Piston engines

There are two primary types of piston engine for power generation, the diesel engine and the spark-ignition gas engine. Of these the diesel engine is the most efficient, reaching close to 50% energy conversion efficiency. The spark-ignition engine burning natural gas can achieve perhaps 42% efficiency but it is much cleaner than the diesel. Indeed it is impossible to obtain authorisation to use a diesel engine for continuous power generation service in some parts of the world.

There are four sources of heat in a piston engine: the engine exhaust, the engine jacket cooling system, the oil cooling system and the turbocharger cooling system (if fitted). Engine exhaust can provide low- to medium-pressure steam and the engine jacket cooling system can provide low-pressure steam. However the most normal CHP application would generate hot water rather than steam. If all four sources of heat are exploited, roughly 70-80% of the energy in the fuel can be utilised.

Piston engine power plants are available in sizes ranging from a few kW to 65 MW. These engines are particularly good at load following; a spark-ignition engine efficiency falls by around 10% at half-load while diesel engine efficiency barely drops over this range. There is no significant penalty in terms of engine wear for variable load operation. Piston engines can also be started quickly, with start-up times as short as 10 s typical.

System Engine Block
Figure 5.1 Block diagram of piston-engine-based CHP system which is a closed-loop head-recovery system

Applications for piston engine CHP plants include small offices and apartment blocks, hospitals, government installations, colleges and small district heating systems. Engines tend to be noisy, so some form of noise insulation is normally required. Emissions of gas engines can normally be controlled with catalytic-converter systems but diesel engines require more elaborate measures to control their higher nitrogen oxides and particulate emissions.

Steam turbines

A steam turbine is one of the most reliable units for power generation available. Modern large utility steam turbines have efficiencies of 46-47% but smaller units employed for CHP applications generally provide efficiencies of 30-42%. These turbines are usually simpler in design too. Steam turbines are available in virtually any size from less than 1MW to 1300 MW.

A steam turbine cannot generate electricity without a source of steam. This is normally a boiler in which a fossil fuel or biomass fuel is burnt. However it can also be a waste-heat boiler exploiting the hot exhaust from a gas turbine. A steam turbine will normally be used in a CHP system only where there is a demand for high-quality, high-pressure steam for some industrial process.

There are a number of ways in which a boiler/steam turbine system can be used in such an application. One method is to take heat directly from the boiler to supply the process, with any surplus being used to drive the steam turbine. Alternatively steam can be taken from the boiler to the steam turbine and then from the turbine exhaust to the process. The pressure and temperature of the steam exiting the turbine can be tailored to suit the

Boiler

Steam

Boiler

Steam

^ Low-pressure steam to site

Electricity to site

Feedwater

Generator

Steam turbine

Figure 5.2 Block diagram of steam turbine CHP system

Feedwater

^ Low-pressure steam to site

Electricity to site

Generator

Steam turbine

Figure 5.2 Block diagram of steam turbine CHP system industrial demand. A third method is to extract steam from the turbine casing at a point before the exhaust. Combinations of all these methods are possible, so that the CHP system can be tuned for maximum efficiency.

The emissions from a steam turbine CHP system will be those of the boiler which generates the steam. Thus the emission-control measure will depend on whether the plant burns coal, wood or gas. Noise is unlikely to be a consideration since a steam turbine CHP system will only be used in an industrial environment.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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