The Kent wind farm subsidy is mostly a waste of money, even measured by UNIPCC’s case for taking drastic action on CO2.
First, two statements and a bit of data.
“… the Kent windfarm. £780m invested to chase £50 ROCs. Offshore is double bubble, so £100/MWh generated.” (Sep 25, 2010 at 1:41 AM | Atomic Hairdryer at BishopHill )
“An effective carbon-price signal could realise significant mitigation potential in all sectors. Modelling studies show global carbon prices rising to 20-80 US$/tCO2-eq by 2030 are consistent with stabilisation at around 550 ppm CO2-eq by 2100. For the same stabilisation level, induced technological change may lower these price ranges to 5-65 US$/tCO2-eq in 2030.” (P.18 UNIPCC Summary for Policymakers)
An alternative for a wind farm is a small power station consisting of diesel engines. The most modern diesel engines can produce less than 500kg of CO2 per MWh. (See note)
So the subsidy should be no more than the trading credit CO2 of 12.5-50 £/tCO2.
Based on these figures, it is possible to state that of the £100/MWh subsidy, at a very minimum £75 is a dead loss to society. At most it could at much as £95. This is before you undertake a present value calculation on the trading credits value in 2030, or start questioning the underlying economic assumptions. Further this is whilst accepting UNIPCC consensus position in its entirety.
For an alternative take, see Christopher Booker in the Telegraph
Note on CO2 output for a diesel power plant
A large container ship engine has around 470kg to 560kg of CO2 output per MW (emission comparison table on page 13), with around 58% engine efficiencies. (See a MAN Diesel & Turbo paper “How to influence CO2” – 5MB pdf). Power-plants can higher up to 90% more efficiencies by heat recovery processes, potentially cutting the CO2 out per MW to 350kg. However, this would need to be verified by actual measurements.
Note on carbon credits v Subsidies
A carbon credit aims at adding to the cost of producing CO2 directly, with the objective of encouraging the most cost-effective means of saving CO2. That is if cost saving is less than the cost of the credit, you purchase the credit. If it is greater, then you make the investment. For power plants it might be very effective for bringing forward investments in newer power plants. It would not be so effective in choosing between new power plants with massive differences in cost per unit of output.
ManicBeancounter 
Steve
/ 09/01/2011Hi Check out you tube for 1984 prototype of the “Automatic Feathering Flat Blade Radial Windmill”. this wind/water energy machine has been Ignored in favor of 17% oil Taxes and spills,embedded old good Boys retirements at patent offices,or just a bribe & go system that temporally has control. a unlinmited solid wall verses a deflection from 500 years ago. do the math.
manicbeancounter
/ 09/01/2011The problem of windmills is not with design efficiencies. It is of providing power when required at the lowest cost. In central England we have just had the coldest December for at least 352 years – and probably the least windiest. If it to save on carbon emissions, if it does not meet the $80 per tonne for CO2 saved (per the very biased Stern Review) then it should be rejected. Windmills not only have much higher set-up costs (including the extra transmission lines), then push up the unit cost of other forms of power generation that have to be available for non-windy peak periods.