By Dave Elliott
Not everyone backs biomass, given the emission/biodiversity/land-use issues, but biomass does offer a range of flexible green fuel options, biogas especially. The World Bioenergy Association (WBA) says bioenergy already contributes over 14% to the global energy mix, and its use is bound to expand. So what are the options? (more…)
By Dave Elliott
This helpful paper from a team at Sheffield University, UK, entitled ‘Great Britain’s Energy Vectors and Transmission Level Energy Storage’, suggests that ‘power to gas’ conversion systems could supply synthetic gas (syngas), made using renewable electricity, for storage in the gas pipe network, so as to balance variable renewables, this being a substantially larger storage option for the UK than pumped hydro.
By Dave Elliott
Is the truth out there? An extended Xmas Whimsy
It’s usual for there to be a spread of viewpoints on most issues, and it’s always worth looking at a range views, including ‘outlier’ ones! On that, this is fun: www.xonitek.com/press-room/company-news/the-stone-age-didnt-end-because-they-ran-out-of-stones/
However at times you can get weary of obsessive time wasters and yearn for clarity! Sadly that may not be easy to achieve.
by Dave Elliott
By their nature, wind and solar energy are variable and there is likely to be excess electricity generated from using these resources at times, and shortages at other times. It is hard to store electricity directly, but the energy can be converted into more easily storable forms.
One of the big hopes for the future is the ‘wind to gas’ idea- using excess wind-derived electricity to produce hydrogen gas by electrolysis for storage and then use, when there is a lull in the wind and high demand, to generate power in a fuel cell or gas turbine. Alternatively, the hydrogen, or methane derived from it (and from captured CO2), can be fed into the gas grid to replace fossil gas. For once the label ‘game changer’ might even be right- perhaps the key to a balanced energy future, compensating for variable inputs from renewables. It seems like an idea whose time has come: www.aspo2012.at/wp-content/uploads/2012/06/Pengg_aspo2012.pdf
Rather than seeing excess carbon dioxide in the atmosphere as a problem to be dealt with, by, for example, expensive carbon capture and underground storage, why not make use of it to produce fuel? The basic chemistry is simple, assuming you have some spare hydrogen: CO2 + H2 = CO + H20. The CO can then being converted to hydrocarbon fuel, for example by via the Fischer–Tropsch process. That of course needs more hydrogen. Fortunately, not only do we have plentiful supplies of CO2 from the air, air movements can also supply the energy, via wind turbines, to make hydrogen, via the electrolysis of water. Problem solved! Except of course each stage in this process is difficult, with conversion efficiency’s of around 60%, unless the waste heat can be recycled/used.
Carbon capture techniques are of course being developed for use with power stations emissions. But there has been a parallel idea of ‘air capture’ – for example the ‘absorption tower’ approach developed by Prof. David Keith from Calgary University, in which a fine mist of strong sodium hydroxide solution is brought into contact with an air flow. The big advantage of that is that you can do it anywhere – not just at power plants. But rather than just storing the resultant mix, the CO2 can be recovered, ready for conversion into a fuel.
That is what a team led by Prof. Tony Marmont are planning to do, using an electrochemical process based on a patented design developed by Prof. Dereck Pletcher, formally of Southampton University.
Prof. Marmont is a long time proponent of renewable energy in the UK and funded the set-up of the CREST organisation at Loughborough University. His team at Beacon Energy in Leicestershire has been working on hydrogen generation, storage and use for some while – with the electricity supplied by their own wind and PV systems, so the next stage should be a bit easier.
In the ‘air fuel synthesis’ (AFS) approach being developed by Marmont, the recovered carbon dioxide will then be reacted with electrolytic hydrogen; either directly to make methanol and thence to petrol via the Mobil Methanol-to-Gasoline route; or via the Reverse Water Gas Shift reaction (as above) with hydrogen, to make carbon monoxide, which in turn will be reacted with more hydrogen in a Fischer–Tropsch reaction to make hydrocarbons. In the latter case, variation of the reaction conditions could enable petrol, diesel or aviation fuel to be made.
It’s a fascinating idea, essentially using renewable energy to do what nature does with photosynthesis – convert atmospheric carbon dioxide back into organic molecules. But it does rely on multiple stages, each with significant energy losses. In terms of road transport applications, you would presumably get a much better return on the wind-generated energy if it was just used in battery electric cars – with the overall conversion efficiency then being 90% or so. So the AFS approach may only be an interim option while electric cars are improved. But liquid fuels have a much higher utility/energy storage density than batteries, and there may well be some application (e.g. for heavy goods vehicles and, crucially, for aircraft) where liquid fuels will have major advantages.
So there could well be a future for ideas like this, and the wind power resource could be up to it in time. The Marmont team calculate that ‘to make all UK oil – 140,000 tons a day – as synthetic, would take a windfarm area 175 miles by 175 miles in the North Sea. To make only aviation, marine and military fuel as synthetic, would take an area 72 miles by 72 miles.’
The Los Alamos Lab in the US has proposed something similar, but with the energy supplied by a nuclear reactor. They gave their idea the somewhat cringeworthy label ‘Green Freedom’.
For good measure they suggested that conventional power station cooling towers could be used for the carbon dioxide trapping NaOH spay system. But as with the wind-AFS approach, it would probably be more efficient to use the nuclear electricity directly in electric cars, a point made elegantly at http://ergosphere.blogspot.com/2010/01/revisiting-green-freedom.html.
Nevertheless, with there being no easy aviation fuel substitutes, Air Fuel Synthesis does still seem worth exploring, as are the various other novel ‘Green Chemistry’ ideas for fuel production being developed around the UK and elsewhere, including the use of biomass as a feed stock for hydrogen production. See for example: www.claverton-energy.com/wp-content/uploads/2010/07/Tetzlaff_Birmingham2010.pdf.
For more on new renewable-energy developments, visit www.natta-renew.org
from which some of the above was drawn. Thanks to Dave Benton for his input on AFS.