SeaFont Pty is on the lookout for the design values for a solar powered water desalination system designed to benefit remote and rural coastal communities. As such the company has sought the services of Brilliance in mind in order for the design to echo and incorporate all the specifications outlined by SeaFont Pty in their client brief. In this report four design sections will be technically analysed in brief to ensure that all the requirements of the project and its design parameters have been addressed with respect to the design specifications from SeaFont Pty. These design sections discussed in the report are
The pump and RO filter
The solar panels, and
The battery
In conducting these technical analyses of the above named components, the design proposal will be able to identify methods to maximize the rate of production of portable and the volume of portable water stored. It will also seek to minimize the capital cost of the equipment and ensure that the budget for the design is not exceeded. The report will also seek to identify some solutions that can be incorporated into the design in order for it to comply with constraints and requirements that SeaFont Pty has detailed in the client brief. In conclusion, the report will also entail any recommendations from Brilliance in Mind to aid the design to meet its specific goals as well as its overall intended purpose.
This section has been further divided into individual sections as is listed on the second page of the SeaFont Pty Ltd client brief. The technical analysis will discuss these individual sections starting with the first design section which is the water tank, followed by a technical analysis of the pump and the RO filter, the solar panels, the battery, and then it will finally conclude with an analysis of the capital cost of equipment.
The stainless steel storage tank is purposed for holding the volume of the potable drinking water resulting from the desalination process when the demand is shorter than the supply. As such, the key design parameters of this section include the volume of the tank (V), as well as the heights of the tank which are the standing height (Hs) and the height of the tank itself (Ht) and width of the tank as they might impact the pressure at the outlet of the tank (Po) which is specified by the client brief to always be at a minimum of 80kPa. The water storage design section also has to factor in the surface area of the tank which is important for the estimation of the amount of material required to design the tank (Ar) as well as the cost per m2 of stainless steel which stands at $120/ m2. For the sake of easily determining the volume of the tank, the shape is expected to be rectangular with a square base and supported by steel frames to make a stand. The total height of the tank and the stand is limited at 12m and the maximum width is 3.5m.
The volume (V) of the tank in litres can be estimated using the dimensions of the tank according to equation 1 below
Where l is the length
w is the width, and
h is the height
Converting to litres is achieved through the relationship stated below
The outlet pressure of the storage tank is also estimated using the relationship between water depth and the stand height (totalling to the total height) to pressure in pascals as is detailed in equation 2 below which is also known as the Pascal’s Law (Moaveni 2016, pg. 314)
Where is the density (kg/m3)
g is (9.81 m/s2) and
H is the height of the fluid column
Given that the height in this design is constrained by the design specifications to be a maximum height of 12m. the different heights are established using the formula below
The tank dimensions of the length, width, and height, can be used to compute the surface area.
Since the tank is taken to have a square base, and is covered at the top, the equation changes to become equation 3 below
Where is the width and
is the height of the tank..
Graph 1 – Graph showing the relationship between tank height and tank dimensions (volume, pressure, surface area).
The value of the surface area is therefore used to compute the cost of the stainless steel which is calculated as follows
Cost of material = $120/m2
In this design section, saline water is pumped to the reverse osmosis filter which then filters and converts the saline water into fresh water stored in the tank, and concentrate that is pumped back into the ocean. As such, this technical analysis outlines the equations used to evaluate the key variables of the desalination unit including the individual flow rates of the water in the different ends of the pump and RO unit. Thus the key parameters to consider in this design section will include flow rates of the potable freshwater going into the tank (VFRw), that of the saline water being pumped into the desalination unit (VFRs) and that of the concentrate (VFRc). The design values of the pump including the output power (Pp), the power input (Pi), and the power recovered device (Pr) are the important pump factors that ought to be considered. It is also important to identify the height to which the potable water rises after the R0 filter (H).
These variables ca be obtained by considering the equation below
Where power = output power
pressure = required pressure of fluid must travel for the filter to work.
The values of flowrate can then be converted to L/s using the following formula
The water flow rate (VFRm) of the potable water is related to the volume of the tank and its dimensions. This is given by the equation below:
Where volume passing a point in litres and
time for the fluid to pass the point in seconds. (Moaveni, 2016)
This is also shown in the figure below.
Graph depicting the linear correlation of the tank volume, the VFRm, and the pump performance.
The power parameters of the pump are also important factors to consider when selecting the pump type that is suitable for the client’s specifications. These power parameters of the pump include the output power (Pp) and the power recovered (Pr). The height the portable water can rise after leaving the desalination unit is also an important factor that will be estimated in this exercise. These equations respectively will be used for the estimation of the power output of the tank and the power recovered.
Where Pin is the power input
Pout is the output power
Pr is the power recovered
The height the portable water can rise after leaving the desalination unit is given by the Pascal’s Law (equation 2)
Where: = density
= acceleration due to gravity
=water demand=4000L/day
H=12m height
in this case the height the pump needs to pump the water is fixed at 12 m. This means that all the available pumps suggested in the client brief are suitable for this design. However, they are limited by the cost of the pumps as the technical analysis also seeks to ensure that the cost budget is not exceeded.
