Understanding Energy

Understanding Energy

14 modules take you from what energy is and the laws that govern it, through the flows of sun, wind, water and life, the long history from muscles and wood to fossil fuels, how electricity is made, moved and stored, how energy is used and wasted, to transitions, climate and what energy means for economies. The backbone is the work of Vaclav Smil, brought up to date with current data from the IEA, Energy Institute, IRENA and others. Each module links to the tiles in the Energy table.

Modules

1. Energy, power and the units that measure them: Energy is the one currency every natural and human process spends, and it comes in a few convertible forms. This module builds the vocabulary and numeracy for the rest of the course: joules, kilowatt-hours and exajoules, the difference between an amount of energy and the rate at which it flows, how much energy fuels pack per kilogram and per litre, and why electricity and nuclear reactions sit at the two extremes of flexibility and concentration.
2. The laws of energy: conservation, entropy and efficiency: Two laws govern every energy system. The first says energy is never created or destroyed; the second says that every conversion degrades its quality until it ends as low-grade heat. Together they set the limits on efficiency, explain why power stations waste most of their fuel, why heat pumps seem to beat 100% and why statisticians argue about how to count primary energy.
3. Earth's energy flows: sun, wind, water, heat and life: Every natural energy source on the Energy table is part of a few planetary flows: sunlight arriving and leaving, the winds and water it drives, a trickle of heat from Earth's interior, and the sunlight captured by plants and passed on to animals. Seeing their sizes side by side, from 174,000 TW of sunlight down to under 4 TW of tides, explains which sources are large, which are concentrated and why human energy use, at about 19 TW, is small beside nature yet big enough to shift its balance.
4. Muscles, wood, water and wind: energy before fossil fuels: For almost all of history, energy came from food eaten by people and animals, from wood and charcoal burned for heat, and from water and wind turning wheels and filling sails. These sources delivered power in units of tens of watts to a few kilowatts and drew on the same limited land, which capped yields, population and output. About two billion people still cook with polluting fuels, mostly wood, charcoal, crop residues and dung, so this old energy system remains part of the present.
5. The fossil-fuel revolution: For almost all of history people lived on the sun's current income: food, wood, wind and falling water. Within about two centuries coal, then oil and gas, and then electricity made from them replaced that income with a vast store of ancient sunlight. The story covers how that happened, why it was slower than popular accounts suggest, and why fossil fuels still supply about 87% of the world's primary energy.
6. Oil, gas and coal: how they form, are found, moved and priced: Oil, natural gas and coal are stores of ancient sunlight, made by geology over millions of years and drawn down by industry in decades. Each fuel is followed from source rock to customer: how it formed, how it is found and produced, how it is moved and refined, how long it might last and why its price swings. It ends with the 2026 closure of the Strait of Hormuz, which the IEA called the largest oil supply disruption in history, and what it revealed about a system built on a few chokepoints.
7. Making electricity: thermal, nuclear, hydro, wind and solar: Almost all electricity comes from spinning a generator, either with steam or hot gas made from fuel, or with moving water and air, or else straight from sunlight in a semiconductor. Each route has its own efficiency, cost, reliability and footprint. In 2025 renewables passed a third of world generation and solar was the fastest-growing source, yet coal still supplied about a third.
8. Power density: why land and space matter: Power density measures how much energy flows through each square metre of land, in watts per square metre. Fossil fuel extraction and thermal plants work at hundreds to thousands of W/m², while most renewable electricity delivers under 10 W/m² and crop biofuels under 1 W/m². Cities and industries use energy at 10 to 1,000 W/m², so the energy transition means gathering diffuse flows from large areas and concentrating them where people live and work.
9. Moving energy: wires, pipes and ships: Energy is rarely used where it is found. Wires, pipes, ships and trains move it, each with its own physics, losses and costs, and the cost of moving a unit of energy 1,000 km differs more than a hundredfold between carriers. Electricity grids must balance every second, fuels carry their own storage with them, and a few narrow straits carry a large share of world trade.
10. Storing energy: from fuel stocks to batteries: Storage moves energy through time, as transport moves it through space. Measured by energy held, fuel stocks still dwarf every electricity store, but batteries are now the fastest-growing part of the power system, beside the old workhorse of pumped hydro. Each store has its own efficiency, duration, cost, wear and material needs, and those decide which job it can do.
11. Using energy: food, materials, mobility and buildings: Final energy splits roughly into thirds between industry, transport and buildings, and within each the energy goes into heat, motion, light or feedstock. This module follows energy into fertiliser, steel, cement, plastics, aluminium, cars, jets, ships, homes and appliances, with the numbers that show where it goes and why some uses are far harder to clean up than others.
12. Efficiency, waste heat and recovery: About seven-tenths of the world's primary energy ends up as waste heat, much of it at low temperatures. This module explains where the losses happen, how efficiency has improved through history and in energy intensity, why rebound limits absolute savings, and how waste heat is graded by temperature and recovered with economisers, heat pumps, organic Rankine cycles, CHP and district heating, along with what each costs.
13. Energy transitions and climate: Energy transitions are changes in fuels, machines and uses that have always taken decades, because the energy system is vast, long-lived and still growing. This module traces the historical record, the progress and limits of decarbonisation so far, the materials that are hardest to clean up, and what CO2 and the IEA's latest scenarios mean for the climate.
14. Energy, the economy and the future: Energy is the physical basis of every economy, but its link with income and well-being weakens beyond a moderate level of use. This module covers energy intensity, quality of life, energy poverty and India's rising demand, the real costs of energy, its role in war and security, and how to read growth curves, forecasts and risks without being misled.