Waste heat recovery
Waste heat recovery involves capturing excess thermal energy produced during industrial processes, electricity generation, and other activities, which would otherwise be wasted. This recovered heat can be utilized for heating and cooling buildings through district heating and cooling systems, enhancing overall energy efficiency and reducing greenhouse gas emissions. Historically, waste heat recovery has roots dating back to the late 18th century with steam engines, evolving into larger urban systems in North America and Europe by the late 19th century. However, the practice saw a decline in the U.S. post-World War II due to urban planning policies favoring remote power generation, while European cities continued to innovate and expand their district heating systems, significantly integrating renewable energy sources.
In many European countries, district heating systems now supply a substantial portion of space and water heating, supported by strong public policies promoting energy efficiency and waste heat recovery. Recent advancements in technology allow electricity generation from lower temperature waste heat through Organic Rankine Cycle turbines, making waste heat recovery an increasingly viable solution for modern energy challenges. Overall, waste heat recovery presents significant opportunities for reducing energy waste, contributing to climate change mitigation, and enhancing energy security worldwide.
Waste heat recovery
Summary: Waste heat can be recovered from many sources and distributed by district heating and cooling systems to improve the efficiency of energy systems and reduce greenhouse gases. Recently developed technologies also allow electricity to be generated from waste heat.
Traditional methods of generating electricity and air conditioning produce large amounts of waste heat. In many cases, this heat can be captured and reused to heat and cool buildings, usually in district heating and cooling systems. This practice, known as combined heat and power (CHP), was widely practiced in the early twentieth century, when electric generating plants were built in cities and connected to steam and hot-water district heating systems, many of which were already in place distributing waste heat from factories. As these small electric generating plants have been closed over time and replaced by large electric generating plants in remote locations in North America, along with the closure of urban factories, these legacy heat distribution systems have often been abandoned, resulting in the loss of efficient capture of waste heat from electric generation. European cities responded to the energy crisis of the 1970s by building modern district heating systems and updating existing ones as a way to use waste energy and reduce oil imports. There are new opportunities to use waste heat in district heating and cooling systems from renewable energy sources, such as wood, both to address the need for energy and to reduce greenhouse gas (GHG) emissions that lead to climate change.
Early Uses of Waste Heat
The wide-scale use of steam engines beginning in the late 18th century soon resulted in the use of waste heat from these engines. A patent was awarded to Oxford brewer Sutton Thomas Wood in 1784 for the beneficial use of waste heat from steam engines. The adoption of high-pressure steam engines in the early nineteenth century greatly increased the amount and temperature of exhaust heat, and as early as 1813 Oliver Evans visited a factory near Baltimore where the exhaust from one of his high-pressure engines was efficiently heating the factory. Many factories in England also used waste heat for heating. Another application of waste heat was to the compound steam engine, which used the exhaust steam from a high-pressure engine to run a low-pressure steam engine. This method was first used in 1771, and over time, engines (and later turbines) incorporated multiple pressure reductions within a single unit.
More Recent District Heating Systems
The notion of large-scale industrial and urban waste heat recovery and distribution emerged at the end of the nineteenth century in North America with the production of electricity from reciprocating engines and with engineers recognizing potential profit from the sale of so-called waste steam. The United States witnessed a proliferation of large-scale waste heat recovery and its use in district energy systems. These included steam distribution systems in cities such as New York, Detroit, Boston, St. Louis, Philadelphia, and Milwaukee. An extensive hot-water system, which included sidewalk snow melting, was developed in Toledo, Ohio, where in 1909 the International District Energy Association was founded. The work to capture and distribute waste heat in North America was paralleled in Europe, in cities such as Hamburg, Stockholm, and St. Petersburg.
Decline of District Heating in the United States
In the United States, the scale and the value of recapturing waste heat descended after World War II. Urban planning practices discouraged proximity of power generation to urban centers and condemned deployment of waste heat in the United States to a legacy of mostly inefficient steam distribution. Policies by the federal Department of Housing and Urban Development that promoted individual boiler systems in buildings during the 1960s and 1970s led to further erosion of waste heat recapture, according to the National Research Council.
