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Natural Gas Combined Cycle

Natural Gas Combined Cycle (NGCC) Plants


Heat Engines Are Inefficient

Do you know that most of our industry, transport and household equipment depend for power directly or indirectly, on heat engines. Another interesting fact about these is that they are very inefficient in conversion of heat to electricity. A car engine will convert less than one fifth of the fuel it consumes into mechanical power. The best efficiency a steam turbine for electrical power can produce is around 34%. And there is a very interesting rule about conversion efficiency.  The energy a turbine can convert is proportional to the cooling it causes in the steam or gas that drives it.

Improving Efficiency

The exact formula is very simple. Conversion efficiency = , where T1 is the inlet temperature and T2 is the exhaust temperature, both expressed in Kelvin. One degree on Kelvin scale, is equal to 1 degree on centigrade scale, but the zero of Centigrade is equal to 273 K. That means the inlet temperature T1 must be high as possible and the exhaust temperature must be as low as possible. The lowest exhaust temperature can be no less than the ambient say, 15ᵒC or 292K, and the upper temperature is limited by the materials needed for high pressure steam.

Currently, practical limits for inlet temperatures are about 860 K. Siemes ST-700 and 900 turbines quote a temperature of 585ᵒC (858K). Do remember that this formula is for the maximum theoretical efficiency of the turbine only. The process of converting from chemical energy to heat energy, and transferring it to steam is a very inefficient process. In addition, the electric generator will also have an efficiency close to ninety percent so that the best overall fuel to electricity conversion is just above 40%, claimed by GE.

Natural Gas Combined Cycle

The Gas Turbine

In case of gas turbines, the process of steam generation is eliminated because the hot gas product of gas combustion itself drives the turbine. As the turbine operation is at a higher temperature (900 to 1,400 °C) but lower pressure, the cost of high temperature material is reduced. The combustion chamber design is simpler, and the heat exchange to steam is eliminated.

The Combined Cycle Gas Turbine

The exhaust gas temperature can be high as 450 to 650 °C (723K to 923K), which is sufficiently high to generate steam for driving a steam turbine. Thus, a gas turbine followed by a steam turbine makes the combined cycle gas turbine (CCGT), giving an overall efficiency above 60%. (62.2% has been reported).

In Comes Solar Heat- Integrated solar combined cycle

The combined cycle gas turbine can benefit from solar heat also. Solar heat can be, in principle, integrated at any stage in the process, but it is much more logical to combine the solar heat at the steam generation level. The system is called Integrated Solar combined cycle (ISCC). The heat can be used to preheat steam or increase the steam mass. Fuel consumption can be reduced or more steam can be generated leading to more power.

The addition of solar heat will reduce the carbon foot-print of the generated power. Power increment as result of the integration of the solar component comes out cheaper than solar electric power. Integrated Solar Combined Cycle Generation plants can come on line faster if solar heat is already stored, or the sun is available. Daily steam start-up losses are also eliminated or reduced.

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