IMPROVEMENT OF RANKINE EFFICINECY OF STEAM POWER

PLANTSP.DHILIP BTG-10-006

R.GOWTHAM BTG-10-007

Despite efforts to develop alternative energy

converters, electricity from steam will continue, for many years, to provide the power that energizes the world economies.

Steam cycles used in electrical power plants and in the production of shaft power in industry are based on the familiar Rankine cycle.

INTRODUCTION

Rankine cycle is an ideal cycle for comparing the performance of

steam plants. It is a modified form of Carnot cycle, in which the condensation

process is continued until the steam is condensed into water.

The Rankine cycle is a thermodynamic cycle which converts heat into work.

The heat is supplied externally to a closed loop, which usually uses water as

the working fluid.

This cycle generates about 80% of all electric power used throughout the

world, including virtually all solar thermal, biomass, coal and nuclear power

plants

In the simple Rankine cycle, steam flows to a turbine, where part of its energy

is converted to mechanical energy that is transmitted by rotating shaft to drive

an electrical generator.

Rankine cycle

The reduced-energy steam flowing out of the turbine condenses to liquid water in the condenser.

A feed water pump returns the condensed liquid (condensate) to the steam generator.

The heat rejected from the steam entering the condenser is transferred to a

separate cooling water loop that in turn delivers the rejected energy to a

neighbouring lake or river or to the atmosphere. The efficiency of a Rankine cycle is usually limited by the working fluid. Without the pressure going super critical the temperature range the cycle can

operate over is quite small, turbine entry temperatures are typically 565°C (the creep limit of stainless steel) and condenser temperatures are around 30°C.

This gives a theoretical Carnot efficiency of around 63% compared with an actual efficiency of 42% for a modern coal-fired power station.

This low turbine entry temperature (compared with a gas turbine) is why the Rankine cycle is often used as a bottoming cycle in combined cycle gas turbine power stations.

The working fluid in a Rankine cycle follows a closed loop and is re-used constantly.

The water vapor and entrained droplets often seen billowing from power stations is generated by the cooling systems (not from the closed loop Rankine power cycle) and represents the waste heat that could not be converted to useful work.

Note that cooling towers operate using the latent heat of vaporization of the cooling fluid.

While many substances could be used in the Rankine cycle, water is usually the fluid of choice due to its favorable properties, such as nontoxic and unreactive chemistry, abundance, and low cost, as well as its thermodynamic properties.

Process 1-2: The working fluid is pumped from low to high

pressure, as the fluid is a liquid at this stage the pump requires little input energy.

Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor.

Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur.

Process 4-1: The wet vapor then enters a condenser where it is condensed at a constant pressure and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase-change.

Process of rankine cycle

3f2 f4ff1 hhhh14operationwarmingduringabsorbedHeat

Heat absorbed during the complete cycle = Heat absorbed during isothermal operation 1- 2 + Heat absorbed during warming operation 4 – 1

Work done during the cycle = Heat absorbed – Heat rejected

The work done by the extraction and boiler feed pumps in increasing the pressure of water from the condenser pressure (P3 = P4) to the boiler pressure (P1 = P2) is very small. Hence neglected

)hh(hx 4f3f3gf3

We know that the efficiency is proportional to: 1-TL/TH

That is, to increase the efficiency one should increase the average temperature at which heat is transferred to the working fluid in the boiler, and/or decrease the average temperature at which heat is rejected from the

working fluid in the condenser.Different methodsDecreasing the of Condenser Pressure (Lower TL)Superheating the Steam to High Temperatures (Increase TH)

Increasing the Boiler Pressure (Increase TH)

Increasing the Efficiency of Rankine Cycle

Lowering the condenser pressure will increase the area

enclosed by the cycle on a T-s diagram which indicates that the net work will increase. Thus, the thermal efficiency of the cycle will be increased.

The condenser pressure cannot be lowered than the saturated pressure corresponding to the temperature of the cooling medium. We are generally limited by the thermal reservoir temperature such as lake, river, etc.

Steam exits as a saturated mixture in the condenser at the saturation temperature corresponding to the pressure in the condenser.

So lower the pressure in the condenser, lower the temperature of the steam, which is the heat rejection temperature.

Decreasing the of Condenser Pressure

Superheating the steam will increase the net work output and the

efficiency of the cycle. It also decreases the moisture contents of the steam at the turbine exit.

The temperature to which steam can be superheated is limited by metallurgical considerations (~ 620°C).

By superheating the stream to a high temperature (from state 3 to state 3'), the average steam temperature during heat addition can be increased.

Superheating the Steam to High Temperatures (Increase TH)

Increasing the operating pressure of the boiler leads to an increase in

the temperature at which heat is transferred to the steam and thus raises the efficiency of the cycle.

If the operating pressure of the boiler is increased, (process 2-3 to process 2'-3'), then the boiling temperature of the steam raises automatically.

For a fixed inlet turbine temperature, the blue area is the net work increased and the gray area is the net work decreased. Also, the moisture content of the steam increases from state 4 to state 4', which is an undesirable side effect.

This side effect can be corrected by reheating the steam, and results in the reheat Rankine cycle.

Increasing the Boiler Pressure (Increase TH)

The Rankine cycle has been modified to produce more output work by

introducing two stage steam turbines, using intermediate heating. Basically, in this modified Rankine cycle, the full expansion of steam is

interrupted in the high-pressure turbine and steam is discharged after partial expansion.

This exhaust steam is passed through a cold reheat line to the steam generator where it gains heat while passing through hot tubes.

This reheated steam is supplied to a low pressure turbine for full expansion and reaches the condenser pressure.

This results in low pressure turbine expansion work. Thus, reheat increases work output because of low pressure turbine expansion work.

The use of reheat also tends to increase the average temperature at which heat is added.

If the steam from the low-pressure turbine is superheated, the use of reheat may also increase the average temperature at which heat is rejected.

Reheat cycle

It may reduce or increase the thermal efficiency depending on the specific cycle conditions viz. thermal (heat addition and heat reduction temperature) or mechanical (condenser vibrations and air leakage, pump vibrations, turbine blade vane deflection etc).

Indeed the the net work of the reheat cycle is the algebraic sum of work of the two turbines and the pump work and the total heat addition is the sum of the heat added in the feed-water and reheat passes through the steam generator.

The presence of more than about 10% moisture in the turbine exhaust can cause erosion of blades near the turbine exit and reduce energy conversion efficiency.

Determination of a suitable reheat pressure level is a significant design problem that entails a number of considerations.

The cycle efficiency, the net work, and other parameters will vary with the reheat pressure level for a given throttle and condenser conditions.

One of these may be numerically optimized by varying the reheat pressure level while holding all other design conditions constant.

The efficiency of a simple Rankine cycle is improved by using intermediate

reheat cycle, enabling improved thermal conditions of the working fluid. However, it cannot reach the thermal conditions as in the case of the

Carnot cycle where heat addition and heat rejection occurs at a specified temperature range.

The regeneration is vital to improve the efficiency as it uses the sensible heat of exhaust steam for the preheating of feed water. Inclusion of a FWH also introduces an additional pressure level into the Rankine cycle as seen in the T-s diagram.

Hence, the extraction pressure level is another parameter under the control of the designer.

The control of steam condenser pressure i.e. condenser vacuum and supply of condenser tube cooling water is another parameter which affects the steam thermal power plant efficiency.

Concluding remarks

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