combined cycle engine cascades achieving high efficiency . Presented - - PowerPoint PPT Presentation

combined cycle engine cascades achieving high efficiency

.

Andy Schroder

Recent Graduate Department of Aerospace Engineering University of Cincinnati Email: info@AndySchroder.com

Mark Turner

Associate Professor Department of Aerospace Engineering University of Cincinnati Cincinnati, Ohio 45221, U.S.A.

Rory Roberts

Associate Professor Department of Mechanical and Materials Engineering Wright State University Dayton, Ohio 45435, U.S.A.

Presented at the ASME TURBO EXPO, June 13th-17th, 2016, Seoul, South Korea

• utline

Introduction Supercritical CO2 Heat Exchanger and Cycle Analysis Combined Cycle Engine Cascades Conclusions

1

introduction .

introduction

∙ Supercritical Carbon Dioxide (S-CO2) Power cycles can possess some favorable qualities of both the Rankine and Brayton cycles. ∙ S-CO2 Power cycles are typically proposed as an alternative or compliment to traditional Rankine and Brayton cycle engines. ∙ Because of their complexity, a S-CO2 engine has not yet been installed into production use. ∙ Ongoing research and development aims to make such engines a reality. The present work seeks to help those efforts and understand if these engines can provide an advantage in combined cycle configurations.

3

about supercritical co2 (s-co2) power cycles

∙ Closed loop configuration. ∙ Main compressor inlet temperature and pressure are at or near the critical point. ∙ Carbon dioxide is the proposed working fluid because it is cheap, inert, and has a critical temperature of 304K (31◦C), which is near typical ambient temperatures of ∼ 294K (21◦C). ∙ High system pressures occur due to the high critical pressure of carbon dioxide (7.4 MPa).

4

carbon dioxide - cp vs temperature

1.4 MPa 2.4 MPa 5.4 MPa 6.4 MPa 7.4 MPa 8.4 MPa 9.4 MPa 10.4 MPa 11.4 MPa 12.4 MPa 20.4 MPa 300. 400. 0.000 5.00 10.0 15.0

Temperature (K) Cp (kJ/kg-K)

1.4 MPa 2.4 MPa 5.4 MPa 6.4 MPa 7.4 MPa 8.4 MPa 9.4 MPa 10.4 MPa 11.4 MPa 12.4 MPa 20.4 MPa 300. 400. 0.000 5.00 10.0 15.0

Temperature (K) Cp (kJ/kg-K)

1.4 MPa 2.4 MPa 5.4 MPa 6.4 MPa 7.4 MPa 8.4 MPa 9.4 MPa 10.4 MPa 11.4 MPa 12.4 MPa 20.4 MPa

5

supercritical co2 power cycle - strengths

∙ Low Pressure Ratio ∙ Large amounts of recuperation possible. ∙ Low back work ratio: Decreased sensitivity of compressor/turbine efficiency on cycle efficiency. ∙ High Power Density

∙ High pressure and high molecular weight. ∙ Fluid densities range from ∼23 kg/m3 to ∼788 kg/m3.

∙ High exergy efficiencies.

6

supercritical co2 power cycle - weaknesses

∙ Nonlinear specific heat mismatch causes difficulties exchanging heat between high and low pressure sides at lower temperatures. ∙ Heating power in recuperators can be 350% of the net output power and 180% of the input heating power. ∙ Closed loop design presents additional system complexities. ∙ High pressures present increased structural loading and seal leakage issues. ∙ Nonlinear property variations near the critical point present turbomachinery design complications as well as challenges maintaining off design operability. ∙ High working fluid densities prohibit efficient low power, low speed, low cost prototypes to be developed.

