Rankine Cycle Efficiency Improvement Techniques
Steam power plants are still the backbone of the total power generation in the Asia pacific. Thus even a small improvement in the form of increasing the efficiency has a tremendous effect on the fuel saving and also reduction in emission of green house gases. Thus one should not miss out any opportunity to find out the ways and means to increase the efficiency of the steam power cycle.
The ideal behind any improvement or modification is to increase the thermal efficiency of the power plant. Thus thermal efficiency improvement techniques are:
- By decreasing average temperature at which heat is rejected from the working fluid (steam) in the condenser. (Lowering condenser Pressure)
- By increasing steam temperature entering the turbine
Lowering The Condenser PressureSteam leaves the turbine and enters the condenser as saturated mixture in line with the corresponding pressure of steam in the condenser. Lowering the condenser pressure always helps in delivering more net work in the turbine as more expansion of steam in turbine is possible. By the help of T-s diagram the effect of lowering the condenser pressure on the performance of the cycle can be seen and understood.
Positive Effects of Lowering the Condenser PressureTo milk the advantage of higher efficiency Rankine Cycle has to operate on lower condenser pressure usually below atmospheric. But the limit for lower condenser-pressure is defined by the cooling water temperature corresponding to saturation-pressure of the area.
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In the above T-s diagram it can be easily seen that the coloured area is the increase in net work out put on account of lowering the condenser pressure from P4 to P4’.
Negative Effects of Lowering the Condenser PressureThe effect of lowering the condenser-pressure is not comes without any side effects. Thus following are the adverse effects of lowering the condenser pressure:
- Additional heat input in the boiler on account decreased condensate re-circulation temperature (effect of lower condenser pressure)
- With lower condenser pressure the possibility of increase of moisture content in steam at the final expansion stage of the turbine increases. Decreases in dryness fraction of steam in later stages of the turbine is undesirable as it results in slight decrease in efficiency and erosion of turbine blades.
Net Effects of Lowering the Condenser PressureThe over all net effect is more towards positive side, since the increase in heat input requirement in the boiler is marginal but the increase in net work out put is more on account of decrease in condenser pressure. Also the dryness fraction of the steam in the latter stages of the turbine are not allowed to drop beyond 10-12%.
Super Heating The Steam to Higher TemperatureSuperheating of steam is the phenomenon in which heat is transferred to the steam to super heat the steam to higher temperature by maintaining the constant pressure in the boiler. The shaded area in the above T-s diagram clearly showing the increase in net work (3-3’-4’-4) on account of increase in superheat temperature of steam.
Additional heat input in the form of energy, leaves the cycle as work i.e increase in work output surpass the additional heat input and heat rejection. Thermal efficiency of the rankine cycle increases on account of increase in steam temperature.
Positive Effects of Increasing the Steam TemperatureOne desirable effect of increasing the increasing the steam temperature is that it doesn’t allow to the last stage mositure % of steam to increase. This effect can be easily seen on the T-s diagram (Fig:2) above.
Negative Effects of Increasing the Steam TemperatureIncreasing the steam temperature results in small increase in heat input. There is a limit to which the steam can be superheated and used in the power cycle. These limiting factors are related to metallurgical proveness at high temperature and economical viability. Presently in supercritical power generating units, steam temperature at turbine inlet is around 620oC. Decision of any further increase in steam temperature can be judiciously taken only after doing the metallugical due diligence and evaluation of the cost-implications.
Net Effects of Increasing the Steam TemperatureFrom the T-s diagram (Fig:2) the net effect of temperature increase is more towards positive side, because the gain from the network output surpasses the increase in heat input and slight increase in heat rejection. So it is always beneficial to increase the steam temperature after accessing the reliability and economic viability.
Increasing Boiler Pressure With Sub Critical ParametersAlternative way of increasing the Rankine cycle efficiency is by increasing the boiler operating pressure and thus in a way related with the temperature at which boiling is taking place in the boiler. Thus the thermal efficiency of the cycle increases. By the help of T-s diagram the effect of Increase in boiler pressure on the performance of the cycle can be clearly seen and understood. Because of increase in boiler pressure, Rankine cycle shifts slightly towards left as shown in the Fig:3 on T-s diagram and thus following can be concluded from it:
- Substaintial increase in net-work, as shown in the pink colour shaded area of the above figure.
- As the cycle shift slightly towards left, so their is decrease in net work during the expansion of steam in the turbine. ( as shown in above fig:3 sheded in grey colour.
- Reduction in the heat-rejection to the cooling water in the condenser.