Energy sources

What each source gives, and whether it runs when you want it. Dispatchable sources run on demand; the others run when the weather or the Moon allows, and need storage or a second source behind them.

Every source passes through the same chain: primary energy → conversion → a carrier (heat, electricity or fuel) → storage → transmission → use.

• Every step loses some energy. A power station burning fuel turns only about a third to two-fifths of it into electricity; the rest leaves as heat. See where electricity comes from.
• Electricity is moved at high voltage because the loss in a wire rises with the square of the current. See moving it.
• Using less is a source too. Insulation, siting and daylight are cheaper than any generation you can build. See the demand step of energy without the carbon bill.

Renewable means naturally replenished. It does not mean harmless, free or carbon-free.

• Steady: hydro and geothermal heat run day and night.
• Variable: wind and sunlight come and go with the weather and the hour.
• Predictable, not on demand: tides can be forecast years ahead but arrive on the Moon's timetable.
• Low-carbon only if regrown: wood and crops. Burning wood gives off more carbon dioxide per unit of heat than coal; see fire and fuel.

Each source's capacity factor, and which are within reach of a rebuilding group, is in the electricity guide's sources table.

Each fuel up the ladder was hotter, denser or easier to move than the one before it. Charcoal and coke are wood and coal with the water and volatile matter baked out: they burn hotter and cleaner, and give off carbon monoxide with no smoke to warn you, which is why they kill in closed rooms. See the way a fire kills.

The table gives the IPCC's default net heating values and carbon dioxide per unit of heat for each fuel. Wood varies most: the IPCC's range for it runs from 7.9 to 31 MJ per kg, so dry it and grade it before you rely on it.

Fossil fuels are the remains of plants and sea organisms buried for millions of years: finite on any human timescale, and the concentrated energy every society so far has industrialised on. Energy without the carbon bill explains why the ladder crosses this bridge deliberately and briefly.

• Carbon: for the same heat, coal gives off about 1.7 times the carbon dioxide of natural gas (see the table in fire and fuel).
• Natural gas is lighter than air and rises. It has no smell of its own, so a sulphur-smelling odorant, mercaptan, is added to make leaks noticeable. A mixture in air ignites anywhere between about 5 and 15% gas.
• LPG, bottled propane and butane, is heavier than air: a leak sinks into drains, cellars and pits, and collects there.
• Coal: coal and coke. Oil: petroleum and refining.

Biomass is wood, crop waste, dung and energy crops: the oldest fuel, and still the one many households cook with.

• It is low-carbon only where as much regrows as is burned. Per unit of heat, wood gives off more CO₂ than coal (112 kg per GJ against 95).
• Indoor smoke kills. The WHO estimates that 2.1 billion people cook on open fires or inefficient stoves burning wood, dung, crop waste, kerosene or coal, and that the smoke caused about 2.9 million deaths in 2021. A chimney that draws, or cooking outside, is the fix: see the way a fire kills.
• Biogas is made by microbes breaking down manure, food waste or sewage without air, in a sealed tank called a digester. It is 45–65% methane, and the leftover digestate is a fertiliser.
• Biogas burns like natural gas, between about 5 and 15% in air, so a leaking digester or gas pipe in a closed room can explode.

• Power in watts ≈ 9.81 × flow (litres per second) × head (metres) × efficiency. 20 L/s falling 10 m gives about 1 kW: see generating it.
• Match the turbine to the head: a Pelton wheel for high head and low flow, a Francis turbine for medium head, a Kaplan or propeller turbine for low head and high flow.
• Pumped storage pumps water uphill when there is power to spare and runs it back through the turbines when there is not.
• From the first wheel to a turbine: water power and the overshot waterwheel.

• Power rises with the cube of wind speed: double the wind and the power rises eightfold. No rotor can capture more than 59% of it (the Betz limit).
• A large turbine's working range: it starts generating at about 3–4 m/s (6–9 mph) and reaches full power at its rated speed. Above that, the blades feather to hold the output there, and above about 25 m/s (55 mph) it shuts itself down.
• Small turbines must never run without a load, and need a dump load, a brake and a fuse: see generating it.
• Mechanical wind power, for grinding and pumping, is on the ladder at wind power, with the post mill build.

• Photovoltaic panels turn light straight into direct current. Commercial panels have gone from under 10% efficient in the mid-1980s to about 15% by 2015, and are now approaching 25%.
• Face fixed panels toward the equator: south in the northern hemisphere, north in the southern.
• Making cells needs semiconductor manufacturing. Until then, salvage existing panels, which lose output only slowly, and protect them.
• Solar thermal is much easier to build: a black absorber under glass heats water to around 40–70 °C, and concentrating mirrors can raise steam. See energy without the carbon bill.

• Power plants need water or steam at roughly 150–370 °C (300–700 °F), so they are built only where it lies near the surface. There are three kinds: dry steam, flash, the most common, and binary cycle, which works at the lower temperatures.
• Cooler water heats buildings, greenhouses and fish ponds directly.
• Ground-source heat pumps work almost anywhere: the ground 3 m down stays at a steady 10–16 °C (50–60 °F), a heat source in winter and a heat sink in summer.

• Tides come from the pull of the Moon and Sun as the Earth turns. They can be predicted years ahead, but they run on the Moon's timetable, not on demand.
• Tidal barrages and lagoons hold water back and release it through turbines; tidal-stream turbines stand in a fast current. Water is about 800 times denser than air, so a slow current carries a great deal of power.
• Few exist at full scale. The largest tidal plant, Sihwa Lake in South Korea, is 254 MW; the oldest, La Rance in France, is 240 MW.
• Wave power is still at the research stage: the United States, for one, has no commercial wave project.

• Fission: a neutron splits a uranium-235 nucleus, which releases heat and more neutrons to split the next. The heat raises steam for an ordinary turbine.
• A pressurised water reactor has two loops. The reactor's water stays sealed in the first loop and heats the second in the steam generator. The second loop's steam turns the turbine, is condensed, and is pumped back to the steam generator.
• Spent fuel stays hot and radioactive: it is cooled in pools of water first, then moved to dry storage.
• It is far beyond a small society. Enrichment, the industrial base, and institutions that must outlast the waste for centuries: see energy without the carbon bill.

• A carrier, not a source. It is made by splitting water with electricity (electrolysis) or, more often today, from natural gas (steam methane reforming), which gives off carbon dioxide. It is only as clean as the energy that made it.
• It holds 120 MJ per kg, nearly three times petrol's 44, but it is so light that it must be compressed to 350–700 bar or cooled to −253 °C to store.
• A fuel cell turns hydrogen and oxygen back into electricity, heat and water.
• Leaks are hard to detect. Hydrogen is odourless, and no odorant is added, because odorants poison fuel cells. It burns in daylight with a nearly invisible flame, and a mixture in air ignites anywhere between 4% and 74% hydrogen. It rises fast, so it collects under roofs and ceilings.

Storage is not a source: every store gives back less than was put in. It matters because wind and sunlight come and go.

• Pumped water: 1,000 tonnes lifted 100 m stores about 270 kWh.
• Hot water: 1 m³ stores about 1.16 kWh for every °C it is heated.
• Batteries cover hours; pumped hydro covers hours to days; hydrogen and chemical fuels cover weeks and seasons.

What a small settlement can build, with the numbers to size it: storing it.