Waste Heat – An Untapped Resource

By Dr. Angnes Tiong Ngieng Tze and Jason Yee Zhi Cheng

With increasing global emphasis on energy efficiency and sustainability, maximising the use of all available resources is paramount. One significant yet often overlooked opportunity is waste heat – the residual heat discharged as a by-product of industrial processes, power generation, transportation, and machinery operations.

Waste heat is typically generated whenever heat engines operate, as dictated by the Second Law of Thermodynamics, which limits the amount of useful work extractable from any heat source. All energy conversions are inherently inefficient, and the proportion of waste heat generated depends on the thermal efficiency of the system. For example, a conventional coal-fired power plant typically operates at 32% – 42% efficiency, with the remaining input energy lost as waste heat through exhaust gases, steam, or cooling systems (BrightHub Engineering, 2022).

Most of this residual heat is simply released into the atmosphere or water bodies without further use. Projections by Firth (2019) suggest that 49.3% – 51.5% of global energy could be lost as waste heat by 2030. Nathler (2022) similarly estimates that total waste heat in the EU is around 300 TWh annually – greater than the United Kingdom’s total yearly electricity demand. Clearly, waste heat represents a substantial, largely untapped energy resource with considerable potential for sustainable energy strategies.

According to the U.S. Department of Energy, between 20% and 50% of industrial energy input ends up as waste heat, and about 18% – 30% of that can feasibly be recovered and reused. The reuse of waste heat offers several key advantages:

• Improved energy efficiency: Capturing waste heat and redeploying it within the same process or for auxiliary uses reduces the need for additional primary energy, improving overall process efficiency.
• Cost savings: Reduced energy consumption leads to significant reductions in operating costs. EnergyNest (2023) estimates that efficiency improvements and cost reductions of up to 60% are achievable with effective waste heat utilisation.
• Reduction of greenhouse gas emissions: Improved efficiency and reduced reliance on primary fuels contribute to lower carbon emissions, making waste heat recovery a meaningful climate change mitigation strategy.
• Enhanced sustainability: Waste heat contributes to environmental degradation when discharged untreated. Capturing and reusing it helps reduce both energy wastage and environmental harm, decreases reliance on fossil fuels, and supports broader sustainability agendas such as the UN Sustainable Development Goals (SDGs 7 and 12) and the transition to a circular economy.

Strategies for Waste Heat Recovery

Waste heat recovery involves capturing hot gases or liquids exiting industrial processes and transferring the retained energy for other uses. The recovery strategy depends largely on the temperature of the waste heat. The U.S. Department of Energy classifies waste heat into:

• High-grade waste heat (>650°C): Generated primarily by heavy industries such as steel, cement, and glass manufacturing. Due to its high temperature, it is of higher quality and can be used for preheating raw materials or generating steam for electricity production.
• Medium-grade waste heat (230–650°C): Common sources include industrial exhaust gases, flue gases, and combustion engines.
• Low-grade waste heat (<230°C): Typically found in warm cooling water or low-temperature exhaust streams; recovery is technically more challenging and less efficient.

Applications of Waste Heat

There are several pathways for repurposing waste heat, each offering distinct benefits:

1. Process heating: In-house reuse is often the most feasible approach, allowing preheating of feedstock, process water heating, or driving absorption chillers for cooling. The advantage lies in reduced heat loss during distribution. This is achieved through technologies such as heat exchangers, recuperators, boilers, and economisers.
2. Space heating: Waste heat can be used to heat indoor office areas or storage facilities, particularly in colder climates. Excess heat can also be sold to nearby consumers such as residential developments or public facilities (e.g., swimming pools), generating additional revenue.
3. Electricity generation: For high-grade waste heat, technologies such as the Steam Rankine Cycle, Organic Rankine Cycle, or thermoelectric generators can convert heat into electricity. The generated power can either supply the facility’s own needs or be exported to the grid.
4. Drying processes: Industries such as food processing, agriculture, and paper production can use waste heat to remove moisture, reducing the need for additional fuel.
5. Heat upgrading for broader applications: If the temperature of the recovered heat is insufficient for intended use, devices such as heat pumps can upgrade it to a higher temperature suitable for other industrial processes.

The potential of these approaches is demonstrated in real-world applications. A case study in Turkey showed that a steel industry’s waste heat recovery system reduced its total energy use by 15% – 30% and cut CO₂ emissions by 10% – 20% (Ozturk et al., 2020).

Challenges in Adoption

Despite its many benefits, widespread adoption of waste heat recovery technologies faces certain obstacles. Low-grade waste heat is difficult to recover efficiently due to technical limitations in heat transfer at low temperatures. The associated costs can also be high, particularly when retrofitting existing facilities. Moreover, integrating waste heat recovery technologies into ongoing processes can be complex, and the intermittent or variable nature of waste heat supply complicates its use in systems that require continuous heating.Nevertheless, these challenges do not outweigh the potential economic and environmental advantages. With growing global energy demand, volatility in fuel prices, and urgent decarbonisation goals, waste heat recovery represents a valuable and underexploited strategy for sustainable energy development. By tapping into this overlooked resource, industries can significantly enhance efficiency, reduce costs, and mitigate environmental impact.

The utilisation of waste heat should be viewed as an integral component of energy transition and sustainable industrial practices. As illustrated, the energy and emissions savings can be considerable, and technologies already exist to facilitate its capture and reuse across a wide range of applications. While technical and economic challenges remain, particularly for low-grade heat, the opportunities for innovation and integration continue to expand.

Given its scale and potential, repurposing waste heat is a pathway worth pursuing for industries seeking to remain competitive while contributing to climate action and global sustainability objectives.

Dr. Angnes Tiong is a Senior Lecturer in the Chemical and Energy Engineering Department of the Faculty of Engineering and Science, Curtin University Malaysia. Her research interests include process heat transfer, computational fluid dynamics, process optimisation, circular economy, and waste heat recovery. She welcomes collaboration and can be contacted at angnesnt@curtin.edu.my.

Jason Yee Zhi Cheng was a final-year Chemical Engineering student at Curtin University Malaysia in 2025.