Delft, 22nd – 23rd September 2011

POTENTIAL OF WATER SPRAYEDPOTENTIAL OF WATER-SPRAYED CONDENSERS IN ORC PLANTS

Stefano Filippini, Umberto Merlo (LU-VE Group)

Matteo C Romano Giovanni Lozza (POLITECNICO DI MILANO)Matteo C. Romano, Giovanni Lozza (POLITECNICO DI MILANO)

Heat rejection in ORC

In ORC cycles, 75In ORC cycles, 75--90% of inlet heat must be rejected 90% of inlet heat must be rejected to the environmentto the environmentto the environmentto the environment

Air cooled condenser is often the preferred choice:Air cooled condenser is often the preferred choice:Air cooled condenser is often the preferred choice:Air cooled condenser is often the preferred choice:large condensing surface and footprintlarge condensing surface and footprinthi h i t t t ( b t 20% f th ORC d l )hi h i t t t ( b t 20% f th ORC d l )high investment cost (about 20% of the ORC module)high investment cost (about 20% of the ORC module)high auxiliary electric consumptionhigh auxiliary electric consumptionl i fl f bi t diti ORC fl i fl f bi t diti ORC flarge influence of ambient conditions on ORC performancelarge influence of ambient conditions on ORC performanceEconomic optimization of condenser design and operation is Economic optimization of condenser design and operation is importantimportantimportantimportant

Heat rejection in ORC

This presentation illustrates an alternative solution to This presentation illustrates an alternative solution to traditional air cooled condenser able to lower electrictraditional air cooled condenser able to lower electrictraditional air cooled condenser able to lower electric traditional air cooled condenser able to lower electric consumption and to improve ORC performance and consumption and to improve ORC performance and efficiency.efficiency.efficiency.efficiency.The solution is applied from several years in The solution is applied from several years in refrigeration and air conditioning fields showingrefrigeration and air conditioning fields showingrefrigeration and air conditioning fields showing refrigeration and air conditioning fields showing excellent results excellent results

Product description

Water spray condenser is a fin-and-tube heat exchanger, operating with dry surface when h bi i l hthe ambient temperature is lower than a

selected design value.

For higher ambient temperatures, the water spray system is activated allowingspray system is activated, allowing significant performance improvements.

R & D activity – Heat exchanger simulation with CFD – dry mode

PathPath lineslines of of Fin design improved by

ComputationalComputational FluidFluidwavywavy finsfins

ComputationalComputational FluidFluidDynamicsDynamics analysis.

PP S dS d T tT tDistributionsDistributions ofof

PressurePressure SpeedSpeed TemperatureTemperature

R & D activity – Heat exchanger simulation with CFD – wet mode

Application of COMPUTATIONAL FLUID DYNAMICS ANALYSIS on a sprayed fin: “WET” application“WET” application.

Water droplets distribution

S S o a sp ayed fi : W applicatioW applicatio .

0.7500

1.0000Diameter Distibution

0.5000

0.0000

0.2500

0 20 40 60 80 100

droplets diameter μm

Quantity Quantity distribution of the distribution of the

•• min diameter 3.5 µmmin diameter 3.5 µm•• max diameter 87.5 µmmax diameter 87.5 µm

di t 17 5di t 17 5Flow of the Flow of the

evaporated evaporated

droplets diameter, μm

distribution of the distribution of the evaporated waterevaporated water

•• avg. diameter 17.5 µmavg. diameter 17.5 µm•• distribution field distribution field 1.251.25

evaporated evaporated waterwater

R & D activity – Heat exchanger testing activity

LULU--VEVE-- Testing stationTesting station

DRY and SPRAY - Performance DRY and SPRAY - Performance

Performance of DRY and SPRAY related to EHLD1S Performance of DRY and SPRAY related to EHLD1S

range, with 30%range, with 30% Ethylene GlycolEthylene Glycol andand ΔΔTwTw=5K=5Krange, with 30% range, with 30% Ethylene Glycol Ethylene Glycol and and ΔΔTwTw 5K5K

Case study: R134a recuperative supercritical ORC cycle for geothermal application

140

160

49

80

100

120

ratu

re, °

C

10

40

60Tem

per

2

1 7

63

0

20

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9Entropy, kJ/kg/K

1

PerformanceDesign conditionsηcyc,gross=16.4%ηheatr rec =91.7%ηnet=11.6%

•Tw=150°C•Treinj-min=70°C•Tamb=23,5°C ηnet 11.6%

Qcond=85.6% of Qin-ORC

Tamb 23,5 C•Tcond=35°C

Calculation model

Given the cycle operating conditions at design point, off-design performance are estimated considering constraints imposed by components design:components design:

• Main heat exchanger heat transfer surface• Recuperator heat transfer surfaceRecuperator heat transfer surface• Turbine nozzle critical section• Condenser design: tube and fin geometry + fan

h i icharacteristic curve

Dependent variables used to respect the given constraints:• Turbine inlet temperature• Recuperator effectiveness• Turbine inlet pressure• Turbine inlet pressure• Condensing temperature

Turbine efficiency corrected by taking into account the isentropicTurbine efficiency corrected by taking into account the isentropic enthalpy drop and outlet volume flow rate.

