Claims of new sources of abundant cheap energy should always be greeted with scepticism. Since the perpetual motion machine, successive technologies – nuclear fission, cold fusion, ethanol – have promised to transform our lives. They have not and will not.
The latest fad is for shale gas. Lots of it is trapped in rocks underneath and around the British Isles (near Blackpool especially) and it was thought it could not be extracted at an economic price. Along came a new technique, known as fracking, which involves underground explosions, and great reserves of oil and gas become viable. Except that, on second thoughts, they do not. As we report today, the energy industry has accepted that shale gas offers limited scope for new supply. Just as with all those previous promises of plenty, it has turned out to be difficult and expensive in practice.
This will come as a relief to the green-minded, worried that new supplies of cheap carbon-based fuel would compound climate change.
Unfortunately, however, environmentalists are just as capable of making unrealistic claims of exciting new energy sources. Peter Hain, who resigned last week from the Shadow Cabinet to advocate the Severn Barrage power plan, is the latest to do so. The Independent on Sunday wanted the tidal barrage to succeed, but we reluctantly accepted that the electricity it might generate would be much more expensive than the main green alternatives. When Chris Huhne, the former secretary of state for energy and climate change, abandoned the scheme in October 2010, we argued that if he, a rigorous economist as well as a committed environmentalist, could not make the numbers add up, no one else was likely to.
Still, let us keep an open mind. Mr Hain says that the consortium that wants to build the barrage can do so without subsidy from the taxpayer. That is the kind of assertion that sounds simple but is not, as it depends on how the cost of damage to the environment from rival carbon-based energy is accounted for.
Nevertheless, that is the policy on nuclear power – that the role of government is not to subsidise it but to help to overcome planning objections – and so, if the consortium really can make it pay, we should wish it well. If they cannot make it pay, it may be that less ambitious schemes for tidal lagoons could be viable.
In the meantime, the green energy policy for which Mr Huhne's successor, Ed Davey, is responsible must focus relentlessly on technology. The main thing that has happened since the coalition was formed two years ago is that the timetable both for new nuclear power and clean coal has slipped. New nuclear electricity is not likely to feed into the National Grid until the 2020s, and we are making disappointing progress towards workable "carbon capture and storage", the technology that would cut the greenhouse effect of burning coal.
Thus, Chancellor George Osborne was right, if provocative, to declare in the Budget: "Gas is cheap, has much less carbon than coal and will be the largest single source of our electricity in the coming years." Only it will not come from shale fracking, but from existing North Sea fields. But three other things need to happen.
One is that the dash for wind power must press on, over the objections of the Tory (and some Labour) backwoods. Wind turbines are only a small part of future energy supply, but they can be built quickly. The next is to try harder to make nuclear power and clean coal work for the long term. And the third is to make a success of the Green Deal – a national plan to improve insulation and save energy.
In energy policy, being the "greenest government ever", as David Cameron promised two years ago, means being straight with people that there are no easy answers.