At any given power rating, rotor diameters are getting bigger. This means more energy can be delivered and, whatever the power rating, the capacity factor goes up.

The significance of capacity factors is often misunderstood, and there are other terms that need clarification, such as "full load hours", "efficiency" and "optimised". Full load hours are not the amount of time that a turbine spends at full load, but the time that it would spend at full load if it always operated at that level. "Equivalent full load hours" would be a better term. In this article, full load hours means exactly that.

**Low wind sites**

At the moment there is a lot of enthusiasm for what is termed large blades. There is nothing particularly new about the concept of putting large rotors on wind turbines with modest power ratings. It has for many years been the obvious route to go down for low wind speed sites. It simply does not make sense to put a wind turbine that has a generator with a high power rating in a location where that rated output will only be realised for a few hours in the year. It makes good sense to use a larger rotor diameter that enables rated power to be achieved for a reasonable proportion of the time.

**Specific rating**

When discussing the relationship between rotor diameter and power rating, the parameter that is usually used is termed the specific rating. This is simply rated power in Watts (W) divided by rotor area in square metres (m2). The average value at present — across a range of current designs — is around 400W/m2. Machines with specific ratings at or around this value include the Repower 5M (126-metre diameter, 5MW output) and the Vestas V27/225 (27-metre diameter, 225kW output) from earlier years.

These machines reach their rated power output at a wind speed of 13 m/s and, at a site with the IEC class 2 average wind speed of 8.5 m/s, spend 13% of the time at full load. So the "real" full load hours in a year are 13% of 8,760, or 1,139 hours. If either of these machines was installed at a site with the IEC class 1 average wind speed of 10 m/s the corresponding figure for full load hours would be around 2,100.

**Higher yields at windy sites**

"Is it possible to increase the yields of an offshore wind-power plant by 15% without having to change its dimensions?" asks turbine manufacturer Repower on its website. "It is.... Our engineers have used the proven 5M wind turbine as a basis for designing the 6M offshore model — and have thus developed a wind turbine with a nominal power of 6.15MW and a rotor diameter of 126 metres that ranks among the most powerful offshore wind power plants in the world," the firm continues.

The 6M has a specific rating of 493W/m2 and reaches its maximum, or rated, output at 14.5 m/s. At a site with a mean wind speed of 10 m/s, it spends about 17% of the time (1,490 hours) at that rating. The thinking behind this use of a higher rating is quite simple — rather than "throw away" a lot of energy at high wind speeds, it makes sense to utilise it, if possible.

**Large blades for low wind sites**

What may be termed the "real" full load hours, depend only on the site wind characteristics and on the rated wind speed; they are not influenced by turbine performance characteristics. With these caveats, a turbine with a rated wind speed of 12 m/s will spend around 2.5% of the time at full load at a site where the mean wind speed is 6m/s, rising to 22% of the time at a site with a mean wind speed of 9m/s. If the rated wind speed is 15m/s, then only 1.6% of the time will be spent at full load on a 7m/s site, rising to 9.4% at a 9m/s site. These trends are illustrated in figure 1.

As the supply of onshore sites with good wind speeds gradually runs out, there is an increasing emphasis on turbines with low specific ratings. They have higher capacity factors than turbines with higher specific ratings, and it is sometimes suggested that their efficiency is higher — but that is not really the case.

**Capacity factors demystified**

Capacity factors have nothing to do with efficiency (see WindStats Vol 23, No 1). High capacity factors indicate a good utilisation of the generator, but that is simply because a lot of the energy content in the wind is not utilised once the rated wind speed is reached. To illustrate the point, if a Vestas V66 2MW machine is installed on a site with a mean wind speed of 8m/s, the energy yield (at 100% availability) will be 5,626MWh a year, the capacity factor 0.32 and the productivity 1,644kWh per square metre of rotor area, per year. Substituting a V80 machine with bigger (80-metre diameter) blades and retaining the 2MW rating pushes the yield up to 7240MWh/yr, and the capacity factor up to 0.41. However, the productivity per square metre of rotor area comes down to 1,440kWh per year. Roughly 700MWh/yr have not been utilised by retaining the 2MW rating, compared with the possible output for a 3MW rating.

Table 1 (top half) illustrates the effects of increasing generator ratings, and the bottom part illustrates the effects of size changes. With the Enercon machines, a 50% increase in rating (2MW to 3MW) increases the yield by 14%, but decreases the capacity factor from 0.52 to 0.40. The effect of installing larger blades — which are 23% longer — increases yields and capacity factors, but productivity – in kilowatt hour per square metre of rotor area – goes down by 5.5%.

So it is possible to have high productivity — in kWh/m2 — or a high capacity factor, but not both, as illustrated in Figure 2. This uses performance data from actual machines. There is some scatter, but the trend is clear; with capacity factors in the 0.25-0.3 range, yields are around 1,500-1,600kWh/m2 per year, but with capacity factors of 0.45-0.50, yields are more likely to be around 1,200-1,300 kWh/m2.

**Can machines be optimised?**

There is often talk about machines being optimised, but, strictly speaking, that probably is not accurate. A wind turbine is only a part of a larger installation — a wind farm — and optimisation implies that all the possible combinations of turbine size and rating have been explored to derive an economic optimum.

The optimisation depends, however, on other factors, such as the remuneration that is to be received for the electricity. If that is generous, it may make sense to use a wind turbine with a fairly high rating, even on a low to medium wind speed site. The converse is also true.

"Tailored" might be a better term to describe the suitability of turbines for particular types of wind regime.