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Gas engines and turbines
There are various systems capable of generating electrical energy that are used in cogeneration plants: steam turbines, gas turbines and gas engines. The latest technological, economic and environmental trends have given rise to the majority of cogeneration plants being based on gas turbines or gas engines.
GAS TURBINES
A gas turbine is a machine capable of both producing mechanical power and providing a high quantity of heat in the form of hot gases. These have a high percentage of oxygen, which allows them to be used as combustion air in an additional burner, thus raising the temperature level of the gases.
A basic diagram of a gas turbine is as follows:
The air compressor carries out the task of raising the pressure of the combustion air (once it has been filtered) before it enters the combustion chamber. This compression can be carried out in one or several stages and consumes a great deal of the power produced by the turbine. The high-pressure combustion of gas and air takes place in the combustion chamber. A gas compressor is often needed in order to ensure that the gas enters at the right pressure. Due to the high temperatures that can be reached during combustion and in order to prevent the life span of the chamber's component parts from greatly decreasing, a high ratio of excess air is used, which both reduces the temperature of the flame and cools down the hottest parts of the chamber.
The conversion of the energy contained in the combustion gases (in the form of high pressure and temperature) to mechanical power (in the form of the rotation of a shaft, known as the power shaft) takes place in the power turbine. The gases that enter the power turbine at a temperature of 1,000 – 1,200 ºC, come out at around 500ºC and at a pressure slightly higher than the atmospheric pressure. The rotation speed of the power shaft tends to be much greater than that necessary to activate an alternator or compressor and a gearbox is usually needed to reduce the number of revolutions. The generator is the element which consumes the mechanical energy provided by the turbine and generates the electrical current.
GAS ENGINES
Gas engines currently provide the greatest efficiency in terms of the conversion of heat energy into electrical energy. The residual heat produced is, however, distributed between different streams of liquid at different temperatures, which makes it more difficult to recover.
The basic diagram of a gas engine for cogeneration plants is as follows:
The combustion of a mixture of gas and air takes place in the combustion chamber. This is cylindrical in shape and inside has a moving piston which draws in the fuel and air at one end, whilst at the other end, it transfers the energy given off during combustion to the power shaft, by means of a rod-crank system. Once the combustion has taken place, the piston begins to move in order to remove the combustion products. In general 15-40% excess air is used and the pressure of the gas at the entrance to the regulator before the chamber is lower than 2 bar. This pressure is easy for the distribution companies to guarantee, and therefore the gas does not usually need to be compressed.
The generator's function is to convert mechanical energy into electrical energy. One particular feature of these engines is their relatively low rotation speed, which makes it possible for the power shaft to be directly attached to the generator.
There are basically three circuits for the evacuation of fluids and cooling, which are as follows: exhaust gas evacuation, engine coolant and lubricating oil. The latter accounts for a very low percentage of the energy given off. The engine is normally cooled using water, depending upon the temperature level of which three different groups can be distinguished:
- "Classic" cooling, in which the water enters at around 70ºC and comes out towards the cooler at 85-90ºC.
- High-temperature cooling, in which, through pressure, the water reaches temperatures of above 100ºC (maximum 120ºC) without changing phase.
- Cooling by boiling: more efficient engine cooling is achieved by allowing the water to vaporise at around 120-125ºC, which also eliminates the need for a recirculation pump, as the system works by natural convection.
