COMBINED CYCLE
The waste heat from the exhaust of an internal combustion engine is generally hot enough to generate medium-pressure steam. In the case of small engine installations, steam production is not normally an economical option unless there is a local use for low-quality steam. In the case of a large diesel installation, however, the engine exhaust can be used to generate steam in a boiler, steam that can drive a steam turbine to produce additional energy. This forms the core of a diesel engine–based combined cycle plant.
Diesel engine combined heat and power systems are rare because they are generally only economical on very large engines. Typical of this sort of application is a generating plant that was installed in Macau in 1987. This plant was equipped with a slow-speed diesel engine with a capacity of 24.4 MW. The engine exhaust was fitted with a waste heat boiler and steam turbine that could generate an additional 1.34 MW when the engine was operating at full power, thus contributing around 5% of the plant output. As a result of this and other measures a fuel-to-electricity conversion efficiency of close to 50% was achieved.
Large engines of this type are frequently derived from marine engines and the original engines upon which they are based are not normally optimized for combined cycle operation. In particular, the cooling system is designed to keep the engine as cool as possible. For best combined cycle performance, however, it is preferable to run the engine as hot as possible, because the higher the exhaust gas temperature, the more efficient the steam turbine cycle. High- temperature operation can also improve engine efficiency because the potential thermodynamic efficiency will increase with operating temperature.
Combined cycle performance of a large diesel engine can, therefore, be improved by modifying engine components such that they can operate continuously at a higher temperature. Such modifications may require more expensive materials capable of withstanding the more extreme conditions. For example, the top of the piston may be made from an alloy that allows it to remain uncooled while exhaust valves are treated with advanced coatings able to resist the high exhaust gas temperature.
These modifications allow a higher temperature exhaust that can be used to generate higher-quality steam to drive a steam turbine. With these measures it may be possible to achieve a fuel-to-electricity conversion efficiency of close to 55%. This is the efficiency target for a plant in Wasa, Finland, installed in 1998. The plant has two 17 MW diesel engines and a single steam turbine. Efficiency in this case is improved by using seawater cooling for the steam turbine con- denser. The additional expense of the waste heat recovery and steam turbine will generally only prove cost effective if the engine is to be used for base-load operation.