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.
How solar heaters work
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.
Converting sunlight to electricity
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.