THERMAL POWER CYCLES

Compiled & presented by : UTKARSH PRAKASH Reliance Power-GET ‘2011

THERMAL POWER PLANT

The basic energy cycle involved in the plant is as follows :

Chemical Energy

Mechanical Energy

Electrical Energy

A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated. The greatest variation in the design of thermal power stations is due to the different fuel sources. Some thermal power plants also deliver heat energy for industrial purposes, for district heating, or for desalination of water as well as delivering electrical power.

EQUIPMENTS

POWER CYCLES CARNOT CYCLE RANKINE CYCLE DIESEL CYCLE OTTO CYCLE BRAYTON CYCLE STIRLING CYCLE COMBINED CYCLES

LAWS OF THERMODYNAMICS The zeroth law of thermodynamics recognizes that if two systems are in

thermal equilibrium with a third, they are also in thermal equilibrium with each other, thus supporting the notions of temperature and heat.

The first law of thermodynamics distinguishes between two kinds of physical process, namely energy transfer as work, and energy transfer as heat. The internal energy obeys the principle of conservation of energy but work and heat are not defined as separately conserved quantities. ∆Q= ∆U + p.dv

Equivalently, the first law of thermodynamics states that perpetual motion machines of the first kind are impossible.

The second law of thermodynamics distinguishes between reversible and irreversible physical processes. It says that the full conversion of heat to the equivalent amount of work is not possible.

Equivalently, perpetual motion machines of the second kind are impossible. The third law of thermodynamics concerns the entropy of a perfect

crystal at absolute zero temperature, and implies that it is impossible to cool a system to exactly absolute zero.

Equivalently, that perpetual motion machines of the third kind are impossible

THERMODYNAMIC PROCESSES Isobaric processes. Isothermal Processes. Adiabatic processes. Isentropic Processes. Isochoric processes. Throttling.

CARNOT CYCLE

7T-s diagram of Carnot vapor cycles.

1-2 isothermal heat addition in a boiler 2-3 isentropic expansion in a turbine 3-4 isothermal heat rejection in a condenser4-1 isentropic compression in a compressor

The Carnot cycle can be thought of as the most efficient heat engine cycle allowed by physical laws. The most efficient heat engine cycle is the Carnot cycle, consisting of two isothermal processes and two adiabatic processes. When the second law of thermodynamics states that not all the supplied heat in a heat engine can be used to do work, the Carnot efficiency sets the limiting value on the fraction of the heat which can be so used. In order to approach

the Carnot efficiency, the processes involved in the heat engine cycle must be reversible and involve no change in entropy. This means that the Carnot cycle is an idealization

CARNOT CYCLE EFFICIENCYIf W= net work output of the system in Carnot cycle, and as the system is carried out through a cycle then there is no change in the internal energy of the system, therefore QH – Qc = W

QH= TH (S2- S1)

The efficiency η is defined to be: (Work output)/(Heat input) η= W/QH = (QH-Qc)/QH

also, Where,W is the work done by the system (energy exiting the system as work), QH is the heat put into the system (heat energy entering the system), TC is the absolute temperature of the cold reservoir, and TH is the absolute temperature of the hot reservoir.

CARNOT CYCLE FEASIBILTY

Carnot's theorem: No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between those same reservoirs.

The Carnot cycle is the most efficient cycle operating between two specified temperature limits but it is not a suitable model for power cycles. Because:

Process 1-2 Limiting the heat transfer processes to two-phase systems severely limits the maximum temperature that can be used in the cycle (374°C for water)

Process 4-1 It is not practical to design a compressor that handles two phases.

Process 2-3 The turbine cannot handle steam with a high moisture content because of the impingement of liquid droplets on the turbine blades causing erosion and wear.

RANKINE TERMINOLOGY The Rankine cycle most closely describes the process by which

steam-operated heat engines most commonly found in power generation plants to generate power.

RANKINE CURVEProcess 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 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor

Process 4-1: The wet vapor then enters a condenser where it is condensed at a constant temperature to become a saturated liquid.

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.

Thermal Efficiency of Rankine Cycle:

Heat Input = Q23 = H3 – H2 Heat Rejected = Q41 = H4 – H1 Work Output = W34 = H3 – H4 Work done by Pump = W12 = H2 – H1 Work output – Pump work W34 – W12 Heat Input Q23

“the rankine cycle has a lower efficiency compared to corresponding Carnot cycle 2’-3-4-1’ with the same maximum and minimum temperatures.”

η = =

Reasons for Considering Rankine Cycle as an Ideal Cycle For SteamPower Plants:

1) It is very difficult to build a pump that will handle a mixture of liquid and vaporat state 1’ (refer T-s diagram) and deliver saturated liquid at state 2’. It ismuch easier to completely condense the vapor and handle only liquid in thepump.

