"WE BELIEVE that we can have the same kind of impact on energy that the mobile phone had on communications.” So says K.R. Sridhar, the boss of Bloom Energy, which on February 24th unveiled what it claims is a revolutionary fuel cell. Thanks to such grandiose pronouncements and a vigorous public-relations blitz, the "Bloom Box”, as the company’s product has been dubbed, has attracted plenty of headlines. But there are good reasons for scepticism.