7

supercritical co2 heat exchanger and cycle analysis .

layout for a stand alone cycle (with reheat)

tank

Heater Cooler

High Temperature Recuperator

Low Temperature Recuperator Total Mass Fraction

Medium Temperature Recuperator

Low Temperature Recuperator Main Mass Fraction

Cooler

ReHeater

Cooler

Generator tank Starter Starter

Recompression Mass Fraction

Main Mass Fraction

Total Mass Fraction

Main R eC PreC 6 6 6 6 5 5 4 4 3 3 2 2 1 1 7 7 7 7 8 8 9 9 10 10 11 11 12 12 13 13 14 14 14 15 15 4 7 Power AC Electricity

∙ Three compressors and several flow splits are used to help mitigate heat transfer issues due to specific heat mismatches. ∙ Four shafts are utilized to better match optimal

• perating speeds of each turbomachinery

component. ∙ Due to the small size of the turbomachinery, as well as the use of multiple shafts, each assembly (except for the power turbine and generator) can be placed inside a pressure vessel to avoid the need for high speed, high pressure seals. ∙ Tanks and a blow down startup procedure are used to eliminate the need to attach a motor to the higher speed shafts.

Thermal Efficiency 49.6% Exergy Efficiency 75.9%

9

layout for a stand alone cycle (with reheat)

tank

Heater Cooler

High Temperature Recuperator

Low Temperature Recuperator Total Mass Fraction

Medium Temperature Recuperator

Low Temperature Recuperator Main Mass Fraction

Cooler

ReHeater

Cooler

Generator tank Starter Starter

Recompression Mass Fraction

Main Mass Fraction

Total Mass Fraction

Main R eC PreC 6 6 6 6 5 5 4 4 3 3 2 2 1 1 7 7 7 7 8 8 9 9 10 10 11 11 12 12 13 13 14 14 14 15 15 4 7 Power AC Electricity 1,000 1,500 2,000 2,500 3,000 3,500 4,000 Entropy [J/(kg)] 300 400 500 600 700 800 900 1,000 Temperature [K]

13

1 2

4 6 7

8 9

5 3 10 14 15 11 12 Constant Pressure Lines 10.06MPa 10.00MPa 20.47MPa 20.39MPa 20.39MPa 20.19MPa 8.24MPa 8.18MPa 2.75MPa 2.52MPa

Line widths scaled by mass fraction.

10

heat exchanger mass flow differences

tank

Heater

High Temperature Recuperator Medium Temperature Recuperator Starter

Recompression Mass Fraction

R eC 6 6 5 5 4 4 3 7 8 9 10 10 11 14 4 P

1,000 1,500 2,000 2,500 3,000 3,500 4,000 Entropy [J/(kg)] 300 400 500 600 700 800 900 1,000 Temperature [K]

13

1 2

4 6 7

8 9

5 3 10 14 15 11 12 Constant Pressure Lines 10.06MPa 10.00MPa 20.47MPa 20.39MPa 20.39MPa 20.19MPa 8.24MPa 8.18MPa 2.75MPa 2.52MPa

Line widths scaled by mass fraction.

11

variable property heat engine cycle analysis code

∙ A thermodynamic cycle analysis code was created from scratch using Python. ∙ Variable fluid properties are implemented as a function of both temperature and pressure using REFPROP. ∙ 0-D counterflow heat exchanger model was developed to account for variable fluid properties, yet maintaining high solution speed. ∙ Design space for the inputs is explored in parallel and can run on as many processors as are available.

12

0-d heat exchanger modeling

∙ Minimum temperature difference is defined instead of an effectiveness or surface area and convection coefficients. ∙ Pressure drop is not computed based on an assumed geometry, but is approximated to be linearly dependent upon temperature drop in the heat exchanger. ∙ Initial guess for the location of the minimum temperature difference and the corresponding unknown boundaries is made by comparing heat capacities of each fluid stream. ∙ A root finding technique is used with the initially guessed heat exchanger minimum temperature difference and unknown boundaries in order to find the actual minimum temperature difference and unknown boundaries.

