Combined Heat and Power using piston engines

Piston engine technology is one of the best known motor technologies. Several types of piston engines are known but the Diesel engine and Otto engines are best known and used exclusively for CHP applications. Since the Otto engines are practically always fuelled by gas, these engines are also called gas engines or gas motors. The power spectrum of the piston engines goes from a few kWe to tens of MWe.

Both engine types have cylindrical combustion chambers wherein a reciprocating piston is located. Through piston rods, the pistons are connected to a crankshaft which transforms the reciprocating movement of the piston into a rotating movement. This can be coupled to a generator for electricity production.

An operating engine also produces a lot of heat at different locations and at different temperatures. The main heat sources are the exhaust gases and the motor bloc cooling. There is also the cooling circuit for the motor oil and there is also the intercooler in case there is a turbo charger.
The useful application of the heat is quite a challenge since the heat is produced at different temperatures. Most of the heat is available as hot water at relatively low temperatures but steam production is not excluded but this will lead to reduced fuel utilisation factor.
Figure 1 illustrates the heat recuperation systems for a piston engine.

Figure 1: Heat recuperation systems in a piston engine.

The compatibility between the desired temperature of the heat and the desired inlet temperature of the water cooling circuit is of prime importance. This determines which heat sources can be used and in what sequence they have to be connected. This is illustrated in figure 1. It follows that the thermal efficiency can vary a lot depending mainly on the required cooling water inlet temperature and the temperature of the return feed from the external users. Nevertheless, a total fuel utilisation factor of between 80 to 90% is feasible.
The large number of moving parts is one of the main disadvantage of piston engines (piston, valves,..). This leads to substantial noise production and intensive maintenance. Maintenance costs for piston engines are higher than for competitive technologies in the same power range, and are a non negligible cost factor in the economic analysis. Piston engines can start-up fast and produce electricity very quickly which explains the use of Diesel engines as emergency stand-by generators. However, the thermal response to a sudden heat demand is much slower. Therefore, piston engines can be used only to deliver a base heat load while sudden peaks in the heat demand must be delivered by dedicated boilers.

For both motor types, attention must be given to the emissions. Exhaust gas cleaning by means of catalysers is needed to comply with strict pollution norms. For Diesel engines, attention must also be given to the emissions of fine particles (soot).

Gas engines

Gas engines are available in sizes from a few kWe to about 10 MWe. These motors are fed by a mixture of fuel and air inhaled in the cylinders and subsequently compressed. This compressed mixture (at end of compression cycle) is ignited by an external spark. The progression of the combustion front leads to a fast increase in cylinder pressure with an expansion through recession of the piston. This is the work phase in the cycle. The exhaust gases are expelled from the cylinder in last phase of the cycle. This engine is known as a 4 stroke engine, since a complete cycle exists of four phases (fuel aspiration, fuel compression, fuel combustion and expansion and combustion gas exhaust).

The electrical efficiency of a gas engine varies between 30 to 40%. To facilitate the ignition of a combustible mixture, the air concentration should neither be too small nor too high. To obtain a good efficiency, a lean fuel mixture with excess air content is recommended. To achieve this, one often switches over from a rich to a lean fuel-air mixture ones the optimal combustion conditions are reached. The methods used are e.g. pre-chamber ignition or diesel pilot ignition. In this way the efficiency can be improved.

Gas engines could also be used as CHP units for small scale applications such as heating and electricity production of private houses. Drawbacks for such special applications are excessive noise and large specific (= cost per unit kWe) investment cost.

Diesel engines

Also in Diesel engines does one recognise the 4 stroke cycle. However, in the compression phase, the temperature of the air in the cylinders is increased beyond the auto ignition point of the fuel-air mixture. At the end of the compression phase fuel is injected leading to spontaneous combustion and expansion of the cylinder volume by pushing the piston outwards (the work phase). In the fourth phase, exhaust gases are expelled from the cylinders. The reciprocating movement of the piston is transformed into a rotating movement of the crankshaft driving the generator to produce electricity.

Diesel motors achieve a somewhat larger electrical efficiency than Otto motors. The thermal efficiency however is somewhat lower. Due to the sulphur content of the diesel fuel, exhaust gas condensation cannot be applied. Diesel engines cater more for larger power levels and are available for the power spectrum from 100 kWe to several tens of MWe. At the moment, gas engines are favoured above diesel engines for CHP applications.

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