Wind

Provided Wind turbines are able to run efficiently, some academics have calculated they will generate the energy taken to build them in less than a year. Others have worked out the typical CO₂ cost per kWh produced over the lifetime of a typical Wind Turbine, and come up with a figure of less than 50g CO₂ per kWh, provided the wind turbines are not built in places where they damage peat bogs. If their construction does disrupt peat bogs the CO₂ cost can rise to about 100g CO₂ per kWh. However the CO₂ per kWh from a coal-fired power station is 1000g CO₂ per kWh.

A link to an academic paper from Edinburgh University with this analysis is listed in references below.

In 2006, a turbine manufacturer, Vesta, also did an analysis of this and came up with a figure of about 7 – 9 months for one of their turbines to “pay” for itself in terms of energy. That was looking at an array of 100 wind turbines in fairly shallow water off the coast of Denmark, so the under-water supports for those turbines were metal poles rather than huge concrete pillars.

They included manufacture of the concrete and steel, transport of parts, erection, demolition as well as end of life dismantling and recycling costs.

They assumed each turbine would produce 14,230 MWh/year

Even if those are wildly optimistic assumptions, if you assume a wind turbine will produce electricity for 20 years, it seems likely that they will be a net positive in terms of energy over their lifetime.