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Efficiency, waste heat and recovery: Key terms, quiz and sources

Understanding Energy / Module 12: Efficiency, waste heat and recovery Wrap-up

Efficiency, waste heat and recovery: review

Key terms

First law of thermodynamics
Energy cannot be created or destroyed, only converted from one form to another.
Second law of thermodynamics
Heat flows from hot to cold, and any engine turning heat into work must reject some heat to a colder place.
Efficiency
Useful output divided by total input for a converter or a chain of converters.
Carnot limit
The maximum share of heat that can become work: one minus the cold temperature over the hot temperature, in kelvin.
Exergy
The part of an energy flow that could in principle be turned into work; high for electricity and hot gas, low for warm water.
Useful energy
The heat, motion or light actually delivered by end-use machines.
Energy intensity
Primary energy used per unit of GDP, such as megajoules per dollar.
Rebound effect
The part of an expected energy saving that is taken back because the cheaper service is used more.
Jevons paradox
The case where efficiency gains raise total energy use by more than the saving, first described for coal in 1865.
Waste heat grade
A classification of waste heat by temperature: high above about 650 C, medium about 230 to 650 C, low below about 230 C.
Pinch analysis
A method for arranging heat exchange between a plant's hot and cold streams to recover the most heat.
Economiser
A heat exchanger that uses boiler flue gas to preheat feedwater.
Recuperator
A heat exchanger that uses furnace exhaust to preheat incoming combustion air continuously.
Regenerator
A heat store of brick or ceramic that alternately absorbs exhaust heat and releases it to incoming air.
Coefficient of performance (COP)
Units of heat delivered by a heat pump per unit of electricity used.
Mechanical vapour recompression
Compressing vapour from an evaporator so it can boil more liquid, a heat pump with a very small temperature lift.
Organic Rankine cycle
A heat-to-power cycle using an organic fluid that boils at low temperature, suited to heat from about 100 to 350 C.
Combined heat and power
Generating electricity and capturing the leftover heat for use, typically reaching overall fuel use of 65 to 80%, and up to about 90% in the best systems.
District heating
A network of insulated pipes delivering hot water or steam from central sources to many buildings.

Try it in the table

  • Open the Combined heat and power tile and find its typical electrical and total efficiencies. Compare them with the 30 to 40% and 65 to 80% figures here, and note which fuels and sizes it lists.
  • In the Industrial heat pumps and vapour recompression tile, look for the highest delivery temperature now available commercially and the typical COP at a 50-degree lift. Which industries does it name as early users?
  • Open the Boiler heat recovery and Recuperators and regenerators tiles. Check how much fuel each typically saves and what limits how cold the flue gas can be made.
  • Compare the Organic Rankine, Kalina and sCO2 cycles tile with the Waste-heat power (steam Rankine) tile. At what temperature does each become the better choice, and what conversion efficiencies do they give?
  • In the Shared and district heat reuse and District heating and cooling networks tiles, find what heat density a network needs to pay, and which sources European networks are adding fastest.
  • Open the Bagasse and black-liquor cogeneration tile and see how much electricity an Indian sugar mill can export per tonne of cane crushed.

Final quiz

Which converter in Smil's tables typically has the highest efficiency?

Large transformers exceed 99%, and big motors 95%. Heat engines such as diesels (about 50%) and gas plants (about 60%) are capped by the second law.

Forman and colleagues found that 63% of global waste heat streams are below what temperature?

Most of the world's waste heat is below 100 degrees Celsius, which is plentiful in energy but low in exergy and hard to turn into power.

Why might a coal plant's condenser loss of about 50% not be a sign of poor engineering?

Rejecting heat to a cold reservoir is required by the second law. The heat is large in quantity but low in quality.

What did the IEA report as global energy intensity improvement in 2025?

Progress was set to reach about 1.8% in 2025, up from about 1% in 2024 but well short of the 4% COP28 goal.

A household buys a more efficient car and drives more as a result. This is an example of:

Using more of the very service that became cheaper is direct rebound. Spending savings on other goods would be indirect.

Which waste heat source belongs in the US DOE's high-grade category, above about 650 degrees Celsius?

Glass furnaces, steel furnaces and coke ovens produce high-grade heat. The other three are low-grade streams.

Why do manufacturers often decline waste heat projects that save energy?

The DOE study noted that firms generally invest only when payback is well under three years and risk is low, which rules out many low-grade projects.

Which statement about CHP is correct?

CHP's advantage comes from using the heat a power-only plant would reject. Without a heat user, it is just a smaller and often less efficient power plant.
Next module: Energy transitions and climate →

Sources for this module