Electricity generation

Contact has 11 power stations in New Zealand generating electricity using hydro, geothermal and thermal energy. We're focused on meeting New Zealand's energy needs in a safe and reliable manner, and through our diverse portfolio of generation assets we are able to respond more efficiently to changing electricity market conditions.

Geothermal generation

Fluid from highly pressurised, natural geothermal systems is brought to the surface by wells that vary in depth from a few hundred metres to 2.5 km. At the surface this fluid is separated into two streams, one of steam and the other of water. The steam is used in a turbine to generate electricity and the hot geothermal water is either injected back into the ground or drained away.

Greenhouse gas emissions from geothermal power plants are significantly less than for a conventional thermal power station, although usually other gases, such as hydrogen sulphide, are emitted with the geothermal steam.

Contact's Wairakei power station was the first in the world to harness hot geothermal water for the production of steam. Now, several countries, including Italy, Iceland, Indonesia, the Philippines, Mexico, China and the USA use geothermal energy for both electricity production and direct use.

The Power from the Earth (3.07MB) booklet, produced for the 50th anniversary of the Wairakei geothermal power station in 2011, provides an introduction to geothermal electricity. It explains what it is, how it works, the history of geothermal energy and why it is so important to a nation and a world requiring ever-increasing amounts of reliable, renewable energy.

For more about Contact’s geothermal experience see Our Geothermal Advantage.

Hydro generation

'Hydro' comes from the Greek word 'hydra', meaning water. Hydroelectric power plants convert the potential energy contained in water into electricity. The water is stored in lakes behind dams. The amount of energy that can be produced depends on how far the water falls from the top of the lake to the turbine and how much water is in the flow. The difference in height is called the 'head' and the higher the head the greater the amount of energy per unit of water that can be converted to electricity. By passing this water through a turbine, the potential energy changes to kinetic energy that is then converted into mechanical, rotating energy, and then into electrical energy.

Thermal generation

Thermally generated power usually converts the chemical energy in fuels (gas, coal or oil) to generate electricity. The main means of generating electricity from fossil fuels are conventional steam, gas turbine, combined cycle or cogeneration plants.

Conventional steam

In a conventional steam plant, the fuel is burned in boilers that produce high pressure and high temperature steam – usually around 540 to 560 degrees Celcius and 100 to 130 bar (one bar is about the pressure of air at sea level). The steam is then put through a turbine, which in turn drives the electrical generator.

Gas turbine

As the name suggests, gas turbine power stations use a 'jet engine'. This provides a high pressure and high temperature gas stream that drives a 'power' turbine that turns an electrical generator. The jet engines can be industrial versions of those used on aircraft or they can be specifically designed industrial engines. Gas turbine power stations tend to be relatively inefficient, but this has been significantly improved in the past few years by good design.

Combined cycle generation

A combined cycle plant uses a combination of a gas turbine, boiler and a steam turbine. They can achieve much higher efficiency than either the gas or steam turbine systems alone. Typical thermal efficiencies of 55 - 58 per cent are achieved from combined cycle plants versus 33 - 38 per cent from conventional steam plants.

In the combined cycle plant, the fuel is burned in the gas turbine. The exhaust gases from the gas turbine contain residual heat, most of which is recovered in a boiler called a 'heat recovery steam generator'. This generates steam to drive a conventional steam turbine. The gas turbine and the steam turbine both drive the electrical generator.

Cogeneration

Many industrial and commercial processes, like pulp and paper manufacture or food processing, require large amounts of heat and power. In conventional processes, the heat is often produced by burning fuels on site and using the hot gases directly to produce steam or hot water. Usually electricity from the national grid drives the machinery.

Cogeneration combines the process of producing heat and power on site, from a single primary energy source, instead of importing power from the national grid.

Cogeneration is nearly two and a half times more efficient in the use of fuel than conventional power generation. A cogeneration facility uses only half, or in some cases as little as 40 percent, of the fuel that a conventional thermal station needs to produce the same amount of useful energy. However, cogeneration plants rely for their efficiency on a good heat load and there is limited scope for this in New Zealand.