Features of the Solar Cyclone Tower Technology. Sensor Network for an Enhanced Solar Cyclone Tower

Contents

1 Introduction

2 History and Context
2.1 History of the SCT
2.2 Manzanares prototype
2.3 Modern developement, Prototypes and projects

3 Presentation of the SCT
3.1 Operating concept
3.2 The SCT's different components
3.3 Estimate of the collector's energetic balance
3.4 Influence of the collector size and chimney height
3.5 Water-harvesting with a Solar Cyclone Tower (SCT)
3.5.1 Nucleation
3.5.2 Water production
3.5.3 Effect of water production on the updraft
3.6 Overview of the SCT under scrutiny
3.7 Choosing an optimum location

4 Opportunities and Advantages
4.1 Agriculture
4.2 Enhancing water harvesting
4.3 Heat and power management
4.3.1 Influence of the ground type
4.3.2 Enhancing soil energy storage
4.4 Food drying and extracting salts
4.5 Carbon Dioxide Capture
4.5.1 Methods
4.5.2 Vortex Tubes
4.6 Biofuels
4.7 Enhancing the collector roof with Organic Photovoltaics (OPVs)
4.8 Advantages

5 Building a Physical Model
5.1 Characteristics
5.2 Conception

6 Challenges in probing environmental variables
6.1 Thermal convection cells and irregularities
6.2 The effect of ambient crosswinds
6.3 Blockage walls to mitigate crosswind

7 Options and solutions for sensing environmental variables
7.1 Real-time data for an active SCT
7.1.1 Insulated storage tanks
7.1.2 Controllable windows
7.1.3 Panels to direct airflow
7.1.4 Controllable swirl vanes
7.2 Temperature
7.3 Humidity
7.4 Sensing airflow
7.5 Measuring solar irradiance
7.6 A simple sensor for humidity and temperature sensing
7.6.1 Layout
7.6.2 Temperature and humidity dependance of the HCZ
7.6.3 Characterisation of the LM
7.7 Developing a model

8 Sensor placement
8.1 Placement Algorithm
8.2 Choosing the optimal number of sensors

9 Summary and Conclusion

Bibliography

Abstract

As the need to transition towards renewable energies becomes increasingly evident and a grow­ing number of regions are subject to more intense water stresses, various power-generation and electricity-production technologiess are being developed. Among these, the Solar Cyclone Tower (SCT) appears to be a promising solution, for both electricity and water production, which is based on a renewable energy source: sunlight.

In this thesis, we explore the different features of the SCT technology, its operating concept and advantages. We also present a physical model which has been built for pedagogical uses.

We discuss how the SCT's water and electricity production can be enhanced. Potential facili­ties which the SCT could host are also considered and the solar tower's overall advantages listed. We explain how various physical phenomena make environmental variables difficult to predict and challenging to probe. However, it is necessary to acquire real-time data on environmental variables such as temperature, humidity, insolation and airflow for a number of different reasons which we will discuss.

We have thus undertaken a study and review of different temperature, humidity, insolation and airflow sensing technologies which are suitable for use in the SCT. We later characterised a simple temperature and humidity sensing device and, finally, presented the outline of a sensor placement algorithm and a method to find the optimal number of sensors.

Declaration

Sina has done a great job with his Masters' Thesis, on sensor arrays for the SCT ('Sensor Network for an Enhanced Solar Cyclone Tower') and has shown a high level of originality and rigorous scholarship. I am delighted with his work, and can confirm that he had undertaken a very large fraction of what he has written in his thesis by himself and most of the rest working with the SCT team. I am also expecting him to get a patent for one of the ideas which came out of his work.

Acknowledgements

People and events which gradually enabled me to successfully achieve my masters' are manifold.

I would like to begin by thanking my parents Kamran Varaei and Farimah Mostaghim and my family for their unfailing support, trust and encouragement in my endeavours.

I would want to thank my supervisor Dr. John Hassard for his stimulating faith in the project, trust in my capacities and for the exciting environment he has been able to create. He has provided me with many useful insights and material for this thesis.

Many elements presented hereafter are the fruit of a collaborative work between the differ­ent members of our group including Ilian Moundib, Patrick Carter-Cortez, Gautam Agarwal and Maryna Voloshina. I would especially like to thank Patrick for his help in characterising the sensor, his reviews of the thesis and suggestions. I also want to thank Massissilia Hamedouche for kindly reviewing my work.

