Energy flows and payback time of a wind turbine

Now we move up in scale a second time. Wind energy (see the Chapter 10 title page figure) is attractive in a number of ways. It is renewable, it is not dependent on fuel supplies from diminishing resources sited in coun­tries other than your own, it does not pose a threat in the hands of hostile nations, and, by its nature, it is distributed and thus difficult to disrupt. But is it energy efficient? Energy has to be invested to build the turbine; how long does it take for the turbine to pay back on the investment?

Table 7.10 lists the bill of principal materials for a 2 MW land-based turbine. The information is drawn in part from a study conducted for Vestas Wind Systems,[27] in part from the Technical Specification of Nordex

Energy,[28] and in part from Vestas ‘ own report[29] scaling its data according to weight. Some energy is consumed during the life of the turbine (design life: 25 years), mostly in the transport associated with maintenance. This was estimated from information on inspection and service visits from the Vestas report, together with the estimated distance traveled.

The net energy demands of each phase of life are summarized in Table 7.10 (in units of MJ in the second column and in kWhr in the third). The turbine is rated at 2 MW, but it produces this power only when the wind conditions are right. This is measured by the capacity factor—the frac­tion of peak power delivered, on average, over a year. We take this to be 25%, giving an annual energy output of 4.4 X 10[30] kWhr/year.[31] The energy payback time is then the ratio of the total energy invested in the turbine (including maintenance) and the expected average yearly energy production:

The total energy generated by the turbine over a 25-year life is about 1 X 108 kW. hr, roughly 20 times that required to build and service it.

The Vestas LCA study for this turbine, a much more detailed analysis of which only some of the inputs are published, arrives at a payback time of eight months using a capacity factor of 50% (correcting to 25% gives 16 months). A recent study at the University of Wisconsin-Madison6 finds that wind farms have a high "energy payback" (ratio of energy produced com­pared to energy expended in construction and operation), larger than that of either coal or nuclear power generation. In the study, three Midwestern wind farms were found to generate between 17 and 39 times more energy than is required for their construction and operation, whereas coal-fired power sta­tions generate, on average, 11 times as much and nuclear plants 16 times as much. And, of course, coal-fired power stations emit CO2.

The construction of the wind turbine itself carries a carbon footprint. Using data CO2 from the data sheets of Chapter 12 and Table 6.7 gives the values shown in the last column of Table 7.11—a total output of 1,400

tonnes of CO2. But the energy produced by the turbine is almost carbon – free. The life output of 1 x 10[33] kW. hr, if generated from fossil fuels, would have emitted 21,000 tonnes of CO2. Thus wind turbines offer power with a much reduced carbon footprint.

The problem with wind power is not energy payback, but the small power output per unit. Even with an optimistic capacity factor of 50%, about 1000 2MW wind turbines are needed to replace the power output of just one conventional coal-fired power station.