Contents
Introduction to the key components of a Cogeneration (CHP) system
A cogeneration system consists of the following key components:
- Prime Mover – in which potential energy of the fuel is converted into rotary kinetic energy.
- Generator or Mechanical Drive – Energy conversion devices that are driven by the prime mover.
- Waste Heat Recovery Unit – Energy conversion devices that recover waste heat from the exhaust stream of the prime mover to generate useful thermal energy.
Cogeneration is the simultaneous production of electricity and useful thermal energy from a single source of primary energy. It is widely recognized as a high-efficiency smart solution for decarbonizing energy-intensive industries which have combined demand for electricity and thermal energy.
Prime Mover
The Prime Mover converts the fuel potential energy into kinetic energy, usually in the form of rotary motion, which is then harnessed for electricity generation or the production of useful mechanical work.
The two types of Prime Movers which are usually adopted in a Cogeneration System are the Combustion Turbine (CT) and the Reciprocating Engine. The summary table below compares and contrasts the key characteristics of these two Prime Movers.
| Characteristics: | Reciprocating Engine Cogeneration | Combustion Turbine Cogeneration |
| Prime Mover: | Reciprocating Engine | Combustion Turbine |
| Heat to Power Ratio: | Up to 1.0 | 1.0 to 2.0 |
| Exhaust Gas Temperature: | Less than 400OC | Up to 600OC |
| Quality of Recovered Heat: | Hot Water/Low-Pressure Steam | Medium to High-Pressure Steam |
| Value Driver: | Electricity | Thermal Energy |
| Power Augmentation: | N/A | Inlet Air Cooling* (Boosts Electricity Output) |
| Supplementary Firing: | N/A | Duct Burner** (Boosts Thermal Output) |
| Electrical efficiency, %: | 30 – 45% | 30 – 36% |
Combustion Turbine
The exhaust stream of the Combustion Turbine contains higher quality waste heat (higher mass flow rate and higher gas temperature translate into higher exhaust gas enthalpy) which enables higher heat to power ratio compared to the Reciprocating Engine. The oxygen content of the exhaust gas is typically around 15% and this allows supplementary burners to be used to increase the thermal output.
Compared to the Reciprocating Engine, the Combustion Turbine has lower electrical efficiency and higher electricity generation cost. The combination of these factors makes the Combustion Turbine the preferred Prime Mover in applications where thermal energy is the main value driver.
The Combustion Turbine also has lower part-load efficiency and slower ramp rate compared to the Reciprocating Engine and is more suitable for Base Load applications.
Reciprocating Engine
The Reciprocating Engine has higher electrical efficiency and lower electricity generation cost. Its lower quality exhaust stream and low oxygen content do not allow supplementary firing to boost thermal output. Hence, Cogeneration with the Reciprocating Engine has lower heat to power ratio. These factors make the Reciprocating Engine the Prime Mover of choice when electrical power is the main value driver.
The superior part-load efficiency and ramp rate of the Reciprocating Engine makes it more suitable for Fluctuating Load applications.
Generator
Prime Movers produce shaft power, which can drive mechanical devices or electrical generators.
The Generator converts the mechanical shaft power of the Prime Mover into electrical power.
In a Cogeneration System, the Generator is an energy conversion device that works on the principle of electromagnetic induction to transform the mechanical shaft power of the Prime Mover into electricity. Synchronous Generators are invariably used in Cogeneration Plants.
Mechanical Drives
In certain use cases, the prime mover can also drive a Mechanical Compressor to produce useful work. Examples of these are the mechanical compressors used to boost gas pressure in a pipeline or to compress natural gas in the production of Liquefied Natural Gas (LNG).
Waste Heat Recovery Unit
The waste heat contained in the exhaust gas of the prime mover is recovered in the Waste Heat Recovery Unit (WHRU) to produce a secondary stream of useful thermal energy. The thermal energy can be in the form of steam, hot water, or thermal oil. In the case of steam production, the WHRU is also called a Heat Recovery Steam Generator (HRSG).
HRSG
The HRSG is used to convert the waste heat in the exhaust stream of the Prime Mover into process steam. In the case of Combustion Turbines, the HRSG is capable of generating steam with pressures ranging from 10bars and below, to more than 100bars. Depending on the process requirement, the steam can be saturated or superheated and the HRSG can be designed to generate multiple pressure levels. In combined cycle applications, dual pressure and triple pressure HRSGs are commonly used.
When higher heat to power ratio is required, supplementary burners are used to increase the thermal energy content of the exhaust gas and increase the steam generation capacity of the HRSG.
Thermally Activated Chillers
If comfort or process cooling is required, special thermally activated devices can be used to directly convert waste heat into cooling energy in the form of chilled water. Examples of these thermally activated machines include direct exhaust absorption chillers and multi-energy absorption chillers.
Steam Absorption Chillers utilize medium-pressure saturated steam as a heat source to drive the absorption cycle, thus turning heat into cooling energy in the form of chilled water.
Direct Exhaust Absorption Chillers are able to utilize the waste heat in the exhaust gas to drive the absorption cycle to generate cooling energy in the form of chilled water. These machines are usually installed at the back-end of the Combustion Turbine.
Multi Energy Absorption Chillers are installed at the back-end of the Reciprocating Engine. It utilizes the waste heat from the exhaust gas as well as hot water from the jacket recovery to drive the absorption cycle to generate cooling energy in the form of chilled water. These machines are capable of utilizing two heat sources to generate chilled water in an efficient manner.
Steam Turbine Driven Chillers
Steam Turbine Driven Chillers are vapor compression chillers that are driven by Steam Turbines. Process steam is used to power the steam turbines. The type of turbine is typically condensing type for optimum efficiency.