A mountain stream is dammed in a high place, to create an artificial lake or reservoir. Farther down the mountain, the power station is equipped with water turbines. These are simply highly efficient versions of the old fashioned water- wheel; effectively they harness the kinetic energy of a carefully channelled waterfall to produce mechanical rotation.

This is another underdeveloped source. If you drill down into the Earth's crust, at first the temperature drops, because the sun's warmth can't penetrate. But deeper, the temperature rises. Volcanoes are evidence of this - molten lava is pretty hot! That well of energy is there to be tapped. As always, the final conversion process is the familiar steam turbine. And, like solar energy, it is environmentally friendly, provided you don't accidentally trigger a local volcano! But it is not as simple as it seems. The process of taking heat from a hot rock cools the rock locally. There's plenty more heat surrounding it, but can it flow quickly enough to your collectors? Again, it's another technology whose time will come, but not a panacea.

Windmills have been around for centuries and all have harnessed the energy of moving air (wind!) through rotating sails or fan blades. Traditionally, the mechanical energy was used directly, to turn a mill wheel. A modern wind turbine simply couples the rotating shaft to an alternator armature. The last link in the chain is always the same - electricity from mechanical rotation.

n a coal or oil fired power station, the fuel is burned (converting its chemical energy into heat) and the heat used to convert water into steam at very high temperature and pressure. This then drives a steam turbine, a device which harnesses the energy in the steam (heat and pressure) to produce rotational movement (mechanical energy). The rotating shaft of the steam turbine is coupled to the armature of the alternator, so the final result is electricity.

Transmission lines are the arteries of New Zealand’s National Grid, owned and operated by Transpower. Transmission lines take power from the generators to substations where it is supplied to local distribution companies and large industrial consumers. Transpower owns and operates 11,806 km of high-voltage transmission lines. Except for the high-voltage direct current (HVDC) link across Cook Strait, all transmission lines carry at least one alternating current (AC) circuit, with a minimum of one conductor per phase. There are three phases per AC circuit. The core of the National Grid is the 220 kV network in each island and the HVDC link between them. The 220 kV lines connect the largest power stations with the main load centres. Provincial centres and smaller power stations are connected by transmission lines operating at 220 kV, 110 kV, 66 kV and 50 kV