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Throughout the United States, an abundant source of emission-free power is being overlooked. This source is waste heat, a byproduct of industrial processes that could reinvigorate American manufacturing, create jobs, lower the cost of energy and reduce overall emissions from electric generation. If not captured and used to generate emission-free renewable-equivalent power, waste heat is released to the atmosphere through stacks, vents, flares and mechanical equipment.
Waste Heat to Power (WHP) works by capturing waste heat with a recovery unit and converting it to electricity through a process called heat exchange. This process produces no emissions because no fuel is burned. By using the waste heat to generate emission-free power, industrial users can route the power back to the facility or sell it to the grid to support clean energy production, distribution and use.
WHP systems use the same technologies deployed in a number of industries including the geothermal industry. The main technologies used by WHP developers are Steam Turbine Technology, Organic Rankine Cycle, Supercritical CO2, Kalina Cycle, Stirling Engine, and emerging technologies such as Thermoelectrics. Through the application of these technologies, industrial waste heat is no longer just a byproduct – it is a valuable resource for emission-free electricity.
Steam Turbine Technology
Steam Turbine Technology is more than 100 years old and has been utilized for WHP systems since the 1970’s. Steam turbines extract the thermal energy from waste heat (steam) and use that energy to drive an electric generator. Steam turbines are generally deployed in larger scale industrial facilities which produce high temperature waste heat.
Organic Rankine Cycle
Organic Rankine Cycle (ORC) technology is a heat exchanging process that utilizes a refrigerant for its working fluid and captures waste heat at lower temperatures and from smaller scale projects than the steam turbine. Since the majority of industrial waste heat in the United States is below 600 degrees Fahrenheit, the ORC’s ability to capture low temperature resources opens up many new applications for waste heat to power (WHP) projects.
Supercritical CO2, Thermoelectrics, Kalina Cycle®, Stirling Engine
Additional technologies make up a smaller but growing portion of the waste heat to power industry. Supercritical CO2, or scCO2, uses supercritical carbon dioxide as the working fluid. Thermoelectrics are solid-state semiconductors that capitalize on a difference in temperature to turn heat into electricity with few or no moving parts. The Kalina Cycle®, which uses a solution of water and ammonia for its working fluid, takes advantage of the different boiling points of the two liquids to extract heat over a wider range of temperatures. And the Stirling Engine uses cyclic compression with gases as the working fluid to capitalize on very low temperature waste heat.
Download the industry trade association’s Fact Sheet on Waste Heat to Power. Visit The Heat is Power Association to find case studies, reports, and more information on WHP. Read the ACORE WHP blog post.
- There are 575 MW of installed Waste Heat to Power capacity in the United States (ICF International)
- According to an EPA report, there is between 7 and 10 gigawatts of Waste Heat to Power capacity in the United States, enough to power 7 to 10 million American homes
- Waste Heat to Power is included in 15 state renewable portfolio standards (The Heat is Power Association)
- Technologies used for Waste Heat to Power are the same technologies used for the Geothermal and other industries
Waste-to-energy (WTE) is a reliable and renewable process of converting waste materials into electricity. Municipal waste is collected by local authorities from residential, commercial, and public origins, disposed in a central location, processed, and then combusted to produce heat and/or power.
In WTE facilities, trash is either burned directly or processed and shredded to produce a fuel before being combusted. The heat from the burning garbage boils water flowing inside boiler tubes and turns the water into steam. The steam can be put to direct use in a heating system or a factory, but it is most often used to turn a turbine-generator to make electricity. After any incombustible residue (ash) cools, magnets and other mechanical devices pull metals from the ash. The remaining ash is as much as 90% smaller in volume than the fuel source, and it is either deposited in a landfill, or in some cases, used to pave roads.
Waste Heat to Power (WH2P) harnesses the electricity-producing potential of a different form of waste. Large industrial facilities, ranging from oil refineries to paper mills, generate large quantities of heat to conduct their operations, and in most cases that heat is simply lost. By implementing a recovery unit to capture the waste heat, industrial users can generate power for their own plants, or sell it back to the grid. WH2P equipment is also used at renewable power plants to capture unused heat.
Waste power can also be generated from landfills. As trash and organic material decays, it releases gases that can be harnessed to produce energy (see Biomass).
To learn more about waste-to-energy, visit the Environmental Protection Agency’s (EPA) overview.
