There has been a lot of hype on low wind speed energy production for years and Sheerwind is just one of the "contestants" of challenging the laws of physics - i.e. there is only so much energy per m3 air / wind power density provided with low wind speeds and you can't harvest more than 100% - also see Betz's law.

For a good overview of novelty wind energy approaches (taken with a grain of salt) - http://barnardonwind.com/2013/06/03/good-and-bad-bets-new-wi...