This was achieved by fitting generators that delivered peak power -- or "rated power" -- at wind speeds in the range 15-18 m/s. The drawback of this approach, however, is that these wind speeds occur infrequently.
When a site’s mean wind speed is 8m/s, for example, wind speeds only reach 15m/s and above about 420 hours a year — less than 5% of the time (see chart below). The parameter that defines the ratio between rated power and size is the "specific rating", defined as rated power per rotor swept area (Watt/square metre) and, until recently, most machines had specific ratings in the 300-500W/m2 range.
However, we have seen a trend towards lower specific ratings in the past few years. The average for machines installed in the US in 2018 was 230W/m2, according to a recent Lawrence Berkeley Laboratory paper.
While a lower rating means less energy production, in kilowatt hour per square metre of rotor area, it delivers a number of benefits.
Cheaper to make
Turbines of any given size with a low specific rating will be cheaper than those with high ratings, since generator costs increase with power output. Because the drivetrain is subject to lower peak loads, smaller savings may accrue from other components.
Transmission costs will also be lower — both for the initial cost of the connection and the ongoing payments to a utility — and this is assigned a monetary value of about $1.60/MWh.
The way in which the costs of transmission reinforcements are shared between generators and consumers varies, but the analysis suggests that the majority of these savings would benefit electricity consumers in the US.
An important characteristic of turbines with low specific ratings is that the variability of the output is lower, because they operate at the rated output for longer periods of time.
This, in turn, reduces balancing costs, which is valued at $0.20/MWh. Another benefit of this reduced variability is that the inter-annual variability — due to variations in the annual mean wind speed — is also less, making investments more attractive, which shaves another $0.30/MWh off the cost of energy.
The authors suggest the trend towards lower rating will continue, citing analysis that shows that a 5MW turbine with a 150W/m2 specific rating and a hub height of 140 metres may have a cost advantage of $6/MWh (on average), compared with equivalent 2018 turbines with a rating of 230W/m2.
Wind-plant output declines less with age than solar PV
Recent reports from authors at Lawrence Berkeley National Laboratory have studied the decline in performance of photovoltaic and wind installations and shown that wind output declines at a slower rate.
The rates of decline for wind are less than they were, and the output from wind farms commissioned during the last ten years declined at a modest 0.17% per year, whereas the corresponding figure for PV was 1.3% a year.
Age-related degradation of the PV cells is the principal reason for the decline in output, whereas with wind it is the decline in the reliability of the mechanical components, due to wear and tear. This leads to increased downtime for maintenance.
Simplified data from both reports are shown in the chart. The "capacity factor index" is the annual capacity factor divided by the initial value.