Duke University

(Team Name)

Task 5

Advisors:

Dr. David Schaad

Dr. Josiah Knight

Team Members:

Wind Pump Water

Matthew Ball

1

Wind Energy Capture

Matthew Ball

Alison Ernst

William Liew

Lyndsey Morgan

Deshira Wallace

Power Transmission

Alexander Brehm

John Peter Dolphin

Trisha Lowe

Nicholas Millar

Peter Perez

Clement Ramos

Water Filtration

Samantha Beardsley

Pim Dangkulwanich

Aaron Lee

Natalia Rossiter-Thornton

Table of Contents

Executive Summary……………………………………………………………………………..3 Task Identification……………………………………………………………………………....4 Conceptual Design Considerations………………………………………….……………….....5

Summary of Available Technologies………………………………………...………………...5 Wind Energy Capture………………………………………………………………………5

Power Transmission………………………………………………………………………..5Water Filtration…………………………………………………………………………….6

Discussion of Conceptual Design……………………………………………………………...7 Wind Energy Capture……………………………………………………………………....7Power Transmission………………………………………………………………………..8Water Filtration…………………………………………………………………………….8

Full Scale Detailed Design…………………………………………………………………….....9 Wind Energy Capture……………………………………………………………………….....9 Power Transmission…………………………………………………………………………..12 Water Filtration……………………………………………………………………………….16Bench Scale Testing Procedures……………………………………………………………….18 Wind Energy Capture………………………………………………………………………...18 Water Filtration……………………………………………………………………………….18Bench Scale Results and Discussion…………………………………………………………...19 Wind Energy Capture……………………………………………………………………..….19Other Considerations…………………………………………………………………………...21 Cost-Benefit Analysis………………………………………………………………………...21 Legal Considerations…………………………………………………………………………22 Health & Worker Safety……………………………………………………………………...22 Waste Generation Considerations…………………………...……………………………......23

Groundwater Retrieval………………………………………………………………………..23Environmental Implications…………………………………………………………………..23Public Involvement Plan……………………………………………………………………...24

References………………………………………………………………………………………25

Appendix I: Audits

2

EXECUTIVE SUMMARY

The primary objective of this task was to design a full-scale wind powered water treatment

system, specifically applicable for treating brackish water in the developing world. Before

beginning any project design, the team separated the broad objective into smaller, more specific

challenges: wind energy capture, power transmission and energy storage, and water treatment.

The wind energy capture team designed a vertical axis Savonius rotor for this application. In

addition to being easy and inexpensive to construct, these rotors are well suited to low wind

speeds and can accept wind from nearly any direction. The necessary compromise is their low

efficiency in comparison to horizontal axis turbines. The primary design consideration is sizing

the swept area to provide enough torque to run the water treatment process, dependent on both

the filtration pressure requirements and the power transmission capabilities. In addition, to

prevent stalling, multiple turbines are stacked vertically and perpendicular to each other,

increasing swept area and ease of startup. Aluminum sheet metal will be used to construct the

rotor, which is light, strong, and easy to work with. The shaft material is steel, as it will be

subject to nearly constant cyclic loading and must transmit a large torque.

The power transmission team’s objective was to use the torque from the turbine to develop

pressure to push water through a filter. They designed a pulley system in which a rope attached

to the turbine shaft lifts a lever arm a certain height above its pivot at the opposite end, using a

cam to engage and disengage the system. This lever arm is situated with several pegs that allow

weight to be added, providing a method of storing potential energy. Near the pivot, two small

area plunger pumps are connected to the lever arm: a low pressure pump to run water through the

pre-treatment process, and a high pressure pump, for the main filtration. As the lever arm rises,

the plungers rise as well and pull water into the pump chambers. Then, as the lever arm falls, the

pumps complete their downstroke and push pressurized water through the system. These pumps

were chosen for their simplistic design and ability to generate high pressure.

Given the quality of brackish water, the water team determined first to implement a pre-

treatment system to filter out the organic material in the water; a granular activated carbon filter

is best suited to this task. The main filtration system chosen was reverse osmosis, which does not

require electricity or large amounts of heat energy, as many filtration methods do. Reverse

osmosis filters do, however, require fairly high pressures that must overcome the natural osmotic

3

pressure of the system. The team chose a spiral-wound thin-film composite membrane. A storage

tank at the output holds the filtered water, and an evaporation pond collects the brine waste.

In order to regulate the flow of the water through the system, the team implemented an

optional control system. Because this monitoring and feedback device would not actually carry

out the filtration, an option for a solar-powered electrical control system was included in the

design. This PID controller will monitor the flow rate and incorporate a feedback loop to provide

nominal adjustments, improving performance and safeguarding against inconsistent flow.

The team chose a sample location of rural Namibia, a highly arid region in southwest Africa,

which meets the WERC ideal location criteria. This system has been designed for a village of

approximately 100 people, requiring a total of 1000 liters per day of water for drinking and

cooking uses. The system was designed conservatively with the assumptions that wind blows for

12.5% of the day, far lower than the 65%-90% average for most wind farms (WNCREI), at a

wind speed of 8.5 meters per second, the highest distribution wind speed in Namibia. This allows

for over 1500 liters per day to be pumped through the system. Based on the percent recovery of

the membrane, conservatively 80%, this provides about 1200 liters per day of clean water. Even

if the wind blows more slowly, less frequently, or both, there is still a sufficient error margin to

provide enough clean water to the residents.

TASK IDENTIFICATION Utilizing wind energy for water treatment where water quality is poor, and access to treatment

and electricity is limited, would greatly alleviate the scarcity of potable water, especially in arid

regions. Fresh water resources are quickly diminishing, leaving areas demonstrating high

population growth and growing agricultural needs without enough drinking water. Although

fresh water is scarce, large supplies of brackish water, with the right treatment, could provide a

solution. This task is to “harness the wind’s energy to power a water treatment process to treat

brackish ground water” (NMSU). The design must use the wind’s mechanical energy without

converting it to electricity, as this conversion results in both efficiency loss and increased costs,

and is impractical in a remote area not connected to an electrical grid. In addition, the design

must be applicable in a third world setting and meet all OSHA requirements.