Influence of ambient temperature

Tamb=35.6°C, fans speed at design value (670 RPM):increased condensing temperatureg preduced heat recovery efficiency (recup. cycle)

140

160

49

80

100

120

ture

, °C

610

40

60

80

Tem

pera

1

2

37

0

20

40

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9Entropy, kJ/kg/K

Influence of ambient temperature –dry operation

Effect of fans speed:Effect of fans speed:ambient temperature = 23 5°Cambient temperature = 23.5 C

5016

4514

e, °C%

Heat recovery  loss

G l ffi i

4012

mpe

rature

Efficiency,  Gross el. efficiency

Condensing  temperature

3510

Te

308

300 400 500 600 700 800 900

fans speed, RPM

Influence of ambient temperature –dry operation

Efficiency decay at ambient temperature = 23.5°C

16

14

16fans efficiency loss

pump efficiency loss

Net efficiency

12

14

10

12

η, %

8

10

i l f d i i i ffi i b f d

800650500350

fans speed, RPM

An optimal fan speed maximizing net efficiency can be found

Influence of ambient conditions –wet operation

0.14dry operation spray=22mc/h

Tamb= 23.5°C; RH=68%

0.13

Tamb= 35.6°C; RH=31% (summer peak)

0.13

ency

spray=45mc/h spray=67mc/h

0.11

0.12

ency

0.11

0.12

Net efficie

0.09

0.10

Net efficie

0.10

300 400 500 600 700 800 900

0.07

0.08

300 400 500 600 700 800 900

dry operation spray=22mc/h

spray=45mc/h spray=67mc/h

fans speed, RPM fans speed, RPM

Thanks to wet operating net efficiency increases by 13% p g y y(Tamb 23,5°C) and by 33% (Tamb 35,6°C).

Also in these cases there is an optimal fan speed maximizing net efficiency

Influence of ambient conditions –wet operation

Net electric efficiency vs. ambient conditions at optimal fan speed

0.16

Rel Humidity0.14

Rel.Humidity=81% 82%

80%76%

0.12

efficiency

76%

68%55%

46%0.10

Net e

dry operation spray 22mc/h 38%31%

0.08

0 5 10 15 20 25 30 35 40

dry operation spray=22mc/h

spray=45mc/h spray=67mc/h

0 5 10 15 20 25 30 35 40

Ambient temperature, °C

Water spray injection improves the net efficiency by 7 ÷ 33%, according to ambient temperature

Economic calculation

Increase of water injection always enhances net electric efficiencyOptimal operations should be optimized to maximize cash flowThe trade-off between water cost and price of electricity is very

dependent on site conditions and local costs

15Cw=1.0€/mc; EE=15c€/kWh

Tamb= 23.5°C; RH=68%

9

12

e, %

Cw=0.5€/mc; EE=15c€/kWh

Cw=0.1€/mc; EE=15c€/kWh

6

flow increase

0

3

Cash f

0 20 40 60 80

Water consumption, mc/h

Economic calculation

Increase of water injection always enhances net electric efficiencyOptimal operations should be optimized to maximize cash flowThe trade-off between water cost and price of electricity is very

dependent on site conditions and local costs

35

40Cw=1.0€/mc; EE=15c€/kWh

Tamb= 35.6°C; RH=31%

25

30

35

, %

Cw=0.5€/mc; EE=15c€/kWh

Cw=0.1€/mc; EE=15c€/kWh

15

20

flow increase

0

5

10

Cash f

0 20 40 60 80

Water consumption, mc/h

Economic calculation

By considering an annual temperature distribution (Central Italy case), an annual calculation can be undertaken at optimized (maximum cash flow) working conditions, for different costs of water

3 512

3.0

3.5

10

12t, %

2.0

2.5

6

8

time, years

mprovem

ent

1.0

1.5

4

Pay back 

al cash flo

w im

0.0

0.5

0

2

0 0 2 0 4 0 6 0 8 1 1 2

Ann

ua

0 0.2 0.4 0.6 0.8 1 1.2

Water cost, €/mc

Conclusions

• Dry & spray technology can be an effective system to improve power output and profitability of ORC plantsp p p p y p

• Actual performance should be evaluated on a annual basis and are strongly dependent on the installationbasis and are strongly dependent on the installation site (climate, water cost and electricity price)

I h i ti i ti ff t f• In a comprehensive optimization process, effects of the investment costs should also be considered, defining the proper condenser size considering thedefining the proper condenser size considering the water spray option

Dry & spray technology will be more convenient in• Dry & spray technology will be more convenient in solar power generation plants, where production is concentrated during the hot hours of the dayconcentrated during the hot hours of the day