The solar system plays the role of charging the batteries during the hours when sunlight is experienced so that the charge is stored in the batteries is used to operate the pump and the filter. The panel is made up of an array of panels with an average rated output (Ppan)of 265W.
The number of solar panels (Np) required is obtained by multiplying the battery bank kWhr rating by the rated output power of the panels (265W).
Energy requirement from the battery pack
The solar panels will also recharge the batteries at the output power of 265W for the period of 6 hrs which will count as the Tc. This period is selected to allow for maximum solar insolation throughout the day.
The price of each solar time
The role of the battery is to accept the charge or the electrical energy harnessed by the solar panels throughout the day via a charging unit, so that the energy stored is used to pump and run the RO filter during the night using a control unit. The pump is expected to be operating for 8 hours (Tc) and thus only two battery types can be considered for the specific design, namely the lead acid battery and the lithium ion battery. The following are the attributes of the batteries:
Table 1 – Summary of the attributes of the batteries
|
Cost |
Efficiency |
Life Span |
|
|
Lead Acid |
$50/kWhr |
70% |
2.5 years |
|
Lithium Ion |
$350/kWhr |
95% |
10 years |
The following are the design parameters that relate to the batteries:
Where S.C= Storage capacity
= energy required for the pump and filter run
Eff batt=
Discharge percentage=
Key variables relating to the batteries are: energy stored in the batteries required to operate pump for 8 hours and the energy required the charge the battery for 8 hours.
(Moaveni,2016)
Table 2 – The important design parameters of the battery design
|
PM-1 |
PM-2 |
PM-3 |
|
|
Eout |
144,000,000 J or 40 kWh |
172,800,000 J or 48 kWh |
201,600,000 J or 56 kWh |
|
Ein (lead acid) |
100,800,000 J |
120,960,000 J |
141,120,000 J |
|
Ein (lithium ion) |
151,200,000 J |
164,160,000 J |
191,520,000 J |
|
Tc (lead acid) |
50,400,000 J |
60,480,000 J |
70,560,000 J |
|
Tc (lithium ion) |
23,400,000 J |
28,080,000 J |
32,760,000 J |
|
Cbat (lead acid) |
$8,000 |
$10,000 |
$12,000 |
|
Cbat (lithium ion) |
$14,000 |
$16,800 |
$19,600 |
Table 2 represents the standard values of the important parameters for these two types of batteries. It details the values of the number of batteries and the total cost of batteries is based on the specified life of the battery and the length of time the charging will be carried out were selected depending on the life span of the battery. Lead acid type batteries are known to be useful up to 2.5 years while the lithium ion batteries are known to be useful for up to 10 years as is detailed in the client brief. This implies that in the course of the 10 year life span designed for the project, the lead acid batteries will be changed 4 times while the lithium ion batteries will operate once throughout the entire life span. These lead acid batteries are also known to be less efficient than the lithium ion ones. This does not necessarily mean that the lithium ion batteries will be the most cost effective option.
The budget of the design project is set at $34,000 total over the 10 year period as is indicated in the client brief. The total cost of the project can be determined by considering the cost of the tank (Ct), that of the pump (Cp), the RO filter (Cro), the energy recovery unit (Ce), the total cost of the selected battery (Cbat) and that of the solar panels (Carrray). The cost of the pump is $4,000 as it is the smallest and yet it can still work. The cost of the RO filter is $5,000 and that of the energy recovery unit is $1,000 as indicated in the client brief . The cost of both the batteries and the solar array are also computed and added to this section
The design should not exceed a maximum budget of $34,000
To guarantee the design parameters for the individual components of this desalination system, the specifications of SeaFont Pty as well as the principles of engineering science were considered and made the foundation of this design. The lowest cost option produces a minimum of 4000 l of water per day and can store about 4 days’ worth of water in case there any fluctuations in demand or the system breaks down. The budget established comes to $ 29,635.87 which is about $4,364.13 under the 10-year long budget for the entire life of the project. The system has been designed to produce about 16,077 litres of potable water a day and the storage is big enough to store water for 4 days is budgeted at $9,304.
Table 3 – Showing 3 possible solutions and the variables used to determine them.
This shows that the design has been able to maximize the volume of the water storage tank and maximize the volume follow rate that the desalination system can convert following the basic principles of engineering science applicable in the design of engineering science around the solar desalination system. In so doing the design was able to stay within the budget of the project and even minimize it to the smallest amount possible. The table above shows the 3three options of best fit and how they conform to the design goals. The recommended option is therefore option 2 which guarantees the maximum water storage volume optimizing the usefulness of the project to the affected community.
References
SeaFont Pty Ltd 2018, Client Brief V2.0, SeaFont Pty Ltd, Southport, Queensland
Mahmoud, M. M. (2003). Solar electric powered reverse osmosis water desalination system for the rural village, Al Maleh: design and simulation. International Journal of Sustainable Energy, 23(1-2), 51-62
Moaveni, S 2016, Engineering Fundamentals an introduction to engineering, 5th Edition, Cengage Learning, Boston
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