Types of Energy: High Carbon, Low Carbon and Zero Carbon

Three bands, four examples each. The marks are not really in the list — they are in being able to say why something sits in one band and not the next one along.

The short answer

Three bands, four examples in each. High carbon: natural gas and LPG, fuel oils, coal and peat, and electricity from non-renewable sources. Low carbon: solar thermal, biomass, heat pumps, and combined heat and power. Zero carbon: wind, tidal, hydroelectricity, and solar photovoltaic.

The paper shows you pictures rather than words. A hydroelectric dam is zero carbon, a coal fired power station is high carbon, and a solar thermal panel is low carbon.

You have to recognise them, not only name them

A picture question is a different skill from a list, so it is worth knowing what each one looks like.

• A coal or gas fired station is the wide cooling towers and the tall thin chimney. Worth knowing which is which: what pours out of the fat towers is water vapour, and the carbon dioxide leaves quietly from the thin one.
• A hydroelectric plant is a dam and a reservoir, or a small turbine house on a river.
• A solar thermal collector has pipework at the top edge, or is a rack of glass evacuated tubes. A photovoltaic panel is a grid of dark cells crossed by fine silver lines, and has no pipes at all.

If you can tell those two panels apart in a photograph you have the mark that catches most people.

Why solar thermal is low and solar photovoltaic is zero

Both sit on a roof and both are driven by the sun, so the split feels arbitrary until you look at what each installation actually does while it is running.

Solar thermal has a pump and a controller, and often a motorised valve, so it draws a small amount of electricity to move the heat it has collected. Photovoltaic has no moving parts and produces electricity rather than consuming any. That is the whole of the difference as far as the bands are concerned.

Being honest about it: on a full life-cycle view nothing is truly zero, because panels, turbines and dams all take energy to build. The bands describe what is released while the thing is in use, which is the only way a classification like this can work.

Electricity sits in two bands, and that is the point

Electricity is not a fuel. It is a way of moving energy from one place to another, and its carbon is decided at the power station rather than at the socket. That is why the answer list splits it: non-renewable generation is high carbon, and wind, tidal, hydro and photovoltaic are zero.

The practical version is that the same heat pump is a different machine at different hours. On a windy afternoon it is running on almost nothing; on a still, cold evening when gas plant is filling the gap, it is not. The grid has changed enormously — coal has gone, wind now supplies more across a year than anything else — but it has not finished changing.

Why low carbon is not a consolation prize

Biomass burns, and you can see it burning. The argument for putting it in the low band is that the carbon coming out is carbon the tree took from the air over the last few decades, so it is a short loop rather than releasing something that has been underground since before there were trees. That argument only holds while the wood is replaced.

Combined heat and power is the one worth understanding properly. An ordinary power station throws away roughly half the energy in its fuel as warm water and steam, because there is nobody near enough to want it. CHP puts the generator where the heat is wanted — a hospital, a block of flats, a district scheme — and uses both outputs. The same fuel does far more work, which is why it earns a band above the fuel it burns.

What comes next

Level 2 takes the high carbon column apart properly: what a fuel actually is, calorific values, what complete combustion produces, and what incomplete combustion produces instead. Carbon monoxide is the point at which this stops being an environmental subject and becomes a safety one.