Several assumptions were made in this design. The first was that it will be applied to a rural

village of about 100 people, providing 10 liters per day for each person. This is about 30% more

than the World Health Organization requires for drinking and cooking uses. Multiple systems

4

can be installed for larger villages that require more water. Namibia, Africa, was chosen as a

sample location and the highest distribution wind speed in that area, 8.5 m/s, was used as the

design speed. The system is also capable of functioning at much lower wind speeds.

CONCEPTUAL DESIGN CONSIDERATIONS

Summary of Available Technologies

Wind Energy Capture

There are three main methods of capturing wind energy: horizontal axis turbines (HAWTs),

vertical axis (VAWTs), and non-conventional. HAWTs are the most popular in large-scale wind

applications due to their high efficiency; a common example is the enormous modern three blade

rotor seen in large windmill farms. There are also two-blade and one-blade variations, which are

less stable. In addition, the small multi-bladed farm windmill, used for pumping water, and the

historic four-bladed Dutch windmill, used for grain production, were considered.

VAWTs are typically less efficient than HAWTs, but they have the benefit of being able to

accept wind from any direction. The Darrieus wind turbine, which looks like a large egg beater

and has a complex aerodynamic structure, and the Savonius wind turbine, which is essentially

two half cylinders offset from each other, are the two main types of VAWTs.

Aside from turbines, there are many nonconventional ways to harness wind power including

balloons, kites, and combinations of the two, which benefit from the ability to access winds at

high altitudes. These technologies are not practical because of the lack of experimentation data.

Two other options include the Windbelt, a power generating device that uses oscillations to

produce power, and the Loopwing, a HAWT with tipless twisted blades, which has very low cut-

in speeds. The main problem with these technologies springs from their complexity.

Power Transmission

Desalinating water is an energy-intensive process, and the variable, intermittent nature of

wind essentially requires an energy storage system. The main non-electrical methods of energy

storage include flywheels, water-pressure storage tanks, and gravitational potential energy;

because a water tower to generate the types of pressure needed would be unreasonably tall, on

the order of 200 feet, this can be done by raising and lowering a weight system.

Flywheels store their energy in the kinetic energy of moving weight. The weight spins around

while attached to an energy source and eventually detaches, connecting to the output device.

Flywheels require sophisticated controls and precise construction, and can be very expensive.

5

Water-pressure storage tanks are the primary method in which RO tanks store their energy.

They rely on a pre-pressurized balloon filled with air resting inside a tank, which increases in

pressure as the tank fills with water, thus pressurizing the water. These tanks cannot take

advantage of low wind speeds, as the pump will have to push the water into the tank at a much

higher pressure than the RO membrane rating. They are also relatively expensive.

Using gravitational potential energy, a weight is raised over a distance and its fall releases

the stored potential energy. The system is inexpensive, since the weight can be provided by any

regularly available, high-density material. Additionally, the force provided to the pump is

unvarying, since the force of gravity is constant over such a negligible distance. In the end, this

design was chosen. It is inexpensive to build and maintain, simplistic and thus more reliable, and

easily scalable. Additionally, animal or human power can be used to lift the weight in cases of

low winds; this allows a consistent supply of clean water despite intermittent wind.

For the pumping mechanism, some options include peristaltic, gear, and plunger pumps.

Peristaltic pumps are used primarily with toxic or fouling fluids. They have a simple design and

are cheap to maintain, but establishing high pressures is difficult, as this is done by pressing the

liquid through its containing tube. Also, the forces on the tubes can destroy or deform them.

Gear pumps can be either internal or external. They are very effective in creating high

pressures and use rotary action, which makes them a natural choice for rotary power provided by

a turbine. The manufacturing technology needed for gear pumps is, however, expensive and

complex. Metal gears are likely to foul and would need to be replaced often.

Plunger pumps use a single piston to force pressurized water through an outlet

valve, then pull water into the chamber as the plunger rises. The technology is

simple with a limited number of moving parts (Figure 1). Provided the plunger is

non-corrosive, it is less likely to foul than a gear pump, but can provide more

pressure and consistent flow than a peristaltic pump. Its major drawback is a linear

motion requirement, so the turbine’s rotary motion must be converted.

Water Filtration

The treatment technologies for the desalination of brackish water fall into two

main categories: thermal distillation and membrane separation. Distillation causes evaporation of

the brackish water due to the absorption of thermal energy, leaving the salts behind and then

6

Figure 1: Plunger Pump

condensing the water vapor. Most thermal processes are not compatible with wind-driven

systems, as wind does not provide much energy in the form of heat (Eltawil 2008).

Electrodialysis is a membrane-based process that separates out salt and ionic impurities from

the intake water by applying an electrical current. The separation is accomplished by using semi-

permeable, ion-selective membranes (CADT, 2008). Electrodialysis is typically used in brackish

water desalination but because of the electricity requirement is not suitable in this case.

Reverse osmosis (RO) is a membrane-based technology that

is standard in water treatment. RO applies pressure to a

concentrated solution so pure solvent can pass through a semi-

permeable membrane, leaving the solute on the other side. A

major challenge is that the pressure applied must overcome the

natural osmotic pressure of the system, which is dependent on

the solution’s ionic concentration, in order to operate (Figure 2).

Furthermore, RO membranes require routine maintenance and necessary pretreatments, such as

organic and/or chlorine removal. For this design, RO is the most viable technology.

Discussion of Conceptual Design

Wind Energy Capture

Multiple methods were considered for wind capture, but non-conventional types were ruled

out, due to the lack of proven results and intricate constructions. The design must be inexpensive

and easy to construct; this makes modern towering HAWTs

problematic. In addition, frequent operation is essential, so the

design must be self-starting and require low wind speeds,

characteristic of VAWTs but not HAWTs. This often means a

slight sacrifice in efficiency, (Figure 3) versus tip speed ratio:

λ= rωV

(1)

Where λ is tip speed ratio, r is rotor’s radius, ω is rotor’s angular

velocity, and V is wind speed. A high torque output to feed into

7

Figure 2: Reverse Osmosis schematic

Figure 3: Efficiencies of Various Rotors (Johnson)

the power transmission is also important. After analysis (Table 1), a Savonius rotor which

selected It is efficient in turbulent winds close to the ground, is simple to construct, has a high

torque output, and can accept wind from any direction.

