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 6th, 2013 38th AIAA Dayton-Cincinnati Aerospace Sciences Symposium

Outline

◮ Overview of Supercritical CO2 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 CO2 (S-CO2) 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

• f 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

CO2 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

• f engines for concentrating solar power applications in mid

2012.

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

Carbon Dioxide - Enthalpy vs Temperature

1.4 MPa 2.4 MPa 3.4 MPa 4.4 MPa 5.4 MPa 6.4 MPa 7.4 MPa 8.4 MPa 20.4 MPa 250. 300. 350. 400. 200. 400. 600.

Temperature (K) Enthalpy (kJ/kg)

1.4 MPa 2.4 MPa 3.4 MPa 4.4 MPa 5.4 MPa 6.4 MPa 7.4 MPa 8.4 MPa 20.4 MPa 250. 300. 350. 400. 200. 400. 600.

Temperature (K) Enthalpy (kJ/kg)

1.4 MPa 2.4 MPa 3.4 MPa 4.4 MPa 5.4 MPa 6.4 MPa 7.4 MPa 8.4 MPa 20.4 MPa

Carbon Dioxide - Temperature vs Entropy

1.4 MPa 2.4 MPa 3.4 MPa 4.4 MPa 7.4 MPa 20.4 MPa

1.00 2.00 3.00 200. 400. 600. 800.

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

1.4 MPa 2.4 MPa 3.4 MPa 4.4 MPa 7.4 MPa 20.4 MPa

1.00 2.00 3.00 200. 400. 600. 800.

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

1.4 MPa 2.4 MPa 3.4 MPa 4.4 MPa 7.4 MPa 20.4 MPa

Supercritical CO2 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-CO2 - ∼35% ◮ Rankine - ∼2% ◮ Open Loop Brayton - 40-80%

◮ High Power Density

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

◮ 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 CO2 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 due to specific heat mismatches. ◮ Four shafts are utilized to better match optimal operating speeds of each turbomachinery component. ◮ Due to the small size of the turbmochinery, as well as the use of multiple shafts, each assembly can be placed inside a pressure vessle 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. ◮ Provisional application for patent(s) filed.

Main tank ReC PreC Power Generator tank COLD COLD HOT HOT COLD 7 6 8 5 9 10 11 12 13 14 15 4 3 2 1 5 7 7 7 2 4

Proposed System Layout - Temperature Entropy Diagram

1800 2000 2200 2400 2600 2800 3000 Entropy [J/kg] 400 500 600 700 800 900 Temperature [K] 1 13 2 4 6 7 8 9 5 3 10 14 15 11 12

7.70MPa 10.96MPa 7.96MPa 4.00MPa

Proposed System Layout - Enthalpy Entropy Diagram

1800 2000 2200 2400 2600 2800 3000 Entropy [J/kg] 5e5 6e5 7e5 8e5 9e5 1e6 1.1e6 Enthalpy [J/kg] 1 13 2 4 6 7 8 9 5 3 10 14 15 11 12

7.70MPa 10.96MPa 7.96MPa 4.00MPa

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), cp=cp(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

• n 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

320 330 340 350 360 370 380 390 400 Temperature, Low Pressure Side, [K] 500 1000 1500 2000 2500 3000 3500 4000 cp, [J/(kg*K)] and C, [J/(kgLowPressure*K)]

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 cp,LowPressure cp,HighPressure CLowPressure CHighPressure

Heat Exchangers - Temperature Variation

320 330 340 350 360 370 380 390 400 Temperature, Low Pressure Side, [K] 2 2 4 6 8 10 ∆T =TLowPressure−THighPressure, [K]

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

0.0 0.5 1.0 1.5 2.0 Heat Capacity Ratio, CHighPressure/CLowPressure

∆T CHighPressure/CLowPressure 1

Design Space Exploration Results

Cycle Efficiency vs PreCompressor and Main Compressor Pressure Ratios

1.0 1.5 2.0 2.5 3.0 3.5 4.0 PreCompressor Pressure Ratio 1.5 2.0 2.5 3.0 3.5 4.0 Main Compressor Pressure Ratio

Maximum Efficiency=0.522438561229

0.30 0.33 0.36 0.39 0.42 0.45 0.48 0.51 0.54 0.57 0.60

Cycle Efficiency

Note: The white region in the lower left corner of the figure represents efficiencies ranging from 0.0 to 0.3.

Design Space Exploration Results

Optimal Recompression Fraction vs PreCompressor and Main Compressor Pressure Ratios

1.0 1.5 2.0 2.5 3.0 3.5 4.0 PreCompressor Pressure Ratio 1.5 2.0 2.5 3.0 3.5 4.0 Main Compressor Pressure Ratio 0.00 0.08 0.16 0.24 0.32 0.40 0.48 0.56 0.64 0.72 0.80

Recompression Fraction at Maximum Efficiency

Design Space Exploration Results

Cycle Efficiency vs Recompression Fraction

0.0 0.2 0.4 0.6 0.8 1.0 Recompression Fraction 0.46 0.47 0.48 0.49 0.50 0.51 0.52 0.53 Cycle Efficiency

Maximum Efficiency=0.522438561229

Web Based Graphical User Interface

Conclusions

◮ Supercritical CO2 Power Cycles have potential for

improvements in efficiency over traditional power cycles or traditional combined cycle power plants, at a dramatically reduced system size.

◮ A new system layout has been presented which may help to

eliminate some of the design challenges with supercritical carbon dioxide engines.

◮ Highly nonlinear fluid properties present significant challenges

in cycle and component design.

◮ A cycle analysis code has been developed, along with a web

based GUI for exploring the design space. These tools will be continually expanded and improved to better understand supercritical carbon dioxide power cycles.

Questions?

http://AndySchroder.com/CO2Cycle/