Power Generation Using Multi Component Working Fluids

P M V SubbaraoProfessor

Mechanical Engineering DepartmentIndian Institute of Technology Delhi

Synthesis of More Appropriate Working Fluids……

Irreversible Heat transfer process : Rankine Cycle

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33

5

7

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8 1kg

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Flue gases

Cooling water

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External Irreversibilities with Rankine cycle

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External Irreversibility-1

External Irreversibility-2

Steam

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Low pressure and low temperature region

Fig 2 Layout of modern Coal fired power plant

Working fluid water – Best performance at high pressure

Heat and mass balance program

Optimal bleed pressure &

4Location of condensation process in a Low pressure steam turbine

(Source Alstom )

Exit at higher velocity

Kinetic Energy loss

Moisture Loss

0.3783 m3/ kg

12.65 m3/ kg

Erosion of a long last stage blade

( Source Alstom )

For example, a long, full speed rotor blade, operating in a non-reheat cycle, may involve wetness levels of about 15% atexhaust.

Without suitable counter-measures this can result in extreme tip-erosion (illustrated in Fig).

Source of Energy Vs Working Fluid• The overall efficiency of a thermodynamic conversion

cycle is a consequence of ; • the energy potential of the source-sink combination, of

internal inefficiencies (losses in turning machinery, in regenerators,etc.) and

• of losses from irreversible heat transfer from a source and to a sink.

• The latter depends mostly on the levels of matching of the apparent heat capacities of the working fluid, source and sink.

• In principle, a divergence in the thermal behaviour between a working fluid and a source or sink can be counteracted by using complex cycle configurations such as evaporation at multiple pressure levels in modern combined cycles or condensation at 2 or 3 decreasing temperatures in cogenerative systems.

Energy Recovery from Hot Gas

Two Phase Fluid

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T 0C

Selection of Another Working Fluid along with Water !!!

• The fluid generally has lower boiling temperature than water.

• Lower freezing point and high stability temperature.• Higher latent heat and low liquid specific heat or near

vertical saturated liquid line so that most of the heat is added during change of phase without the need for the complexity of regenerative feed heating to ‘Carnotize the cycle’ to realize high cycle efficiency.

• Small specific volume and low viscosity.• The fluid should to be non-corrosive, non-flammable,

non-toxic and safe to use. • Good availability and low cost.

T-S and T-X DIAGRAMS : Binary Components

Mixtures as Working Fluids

• Any single component working fluid, due to pinch, approach point limitations and a constant boiling point, cannot cool the gases to low temperature.

• Single component fluid can recover only about 15-20 % of the energy is that recovered by a two phase fluid.

• Multiple pressure systems could recover more energy, but added the complexity of the system and cost.

• The main characteristic is that the boiling of working fluid occurs over a range of temperatures.

• By virtue of varying boiling point, two component working fluid is able to "match" or run parallel to the gas (source) cooling temperature line while recovering energy and hence the final exit gas temperature can be low.

• The condensation of two component working fluid also  occurs over a range of temperatures and hence permits additional heat recovery in the condensation system.

• The condenser pressure can be much higher in two component fluid cycle, and the cooling water temperatures do not impact the power output of the turbine .

• Thermo-physical properties of mixture can also be altered by changing the concentration of one component.

• This helps to recuperate or regenerate energy in the condensation system.

• Modifications to the condensing system are also possible by varying the mixture concentration and thus more energy can be recovered from the exhaust gases.

• Expansion in turbine can give a saturated vapor in two component fluid cycle compared to wet steam.

• Conventional equipment such as steam turbines can be used in two component fluid cycle.

Rankine Cycle Vs Novel Cycle

• Binary working fluid -- A mixture of two fluids.– Variable temperature Boiling.

• Varying concentration of binary fluid in different parts of the cycle.– Varying thermodynamic properties.

• Internal Recovery of Heat.– Energy recovery from a one stream of working fluid to another

stream of working fluid.

• 15 -- 25% improvement in efficiency.– Less Irreversibility.

Brief History

• The technology is the creation of Dr. Alexander Kalina, a Russian scientist.

• He left a high position in Soviet Union 30 years ago to come to US.

• Formed Exergy Inc. to develop and commercialize an advanced Thermodynamic Cycle.

• 1993, General Electric signed an agreement with Exergy for a world wide exclusive licensing rights to use the technology for combined cycle systems in 50 MW to 150 MW range.

• GE and Exergy working on a combined cycle plant that will operate on an overall efficiency of 62%.

Simple Kalina CycleThe pump pressurized the saturated liquid (5) which is leaving from the condenser and it is sent in to the high temperature recuperator (6). The liquid takes off the heat from the two phase dead vapour (3). The pressurized hot liquid (sub-cooled state) enters (1) into the vaporizer where the liquid is converted in to vapor (2) by utilizing the latent or sensible heat of the hot source (1s-2s). The saturated vapor (2) from the vaporizer is expanded in the turbine up to its condenser pressure. The two phase mixture after giving a part of it’s latent heat to the incoming liquid (4) enters in to the condenser, where cooling water enters (1w), takes away all the heat available in the two-phase mixture, and leaves at higher temperature (2w).The saturated liquid is pressurized in the pump and the cycle repeats.

T-S and T-X DIAGRAMS

Ammonia water cycle modeling

• The mathematical models for Ammonia water cycle are constructed using the theory of thermodynamics.

• The whole system is divided into many components namely vaporizer, steam turbine, condenser, high temperature recuperator etc.

• According to the characteristic construction of each component, appropriate assumptions are introduced.

• Steady State Steady Flow Models are developed.

Vaporizer

The heat transfer rate QV (W) between the source and the working fluid is calculated by

Optimization

• In this case, efficiency of the cycle is considered as the objective function to be optimized.

• The Ammonia water cycle has four variables.

• Fraction of ammonia (x)

• Turbine inlet pressure (P3)

• Heat source inlet temperature T1S

• Heat source outlet temperature T3S.

• The cycle performance depends on the values for these four variables that are free to change during optimization.

• Each combination of the eight values represents a unique operating condition of the cycle.

• Searching for optimum values for these variables are the task of this optimization work.

• Consequently, the objective function to be maximized can be written as,

The objective function is solved with the help of most power fulloptimization methodology ‘Monte Carlo’.

First law efficiency of the cycle is defined as

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hh

hhcycle

Variation of first law efficiency at different steam inlet conditions of simple

Saturation Pressure of Rankine Cycle (bar)

Effect of variation in fraction of ammonia at the evaporator inlet on first law efficiency

Condenser pressure in the Ammonia water cycle largely depends on cooling water inlet temperature and fraction of Ammonia in the Ammonia water mixture. For the same cooling water inlet temperature, decreasing the Ammonia mass fraction at the condenser inlet will reduce the condenser pressure and it will leads to larger expansion process in the turbine and hence more power output and higher efficiency

The Kalina Cycle : Nine Components

Modern Kalina Cycle

Effect of variation in fraction of ammonia at the evaporator inlet on first law efficiency

The following modifications are suggested for the proposed Ammonia water cycle when compared to KCS 34.1.Super heater is added in the cycle to utilize the superheated steam at low temperature and pressure. The saturated vapor from the separator is superheated in the super heater before entering the steam turbine.2.The additional feed water is included in the system, which utilize the sensible heat of low grade to heat the sub-cooled water coming it from the condenser of an Ammonia water cycle

Kalina Cycle with Subcooler

Effect of turbine inlet pressure on first law efficiency

The Superheat Kalina Cycle

Comparison of Exergy destruction in various components of the Ammonia water cycles