INVESTIGATION OF SCO2 AIR-COOLED HEAT EXCHANGERS FOR APPLICATION IN CSP CYCLES

ANDREW LOCKPHD CANDIDATE - UNIVERSITY OF QUEENSLAND

sCO2 COOLING BACKGROUND

sCO2 Brayton cycle is proposed for CSP - benefits include compact equipment and high efficiencies.

Cycle modelling has shown direct sCO2 cooling system (sCO2–air) likely to be most economical.

Past sCO2 cooling research is focussed on HVAC, small tubes, or CFD.

Minimal past research focussed on sCO2-air heat rejection for power systems.

COOLING SYSTEM DESIGN PARAMETERS

Cycle efficiency is sensitive to cycle minimum temperature (CIT).

Cooling system pressure drop decreases cycle efficiency. (1% Δɳ ≈ 100 kPa ΔpCT )

Effect of: CIT on cycle efficiency (left); and pre-cooler pressure drop on cycle efficiency (right). Turbine inlet conditions: 610 °C and 20 MPa.

30 35 40 45 50 550.45

0.46

0.47

0.48

0.49

0.50

CIT ( °C)

ηth

PTO = 8 MPa

PTO = 9 MPa

NUMERICAL HEAT EXCHANGE MODEL

Heat exchanger arrangement: 4 rows, 4 passes

Heat exchanger arrangement: 4 rows, 2 passes

Heat exchanger arrangement: 4 rows, 1 pass

TWC

TWH

TPF,i

Tair,j

Pair,j

PPF,i

TPF,i+1

Tair,j+1

Air

Wall

Process Fluid

NUMERICAL HEAT EXCHANGE MODEL

Qa,BC,in Qa,BC,out

Qa,conv

Qcond

QPF,conv

ṁa,inṁa,out

QPF,BC,outQPF,BC,in

QPF,cond,out QPF,cond,in

Qwall,cond,out Qwall,cond,in

ṁPF,out ṁPF,in

Further developed an existing Python code developed at UQ1,2 for air-cooled cross flow HX.

Solves equations in each cell for:

1. energy conservation

2. continuity

3. momentum (pressure).

Can solve with temperature, pressure, or mass flow rate boundary conditions.

1. JAHN, I. H. J. (2017), CODE FOR THE DESIGN AND EVALUATION OF EXCHANGERS FOR COMPLEX FLUIDS, MECHANICAL ENGINEERING TECHNICAL REPORT 2017/04, THE UNIVERSITY OF QUEENSLAND, AUSTRALIA,2. JAHN, I. H. J. (2017), CODE FOR THE DESIGN AND EVALUATION OF HEAT EXCHANGERS OPERATING WITH COMPLEX FLUIDS, THE JOURNAL OF OPEN SOURCE SOFTWARE, 2017

MODEL VALIDATION

• Validated for multiple cases against commercial software ASPEN Exchanger Design and Rating (V10), using similar heat transfer correlations.

• Differences in ΔT between models typically <0.2%.

• Differences attributed to different discretisation, metal material properties, ASPEN fluid property table interpolation.

Example of model validation with Aspen commercial software (H2O case)

WHAT DOES THIS MODEL CONTRIBUTE?

1. Accurate properties for non-linear fluids – uses CoolProp1 EoS properties for each node.

2. Modelling in high detail (250+ elements).

3. Calculates internal wall temperature - important for accurately predicting sCO2 behaviour.

4. Can compare correlations, and be integrated into other models (i.e. natural draft cooling tower model).

1. BELL, I. H., WRONSKI, J., QUOILIN, S., & LEMORT, V. (2014). PURE AND PSEUDO-PURE FLUID THERMOPHYSICAL PROPERTY EVALUATION AND THE OPEN-SOURCE THERMOPHYSICAL PROPERTY LIBRARY COOLPROP. INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH, 53(6), 2498-2508. DOI:10.1021/IE4033999

MODEL FLUID CORRELATIONSFluid Property Correlation(s) Reference

Internal fluid Nusselt Number

Gnielinski Gnielinski, V. (1975). Neue Gleichungen für den Wärme- und den Stoffübergang in turbulent durchströmten Rohren und Kanälen. Forschung im Ingenieurwesen, 41(1), 8-16. doi:10.1007/bf02559682

ASPEN “SM2” AspenTech, Aspen Exchanger Design and Rating V10, computer program.

Yoon et al. (sCO2 specific) Yoon, S. H., Kim, J. H., Hwang, Y. W., Kim, M. S., Min, K., & Kim, Y. (2003). Heat transfer and pressure drop characteristics during the in-tube cooling process of carbon dioxide in the supercritical region. International Journal of Refrigeration, 26(8), 857-864. doi:10.1016/s0140-7007(03)00096-3

Wang et al. (sCO2 specific) TBC

Pitla et al. (sCO2 specific) Pitla, S. S., Groll, E. A., & Ramadhyani, S. (2002). New correlation to predict the heat transfer coefficient during in-tube cooling of turbulent supercritical CO2. International Journal of Refrigeration, 25(7), 887-895. doi:10.1016/s0140-7007(01)00098-6

Internal fluid friction factor

Laminar flow equation Incropera, F. P. (2007). Fundamentals of heat and mass transfer: John Wiley.

Turbulent flow equation Incropera, F. P. (2007). Fundamentals of heat and mass transfer: John Wiley.

