What Is Waste Heat?

Waste heat is thermal energy released as a byproduct of industrial processes, power generation, and mechanical systems that is not captured for useful work in the primary process. It represents a loss of thermodynamic potential: any conversion of fuel or electrical energy to mechanical work or chemical products leaves a fraction of the original energy as low- to medium-temperature heat that must be rejected to the environment. In large industrial facilities such as steel mills, cement plants, glass furnaces, and chemical refineries, waste heat losses can represent 20 to 50 percent of total fuel input, making recovery a priority both for energy cost reduction and for meeting industrial decarbonization targets.

The engineering of waste heat recovery draws on thermodynamics, heat transfer, fluid mechanics, and process engineering. The grade of waste heat, meaning the temperature at which it is available, determines which recovery technologies are applicable and what useful output can be produced. High-temperature exhaust streams above 650 degrees Celsius can generate steam at conditions suitable for electricity generation; medium-temperature streams between 230 and 650 degrees Celsius feed preheaters and waste heat boilers; low-temperature streams below 230 degrees Celsius are candidates for absorption cooling, organic Rankine cycles (ORC), or district heating networks.

Heat Recovery Systems and Boilers

Waste heat boilers are the most widely deployed recovery technology for medium- and high-temperature gas streams. They are shell-and-tube or water-tube heat exchangers that extract energy from hot flue gases or process exhaust, transferring it to water or steam for use elsewhere in the facility. Unlike the heat recovery steam generators (HRSGs) typically associated with clean gas-turbine exhaust, industrial waste heat boilers are engineered to handle dirty, particle-laden gases from furnaces, kilns, and chemical reactors, incorporating dust hoppers, soot blowers, and corrosion-resistant alloys. Economizers, a simpler variant, preheat boiler feedwater using exhaust gases at lower temperatures where steam generation is not practical. The U.S. DOE report on waste heat recovery technology and opportunities in U.S. industry surveys the temperature ranges, equipment options, and recovery potential across major industrial sectors.

Cogeneration and Combined Heat and Power

Cogeneration, also called combined heat and power (CHP), is the simultaneous production of electricity and useful thermal energy from a single fuel input, recovering heat that a simple-cycle power plant would reject to a cooling tower or river. A typical CHP system uses a gas turbine or reciprocating engine to generate electricity, then captures the exhaust heat in an HRSG to produce steam or hot water for process or space heating loads. Thermal efficiencies exceeding 80 percent are achievable in well-matched installations, compared to approximately 33 to 45 percent for electricity-only generation. The UNFCCC technology description of waste heat recovery systems identifies CHP as one of the most effective pathways for simultaneously reducing energy costs and greenhouse gas emissions in manufacturing.

Industrial Applications and Efficiency Metrics

The economic case for waste heat recovery depends on the temperature and flow rate of the available stream, the capital cost of recovery equipment, and the value of the recovered energy output. Payback periods for industrial WHR projects typically range from two to seven years depending on energy prices and process compatibility. Organic Rankine cycle systems convert low-grade heat, such as engine jacket water at 90 to 150 degrees Celsius, to electricity through a working fluid with a lower boiling point than water. Thermoelectric generators, which use semiconductor junctions to convert temperature differentials directly to voltage, are applied in distributed settings such as automotive exhaust recovery. A review of boiler waste heat recovery technologies in the medium-low temperature range published in the journal Energy quantifies the technical and economic performance of competing approaches at the temperature conditions most common in food, chemical, and light manufacturing facilities.

Applications

Waste heat has applications in a wide range of disciplines, including:

• Industrial process integration and energy cost reduction
• District heating and cooling networks fed by urban waste heat sources
• Combined cycle and cogeneration power plants
• Automotive engine thermal management and exhaust energy recovery
• Data center cooling systems using server exhaust heat