Repsol YPF
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Comparison between gas engines and gas turbines
In comparison with gas turbines, cogeneration plants that have gas engines are characterised by a higher electrical performance with considerably smaller investments, as is illustrated in the following graph:
Turbines are recommended when there is a constant demand for heat with a higher temperature than 110ºC, or in facilities that are extremely powerful. Both these technologal options share a compact build that only takes up a very small space.
Thermodynamic cycle
The thermodynamic cycle, for engines and turbines alike, is composed by 4 stages: admission, compression, combustion and release. In both cases, the air undergoes a compression process before combustion takes place. The combustion process is carried out in a cooling chamber.
- The pressure levels that are reached with turbines are higher than those attained using engines.
- The combustion chamber temperatures that are reached using turbines tend to be higher than the ones for engines.
- The products of combustion in engines tend to have lower temperatures than is the case for turbines, of around 350ºC compared with 500ºC.
- The speed of the revolutions of the engine shafts varies between 1,000 and 1,500 r.p.m., while for turbines, the speed of revolutions varies between 10,000 and 15,000 r.p.m.
- Engines are water-cooled, whereas turbines are refrigerated using air.
- Engines have a complex cooling and lubrication system, which uses oil and water, so that apart from electricity, heat is available in the form of water at a low temperature (40ºC), water at a high temperature (90ºC) and combustion gases.
- In turbines, there are two sources of heat: oil (in negligible quantities) and the turbine's gases, which are a mixture of combustion gases and the air for refrigeration from the chamber, resulting in the O2 content fluctuating around the 15% mark.
- The oxygen content of the exhaust gases makes postcombustion possible with turbines, whereas it is not possible with engines, leading to a need to resort to conventional boilers for support.
- For the same quantity of useful heat at high temperatures (> 90ºC) and of generated electricity, the global performance level of the cogeneration plant is higher with gas turbines.
Electrical performance
The electrical performance of a plant is one of the most important elements of a cogeneration plant from an economic perspective. The electrical performance for gas turbines tends to fluctuate between 20% and 30%, while the electric performance of engines is of between 35% and 40%. Turbines require a reduction kit to adapt them to the alternators that are connected to the electric grid's frequency, which rotate at 1,500 rpm.
The efficient use of heat
In the case of gas turbines, the main source of heat just comes from the exhaust gases. The heat drawn from the cooling of the oil is negligible.
Engines provide two sources of heat. On the one hand, exhaust gases are produced at temperatures of between 350 and 400ºC to generate steam or to use it for other processes (drying, the heating of process agents, etc.).
On the other hand, 30% of the energy that is consumed is available in the form of water for engine cooling at temperatures of between 90ºC and 100ºC that can be used directly.
