The ridiculous fashion for urban wind turbines is still showing no sign of abating with the erection of BSkyB’s new turbine at its West London studio complex. Perhaps the continuing political insistence for ineffective on-site renewable generation is to blame. It is not just successive national governments and fashion-following local planning regulations, but all too often we find that corporates are now playing to the populism of green. This collective disregard for engineering reality forces building owners and developers to pay for sub-optimal solutions and forces architects and engineers to try and justify the essentially unjustifiable in defence of what has been forced on them. AJ Footprint 25th April

If you ask a primary school class where we should build wind turbines, the answers usually range from “on top of hills” to “out at sea”, anywhere it is windy. By the time those children arrive at the final year of their architectural degrees the answer has often become “attached to my building as an icon”.

Unfortunately the very nature of buildings is to disrupt the smooth flow of wind which is essential for efficient energy generation. The increased friction due to surface roughness in urban areas reduces the potential power in the wind dramatically. At the height of BSkyB’s turbine, it is only half that of rural areas. In city centres the power available may be just 15% of the open country equivalent (full explanation here).

This location effect is generally accounted for by applying a capacity factor to the theoretical maximum generation of a turbine. The rule of thumb for UK wind power is to assume a capacity factor of 30%-35% for good onshore installations. The generation figures quoted for BSkyB indicate a capacity factor of just less than 15%. Thus the same turbine, at the same cost, could generate more than twice as much electricity if it was not shackled to a building. This doubling in output would more than offset the grid distribution losses (around 6%) to deliver the electricity back to BSkyB in West London.

Two identical Enercon E70 Turbines. The one on the left produces 3.5GWh pa whilst the one on the right produces 5.7GWh pa. The difference is due to surface friction.

Apart from the very obvious branding potential, urban wind turbines have little going for them. It is time that politicians, national, local and corporate, stopped interfering and let engineers and architects make the best technical systems decisions for genuinely sustainable development.