Although the performance of the gas turbine is not particularly appealing when compared to the efficiencies possible in diesel and gasoline engine power plants, a simple gas turbine has weight, size, and vibration advantages over the engine, as well as size and cost advantages over a small steam plant. It is also superior to both in terms of water consumption, as the simple gas turbine plant requires almost no cooling water.
Even if the design of the components used in gas turbine power plants is improved, the efficiency and specific output of the simple gas turbine cycle are quite low. The efficiency disadvantage can be overcome at the expense of increasing the complexity of the gas turbine plant.
Methods for Improving Gas Turbine Efficiency
Regeneration
As the temperature of the turbine exhaust is higher than the temperature at the end of compression, we can consider using Ericsson’s concept of regeneration. In this case, the exhaust emits heat into the surrounding air. This type of heat transfer is known as regenerative heating, and the heat exchanger used for it is known as a regenerator. This causes the final exhaust gases to cool, resulting in a reduction in heat rejection.
In theory, if the heat exchanger is large enough and the flow is slow enough, the air from the compressor can be heated reversibly to temperature 4 at state b. At the same time, the exhaust cools to temperature 2 at state a. Some of the previously discharged heat h4 – ha is exchanged within the system, reducing the heat to the sink to ha – h1.
Furthermore, instead of h3 – h2, only the heat equal to h3 – hb must be added. As a result, less fuel is required, and this additional piece of equipment should significantly improve the efficiency of the ideal cycle.
With a regenerator and a fixed initial temperature of T1, the thermal efficiency increases as T3 increases and decreases as the pressure ratio increases. The cycle efficiency increases as the pressure ratio increases without the regenerator.
Regenerator effectiveness:
However, because the regenerator is not perfect, the actual state of the air will be d’. As a result, the regenerator will not be completely efficient. The Effectiveness of the regenerator determines the regenerator’s performance.
Intercooling
The network of the gas turbine cycle can be increased by either reducing or increasing the compressor work. The nature of constant pressure is used to reduce compressor work. The vertical distance between two constant pressure lines decreases to the left and increases to the right.
Surface coolers that are water-cooled are typically used. Low and high-pressure stages must be encased separately. The turbine is said to work on the Open Cycle if the entire flow comes from the atmosphere and is returned to the atmosphere.
The method of heating the air after compression is the primary distinction between open and closed-cycle gas turbines. In the case of an open cycle gas turbine, the fuel is burned in the air itself to raise its temperature, i.e., the fuel is mixed with air, and the combustion products are then passed on to the atmosphere via the turbine. The compressor is refilled with fresh air for the next cycle, and the processes are repeated.
A closed-cycle gas turbine, on the other hand, circulates the same air or working substance repeatedly. The working substance is heated in a heat exchanger, and a separate hot gas is obtained by burning the fuel in a combustion chamber with an additional supply of air. The heat exchanger is of the shell and tube type, which prevents the working substance from coming into contact with combustion products.
The majority of gas turbines in use are open-cycle plants. However, recent developments have enabled closed-cycle plants to operate, producing more than 1500 kW with a gas turbine inlet gas temperature of 800°C and thermal efficiency of the order of 30%.
At state 1, cold gas enters a compressor, where shaft work on the compressor is performed to increase pressure and temperature. The gas exits the compressor in state 2. This gas enters the heater (heat exchanger), which supplies heat at a constant pressure. The temperature of the gas rises, and it exits the heater in state 3. This gas from the heater enters the gas turbine, where it is expanded to a lower pressure, lowering the temperature and producing shaft work. A portion of this shaft work is used to power the compressor, while the remainder is supplied to the load or useful power or net power.
Reheating and Reheat Cycle
Another variation on the simple Brayton cycle is the use of two turbines instead of one, with one driving the compressor and the other producing network output. In the space between these two turbines, we may or may not install another combustor.
Reheating is the process of raising the temperature of partially expanded gas by burning more fuel in a device known as a Reheater. Intercooling, reheating, and regeneration are provided by a gas turbine (Brayton Cycle).
All of the modifications to the simple cycle can be used separately or in combination. They are capable of increasing plant efficiency to more than 30%, effectively eliminating any advantage of fuel efficiency enjoyed by diesel or condensing steam plants.
Water Injection
Injecting water into the working air at the compressor’s entrance is one method of improving the performance of the gas turbine. The compressed air is cooled in this manner by absorbing the latent heat of the vaporization of water from the air.
The total mass flow of the working medium is increased by the mass of the injected water, and thus the power output of the cycle is increased, as is the work ratio, in addition to the lowering of the air rate.
On jet-powered aircraft, a water injection system is commonly used as a power book for takeoff and emergencies. Water injection for a long duration or continuous water injection is possible for marine or land-based gas turbines. The water that will be injected must be pure or it will cause corrosion or deposits on the blades.
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