- The U.S. has 89 waste-to-energy plants nationwide, generating the equivalent of 2.5 GW of energy while annually disposing of 29 million tons of trash. (Solid Waste Association of North America (SWANA)
- Waste-to-energy plants annually recover and recycle 1.6 million tons of ferrous and non-ferrous metals, plastics, glass, and combustion ash. (EPA)
- More than $1 billion has been invested to upgrade air quality control systems in American waste-to-energy facilities under the Clean Air Act. (Energy Recovery Council (ERC)
- Improvements to municipal waste combustion (MWC) units have resulted in a 90% reduction in particulate matter emissions from their 1995 levels. (EPA)
Hydroelectric power is the world’s largest producer of renewable energy. Through the conversion of the kinetic energy of flowing water into electricity, hydroelectric power provides a steady and reliable source of renewable energy.
The same physics lie behind the design of all hydroelectric systems: A dam is used to capture and store water; pipes, or penstocks, carry the water from a high reservoir, downhill, toward turbines in a power station, with the strength of the natural pressure of the surging water often increased by nozzles affixed to the end of the pipes; the water strikes the turbines, rotating them and driving a generator that produces electricity.
The water’s flow can be utilized in a variety of ways. Conventionalhydroelectric power plants use a one-way flow of water. Systems with one-way water flow can also be designed as “storage” plants, reserving enough water in their dams to offset seasonal impact on their water flow. Alternatively, “run-of-the-river” plants have limited or no reservoir capacity and rely on the natural flow of waterways to produce electricity.
Other hydroelectric systems are designed as “pumped storage” plants. This means that after the water has produced an initial quantity of electricity, it is diverted from the turbines into a lower reservoir below the dam. During off-peak hours, or through dry-weather conditions, the water in this lower reservoir can be pumped back up and reused to supply a steady stream of electricity to the plant’s customers during peak use times.
For more on hydroelectric power, visit the National Hydropower Association.
- The U.S. has the second largest installed capacity of hydropower globally (including pumped storage), amounting to about 100 GW. (Environmental and Energy Study Institute (EESI))
- The U.S. accounted for 9.4% the world’s hydropower consumption in 2011, a nearly 25% increase over 2010. (BP)
- The U.S. hydropower industry could install 23,000 MW to 60,000 MW of new capacity by 2025 depending on policy changes. (National Hydropower Association (NHA))
- In the U.S. alone, there is over 400 GW of untapped hydropower resource potential (inland and ocean). (NHA)
Marine energy technologies harness the natural movement or temperature of bodies of water to produce energy, and include wave, tidal, marine current, and ocean thermal energy conversion (OTEC). The technical potential for marine energy is very large, though most technologies are in development stages.
Tidal Energy: Tidal energy installations come in two general forms: tidal barrage energy and tidal stream generators. A tidal barrage is usually a structure constructed at the mouth of a bay or in a river estuary that captures water during high tide and pushes it through a hydro turbine during low tide, generating electricity. The basic principles of hydropower apply, even though tidal energy deals with a massive volume of water moving very slowly. Tidal stream generators convert the kinetic energy of moving water into electricity through the use of horizontal, vertical, and oscillating turbines, which can resemble underwater wind turbines. Electricity is generated by tidal water flowing both into and out of a bay, and the flow of water is not restricted.
Marine Current Energy: As with tidal stream generators, marine current power utilizes submerged turbines to capture energy from the movement of ocean water, but only within marine currents where water moves in just one direction. In the U.S., the Florida Current and the Gulf Stream are reasonably swift and continuous currents moving close to shore in areas where there is a demand for power.
Wave Energy: Wave energy devices either float on the surface of the ocean or are fixed to the ocean floor, and can be employed both on the shoreline and in deep waters. A number of technologies have emerged to capture energy from the movement of waves, including floating devices that bend with the waves and others that utilize pressure fluctuations in tubes caused by waves going up and down.
Ocean Thermal Energy Conversion (OTEC): OTEC uses the temperature difference from the warmer surface of a body of water to its cooler, lower depths for electricity generation, water desalinization, air conditioning, and other purposes. There are three general forms of OTEC technologies, including open cycle, closed cycle, and hybrid cycle systems.
For a more in-depth account of marine energy technologies, visit the National Renewable Energy Laboratory’s Ocean Energy Technology Overview.
- Industry estimates have placed untapped U.S. wave potential at 90 GW. (National Hydropower Association (NHA))
- Off the coasts of Florida, the potential to generate between 4 and 10 GW from a variety of marine energy sources may exist. (NHA)
- According to preliminary surveys, the feasible global potential for marine energy could reach 450 GW. (Blue Energy)
- Wave energy contains roughly 1,000 times the kinetic energy of wind. (Ocean Energy Council)
Geothermal energy systems rely on two basic components: the heat beneath the earth’s crust, and the subterranean waters that the earth’s heat will turn to steam. In most geothermal systems, accessing these components involves drilling as deep as two miles below the surface of the earth.
