The Challenge of Hydrogen Storage
Meera - Despite the safety on-board, the storage of compressed hydrogen within these tanks has its own set of problems which Alex Bhevan is trying to overcome.
Alex - The key thing with hydrogen is it's a storage in a small space using as little energy as possible to store it. If we look at 1kg of hydrogen, 1kg of hydrogen will occupy around 11 cubic meters that's STP which is standard temperature and pressure. What we want to do is to be able to store 1kg of hydrogen in less than 11 cubic meters.
Meera - What are the problems of compressing it so much?
Alex - Well as you compress hydrogen, obviously, you're going to consume energy in the compression process if you move the mechanical pistons. And currently, with the sort of compression technology we have, around 15% of the usable energy from the hydrogen is lost on compression.
Meera - As a result, scientists are already looking into alternative materials that can store and release hydrogen gas efficiently including structures called metal organic frameworks or MOFs and also, carbon nanotubes. But Alex is looking into the potential of other materials or compounds that store and release hydrogen readily; compounds known as metal hydrides.
Alex - If we use metal hydrides, this can offer a very low pressure storage technology and it's possible to use at very low pressures. In fact, at such a low pressure, it can be lower than the pressure in a car tire and yet, we can store a vast quantity of hydrogen in a very small space.
Meera - How do these really work to store hydrogen?
Alex - Okay, well if we take a metal like magnesium which is very good at storing hydrogen - it can store up to a maximum of around 8% w/w of hydrogen. The process is, you get gaseous hydrogen above the metallic magnesium. This hydrogen sort of disassociates and goes into the magnesium crystal and packs at very discrete sites. That's a metal hydride.
Meera - And is this possible in a range of metals?
Alex - Sure. I mean, all metals can absorb hydrogen to some extent, but some are very good absorbing hydrogen.
Meera - And how would this work? So the hydrogen can I guess easily go in and be captured within the metal, but you'll want it back again. So how easily I guess is it released?
Alex - Thankfully, we have a lot of these materials. Some of them operate at room temperature and an equilibrium is formed between a gaseous phase above the metal and the hydrogen stored within the metal. So as we reduce the pressure outside the metal, hydrogen then comes out of the metal to reform this equilibrium. It's very similar to a butane gas lighter. If you look at that where you got an equilibrium between the liquid butane and the gaseous butane above.
Meera - It sounds very feasible, so the hydrogen goes into these metals and then you change the pressure above it and it comes back out when you want it. What are the challenges? What are limitations?
Alex - One of the key challenges is to get this material into the automotive applications and that particular application, weight is absolutely critical. And these materials can absorb a reasonable amount of hydrogen but they are heavy, and what we have here, this is a metal hydride. This one is based on the elements of lanthanum and nickel. And this stores around 1.4% w/w of hydrogen. As you can feel, it's quite heavy.
Meera - Yes, extremely heavy in fact. So what this is - it's quite a small bottle, say about 15cm in height but packed quite densely with this hydride, and it's extremely heavy. It's really causing quite a drop in my hand as I hold it.
Alex - Yes, we need to sort of continue our quest to develop new materials and search for new materials that can make life a bit easier.
Meera - While the search for lighter storage materials in cars therefore continues, the use of metal hydrides to power boats has been found to work well. But there's another fundamental issue for those hoping to own a hydrogen car.
Alex - You're used to the fact of petrol stations being every 3 miles from your house or from wherever you are.