2) In the rankine cycle, the vapor may be superheated at constant pressure from3 to 3” without difficulty. In a Carnot cycle using superheated steam, thesuperheating will have to be done at constant temperature along path 3-5.During this process, the pressure has to be dropped. This means that heat istransferred to the vapor as it undergoes expansion doing work. This is difficultto achieve in practice.

Second law analysis of Rankine cycle

The Rankine cycle is not a totally reversible cycle, it is only internally reversible, since heat transfer through a finite temperature difference (between the furnace and the boiler or between the condenser and the external medium) can results in irreversibilities.

The second law of thermodynamics can be used in order to reveal the regions where the largest irreversibilities within Rankine cycle occur.

  It will be possible, therefore, to act on these regions to reduce the

irreversibilities.  To do this we must compute the exergy destruction for each component of

the cycle. 

MEAN TEMPERATURE METHOD

Tm1

In rankine cycle heat is added at a constant pressure but at infinite temperaturesIf TM1 is the mean temperature of the heat addition as shown in the figure so that the area under the curve 2 to 3” is equal to the area under 6 and 7 then the heat added is Q23” = Tm1 (S3”- S2) Tm1 = (H3”- H2)/(S3” – S2)Heat rejected, Q4”1 = H4” – H1 = T2 (S4” – S1)

η = 1 – Q23”/Q4”1

η = [1 – Tm1/T2]

T2

6 7

“The higher the mean temperature of heat addition, higher will be the Rankine cycle efficiency.”

DEVIATION OF ACTUAL VAPOUR POWER CYCLES FROM IDEALIZED CYCLE

(a) Deviation of actual vapor power cycle from the ideal Rankine cycle. (b) The effect of pump and turbine irreversibilities on the ideal Rankine cycle.

The actual vapor power cycle differs from the ideal Rankine cycle as a result of irreversibilities in various components. Fluid friction and heat loss to the surroundings are the two common sources of irreversibilities.

Isentropic efficiencies

HOW TO IMPROVE EFFICIENCY

18The effect of lowering the condenser pressure on the ideal Rankine cycle.

The basic idea behind all the modifications to increase the thermal efficiencyof a power cycle is the same: Increase the average temperature at which heat is transferred to the working fluid in the boiler, or decrease the average temperature at which heat is rejected from the working fluid in the condenser.Lowering the Condenser Pressure (Lowers Tlow,avg)

To take advantage of the increased efficiencies at low pressures, the condensers of steam power plants usually operate well below the atmospheric pressure. There is a lower limit to this pressure depending on the temperature of the cooling medium Side effect: Lowering the condenser pressure increases the moisture content of the steam at the final stages of the turbine.

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The effect of superheating the steam to higher temperatures on the ideal Rankine cycle.

Superheating the Steam to High Temperatures (Increases Thigh,avg)

Both the net work and heat input increase as a result of superheating the steam to a higher temperature. The overall effect is an increase in thermal efficiency since the average temperature at which heat is added increases.Superheating to higher temperatures decreases the moisture content of the steam at the turbine exit, which is desirable.Constraint: The temperature is limited by metallurgical considerations. Presently the highest steam temperature allowed at the turbine inlet is about 620°C.

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Increasing the Boiler Pressure (Increases Thigh,avg)

The effect of increasing the boiler pressure on the ideal Rankine cycle.

For a fixed turbine inlet temperature, the cycle shifts to the left and the moisture content of steam at the turbine exit increases. This side effect can be corrected by reheating the steam.

A supercritical Rankine cycle.

Today many modern steam power plants operate at supercritical pressures (P > 22.06 MPa) and have thermal efficiencies of about 40% for fossil-fuel plants and 34% for nuclear plants.

THE IDEAL REHEAT CYCLE

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How can we take advantage of the increased efficiencies at higher boiler pressures without facing the problem of excessive moisture at the final stages of the turbine?1. Superheat the steam to very high temperatures. It is limited metallurgically.2. Expand the steam in the turbine in two stages, and reheat it in between (reheat)The ideal reheat Rankine cycle.

The average temperature at which heat is transferred during reheating increases as the number of reheat stages is increased.

The single reheat in a modern power plant improves the cycle efficiency by 4 to 5% by increasing the average temperature at which heat is transferred to the steam.The average temperature during the reheat process can be increased by increasing the number of expansion and reheat stages. As the number of stages is increased, the expansion and reheat processes approach an isothermal process at the maximum temperature. The use of more than two reheat stages is not practical. The theoretical improvement in efficiency from the second reheat is about half of that which results from a single reheat.The reheat temperatures are very close or equal to the turbine inlet temperature. The optimum reheat pressure is about one-fourth of the maximum cycle pressure.

The End