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Waste Heat to Power Technologies
Generating power from waste heat typically involves using the waste heat
from boilers to create mechanical energy that then drives an electric
generator. While these power cycles are well developed, new technologies are
being developed that can generate electricity directly from heat, such as
thermoelectric and piezoelectric generation. When considering power
generation options for waste heat recovery, an important factor to keep in
mind is the thermodynamic limitations on power generation at different
temperatures. The efficiency of power generation is heavily dependent on the
temperature of the waste heat source. In general, power generation from
waste heat has been limited to only medium to high temperature waste heat
sources. However, advances in alternate power cycles may increase the
feasibility of generation at low temperatures. While maximum efficiency at
these temperatures is lower, these systems can still be economical in
recovering large quantities of energy from waste heat. The table below
summarizes different power generation technologies.
Options for Heat Recovery via Power
Generation
| Thermal Conversion Technology |
Temperature Range |
Typical Sources of Waste Heat |
Capital Cost ($/kW) |
| Traditional Steam Cycle |
Medium High |
Exhaust from gas turbines, reciprocating engines, incinerators,
and furnaces. |
$1,500 - $2,500 |
| Kalina Cycle |
Low Medium |
Gas turbine exhaust, boiler exhaust, cement kilns |
$2,000 - $3,000 |
| Organic Rankine Cycle |
Low Medium |
Gas turbine exhaust, boiler exhaust, heated water, cement kilns |
$2,000 - $3,500 |
| Thermoelectric Generation |
Medium High |
Not yet demonstrated in industrial applications |
N/A |
| Piezoelectric Generation |
Low |
Not yet demonstrated in industrial applications |
N/A |
| Thermal Photovoltaic |
Medium High |
Not yet demonstrated in industrial applications |
N/A |
Source:
Waste Heat Recovery: Technology and Opportunities in U.S. Industry (DOE,
2008);
Waste Heat Recovery in Industrial Facilities (EPRI 2011)
Courtesy: Siemens
The most frequently used system for power generation from waste heat
involves using the heat to generate steam, which then drives a steam
turbine. The traditional steam Rankine cycle is the most efficient option
for waste heat recovery from exhaust streams with temperatures above about
650-700°F [340-370°C]. At lower waste heat temperatures, steam cycles become
less cost-effective, since low-pressure steam will require bulkier
equipment. Moreover, low-temperature waste heat may not provide sufficient
energy to superheat the steam, which is a requirement for preventing steam
condensation and erosion of the turbine blades. Therefore, low-temperature
heat recovery applications are better suited for the organic Rankine Cycle
or Kalina cycle, which use fluids with lower boiling point temperatures
compared to steam.
Appropriate for: Waste heat recovery; medium and large CHP
Further information:
- View a list of waste-heat-to-power manufacturers and project developers on our Resources page.
The Organic Rankine Cycle (ORC) operates similar to the steam Rankine
cycle, but uses an organic working fluid instead of steam. Options include
silicon oil, propane, haloalkanes (e.g., "freons"), isopentane, isobutane,
pxylene, and toluene, which have a lower boiling point and higher vapor
pressure than water. This allows the Rankine cycle to operate with
significantly lower waste heat temperatures - sometimes as low as 150°F
[66°C]. The most appropriate temperature range for ORCs will depend on the
fluid used, as fluids' thermodynamic properties will influence the
efficiency of the cycle at various temperatures.
In comparison with water vapor, the fluids used in ORCs have a higher
molecular mass, enabling compact designs, higher mass flow, and higher
turbine efficiencies (as high as 80-85%). However, since the cycle functions
at lower temperatures, the overall efficiency is only around 10-20%,
depending on the temperature of the condenser and evaporator. While this
efficiency is much lower than a high-temperature steam power plant (30-40%),
it is important to remember that low-temperature cycles are inherently less
efficient than high-temperature cycles. Limits on efficiency can be
expressed according to Carnot efficiency - the maximum possible efficiency for
a heat engine operating between two temperatures. A Carnot engine operating
with a heat source at 300°F [150°C] and rejecting it at 77°F [25°C] is only
about 30% efficient. In this light, an efficiency of 10-20% is a substantial
percentage of theoretical efficiency, especially in comparison to other
low-temperature options, such as piezoelectric generation, which are only 1%
efficient.
ORC technology is not particularly new; at least 30 commercial plants
worldwide were employing the cycle before 1984. Its applications include
power generation from solar, geothermal, and waste heat sources. Waste heat
recovery can be applied to a variety of low-to medium-temperature heat
streams. Although the economics of ORC heat recovery need to be carefully
analyzed for any given application, it will be a particularly useful option
in industries that have no in house use for additional process heat or no
neighboring plants that could make economic use of the heat.
Appropriate for: Waste heat recovery
Further information:
Courtesy: Raser Technologies
The Kalina cycle is a variation of the Rankine cycle, using a mixture of
ammonia and water as the working fluid. A key difference between single
fluid cycles and cycles that use binary fluids is the temperature profile
during boiling and condensation. For single-fluid cycles (e.g., steam or
organic Rankine), the temperature remains constant during boiling. As heat
is transferred to the working medium (e.g., water), the water temperature
slowly increases to boiling temperature, at which point the temperature
remains constant until all the water has evaporated. In contrast, a binary
mixture of water and ammonia (each of which has a different boiling point)
will increase its temperature during evaporation. This allows better thermal
matching with the waste heat source and with the cooling medium in the
condenser. Consequently, these systems achieve significantly greater energy
efficiency.
Appropriate for: Waste heat recovery
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