I agree that you have a point - eventhough they are (at least) trying to paint these "diesel generators" green(er).They mainly get the juice from diesel generators, because most locations can't handle the amount of power that is needed for the event. Most inner city locations are not prepared for these kind of power requirements.
As Formula E wants to play a key role in advancing the technology surrounding electric car ownership not just the cars themselves, the bold decision was taken that the charging of the racing cars had to come from a sustainable source.
To this end, a British company called Aquafuel Research Ltd was commissioned to build a pair of mobile generators that could be packed inside a shipping container and freighted to each race. But what makes these generators special is that they run on a fuel that is virtually emission free – glycerine.
“What we do is provide an electric power and distribution system in a very controlled way so that the cars can be charged in an hour in a very safe and reliable way that is fair to all the teams,” says Paul Day, the CEO of Aquafuel. “The really beautiful thing about it is that we do that with our own patented renewable energy generators using a really clean fuel – clean in terms of carbon emissions and clean in terms of the local air quality, very low in NOx emissions, very low in particulate emissions – there’s no smoke.”
The generators are based on standard production diesel engines – a Cummins KTA50 to be precise – that have been adapted with Aquafuel’s patented technology to run on glycerine. The fuel itself is a by-product of the bio-diesel productions process. Traditionally its commercial uses have been mainly in cosmetics and industry, but six or seven years ago, with the new biodiesel supply outstripping traditional demands, Day started to look at other applications for the sweet-tasting, colourless and gloopy liquid.
https://www.fiaformulae.com/en/news/2016/june/insight-formula-e-s-emission-free-charging/