Mapping the Design Space of a Supercritical Carbon Dioxide Power - PowerPoint PPT Presentation

Mapping the Design Space of a Supercritical Carbon Dioxide Power Cycle Andrew Schroder Mark Turner University of Cincinnati Wednesday, March 6 th , 2013 38 th AIAA Dayton-Cincinnati Aerospace Sciences Symposium

Outline ◮ Overview of Supercritical CO 2 Power Cycles ◮ Proposed System Layout ◮ Variable Property Heat Engine Cycle Analysis Code ◮ Heat Exchangers with Nonlinear and Dissimilar Specific Heats ◮ Results of the Design Space Exploration ◮ Conclusions

About Supercritical CO 2 (S-CO 2 ) Power Cycles ◮ Closed loop configuration. ◮ 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). ◮ Possible applications: ◮ Base load terrestrial electrical power generation ◮ Marine, Aviation, and Spacecraft electrical power generation ◮ Possible Configurations: ◮ Bottoming cycle using waste heat from a traditional open loop gas turbine (traditional brayton cycle) ◮ Primary cycle with nuclear and solar energy heat sources ◮ Primary cycle with the combustion of fossil fuels as a heat source

State of the Art ◮ The earliest reference to a supercritical carbon dioxide power cycle is that of a patent by Sulzer in 1948. ◮ Vaclav Dostal revived interest in supercritical carbon dioxide power cycles with the publication of his doctoral thesis in 2004. ◮ Sandia National Laboratories has developed two supercritical CO 2 test rigs with their contractor, Barber-Nichols and has successfully achieved startup of both a main compressor/turbine and recompressor/turbine loop. Their efforts are focused towards nuclear power applications. ◮ Echogen Power Systems has been developing an engine for waste heat recovery applications. ◮ The United States Department of Energy began development of engines for concentrating solar power applications in mid 2012.

Carbon Dioxide - c p vs Temperature 15.0 15.0 8.4 MPa 8.4 MPa 8.4 MPa 7.4 MPa 7.4 MPa 7.4 MPa 10.0 10.0 9.4 MPa 9.4 MPa 9.4 MPa Cp (kJ/kg-K) Cp (kJ/kg-K) 6.4 MPa 6.4 MPa 6.4 MPa 10.4 MPa 10.4 MPa 10.4 MPa 11.4 MPa 11.4 MPa 11.4 MPa 5.00 5.00 12.4 MPa 12.4 MPa 12.4 MPa 5.4 MPa 5.4 MPa 5.4 MPa 20.4 MPa 20.4 MPa 20.4 MPa 2.4 MPa 2.4 MPa 2.4 MPa 1.4 MPa 1.4 MPa 1.4 MPa 0.000 0.000 300. 300. 400. 400. Temperature (K) Temperature (K)

Carbon Dioxide - Enthalpy vs Temperature 600. 600. 20.4 MPa 20.4 MPa 20.4 MPa Enthalpy (kJ/kg) Enthalpy (kJ/kg) 8.4 MPa 8.4 MPa 8.4 MPa 400. 400. 7.4 MPa 7.4 MPa 7.4 MPa 6.4 MPa 6.4 MPa 6.4 MPa 5.4 MPa 5.4 MPa 5.4 MPa 4.4 MPa 4.4 MPa 4.4 MPa 3.4 MPa 3.4 MPa 3.4 MPa 2.4 MPa 2.4 MPa 2.4 MPa 200. 200. 1.4 MPa 1.4 MPa 1.4 MPa 250. 250. 300. 300. 350. 350. 400. 400. Temperature (K) Temperature (K)

Carbon Dioxide - Temperature vs Entropy 800. 800. 20.4 MPa 20.4 MPa 20.4 MPa 600. 600. Temperature (K) Temperature (K) 400. 400. 7.4 MPa 7.4 MPa 7.4 MPa 4.4 MPa 4.4 MPa 4.4 MPa 3.4 MPa 3.4 MPa 3.4 MPa 2.4 MPa 2.4 MPa 2.4 MPa 1.4 MPa 1.4 MPa 1.4 MPa 200. 200. 1.00 1.00 2.00 2.00 3.00 3.00 Entropy (kJ/kg-K) Entropy (kJ/kg-K)

Supercritical CO 2 Power Cycle Strengths ◮ Low Pressure Ratio (optimal overall pressure ∼ 3 to 6) ◮ Large amounts of recuperation possible. ◮ Low back work ratio ◮ Decreased sensitivity of compressor/turbine efficiency on cycle efficiency. ◮ S-CO 2 - ∼ 35% ◮ Rankine - ∼ 2% ◮ Open Loop Brayton - 40-80% ◮ High Power Density ◮ High pressure and high molecular weight. ◮ Fluid densities range from ∼ 23 kg/m 3 to ∼ 788 kg/m 3 . ◮ Narrow heat addition and heat rejection temperatures does not require evaporative cooling, but still approximates a Carnot cycle better than an open loop Brayton cycle. ◮ High real cycle efficiency predicted ◮ > 50% @ 923K (650 ◦ C) turbine inlet temperature

Supercritical CO 2 Power Cycle - Weaknesses ◮ Nonlinear specific heat mismatch causes difficulties exchanging heat between high and low pressure sides at lower temperatures. ◮ Closed loop design presents additional system complexities. ◮ High pressures present increased structural loading and seal leakage issues. ◮ 20MPa to 30MPa maximum pressure typically proposed ◮ 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.

