# The laws of energy: conservation, entropy and efficiency: The first law: energy is conserved Source page: https://www.materialinsights.org/learn/energy/laws-of-energy/p1/ [Understanding Energy](https://www.materialinsights.org/learn/energy/) / [Module 2: The laws of energy: conservation, entropy and efficiency](https://www.materialinsights.org/learn/energy/laws-of-energy/) Part 1 of 5 ## The first law: energy is conserved The first law of thermodynamics is the strictest rule in energy accounting: in a closed system the total amount of energy never changes. It cannot be made from nothing and it cannot vanish. What does change is how much of it ends up doing what you wanted. That share is efficiency, and it varies from a few per cent to almost a hundred. ### 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. Process flowchart Where the energy in coal goes at a power station Inputs **Thermal coal**100 units of chemical energy Process steps 1 **Boiler** about 88-90 units become heat in steam; 10-12 leave in flue gas and ash 2 **Steam turbine** about 40 units become shaft work; the rest stays as heat in exhaust steam 3 **Condenser** about 50 units of low-temperature heat go to cooling water or towers 4 **Generator and station use** a few units go to pumps, fans and mills inside the plant Outputs **Electricity sent out**about 35-38 units **Heat to air and water**about 62-65 units Totals always balance to 100: the first law in action. Source: Smil, Energy: A Beginner's Guide (2006), via Smil Mastery Course, M1 'Conservation and efficiency' Common misreading #### The water-powered car Every few years a story appears of a car that runs on water, usually by splitting it into hydrogen and oxygen on board and burning the hydrogen. The first law rules it out. Splitting water needs at least as much energy as burning the hydrogen gives back, and real electrolysers and engines lose a large share on top of that. Water is what is left after hydrogen has burned. It is ash, not fuel. Any claim of net energy from water alone is a perpetual motion machine in disguise. - 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. Data Typical conversion efficiencies | Converter | Conversion | Typical efficiency | |---|---|---| | Open wood fire for cooking | chemical → heat in pot | under 10% | | Incandescent lamp | electricity → visible light | 2-5% | | Steam locomotive | chemical → motion | 6-10% | | Car petrol engine (real driving) | chemical → motion | 20-30% | | Large marine diesel | chemical → motion | about 50% | | Coal power station | chemical → electricity | 35-45% | | Combined-cycle gas plant | chemical → electricity | about 60% | | Large hydro turbine and generator | potential → electricity | about 90% | | Large electric motor | electricity → motion | over 95% | | Large transformer | electricity → electricity | over 99% | Source: Smil, Energy: A Beginner's Guide (2006), via Smil Mastery Course, M1 'Conservation and efficiency' In the data #### Light per unit of fuel Lighting shows how much converters matter. A candle or kerosene lamp turns a tiny fraction of a per cent of its fuel into visible light. An incandescent bulb did better, a few per cent of the electricity. Modern LEDs produce more than ten times as much light per watt as incandescent bulbs. Across two centuries, the light obtained from a unit of primary energy has risen by several hundred times, and the cost of light has fallen even faster. Better converters, not cheaper fuel alone, made light abundant. - 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. Process flowchart Two chains from fuel to motion Inputs **Petrol in a tank**100 units **Coal at a power station**100 units Process steps 1 **Petrol engine and drivetrain** petrol → motion at the wheels, about 20-25% 2 **Coal plant and grid** coal → electricity at the charger, about 33% 3 **Charger, battery and motor** electricity → motion at the wheels, about 75-85% Outputs **Motion from petrol**about 20-25 units **Motion from coal electricity**about 25-28 units With electricity from Solar photovoltaics or Onshore wind there is no fuel step, so the comparison turns on cost and emissions, not combustion losses. Source: Smil, Energy: A Beginner's Guide (2006), via Smil Mastery Course, M1 'Conservation and efficiency' Case #### Fans, ACs and the rebound India's efficiency labelling for ceiling fans and air conditioners has made new models far more efficient than those of fifteen years ago. Yet household electricity use for cooling keeps climbing. Much of that is rising incomes and hotter summers, but some is probably rebound: an efficient inverter AC is cheaper to run, so families run it longer and buy a second unit. The saving per unit of cooling is real; the saving in total demand is smaller. Both effects belong in any forecast of power demand. - 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 [H₂O Water](https://www.materialinsights.org/commodities/water/)[H2 Hydrogen](https://www.materialinsights.org/chemicals/hydrogen/)[O2 Oxygen](https://www.materialinsights.org/chemicals/oxygen/)[Elz Electrolysers](https://www.materialinsights.org/energy/e_electrolysers/) ### Check your understanding A chain has three steps at 40%, 90% and 50% efficiency. What is the overall efficiency? Multiply: 0.4 x 0.9 x 0.5 = 0.18, so 18%. Averaging the steps would be wrong. A power station is 38% efficient. Where does most of the other 62% go? The first law says it cannot vanish. Most goes to the condenser cooling water or towers, and some leaves in hot flue gas. What did Jevons observe about more efficient steam engines? Cheaper steam power found many new uses, so total coal use rose: the Jevons paradox, an extreme form of rebound. [← Module contents](https://www.materialinsights.org/learn/energy/laws-of-energy/)[Mark read · Part 2: The second law: entropy and the quality of energy →](https://www.materialinsights.org/learn/energy/laws-of-energy/p2/) ## Other sections - [Module overview](https://www.materialinsights.org/learn/energy/laws-of-energy/index.md) - [The second law: entropy and the quality of energy](https://www.materialinsights.org/learn/energy/laws-of-energy/p2/index.md) - [The limits to efficiency: Carnot and beyond](https://www.materialinsights.org/learn/energy/laws-of-energy/p3/index.md) - [Heat and how it moves](https://www.materialinsights.org/learn/energy/laws-of-energy/p4/index.md) - [Exergy and the three stages of energy accounting](https://www.materialinsights.org/learn/energy/laws-of-energy/p5/index.md) - [Key terms, quiz and sources](https://www.materialinsights.org/learn/energy/laws-of-energy/review/index.md)