Investigation of Supercritical CO₂ Rankine Cycles for Geothermal Power Plants Presented to

Supercritical CO₂ Power Symposium Boulder, CO May 25, 2011 A.S. Sabau, H. Yin, L. Qualls, J. McFarlane By

Lou Qualls Oak Ridge National Laboratory [email protected]

Project Overview • Measuring fluid properties near the critical point • Performing power cycle analysis • Confirming performance experimentally

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Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

There are numerous locations in the US with accessible moderate and low temperature geothermal energy sources

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Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

Organic Rankine Cycles The organic Rankine cycle for the extraction of geothermal energy is an economical and reliable technology but it generally has low conversion efficiency 6 5 H eat exchanger / Cond ensor

Internal heat exchanger

1

Pump

H eat exchanger / Evap orator

2

3

Turbine

4

The WinEagle power plant (Barber-Nichols) produces about 700 kW from relatively low temperature (115° C (240° F)) geothermal water . The plant has demonstrated 98% availability. 4

Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

S- CO₂ power conversion systems •S- CO₂ appears to be competitive with ORC as a low source temperature power system. •Calculations indicate that mixing CO₂ with other fluids can -Alter critical point •Adapt to source or sink temperature •Standardize component fabrication -Improve plant performance •Multiple compression and cooling stages are required to attain optimal efficiency.

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Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

Closed form solutions to the equations modeling cycle performance require an iterative process Coupled components require sequential analysis in which the solution for the output parameters of one component is advanced to the next component as input Heater and ReHeaters

7204.99 kw

Fluid

Courtesy of S. Wright, SNL

CO2 (0.9) Butane (0.1)

Compressors K

Turbines

838. kwkw

C

B

A

959. kw

877. kw

376.

550.

kw

kw

320.00 K 448.4 kw Gas

50

8623 kPa

kg/s

403.46 K 22241 kPa

Pratio 2.64 5904.46 kw

GenPwr.Net = 1300.53 kW Flow Split 0.8272

Efficiency = 0.181 Recup

8.64

6706.41 kw 335.89 K

6

kg/s 335.89 K 41.36 kg/s

Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

Non-linear property variation near the critical point tends to make the solution unstable CO2

Courtesy of T. Conboy, SNL

Cp at various pressures as a function of temperature. 7

Density, thermal conductivity, and viscosity of CO₂ at a constant pressure of 8.5 MPa as a function of temperature.

Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

Developed a robust solver to accurately calculate state variables for geothermal power cycles – Supercritical and transcritical – Multiple fluids including mixtures – Uses Newton-Raphson solution technique – Demonstrated that it is a robust solution tool – Using to perform S- CO₂ + X mixture trade studies 8

Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

The solver is robust and efficient •Case 1 indicates initial values do not influence the final results of the simulation •Convergence generally requires between 10 and 20 iterations Input Parameters/cases P1 (MPa) T1 (°C) Cycle pressure ratio Brine inlet temp. (°C) Brine flow rate (kg/s) Recuperator area (m2) Numerical ΔT_e (°C) parameters ΔT_c (°C) ΔT_i (°C) Δh_i (kJ/K) Computed thermal efficiency Computed outlet Tc [oC] Computed condenser area (m2) Computed recuperator effectiveness

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Managed by UT-Battelle for the U.S. Department of Energy

1-a 6 20 1.4 90 30 6 4 4 4 10

1-b 6 20 1.4 90 30 6 15 4 4 10

1-c 6 20 1.4 90 30 6 26 10 1 1

104 0.156

104 0.156

104 0.156

Temperature at the turbine inlet vs. Number of iterations A. L. Qualls, 25 May 11

Thermal Efficiency Preliminary Investigations Using the Solver • Case 2: Effects of recuperator on η • Case 3: Effects of brine temperature on η • Case 4: Effect of pump ratio on η • Case 5: Effects of inlet pump pressure on ηtranscritical • Case 6: Effects of inlet pump pressure on ηsupercritical

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Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

Case 2: Recuperator Effects P-h thermodynamic cycle representation of Case 2-b

P-h cycle diagram for several recuperator configurations (cases 2a, b, and d).

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Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

Case 2: Recuperator Effects • η increases with increased recuperator area – Brine temperature raised and held constant and recuperator area varied

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Managed by UT-Battelle for the U.S. Department of Energy

Input Parameters/cases P1 (MPa) T1 (°C) Cycle pressure ratio Brine inlet temp. (°C) Brine flow rate (kg/s) Recuperator area (m2) Numerical ΔT_e (°C) parameters ΔT_c (°C) ΔT_i (°C) Δh_i (kJ/K) Computed thermal efficiency Computed outlet Tc [oC] Computed condenser area (m2) Computed recuperator effectiveness