Criterion Savonius Modern 3-Blade Modern 2-Blade Darrieus Dutch LoopwingSelf-Start 5 1 1 -1 -1 -1 1 1

Low Cut-In Speed 5 1 0 -1 -1 1 -1 1Durability 4 0 1 1 1 0 1 0

Ease of constructability 3 1 1 -1 -1 -1 0 -1Ease of maintanence 4 1 1 -1 -1 -1 1 -1

Efficiency 3 -1 -1 1 1 0 -1 1Low Cost 5 1 1 -1 -1 -1 1 -1

Torque Output 3 1 -1 1 1 0 0 1Practicality (proven performance) 3 1 1 1 1 0 1 -1Accepts wind from any direction 5 1 -1 -1 -1 1 -1 0

8 6 4 4 2 5 4 Plusses 1 3 6 6 4 3 3 Minuses 7 3 -2 -2 -2 2 1 Sum

Importance (1-5)

American Farm Windmill

Table 1: Pugh Decision Matrix for Wind Energy Capture Technologies

Power Transmission

The pump design, evaluated in Table 2, is reflective of the wind turbine choice, the energy

storage design, and the desalination choice. Based on the high pressure required by the RO filter

and the low rotational speed of the Savonius turbine, a custom-made pump was needed.

Criterion Peristaltic Pump Internal Gear Pump Vane PumpPressure 5 1 -1 1 1 1 1

Flow rate 4 0 -1 0 0 0 0Cost 4 1 1 0 0 -1 1

Ease of maintenance 3 0 1 1 -1 -1 1 Ease of constructability 3 -1 0 0 -1 0 0Necessary wind speed 3 0 -1 -1 0 0 0

Control simplicity 3 -1 1 0 0 -1 0Noise 2 -1 0 1 0 -1 0

2 3 3 1 1 3 Plusses 3 3 1 2 4 0 Minuses -1 0 2 -1 -3 3 Sum 1 -2 7 -1 -7 12 Wt Sum

Importance (1-5)

External Gear Pump

Axial Piston Pump

Plunger Pump

Table 2: Pugh Decision Matrix for Pump Technologies

The pretreatment system also requires a pump, but a low-pressure (LP), rather than a high-

pressure (HP), pump is adequate. To reduce complexity, a similar design with adjusted

dimensions was used, sized to find the best compromise between force and plunger area:

P= FA (2)

8

To eliminate the need for a water reservoir between the pretreatment and RO systems, the

pumps are designed to pump the same volume of water per stroke. Thus, the HP pump has a

larger stroke height and smaller piston area then the LP pump, which is closer to the fulcrum.

Water Filtration

There are three different structures primarily used in RO

membranes: spiral wound, tubular, and flat sheet. Spiral wound

membranes are composed of a central perforated tube, surrounded

by two layers of membrane with a permeate collection material

between them, wrapped spirally around the tube (Figure 4). The

primary advantages of the construction are its compact design and low cost (Wagner 2001).

Tubular membranes are composed of small tubes and have a high ability to tolerate suspended

solids (Koch 2008). Lastly, flat sheet membranes are constructed by layering flat sheets of the

membrane material between spacers and supports. They can typically tolerate very high

pressures, but are much more expensive (EPA 1996). Spiral-wound was chosen (Table 3).

Criterion Tubular Flat Sheet Spiral Wound Ease of maintenance 4 -1 1 1Tendency to fouling 3 1 0 0Pretreatment Requirement 3 1 0 -1Cost 5 0 -1 1

2 1 2 Plusses 1 1 1 Minuses 1 0 1 Sum 2 -1 6 Wt Sum

Importance (1-5)

Table 3: Pugh Decision Matrix for Membrane Structure

Currently there are three main types of membrane material: cellulose acetate, aromatic

polyamide, and thin-film composite. Table 4 below describes the choice of TFC. They exhibit a

high tolerance and are more readily adaptable to a variety of environments. Although TFCs are

generally more expensive, wide use has driven down previously exorbitant costs.

9

Figure 4: Spiral wound membrane

Criterion Cellulosic Thin Film

Composite Rejection of organics 3 -1 0 1

3 0 1 1Water flux 5 0 -1 1pH tolerance 1 -1 0 1Temperature stability 3 0 0 1Oxidant tolerance 4 1 -1 -1Compaction tendency 5 -1 -1 1Biodegradability 4 1 -1 -1Cost 5 1 0 -1

3 1 6 Plusses 3 4 3 Minuses 0 -3 3 Sum

Importance (1-5)

Aromatic Polyamide

Rejection of low molecular weight organics

Table 4: Pugh Decision Matrix for Membrane Material

FULL SCALE DETAILED DESIGN

Wind Energy Capture

The design of the turbine required two main challenges: determining the ideal shape, and

sizing this turbine for the project needs. Many reports were studied in order to optimize

geometry. Two types of Savonius rotors were considered, the S-rotor and the double hook rotor.

Because no report compared the two, the group built both.

For the S-rotor, the main geometric variables are the gap size (the horizontal overlap between

the two half-cylinders) and the aspect ratio AR, which is the ratio of the height of the rotor to its

radius. The ideal gap size ranges between 10%-15% of the radius of the rotor (Blackwell et al),

and efficiency also increases slightly with aspect ratio up to an aspect ratio of about 5

(Alexander, Holownia). Several reports showed that the vertical gap (seen as ‘a’ in Figure 6) has

a value of zero at the best performance (Alexander et al; Modi et al). An AR of 2.5 was chosen,

the most common AR tested (Alexander et al). Increases in efficiency beyond an aspect ratio of

2.5 were negligible, and not worth the risk of failure due to a slimmer, less stable turbine.

Because of these results, the vertical gap was set at zero. The group built several rotors to

corroborate the results of the papers with regard to horizontal gap size.

10

Figure 5: S-rotor (Alexander, Holownia)

Figure 6: Double Hook Rotor (Modi, Roth, Fernando)

For the double hook rotor, the main variables are the p:q ratio and the angle θ (Figure 6).

Tests by the University of British Columbia showed rotor efficiency peaks at a b:d ratio of 0.22,

as well as a p:q ratio of 0.4. These ratios were set for our tests, and two values of θ were tested.