Petukov et al. Petukhov, B. S. (1970). Heat Transfer and Friction in Turbulent Pipe Flow with Variable Physical Properties. In J. P. Hartnett & T. F. Irvine (Eds.), Advances in Heat Transfer (Vol. 6, pp. 503-564): Elsevier.

Haaland Haaland, S. E. (1983). Simple and Explicit Formulas for the Friction Factor in Turbulent Pipe Flow. Journal of Fluids Engineering, 105(1), 89-90. doi:10.1115/1.3240948

External air heat transfer coefficient ASPEN HTFS-3 AspenTech, Aspen Exchanger Design and Rating V10, computer program.

External air pressure drop ASPEN HTFS-3 AspenTech, Aspen Exchanger Design and Rating V10, computer program.

EFFECT OF DIFFERENT NUCO2 CORRELATIONS

L

Design 1: L = 6mDesign 2: L = 10m

Design 3: L = 6mDesign 4: L = 10m

Tout = 35 °CTin = 70 °CPin = 8 MPaṁ = variable

Property Value

Tube SS316, OD 25.4mm, WT 2.1mm

Fins Al 6061, OD 57.15mm, t=0.4mm, 433 FPI

Bundle arrangement Staggered, trans. pitch 58mm

To compare different NuH correlations, overall heat transfer coefficient U is calculated for a similar CO2 ΔT.

𝑈𝑈 = 𝑄𝑄𝐴𝐴 ΔT Δ𝑇𝑇 = ∑𝑖𝑖=1

𝑛𝑛 Δ𝑇𝑇𝑖𝑖𝑛𝑛

Tamb = 20 °Cvface = 2 m/s

EFFECT OF DIFFERENT NUCO2 CORRELATIONS

+122%

+24%

+63%

+15%

L

Design 1: L = 6mDesign 2: L = 10m

Design 3: L = 6mDesign 4: L = 10m

EFFECT OF DIFFERENT NUCO2 CORRELATIONS

Design 2 – local internal (CO2) heat transfer coefficient.

Conclusion: modelling using current literature results in large variabilities.

L

Design 2: L = 10m

INCORPORATING MODEL INTO DRAFT EQUATION

Instead of fixed air face velocity, air velocity can be a function of air outlet temperature:

𝑣𝑣𝑎𝑎, 𝑖𝑖𝑖𝑖= 𝑓𝑓(𝑇𝑇air,out , ...)

Using cooling tower scaling relationship (d/h), for a given total mass flow rate can solve for:

• natural draft velocity

• tower size (diameter and height)

Natural draft cooling tower resistance model, from Kroger1

1. KRO ̈GER, DETLEV G (2004). AIR-COOLED HEAT EXCHANGERS AND COOLING TOWERS. PENWELL CORPORATION

EFFECT OF BUNDLE ARRANGEMENT ON NDDCTL = 10m

Design 1

Design 2

Design 3 Note: P = 20-8 MPa, T= 35-610 °C. Maximum resultant CO2 pressure drop <15 kPa)

Cooling tower for 25MWe CSP plant:

Cooling Tower Height Heat Exchanger Face Area

+49%

EXPERIMENTAL TESTING

Project underway to build and test a sCO2 air-cooled heat exchanger within wind tunnel at UQ Pinjarra sCO2 facility.

First known experimental testing of sCO2-air heat exchange in conventional 25mm finned tube heat exchangers.

Wind tunnel able to turn 90º to investigate effect of buoyancy.

EXPERIMENTAL TESTING - BUOYANCY

Modelling so far ignores buoyancy effects, assumes isotropic heat flux around tube circumference.

CFD shows buoyancy may significantly impact heat transfer behaviour, and direction of air flow (vertical/horizontal) may be an influence.

Air flow behaviour over finned tube, from Neal1 Cooled sCO2 buoyant behaviour, from Wang2.

(left – vertical velocity, right – density)

1. NEAL, S. B. H. C., & HITCHCOCK, J. A. (1967). A STUDY OF THE HEAT TRANSFER PROCESS IN BANKS OF FINNED TUBES IN CROSS FLOW, USING A LARGE SCALE MODEL TECHNIQUE.2. WANG, J., GUAN, Z., GURGENCI, H., VEERARAGAVAN, A., KANG, X., SUN, Y., & HOOMAN, K. (2018). NUMERICAL STUDY ON COOLING HEAT TRANSFER OF TURBULENT SUPERCRITICAL CO 2 IN LARGE HORIZONTAL TUBES. INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 126, 1002-1019

CONCLUSIONS

1. Performance of CSP plant sCO2 cooling systems is uncertain - predictions using literature and commercial software produce variable results.

2. Design and arrangement of cooling bundles may have large effects on the size of cooling infrastructure.

3. Influence of buoyancy on sCO2 in large tubes under non-isotopic heat flux is a significant unknown.

4. Experimental testing power plant scale sCO2 air-cooled heat exchangers recommended (before large scale pilot testing).

5. Future research aims to optimise design of cooling system, to reduce capital cost of CSP.

ACKNOWLEDGEMENTS

My supervisors - Assoc. Prof Kamel Hooman and Dr Zhiqiang Guan

The Australian Solar Thermal Research Institute (ASTRI)

University of Queensland solar thermal heat exchange research group and Professor Hal Gurgenci

This research is supported by an Australian Government Research Training Program (RTP) Scholarship