# 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. Source page: https://www.materialinsights.org/learn/energy/efficiency-and-recovery/ [Understanding Energy](https://www.materialinsights.org/learn/energy/) / Module 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. 5 parts · 15 sections · about 50 minutes · 0 of 5 parts read ### By the end you will be able to - Explain efficiency, the difference between primary, final and useful energy, and why about 72% of primary energy is lost - Use Carnot's limit to explain why some losses are unavoidable and why waste heat temperature matters - Interpret energy intensity and its pitfalls, and compare today's global efficiency progress with the COP28 target - Describe the Jevons paradox and rebound effects, and distinguish relative from absolute savings - Classify waste heat by temperature grade and match it to recovery technologies, with their rough costs and conditions for paying back ### Contents 1. [Part 1 Where energy is wasted](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/p1/) Energy is never destroyed, but most of it does not end up doing what we wanted. Roughly seven-tenths of the world's primary energy leaves the system as unwanted heat. This part sets out how efficiency is measured, where along the chain the losses happen, and which losses physics makes unavoidable. - Efficiency is useful output divided by input, and converters range from 2% to 99% - About seven-tenths of primary energy ends up as waste heat - The second law sets a floor under some losses, so not all waste can be recovered 2. [Part 2 Efficiency gains in history and energy intensity](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/p2/) Better converters built the modern world as much as more fuel did. Steam engines, lamps, furnaces and ammonia plants all became many times more efficient. At the scale of whole economies, these gains show up as falling energy intensity, the energy used per unit of GDP. This part traces that history and today's pace. - History's converters improved tenfold or more, again and again - Energy intensity measures an economy's efficiency, but it hides a lot - Global efficiency is improving at 1 to 2% a year, half the rate governments promised 3. [Part 3 Rebound, Jevons and the limits of efficiency](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/p3/) If machines keep getting better, why does the world keep using more energy? The economist William Stanley Jevons asked this about coal in 1865. His answer, that cheaper useful energy invites more use, is still debated. This part explains rebound, separates relative from absolute savings, and uses Smil's scepticism to test common claims. - Jevons saw that better steam engines raised Britain's coal use, not lowered it - Relative savings are real; absolute savings need efficiency to outrun growth - Smil's test for energy claims: check scale, speed and physics 4. [Part 4 Waste heat by temperature grade](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/p4/) Waste heat is not one thing. A stream of 900-degree furnace gas and a pond of 35-degree cooling water both count as losses, but they are worth very different amounts. Temperature decides what a stream can be used for and how much it costs to capture. This part maps where waste heat comes from and how to match it to a use. - Industry throws away a fifth to a half of its energy, mostly in exhaust and cooling water - Temperature sets how much of the heat can become work - Recovery works when a source meets a sink: same place, same time, right temperature 5. [Part 5 Recovery technologies and what they cost](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/p5/) Recovering heat is old engineering: heat exchangers, boilers, turbines and heat pumps. What changes is the economics, which depend on temperature, hours of use, fuel prices and whether a buyer for the heat exists. This part walks through the main technologies, from the cheapest to the most capital-heavy, with rough costs and the conditions under which they pay. - Economisers, recuperators and condensing boilers are the cheapest recovery - Heat pumps and vapour recompression lift low-grade heat to useful temperatures - Turning heat into power, and sharing it through CHP and district heat 6. [Wrap-up Key terms, try it in the table, final quiz, sources](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/review/) [Start: Part 1, Where energy is wasted →](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/p1/) ## Other sections - [Where energy is wasted](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/p1/index.md) - [Efficiency gains in history and energy intensity](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/p2/index.md) - [Rebound, Jevons and the limits of efficiency](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/p3/index.md) - [Waste heat by temperature grade](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/p4/index.md) - [Recovery technologies and what they cost](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/p5/index.md) - [Key terms, quiz and sources](https://www.materialinsights.org/learn/energy/efficiency-and-recovery/review/index.md)