Technical Analysis Of Solar Desalination System For SeaFont

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In this project, SeaFont Pty Ltd proposes that a water desalinations system that is powered by solar is designed for the benefit of remote coastal communities. For this reason, the company is seeking to invite companies that are qualified and experienced in the design of similar systems so they could propose different design ideas that could solve the problems suitably (Fiorenza, Sharma& Braccio, 2003). The expected design is expected to compose of three main components namely the unit responsible for the solar power provision, the unit that will carry out the actual desalination process through reverse osmosis and the unit that will be used to store the desalinated, fresh water after the process (Gálvez, García-Rodríguez, & Martín-Mateos, 2009)

The following figure indicates the layout of the design as is expected.

This technical analysis report will analyse the functionality of the design to guarantee durability of the system and easy maintenance due to the remoteness of the communities that the design is targeted for. The design will entail specifically analyse the use of the right materials in the design to guarantee the economic feasibility and durability of the design (Hasnain & Alajlan, 2008) . The first parameter that will be addressed in this technical analysis will include the amount of water the system will be processing at a time as indicated by the depth of the water in the controller. Other factors to consider will include the  effectiveness of the desalination system which  will be indicated by the pressure of the water entering the desalination unit’s reverse osmosis filter, and the pressure of the water leaving it both as the fresh water and as the concentrate. The suitability of the size of the  system for the demand that the designed system is expected to supply will also be considered by analysing the volume of the tank and the  height differences to bring about pressure variations for the water to be drawn (Karagiannis & Soldatos, 2008).The report will analyse different variables in each of the three components as is detailed in the client The report will analyse different variables in each of the three components as is detailed in the client brief version of the design on the Brilliance in Mind Sharepoint. Some of the parameters to be considered include the stand height (H_s) and the tank height (H_T) of the water storage unit, as well as the volume (V) of the  entire unit. They also include the pressures of the liquid going into the Reverse osmosis filter (P₁) as well as the pressure of the fresh water (P₂) and that of the concentrate (P₃).   The depth of water in the controller (D) will determine the volume of the inlet to identify the Efficiency of the RO filter. These individual variables will be analysed in this report to bring out the any relationships that will impact the design of the system as well as the parameters that will influence the design parameters of the system (Khawaji, Kutubkhanah, & Wie, 2008).

Design Section 1- The Storage Tank

The design layout functions through the use of solar energy from the solar panels passing through the batteries to be converted into electrical energy that can be used to pump the sea water through a filter in the controller. The controller controls the pump on a simple timer such that the depth of water in it does not overfill before being drawn into the pump.  This process increases the pressure of the water entering the reverse osmosis filter. The filter then extracts fresh water due to the high pressure flow of the salty water from the input separating the contents into fresh water and the concentrate drawn from different pipes from the outlet of the desalination unit (Tiwari & Tiwari, 2008). The fresh water drawn is stored in a stainless steel tank from which it can be supplied to the beneficiaries of the project. The concentrate is then let out back into the ocean  by being allowed to pass  through an energy recovery device which recovers  50% of the  energy from the high pressure flow of the concentrate. The recovered energy is returns to the pump.  

The stainless steel tank serves the function of holding the fresh water extracted until a demand arises. The pressure of the water at the exit point of the tank is expected to be at 80kPa while it is being drawn for use by the community regardless of the height of water in the tank (H_T). For ease of volume determination, the design of the tank is to be of a rectangular shape with a square base and top. The stainless steel tank will also be supported by steel frames to make a stand. The total heigh of the tank and the stand is limited at 12m,, while the maximum width expected will be 3.5m.

In order to maximize the volume of the storage tank and guarantee that the pressure at the drawing point of the tank is maintained, the design volume ought to be computed using the formula for obtaining volume of a rectangular vessel. The volume of determined by: 

(Peñate & García-Rodríguez, 2012). Since the base is a square, the length and the width are equal and are limited to a maximum of 3.5m

The height of the tank will be determined by the height of the stand that will allow the minimum pressure of the tank to be 80kPa. Given that the maximum height permissible is 12m: 

Surface Area

Hs is a function of the pressure pf the tank and can be determined through

Since no atmospheric pressure will also be acting on the surface of the water in the tank, the pressure at the bottom of the tank can be kept at 80 Kpa through using the following formula:

Given that the pressure is known as well as the density of fresh water and the acceleration due to gravity is also known, the minimum height of the tank can be determined through substituting these values

Thus 

The maximum height of the tank is 3.80 m.

The maximum volume can thus be computed as 

Considering placing the tap at the bottom of the tank the pressure at outlet when tank is full will be given by:

 so the minimum pressure at drawing point is achieved.

When the tank is half full, the pressure at the drawing point is: 

which also qualifies

When the level of water is as low as 20cm, 

Which also qualifies as it is larger than 80kpa. (Laborde, et al., 2001 

For the surface area of the metal sheet to fabricate the tank will be the area of both the bottom and top surfaces, as well as the area of all of the four wall s of the rectangular tank, assuming that it will have a cover at the top.