13

heat exchangers - temperature and specific heat variation

300 320 340 360 380 400 420 440 Temperature, Cooled Side, [K] 500 1000 1500 2000 2500 3000 cp, [J/(kg*K)] and C, [J/(kgCooled*K)]

cp,Cooled cp,Heated CCooled CHeated

300 320 340 360 380 400 420 440 Temperature, Cooled Side, [K] 5 10 15 20 ∆T =TCooled−THeated, [K]

∆T CHeated/CCooled 1

0.0 0.5 1.0 1.5 2.0 Heat Capacity Ratio, CHeated/CCooled Cooled Side Inlet: Temperature=450.0K, Pressure=8.0MPa, Mass Fraction=1.00 Heated Side Inlet: Temperature=305.0K, Pressure=18.5MPa, Mass Fraction=0.6000 ∆Tmin=5.0 K, Pressure Drop=0 Pa/K, Inlet Pressure Ratio=2.3, φ=0.57, ε=0.98

14

cycle optimization constraints

Parameter Minimum Maximum PreCompressor Pressure Ratio 1.0 4.0 Main Compressor Pressure Ratio 1.1 4.1 Recompression Fraction 0.000 0.991 Low Temperature Recuperator Main Fraction High Pressure Com- ponent Mass Fraction 0.001 0.991 Main Compressor Outlet Pressure 2 MPa 35 MPa Maximum Temperature 923 K [650◦C] 923 K [650◦C] Minimum Temperature 306 K [33◦C] 306 K [33◦C] Main Compressor Isentropic Efficiency 0.850 0.850 PreCompressor Isentropic Efficiency 0.875 0.875 ReCompressor Isentropic Efficiency 0.875 0.875 Power Turbine Isentropic Efficiency 0.930 0.930 Main/Re/Pre Compressor Turbine Isentropic Efficiency 0.890 0.890 Heat Exchanger Minimum Temperature Difference 5 K 5 K Heat Exchanger Pressure Drop 500 Pa/K 500 Pa/K

15

combined cycle engine cascades .

general topping cycle with optional fuel cell

Generator Cool Intake Air

CH4

AC Electricity

O2 Pressurized Methane (CH4) Fuel N2 O2 N2 CO2

Electrolyte Anode Cathode

Heat Generation Heat Generation Heat Generation

DC Electricity

H2O H2O CO2

Combustor

Solid Oxide Fuel Cell

ηc = 84.0% ηt = 90.0% PRc = fixed at 37.15 (with fuel cell), optimized but limited to 45.00 (without fuel cell) Turbine Inlet Temperature = 1,500 K [1,227◦C] (with fuel cell), 1,890K [1,617◦C] (without fuel cell) Fuel Cell Excess Air = 26.3% Fuel Cell Fuel Utilization = 80.0% Fuel Cell Electrochemical Efficiency = 58.5% (HHV), 65.0% (LHV)

17

intermediate and bottoming engines (no reheat)

Waste Heat Waste Heat AC Electricity

tank

Cooler

High Temperature Recuperator

Low Temperature Recuperator Total Mass Fraction

Medium Temperature Recuperator

Low Temperature Recuperator Main Mass Fraction

Cooler Cooler

Generator tank Starter Starter

Recompression Mass Fraction Main Mass Fraction Total Mass Fraction

Main R eC PreC 6 6 6 6 5 5 4 4 3 3 2 2 1 1 7 7 7 8 9 9 10 10 11 11 12 12 13 13 14 14 14 15 15 4 7 Power

Heat Exchanger

18

general combined cycle engine

Engine 1: Low/Medium Pressure T

• pping Cycle (Airbreathing Gas T

urbine with Fuel Cell) Engine 2: High Pressure Intermediate Cycle (S-CO2 Engine) Engine 3: High Pressure Intermediate Cycle (S-CO2 Engine) Engine 4: High Pressure Bottoming Cycle (S-CO2 Engine)

Hot Exhaust Warm Exhaust Gases

Waste Heat Waste Heat AC Electricity

Waste Heat Waste Heat AC Electricity

Waste Heat Waste Heat AC Electricity

Generator Cool Intake Air

CH4 AC Electricity O2 Pressurized Methane (CH4) Fuel N2 O2 N2 CO2

DC Electricity H2O H2O CO2

Solid Oxide Fuel Cell

19

combined cycle engine

200 400 600 800 1000 1200 1400 1600 Temperature [C] 1,000 1,500 2,000 2,500 3,000 Entropy [J/(kg*K)] 400 600 800 1,000 1,200 1,400 1,600 1,800 Temperature [K] Combined Cycle Efficiency: 64.95% Line widths scaled by mass fraction. Air cycle entropy reference is arbitrary and does not follow the same conventions as CO2. Combined Cycle Efficiency: 64.95% Line widths scaled by mass fraction. Air cycle entropy reference is arbitrary and does not follow the same conventions as CO2.

