Figure 1: Sketch of the Solar Desalination water tank
1 Sea water inlet: allows sea water to be pumped into the evaporation chamber, while acting as a cooling coil for water to condensate over in the condensation chamber.
2 Evaporation tank: a tank to hold sea water at 20kPa to allow water to boil at 60°C.
3 Heat rods: transfers heat absorbed from the solar absorber directly into the evaporation tank where the sea water will sit.
4 Solar absorber: absorbs heat energy from the sun’s rays.
5 Acrylic dome: allows sun light (energy) to reach the solar absorber, while preventing wind convection that may cool the absorber.
6 Condensation chamber: a tank to hold fresh water condensed from vapour from the evaporation tank.
7 Insulation Layer: made of polystyrene and surrounded by a concrete barrier prevents any heat loss from the tank. The concrete supports the acrylic dome structure.
8 Fresh water outlet: allows fresh water from the condensation chamber to be pumped out for distribution.
9 Salt gradient: absorbs and stores heat transferred from the solar absorber to allow a steady boil during times of absence of sun light (usually at night).
10 Brine discharge: pumps out the brine solution left over from the evaporated sea water.
Figure 2: P&ID of Solar Desalination tank
- Sea water is pumped through the sea water inlet pipes into the evaporation tank.
- Air is sucked out until pressure reaches 20kPa.
- Solar absorber absorbs heat from the sun and transfers the heat into the sea water through the heating rods.
- Water starts to boil at 60°C and begins to evaporate. The vapour rises to the top of the chamber, and circulates into the condensation chamber.
- Vapour condenses over the sea water inlet pipes to condense into fresh water. Energy from the condensation process is transferred back into the sea water inlet pipes and the evaporation chamber walls to regenerate the heat.
- Fresh water is pumped out through the fresh water outlet pipes for distribution.
- Brine solution is discharged once all the water has evaporated from the salt.
2.2 Site
Figure 3: Map location of Hastings
The most important criterion in choosing a site for a desalination plant is the availability of water. Hastings was chosen as it satisfied this criterion, as well as having the following points:
- Relatively low salinity sea water readily available
- In close proximity to a water treatment plant at Westernport
- Government proposed area
Figure 4: GPS photographic view of Hastings
3. Calculations
3.1 Water Calculations
Total solar exposure based on the annual daily solar exposure taken from the Bureau of Meteorology = 18 MJ/m2
Area of absorber = 78.54 m2
Area of ellipse glass = 7.854 m2
Total area = 78.54 m2 + 7.854 m2
= 86.394 m2
Efficiencies of: Glass (acrylic) transmittance = 93%
Solar absorber disc = 95%
Total solar energy absorbed to heat water = 93% × 95% × 86.394 × 18 MJ
= 1.3739x109 J
Amount of energy required to desalinate 1kg of water = 71570 J
Amount of water evaporated = 1.3739x109 J
71570 J/kg
= 19196.92 Litre
Total tanks required to meet targeted amount of water production = 2605
Average water consumption in Melbourne per day = 1000 MLitres
Targeted water production per day = 50 MLitres
Average water consumption in Melbourne = 500 GLitres
per year according to household usage
Targeted annual water production = 18.25 GLitres
= 3.65% of 500 GLitres
For example calculations in a specific day in 1/2hr intervals, see APPENDIX A1.
For detail calculations based on the annual daily solar exposure taken from the Bureau of Meteorology = 18 MJ/m2, refer to APPENDIX A2.
3.2 Structural Integrity
For optimum efficiency and operating conditions, the tank where the distillation process occurs must be at a pressure below 101kPa (atmospheric pressure) to allow water to boil below 100°C. The minimum pressure to satisfy this condition is 20kPa, which must be held constant inside the tank. This will cause an external pressure of 81kPa from the atmosphere onto the outside surface of the tank. To avoid deformation, or in the worst case scenario structural failure, a suitable wall thickness must be determined.
