Laboratorij za termoenergetiko

Energy management

Water and steam

Exergy

Exergy by heat transfer

Exergy in the case steam turbine expansion

Properties of water and steam

Energy Management 2

Properties of water and steam- density

Energy Management 3

water

Moist steam

Superheated steam

Properties of water and steam - volume

Energy Management 4

water

Moist steam

Saturated steam

Properties of water and steam - entalphy

Energy Management 5

water

Moist steam

Superheated steam

Properties of water and steam – Molliere diagram h-s

Energy Management 6

Properties of water and steam - table

Energy Management 7

tlak

specifični volumen

specifična entalpija

specifična entropijatemperatura

sprememba

agregatnega

stanja

voda

para

Properties of water and steam - table

Energy Management 8

tlak nasičenja

specifični volumen

specifična entalpija

specifična entropija

vrela voda nasičena para

v = v' + x(v" - v')

h = h' + x(h" - h')

s = s' + x(s" - s')

suhost pare:

x =mpara

mpara + mvoda

Exergy

Exergy is the convertible part of energy and can be described in several ways

it is the energy that can be completely converted into any other form of energyat given surroundings conditions

it is the largest quantity of work that can be produces in a technical devicefrom working media with given starting parameters

it is the smallest required quantity of work to raise working media fromsurroundings conditions to any other condition provided that heat is broughtinto the process only from the environment

Exergy depends on surroundings conditions which limits the 'usability' of enerycarried by working media. If working media is in balance with the surroundingsno energy can be extracted from it without using additional source of energy.

Anergy is the part of energy that cannot be converted into any other form ofenergy including exergy. Internal energy of the environment is pure anergy.

Energy Management 9

Exergy

Energy Management 10

Exergy

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Specific exergy of working media

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ideal(reversible)

process:

• internal energy of the working media, U1

• volume work for inserting the work media into the system, p1 V1

• internal energy of the working media, Uamb

• volume work to eject the work media from the system, pamb Vamb

• heat dissipated,Qout = Tamb(S1 - Samb)

• acquired workWt = Wt,max

U1 + p1 V1 = Uok + pamb Vamb + Qod + Wt,max

Wt,max = H1 – Hamb – Tamb(S1 – Samb)

wt,max = e = h – hamb – Tok(s – samb)

Examples of defining properties of water and steam

Using the tables of water and water vapor properties, determine the volume of 2.5 kg of water/steam that has

• a temperature of 60 ° C and a pressure of 1 bar

• temperature 150 ° C and pressure 1 bar

• temperature 150 ° C and pressure 20 bar

Determine the state of 4 kg of water/steam at 160 ° C and

• pressure 10 bar

• volume 0.8 m3

• specific enthalpy of 2780 kJ/kg

Energy Management 13

Examples of determining the properties of water and steam

Linear interpolation

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pnpn

pp yy

xx

xxyy

primer parameter 1 parameter 2 iskano

1 p = 20 bar T = 180 °C v =

2 p = 5 bar T = 306 °C h =

3 p = 8 bar h = 3000 kJ/kg T =

4 T = 50 °C s = 6,75 kJ/kgK h =

5 p = 1,9 bar T = 120 °C s =

6 p = 7 bar x = 0,813 v =

7 p = 105,3 bar v = 0,02 m3/kg T =

Examples of determining the properties of water and steam

Linear interpolation

Energy Management 15

pnpn

pp yy

xx

xxyy

primer parameter 1 parameter 2 iskano

1 p = 20 bar T = 180 °C v = 0,001127 m3/kg

2 p = 5 bar T = 306 °C h = 3077 kJ/kg

3 p = 8 bar h = 3000 kJ/kg T = 273,1 °C

4 T = 50 °C s = 6,75 kJ/kgK h = 2163,15 kJ/kg

5 p = 1,9 bar T = 120 °C s = 7,1517 kJ/kgK

6 p = 7 bar x = 0,813 v = 0,2220 m3/kg

7 p = 105,3 bar v = 0,02 m3/kg T = 341,0 °C

The exergy losses in the case of heat transfer

In case of a water-water heat exchanger find the loss of exergy flow and analize dependenceof transferred exergy flow and required heat transfer area on inlet temperature of the colderwater. The warm water enters with 90 °C and exits with 60 °C while mass flow rate is17 kg/s. Mass flow rate of the cold water is 12,8 kg/s and its inlet and outlet temperaturesare 30 °C and 70 °C, respectively.

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The exergy losses in the case of heat transfer

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0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

10 20 30 40 50

rela

tivn

a p

ovr

šin

a p

ren

osn

ika,

rela

tivn

a iz

gub

a e

kse

rgije

vstopna temperatura hladne snovi / °C

površina izguba eksergije

Exergy loss in a heat exchanger - mixer

For the case of of mixer heat exchanger shown in the figure calculate the global entropy (systemand surroundings) if ambient temperature is 20 °C. Calculate lost exergy due to mixing of bothflows?

Energy Management 18

Energy and exergy flows in the turbine

team is entering a turbine with a pressure of 110 bar and a temperature of 530 °C andexpands to a pressure of 0,06 bar. Steam flow rate is 15 kg/s. Surrounding conditions are1 bar and 25 °C. Calculate

- turbine power

- inlet and outlet exergy flows and

- energy and exergy balance

for an ideal turbine with thermal efficiency 1 as well as a real turbine where outlet steamdryness is 0,84. Verify the validity of Gouy-Stodola theorem.

Energy Management 19

Reduction cooling station

Energy Management 20

For the system shown in the figure find missing parameters as well as energy and exergyflows for two operating regimes:a) all steam from the boiler is directed throgh the turbineb) parameters of superheated steam are reduced in reducing and cooling stationConstruct a Rant chart for both regimes. Surrounding conditions are 1 bar and 20 °C.

Reduction cooling station

Energy Management 21

reducirno-hladilna postaja turbina