Cogeneration (cogen) through combined heat and power (CHP) is the simultaneous production of electricity with the recovery and utilization of 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 or a gas boiler for onsite heating. To reduce transportation and distribution losses, combined heat and power plants are typically embedded close to the end user improving also the overall distribution losses and the performance of the electricity transmission and distribution network. 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
A CHP plant with its high efficiency, compared with a conventional one used for electricity production 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
The 1, 2 and 3 options 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.
Typically, the engine exhaust gases leave the engine at between 400 and 500˚C. These exhaust gases can be used directly for drying, in a waste heat boiler to generate steam, or through an exhaust gas heat exchanger when they are also combined with the heat which comes from the cooling circuits. A lower grade of heat can also be recovered from the second stage intercooler. Alternatively new technologies are available for the conversion of heat to further electricity, such as the Organic Rankine Cycle Engine.
CHP applications are many and of different kind. The areas where CHP applications are usually used for are for commercial, residential and industrial application reasons.
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. At a first stage the energy in the fuel gas input is converted into mechanical energy through the combustion of the gas in the engine’s cylinders and their resulting turning action of the engine’s crankshaft. This mechanical energy is 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).