To all my friends and family who have encouraged me and prayed for my success.

I would finally want to express gratitude to Baha'u'llah's and Abdu'l-Baha's inspiring words which guided me through my studies.

"The source of crafts, sciences and arts is the power of reflection. Make ye every effort that out of this ideal mine there may gleam forth such pearls of wisdom and utterance as will promote the well-being and harmony of all the kindreds of the earth." Baha'u'llah

Physical dimensions

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List of Figures

1.1 An overview of the overall global primary energy consumption [2, 3]

1.2 CO2 concentrations from 1700 to 2019 [4]

2.1 (a) Da Vinci's rotating spit (1452-1519), (b) Cabanyes's Solar Tower [5]

2.2 Overview of the Manzanares plant built in the 1980s [6]

2.3 Daily operating hours of the Manzanares prototype in 1987 [7]

2.4 Comparison of calculated and measured monthly average of daily energy production by the Manzanares plant in 1987 [7]

2.5 Locations of the different feasibility (red spots) and experimental (green spots) stud­ies which have been undertaken

3.1 Solar Cyclone Tower (SCT) description. The cooler ambient air (1) enters the collector(2). Solar radiation (3) heats the air. Turbines (4) produce electricity from the updraft as the air rises in the tower (5)

3.2 Transmittance and reflectance of fused Silica (0-2 ^m) [8]

3.3 Estimateoftheenergeticbalanceofthecollector,placedinAbuDhabi,withoutany ambient wind [8-11,11]

3.4 Driving force with respect to collector radius at different solar radiation levels (q) [12]

3.5 Output power of the SUT with respect to Chimney height at different solar radiation levels (q) [12]

3.6 An overview of the Solar Cyclone Tower configuration proposed by Kashiwa & Kashiwa [1]

3.7 Schematic of the expansion cyclone separator [1]

3.8 Freshwater production versus height for 0 < H < 0.5km (a) and 0.5 < H < 1.0km (b)[1]

3.9 Electric power production versus height for 0 < H < 0.5km (a) and 0.5 < H < 1.0km (b)[1]

3.10 Artist view of the SCT chimney [13]

3.11 World Map of Global Horizontal Irradiance [14]

3.12 An average exposure of users to water stress in each country [15]

3.13 Nine possible locations for for 10km diameter SCTs in Qatar. Acknowledgements to Dr. John Hassard

3.14 World Map of a = RH • SSRD computed on a monthly-average basis. RH is taken at 1000hPa

3.15 World Map of a = RH • SSRD computed on with hourly data. RH is taken at 1000hPa

4.1 Models for lettuce grown in southwestern Arizona using hydroponic vs. conventional methods (Error bars indicate one standard deviation) [16]

4.2 Overview of a SCT with seawater ponds. Air flows through the collector and gets heated. Water ponds humidify the warm air which flows above it(1), the air then goes through the swirl vanes (2), the turbines , and freshwater is finally harvested as air flows through the Expansion Cyclone Separator(3) and ascends in the tower

4.3 The influence of various ground types on the SUT's power output [17]

4.4 Water storage tubes during the day [18]

4.5 Water storage tubes during the night [18]

4.6 Effect of heat storage using water-filled black tubes in the collector [19]

4.7 Schematic representation of the solar chimney prototype as a solar dryer [20]

4.8 Overview of the operating concept of a counterflow vortex tube [21]

5.1 Picture of the SCT model built

5.2 Visualisation of the SCT model running. We see the steam, illuminated by the blue LEDstrip, rising(from(a) to(b)) as it swirls up and finally flowing out of the tower (c)

5.3 Design of the chimney for the SCT model on SolidWorks

5.4 Overview of the SCT model and its components. The chimney(1), the collector (2), a fan (3), EPE foam used as air sealers(4), notches in EPE foam to create a swirling airflow (5), a beaker fil led with water (6), a fogger (7) and a LED strip

6.1 Instantaneous temperature in the collector for a Manzanares-sized(120m radius) collector. The chimney is located at the right of the image [22]. Central tower on the right, blue:300K, red: 360K

6.2 Influence of ambient crosswind on local pressure distributions at the chimney bot­tom. The simulations are carried out for ambient surface crosswinds of 5m s-[1] (a), 10m s-[1] (b) and 15m s-[1]

6.3 VelocityVectorsandenthalpyhdistrubutioninthecollector(Aviewfromthetop)[23] 49 An overview of the wall which could be built to mitigate the effects of crosswind

