Optimal Design of Binary Cycle Power Plants for water-dominated, medium-temperature

geothermal fields

Energy Conversion Technologies 2

Riccardo Pagotto

Outline

!  The Geothermal Energy and Geological settings

!  Binary Cycle Power Plant

!  Analytical aspect: merit parameters of a Binary Cycle Power Plant

!  Performance evaluations

Geothermal Energy and Geological setting

Source:  geo-­‐energy.org  

!  Geothermal gradient (crust)

Extensive settings ≈50-60°C/km

Compressive settings ≈ 15-20°C/km

Avg crust ≈ 30°C/km

1

Source:  Geophysic  Booklet,  Claudia  Pasquero  (2011)  

3

2 Source:  geo-­‐energy.org  

Source:  geo-­‐energy.org  

!  Heat Flux

Avg ≈ 87 mW/km2

BCPP

!  BCPP

Heat Recovery Cycle (HRC)

Recovery Heat Exchanger (RHE)

Cooling System (CS)

!  Tempature’s range

Tgeo 110–160 °C

Trej 70–100 °C

Tcond 30–40 °C

Source:Optimal Design of Binary Cycle Power Plantsfor water-dominated,medium-temperature geothermal fields; A.Franco, M.Villani (2009)

!  Total installed power

worldwide ≈ 700 Mwe

Plant size 500 kW – 10 MW

Efficiency 7 – 12%

Source:Indian Istitute of Technology

HRC

RHE

CS

2

Source:learnthermo.com  

3

Source:learnthermo.com  

Source:Optimal Design of Binary Cycle Power Plantsfor water-dominated, medium-temperature geothermal fields; A.Franco, M.Villani (2009)

1-­‐BCPP  op/mal  design !  BCPP purpose

is minimizing

m [kg/s]

β [kg/MJ]

Analytical Aspect: merit parameters of a BCPP

Q0 = Heat Flow Rate [W]

EX = Energy Flow [W]

EX0 = Energy Flow [W]

Reference state, T0 = 25°C Source:Optimal Design of Binary Cycle Power Plantsfor water-dominated,medium-temperature geothermal fields; A.Franco, M.Villani (2009)

1-­‐First  Law  Efficiency

1

2

2-­‐Second  Law  Efficiency

2

Β = specific brine consumption [kg/MJ]

I = Energy Loss Flow [W] = Σ energy loss into steam turbine, recovery heat exchanger, condenser and rejected

Source:Optimal Design of Binary Cycle Power Plantsfor water-dominated,medium-temperature geothermal fields; A.Franco, M.Villani (2009)

Thermodinamic  phase  diagram   Synthetic refrigerant:

R134a, R401a, R152a

Hydrocarbons:

Isobutane, n-pentane

Performance Evaluation

Source:Optimal Design of Binary Cycle Power Plantsfor water-dominated,medium-temperature geothermal fields; A.Franco, M.Villani (2009)

1-­‐  BCPP,  Poten/al  and  Limits  (Tgeo-­‐Trej  °C)

1

2

2-­‐  BCPP,  Poten/al  and  Limits  (Tgeo-­‐Trej-­‐Tcond  °C)

!  Tested fluids

Hydrocarbons: Isobutane, n-pentane

Syn. refrigerants: R134a, R152a

Mixture: R401A, R407C !  Diagram n°1

Hydrocarbons ! small ΔT

R152a ! large ΔT

!  Diagram n°2

Hydrocarbons ! small ΔT

R152a ! large ΔT

!  Temperature system 160-70-30 (°C)

First Law Efficiency = 12%

Second Law Efficiency = 45%

Conclusion !  Advantages

✓  Binary plants with dry cooling systems represent a sustainable way to exploit low-temperature, water-dominated geothermal fields

✓  No additional water is required and emissions of chemical pollutants (Hg + As) and greenhouse gases are close to zero

✓  Best results obtained with Isobutane and R152a

!  Disadvantages

✗  Geological aspects (depth of the reservoir ! Tgeo, chemical composition of the geofluid and rock permability)

✗  Temperature’s variation ! efficiency variation

✗  Gross Power absorbed by BCPP ≈ 10-12%

✗  Quite high costs ≈ 2000-4000 €/kW ! limited distibution

Source:Optimal Design of Binary Cycle Power Plantsfor water-dominated,medium-temperature geothermal fields; A.Franco, M.Villani (2009)

Assessment of the review

•  Positive: The article gives an exhaustive background on Binary Cycle Power Plant

Completeness: The article includes a general overview, but also a thorough mathematical analysis and a technical description of the power plant

Correctness: The paragraphs are well set up and related, in order to provide a better

understanding of the issue

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