As will be discussed with the results, the double hook rotor with θ = 135o proved to have the

best overall performance. In addition, research showed these types of rotors have higher static

torques than similar rotors with three, as opposed to two, half-cylinders or ‘hooks.’ To combat

this high static torque, two identical rotors were stacked vertically and perpendicularly. This

increases the likelihood of the wind easily catching a blade.

Once the most effective shape was determined, the turbine needed to be sized for the system’s

needs. In this design challenge, the team essentially worked backwards; first, a specific pressure

needed by the water filtration system was determined based on the filter properties. Then, the

power transmission system was sized based on the pressure needed, calculating a specific torque

required. This originated from preliminary estimates calculated based on a turbine of reasonable

size. Finally, the turbine was formally sized based on this torque and bench scale results, using

an iterative method. The detailed calculation for these numbers can be seen following. The

turbine was sized using the following variables:

T = torque (N·m), Pt = power turbine delivers (W), ω = angular velocity (radians/sec), V = wind

speed (m/s), ρair = density of air (kg/m3), A = swept area of turbine, Cp = performance coefficient

of rotor, λ = tip speed ratio, r = radius of rotor (m), h = rotor height (m)

T=P t

ω (3)

From this equation, it is necessary to determine ω in order to calculate the required P t, which will

in turn determine the necessary area of the rotor. Rearranging Equation (1):

λ=ωrV (1) so ω=Vλ

r (1.a)

The power delivered by any wind turbine is:

Pt=12

ρair CP A V 3 (4)

Combine the two equations, replacing the rectangular swept area A with the double-hook area

formula of the rotor, keeping in mind that the sum of q and p (Figure 6) is the equivalent radius

of the rotor. Multiply by two to account for the stacking of two perpendicular rotors:

11

T=

12

ρair CP A V 3

Vλq+ p

= 12 λ

ρair CP (2 · 3.54 · (q+ p ) ·h )V 2(q+ p)=1λ(3.54 ρair CPV 2)(1.4 q)2 h (5)

Given the following known and experimentally determined values, which will be discussed in

the Results section, q2h can be calculated in terms of T. The ideal aspect ratio, or h:2q, is 2.5, so

the following relation leads to the desired size:

ρair = 1.23 kgm3 , Cp = 0.075, V = 8.5 m/s, λ = 0.5

hq=5 →h=5q (6) and T=(92.5 )q25 q→ q=[ T

5 (92.5 )]

1/3

To obtain preliminary output values (Table 5), rough dimensions based on a practical size of the

wind turbine were chosen, assuming two perpendicular stacked rotors with height 3 meters and

radius of 1.2 meters, as well as efficiency of 15% and TSR of just over 1, based on Figure 3.

Torque (N·m) 120Angular Velocity (RPM) 70Power (Watts) 800

Table 5: Preliminary Output Estimates

Then, after testing the bench scales, determining actual efficiencies, and sizing the system up to

account for conservative loss estimates, the following parameters were determined:

In addition to size and shape of the system, modes of

failure must be analyzed in order to prevent them to a

reasonable degree. The most likely modes of turbine

failure are fatigue due to cyclic loading on the shaft and

shear failure of the aluminum rotors. The shaft bears the

torsional stress in the system, so a 0.6 m diameter steel

shaft with high shear and tensile strength was chosen to

alleviate this type of failure. The shear strength of

aluminum is 7 MPa, so it would take a gust of wind

characteristic of a Level 5 hurricane to cause shear

failure of the rotors.

Power Transmission

12

Table 6: Turbine Specifications

TurbineA 5.92 1.80B 6.81 2.07C 8.69 2.65

(P:A )max (W/m2) ηmax (%)

Wind spins the vertical axis turbine, generating torque. Attached to the top of the turbine is a

rod, wrapped in rope. The rope extends through a pulley system (Figure 8). A cam attaches the

lever to the rope, and the rope pulls up the lever arm, developing gravitational potential energy.

The raising arm creates suction in the pumps, pulling water into the pump chambers. Upon

reaching maximum height, the cam releases, dropping the arm back down and converting the

potential energy of the falling weight into pressure head, forcing brackish water though the

pumps. The torque created by the weight causes a larger force in

the two pumps, since they are closer to the fulcrum. The energy storage system effectively

decouples the wind energy and pumping energy, allowing each system to

operate more efficiently.

The turbine is rated for producing power, torque, and angular velocity based on wind speed.

Raising the weight requires a constant force, so changing wind speeds will change the rate at

which the weight is raised. Another important consideration is turbine shaft diameter.

Fwt=τwt

rwt (7)

Fwt is the force of the tension in the rope attached to

the wind turbine, determined by the torque produced

and the radius of the turbine shaft. An efficiency

factor must be included to account for frictional

losses in the system.

Fwt=Fwindshaft

ηpulley

(8) ->> Fwindshaft=Fwt × ηpulley

13

Figure 7: Energy Storage System

Wind Turbin

e

Pulley Syste

m

WeightPump

Rotational

Force to

Linear Force

Kinetic

Energy to Potential Ener

gy

Potential

Energy to

Water Pressur

e

Figure 9: Energy Storage Flow Chart

Figure 8: Pulley motion

This gives a balance of forces around the weight arm joint as shown in Figure 10. The torques

around the joint need to be analyzed in two separate motions.

T wa joint=Fwindshaft × Lpulley−Fweigh t × Lweigh t (9)

For the upward stroke, the torque around the weight-arm joint on the upward stroke is created

by the upward force of the pulley rope connected to the wind turbine minus the torque created by

the downward pull of the weight. The forces of the pumps are ignored because negligible force is

required to pull water up through the height of the piston chamber.

T wa joint=

(F ¿¿ pump1 × Lpump1+F pump2 × Lpump2)ηpumps

−Fweig h t × Lweigh t ¿ (10)

On the downward stroke, the torque created by the falling weight forces down the plungers in

the pump chambers, increasing the pressure of the water. This pressure creates an upward force

that creates a torque around the weight arm joint that acts in the opposite direction of the torque

created by the falling weight. In addition, the system is designed in order for Lweig ht to be

variable; weight can be placed on any number of different pegs along the arm. The farther away

it is placed from the fulcrum, the greater force it exerts on the pumps; however, it also increases

the cut-in speed of the wind turbine because a greater force is required to create enough torque to

lift the weight arm. Thus, regions with higher winds could adjust the weight size and placement

to take advantage of these wind speeds; additionally, users of the system could adjust the weights

seasonally or even daily to make the most of gusty days.