6.4 Influence of ambient crosswind on local pressure distributions at the chimney bot­tom for a SUT with blockage. The simulations are carried out for ambient surface crosswinds of 5m s-[1] (a), 10m s-[1] (b) and 15m s-[1](c)

6.5 Solar Radiation Spectrum at Sea level (300nm - 2500nm) plotted from the ASTM G173-03 reference data for solar irradiance at Air Mass 1.5 [24]. The three red lines represent three photosensors, their respective wavelength range, and detected irradiance

7.1 Simple sensor design under scrutiny

7.2 Impedance-Relative humidity curve for different temperatures [25]

7.3 Output voltage (±5mV) obtained for the LM35 at constant humidity (47%± 1). Temperatures measurements displayed are from a Thermocouple (±1.5°C [26]) and an MT-903 (±0.5°C [27]))

7.4 Output voltage (±5mV) obtained for the LM35 at constant humidity (47%±1). Temperatures measurements displayed are from a Thermocouple (±1.5°C [26]) and an MT-903 (±0.5°C [27])

7.5 Different energetic transfers to be determined at any time. The incoming solar radiation (1); the radiation reflected by the collector roof (2) and the one transmitted (3); the radiation reflected by the collector ground (4) and the one absorbed by it (5); the ascending short-wave radiation reflected by the roof (6) and the one transmitted (7); the thermal energy entering (8) and leaving (9) a particle of air by convection; the surface longwave radiation (10) and the one transmitted (11) or reflected by the roof

7.6 Outline of the proposed algorithm

7.7 ANSYS Fluent simulation of the 2D velocity profile for a SCT with the dimensions presented in section 3. Acknowledgements to Ilian Moundib

7.8 Overview of different steps of the algorithm

7.9 Axial velocity profiles at node points along the axis of the chimney for four different grid resolutions [28]

7.10 Design of a panel which can be used to direct airflow

7.11 Overview of two potential approaches for temperature sensing

8.1 Two capacitor-based humidity sensors

8.2 Overview of the design of an airflow measuring device using MEMS pressure sensors

8.3 Overview of the pyranometer and its positioning throughout the collector

8.4 Axial velocity profiles at node points along the axis of the chimney for four different grid resolutions

List of Tables

2.1 Key dimensions of Manzanares plant [29,30]

3.1 Key dimensions of the SCT design under scrutiny

4.1 Average properties ofGranite, Limestone and Sandstone according to Holman 1992 [31]

4.2 Caption

4.3 Caption

5.1 Synthesis of the SCT model's dimensions and materials

7.1 Comparison of in-situ temperature sensing and remote temperature sensing

7.2 Comparison of the characteristics of two humidity sensors: the HCZ-J3 [25] and the HS1101LF [32]

7.3 Measurements for a (74 ± 3)% RH. Temperatures measured with the Thermocouple (TThC), with the MT-903 (TMT-903); the measurement difference (AT ); the output voltage of the LM35 (VTEMP)

7.4 Measurements for a (47 ± 1)% RH. Temperatures measured with the Thermocouple (TThC), with the MT-903 (TMT-903); the measurement difference (AT ); the output voltage of the LM35 (VTEMP)

Chapter 1

Introduction

There has been an unprecedented growth in energy consumption over the last decades. Energy needs have more than tripled in the last 60 years [3, 33] and regions such as countries from the CIS (Commonwealth of Independant States) and the Asia/Pacific have known, over the last decade, an energy consumption growth rate of over 4.1% per annum (see figure 1.1). Jointly, global CO2 emissions have increased significantly (see figure 1.2) , from just over 260ppm in the pre-industrial era [34] to over 409ppm today (Mauna Kea, September 2019) [4]. This increase in atmospheric CO2 concentration has an undeniable effect on the global temperature balance and thus, on innumerable atmospheric phenomena [35-38], water resources availability [39, 40], on the biosphere [41, 42], etc.

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Global energy consumption by region, measured in terawatt-hours (TWh). Note that this data includes only commercially-traded fuels (coal, oil, gas), nuclear and modern renewables used in electricity production. As such, it does not include traditional biomass sources.

