Cool alternative energy storage technologies

Exponential Investor is all over energy storage – and we certainly get a lot of engagement from readers on this subject. I’ve been sitting on a couple of interesting letters for a while, and I think it’s finally time to give them an airing. They’re not long – but each of them unpacks a whole load of exciting technology innovations. They’re reproduced below, in edited form:

From: John
Subject: Energy storage

[Elon] Musk may be grabbing the headlines (because he is big and brash), but don’t ignore fuel cells and related hydrogen storage (check Siemens and ITM Power). Also, check out Nissan’s developing philosophy of the inter-relationship between batteries in cars and their potential for “dual-use” as domestic storage systems.

Fuel cells are charged with hydrogen, which takes no longer to fill up with than when filling a car tank with petrol. That’s a huge improvement on the time taken to charge a battery – however “fast” the charge rate.

The Tesla Powerwall was £5,000.00 – and is now likely to cost more, with the fall in the £. All the European car manufacturers are developing electric models. They all need batteries and battery costs will become very competitive. Musk will not have it all his own way in future.

John’s right that battery costs will fall – it’s one of Exponential Investors big themes. Hydrogen has a lot of potential as a road fuel (check our ITM coverage coming later this week). ITM’s power-to-gas technology is great, with Siemens’ SILYZER alternative being similarly impressive. I expect to see a lot more of this, as we move to a renewables-heavy grid. There’s going to be a lot of cheap, off-peak electricity, and we’ll need to find a use for it. One useful technology is desalination.

Nissan/Enel’s vehicle-to-grid (V2G) trial is very interesting, too. That’s where your car battery becomes a home battery, as soon as you park up. Musk is convinced that V2G is a dog technology – but I’m personally fairly positive. I just can’t see why you’d not use all those car batteries for storage, and for frequency regulation (controlling sudden peaks and troughs of demand on the grid). After all, if you’ve paid for the battery, you’d want to work it as hard as possible – wouldn’t you?

From: Martin
Subject: Energy storage and flow batteries

I’d always understood capacitor energy density for storage to be limited. The last I read on this they were looking at nano surfaces in capacitance devices. This increases effective plate areas and, therefore, the charge quantities within a given volume.

Have you looked at flow batteries? A university in the US was looking at cheaper chemicals.

Firstly, to address Martin’s discussion of capacitors, it’s worth checking out our recent article – but he’s right about their low storage density, at least for now! However, this doesn’t matter for static applications. Where capacitors definitely win out is speed of response; and potentially in their costs of manufacturing at scale (which isn’t limited by raw material supplies). Their fast charging times also have a huge benefit for mobile applications; if your phone or car can be charged in five minutes, you probably don’t mind plugging it in several times a day (see one company working on this here).

However, the big story here comes in the brief mention Martin makes of flow batteries (Exponential Investor has covered a firm developing them, before).

Let’s start, by explaining what they are. We can start by using your car’s lead-acid battery as a comparator. This is a liquid-filled battery – as I was reminded, when I accidentally tipped one on to the floor of my car. However, this liquid doesn’t flow – at least not in normal use. The liquid is retained in the battery itself – and no external tanks are used, to extend the battery’s capacity. Fitting tanks to a lead-acid battery wouldn’t help. You could store all the additional acid you wanted, but the plates would be the limiting factor – because it’s the plates which react, not the electrolyte.

In some battery chemistries, however, the reaction takes place in the electrolyte liquid itself – not on the plate. As such, you can extend the battery capacity infinitely, simply by fitting larger tanks – and that’s the principle behind the flow battery.

Exponential Investor has previously touched on flow batteries – in the context of their use in drones. The big plus for flow batteries is that they can store a lot of electricity at a relatively low cost – as you can just fit large liquid tanks to them. However, they’re likely to be more expensive than comparable medium-term storage approaches – such as compressed air.

These competing liquid air and compressed air technologies can be scaled up fairly easily – as air is free, you just need bigger tanks. There’s a slew of firms operating flow battery technology – including the UK’s redT (which several readers have mentioned to me). This tech is up and running in some areas – which is a very promising sign. However, the use of mechanical pumps, and dangerous chemicals, mean that it’s most suitable for grid storage – although nanoFlowcell has made a vehicle version.

Although redox flow batteries are a bit of a diva to set up or repair, they’re extremely well-behaved in use. They start almost instantly, hold charge indefinitely, and can be deep-cycle charged for hundreds of thousands of cycles. Accordingly, they’re pretty attractive for operators – as they give multiple revenue streams. Currently, fast-response power response is purchased separately to longer-term energy storage.

I admit, these technology calls are tough to make – and I recently had to eat humble pie over capacitors, which I’ve now realised could be a huge deal.

If you know of any awesome technologies we should be covering, please do let me know:


Andrew Lockley
Exponential Investor

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