F pump=Pressure pump × Area pump(11)

The required pressure of the pump is dependent on the RO membrane used and the input

water quality. While the pressure requirements of RO membranes have been significantly

reduced in recent years, they are still substantial.

14

Figure 10:

Torque on

Weight Arm

Lpump is adjustable similar to Lweight. Moving the pumps further from the weight-arm joint

decreases the force available to them but increases the volume per stroke.

Another consideration is that there are two pump systems: a LP pump for pre-treatment and a

HP pump for RO filtration; these two pumps must have the same volume per stroke. If the HP

pump had a higher volume, it would run dry because the LP pump would not be providing

enough water to it; if reversed, the HP chamber would fill and become damaged, possibly

backing up the LP filter. Thus,

V pump 1=¿ V pump 2 (12)

and

Pressure pump1 ≪ Pressure pump2 (13)

The second equation means that the area of pump 1’s chamber must be increased relative to

pump 2. By increasing area, pressure is reduced. This would also lead to an increase in the

volume per pump stroke, which is remedied by moving the pump closer to the weight-arm joint,

reducing the stroke length, in turn reducing the volume of water pumped.

The biggest advantage to the energy system is that it can be custom designed to operate in any

environment. By changing the mass and placement of the weight, and size and placement of the

pumps, areas with high winds can extract the wind power more efficiently and areas with low

winds can still produce significant amounts of water. The shaft size of the turbine is also an

important consideration. The force in the pulley rope can be determined as follows:

F=Tr=120

0.3=400 N (14)

The weight arm uses lever principles to allow a smaller force generated by the wind turbine to

lift a large amount of weight. The attachment to the pulley system is located further from the

point of rotation than the weight (Figure 11).

The following equations can be used to determine the values in Table 7:

15

Upstroke Time: U .T .=pulleyheight ∙ 2∙ π

ω∙ π ∙ shaft diameter= 2.5 ∙ 2∙ π

7.33 ∙ π ∙0.6=1.14 seconds (15)

Max Arm Angle:

M . A . A .=sin−1 pulleyheight

armlength=2.5

5=30 degrees (16)

Height of Weight: Hweight=Lweight ×sin (M . A . A .) (17)

As shown in Table 7, the torque of

the pulley system exceeds the torque of

the weight arm, and so the turbine will

be able to lift the arm. At 8.5 m/s, the

arm will take 1.14 seconds to rise

through its 30 degree angle, raising the

arm through 2.5 vertical meters and the

weight through 1.5 vertical meters. This allows the weight arm to store just over 900 joules of

potential energy. This is summarized in Table 8 and is calculated in the following equations:

Energy Per Stroke: Epump=∆ h∙ ρwater ∙Volstroke

ηpump ❑ (18)

Given that for water at 25oC: Enthalpy(hatm)=104.92 kJ

kg ; h30 psi=105.01 kJ

kg ; h225 psi=106.26 kJ

kg

The energy delivered can be determined as follows, with the first set of numbers in the equation

representing the energy in the weight and the second representing the energy in the weight-arm.

E=m× g × ∆ h=50× 9.8 ×1.5+15× 9.8× 1.2 = 911.4 J (19)

Figure 12: High-Pressure Pump Figure 13: Low-Pressure Pump

For the design of our energy storage device, we wanted to keep the weights under 100kg to

simplify construction of the weight arm and allow for quicker strokes, and thus consistent water

16

Figure 11: Weight Arm Dimensions

Table 7: Weight Arm Properties

Pulley System Weight Arm

Force (N) 400 490 151.7

Distance (m) 5 3 2.5

Torque (N-m) 2000 1849.4

Upstroke Time (s) 1.14

Max Arm Angle (deg) 30

Height (m) 2.50 1.5 Varies

flow. The pump specifications are shown in Table 8; figures 12-13 show the pumps’ dimensions.

The torque and energy delivered by the weight arm exceed the requirements for the pumps

(Table 8). They can deliver over a fifth of a liter per stroke and require a fewer than 5500 strokes

a day to meet the 1000 liter goal of the unit. If the target wind speed of at least 8.5 m/s occurs for

12.5% of the day, the unit will be able to pump about 1200 liters of water. Being conservative,

we made low estimates on the pump efficiencies, below literature and store-bought pump values.

Water Filtration

As determined previously, the best

composition for the RO membrane for

our purposes is a spiral-wound, thin film

composite membrane. It was important

that the membrane be able to treat water

with high TDS levels, but do so with the

lowest applied pressure possible. The

TDS of the feed water, necessary pressure

and resultant flow rate are all intimately

linked; higher TDS requires higher pressures (due to the higher osmotic pressure of the water),

while higher pressures result in higher flow rates and higher energy consumption. Balancing all

of these considerations, a Low Energy Brackish Water Reverse Osmosis Membrane

(manufacturer information withheld) was chosen. When tested with a 2000 ppm NaCl solution,

the applied pressure was 150 psi and the flow rate was 2700 gallons/day. The goal is supply

enough pressure to maintain a flow rate near 2700 gallons/day. Assuming 80% recovery and

operation of the membrane for about 3 hours/day, this results in a permeate flow of 270

gallons/day, just over 1000 L/day. The pressure necessary to maintain this flow rate with these

particular conditions must be determined experimentally. Based upon the manufacturers’

specifications, we have estimated that a pressure over 200 psi is necessary.