By the 2020s, in the United States, approximately 16 percent of electricity was produced in large, remote coal-burning power plants that consumed far more energy to make unused heat than to generate useful electricity. These plants are characterized by high conversion losses and inefficiencies of up to 69 percent because of the lack of waste heat recapture and lack of proximity to urban regions, according to John Randolph and Gilbert Masters. District heating systems face another challenge; even if power plants were closer to urban areas, there is little energy distribution infrastructure available to use the waste heat. A very small handful of systems, such as the district heating and cooling system serving St. Paul, Minnesota’s, central business district, are being extended and upgraded to modern standards.
District Heating in Europe
The story has been different In Europe. Since the 1970s, many European (and with greater frequency Asian) cities have successfully built energy systems in which waste heat from large utilities or industrial sources has been captured and channeled efficiently via cogeneration (and increasingly via microcogeneration) plants and district-heating systems. Urban planning policies in European countries such as Germany encourage the development and expansion of district energy systems by promoting density and the inclusion of larger-scale energy sources within an urban area and appropriate energy supply zoning policies. As a result, a growing percentage of homes and buildings in Europe are served by district heating (and cooling) systems. For example, a wood-chip-fired district heating system was installed in the town of Ostfildern outside Stuttgart, Germany, at a former air force base that was redeveloped into housing and businesses.
District heating is intensively used in Denmark, with about 70 percent of space and water heating supplied by district heating. District heating supplies approximately 50 percent of demand in other countries, such as Poland, Sweden, and Finland. Most district heating systems heat hospitals, large housing complexes, industrial areas, and universities.
District heating is increasingly recognized as a priority in public policy. In 2004, the European Union enacted cogeneration Directive 2004/8/EC. The directive calls on member states to remove barriers to cogeneration, through guarantees that electricity from CHP systems will be transmitted and distributed in a nondiscriminatory manner, and to implement other policies to promote its use. In 2007, at the Meseberg Summit, Germany became one of the first countries to elevate heat recovery to a national strategic priority.
Waste heat recovery in many European cities has traditionally used conventional nonrenewable fuels such as coal, gas, and oil. Heat recovery using CHP sources is increasingly being combined with renewable energy sources, including biomass and solar thermal. Industrial waste heat is also finding its way into the district energy source mix. District energy systems, combined with multiple waste heat sources, are seen as a critical technology to reach challenging greenhouse gas reduction and energy efficiency targets. This need is not lost on Asia: Many of the major urban developments in South Korea and China are rapidly deploying large-scale district energy systems served by CHP sources.
Modern developments in waste heat technology allow electricity to be generated from temperatures down to 200 degrees Fahrenheit (91 degrees Celsius). Organic Rankine cycle turbines are used for this purpose and are similar to Rankine cycle steam turbines but use fluids suitable for use in low-temperature turbines.
Bibliography
Danish Board of District Heating. “District Heating History.” dbdh.dk/district-heating-history/.
Declaye, Sébastien. Design, Optimization, and Modeling of an Organic Rankine Cycle for Waste Heat Recovery. June 2009. hdl.handle.net/2268/72915.
Euroheat and Power. “District Heating and Cooling.” www.euroheat.org/Default.aspx?ID=4.
Infosource Europe. “EU Energy Policy: Cogeneration Directive.” December 2008. www.inforse.dk/europe/eu‗cogen-di.htm.
International District Energy Association. “What Is District Energy?” www.districtenergy.org/what-is-district-energy/.
National Research Council, Committee on District Heating and Cooling. District Heating and Cooling in the United States: Prospects and Issues. Washington, DC: National Academy Press, 1985.
Ononogbo, C., et al. "Opportunities of Waste Heat Recovery from Various Sources: Review of Technologies and Implementation." Heliyon, vol. 9, no. 2, 2023, doi.org/10.1016/j.heliyon.2023.e13590. Accessed 7 Aug. 2024.
Quoilin, Sylvain, and Vincent Lemort. “Technological and Economical Survey of Organic Rankine Cycle Systems.” Paper presented at the European Conference on Economics and Management of Energy in Industry, Vilamoura, Portugal, April 2009. orbi.ulg.ac.be/handle/2268/14609.
United Kingdom District Heating Association. “Introduction.” ukdea.org.uk/introduction/.