The laws of energy: conservation, entropy and efficiency: The first law: energy is conserved

1.1 Energy is never created or destroyed, only converted

The first law says that energy can change form but the total stays constant. When coal burns in a power station, every joule of chemical energy in the coal reappears somewhere: as electricity sent out on the wires, as heat carried away by cooling water, as hot flue gas leaving the stack, as warmth radiating from pipes and walls. Add them up and you get back exactly the energy that went in.

The law was hard won. In the 1840s the German physician Julius Robert Mayer argued that heat and mechanical work were two forms of one thing. James Joule in Manchester measured how much a falling weight warmed water when it turned a paddle, establishing a fixed exchange rate between work and heat. Hermann von Helmholtz generalised the idea, and by the 1850s, with Rudolf Clausius and William Thomson, the conservation of energy had become the first law of a new science of thermodynamics.

In practice the first law is a bookkeeping rule. It means every energy flow diagram must balance. If a claim implies a machine delivers more energy than it receives, the claim is wrong, whatever the inventor says. Perpetual motion machines, which promise output without input, break the first law and have been refused by patent offices for over a century.

The first law also explains what efficiency measures. Because nothing is lost from the universe, efficiency is not about energy disappearing. It is about how energy is divided between the form you wanted, such as electricity, motion or light, and forms you did not want, which are nearly always heat. A 40% efficient power station is not destroying 60% of its fuel. It is turning that 60% into warm water and warm air.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'Conservation and efficiency'
• Jaffe and Taylor, The Physics of Energy (2018) · via Smil Mastery Course, M1 'Entropy and the second law', 'Heat and how it moves'

1.2 Efficiency is useful output divided by input, and it spans a huge range

Efficiency is the useful energy out of a device divided by the energy put in. It is always below 100% for a single conversion, but the range is enormous, and knowing a few benchmark values lets you judge almost any technology claim.

At the bottom are open fires used for cooking, which put well under 10% of the wood's energy into the pot; most heats the air around it. Old incandescent bulbs turned only 2 to 5% of electricity into visible light, the rest into heat. Steam locomotives managed 6 to 10%. Smil notes that Thomas Newcomen's early steam engines of the 1710s converted under 1% of coal's energy into work, and James Watt's improved engines of the 1780s reached only a few per cent, yet that small gain helped launch the industrial age.

In the middle sit car petrol engines, at 20 to 30% under typical driving and up to about 40% at their best point. Large slow-speed diesel engines on ships approach 50%. Modern coal plants reach 35 to 45%, and combined-cycle gas plants about 60%.

At the top are devices that do not pass through heat. Large electric motors exceed 95%. Big transformers exceed 99%. Electric heaters turn essentially 100% of electricity into heat, though, as Part 2 explains, that is not as impressive as it sounds.

Smil argues that the rise of modern civilisation owes as much to better converters as to more fuel. Each improvement in a prime mover, lamp or furnace multiplied the useful energy that a given amount of fuel could provide. The global energy system of 2024 delivers far more light, motion and heat per tonne of coal or barrel of oil than that of 1900.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'Conservation and efficiency'
• Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M1 'Conservation and efficiency'

1.3 In a chain, losses multiply, and cheaper useful energy invites more use

Real energy services pass through several converters, and the overall efficiency is the product of each step's efficiency, not the average. Small losses compound quickly.

Take electric light from coal. A power station might be 38% efficient, transmission and distribution 93%, and an incandescent bulb 5%. Multiply them: 0.38 x 0.93 x 0.05 is about 0.018. Less than 2% of the coal's energy becomes light. Swap the bulb for an LED and the chain improves six or seven times, without touching the power station. The lesson for investors is that the cheapest efficiency gains are often at the end of the chain, where losses have already been multiplied by everything upstream. Saving one unit of electricity at a lamp or motor saves roughly three units of coal at the plant.

The same arithmetic compares cars. A petrol car turns perhaps 20 to 25% of its fuel's energy into motion at the wheels. An electric car turns about 75 to 85% of the electricity it takes from the grid into motion. If that electricity came from a 36% efficient coal plant, the full chain is roughly 25 to 30%, little better than petrol. If it came from solar panels or wind, the comparison is transformed, because there is no fuel to waste in the first place.

Efficiency has a twist. In 1865 the economist William Stanley Jevons observed that more efficient steam engines had increased Britain's total coal use, because they made steam power cheap enough for many new uses. Economists now call the general tendency the rebound effect: when a service becomes cheaper, people use more of it. Rebound rarely cancels the whole saving, but it means efficiency alone seldom cuts total energy use as much as engineering calculations suggest.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'Conservation and efficiency'
• Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M1 'Conservation and efficiency'

See it in the table

Check your understanding

A chain has three steps at 40%, 90% and 50% efficiency. What is the overall efficiency?

A power station is 38% efficient. Where does most of the other 62% go?

What did Jevons observe about more efficient steam engines?