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Figure 1.1: An overview of the overall global primary energy consumption [2, 3]

Reducing greenhouse gas concentrations is an increasingly pressing question which can be re­alised, through a gradual decarbonisation of the power sector [43], better energy storage and can be strengthened with the large-scale use of carbon-capture facilities. Among other things, a transition towards renewable energies is indispensable. However, in spite of an increasing political awareness on the need to lower carbon emissions (e.g. Paris Agreement on Climate Change 2015 [44]) and a rising role of renewable energies in meeting the power demand, renewable energies accounted only for one-third of the growth in energy needs in 2019 and the share of renewable power remains under 10% of the global overall energy production [43]. It thus seems that, despite the vast array Abbildung in dieser Leseprobe nicht enthalten of renewable energy production technologies which have been developed in the last decades (using solar power, wind power, tidal power, etc.), there is still room for improvement.

Concurrently, access to a sustainable source of clean water remains a priority: in 2015, 605 million people worldwide didn't have access to clean drinking water [45], 844 million people still lacked a basic drinking water service [46] and more than 20 countries are under considerable water stress [47]. Furthermore, this pressure on water supply is predicted to increase as a consequence of a global population increase and global warming [48], especially in the Sahel and the Middle East [49]. This water stress is also predicted to increase in the Middle East due to the rise in desalination costs. Indeed, the Middle East is home to 70% of the world's desalination plants [50] and a substantial amount of brine is constantly discharged in the Arabo-Persian Gulf and the Red Sea. As a consequence, the salinity will reach a peak in the years to come, making desalination too expensive for it to remain a viable solution [51].

Interestingly, many of the regions under water stress are also sub ject to high insolation. The Solar Cyclone Tower (SCT) technology offers a sustainable way to generate considerable amounts of electrical power from solar power [5, 7, 30, 52] while producing clean drinkable water [1]. Accord­ing to Schlaich 1995 [7], a sufficiently large solar chimney can produce up to 200 MW. Moreover, adopting a cyclonic design as suggested by Kashiwa & Kashiwa (see section 3.5) [1], the SCT can produce up to 50 10[9] kg year-[1] (consumption of about 250 000 urban dwellers) for a 1 km high tower and an average specific humidity of q A =18 g kg-[1] [1].

We will, throughout the thesis, describe the different features of the SCT technology, its oper­ating concept and advantages. The need to measure environmental variables will then be justified, and we will present technological solutions adapted to the SCT's characteristics and challenges.

After having explained how the concept of using updraft to produce work has evolved into the modern version of the SCT (chapter 2), we will present its operating concept and various features (chapter 3). In chapter 4, we will discuss how the SCT's water and electricity production can be enhanced. Potential facilities which the SCT could host will also be considered and the solar tower's overall advantages listed. We will then, in chapter 5, describe a physical model which has been built for pedagogical uses. Chapter 6 will deal with the phenomena which make environmental variables difficult to predict and challenging to probe. In chapter 7, we will discuss the usefulness of a sensor network in a SCT, explore different technological solutions, present the characteristics of a simple sensing unit and suggest a potential path for improvements. We will finally, in chapter 8, discuss a potential placement algorithm and method to find the optimal number of sensors to be used.

Note that, throughout the literature, the SCTs are interchangeably called Solar Updraft Towers (SUTs), Solar Thermal Chimneys (STCs), Solar Thermal Power Plants (STPPs), Solar Updraft Power Plants (SUPPs), Solar Chimney Power Plants (SCPPs), Solar Chimney Plants (SCPs), etc. Each denomination allows the author to emphasise on some particular aspects of the technology. We will refer to it as a SUT in chapter 2, as we will discuss different designs and uses of warm air updraft to produce work. In the rest of the thesis, we will refer to the technology we're investi­gating as a SCT as one of its main features is the production of water by inducing a swirl in the chimney.

Chapter 2

History and Context

The Solar Updraft Tower (SUT) relies on physical principles which have been known for centuries: the greenhouse effect and buoyancy. In this chapter, we will see how the concept of using the updraft to produce work has evolved into the modern idea of a solar tower (see section 2.1). We will then discuss the principal prototype of a SUT built to this day: the Manzanares prototype (see section 2.2). This prototype constituted an important milestone towards the further development of this technology. In section 2.3, we will give an overview of the different studies and prototypes which have been undertaken throughout the world.

2.1 History of the SCT

The simple idea of extracting energy from a buoyant updraft of heated air has appealed to many scientists, engineers and inventors throughout history. This idea traces back as far as the 1500s, when Leonardo Da Vinci sketched an automated rotating chicken spit, which uses the updraft of hot air from the fire placed beneath it (see figure 2.1(a)) [53, 54]. Similar designs based on the same operating principle have also been produced by predating Islamic scholars [30].

