Wind power can also be used on a small scale to reduce greenhouse gas emissions and household energy costs. While roof-top wind turbines are not as efficient as those used for large-scale wind farms, they have the advantage of not being subject to energy losses from transmission and distribution. Nalanie Mithraratne (2009) performed a life cycle assessment of 1.5 kW Swift wind turbines in New Zealand to determine their net energy savings and emissions reductions. She found that, for households in New Zealand, a roof-top turbine would take 7–11 years to make returns on energy, and 10–16 years to compensate for the CO2 emitted in its manufacture, transport, and maintenance. — Noah Proser
Mithraratne, N., 2009. Roof-top wind turbines for microgeneration in urban house in New Zealand. Energy and Buildings 41, 1013–1018.
First, Mithraratne had to determine what locations and turbines would be viable for micro-scale wind power production. In the urban environments being considered, wind resources are affected by nearby buildings and trees as well as the architecture of the building the turbine is mounted on. These obstacles can significantly reduce wind energy production. Additionally, the turbines cannot exceed community noise standards and must be light enough to be installed on an average household’s roof. The inherent difficulties involved with urban wind power effectively limit turbines to areas with average wind speeds of at least 5.5 m/s. Mithraratne also suggests that turbines should only be installed on buildings that are 50% higher than the surrounding structures.
The life cycle analysis of the turbines revealed that the manufacturing process accounts for nearly 80% of the energy used and roughly 70% of the greenhouse gases emitted in the turbine’s life. The author also evaluated the energy costs of transporting, installing, maintaining, and, finally, decommissioning the turbines. Overall, one turbine can be expected to emit 2312 kg of CO2, while generating 10520–16820 kWh of electricity during its 20-year lifespan. Thus, using urban wind power can create a net reduction of 539–2246 kg of CO2. The wide range of this statistic is due to the different scenarios Mithraratne considered, which involved different maintenance regimes and disposal techniques.
It is important to note that the life cycle analysis presented here was focused on urban wind power in New Zealand. Transportation of the turbines from the UK was a large factor in the energy costs and emissions in this scenario (roughly 18%). Clearly, less remote locations would have lesser transportation costs, and, correspondingly, higher net CO2 reductions with quicker returns on investments. Though urban wind power is unlikely to make up any large portion of worldwide energy production, it could be a useful and practical addition to the grid.