Engine Work Fraction Marginal Combined Cycle Efficiency Engine Efficiency Engine Exergy Efficiency Type Number % % % % Gas Turbine 1 70.05 45.49 45.49 54.28 S − CO2 Engine 2 18.60 12.08 49.59 75.02 S − CO2 Engine 3 9.45 6.14 33.53 63.79 S − CO2 Engine 4 1.90 1.23 14.14 46.10 Combined 100.00 64.95 64.95 77.5

20

engine number 2: s − co2 cycle, temperature entropy diagram

80 160 240 320 400 480 560 640 Temperature [C] 1,000 1,500 2,000 2,500 3,000 Entropy [J/(kg*K)] 300 400 500 600 700 800 900 Temperature [K]

13 1 2 4 6 8 9 5 3 10 11 14 Critical Temperature: 304.13K Critical Pressure: 7.377MPa Constant Pressure Lines 8.20MPa 8.18MPa 32.40MPa 32.35MPa 32.35MPa 32.12MPa 6.25MPa 6.06MPa

800 1,220 1,640 2,060 2,480 2,900 3,320 3,740 4,160 4,580 5,000 cp , Specific Heat at Constant Pressure [J/(kg*K)] Cycle Efficiency: 49.59% Line widths scaled by mass fraction.

21

engine number 3: s − co2 cycle, temperature entropy diagram

60 120 180 240 300 360 420 Temperature [C] 1,000 1,500 2,000 2,500 3,000 Entropy [J/(kg*K)] 300 400 500 600 700 Temperature [K]

13 1 2 4 6 8 9 5 3 10 11 14 Critical Temperature: 304.13K Critical Pressure: 7.377MPa Constant Pressure Lines 13.19MPa 13.18MPa 34.35MPa 34.33MPa 34.33MPa 34.19MPa 8.14MPa 8.05MPa

800 1,220 1,640 2,060 2,480 2,900 3,320 3,740 4,160 4,580 5,000 cp , Specific Heat at Constant Pressure [J/(kg*K)] Cycle Efficiency: 33.53% Line widths scaled by mass fraction.

22

engine number 4: s − co2 cycle, temperature entropy diagram

30 30 60 90 120 150 180 210 Temperature [C] 1,000 1,200 1,400 1,600 1,800 2,000 Entropy [J/(kg*K)] 250 300 350 400 450 500 Temperature [K]

13 1 2 4 6 8 9 5 3 10 14 15 11 12 Constant Pressure Lines 17.85MPa 17.84MPa 25.13MPa 25.12MPa 25.12MPa 25.07MPa 8.46MPa 8.44MPa

800 1,220 1,640 2,060 2,480 2,900 3,320 3,740 4,160 4,580 5,000 cp , Specific Heat at Constant Pressure [J/(kg*K)] Cycle Efficiency: 14.14% Line widths scaled by mass fraction.

23

efficiency vs s − co2 engine peak pressure & topping cycle turbine inlet temp

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Main Compressor Outlet Pressure [Pa] 1e7 63.0 63.2 63.4 63.6 63.8 64.0 64.2 64.4 64.6 64.8 65.0 Cycle Efficiency [%] 1200 1300 1400 1500 1600 1700 1800 1900 Gas Turbine Rotor Inlet Temperature [K] 48 50 52 54 56 58 60 62 64 66 Cycle Efficiency [%]

306.0 K 310.4 K 314.8 K 319.2 K 323.6 K 328.0 K

Maximum Thermal Efficiency=65.01%

24

efficiency vs number of engines & topping cycle comp isentropic efficiency

1 2 3 4 5 6 Maximum Allowable Number of Engines 46 48 50 52 54 56 58 60 62 64 66 Cycle Efficiency [%] 0.80 0.82 0.84 0.86 0.88 0.90 Gas Turbine Compressor Isentropic Efficiency 64.00 64.25 64.50 64.75 65.00 65.25 65.50 65.75 66.00 66.25 66.50 Cycle Efficiency [%]

