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Combined heat and power (CHP), or cogeneration, is the simultaneous generation of useful mechanical and thermal energy in a single, integrated system. CHP can be configured as a topping or bottoming cycle. In a typical topping
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Combined heat and power (CHP), or cogeneration, is the simultaneous generation of useful mechanical and thermal energy in a single, integrated system. CHP can be configured as a topping or bottoming cycle. In a typical topping cycle system, fuel is combusted in a prime mover, such as reciprocating engines, combustion or gas turbines, steam turbines, microturbines, or fuel cells, which drives the overall system to generate electricity. In a bottoming cycle system, also referred to as "waste heat to power," fuel is combusted to provide thermal input to a furnace or other industrial process and heat rejected from the process is then used for electricity production.
CHP involves the recovery of otherwise-wasted thermal energy to produce useful thermal energy or electricity. This can significantly increase industrial efficiency and decrease unnecessary fuel consumption. CHP already has a large presence in the U.S., with 4,300 installations in all 50 states and 130 GW of remaining technical potential for electricity generation. Additionally, CHP is recognized for its potential to significantly reduce carbon emissions and to reliably generate power, even during emergency situations. Although CHP represents a significant up-front investment, it can pay for itself within a few years.
A. Types of CHP Prime Movers:
|CHP SYSTEM||ADVANTAGES||DISADVANTAGES||AVAILABLE SIZES|
|Gas Turbine||High reliability.
High-grade heat available.
No cooling required.
|Require high-pressure gas or in-house gas compressor.
Poor efficiency at low loading.
Output falls as ambient temperature rises.
|500 kW to 300 MW|
|Microturbine||Small number of moving parts.
Compact size and lightweight.
No cooling required.
Relatively low mechanical efficiency.
Limited to lower temperature cogeneration applications.
|30 kW to 250 kW with multiple unit packages up to 1,000 kW|
|Spark ignition (SI) reciprocating engine||High power efficiency with part-load operational flexibility.
Relatively low investment cost.
Has good load following capability.
Can be overhauled on site with normal operators.
Operate on low-pressure gas.
|High maintenance costs.
Limited to lower temperature cogeneration applications.
Relatively high air emissions.
Must be cooled even if recovered heat is not used.
High levels of low frequency noise.
|1 kW to 10 MW in DG applications|
|Compression ignition (CI) reciprocating engine (dual fuel pilot ignition)||High speed (1,200 RPM) ≤4MW|
|<80 MW for Low speed (60-275 RPM)|
|Steam Turbine||High overall efficiency—steam to power.
Can be mated to boilers firing a variety of gaseous, liquid or solid fuels.
Ability to meet more than one site heat grade requirement.
Long working life and high reliability.
Power to heat ratio can be varied.
|Slow start up.
Very low power to heat ratio.
Requires a boiler or other steam source.
|50 kW to several hundred MWs|
|Fuel Cells||Low emissions and low noise.
High efficiency over load range.
Fuels require processing unless pure hydrogen is used.
Sensitive to fuel impurities.
Low power density.
|5 kW to 2 MW|
Source: EPA Combined Heat and Power Partnership
The installed cost for most CHP technologies consists of the total equipment cost plus installation labor and materials, engineering, project management, and financial carrying costs during construction.
Operation and Maintenance (O&M) Cost
Non-fuel operation and maintenance (O&M) costs typically include routine inspections, scheduled overhauls, preventive maintenance, and operating labor. O&M costs are comparable for gas turbines, gas engines, steam turbines, and microturbines, and only a fraction higher for fuel cells.
Start-up times represent how long it takes CHP units to get online and be able to provide electricity and thermal output. Start-up times for the five CHP technologies described above can vary significantly depending on the technology and fuel used.
Availability indicates the amount of time a unit can be used for electricity and/or steam production. Availability generally depends on the operational conditions of the unit.
The ability to produce useful thermal energy from exhaust gases is the primary advantage of CHP technologies. This value is equivalent to the gross useful thermal output of the CHP system minus the thermal input.
|FEATURE OF CHP TECHNOLOGY||TYPE OF CHP PRIME MOVER|
|Gas Turbine||Microturbine||Reciprocating Engine||Steam Turbine||Fuel Cell|
|Installed Cost ($/kW)||Ranges from 1,200 to 3,300||Ranges from 2,500 to 4,300||Ranges from 1,500 to 2,900||Ranges from 670 to 1,100||Ranges from 5,000 to 6,500|
|Non-fuel O&M Cost ($/kWh)||Ranges from 0.009 to 0.013||Ranges from 0.009 to.013||Ranges from 0.009 to 0.025||Ranges from 0.006 to 0.01||Ranges from 0.032 to 0.038|
|Start-Up Time||10 min–1 hr||60 sec||10 sec||1 hr–1 day||3 hrs–2 days|
|Uses for thermal Output||Heat, hot water, low-pressure and high-pressure steam||Hot water, chiller, heating||Space heating, hot water, cooling, low-pressure steam.||Process steam, district heating, hot water, chilled water||Hot water, low-pressure and high-pressure steam|
Source: EPA Combined Heat and Power Partnership
CHP systems are major investments. For instance, a 1 MW recipr