't need that high power, we don't need that fast charge capability, so we can start looking at things like flow batteries where you're working with completely different metals and a completely different system design as well.

Chris - So what's a flow battery then?

Kathryn - So a redox flow battery is where all your charge goes into the solution; so where we talked about electrodes and then an electrolyte in between them earlier in normal batteries, your electrodes are what matter and that's where all the charge is kept; but in a flow battery it's all kept in the electrolytes. So you have these chemicals in solution that can take on charge and they can give back charge depending on which way you want your flow to go in your battery. And then you store these solutions in big tanks so they're separate from the electrodes. And that means you can scale up your tanks depending on how much storage you need, or you can scale up the number of electrodes in the middle depending on how much power you need.

Katie - Kathryn, you mentioned that the electrolyte is the important thing here. So what's novel in this area then?

Kathryn - What we use here that's the state of the art is vanadium. So vanadium ions in solution, nothing is solid in there. They are charging and discharging, different states, depending on what you need. But also we are moving away from metal-centred flow batteries as well. So we're trying to look at technologies where we don't have metals in there at all bcause they have two political issues associated with them.

Chris - Kathryn, if I may: if you've got a battery system that's built around a liquid, you're saying the electrolyte is the charged thing and the electrodes are less important, that sounds to me very much like the petrol that we put in a car tank. So have we not got a really nice solution here which uses much of the infrastructure we already have for dispensing an energy-rich fuel into a car? You could just fill up your car battery with a charged electrolyte, and then when the charge has been used up you drain that depleted electrolyte out and replace it with some replete, charged electrolyte again.

Kathryn - It is not a crazy idea actually. But unfortunately vanadium flow batteries and most flow batteries, the energy density is too low. So you're looking at something that's ten times less dense in energy than a lithium ion battery. So it wouldn't be feasible because petrol has a very high energy density, even though the system is quite inefficient. But there are people researching... so up in Glasgow, for example, there are people who have been researching these new materials which would deliver very high energy density# liquid electrolytes. So that could be an option in the future. But it's quite far away at the moment.

Chris - And what about price, Kathryn? Because I've been talking to some people who live in, for instance, remote parts of Australia. They're very reliant on solar, for example. And so for them, the ability to store a lot of charge in a decent battery, it can be a life-changing thing. But how much does it cost to install a rig for say a domestic setup?

Kathryn - It depends on the rig you have, but you're looking at installing it coupled with solar, that would influence the cost as well. And then you've got your battery management system. So there's lots of different components, it's not all dependent on the battery itself. But you're looking at tens of thousands of pounds for a home storage system.

Chris - Pretty pricey then.

Kathryn - But it's equivalent to an electric vehicle.

Chris - But then I suppose if you've got the electric car parked in the garage and the stationary battery in the utility room, are we getting to the point where everyone's basically got their own power station at home? Is this not overkill?

Kathryn - I don't... I personally wouldn't want to cycle my car battery because that battery is not designed to do many cycles. I would want that load of a long lifetime for a home battery storage system to be on something that's designed for that purpose.