1. Comparison of a scroll, a screw, a roots, a piston expander and a Tesla turbine for small-scale organic Rankine cycle
Parralel disk, viscous effects
Hello! First I would like to thank the organizers to allow me presenting my work. Its is focused on the…Design, modeling and experimentation of a reversible HP-ORC prototype
First Innogie ask us a theroretical modelisation of the prototype. As the results were promising we build a test rig, then whe the test rig will be fully operationnal, innogie will sell these Reversibles units
O. Dumonta, L. Tallurib, D. Fiaschib, G. Manfridab and V. Lemorta
aThermodynamics Laboratory, University of Liège (Belgium)
bDepartment of Industrial Engineering,  Università degli Studi di Firenze (Italy)

2. Conclusions and prospects
Table of contents
Introduction
Modeling
Discussion
Conclusions and prospects

3. Introduction State of the art
Strong influence of expander on ORC performance
(Almost) no experimental comparison of expander in litterature
No single technology identified as optimal (cost, efficiency, compactness, working conditions..)
3 main technologies : Scroll, Screw, Piston (+roots+tesla)
Parameter
Scroll
Piston
Screw
Roots
Tesla
Displacement [l/s]
[ ]
[ ] [12] -
N/A
Power [W]
[ ,000] [10]
[ ,000] [10]
[2,000-2e5] [10]
[ ]
0.005 –
Max. rotational speed [RPM]
10,000 [12]
12,000 [10]
21,000 [10]
20,000 [10]
Built-in volume ratio
[ ] [10]
[2-14] [10]
[n.a.-8] [12] ~1
Maximum pressure [bar]
~40
70 [12]
Max. temperature [°C]
250 [12]
560 [12]
Two-phase flow handling
Yes [12]
Low [12]
Yes
Yes [13]
Isentropic efficiency [%]
87 [1]
84 [12]
47 [12] 59

4. Experimental setup Hydraulic scheme
Test-rig : 3 kWe ORC (SUN2POWER)
Fluid : R245fa and ~5% oil (but not for piston)
Scroll: AC/DC convertor + variable resistor
Piston : Asynchronous generator with four quadrants variable-frequency drive
Screw : Air-cooled Eddy-current brake
Roots : modified engine supercharger
Tesla : Prototype from Firenze university

5. Conclusions and prospects
Table of contents
Introduction
Modeling
Discussion
Conclusions and prospects

6. Important note Important note
Perfectly objective comparison between different types of expanders not possible :
Different level of maturity for expanders
Not an expander sized for the test-rig (mass flow, pressure and temperature affect the performance of the expanders (not necessarily in the same way for each one).
Sizing fluids for those expanders are not the one used in this ORC (R245fa).
Nominal working conditions in terms of pressure and temperature are different for each technology (higher pressure and temperature for the piston for example).
BUT:
No such a comparison in litterature
Same test-rig and fluid
Models calibrated to predict optimal performance

7. Semi empirical model Advantages
Low CPU time + robust + extrapolation + general formalism
Supply DP, mechanical losses, leakage, under(over)-expansion, heat transfers
Calibration of 7 parameters based on experimentation
Performance extrapolation with speed optimisation
Tesla deterministic model
2D code for the viscous flow solution, considering real compressible fluid properties

8. Conclusions and prospects
Table of contents
Introduction
Modeling
Discussion
Conclusions and prospects

9. Model Assumptions Larger range of pressure ratio explored
Larger efficiency for the screw and scroll
Rather constant efficiency for the piston and Tesla
So when the fluid is admitted in the cylinder, a part of the volume is already taken by some fluid
trapped into this clearance volume. The effect of the clearance volume decreases when the pressure
ratio increases because the mass flow increases and the then, the amount of fluid trapped into the
clearance volume have less impact.

10. Model Operating maps Extreme conditions Flexibility Low temperature
Optimal performance of a machine for a given condensation and supply temperature temperature
!!!All axis are positive!!!
Extreme conditions
Flexibility
Low temperature
Efficiency

11. Conclusions and prospects
Table of contents
Introduction
Modeling
Discussion
Conclusions and prospects

12. High Pressure and temperature
Conclusion
High Pressure and temperature
Wet expansion
Compactness
Flexibility
Efficiency
Piston + -
Screw
+++
Scroll ++
Roots
Tesla turbine
Screw expander minimum power ~10 kW  wrong!
Scroll expander are the best because of higher isentropic efficiency  wrong!
Piston expander not interesting because of low efficiency  wrong!
Not a single volumetric expander is the optimal solution!
Tesla optimal for low volume ratio
Perspectives
Maturity of technology
Other technologies: Vane, Wankel, …
Economic aspects

13.  The most volumetric of the turbines
Conclusion - Tesla
 The most volumetric of the turbines

14. Important note Important note
Perfectly objective comparison between different types of expander not possible :
Different level of maturity for expanders
Not an expander sized for the test-rig (mass flow, pressure and temperature affect the performance of the expanders (not necessarily in the same way for each one).
Sizing fluids for those expanders are not the one used in this ORC (R245fa).
Nominal working conditions in terms of pressure and temperature are different for each technology (higher pressure and temperature for the piston for example).

15. Open source papers  http://orbi.ulg.ac.be
Thank you!
Open source papers  15

17. Semi-empirical models Extrapolation
Optimal conditions not reached on the test-rig  model to extrapolate
Oh = 5 K (Tamb=25°C)
RPM optimised
Rp adjusted with Pexp,ex
10 bar
30 bars

18. (constant speed/ variable speed) Screw expander Piston expander
Parameter
Scroll expanders
(constant speed/ variable speed)
Screw expander
Piston expander
Swept volume [cm3]
20.2
12.74
19.96
195
Volume ratio [-]
2.2
2.19
2.5
4.74
Maximum inlet temperature [°C]
140
130
250
Maximum inlet pressure [bar] 28 25 16 40
Rotational speed range [RPM]
3,000
[800-8,000]
20,000
[1,000-4,000]
Nominal shaft power [W]
2,277
2,000
4,000

19. Experimental results Isentropic efficiency
Same trend for each technology
Low pressure ratio  over-expansion losses
High pressure ratio  under-expansion, mechanical losses and pressure drops
Higher scroll efficiency
Low efficiencies because of test-rig limitations (pressure drops, mass flow rate…)
𝜂 𝑠 = 𝑊 𝑠ℎ 𝑚 𝑟 ℎ 𝑒𝑥𝑝,𝑠𝑢 − ℎ 𝑒𝑥𝑝,𝑒𝑥,𝑠
So when the fluid is admitted in the cylinder, a part of the volume is already taken by some fluid
trapped into this clearance volume. The effect of the clearance volume decreases when the pressure
ratio increases because the mass flow increases and the then, the amount of fluid trapped into the
clearance volume have less impact.

20. Semi-empirical model Operating maps
Optimal performance of a machine for a given condensation and evaporation temperature
!!!All axis are positive!!!
Extreme conditions
Flexibility
Efficiency