Tmax is the temperature of the gas at the inlet to the gas turbine, Tmin is the ambient temperature and K is internal losses. As a result, there are three theoretical methods of increasing efficiency: increasing inlet temperature, decreasing ambient temperature and reducing internal losses. Theoretically, a gas turbine could achieve efficiencies of up to 65%. At present, simple open-cycle turbines achieve efficiencies of about 40%. in addition, it is possible to use waste heat from the outlet of the gas turbine to improve efficiency of use. This is where the very high overall efficiencies from cogeneration come from. The basic gas turbine cycle is shown in Figure 3. Air is compressed from point 1 to point 2. This increases the pressure as the volume of space occupied by the air is reduced. The air is then heated at constant pressure from point 2 to point 3. This heat is added by injecting fuel into the combustor and continuously igniting it. The hot compressed air at point 3 is then allowed to expand (point 3 to point 4), reducing the pressure and temperature and increasing the volume. This represents flow through the turbine to point 3' and then flow through the power turbine to point 4. The combustion cycle is completed by decreasing the temperature, with heat being absorbed into the atmosphere. 2:! "' "' ., p Healadded through combustion 023 2 3 W12 W33- Output work ta \ • ---'---► run compressor Pressure İS İncreas� through compressor as volum İS redııced ----------------� Start 1 041 4 Usefııl work avaİlable lor shalt power ar lhrust Exhaust heat 0+------------------- 0 V Volume of aİr Figııre 3. ldealized gcıs ıurbi11e lıeaı cycle (E11ergy Solutio11s Centeı) This cycle is the simple gas turbine cycle, called the Brayton cycle. However, additional equipment and techniques can be used to increase the efficiency of the cycle. These modifications include: regeneration, intercooling and reheating. in general, efficiency is influenced by: O Energy used by the air compressor- if less energy is used to compress the air, more energy is available at the output shaft O Temperature of the gas leaving the combustors and entering the turbine- the higher the temperature, the greater the efficiency OTemperature of the exhaust gas from the turbine- the lower the temperature, the greater the efficiency OMass flow through the gas turbine- in general, higher mass flows result in higher efficiencies O Pressure drop across inlet air filters- increased pressure loss decrease efficiency /�����f ��mtıo� WO�l� ARTICLE / MAKALE O Pressure drop across exhaust gas silencers, ducts and stack- increased pressure loss decreases efficiency. There has been considerable work done to improve the efficiency of gas turbines, mainly on increasing turbine entry- gas temperatures and increasing the efficiency and capability of the compressor. Various methods have been used to improve efficiency in these areas. These include: O Using the exhaust gas to heat the air from the compressor this is most eftective in cold weather O Dividing the compressor into two parts and cooling the air between the two parts O Dividing the turbine into two parts and reheating the gas between the two parts O Cooling the inlet air-this is mainly used in hot weather O Reducing the humidity of the inlet air O lncreasing the pressure of the air at the discharge of the air compressor O Regularly washing or otherwise cleaning the fouling of tur- bine and air-compressor blades. However, there is a trade-off with ali of these methods of in creasing efficiency. They ali increase costs, and some reduce the available power output of the gas turbine. The final design choice will be the most appropriate compromise that balances cost, power and efficiency tor each specific application. These different approaches can be broken down into five main categories: O lncreased inlet temperature O Regeneration O Compressor intercooling O Turbine reheat O Steam/water injection lncreasing Turbine inlet Temperature The most obvious way of increasing the efficiency of a gas turbine is to increase the inlet temperature. Efficiency is related to both the inlet and outlet temperatures of the turbine; the higher the difference between the two temperatures, the greater the thermal efficiency of the turbine. There is an absolute limit to how low the outlet temperature can go, so increasing the inlet temperature is an obvious method of increasing efficiency. However, increases in inlet temperature have already reached the point where the temperatures are actually higher than the melting point of some of the metals used in the turbine. Cooling of the first rows of turbine blades is therefore imperative, and any further increases in inlet temperature will require improvements in cooling techniques. This could involve increased use of steam cooling, increased flow of the cooling fluid, or increased effectiveness of heat transfer with the cooling fluid. in addition, different materials with improved heat-resisting properties and improved thermal barrier coating can also assist in allowing the elevation of the inlet temperature. Nonetheless, the cost involved in increasing the efficiency of the turbine through raising the inlet temperature is becoming increasingly prohibitive. The materials involved ENERJİ & KOJENERASYON OÜNYASI 5
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