Trigeneration or District Energy Systems, Providing up to 50% Greater System Efficiency and Fuel Savings Over Typical Cogeneration
"Trigeneration,” also referred to as "district energy," or "integrated energy systems," is dramatically more efficient and environmentally friendly than "cogeneration."
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.
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.
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