The Hydrogen Economy: Fuelling the Future

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.

Meera - This is where the work of the biosciences and chemical engineering departments at the University of Birmingham comes in. Researcher Mark Redwood is working on obtaining hydrogen gas from a natural and widely available source: biomass, obtained from organic waste.

Mark - The thing about organic waste is it's really diverse. It's all sorts of different things. For example, there's the caterings wastes, the plate scrapings that come off the plates in kitchens and after the whole BSE problem, you can no longer feed that back to animals because you might accidentally feed pork back to a pig. So nowadays, there's a lot of regulations and difficulties with disposing of all sorts of kinds of wastes. There are things like apple pomace, or spent grain that comes out of brewing, just excess food that doesn't get eaten or go spoils in the supermarkets. Altogether, there's over 100 million tons of suitable biodegradable wastes every year just in the UK.

米拉-所以,你如何设置abo血型ut turning this into hydrogen?

Mark - We would build what we call an integrated biohydrogen refinery where we'd use a diverse range of techniques to squeeze any sort of organic wastes or biomass into different fractions. And then ultimately, all of those get put into a bioreactor using special bacteria of different types to turn the waste into hydrogen. It's about using the calories, using the food value in all different sorts of wastes to make the bacteria grow and to produce hydrogen. And then there are special ones that also use sunlight for an extra boost.

Meera - And so, what's the actual process? So what is it in the food waste that's then converted into hydrogen and what else do you get as a by-product?

Mark - Well, in any form of bio waste, it's the same feedstocks that give us food value when we eat food. It's the carbohydrates, the proteins, the fats, all those things are suitable for bacteria to breakdown and grow, and make into hydrogen. Now, what we do is we start off with something that's quite sugary. First thing we do is put it into a fermentation which is like what happens when yeast works in brewing where you use E. coli bacteria or other bacteria in a dark fermentation, break down the sugars and produce hydrogen and carbon dioxide, and a bit of bacterial growth.

Meera - And when you say dark fermentation, you basically just mean without light.

Mark - Yeah, just without light.

Meera - And fellow scientist Rafael Orozco is leading the work using bacteria in this dark fermentation process.

Rafael - There is a range of bacteria that can do the job, converting the sugars to hydrogen. We used facultative anaerobes, so just E. coli, in different strains of E. coli because these are very easy strain to work with. It's easy to grow and the fermentation conditions are mild. For experiment purposes, it's a very, very useful microorganism.

Meera - And what does this E. coli work on? So it takes sugars and what does it do to then somehow get hydrogen and other by-products from it?

Rafael - It converts sugars to hydrogen and a mixture of organic acids, following a specific metabolic pathway.

Meera - And how efficient is that process then? So, for all of the sugars coming in, how much hydrogen do you get out from this initial use of bacteria like E. coli?

Rafael - Well in this case, the maximum yield will be like 2 moles of hydrogen per mole glucose through the development of our fermentation techniques and culture media and conditions, we have achieved continuous fermentation with 80% of that maximum potential.

Meera - Whilst this dark fermentation is showing good yields of hydrogen, the team are now further improving on the efficiency of this process by using the organic acids released when their E. coli bacteria breakdown these sugars. Mark Redwood again.

Mark - Now, the dark fermentation also produces organic acids. These are things like vinegar or butyrate. These organic acids are the preferred foods for photosynthetic bacteria, purple bacteria that use sunlight to convert the waste product of dark fermentation into hydrogen. And actually, if you took all of those organic acids from a dark fermentation and put them into the photo fermentation using sunlight, you'd get about 5 to 10 times as much hydrogen from the photo fermentation as you got from the dark fermentation.

Meera - So, how do you focus in your research in this area? So you've shown that it can produce so much more hydrogen. So what do you actually look into to make this happen on a larger scale?

Mark - Well, at the moment, we're developing reactors that can be used to take the technology out from the lab into the real world and a big part of that is that these reactors will have to be very large because sunlight itself is not that intense. So, in order to get a lot of sunlight energy, you have to cover a lot of space. And we've come up with one solution to that problem and which is what we call dichroic beam sharing and it uses a really well-known bit of technology called the dichroic mirror. So, I've got one of them here and that's why we're outside in the sunlight. And we've got a little 2 inch across circular mirror. It's just a normal piece of glass which has on it an organic coating which is a dichroic coating and that interacts with the sunlight and it splits it. So, some of the wavelengths, some of the colours get transmitted and some of the colours get reflected. This is actually really useful because those colours can be fed to different organisms making different biofuels because some organisms like to use red light and some organisms like to use other colours of light. Then it happens to be that the purple bacteria that are really good at using organic acids work really well on the reflected light and other things - like algae, green things, higher plants, grass, crops - those work really well on the transmitted light which is mostly red. So, that means if we could use this dichroic mirror to split sunlight, we can drive 2 bioreactors in the same space and effectively double the amount of biofuel it can produce without needing anymore land.

Meera - So this is very much a multistep process, bringing different biological processes together. That was Mark Redwood from the University of Birmingham.

Meera - So, chemical processing enables the team to branch out to a wider range of waste. Chemical Engineer Bushra Al-Dhuri is exploring these untapped sources of hydrogen.

Bushra - Wastes come from various sources as we know. There is the kitchen waste, there is the municipal waste, and with this, we're talking about cardboards, papers, all sorts of things. Now industrial waste means like paper waste because it is also a cellulosic material and then we have wood, wood chips, wood that is used for old cupboards, all sorts of woods, also going further to forestry.

Meera - You're listening to a special edition of the Naked Scientists with me, Meera Senthilingham. This week, we're exploring the role and production of hydrogen in the race to find green sources of energy. We've heard so far about the many roles bacteria can play in converting various organic sources of waste such as food waste into hydrogen for use thereafter in hydrogen fuel cells. But whilst the use of bacteria shows high efficiency in breaking down simple organic waste, like sugars, the ultimate aim is to tackle more complex forms of waste as well in order to produce the maximum amount of hydrogen possible and from a greater diversity of currently unused resources. But therefore, this is also where the process becomes more complicated as Lynne Macaskie explains.

Lynne - If we go to a more complicated waste like an agricultural waste then that's more likely to be starch or cellulose, and that presents some problems because many microbes actually don't like starch or cellulose. They prefer sugar. So, often what you can do is treat the starch or cellulose beforehand to make sugar or you can actually have mixed cultures. You can use microbes that act at high temperatures for example, or use a chemical system beforehand to break down the cellulose.

Meera - Chemical processes are the main focus at the moment, not only for waste containing cellulose but also lignin found commonly in wood waste as these compounds are also more complex. Too complex in fact, the bacteria which are much better suited to metabolising simple sugars.

Lynne - The microbial routes, it's very eco-friendly, but it's a bit restrictive because breaking down for example lignin, the component of wood, is still very problematic because it's quite hard to break this down biochemically. It's very slow. So, chemical processing comes into its own when you have recalcitrant - that's difficult - materials. And often, the only convenient way to treat these is to use gasification which is basically thermochemical treatment to just completely convert the raw material into gasses.

Meera - So, I guess at the moment, it's perhaps the chemical process that's more accessible or more feasible.

Lynne - At the moment, chemical processing is the main contender, but I see a future for biotechnology. In fact, I see a future in hybridising the two technologies together so that biotechnology can look at a fraction of the waste and chemical processing can look at the fraction which bacteria may find it harder to look at. So, it's horses for courses really. I mean, each waste is going to be completely different. A chocolate waste is very easy to treat biologically. Wood waste is not. And really, that's the cutting edge of this area at the moment - how do we treat very difficult waste?