Using Australian Standards AS1210 for design and construction of pressure vessels, the following wall thicknesses were found:
All wall thicknesses are able to withstand the external pressure of 81kPa.
Figure 5: Dimensions of pressure vessel.
Detailed calculation is in APPENDIX B.
4. Pumps
In addition to being able to transport the respective fluid, each pump also has to meet several criteria in order for selection. These include:
- Operable between 20-80°C.
- Can undergo a pressure change of 81.3KPa.
- Cost effective/economical, efficient
- Considerable corrosion is avoided
- Low maintenance
Fresh Water pump
- Pump used to remove fresh water from pressure vessel.
- Chosen: Uniform pressure centrifugal pump, model 15075HS18.
- Operates on ¾ hp motor powered by 115V.
-
Evaporation rate of fresh water is 0.8 m3/hr. Maximum discharge capacity of this pump is 5.7m3/hr.
- This pump is to be switched on for one hour, approximately every five hours, to remove the fresh water.
- The energy requirement of this pump is 4027KJ/day. This is for one tank. We are operating 2605 tanks simultaneously.
Figure 6: Fresh water pump; model 15075HS18
Brine pump
- Pump used to remove brine and other impurities from pressure vessel.
- Chosen: Diaphragm pump, model E5PS2X119.
- Operates on 1/16 hp motor.
-
Brine product is produced at 0.2 m3/hr. Maximum discharge capacity of this pump is 3.18m3/hr
- The brine product is left to build up for the first eleven hours of the operational day, then pumped out during the last hour of operation in the day.
- The energy requirement of this pump is 167.8KJ/day/tank.
Figure 7: Brine pump; model E5PS2X119
Air-vacuum pump
- Pump used to remove air to maintain partial pressure at 20KPa.
- Chosen: Centrifugal pump, model Mountain 8403-1.5CFM.
- Operates on a 1/6 hp motor.
- This pump will operate at the same time as the fresh water pump. That is, will be switched on for one hour, approximately every five hours.
- The energy requirement of this pump is 895.0KJ/day/tank.
Figure 8: Air-vacuum pump; model Mountain 8403-1.5CFM
Salt water /Ocean water pumps (×2)
- Pump chosen to transport the ocean water into pressure vessel. These two pumps are identical.
- Chosen: Centrifugal pump, model ZC2/ZCH2
- Operates on a 1/3 hp motor.
- The two ocean water pumps will operate simultaneously, and at the same times as the fresh water and air-vacuum pumps.
- The energy requirement of both these pumps is 3580.0KJ/day/tank.
- Two pumps are used because of the long distance the ocean water must travel in order to reach the desalination plant. Therefore, the water will lose an appreciable amount of head. The pumps increase the fluid’s velocity, thus increase the head, giving it the energy required to enter the low pressure vessel.
Figure 9: Salt water pump; model ZC2/ZCH2
5. Materials
One of the major criteria for this desalination project is the material selection. Thus, selecting appropriate materials are important in order to attain the best result. The materials selected are mainly for the main components of the plant, which are the solar absorber, solar retainer, tank, pipes and valves. The materials chosen are dependant on its strength, quality, corrosion resistance, cost and its environmental impacts.
The table below shows summary of materials chosen for different components of the plant. It also includes the reasons of choice and possible corrosion with a solution.
Solar absorber
This component is a vital component as it absorbs as much sunlight as possible from the sun and provides heat into the pressure vessel to evaporate seawater. It is made of an aluminium plate, which is coated with black chromium. The black chromium coating allows the aluminium plate to absorb sunlight more efficiently to heat the metallic rods for evaporating water. Therefore, this combination of aluminium plate coated with black chromium provides a higher thermal conductivity and resistance to corrosion.