2-a 6 20 1.4 120 30 None 4

2-b 6 20 1.4 120 30 6 4

2-c 6 20 1.4 120 30 18 4

2-d 6 20 1.4 120 30 30 10

3.9

4.09

4.82

5.48

23.3 123

22.7 122

21.4 118

20.4 112

None

0.131

0.217

0.226

A. L. Qualls, 25 May 11

Case 3: Brine temperature effects • η increases with higher brine temperatures – Higher brine temperatures increase change in enthalpy over the evaporator Input Parameters/cases P1 (MPa) T1 (°C) Cycle pressure ratio Brine inlet temp. (°C) Brine flow rate (kg/s) Recuperator area (m2) Numerical ΔT_e (°C) parameter ΔT_c (°C) s ΔT_i (°C) Δh_i

3-a

3-b

3-c

3-d

6 20 1.4 104 30 6 4

6 20 1.4 120 30 6 4

6 20 1.4 140 30 6 4

6 20 1.4 160 30 6 10

116

122

125

128

0.152

0.131

0.115

0.103

(kJ/K)

Computed thermal efficiency Computed outlet Tc [oC] Computed condenser area (m2) Computed recuperator effectiveness 13

P-h cycle diagram for various brine temperatures (case 3-b, c, and d).

Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

Case 4: Pump Ratio Effect • There is an optimal pressure ratio to increase η • η increases initially with increasing pressure ratio, but decreases after reaching the optimal pressure • Optimal pressure ratio found to be 2.2 Input Parameters/cases P1 (MPa)

4‐a

4‐b

4‐c

4‐d

4‐e

4‐f

4‐g

4‐h

6

6

6

6

6

6

6

6

T1(˚C)

20

20

20

20

20

20

20

20

Pressure Ratios

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

Brine Inlet Temp (˚C)

160

160

160

160

160

160

160

160

Brine Flow Rate (kg/s)

120

120

120

120

120

120

120

120

Condenser Area (m^2)

15.2

15.2

15.2

15.2

15.2

15.2

15.2

15.2

6

6

6

6

6

6

6

6

0.103

0.103

0.103

0.103

0.103

0.103

0.103

0.103

10

10

10

10

10

10

10

10

Computed Cycle Efficiency

4.44

6.24

7.39

8.05

8.33

8.24

7.92

7.26

Computed outlet Tc (˚C)

17.7

17.5

17.29

17.1

17

16.8

16.7

16.6

Recuperator Area (m^2) Recuperator effectiveness ΔT_e (˚C)

Maximum η 14

Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

Case 5 & 6: Inlet pump pressure effect 6

5 H eat ex ch an g er / C on d en sor

Transcritical

Pump

Turbine

4

Input Parameters/cases

5

Input Parameters/cases

6a_20

6b_20

6c_20

6d_20

Inlet pressure of pump (MPa)

5

Inlet pressure of pump (MPa)

5

5.6

6.2

6.8

Inlet temperature of pump (°C)

14.2

Inlet temperature of pump (°C)

15

20

25

28

Cycle pressure ratio

2.4

Cycle pressure ratio

2.4

2.143

1.935

1.765

Brine inlet temperature (°C)

115

Brine inlet temperature (°C)

160

160

160

160

Brine flow rate (kg/s)

32

Brine flow rate (kg/s)

120

120

120

120

Evaporator area (m2)

20

Evaporator area (m2)

20

20

20

20

Recuperator area (m2)

6

Recuperator area (m2)

6

6

6

6

Computed condenser area (m2)

72

Computed condenser area (m2)

22

18

16

17

η≈10% 15

3

H eat ex ch an ger / Ev ap or ator

2

1

Supercritical

Internal heat exchanger

η≈12.5%

Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

Case 6: Inlet pump pressure effect (supercritical) P-h cycle with evaporator area 6m2

P-h cycle with evaporator area 10m2

P-h cycle with evaporator area 20m2

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Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

Case 6: Inlet pump pressure effect (supercritical) • Two methods of increasing η

Computed thermal efficiency [%]

• η increases with increased condenser pressure and evaporator area>10 m2 • η increases with decreased condenser pressure and evaporator area<10 m2

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13 12 11 10 A =6 E 9

A =10

8

E

p5.0 p5.6 p6.2 p6.8

A =20 E

7 6 5 6

8

10 12 14 16 18 2 Evaporator Area [m ]

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Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

Recent work at ORNL and SNL has demonstrated the ability to raise and lower the critical temperature using mixtures

• Work is on-going to measure the thermophysical properties near the critical point for selected mixtures • Mixtures tend to have complex behavior

• The solver will be used to investigate more realistic cycles with better known thermophysical properties

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Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11

Conclusions • ORNL has developed a solver to investigate supercritical and transcritical Brayton power cycles • Preliminary investigation demonstrates the routine is stable and robust • Using this solver, the use of supercritical fluid mixtures will be investigated for low temperature geothermal power systems – adapt to different or changing source and sink temperatures

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Managed by UT-Battelle for the U.S. Department of Energy A. L. Qualls, 25 May 11