I submitted a letter to the magazine "New scientist", based on my previous posting "Biohydrogen - a Preposterous Idea" (which it is) and that was published in the 8 April issue. I referred to this blog (http://ergobalance.blogspot.com) for details of how I arrived at this conclusion. The text is the following:
Peter Abrahams (Letters, 11 March) raises the question of how much water would be required to produce hydrogen fuel by farming algae, in response to the 25 February articles "The parched planet" (p 32) and "Green gold" (p 37). While I do not have the figures to answer this question, I have made a calculation on hydrogen production by bio-fermentation, which might prove illustrative. To replace the 54 million tonnes (and rising) of petroleum fuel currently used annually in the U.K. would require 1.78 x 10*11 cubic metres of H2. To produce this amount would need a total volume of 152 cubic kilometers of fermentation vessels, which is rather more than the entire reserve of freshwater available in the U.K. (148 km*3), assuming we had no other need for it. To grow the sugar crop for fermentation would require around 520,000 km*2 of land, which is more than twice the entire area of the U.K. mainland (most of which, of course, is not arable). The waste, mainly butyric acid (essence of sweat) and acetic acid (raw vinegar) produced would amount to around 400 million tonnes, or more than sufficient to fill Lake Windermere...and that is every year, in perpetuity. The details of these and other calculations on meeting future fuel requirements by renewables, nuclear etc are available at: http://ergobalance.blogspot.com, which your readers might find interesting.