Proposed System Layout ◮ Three compressors and several flow splits are used to help mitigate heat transfer issues tank HOT due to specific heat 6 5 mismatches. ◮ Four shafts are utilized to COLD 2 1 Main better match optimal operating 7 speeds of each turbomachinery 15 component. ◮ Due to the small size of the 4 COLD turbmochinery, as well as the ReC use of multiple shafts, each 7 COLD assembly can be placed inside a 11 12 14 pressure vessle to avoid the 13 PreC need for high speed, high 7 HOT pressure seals. 2 3 ◮ Tanks and a blow down startup tank 7 procedure are used to eliminate 8 the need to attach a motor to Power Generator 10 the higher speed shafts. 4 9 ◮ Provisional application for 5 patent(s) filed.

Proposed System Layout - Temperature Entropy Diagram Temperature [K] 6 8 7.70MPa 900 10.96MPa 7 7.96MPa 4.00MPa 5 9 800 700 600 500 10 4 400 3 14 11 2 15 12 1 13 Entropy [J/kg] 1800 2000 2200 2400 2600 2800 3000

Proposed System Layout - Enthalpy Entropy Diagram Enthalpy [J/kg] 8 6 7.70MPa 10.96MPa 7 1.1e6 7.96MPa 4.00MPa 9 5 1e6 9e5 8e5 7e5 6e5 10 4 11 14 12 3 5e5 15 13 2 1 Entropy [J/kg] 1800 2000 2200 2400 2600 2800 3000

Variable Property Heat Engine Cycle Analysis Code ◮ Cycle analysis code created from scratch. ◮ Developed with Python, NumPy, SciPy, and matplotlib. ◮ Variable fluid properties are utilized. ◮ i.e. h=h(T,p), c p =c p (T,p), s=s(T,p) ◮ Fluid property data tables used from http://webbook.nist.gov/ ◮ Specialized heat exchanger model was developed to account for variable fluid properties, yet maintaining high solution speed. ◮ Cycle iteratively solved for unknown pressures. ◮ Inputs include maximum temperature, minimum temperature, compressor pressure ratios, turbomachinery component efficiencies, heat exchanger pressure drop, main compressor inlet pressure, and mass fraction for flow splits. ◮ Design space for the inputs is explored in parallel and can run on as many machines and processors as are available.

Variable Property Heat Engine Cycle Analysis Code Limitations and Assumptions ◮ Currently the code only supports gases and supercritical fluids. Liquids and and liquid vapor mixtures are not yet supported. ◮ Heat source currently modeled is that of a constant heat flux (i.e. solar) or a highly regenerated combustion system (heater efficiency is assumed to be 100%). ◮ Pumping power for the ambient pressure side of the heaters and coolers are assumed to be low.

Heat Exchangers - Overview ◮ Heatric counterflow diffusion bonded heat exchanger are typically proposed due to their high convection heat transfer rates and high pressure and temperature capability. ◮ Because of the very high convection of the diffusion bonded heat exchangers, the current heat exchanger model assumes the limiting case where convection approaches infinity. ◮ With high convection assumed: ◮ The temperature difference between the high pressure to the low pressure side of the heat exchanger is assumed to be purely due to specific heat mismatches. ◮ At at least one point in the heat exchanger there will be zero (or approximately zero) temperature difference between the high and low pressure side. ◮ Pressure drop is not computed, but is another parameter varied as part of the design space exploration.

Heat Exchangers - Specific Heat Variation Low Pressure Inlet Temperature=400.0K, Low Pressure=9.5MPa, Mass Fraction=1.00 High Pressure Inlet Temperature=326.0K, High Pressure=15.0MPa, Mass Fraction=0.58 Pressure Ratio=1.6 4000 c p,LowPressure c p,HighPressure C LowPressure 3500 C HighPressure c p , [J/(kg*K)] and C, [J/(kg LowPressure *K)] 3000 2500 2000 1500 1000 500 0 320 330 340 350 360 370 380 390 400 Temperature, Low Pressure Side, [K]

Heat Exchangers - Temperature Variation Low Pressure Inlet Temperature=400.0K, Low Pressure=9.5MPa, Mass Fraction=1.00 High Pressure Inlet Temperature=326.0K, High Pressure=15.0MPa, Mass Fraction=0.58 Pressure Ratio=1.6, Effectiveness=0.82 10 2.0 ∆ T 0 C HighPressure /C LowPressure 1 Heat Capacity Ratio, C HighPressure /C LowPressure 8 1.5 ∆ T = T LowPressure − T HighPressure , [K] 6 4 1.0 2 0.5 0 2 0.0 320 330 340 350 360 370 380 390 400 Temperature, Low Pressure Side, [K]

Design Space Exploration Results Cycle Efficiency vs PreCompressor and Main Compressor Pressure Ratios Maximum Efficiency=0.522438561229 0.60 4.0 0.57 3.5 0.54 Main Compressor Pressure Ratio 0.51 3.0 0.48 Cycle Efficiency 0.45 2.5 0.42 2.0 0.39 0.36 1.5 0.33 0.30 1.0 1.5 2.0 2.5 3.0 3.5 4.0 PreCompressor Pressure Ratio Note: The white region in the lower left corner of the figure represents efficiencies ranging from 0.0 to 0.3.