Efficiency, waste heat and recovery: Recovery technologies and what they cost

5.1 Economisers, recuperators and condensing boilers are the cheapest recovery

The simplest recovery returns waste heat to the process that made it. A boiler economiser is a heat exchanger in the flue that uses hot exhaust gas to preheat the water going into the boiler. A recuperator does the same for a furnace, using exhaust to warm the incoming combustion air. A regenerator stores heat in a mass of brick or ceramic, then releases it to the incoming air when the flow is reversed, which allows very high preheat temperatures in glass and steel furnaces.

The gains are dependable. The US Department of Energy's rule of thumb is that boiler efficiency rises by about one percentage point for every 40 degrees Fahrenheit, about 22 degrees Celsius, that the flue gas is cooled. Its tip sheet adds that an economiser can cut fuel use by 5 to 10% and often pays for itself in under two years. Preheating furnace air with a recuperator or regenerator can cut fuel use by a quarter or more at high furnace temperatures, because less fuel is spent heating the air itself.

Condensing boilers go further. Burning natural gas makes water vapour, which carries away about a tenth of the fuel's energy as latent heat. If the flue gas is cooled below about 55 degrees, the vapour condenses and gives up that heat. This is why modern condensing gas boilers reach over 90% on a gross basis, against 70 to 80% for older types. They need corrosion-resistant heat exchangers and return water cool enough to condense the vapour.

These measures are the base of the pyramid. They work at the source, need no outside buyer and use well-known equipment. The DOE study observed that manufacturers generally invest in recovery only when payback is well under three years and risk is low. Economisers, recuperators and better insulation often clear that bar. Their limit is that they can recover only what the process itself can reuse.

• BCS Inc. for US Department of Energy, Waste Heat Recovery: Technology and Opportunities in U.S. Industry (2008) · web
• US Department of Energy, Advanced Manufacturing Office, 'Use Feedwater Economizers for Waste Heat Recovery', Steam Tip Sheet #3 (2012) · web

5.2 Heat pumps and vapour recompression lift low-grade heat to useful temperatures

Most waste heat is too cool to reuse directly. A heat pump can lift it. It uses an electric compressor to take heat from a cool stream, such as cooling water at 30 to 40 degrees Celsius, and deliver it at a higher temperature, such as 80 to 150 degrees for process heat or district heating. The smaller the temperature lift, the less electricity each unit of heat needs.

The physics is the Carnot limit in reverse. Lifting heat from 40 to 90 degrees has a theoretical coefficient of performance of about 7: seven units of heat delivered for one of electricity. Real machines achieve about half of that, so three to four units. Lifting from 40 to 150 degrees is harder, with real values nearer two. Industrial heat pumps are now commercial up to about 150 degrees, with demonstration units going higher.

Mechanical vapour recompression is a special case. In evaporators and distillation columns, the vapour boiled off a liquid is compressed slightly, which raises its temperature just enough to boil more liquid. Because the lift is small, coefficients of performance well above ten are common. It is widely used in dairy, sugar, salt, pulp and chemical plants.

Cost depends on scale. The DOE study gave installed costs for electric closed-cycle industrial heat pumps of roughly 450 to 700 US dollars per kilowatt of heat at 0.5 megawatts, falling to 240 to 420 at 4 megawatts. The study took those figures from a 1997 IEA compilation, so they are badly dated; real costs today are higher. The economics hinge on the ratio of electricity price to fuel price. Where electricity costs more than three times as much as gas per unit of energy, as in many countries, a heat pump needs a good coefficient of performance and long running hours to beat a gas boiler.

• BCS Inc. for US Department of Energy, Waste Heat Recovery: Technology and Opportunities in U.S. Industry (2008) · web
• Jaffe and Taylor, The Physics of Energy (2018) · via Smil Mastery Course, M1 'Conservation and efficiency'

5.3 Turning heat into power, and sharing it through CHP and district heat

When no user for the heat exists, the remaining option is to make electricity, which can go anywhere. High- and medium-grade heat can raise steam for a conventional steam turbine. This waste-heat power is common at steel works, cement kilns and glass plants. For cooler or smaller streams, from about 100 to 350 degrees Celsius, organic Rankine cycles replace water with an organic fluid that boils at a lower temperature. They are compact and reliable but convert only around 10 to 20% of the heat into electricity: the UK government's Energy Technology List sets minimum net efficiencies of about 7 to 10% for units using heat up to 150 degrees and about 20% for heat above 350 degrees. Their cost per kilowatt is high. They pay where heat flows steadily for most hours of the year and power is expensive.

The bigger prize is to avoid wasting the heat in the first place by making heat and power together. A combined heat and power plant, also called cogeneration, generates electricity and captures the turbine or engine exhaust for steam or hot water. It may turn 30 to 40% of its fuel into electricity and another 35 to 45% into useful heat. The US Environmental Protection Agency gives typical total efficiencies of 65 to 80%, with some systems approaching 90%, against 50 to 55% for grid power plus a separate boiler. Producing the same electricity from the grid and the same heat from a boiler typically needs roughly a sixth to a half more fuel, depending on how efficient the grid's power plants are. India's sugar mills, burning bagasse to make steam and power for themselves and the grid, are a large example.

District heating extends the idea to whole towns, piping hot water from CHP plants, incinerators, industrial sources and large heat pumps to buildings. Euroheat & Power's DHC Market Outlook 2025, as reported by Enlit, found that in the EU renewables and waste heat made up 44.1% of the district heating mix by 2023, with waste heat growing 20% to reach 4.1%. Large heat pumps grew 44% and electric boilers 80% as shares of supply compared with 2022. Over 3,500 kilometres of network were added in 2023, and national plans foresee at least 8.5 million more household connections by 2030. Networks are capital-heavy and need dense demand to pay.

• Euroheat & Power, DHC Market Outlook 2025 (as reported by Enlit, 2025) · web
• BCS Inc. for US Department of Energy, Waste Heat Recovery: Technology and Opportunities in U.S. Industry (2008) · web
• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'Conservation and efficiency'
• US Environmental Protection Agency, Combined Heat and Power Partnership, 'CHP Benefits' · web
• UK Department for Energy Security and Net Zero, Energy Technology List: Organic Rankine Cycle heat recovery equipment · web

See it in the table

Check your understanding

Why does mechanical vapour recompression often reach a coefficient of performance above ten?

A CHP plant turns 38% of its fuel into electricity and 45% into useful heat. What is its overall fuel use?

According to Euroheat & Power, what share of the EU district heating mix came from renewables and waste heat by 2023?