I am a bit of a contrarian and armchair economist. This is an interesting article.
Osborne tosses £3bn gift to the green elite
While he makes some interesting points he fails to note that a public subsidy is not identical to a making a loss. A public subsidy can be justified when it produces externality benefits. In addition, subsidies can be redistributive because they benefit all citizens rather than just tax payers and even tax payers may receive more in benefits than they pay in taxes.
The green bank may turn out to be an economically effective subsidy because it faciliates the production of public goods such as CO2-free electricity that would not otherwise be produced.
Of course as a free-marketeer I would not have chosen this as an optimal solution. If my goal was to reduce C02 emissions I would simply impose a C02 tax and then then the market solve the problem. This is not politically possible but many nations are making an attempt to move in this direction.