Dome
This component is specially made of acrylic, which is manufactured into triangular plates and connected together to form a dome. It is located above the solar absorber and acts as a “roof” to the tank. Due to acrylic’s flexible property, it could be easily manufactured into any shape, which is the reason for the use of this material. It also has a high breakage resistance of sustaining impact 6 to 17 times greater than ordinary glass. In addition to its good resistance to weather, acrylic is the most ideal material to be used for the dome.
Safety:
Acrylic sheets are less hazardous as it breaks into large relatively dull edged pieces which are dispersed at low velocity, due to its light weight.
Tank consists of 4 layers:
-
The inner-most layer (pressure vessel), which is in contact with seawater, is made from 70-30 Cu-Ni alloy. The material has been widely proven for sea water handling due to its electrode potential that is adjusted to be neutral with regard to seawater which contains chloride about 1.94%. The resistance to sea water corrosion is resulted from the formation of a thin and adherent protective surface layer of nickel. Evidently, this copper-nickel alloy is greatly used in manufacturing ships and submarines. However, chloride pitting and crevice corrosion might occur due to any possibilities of cracking initiated in the vessel.
(Issues regarding Copper-Nickel alloys – stated in APPENDIX D)
- The second layer which consists of a tank containing fresh water is made from austenitic Stainless steel 316. Due to its ability to be easily fabricated aids in the manufacturing of the shape of the tank. Its high strength is also able to sustain any force that is exerting on the tank. This material on the other hand might experience pitting and crevice corrosion due to the chloride content in seawater.
-
A layer of insulation is being wrapped around the stainless steel tank, which is made from polystyrene (0.033 W/mK thermal conductivity), contains the heat in the vessel. It also has high resistance to oxidation at high temperatures and galvanic corrosion.
- The outer-most layer is made from concrete as it is strong and act as a strong foundation for the whole tank. In order to increase the strength of the concrete, reinforcement steel bars are inserted into the concrete. However, there might be localized and reinforced cracking corrosion occurring in the long-term.
(Definitions of corrosion and possible solutions can be found in APPENDIX D)
Some Examples of Corrosion:
(Biofouling)
(Metallic Corrosion) (Stress-Corrosion Cracking)
Pipelines
For all of the pipes transporting seawater and brine are made of Polyvinyl Chloride (PVC) pipes. However, Chlorinated PVC pipes are used for freshwater transportation (An issue of PVC - stated in APPENDIX D). PVC is chosen for its high resistance to hot and cold conditions, especially in low pressure system. It is cheap, durable and has high tensile strength, with a low maintenance cost.
Valves
The valves materials that are used in the water supply industry are generally made of coated cast iron or steel but with internal trim in non-ferrous material. Monel and aluminium bronze AB1 or AB2 are often used with aluminium bronze CA104 for spindles due to high strength. Aluminium bronze is much to be preferred because of the liability of high tensile properties to selective phase dealloying (dezincification) in some waters compare to other. However, aluminium bronze is not generally used in small stop-valves for domestic water installations except for valves that are to be installed underground.
6. Emissions and Efficiency
Electricity generated from brown coal, is one of the most CO2 intensive forms of energy. To calculate the greenhouse gas emissions, we have assumed that all processes are using the existing electricity grid. In Victoria, 1 kWh of electricity generates 1.467kg CO2.
Table 1 shows the CO2 generation for four main desalination processes. There are, however, no direct CO2 emissions from our solar desalination plant. The only production of CO2 is from the powering of pumps which is shown in Table 2.
Table 1: Comparisons of CO2 generation for different processes
For our solar desalination plant, we are producing 500 Mega litres of fresh water per day. Therefore, the total CO2 generated from the pumping of water is 94.9 tonnes per day. From Table 1, it is clear that our solar desalination plant emits less C02 than the other desalination methods.
In comparison with seawater reverse osmosis (SWRO) which is mainly used in Australia, the production of greenhouse gases is approximately 2.542 kg less per tonne of desalinated water by using our design.
Table 2: CO2 emissions for the different pumps per kg of desalinated water
Efficiency
Based on the energy required to produce 1kg of fresh water in Table 1, the efficiency of our system is calculated using BWRO as the reference point.
