Quezi

Wind power at Altamont Pass (photo by Kevin - CC-BY)

Green energy is a somewhat nebulous term, but we’ll take it to refer to energy sources that are carbon neutral, and are replenished naturally within timescales of the order of a human lifetime. That gives us quite a few sources of green energy.

Hydro power harnesses the energy of moving water. This can take the form of a water-wheel which drives machinery (as was common for the milling of wheat in past centuries), or of hydro-electricity (a generator powered by moving water), or of mechanical action powered by movement of the water (such as a hydraulic ram used to pump water). A dam can be used to hold back the water when its power is not required. The up-and-down movement of waves can also be turned into electricity, as can the ebb and flow of the tides.

Solar power harnesses the energy of insolation (incoming radiation – light and heat – from the sun). Photoelectric panels (solar cells) convert the sun’s energy directly into electricity. Solar thermal water heating panels convert the sun’s energy directly into heat.

Ground source heat pumps are an indirect form of solar power, and pump heat out of the earth. The heat was stored there as a result of the heating of the earth by the sun – and also indirectly by the sun’s heat captured by rainwater that in turn warms the earth. The heat pump requires a source of electricity to operate (typically consuming about a quarter the amount of energy that the pump can deliver from the ground heat). Unless that electricity has been generated from a green energy source, a ground source heat pump cannot be considered a form of fully green energy. In combination with a green energy electricity source, a ground source heat pump is probably the greenest way to heat a house in winter in cold climates.

In a few places, geothermal power can obtain heating or electricity from the earth’s stores of volcanic heat. For all practical purposes, this counts as green energy because the heat is replenished from the earth’s core (which holds enormous stores of heat), but strictly speaking it’s not green energy because the the heat of the earth’s core is not limitless.

Wind power harnesses the energy of the wind, either by using a generator to produce electricity, or by using the wind’s power to directly drive a pump or other kind of machine.

Burning wood, vegetable oil, or other organic matter (collectively known as biomass) is carbon neutral if the burned material is replaced by growing more. The carbon given out by burning the biomass exactly equals the carbon taken in during its growth. The heat produced by burning the biomass can be used to generate electricity or to propel a vehicle, or for heating. Biomass can have associated ethical problems if it displaces the growing of food.

Combined heat and power installations are sometimes erroneously described as green energy. These electrical generators produce power by burning gas but capture the otherwise wasted heat and use it for heating. Therefore they can be more efficient than centrally-generated power (where waste heat is dumped into the environment), but unless they are burning biomass they cannot be said to be green.

Nuclear power generates no carbon dioxide, but the fuel source cannot be replenished and it is therefore not a form of green energy as we have defined it. Were it not for the environmental and security problems associated with radioactive fuel and waste products, nuclear power would be clean even if not technically green. (see proposed UK nuclear power sites)

Electricity and heat can be produced by the incineration of waste, but the environmental issues associated with this are great enough that one would be hard-pressed to describe it as “green energy”.

Gas can be produced biologically from the decomposition (”anaerobic digestion”) of sewage, landfill, or marsh vegetation. The resulting gas can be burned for heat or electricity generation, and is considered by many to be green energy.

To harness any form of green energy requires the manufacture, transport, installation, maintenance and eventual disposal of machinery, and that will have a carbon cost associated with it. Because this environmental cost varies greatly with different energy sources, some forms of green energy are much “greener” than others – but that’s another question for another day.