However, the idea of using solar radiation to cause the updraft was first proposed in the Proyecto de motor solar by Isodoro Cabanyes in 1903 [55]. As presented in figure 2.1, the design consisted of a glass structure in which air is heated. The heated air would then be directed towards a pen­tagonal fan inside a brick structure, electricity would be generated and the air would exit through a 63.87 m tall chimney [5, 30, 56]. Another interesting design was proposed in 1926 by the French engineer Bernard Dubos. He proposed to build large Solar Updraft Towers (SUT) and place them on the slope of high mountains (in North Africa) to avoid complex structural issues linked with building a high chimney [57].

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Figure 2.1: (a) Da Vinci's rotating spit (1452-1519), (b) Cabanyes's Solar Tower [5]

Nonetheless, it was not until 1975 that Robert E. Lucifer filed the first patent on such a technol­ogy in Australia [58], Canada [59] and the USA [60]. Moreover, serious interest in such structures is only recent and has been particularly strengthened by the construction of the large scale research plant in Manzanares, Spain [30].

2.2 Manzanares prototype

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Figure 2.2: Overview of the Manzanares plant built in the 1980s [6]

To this day, the largest and most complete prototype remains the experimental plant of Man­zanares, Spain, data was collected from mid 1986 to early 1989. The measurements and charac­teristics (presented in table 2.1) of the Manzanares plant are often used as default dimensions and settings. Likewise, the extensive data gathered is still used to validate theories.

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Table 2.1: Key dimensions of Manzanares plant [29, 30]

The plant proved to be very reliable. By observing experimental data from the tower built in Manzanares (see figure 2.3), we notice that, even with a very simple SUT design, power production is relatively steady and reliable. The power plant has indeed been able to run 95% of the time [7]. In fact, in figure 2.3, we notice that the number of daily operating hours has been relatively steady throughout 1987 (outside of planned interruptions) with approximately 8 to 12 hours a day during the summer and 4 to 8 hours a day during the winter.

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Figure 2.3: Daily operating hours of the Manzanares prototype in 1987 [7]

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Figure 2.4: Comparison of calculated and measured monthly average of daily energy production by the Manzanares plant in 1987 [7]

2.3 Modern developement, Prototypes and projects

Following the success of the Manzanares prototype and given the many advantages of Solar Tow­ers, over 14 feasibility studies [61-74] and 15 experimental studies [75-89] have been undertaken in different regions of the world (See figures 2.5) [90].

Many other projects have thus confirmed CFD models, the analytical models and the reliability of SUTs as power plants: for instance, Maia et al. [91] have validated their model with a prototype and find values within 2% of their model. Overall, the SUTs have proved to be reliable and can be built with relatively common materials for its structure (steel, glass, concrete).

Editorial note: Figure 2.5 has been removed due to copyright issues.

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Figure 2.5: Locations of the different feasibility (red spots) and experimental (green spots) studies which have been undertaken

In 2008, Kashiwa & Kashiwa suggested using the updraft both for electricity and water produc­tion [1] using a cyclonic design. The SCT's operating concept will be discussed in the next chapter. It is important to note that no SCT prototype aimed at harvesting atmospheric water has been built yet (2019). However, the theory presented in Kashiwa & Kashiwa 2008 has been reviewed and further investigated analytically [92, 93]. It is our hope that the SCT model be assessed both experimentally and through CFD simulations in the coming years.

Chapter 3

Presentation of the SCT

The SCT uses updraft to produce both electricity and water. In this chapter, we will discuss the main features of the SCT under scrutiny. In section 3.1, we will first describe its elementary operating principle. We will then (in section 3.2) present the SCT's prinicipal components and their role. As mentioned earlier, solar radiation is at the core of the SCT's functioning. We will thus give an estimate of the collector's energetic balance in section 3.3. We will then describe the relationship between chimney height, collector size and energy production (see section 3.4).

Subsequently, in section 3.5.2, we will discuss the operating concept, theory and relations behind cyclonic water harvesting. We will try to understand how Kashiwa & Kashiwa's results [1] scale with the SCT studied. We will then give an overview of the SCT under scrutiny (see section 3.6) and explore the different criteria to consider when it comes to choosing a location (see section 3.7).