The Membranes Brackish Water Low Energy RO Membrane is a spiral wound, thin film

composite element. It is 4 in. in diameter and 40 in. long with a minimum salt rejection of 98%,

bringing the TDS of the permeate far below the WHO recommended 1000 ppm. It is also

competitively priced at $291.60. A housing unit is also necessary to contain the membrane as it

treats the water. A stainless steel, high pressure housing unit is appropriate due to the high

17

Table 8: Pump Characteristics

  Pumps

  High Pressure Low Pressure

Pressure (psi) 225 ~30

Diameter (cm) 2.50 5.00

Force req'd (N) 1522.51 624.62

Efficiency (%) 50 65

Distance from joint (m) 1 0.25

Torque (N-m) 1522.51 156.15

Energy Per Stroke (J) 632.29 32.58

Water Per Stroke (ml) 235.64

pressures (>200 psi) expected; this housing is expected to cost around $230.00. However, this

membrane has no tolerance for free chlorine and will foul if exposed to feed water with a high

concentration of organics and microorganisms, requiring a pretreatment process. One of the most

cost-effective and easily managed pretreatment strategies is a granular activated carbon (GAC)

filter, chosen for this application. GAC filters are easy to install and only require periodic

cleaning that can be automated via a backwash process. GAC is highly efficient in removing

organics, chlorine, and certain ions, which all contribute to the dissolution of membrane

integrity. One of the major design considerations for a GAC treatment system is the empty bed

contact time (EBCT), which is defined as the total bed volume divided by the flow rate.

Assuming that the given water has low to moderate amounts of organic material, an EBCT of

approximately 10-15 minutes is adequate. Using the flow rate assumptions stated above, the 270

gallons/ 3 hours translates to a necessary volume of 0.05678m3 (using a 10-minute EBCT). For

ease of design and construction, a cylindrical GAC filter made from concrete, or a locally

available material, with a diameter of 0.5 meters and a length of 1 meter will be more than

sufficient in treating the water for RO process. The water will simply have to be allowed to

trickle down the filter and then routed to the membrane. Also, because minimal head loss is

expected through this short amount of piping and the system is designed generate excess

pressure, this was not a major concern.

BENCH SCALE TESTING PROCEDURES

Wind Energy Capture

In order to determine the power generated and the turbine efficiency, two main tests were

conducted simultaneously on the rotors. The first was to test the angular velocity of each turbine

over a range of speeds. This was done by placing a leaf blower at a variety of distances and

settings centered in front of the turbine and recording the RPM using a no-contact tachometer.

An anomemeter was used to determine the exact wind speed at these various distance-power

setting combinations. The second set of tests determined the power and torque each generated.

This was done by attaching a mass-pulley system to the shaft and measuring the time the turbine

took to pull the mass up over a certain distance. The potential energy of this rise was divided by

the time to determine the power generated. Then, torque was determined using Equation (3).

Photographs of testing can be seen below.

18

Photograph 1: Tachometer Tests Photograph 2: Testing Set Up

Water Filtration

To determine the performance of the RO filter in practice, a simple test will be employed. The

cylindrical filter will be attached to piping on either side, and a pump will be used to generate

pressure and pump modeled brackish water through the filter. The quality of the water will be

tested both before and after filtration, using TDS as a quality standard, using a TDS meter. The

water use efficiency of the filter, the ratio of potable water coming out of the filter to total

brackish water going into the filter, will be determined. These tests will be performed over a

range of pressures, from 50 psi to 250 psi. The following equation was employed:

Fw=K (∆ p−∆ π ) (20)

where

Fw = water flux in gal/day-ft2

K = mass transfer coefficient for a unit area of the membrane, (gal/day-ft2-psi)

p = pressure difference between the feed and product water (applied pressure)

= osmotic pressure difference between the feed and product water.

The osmotic pressure difference is calculated using average TDS values. The flow rate will

depend on the applied pressure under the following relationship:

Fw( galday

)=2.563 galday psi

( ∆ p−41 ) psi

BENCH SCALE RESULTS AND DISCUSSION

Wind Energy Capture

19

In Charts 1-2 are the results from the Savonius wind turbines. The first chart displays power

coefficient versus tip speed ratio for each turbine, and the second shows torque coefficient

similarly. The table shows the maximum efficiency and power to area ratio of each rotor. Rotors

A, B, and C are the S-rotors, with gaps of 0%, 15%, and 30%, respectively. Rotors D and E are

the double hook rotors, with θ = 135o and 90o, respectively.

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.000

0.01

0.02

0.03

0.04

0.05

0.06

0.07

ABCDE

100·λ (Tip Speed Percentage)

CP Chart 1: CP versus Tip Speed Percentage

0 10 20 30 40 50 60 70 800

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

ABCDE

100·λ (Tip Speed Percentage)

CT Chart 2: CT versus Tip Speed Percentage

20

TurbineA 5.92 1.80B 6.81 2.07C 8.69 2.65D 22.96 6.08E 16.92 5.69

(P:A )max (W/m2) ηmax (%)

Table 10: Rotor Performance

From these results, it is seen that double hook rotor with θ = 135 o has the highest efficiency

and thus was chosen as the final design, as the variation between designs in expense and

difficulty of construction was negligible. Though the efficiency of the bench scale was just over

6%, research shows that efficiency increases with rotor size. Because the full scale is over ten

times the size of the bench scale, an efficiency of over 9% would be expected (Alexander et al).

However, to be conservative, an efficiency of 7.5% was chosen for design. Conservative values

were also chosen in terms of the best tip speed ratio; the bench scale had its peak performance at

about λ = 0.3, but the turbine is more likely to spin slightly faster, so a tip speed ratio of λ = 0.5

was used. This guarantees that the system over-designs for torque; that is, it is very likely that

more torque is being produced than calculated. As previously discussed, failure of the shaft due

to large torque transmission is not a serious concern, and this over-design ensures that the turbine

will in fact achieve the torque necessary to power the system. Using these experimentally

determined numbers, the dimensions in Table 6 were determined.

In addition, Chart 2 shows that for the majority of tip speed percentages, with the exception of

fairly low values, Rotor D also produces the highest torque. In general, it makes intuitive sense

that this rotor would demonstrate the best performance; the straight edge that extends into the

inside of the opposite blade on the double hook rotor allows wind to flow in between that space

and push on both blades simultaneously. Having a higher efficiency associated with a larger

angle θ also is logical, as the wind has more curved area to ‘catch’ and induce spinning.

Finally, cut-in speed was tested; the bench scale had a cut-in speed of less than 0.5 m/s, and

using the ratio of rotational inertia of the bench scale to the full-scale, a cut-in speed of about 2.5

m/s was estimated.

Due to time constraints, the entire system was not able to be tested before the completion of

this paper; however, data will continue to be collected and analyzed for the WERC competition.