Efficiency = × 100%
= 93.00%
Our solar desalination plant has an increased efficiency of 93%.
7. Financial Analysis and Market Assessment
7.1 Concept Design
By 2021, it is anticipated there will be an increase of 800,000 people (Figure 1 in the appendices) living in the Melbourne metropolitan area and by 2055, the population increases to 4.6 millions. This is a 31% increase from today’s population of 3.6 million. These projections are based on overseas migration, fertility and death rates and net migration from Melbourne to regional Victoria. However, population growth can be difficult to accurately forecast and variations to assumptions may affect Melbourne’s population growth. While per capita demand for water has increase in the past 10 years because of population growth (Figure 2 in the appendices). Climate change is also an issue where the drought has been occurring in the last decades.
The current water consumption by Victorians is as follow:
(Source: )
As the graph (Figure 4) shows a drop in the water consumption is because the water restriction program has been applied. However, the graph shows an exponentially increasing water demand due to the population growth.
To summarize the problems, the survey has been made to a 100 people in Victoria;
- 71 percent report have low-flow toilets.
- 61 percent say they can’t do much more to conserve water.
- 52 percent regularly drink tap water.
- 51 percent rated future water supplies as a very serious problem
7.2 Alternative
Solar Desalination is one of the options to overcome these problems among other desalination processes (Reverse Osmosis, Multi Stage Flash Desalination, etc.). The reason that this type of desalination is chosen is because it has many advantages such as;
-
- Excellent product water quality for drinking and every other usage as its process is taken from natural convection loop.
- - UV-disinfection unit for safe storage of the produced water over a longer period (prevents re-contamination with bacteria or viruses)
- - Energy efficient operation by internal energy recovery method
- - No chemical pretreatment needed
- - Long Refurbishment Intervals (every 10 years)
- - Nearly any raw water can be used
- - Low temperature waste heat
- - No moving parts ,therefore Low Maintenance Cost
- - No pre-treatment of raw water
The solar desalination project aimed at the household market which takes 9% of the total usage of the water consumption in Victoria (5.7 Millions ML)
From the appendices attached, it is said that the targeted amount of water produced annually is 18,250,000,000 L. With the charging price per 100L of 50 cents, it is expected that the annual income should come about AU$ 91,250,000.
In this plant, 2605 tanks are mass produced where the cost of one tank is approximately AU$50,000. Therefore the total capital cost described in the appendices is around AU$147,932,190 and the maintenance cost over the years is about AU$13,500,000 (apart from the refurbishment cost every 10 years time of AU$4 millions). From the given government budget as in 2005-2006 budget plan, it is said that government will provide AU$227 millions to any water related project which includes;
- Contribution to COAG Living Murray;
- Smart urban water initiative and recycling,
- Protecting and repairing water resources,
- Boosting water smart farms and sustainable irrigation practices; and
- Water security for cities, farms, and the environment.
This shows that the Solar Desalination plant will meet the government budget as it is also a big investment project that has never been applied in Victoria. With the rate of return of 8% a year, the total cost of construction and the capital cost of the plant will be covered up in 6 years time (by 2012).
Comparison to other desalination plant:
The table shows that in conclusion, solar desalination provides cost efficient since its process is taken from the natural convection loop, therefore the cost of maintenance is lower. It is also energy efficient since it is taking the sun’s energy to evaporate the sea water to become pure water.
The final conclusion on this decision making is that the solar desalination is chosen by the fact that it has energy and cost efficient. Hence, it also provides a ‘greener’ process since the emission is much less than that of other desalination plants.
8. Conclusion
The design comprises of 6 main aspects concerning design concepts, calculations for water output, structural integrity, choice of pumps for water pipeline network, materials selection, carbon-dioxide emissions and financial analysis. Each aspect has been critically assessed to meet all standard requirements to produce a fully operational and environmentally friendly solar desalination plant.