25

combined cycle engine with fuel cell

200 400 600 800 1000 1200 1400 1600 Temperature [C] 1,000 1,500 2,000 2,500 3,000 Entropy [J/(kg*K)] 400 600 800 1,000 1,200 1,400 1,600 1,800 Temperature [K] Combined Cycle Efficiency: 65.84% (HHV), 73.09% (LHV) Line widths scaled by mass fraction. Air cycle entropy reference is arbitrary and does not follow the same conventions as CO2. Combined Cycle Efficiency: 65.84% (HHV), 73.09% (LHV) Line widths scaled by mass fraction. Air cycle entropy reference is arbitrary and does not follow the same conventions as CO2.

Engine Work Fraction Marginal Combined Cycle Efficiency Engine Efficiency Engine Exergy Efficiency Type Number % HHV, % LHV, % % % Fuel Cell 1 71.14 91.15 46.84 60.01 52.00 66.63 52.00 (LHV) 66.63 (LHV)

• Gas Turbine

20.01 13.17 14.63 30.47 (LHV) S − CO2 Engine 2 6.44 4.24 4.71 41.00 69.99 S − CO2 Engine 3 2.41 1.59 1.76 23.02 55.52 Combined 100.00 65.84 73.09%

• 26

conclusions .

novelty of the current work

∙ Combined cycle configurations using supercritical carbon dioxide power cycles in conjunction with a fuel cell and gas turbine has been explored and optimized. ∙ A unique shaft layout and startup procedure are used in conjunction with a series of intermediate and bottoming engines. ∙ With the multi-shaft configuration, turbomachinery can be placed inside pressure vessels to avoid high pressure ratio seals. ∙ A custom variable property cycle analysis code was developed and used. ∙ A combined cycle efficiency of 64.95% was predicted for the combined cycle without a fuel cell with a turbine inlet temperature of 1,890 K [1,617◦C] and a rejected heat temperature of 306 K [33◦C]. ∙ A combined cycle efficiency of 73.09% was predicted for the combined cycle with a fuel cell with a rejected heat temperature of 306 K [33◦C].

28

conclusions

∙ Supercritical CO2 Power Cycles have the potential for high efficiencies at low turbine inlet temperatures. ∙ Highly nonlinear fluid properties present significant challenges in cycle and component design. ∙ Appropriate modeling of heat exchangers is critical in assessing correct cycle performance. ∙ Supercritical carbon dioxide power cycles can be very beneficial in combined cycle configurations, provided multiple supercritical carbon dioxide power cycles are used and each cycle is optimized individually.

29

recommended future work

∙ Allow for variable turbomachinery efficiencies which are dependent on the inlet conditions and pressure ratio. ∙ Improve pressure drop relationships for heat exchangers in the 0-D heat exchanger solver. ∙ Support condensation and boiling in heat exchangers. ∙ Further investigate the use of CoolProp as a replacement for REFPROP. ∙ Incorporate a cost model into the cycle optimization process. ∙ Conduct preliminary design and numerical simulations of turbomachinery components.

30

Questions?

31

selected temperatures: combined cycle

Engine Exhaust Gas Heat Exchanger Power Turbine Main Compressor Type Number Inlet Temperature Outlet Temperature Exit Temperature Exit Temperature K [◦C] K [◦C] K [◦C] K [◦C] Gas Turbine 1

• 903 [630]

903 [630] 925 [652] S − CO2 Engine 2 903 [630] 645 [372] 698 [425] 348 [75] S − CO2 Engine 3 645 [372] 441 [168] 494 [221] 329 [56] S − CO2 Engine 4 441 [168] 342 [69] 348 [75] 313 [40]

32

selected temperatures: combined cycle with fuel cell

Engine Exhaust Gas Heat Exchanger Power Turbine Main Compressor Type Number Inlet Temperature Outlet Temperature Exit Temperature Exit Temperature K [◦C] K [◦C] K [◦C] K [◦C] Fuel Cell + Gas Turbine 1

• 739 [466]

739 [466] 923 [650] S − CO2 Engine 2 739 [466] 523 [250] 563 [289] 346 [73] S − CO2 Engine 3 523 [250] 373 [99] 385 [111] 334 [61]

33