OTHER CONSIDERATIONS

Cost Benefit Analysis

21

Pump $50 2 $100 GAC pretreatment Filter $23 1 $23 RO Filter $290 1 $290 Filter Housing $230 1 $230 Piping $1.50/ft 15 $25 Storage tank $120 1 $120 Installation $12/hr 4 $48 Land $58/acre 0.5 acres $30 TOTAL CAPITAL $1,675

Annual Filter Replacement 1 RO, 2 GAC $340 Annual Pump Maintenance $12/hr 25 hours $300 Annual Rope Replacement $0.1/ft 1000 ft $100

Miscellaneous Maintenance $12/hr 25 hours $300

$1,040 TOTAL ANNUAL COST $2,715 EXPECTED LIFETIME 10 yearsANNUAL LIFETIME COST $1,200

--- 1000 L/day $0.003/L

$290 (RO), $23(GAC)

TOTAL OPERATION AND MAINTENANCE

ANNUAL RESULTANT WATER COST

Table 10: Cost Summary

Most of the benefits of the system, including health benefits, improved quality of life, time

savings, and community pride, are intangible and will be discussed rather than tabulated. The

World Health Organization (WHO) predicted that in developing regions the return on a US$1

investment accounts for a US$5 to $12 return on investment due to better health outcomes and

reduced healthcare costs (WHO, 2004). The actual benefits are dependent on the scale of the

intervention.  In general, the costs of the interventions include capital and annual variable costs,

such as operation and maintenance.  The benefits are measured less concretely with criteria such

as time savings associated with better access, the gain in productivity due to fewer disability

days, health sector and patients costs saved and the value of prevented deaths (WHO, 2004). In

the developing world, most available technologies are only capable of producing clean water at

the household level and are not capable of salt removal; this system overcomes these challenges.

Legal Considerations

22

Whether in the developed or developing world, many cities and towns have ordinances that

ensure that structures are safe, proper and compatible with existing or planned development

(Iowa Energy Center). However, ordinances and building codes are rare in rural areas, where

restrictions on the use of wind turbines are not in place. An equivalent to a structured legal

system is the local social structure. Prior to installing the water purification system we must take

into account the value the residents place on the preservation of a proposed site and employing

compromise. The owner of the system faces liability if their property poses a threat to the general

public. Events such as the loss of a blade or a tower collapse can cause unwanted property

damage or injury, particularly to children. To reduce the chance of damage or injury, the system

will have at minimum a set-back distance of at least one tower height. In addition, an optional

safety fence may be constructed to ensure that unauthorized personnel cannot access the turbine.

Health & Worker Safety

Industrial wind turbines have often been associated with noise pollution which is under

investigation for its effect on humans, but there has not been conclusive evidence supporting that

hypothesis. Another health concern is potential injury due to mechanical malfunctioning.

Physical injuries to pedestrians can be avoided by creating fences around the wind turbine.

Additionally, all the residents must receive safety training prior to using the device. Finally,

diarrheal diseases are a major cause of morbidity throughout the world, particularly in

developing regions. WHO estimates that improved water supply can reduce diarrheal morbidity

by 32% (WHO, 2004). Other diseases related to water consumption such as schistosomiasis,

trachoma and intestinal infections affect millions of people worldwide. With the access of clean,

safe drinking water morbidity can be reduced greatly.

Worker safety concerns stem largely from physical injuries caused by the wind turbine itself.

Proper training for installers and subsequent maintenance workers will be required to reduce

excess injuries at the work site.

Waste Generation Considerations

A concentrated brine stream is produced that contains about 99% of the salts in about 20% of

the volume of the feed water, this yields a solution with a TDS of approximately 18,109 ppm. A

solution with this salt content must be disposed of, but the disposal options are very limited due

to the intended applications of the system. Since the system will be employed in rural, third

world areas, mechanically complex options, such as deep well injection or aquifer recharge are

23

unreasonable. Also, applications that risk salination of groundwater are not ideal. Because it is

likely that land is abundant and inexpensive in locations where the system would be employed,

an evaporation pond is the best option for brine disposal. Evaporation ponds further concentrate

brine until the solubility limits of the salts are reached, causing precipitation of the salts. The

crystals can either be utilized or disposed of with other sources of solid waste. The ponds are

sized depending on the evaporation and precipitation rates of the area (Svensson, 2005).

Groundwater Retrieval

In the proposed design only one wind turbine is implemented to power the water treatment

system. However, pumping ground water to the level of the system is also a practical necessity.

In order to provide for this, a second turbine can be installed, transferring ground water to a

reservoir that the treatment system can draw from. For simplicity’s sake, the second turbine

should be identical or very similar to the first with the possible exception of scaling factors.

Environmental Implications

In rural arid or semi-arid regions the capitalization of wind as a renewable resource would be

optimal. The installation of a wind turbine and water purification system will have minimal

negative effects on land use since the system is not large. The concentration of particulate matter

and noxious fumes should remain the same in comparing wind input and wind output. The water

produced will be of higher environmental quality due to a decreased TDS load. However, the

secondary product, brine, can have various environmental implications if it is not disposed of

properly. The proposed design conforms to Millennium Development Goal number seven, which

states that future development must take environmental stability into account. Moreover, the

United Nations’ goal is to halve, by 2015, the proportion of people without sustainable access to

safe drinking water and sanitation. The incorporation of sustainable water retrieval and treatment

systems allows for greater potential of reaching that goal.

Public Involvement Plan

The success of the proposed project is contingent on the support from the community. Prior

to the installment of the system, the local leaders will be informed of the purpose and benefits of

the project. Once the approval from leaders is attained, the public will learn more about the

project through various seminars. Each seminar will target a specific demographic within the

community in hopes of making more relevant to each group. The members of the water treatment

24

program will also survey the public about their expectations for the project and also use the time

to address their questions and concerns related to the effectiveness of the system.

Once the public understands the overall goal of the project, interested men and women will be

selected to give input on how to build the system as effectively and efficiently as possible. After

deliberations, the final concept will be presented to the public for their overall critique and

approval. After the final design decision, financing for the project must be arranged. In order to

increase the feeling of ownership among the residents, the funds may potentially come from

them. The idea behind ensuring that the funds come from the region itself is that the public will

be more invested in the success of the project than if funds came from elsewhere.

