Filling up with hydrogen moves a step closer
3 Feb 2011
Oxfford, UK – Cella Energy Ltd, a new spin-out company from STFC’s Rutherford Appleton Laboratory, is developing a technology that allows hydrogen to be stored in a cheap and practical way, making it suitable for widespread use as a carbon-free alternative to petrol.
Hydrogen, which produces only pure water when burned, is considered an ideal solution to cutting carbon emissions from petrol, which are estimated to cause 25 per cent of all carbon release. Until now, attempts to store it have not been consumer friendly so this has not been a viable option.
Cella Energy, which already has one investor in specialist chemical company Thomas Swan & Co Ltd, who signed an agreement on 24 Jan, believes it has found the answer.
Working with the London Centre for Nanotechnology at University College London and University of Oxford, scientists from STFC’s ISIS neutron source have developed a way of making tiny micro-fibres 30 times smaller than a human hair. These form a
tissue-like material that is safe to handle in air.
The new material contains as much hydrogen for a given weight as the high pressure tanks currently used to store hydrogen and can also be made in the form of micro beads that can be poured and pumped like a liquid.
It could be used to fill up tanks in cars and aeroplanes in a very similar way to current fuels, but crucially without producing the carbon emissions.
“In some senses hydrogen is the perfect fuel; it has three times more energy than petrol per unit of weight, and when it burns it produces nothing but water. But the only way to pack it into a vehicle is to use very high pressures or very low temperatures, both of which are expensive to do.
Our new hydrogen storage materials offer real potential for running cars, planes and other vehicles that currently use hydrocarbons on hydrogen, with little extra cost and no extra inconvenience to the driver”, said Professor Stephen Bennington, lead scientist on the project for STFC.
Stephen Voller, the new CEO of Cella Energy Ltd said; “Consumers want to be able to travel 300-400 miles before they have to refuel. And when they do have to fill up they want to be able to do it as quickly as possible. Existing hydrogen storage methods do not meet these consumer expectations, but the ones we are developing have the potential to do just this”.
Tim Bestwick, director STFC Innovations Ltd said; “We’re delighted that Thomas Swan & Co Ltd has chosen to invest in Cella Energy.
“We believe they will be a great partner with nearly 90 years of experience in making high performance chemical products including nanomaterials,” he said.
Process engineering
Complex chemical hydrides now exist that store hydrogen in concentrations that are well above 10 wt%. For example, ammonia-borane is 19.6 wt% hydrogen, 12 wt% of which is released at temperatures below 150°C. However, these materials have slow desorption kinetics and can release other chemicals such as ammonia or borazine which could poison a fuel cell.
Many are also difficult to handle in that they degrade rapidly in air. These issues can be solved using our nanotechnology.
Cella Energy have developed a method using a low-cost process called coaxial electrospinning or electrospraying that can trap a complex chemical hydride inside a nano-porous polymer.
This speeds up the kinetics of hydrogen desorption, reduces the temperature at which the desorption occurs and filters out many if not all of the damaging chemicals. It also protects the hydrides from oxygen and water, making it possible to handle it in air.
The coaxial electrospinning process that Cella uses is simple and industrially scalable, it can be used to create micron scale micro-fibres or micro-beads nano-porous polymers filled with the chemical hydride.
Cella Energy believes that this technology can produce an inexpensive, compound material that can be handled safely in air, operates at low pressures and temperatures and has sufficiently high hydrogen concentration and rapid desorption kinetics to be useful for transport applications.
The company’s current composite material uses ammonia borane NH3BH3 as the hydride and polystyrene as the polymer nano-scaffold. Ammonia borane in its normal state releases 12wt% of hydrogen at temperatures between 110°C and 150°C, but with very slow kinetics.
In Cella’s materials the accessible hydrogen content is reduced to 6wt% but the temperature of operation is reduced so that it starts releasing hydrogen below 80°C and the kinetics are an order of magnitude faster. Although ideal for our proof-of-concept work and potentially useful for the initial demonstrator projects it is not currently a viable commercial material: it is expensive to make and cannot be easily re-hydrided or chemically recycled.
Cella is now working on other hydride materials, these have slightly lower hydrogen contents but it is possible to cycle them into the hydride phase many hundreds of times and we are encapsulating these in hydrogen permeable high-temperature polymers based on polyimide.
There are two ways to use these materials:
Pure hydrogen solution zero carbon emissions as a way of storing and delivering hydrogen safely for use in an internal combustion engine or a fuel cell
For use as a fuel additive to reduce the carbon emissions from a hydrocarbon fuel such as gasoline, diesel, JP-8, jet-fuel or kerosene
Cella can manufacture the materials in the form of micron-sized beads. This makes it possible to move the beads like a fluid. This opens up a number of opportunities:
It is no longer necessary to try and rehydrogenate the material within the vehicle. For most hydrogen storage materials this releases megajoules of energy. If the refuelling is to be done in a few minutes, this requires cooling to remove several hundred kilowatts of power.
To facilitate rehydrogenation in the 3-4 minutes that the DOE targets stipulate, the thermodynamics require high temperatures and pressures of around 100bar.
This requires substantial engineering and as such we don’t believe that on-car rehydrogenation is reasonable. With a fluidized hydride, it is possible to quickly fill or remove the material from the vehicle so that it can be recycled or rehydrided elsewhere.
It is possible to move the material within the vehicle making it possible to separate the storage from thermolysis. The beads are stored in a fuel tank, which does not need to contain high pressures or be heated and cooled, therefore it can be a simple lightweight plastic tank of complex shape similar to that used in current vehicles.
The hydride beads are then pumped to a hot cell where waste heat from the engine exhaust is used to drive the hydrogen into a small buffer volume.
The hydrogen buffer is maintained at a pressure suitable for the internal combustion engine ICE or fuel cell and which is sufficient in volume to be able to restart the vehicle.
Once the hydride has been heated and the hydrogen driven off, the waste beads are stored in another lightweight plastics tank.
