Renewable energy comes from sources that are replenished as fast as they are used. Examples include energy from the sun (solar), wind, moving water, and plants such as pine forests, which supply firewood. This energy is harnessed to drive generators that produce electric power.
Wind and solar energy are sustainable, clean sources of energy that have the potential to make a significant contribution to New Zealand’s economy. New Zealand is rich in renewable energy resources and, more than other developed countries, already meets much of its energy needs by harnessing the power stored in rivers, lakes, geothermal fields and woody plants (known as biomass).
Nearly a third of the total energy consumed – including electricity, heat and transport fuels – comes from renewable sources. About 70% of all electricity is generated by renewable energy.
Water power (also known as hydro-electric power) and geothermal energy are the main, well-established renewable sources in New Zealand, and they make up the lion’s share of the total renewable energy supply.
New Zealand’s largest rivers, the Waikato in the North Island and the Clutha in the South Island, flow though several large dams and power stations, and there are many smaller hydro-electric stations throughout the country. Geothermal springs and vents have powered electricity generators since the 1950s, and have also been used for domestic and industrial heating.
Bioenergy – from firewood and solid or liquid waste products – is well established, contributing 5–6% of the energy used. The forestry industry, for example, uses waste woody biomass to produce both heat (from firewood) and electricity (to fire up turbines), and several landfill and sewage facilities extract methane to do the same.
New Zealand’s use of energy has doubled every 22 years over the past century. While renewable energy has been an important source, particularly because of the use of hydro power for electricity, New Zealand has increasingly relied on non-renewable, fossil fuels such as coal, oil and gas. In particular, the country’s largest gas field, Māui, has provided a cheap source of energy since the late 1970s. However, there is a growing interest in renewable energy as a result of dwindling gas reserves, mounting pressure to reduce carbon dioxide emissions, and power crises caused by fluctuating hydro lake levels.
In 2001 New Zealand’s first national energy efficiency and conservation strategy was released. It outlined some ground rules, including a 20% improvement in energy efficiency and a 22% increase in the supply of renewable energy, both to be achieved by 2012.
Wind generation is making an increasingly significant contribution, although it still forms less than 1% of annual energy production. Solar energy is not yet extensively used. Less than 2% of homes had a solar water-heater in 2004, but interest is growing.
In the future, emerging renewable technologies such as harnessing the ocean’s waves and currents are expected to become more economic. Ocean waves are produced by wind. The powerful ocean swells reaching the coast carry energy that could be used for electricity production. Scientists from the National Institute of Water and Atmospheric Research have determined that the western coast of New Zealand has the best prospect for small-to-medium scale generation of wave power.
More than a third of New Zealand’s energy is used in transport, and alternative fuels under consideration include biodiesel from animal fat and ethanol from whey as a blending component in petrol.
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.
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.
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.
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.
Solar energy – the energy from sunlight – is the most abundant form of renewable energy. Sunlight can reach the Earth’s surface with a maximum intensity of more than 1,000 watts per square metre. Annual sunshine hours in New Zealand range from about 1,600 in Invercargill to over 2,400 in Blenheim, and the main centres receive about 2,000 hours. While the total household rooftop area in New Zealand is exposed to solar energy that equates to about twice the total national energy use, the resource is relatively low in intensity for much of the day, and available only intermittently. Two technologies are used to capture solar energy: solar water-heating systems and photovoltaic conversion of radiant energy to electricity.
The amount of energy received from the sun in New Zealand is similar to Australia and higher than in Europe. An average New Zealand home receives 15 to 30 times more energy from the sun than it uses in electricity and gas. The most common use of this renewable energy is to heat water.
Conventional electric water-heating systems are the biggest energy guzzlers in most households, making up around 40% of the power bill. While solar water-heaters do not produce any electricity directly, they replace the use of considerable amounts of natural gas or electricity that are produced at least partly from non-renewable fossil fuels.
In most New Zealand locations, a solar water-heater can produce up to 15 kilowatt-hours per day in summer – more than enough for an average four-person household. But in winter it will provide nine kilowatt-hours, less than the daily requirement. Therefore solar water-heaters normally have to be backed up by electricity, gas or woodburner water-heaters.
In 2003, solar hot-water technologies contributed the equivalent of about 51 gigawatt-hours of electricity (0.1% of New Zealand electricity consumption). Each year about 1,800 solar water units are installed, mostly in family homes. The demand is growing as the benefits are recognised.
Great Barrier Island is New Zealand’s largest community without a power plant or connection to the national electricity grid. There are no power poles or lines – instead the island’s homes are festooned with wind turbines, solar panels and solar water-heaters. In 1989 residents turned down a proposal for three central diesel-powered power plants, and today use only as much power as they can generate from renewable sources, doing without luxuries such as street lighting and night landings at the small airstrip.
To use the sun’s heat efficiently, solar water-heaters have to trap the energy in a collector panel (either a flat sheet or envelope of darkened metal, or an evacuated (vacuum) tube, which all absorb the heat). This heat is then transferred to water flowing through the panel, or to tubes attached to the panel. The heated water then flows into a storage cylinder. In thermo-siphoning systems, the collector and water cylinder are both mounted on the roof, and the hot water rises naturally to the top of the cylinder.
To convert solar energy to electricity, photovoltaic (PV) cells are used. Usually made of silicon, they are designed to generate an electric current when exposed to sunlight. New Zealand receives on average about 4 kilowatt hours of raw solar energy per day for each square metre horizontal to the ground. Solar electric PV panels currently available can convert up to 15% of the available solar energy into electricity. On average, for a fixed panel on a roof, the percentage converted is lower because the sun changes position through the day, and across the seasons. The average New Zealand household uses about 22 kilowatt hours of electricity per day. To generate this quantity from sunlight would require about 45 square metres of PV panels on the roof. Since the average roof has much more area than that, this is easily achievable.
PV electricity is well established at certain remote sites. New Zealand’s coastline is dotted with solar-powered lighthouses, and kilometres of electric fences on farmland and nature reserves up and down the country are fed by PV-powered solar panels. In Antarctica, at latitudes of more than 70° south, the 24-hour summer sunlight powers instruments, meteorological stations and machinery.