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"Trigeneration,” also referred to as
"district energy," or "integrated energy systems," is dramatically more efficient and
environmentally friendly than "cogeneration."
A well-designed
trigeneration plant will surpass the efficiency of a cogeneration plant
by about 50%, and a utility power plant by 300%, of similar size.
A
trigeneration plant, defined in non-engineering terminology, is most often described as a
cogeneration plant that has added absorption chillers - which takes the "waste
heat" a cogeneration plant would have "wasted," and converts this
"free energy" that would have been wasted by cogeneration, into useful energy in
the form of chilled water.
The
trigeneration power and energy process produces at least three different forms of energy from the primary
energy source, namely, hot water, chilled water (for air
conditioning) and power generation
(electrical energy). Many times, steam is also produced from the
trigeneration power plant - especially at hospitals, which are an ideal
location for trigeneration plants.
Trigeneration
has also been referred to as CHCP (combined heating, cooling and power generation), this
option allows having greater operational flexibility at sites with demand for energy
in the form of heating as well as cooling. This is particularly relevant in tropical
countries where buildings need to be air-conditioned and many industries require process
cooling. |
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Example of a Gas Turbine Based
Trigeneration Facility
Although cooling can be provided by conventional vapor compression chillers driven by
electricity, low quality heat (i.e. low temperature, low pressure) exhausted from
the
cogeneration plant can drive the absorption chillers so that the overall primary
energy consumption is reduced. Absorption chillers have recently gained widespread acceptance due
to their capability of not only integrating with cogeneration systems but also because
they can operate with industrial waste heat streams. The benefit of power generation and
absorption cooling can be realized through the following example that compares it with a
power generation system with conventional vapor compression system.
Trigeneration Example by the numbers
A
factory requires 1 MW of electricity and 500 refrigeration tons* (RT). The gas
turbine generates electricity required for the on-site energy processes as well as the
conventional vapor compression chiller.
Assuming an electricity demand of 0.65
kW/RT, the compression chiller needs 325 kW of electricity to obtain 500 RT of cooling.
Therefore, a total of 1325 kW of electricity must be provided to this factory. If the gas
turbine efficiency has an efficiency of 30 per cent, primary energy consumption would be
4417 kW.
However, a cogeneration system with an absorption chiller (thereby making this a
"trigeneration" plant) can provide the same energy service (power and cooling)
by consuming only 3,333 kW of primary energy versus 4417 kW thereby saving nearly 25% in
primary energy usage. This is why a trigeneration plant is even more efficient
than a cogeneration plant. |
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This
example clearly points out the advantages of trigeneration over
typical cogeneration plants. A
trigeneration plant (with an absorption chiller) can save about 24.5 per cent
of primary energy in comparison with a cogeneration plant and vapor compression
chiller.
Additionally, a smaller prime mover leads to not only lower capital cost
but also less standby charge during the system breakdown because steam needed for the
chiller can still be generated by auxiliary firing of the waste heat boiler.
Since many industries and commercial buildings need combined power and heating and
cooling, trigeneration plants have very high potentials for industrial and commercial
application - with the associated energy and economic savings inherent with
trigeneration.
* Note : A refrigeration ton (RT) is defined as the transfer of heat at the rate of 3.52
kW, which is roughly the rate of cooling obtained by melting ice at the rate of one ton
per day.
This
example and information courtesy of ASHRAE
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