Our aim was to produce cost-abatement curves for comparison with other technology options; estimates of the unit cost and installable capacity of wind energy were means to this end. Nonetheless, the $20/tonne result above would require some 2875 GW of wind worldwide, of which 103 GW would be in the EU. Greenpeace's "Wind Force 10" world target is for 1210 GW and the European Wind Energy Association's EU target is 150 GW.
You state that wind power in "most of Europe, the Americas and Asia" displaces existing coal-fired generation, with CO2 savings of just under 1 kg/kWh. Our report argues that new wind turbines, for example in the EU, will mainly avoid the construction of new gas fired combined cycle plants, resulting in emissions savings of about 0.3-0.4 kg/kWh. In some other regions, such as China, wind turbines will mainly avoid construction of new coal plants, with consequently higher savings.
Our cost-abatement curves were produced by comparing total power sector costs and CO2 emissions with and without wind energy. In the former case in the EU, we expect new gas-fired generation to meet new and replacement base-load needs; gas displaces coal. The introduction of wind results in less gas being built, rather than more coal being retired, because the marginal cost of existing coal-fired generation is lower than the full cost of new gas or wind generation. It is more expensive to displace existing coal than new gas, even if the stranded costs associated with early retirement are excluded.
Finally, the study was carried out by Garrad Hassan and ECON for the International Energy Agency Greenhouse Gas R&D Programme, not the International Energy Agency. The two have separate memberships.