3.1 Operating concept

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Figure 3.1: Solar Cyclone Tower (SCT) description. The cooler ambient air (1) enters the collec- tor(2). Solar radiation (3) heats the air. Turbines (4) produce electricity from the updraft as the air rises in the tower (5)

A SCT converts solar radiation into electrical power and relies on well-known physical princi­ples: the greenhouse effect and buoyancy. As illustrated in figure 3.3, the SCT consists of three main components (see figure 3.3): The collector (2), a set of wind turbines (4) and the central chimney (5) [7].

Cooler ambient air (1) enters the structure at the edges of the collector. Solar radiation (3) heats the air as it travels radially through the collector (2). A tall chimney is located at the center of the greenhouse canopy (5), through it, buoyant hot air can rise. Hot air is thus accelerated towards the centre of the canopy and wind turbines can extract energy from the airflow generated by the buoyancy pressure difference between the tower and the collector. The air finally exits the structure through the central chimney [22, 30].

Overall, the SCT's electricity production is directly proportional to solar radiation, collector area as well as the chimney height and diameter [7, 94].

3.2 The SCT's different components

The Collector: The collector consists of a large circular greenhouse (5 km diameter in our case) and can have a glass or a plastic film roof. The air within it is heated up by simple greenhouse effect, retaining long-wave radiations and transmitting short-wave radiations [7]. The collector is open at its periphery in order to allow air to flow (1) and the height of the roof increases as it approaches the central tower in order to facilitate the flow of hot air towards the chimney.

The Chimney: This is a key component of the SCT. It can be likened to a long tube located at the centre of the greenhouse canopy through which buoyant hot air can rise. The temperature and pressure differential between the top of the tower and its bottom draws the air up, creating an significant airflow.

The Turbines: Hot air is thus accelerated towards the centre of the canopy and the turbines can extract energy from the airflow [22, 30]. The output electricity is proportional to the debit of air flow and the pressure gradient at the turbine [7] (see section 3.4).

3.3 Estimate of the collector's energetic balance

We consider the average Global Horizontal Irradiance in Abu Dhabi for the month of April [11]:

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Considering a glass-roofed greenhouse, we can draw an estimate of the energetic balance. Ac­cording to A. Al-Mahdouri et al. [8], the reflection of fused silica is of ^7% and is relatively constant in the wavelengths associated with solar radiation: 0 - 2^m (see figure 3.2). Therefore, the shortwave radiation reflected by the roof is:

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Figure 3.3: Estimate of the energetic balance of the collector, placed in Abu Dhabi, without any ambient wind [8-11,11]

3.4 Influence of the collector size and chimney height

The buoyancy gradient in the chimney causes a static pressure differential [12] directly proportional to the height of the chimney (see figure 3.5):

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Where H is the height of the central chimney, p0,z is the ambient air density at height z, pi,z is the air density within the chimney at height z Ming et al. derived the maximum power output of the system (Equation 3.8) and its maximum efficiency (Equation 3.9) for a steady adiabatic solar chimney of cylindrical geometry and neglecting viscous friction [12]:

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Figure 3.4: Driving force with respect to col­lector radius at different solar radiation lev­els (q) [12]

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Figure 3.5: Output power of the SUT with re­spect to Chimney height at different solar radia­tion levels (q) [12]

Where the Driving force is the absolute value of the relative static pressure at the bottom of the chimney [12]:

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3.5 Water-harvesting with a Solar Cyclone Tower (SCT)

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In addition to producing electricity, Kashiwa & Kashiwa have suggested the use of the SCT as a means to extract freshwater from the atmosphere [1]. The overall operating principle and outline of a solar tower remains the same as presented by Schlaich [7] and Haaf [53, 53] (see figure 3.3). However, one of the main differences in the configuration they propose for water harvesting is the presence of swirl vanes placed in the flowpath between the collector and the chimney (see figure 3.6 and 3.7).

As seen in figure 3.6, ambient air enters the collector at its edges and gets heated as it flows through the collector. Next, swirl vanes cause the radial inflow of air to spin. The swirling airflow then accelerates as it approaches the central tower and, by energy conservation, the increasing velocity causes pressure, temperature and density to decrease [1]. Concurrently, pressure, tem­perature and density will lower as the air ascends adiabatically in the tower [69]. Eventually, updrafting surface air will reach its dew point and condensation will occur [1].

