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 a reduction in emission of greenhouse gases.

Thus one should not miss out on any opportunity to find out the ways and means to increase the efficiency of the steam power cycle.

The idea behind any improvement or modification is to increase the thermal efficiency of the power plant. Thus thermal efficiency improvement techniques are:

• By decreasing the 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 Pressure

Steam leaves the turbine and enters the condenser as a saturated mixture in line with the corresponding pressure of steam in the condenser. Lowering the condenser pressure always helps in delivering more network in the turbine as more expansion of steam in the turbine is possible.

With the help of the 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 Pressure

To milk the advantage of higher efficiency, the 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.

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 Pressure

The effect of lowering the condenser-pressure does not come 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 Pressure

The 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 Temperature

Superheating 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 Temperature

One 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 Temperature

Increasing 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 Temperature

From 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 Parameters

Alternative 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.

Thus net-effect is marked increases in the thermal efficiency of the cycle on account of these measures.

Increasing the Boiler Pressure with Super Critical Parameters

In order to increase the thermal efficiency of the Rankine cycle, super-critical pressure is used in steam-generators used in the present time. When the steam generators operates above 22.06Mpa then the steam generators are called super-critical steam-generators and the plant is called super-critical power generation plant. Because of the higher operating pressures these plants are know for giving higher efficiencies.

Re-Heat Rankine Cycle

Re-heat Rankine cycle is for taking the advantage of increased cycle efficiency at higher boiler pressure without compromising on moisture content of the steam in the last stages of the turbine.

Higher cycle efficiency is possible with re-heating cycle that too without compromising on dryness fraction this is possible by expanding the steam in turbine in two stages by re-heating it in between. Re-heating is practically acceptable way of tackling the problem of excessive moisture in the last stages of the turbine.

Theoretical Way of Reducing the Last Stage Moisture

Theoretically one way is to super heat the steam to a higher temperature before steam enters the turbine but there is a limit above which metallurgical limitations of handling high steam temperature prevents it from further increase beyond 620oC. Super critical power plants are running in India are running with inlet steam temperature of around 593oC.

Modified Rankine Cycle

Practical way of successfully reducing the last stage moisture in large turbine (200 MW and above) is by slightly modifying the simple Rankine cycle with re-heat cycle as shown below in Figure:5

Re-Heat Cycle Differs from Rankine Cycle in Following Aspects

Expansion of steam in reheat cycle happens in two stages. In the first stage steam expands in the High Pressure turbine (HP turbine) and the exhaust of the HP turbine is then send back to the steam generator for re-heating. Steam outlet from the re-heater in the 2nd stage steam generator re-heating is directed to the Low-pressure-turbine (LP Turbine) for final-expansion over the last stages of the turbine with high dryness fraction then exhaust to the condenser.

Analysis of Re-Heat Cycle is as Follows

Heat input during the cycle (2-3-4-5) is

Turbine work output for the cycle is

Thus by adopting a single reheat cycle in a thermal power plant cycle efficiency can easily is enhanced by another 4 to 5 percentage.

What is the Practical Limit of Re-Heating?

Theoretically if we increase the number of reheating stages then the number of expansion in the turbine can also be increased in order to get the more turbine output and thus higher cycle efficiency.

But practically more than two stages of reheat is not practical. It has been seen and experienced that the theoretical improvement in the efficiency of the cycle from 1st to 2nd reheat is reduced from 5 percent to less than 2.5 percent.

Also it is observed that with a sub-critical pressure double reheat cycle are having a more super-heated exhaust loss in the condenser than with super-critical-cycle-parameters. So double-reheat cycles are avoided with sub-critical parameters.

From third reheat cycle onwards the gain of cycle efficiency starts diminishing, so not justifiable to incurred additional cost and complexity.

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