This value is used to compute the cost of the material which is calcu;ltaed as follows

Cost of stainless steel = $120/m²

(Chaibi, 2000) 

Mass

Finally the mass of the tank plus the stored water will be estimated using the relationship between the mass  density and volume

Density of steel= 7860 kg/m3

Volume of the steel = volume of outer dimensions –volume of inner dimensions.  

Assuming a thickness of 2mm. the volume of the steel is given by :

(3.5*3.5*3.8) – (3.498*3.498*3.798) 

Volume of the water = volume of the space inside the steel container 

Density of water= 1000kg/m3 

For water:

For steel:

The total mass is thus given as: 

The pump used at the input of the desalination unit must deliver saline water at a pressure of 5Mpa so that the RO filter may deliver an efficiency of 25%. This means that only 25% of the water entering the filter will be converted into fresh water, while the rest will leave the filter as concentrate. The fresh water is also designed to exit the filter at a pressure of 150kPa while the concentrate will exit at a pressure of 4.85Mpa. 50% of the power used to run the pump will be derived from the energy recovery device without any losses. There will not be any pressure alterations due to height differences in this unit as the RO filter and the pump which make up the desalination unit will be placed on the ground for eased maintenance.

Mass

Taking the pump to be used as having a maximum power output of 5000W and a maximum pressure of 5500KPa,

The flowrate of potable water is computed first by estimating the power output of selected pump and the pressure at that point. 

On the other hand, the flow rate of saline water is given by( AlMadani, 2003) 

Thus the power is  (5000W) 

Taking the depth of water to be at 1m 

the flow rate of concentrate VFRC (in L/s) through the energy recovery device 

Power output of the Pump

5000*85% = 4250 W 

the power recovered by the energy recovery device (PR)

50%*4250W= 2125W

The electrical power input to the pump (PI) the height to which the potable water can be raised after leaving the Ro filter (H)( AlMadani, 2003) 

The table below summarizes the parameters of the design established in this report. 

Conclusion

The design of the solar powered desalination system for use by a remote coastal community was identified using the following equations.  

In this project, the design presented seeks to optimize the rate of filtering in the desalination unit in order for more fresh and potable water to be obtained by using this system. Further, the design seeks to ensure that the maximum amount of water is stored without the total budget of the project being exceeded.  This is achieved through the computation of the maximum volume of the tank after identifying the heights of the tank and the stand through guaranteeing that the pressure limitation at the bottom of the tank is not exceeded. This will guarantee that the capital cost of the equipment and materials required for the project is minimized and thus a greater benefit to the community through the project. The system  designed should be able to provide potable drinking water towards the storage at a capacity of a minimum of 4000L every day which tis estimated by the total volume of the tank which stands at 46.47m3. Further the storage ought to be able to store a minimum water supply of about 4days for the workers, just in case the system breaks down to avoid a lack of reliability and dependability. The minimum pressure at the drawing point also ought to remain 80kPa regardless of the water level in the tank.  The height of the total water storage is limited at 12m explaining why the 2 heights have to add up to 12m.  Since the design of the tank is such that the top of the tank will be sealed, it cannot be pressurized by the atmospheric conditions.. The width of the tank is also limited to a maximum of 3.5m. The design assumes structure of the tank will be made from a flat stainless steel sheet of a width of 2mm and thus the volume of the steel is computed by subtracting the volume of the space from the volume of the entire structure. The pump to be used is also expected to be a complete and the losses made in this design due to fittings and piping frictions are to be ignored. 

References

AlMadani, H. M. N. (2003). Water desalination by solar powered electrodialysis process. Renewable Energy, 28(12), 1915-1924.

Chaibi, M. T. (2000). An overview of solar desalination for domestic and agriculture water needs in remote arid areas. Desalination, 127(2), 119-133.

Fiorenza, G., Sharma, V. K., & Braccio, G. (2003). Techno-economic evaluation of a solar powered water desalination plant. Energy conversion and management, 44(14), 2217-2240.

Gálvez, J. B., García-Rodríguez, L., & Martín-Mateos, I. (2009). Seawater desalination by an innovative solar-powered membrane distillation system: the MEDESOL project. Desalination, 246(1-3), 567-576.

Hasnain, S. M., & Alajlan, S. A. (2008). Coupling of PV-powered RO brackish water desalination plant with solar stills. Desalination, 116(1), 57-64.

Karagiannis, I. C., & Soldatos, P. G. (2008). Water desalination cost literature: review and assessment. Desalination, 223(1-3), 448-456.

Khawaji, A. D., Kutubkhanah, I. K., & Wie, J. M. (2008). Advances in seawater desalination technologies. Desalination, 221(1-3), 47-69.

Laborde, H. M., Franca, K. B., Neff, H., & Lima, A. M. N. (2001). Optimization strategy for a small-scale reverse osmosis water desalination system based on solar energy. Desalination, 133(1), 1-12.

Peñate, B., & García-Rodríguez, L. (2012). Current trends and future prospects in the design of seawater reverse osmosis desalination technology. Desalination, 284, 1-8.

Tiwari, G. N., & Tiwari, A. K. (2008). Solar distillation practice for water desalination systems. Anshan Pub.