The construction of the system will be a partnership between the water treatment members

and the local residents. During the construction period, classes will be held explaining the

function and repairs of various components. This increases sustainability and decrease costs of

hiring an engineer from another area. At the system’s completion, a demonstration will be held

and the water treatment team will get feedback about water quality and satisfaction with the

project. A follow-up survey of the system will be taken after a month of use, three months and a

year of use to ensure that each part is working properly and it is benefiting the community.

25

REFERENCES

Ackermann, T., Soder, Lennart (2002). "An overview of wind energy-status 2002." Renewable and Sustainable Energy Reviews 6: 67-128.

Alexander, A. J., & Holownia, B. P. Wind tunnel tests on a savonius roter. Journal of Industrial Aerodynamics, 1978, Retrieved January 24, 2009, from https://wiki.duke.edu/download/attachments/13373206/Loughborough+report.pdf.

Aly, Narmine H. and Adel K. El-Fiqi. (2003) “Mechanical vapor compression desalination systems—a case study,” Desalination, 158: 143-150.

Beardmore, R. (2008, June 14). Gear Efficiency. Retrieved February 15, 2009, from RoyMech: http://www.roymech.co.uk/Useful_Tables/Drive/Gear_Efficiency.html

Blackwell, B.F.; Sheldahl, R.E.; Feltz, L.V. (1977). Wind Tunnel Performance for Two and Three-Bucket Savonius Rotors. Sandia Laboratories. Albuquerque, NM and Livermore, CA. The United States Energy Research and Development Administration, Contract AT (29-1)-789.

Committee on Advancing Desalination Technology, National Research Council, 2008. Desalination: A National Perspective.

Elimelech, M., Zhu, X., Childress, A.E., Hong, S. (1997). “Role of membrane surface morphology in colloidal fouling of cellulose acetate and composite aromatic polyamide reverse osmosis membranes.” Journal of Membrane Science, 127, 101-109.

Eltawil, M., Zhengming, Zhao, Yuan, Liqiang (2008). Renewable Energy Powered Desalination Systems: Technologies and Economics-State of the Art. Twelfth International Water Technology Conference, Alexandria, Egypt.

EPA. (1996) “Capsule Report: Reverse Osmosis Process,” Washington, D.C.: Office of Research and Development.

"Hydrostatic Pumps". Hydraulic Equipment and Tools Marketplace. Hydraulic Equipment Manufacturers. Retrieved 25 Jan 2009, from http://www.hydraulic-equipment-manufacturers.com/hydraulic-articles1.html.

“Integrated Wind Energy/Desalination System” (2006). GE Global Research. NREL SR-500-39485.

Johnson, G. L. (2001). Wind Energy Systems. Manhattan, KS, Prentice-Hall. 2.

Koch Membrane Systems, “Tubular Membranes Provide a Constant Flow of Solid Solutions.” 15 February 2008. Available < http://www.kochmembrane.com/prod_tubular.html>

Krivchenko, G. (1994). Hydraulic Machines: Turbine and Pumps. 2nd ed. Boca Raton, FL: CRC Press.

Leighton, C. W. H., Martin, D.D., Lindemann, W.C. (1983). “Inability of Microorganisms To Degrade Cellulose Acetate Reverse-Osmosis Membranes”. Applied and Environmental Microbiology, 45, 418-427.

26

Liu, C.C.K., et al. “Experiments of a prototype wind-driven reverse osmosis desalination system with feedback control.” Elsevier. Desalination 150 (2002) 277-287.

Lonsdale, H. K., Merten, U., Riley, R.L. (1965). Transport Properties of Cellulose Acetate Osmosis Membranes. Journal of Applied Polymer Science, 9, 1341-1362.

Loopwing Co. (2008). "Loopwing Characteristics." Retrieved January 22, 2009, from http://www.loopwing.co.jp/en/entop.html.

Lopez-Ramirez, J. A., Coella Ovieda, M.D., Quiroga Alonso, J.M. (2006). Comparative studies of reverse osmosis membranes for wastewater reclamation. Desalination, 191, 137-147.

Mallevialle, J., P. Odendaal, and M.R. Weisner, eds. 1996. Membrane Processes in Water Treatment. New York: MacGraw-Hill Publishers.

Modi, V. J., Roth, N. J., & Fernando, M.S. Optimum configuration studies and prototype design of a wind-energy-operated irrigation system. Journal of Wind Engineering and Industrial Aerodynamics, 16, Retrieved January 24, 2009

“Reverse Osmosis Water Treatment” (2001). Water Systems. Retrieved January 19, 2009, from http://www.aquatechnology.net/reverse_osmosis.html

Singh, R. (2005). “Introduction to Membrane Technology”. Hybrid Membrane Systems for Water Purification. Colorado Springs, CO, Elsevier.

Svensson, Marcus. (2005). “Desalination and the Environment: Options and considerations for brine disposal in inland and coastal locations” Swedish University of Agricultural Sciences, Sweden.

Thompson, M., Miranda, M. (2000). “Theory, testing and modelling of a Clark pump.” CREST, Loughborough University of Technology, UK.

von Gottberg, A., Pang, A., & Talavera, J. Optimizing water recovery and energy consumption for seawater RO systems. GE Water and Process Technologies, Retrieved Feb 14, 2009, from http://www.gewater.com/pdf/what_we_do/water_scarcity/TP1021EN.pdf .

Fluid Equipment Development Company, LLC (FEDCO). (2006). Frequently asked questions about energy recovery for RO systems [Brochure]. Monroe, MI: http://www.fedco-usa.com/pdf/faq-hpb.pdf

Wagner, J. (2001). Membrane Filtration Handbook: Practical Tips and Hints (2nd ed.). Minnesota: Osmonics, Inc.

“Western North Carolina Renewable Energy Initiative.” Appalachian State University. (2007). Retrieved January 19, 2009, from http://www.wind.appstate.edu/reports/WindEnergyFactSheetWNCREIFeb07.pdf

“Wind energy manual: Legal issues in wind energy.” Iowa Energy Center. Retrieved February 14, 2009, from web site: http://www.energy.iastate.edu/renewable/wind/wem/legal_issues.htm

World Health Organization. (2004). Water, sanitation and hygiene links to health (Nov 2004). Geneva: WHO. http://www.who.int/water_sanitation_health/factsfigures2005.pdf

27