Cogeneration & CHP
Cogeneration (cogen) through combined heat and power (CHP) is the simultaneous production of electricity with the recovery and utilisation heat. Cogeneration is a highly efficient form of energy conversion and it can achieve primary energy savings of approximately 40% by compared to the separate purchase of electricity from the national electricity grid and a gas boiler for onsite heating. Combined heat and power plants are typically embedded close to the end user and therefore help reduce transportation and distribution losses, improving the overall performance of the electricity transmission and distribution network (see district energy for more details). For power users where security of supply is an important factor for their selection of power production equipment and gas is abundant, gas-based cogeneration systems are ideally suited as captive power plants (i.e. power plants located at site of use).
Benefits of Gas Engine CHP
The high efficiency of a CHP plant compared with conventional bought in electricity and site-produced heat provides a number of benefits including
- On site production of power
- Reduced energy costs
- Reduction in emissions compared to conventional electrical generators and onsite boilers
Heat Sources from a Gas Engine
The heat from the generator is available in from 5 key areas:
- Engine jacket cooling water
- Engine lubrication oil cooling
- First stage air intake intercooler
- Engine exhaust gases
- Engine generator radiated heat, second stage intercooler
1, 2 and 3 are recoverable in the form of hot water, typically on a 70/90˚C flow return basis and can be interfaced with the site at a plate heat exchanger.
The engine exhaust gases typically leave the engine at between 400 and 500˚C. This can be used directly for drying, in a waste heat boiler to generate steam, or via an exhaust gas heat exchanger combining with the heat from the cooling circuits. 5. The heat from the second stage intercooler is also available for recovery as a lower grade heat. Alternatively new technologies are available for the conversion of heat to further electricity, such as the Organic Rankine Cycle Engine.
A variety of different fuels can be used to facilitate cogeneration. In gas engine applications CHP equipment is typically applied to natural gas (commercial, residential and industrial applications), biogas and coal gas applications.
CHP System Efficiency
Gas engine combined heat and power systems are measured based upon the efficiency of conversion of the fuel gas to useful outputs. The diagram below illustrates this concept.
Firstly the energy in the fuel gas input is converted into mechanical energy via the combustion of the gas in the engine’s cylinders and their resulting action in the turning of the engine’s crankshaft. This mechanical energy is in turn used to turn the engine’s alternator in order to produce electricity. There is a small amount of inherent loss in this process and in this example the electrical efficiency of the engine is 40% (in reality GE Jenbacher gas engines are typically between 40-48.7% electrically efficient).
(Diagram key: HE 1 – Mixture intercooler, HE 2 – Oil exchange heater, HE 3 – Engine jacket water heat exchanger, HE 4 – Exhaust gas heat exchanger)