Waste Heat to Power Systems Page: 3 of 9
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removing the contaminants often removes the heat at the same time. Other waste heat sources are difficult to
recover because of equipment configuration or operational issues.
Steam Rankine Cycle (SRC) -The most Working Fluid
commonly used system for power
generation from waste heat involves using
the heat to generate steam in a waste heat
boiler, which then drives a steam turbine. Exhaust Heat Turbine Power
Steam turbines are one of the oldest and Heat In Recovery Generator Out
most versatile prime mover technologies. Boiler
Heat recovery boiler/steam turbine
systems operate thermodynamically as a
Rankine Cycle, as shown in Figure 2. In the Condenser
steam Rankine cycle, the working fluid- 0
water-is first pumped to elevated Pump
pressure before entering a heat recovery
boiler. The pressurized water is vaporized
by the hot exhaust and then expanded to Heat Out
lower temperature and pressure in a
turbine, generating mechanical power Figure 2: Rankine Cycle Heat Engine
that can drive an electric generator. The
low-pressure steam is then exhausted to a condenser at vacuum conditions, where heat is removed by
condensing the vapor back into a liquid. The condensate from the condenser is then returned to the pump and
the cycle continues.
Organic Rankine Cycles (ORC) - Other working fluids, with better efficiencies at lower heat source temperatures,
are used in ORC heat engines. ORCs use an organic working fluid that has a lower boiling point, higher vapor
pressure, higher molecular mass, and higher mass flow compared to water. Together, these features enable
higher turbine efficiencies than in an SRC. ORC systems can be utilized for waste heat sources as low as 300 OF,
whereas steam systems are limited to heat sources greater than 500 OF. ORCs have commonly been used to
generate power in geothermal power plants, and more recently, in pipeline compressor heat recovery
The Kalina Cycle is another Rankine cycle, using a mixture of water and ammonia as the working fluid, which
allows for a more efficient energy extraction from the heat source. The Kalina cycle has an operating temperature
range that can accept waste heat at temperatures of 200 F to 1,000 F and is 15 to 25 percent more efficient than
ORCs at the same temperature level. Kalina cycle systems are becoming increasingly popular overseas in
geothermal power plants, where the hot fluid is very often below 300 F.1
The three types of Rankine power cycles discussed above overlap to a certain degree. There are advantages to
* SRCs are the most familiar to industry and are generally economically preferable where the source heat
temperature exceeds 800 F.
1 A Rankine cycle operating with a liquid waste heat source can be designed around lower temperatures than for one based
on a gaseous heat source, such as industrial process flue gases. The minimum liquid waste temperature for economically
feasible operation is 200 F.
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United States. Environmental Protection Agency. Combined Heat and Power Partnership. Waste Heat to Power Systems, text, May 30, 2012; Washington, D. C.. (digital.library.unt.edu/ark:/67531/metadc949408/m1/3/: accessed August 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.