In direct-use geothermal systems, the earth’s natural steam is piped directly into buildings to warm them in winter and, perhaps surprisingly, to cool them in summer. While the temperature on the surface of the earth varies throughout the year, the temperature in the upper ten feet of the earth remains fairly constant, usually between 50 and 60 degrees F. The benefits of this constant temperature can be accessed by pumping the water from springs or reservoirs near the earth’s surface into buildings for interior climate control.
Direct-use of geothermal heat is often achieved through the use of a heat pump, which efficiently extracts the earth’s thermal energy. Besides maintaining indoor temperatures, geothermal heat can be used to heat greenhouses, heat water at fish farms, pasteurize milk, and provide the heat required for range of industrial processes. A large centralized geothermal pump can even provide heating for an entire community, known as “district heating.”
Geothermal energy is also used to drive electric generators in a number of ways:
Dry steam systems are the oldest and simplest application of geothermal power, in which the steam released from a geothermal reservoir is captured and used to rotate turbines which generate electricity.
Flash steam systems utilize a more technologically sophisticated method of electrical generation and are the most widely deployed. These systems use intense pressure to keep water in liquid form, even as it is heated to temperatures well above its boiling point at sea level. The water is then exposed to an abrupt drop in pressure, causing it to convert in a flash to steam, which more efficiently rotates the steam turbines to generate electricity.
Binary cycle systems direct the earth’s hot water upward to a heat exchanger above ground, where the heat is transferred to a pipe containing a fluid with a much lower boiling point than water (usually isobutane or isopentane gas). The transferred heat vaporizes the liquid, and that steam rotates turbines to produce electricity. The advantage of this system is that it can make use of geothermal reservoirs that have lower temperatures, increasing the places where geothermal systems can be located.
Enhanced geothermal systems (EGS) (or hot dry rock systems) may be yet another avenue into the earth’s deep power potential. Rather than harvesting the heated water already in the earth, this method involves manufacturing steam by piping surface water down into the hot but dry rocks in the earth’s crust. A main benefit of this system is that it does not require the high temperature geothermal resources of other geothermal electric technologies, and it can be used nearly anywhere on the planet. While the technology’s potential is great, further research and development is required before it can be deployed at scale.
To learn more about geothermal systems, visit U.S. Department of Energy’s geothermal overview.
- The U.S. has approximately 3,200 megawatts (MW) of installed geothermal capacity, accounting for about 28% of global capacity. (Geothermal Energy Association (GEA))
- As of April 2012, there were 147 projects identified under development in the U.S. (130 of which are confirmed by developing companies), with roughly 5,000 MW of power potential. (GEA)
- In 2011 and early 2012, five additional geothermal plants came online, with a gross capacity of approximately 91 MW. (GEA)
Biomass refers a wide range of biological materials used as sources of energy. While wood products are the most common form of biomass power, a host of feedstocks can be used for electricity generation, including a variety of crops, agricultural waste, yard clippings, and even municipal solid waste (MSW). In addition to producing electricity, biomass can also be used for space and domestic water heating, process heat, and the thermal portion of combined heat and power, as well as for transportation (see Biofuels).
A variety of processes convert biomass materials into electricity:
Incineration/Direct-Firing is the most common method of generation. The biomass feedstocks are burned directly to produce steam, which in turn rotates turbines to generate electricity.
Gasification refers to heating biomass while restricting the amount of oxygen and/or steam in the gasifier. This process produces a synthetic gas known as “syngas,” containing carbon monoxide and hydrogen. The syngas can be burned in gas engines, used to produce methanol and hydrogen, or transformed into a synthetic fuel via the Fischer Tropsch (FT) process.
Co-Firing is the burning of biomass in conjunction with a non-renewable feedstock (often coal) to reduce emissions and, in certain cases, increase output and efficiency. Nitrogen, sulfur oxides, and carbon dioxide emissions are considerably lessened by a biomass co-firing arrangement.
Anaerobic Digestion is a biological process in which microorganisms break down biomass and release biogas, which consists of methane, carbon dioxide, and certain other gasses. The biogas can be subsequently captured and combusted to generate electricity. Anaerobic digestion is used in a variety of locations as a way to manage waste, including at dairy farms, landfills, and wastewater and sewage treatment facilities.
Landfill gas is produced as a result of anaerobic digestion at landfills. Landfill operators collect the gas produced by the decomposition of solid waste and use it to generate electricity.
Pyrolysis is the process of heating biomass in the absence of oxygen. The result of the process is a substance known as pyrolysis oil, which then can be converted into biofuel or used to generate electricity.
- Wood and waste biomass power together are projected to account for roughly 30% of the renewable energy produced in the United States in 2012. (Energy Information Administration (EIA))
- At a projected 6% annual growth rate, biomass is expected to be among the fastest growing renewable energy sources. (EIA)
- Biomass power capacity is forecast to reach to reach 20.2 GW by 2035. (EIA)