Wind energy can be used to turn the blades of a turbine, which spin a shaft, which connects to a generator and makes electricity.
New Zealand straddles the roaring forties, and both main islands lie across the prevailing westerly winds. While winds over the ocean are generally stronger and less turbulent than wind on the land, there are several land sites that provide reliable wind energy, most notably the Tararua Ranges in the North Island. These mountains are notorious among trampers for consistent and strong winds, so it is not surprising to find the two largest wind farms of the Southern Hemisphere along their ridge lines. A mountain range funnels the wind, creating flows strong enough to maintain an average wind speed of over 10 metres per second.
Tararua wind farms
Wind turbines need to be between 30 and 130 metres high to harness higher wind speeds and less turbulent flows, and they must be in windy, open terrain to be economically viable.
At the Tararua wind farm north of Wellington the blades of 103 turbines, each with a 660-kilowatt generating capacity, are working on 99 out of 100 days – almost half the time at maximum capacity. The farm produces electricity equivalent to operating for 4,000 hours at maximum output each year. In Wales, Scotland or western Ireland the output is typically around 3,000 hours, and in Germany, with the largest wind-power industry in the world, only 2,000 hours each.
The nearby Te Āpiti wind farm, completed in 2004, consists of 55 1.65 megawatt turbines. Combined, the two wind farms provide enough power to meet the needs of approximately 75,000 average New Zealand homes. This represents a similar output to that of the Wairākei geothermal plant, or about one-fifth the output of Manapōuri – New Zealand’s largest producer of hydroelectricity.
The weather in New Zealand is very changeable – it’s often said that you can have four seasons in one day. Designing wind turbines to withstand the country’s powerful, unpredictable winds is a challenge. In 2005 a prototype wind turbine in Canterbury was hit hard when the wind suddenly reversed direction and strengthened, from north-westerly to south-westerly, in about 90 seconds. The rotor was ripped out and the blades plunged to the ground.
Wind power economy
In New Zealand the wind blows strongly enough for wind-power companies to survive without the government subsidies available in other countries. By providing greater diversity in the way electricity is generated in New Zealand, wind farms make the country less vulnerable to power shortages. A further advantage is that they do not emit greenhouse gases. In future, wind power is expected to become even more cost-effective as turbine technology gets cheaper. Also, power companies will benefit from carbon credits granted for reducing carbon dioxide emissions.
The Hayes windmill
In the early 1900s the renowned Hayes engineering works in Central Otago was powered by New Zealand’s largest windmill. For 17 years the 19-metre-tall windmill fed power to machines by a complicated system of overhead shafts, belts and pulleys. In 1927 it was replaced by a more reliable source of power – hydroenergy.
The wind farms in the Tararua Ranges have generally been well received by local communities. The residents of the nearest city, Palmerston North, promote them as a tourist attraction. However, because the best sites are often on prominent ridgelines, locals are not always willing to have farms in their vicinity. Residents of Makara, a coastal community near Wellington, have expressed concern about the visual impact of wind turbines on their rural landscape.
Noise is another issue raised by some communities, but with modern designs this is rarely a problem. Wind farms must comply with a national standard which states that noise at the boundary of any residential site must not exceed the greater of 40 decibels or background noise plus 5 decibels. Local authorities may impose even lower acceptable noise levels.