Maximising the efficiency of algal raceways for the production of biomass

270 Pages·2017·6.99 MB·English

by  Ed Musgrove

University of Southampton Research Repository
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UNIVERSITY OF SOUTHAMPTON

FACULTY OF ENGINEERING AND THE ENVIRONMENT

Water and Environmental Engineering Group

Investigating the performance of paddlewheels used in
microalgae raceways for the production of biomass

by

Ed Musgrove

Thesis for the degree of Doctor of Philosophy

June 2017

UNIVERSITY OF SOUTHAMPTON

ABSTRACT
FACULTY OF ENGINEERING AND THE ENVIRONEMENT
Water and Environmental Engineering Group
Doctor of Philosophy
Investigating the performance of paddlewheels used in microalgae raceways for the
production of biomass
Ed Musgrove

It has been suggested that biofuels produced from microalgae may be a more sustainable
alternative to other types currently produced, although currently the production of microalgae
for this purpose has the potential for the overall energy balance to be negative. Microalgae are
today cultivated in oval ponds of up to 10 m width and 500 m length, with water depths of 200
to 300 mm. The water must be kept in motion to avoid sedimentation. This is usually done
with paddlewheels which have 6 to 8 blades, and a typical diameter of around 1.20 m. The
energy demand for the continuously running wheels is one of the main cost factors, whilst the
wheel efficiency is typically estimated as only 10 %. Very little is known about the effect of
blade number or rotational speed on wheel efficiency. This research aimed to improve the
paddlewheel as a propulsion mechanism in order to reduce the energy required.
Theoretical work and 1:5 scale physical model tests were conducted to analyse the parameters
affecting paddlewheel performance, to develop a consistent and improved model of the
hydraulics of algae ponds, to define optimum configurations and to develop appropriate
design tools. The results indicate that the number of blades, rpm and immersion depth have a
great effect on the efficiency, with optimum values of over 60 % being achieved with higher
blade numbers and lower rpm. Using an insert to reduce the backflow around the blades
increased the efficiency and discharge of a 12-bladed wheel especially for the lower rotational
speeds tested.
A new theoretical equation to calculate the efficiency of the wheel was derived and validated
against the physical model. It was found that the leakage of the fluid beneath the blades was
the main loss factor in the efficiency of the wheel and should be minimised by using the insert
where possible.

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Contents
ABSTRACT ........................................................................................................................................ i
List of figures ................................................................................................................................ vii
List of tables ................................................................................................................................. xv
Declaration of authorship .......................................................................................................... xvii
Acknowledgments ....................................................................................................................... xix
List of variables ........................................................................................................................... xxi
List of abbreviations ................................................................................................................. xxvii
1 Introduction .......................................................................................................................... 1
1.1 Concern with current energy mix ................................................................................. 1
1.2 Renewable energy sources and biofuels ...................................................................... 2
1.2.1 First generation biofuels ....................................................................................... 3
1.2.2 Third generation microalgal biofuels .................................................................... 5
1.1 Conclusion ..................................................................................................................... 8
1.2 Structure of the report .................................................................................................. 9
2 Microalgae cultivation ........................................................................................................ 11
2.1 Cultivation conditions ................................................................................................. 11
2.2 Cultivation techniques ................................................................................................ 12
2.2.1 Closed systems - Photobioreactors ..................................................................... 12
2.2.2 Open systems ...................................................................................................... 13
2.2.3 Photobioreactors vs raceways ............................................................................ 18
2.3 Conclusion ................................................................................................................... 19
3 Raceway design and parameters ........................................................................................ 21
3.1 Size .............................................................................................................................. 21
3.1.1 Depth ................................................................................................................... 21
3.1.2 Width................................................................................................................... 24
3.1.3 Surface area ........................................................................................................ 25
3.2 Bends ........................................................................................................................... 26
3.2.1 Flow deflectors .................................................................................................... 28
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3.2.2 Islands .................................................................................................................. 32
3.2.3 Other designs ....................................................................................................... 34
3.2.4 Head loss .............................................................................................................. 35
3.3 Gas transfer ................................................................................................................. 37
3.3.1 Gas type ............................................................................................................... 38
3.3.2 Effect of pH and alkalinity on gas transfer .......................................................... 40
3.3.3 Surface transfer ................................................................................................... 41
3.3.4 Coverings ............................................................................................................. 44
3.3.5 Carbonation columns .......................................................................................... 45
3.3.6 Carbonation sumps .............................................................................................. 46
3.4 Fluid mixing .................................................................................................................. 49
3.4.1 Fluid velocity ........................................................................................................ 49
3.4.2 Turbulence ........................................................................................................... 50
3.4.3 Raceway mixing ................................................................................................... 54
3.5 Propulsion .................................................................................................................... 60
3.5.1 Airlift pumps ........................................................................................................ 60
3.5.2 Centrifugal and other pumps............................................................................... 61
3.5.3 Archimedes screws .............................................................................................. 62
3.5.4 Paddlewheels ....................................................................................................... 62
3.6 Computational models ................................................................................................ 79
3.6.1 Computational Fluid Dynamic models................................................................. 79
3.6.2 Growth models .................................................................................................... 82
3.7 Conclusion ................................................................................................................... 84
3.8 Aims ............................................................................................................................. 87
3.8.1 Objectives ............................................................................................................ 87
4 Paddlewheel theory ............................................................................................................. 89
4.1 How a paddlewheel works .......................................................................................... 89
4.2 Previous theoretical models for paddlewheels ........................................................... 90
4.2.1 Discharge ............................................................................................................. 90
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4.2.2 Losses .................................................................................................................. 93
4.2.3 Shaft power ....................................................................................................... 100
4.2.4 Efficiency ........................................................................................................... 102
4.3 Hydrostatic pressure lifting wheel ............................................................................ 103
4.3.1 Ideal theory ....................................................................................................... 103
4.3.2 Paddlewheels with finite radius ........................................................................ 105
4.4 Conclusion ................................................................................................................. 118
5 The paddlewheel as water propulsion technology ................................................................... 121
5.1 Experimental methodology....................................................................................... 121
5.1.1 Measuring the paddlewheel efficiency ............................................................. 122
5.1.2 Model construction, materials and equipment ................................................ 130
5.1.3 Parameters investigated ................................................................................... 139
5.1.4 Scaling ............................................................................................................... 144
5.2 Model experimental results and discussion ............................................................. 147
5.2.1 Wheel rotational speed and blade number ...................................................... 147
5.2.2 Fluid discharge .................................................................................................. 153
5.2.3 Fluid depth and head difference ....................................................................... 156
5.2.4 Length of raceway ............................................................................................. 161
5.2.5 Insert ................................................................................................................. 165
5.3 Model validation ....................................................................................................... 172
5.3.1 Theoretical model validation ............................................................................ 172
5.3.2 Experimental model validation ......................................................................... 179
5.4 Designing a paddlewheel driven system ................................................................... 183
5.5 Conclusion ................................................................................................................. 185
6 Case studies ...................................................................................................................... 189
6.1 Wastewater treatment plant of El Torno, Chiclana .................................................. 189
6.2 Earthrise Farm ........................................................................................................... 193
6.3 Parry Nutraceuticals .................................................................................................. 197
6.4 Cyanotech Corporation ............................................................................................. 200
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6.5 Conclusion ................................................................................................................. 202
7 Conclusion ......................................................................................................................... 205
7.1 Further work .............................................................................................................. 212
Bibliography ............................................................................................................................... 215
Appendix A – Journal papers and conferences ......................................................................... 227
Appendix B – Paddlewheel experiment technical drawings ..................................................... 229

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