Comparison of Enhanced Organic Rankine Cycles for Geothermal Power Units

Paola Bombarda Mario Gaia Costante Invernizzi Claudio Pietra

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Exploitation of geothermal sources: approach used in this study

Focus on liquid-phase sources •  exploitation of low temperature sources •  avoid pollutant emissions and reduce scaling, possibly by

means of geothermal fluid pressurization •  full reinjection possible

Adoption of state of the art technology + innovative cycle configuration => no relevant operating or investment risk

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

An important challenge in binary cycle design: the improvement of the heat introduction process

Several improved solutions available with respect to the base single level ORC: Multilevel evaporation cycle, Kalina cycle, supercritical cycle, zeotropic mixture cycle…….

Aim of this work: Comparison of innovative, non conventional, potentially advantageous technical solutions based on current technology

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Design criteria: best utilization of the heat exchange surface

HOT SIDE HEAT EXCHANGERS • LARGE SURFACE AREA • EXTENDED SURFACES (various kinds of finned tubes) • PLATE HEX (plate/ shell and plate/ etc) •  INNOVATIVE MATRICES

4

Reducing ∆Tpp :

MINOR INCREASE OF EXCHANGED THERMAL POWER MINOR INCREASE OF EVAPORATION TEMPERATURE

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Design criteria: best utilization of the heat exchange surface

5

Possible improvements of the T-Q diagram, besides two level evaporation:

-”smooth” the sharp edge

-eliminate the sharp edge

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Pinch Point Smoother

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PINCH POINT SMOOTHING: a fraction of the working fluid flow evaporates at lower pressure and it is mechanically compressed to the turbine inlet pressure

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Organic Flash Cycle, arrangement: double FLASH recently proposed in literature (Ho, Mao and Greif 2012)

HEAT EXCHANGER

FLASH CYCLE: heat introduction occurs in liquid phase; vapour is generated by a flash process

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Performance evaluation: basic assumptions

Geothermal fluid temperature Variable in the range 125-150°C Reinjection temperature unconstrained Geothermal fluid volumetric flow 150 l/s Ambient temperature 15°C (as per ISO standard) Condenser cooling medium Air

Power cycle working fluid R245fa Working fluid condensation temperature

30°C

Air cooler electric parasitic consumption, percentage of condensation heat

1%

Heat Exchangers minimum ∆Tpp 2 °C isoentropic/hydraulic mechanical-electrical

Turbine efficiency 0.85 0.97 Pumps efficiency 0.75 0.95 Compressor efficiency 0.8 0.96

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Performance evaluation: selected configurations and optimization

Optimizing function: Net plant power

Reference case: •  Single evaporation level

Enhanced cycles: • Double evaporation level • Pinch Point Smoother

• Double OFC

Optimized parameters

Evaporation temperature

Evaporation temperatures Evaporation temperatures + mass fractions Flash pressures

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Performance comparison: Tgeo 125-150°C

Comparison at constant minimum ∆Tpp : gross power

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Performance comparison: Tgeo 125-150°C

Comparison at constant minimum ∆Tpp : net power

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Performance comparison: Tgeo 125-150°C

Required “U·S”at constant minimum ∆Tpp

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Performance comparison:Tgeo=150°C

Comparison at constant “U·S” (i.e. at roughly the same heat exchanger cost)

Extended comparison, considering also propane and R134a supercritical cycles

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

The answer to the challenge: T-Q diagrams (U·S=const)

double flash

supercritical, propane

supercritical, R134a

two pressure level ORC

base, single pressure level ORC

single pinch point smoother

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Performance comparison:Tgeo=150°C

Comparison at constant “U·S” (i.e. at roughly the same heat exchanger cost)

Extended comparison, with supercritical cycles

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Performance comparison: Tgeo 125-150°C

Comparison at constant “U·S”

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Conclusions • The two pressure level emerges as the best solution, followed by the Pinch Point Smoother solution, giving a definite advantage compared to the base single pressure level ORC • The Pinch Point Smoother solution could be a feasible option for plant retrofit • Simple supercritical cycles have performance similar to the two pressure level cycle, though more demanding in terms of maximum cycle pressure • The double FLASH cycle solution seems not attractive, at least with HFC245fa in standard geothermal applications; it could however be attractive in special cases, e.g. with very high pressure geothermal fluids.

P. Bombarda

M. Gaia

C. Invernizzi

C. Pietra

Thank you for attention Paola Bombarda

paola.bombarda@polimi.it Gecos Group - http://www.gecos.polimi.it/

Politecnico di Milano