Kashiwa & Kashiwa also suggest the use of an expansion cyclone separator (see figure 3.7) which resembles a converging-diverging nozzle with a swirling airflow passing through it [1]. In the converging part, temperature can drop well below the dew point and the air/water vapour mixture reaches saturation and forms a fog. The intense turbulence and the presence of aerosols causes droplets to form. The centrifugal force then pushes the droplets towards the tower walls where it can be collected via a small opening where the spinning film of water is guided into a collection trap [1] and collected for treatment and use.

3.5.1 Nucleation

Nucleation is the process of droplets formation in a condensed phase. Nucleation can occur in a clean environment (pure mixture), in which case it is referred to as homogeneous nucleation, or in a dusty mixture, in which case it is referred to as heterogeneous nucleation .

At the dew point, the air is saturated in water vapour. Temperature must then drop even further and reach a so-called subcooling state for the probability of condensation to exist. In an environment devoid of dust or aerosols (homogeneous nucleation), temperature would have to drop significantly below the dew point for the condensed phase to nucleate (form droplets) [1]. Once condensation has occured, the air parcel has to reach its saturated state again and undergo the same process again.

However, nucleation can be facilitated by the natural (or forced) presence of dust or biological aerosols in the SCT [96-101]. Indeed, in the case of a heterogeneous condensation, water condenses immediately at saturation and condensation continues to occur as the temperature lowers, keeping the mixture at saturation [1].

Nucleation can also be activated by the intentional introduction of aerosols, the use of highly oxygenated biogenic vapours [102] or bacteria and water condensation can be enhanced by the use of different water-harvesting technologies [103-107].

3.5.2 Water production

Water production rate R W, according to Kashiwa & Kashiwa [1], depends on several parameters: the specific humidity of ambient surface air q A, specific saturation humidity q S, the mass flow rate of air through the chimney AM (3.14) and the efficiency of separation nS:

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Kashiwa & Kashiwa computed the liquid water production rate considering six assumptions:

1. a greenhouse temperature gain of 15K,
2. a chimney 12 times longer than its diameter: H = 12,
3. a chimney diameter of about dKash =42m
4. a friction coefficient f = 0.01 and a point loss coefficient e = 1.50,
5. a minimum separator temperature of 263K,
6. an efficiency of separation of nS =0. 80

The results are presented in figure 3.8. As we would expect from the previous equations 3.13 and 3.14, freshwater production is proportional to both the specific humidity of surface air qA and to the chimney height d [1].

Those assumptions apply relatively well to our design. However, in our case H = [1000] = 5. Thus, we can deduce the water production rate of our tower relative to the water production rate predicted by Kashiwa & Kashiwa [1]:

In the following calculations, the water production rate, chimney height and chimney diameter as calculated by Kashiwa & Kashiwa [1] are noted, respectively: RWKash, HKash and dKash Thus, under the assumptions listed above, and considering the model applies to the dimensions we are considering, water production is proportional to the chimney diameter.

3.5.3 Effect of water production on the updraft

The latent heat released during condensation can increase the updraft as it can cause a significant temperature drop in the chimney. Harvesting water can thus, according to Castilla [108], more or less double the power obtained.

Kashiwa & Kashiwa [1] state that the total temperature rise due to the latent heat released during condensation is:

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The electrical power generated Pe is thus given by the experimental correlation [1]:

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Where nt is the turbine efficiency and AN is the motive potential due to buoyancy [1] :

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The results are plotted in figure 3.9 with the same assumptions as in 3.5.2.

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It yields that, under the assumptions listed in subsection 3.5.2, and considering that the model applies to the dimensions we are considering, the updraft is proportional to the chimney diameter.

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Figure 3.9: Electric power production versus height for 0 < H < 0.5km (a) and 0.5 < H < 1.0km

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3.6 Overview of the SCT under scrutiny

As seen sections 3.4 and 3.5.2, the updraft and water production both depend on the height of the chimney and the collector's radius. The chimney height is thus chosen to be of 1km (providing thus a temperature gradient of at least 10°C) and the diameter is chosen to be of 5km. The chimney expands at its output (6% opening angle) in order to facilitate updraft.

The tower will be made essentially of steel girders in a diamond tessellation pattern, as pre­sented in figure 3.10. The spaces between the girders will be filled with toughened polyacrylamide sheets. Guy-ropes will provide additional support to the tower. The collector is also most likely to be built of a tessellated steel lattice.

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Figure 3.10: Artist view of the SCT chimney [13]

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Table 3.1: Key dimensions of the SCT design under scrutiny

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