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Introduction

Similar to Solar Water Disinfection System (SODIS) initiated by Professor Aftim Acra, solar water disinfection system with solar distillation is a water purification system at household level based on solar radiation treatment with additional use of solar heating. Since conventional SODIS is only ideal to disinfect small quantities of low turbidty, micro-biologically contaminated water, a solar heated still is added to the conventional SODIS to pre-filter the contaminated water.

Contaminated water is first distilled by the low-temperature distillation system to remove any non-volatile solid impurities such as salts, sediment and heavy metals. However, the distilled water is not drinkable yet since the water may still contain some pathogenic microorganisms . To solve this problem, the distilled water is then contained in clean, transparent PET bottles and are exposed to the sunlight for a certain amount time (depending on the intensity of the sun light) allowing the radiations to deactivate any waterborne pathogens[1] in the contaminated water. Solar water disinfection is an effective way to purify drinking water as it is recommended by World Health Organization[2]. The system uses only solar energy and can be built using recycling materials, thus, the system is environmentally sustainable.

Principle

Water distillation is a physical process that filter solid impurities out of fluid based on the difference in the volatility. At a given temperature and pressure, substances with higher volatility (water in this case) vaporizes more readily than the substances(solid impurities) with lower volatility. The water vapor is then directed to a cool region which condenses the water vapor back to liquid state, leaving all the non-volatile solid impurities such as salts, sediment and heavy metals behind. However, the distilled water may not be suitable for drinking since it may still contain some volatile organic compounds[3] or/and pathogenic microorganisms (bacteria, viruses and in some cases protozoans). The rate of vaporization is proportional to the vapor pressure, fluid surface area and the fluid temperature.

The principle of SODIS is based on Ultraviolet water treatment . It uses two components of the sunlight for the water disinfection process :Ultraviolet radiationand infrared radiation. UV-A radiation(wavelength 320-400 nm) interacts with the DNA, nucleic acids and enzymes of the organic cell, destroys the cell molecular structures which leads to cell deaths. UV-A radiation also reacts with oxygen dissolved in the water producing highly reactive forms of oxygen (oxygen free radicals and Hydrogen peroxide], that can help the germicidal process. Infrared radiation is a long-wave form of sun radiation, it can be felt as heat, as it is responsible for raising the fluid temperature. Studies had proven that 99.9%[4] of microorganisms in the water are eliminated if the water is heated to 50-60°C for one hour. In order to disinfect contaminated water for drinking effectively, it is recommended to expose the contaminated water to full sunlight using clear Polyethylene terephthalate (PET) bottles for 6[5] hours.If water temperatures exceed 50°C, one hour of exposure is sufficient to obtain safe drinking water.When the weather is cloudy for more than 50% , the contaminated water need to be exposed for 2 consecutive days. The treatment efficiency can be improved by raising the fluid temperature and exposing the contaminated water to additional reflecting surfaces such as aluminium or corrugated iron sheets.

Design

The system is consisted of three main components: the solar energy collector, the solar distillation system and the solar water disinfection system. The solar energy collector is a device that collects solar radiation and converts it into thermal energy for the SODIS and the solar distillation process. Solar Distillation system is similar to the conventional water distillation system, except it does not vaporize the water at boiling temperature. Solar water disinfection system takes the distilled water from the solar distillation system and disinfects it to drinkable water with utilization of solar radiation. The process can be summarized in Figure 1. Insulated or thermal resistive pipes are used to connect all three systems and the piping system should be as short as possible to minimized the heat losses.


Figure 1: SODIS with Solar Distillation Schematic Layout

Solar Energy Collector

The idea is first developed by Cansolair Inc., converting solar energy to house heating energy using aluminum can. Solar energy collector is composed of columns of painted black aluminum can, a frame to hold the columns and a ventilation for the heat transportation. Before all the cans are glued together to form a collected column, the top and the bottom of aluminum can is need to be removed. When placed under the sunlight, the columns absorb the solar radiation and heat is convected to the air inside the columns. Due to difference in the air density , warm air would raise to the top of the columns and cool air would be sucked into the columns from the bottom. The warm air flow is then collected at the top of the columns. The columns are painted in black to enhance the radiation absorbability and the size of the columns can be varied for different requirement.

Figure 2: Solar energy collector

Solar Distillation System

Solar distillation system is composed of a vaporizer that holds the water, a vapor condenser that collects and condenses steam and a water collector that collects distilled water. The rate of vaporization is proportional to the fluid surface area and the fluid temperature. The improve the performance of the still, the vaporizer should be made as large as possible. Also, at the bottom of the vaporizer, there are some serpentine gas channels where warm air flow from the solar energy collector is directed into. Due to the temperature difference between the water and the air flow, heat is transfered into the vaporizer, causing the water temperature to raise, thus, speed up the vaporization process. Other methods such as using a thermal conductive materials, painting the vaporizer to black and using some reflective surfaces to concentrate the radiation can be used to improve the performance of the system.


Figure 3:Solar Distillation system


The evaporation rate can be calculated as below [6]:

where

  • Q is the daily output of distilled water (Liters/day)
  • is the efficiency of the still, as the fraction of the energy transfered to the water to the total absorbed solar energy.
  • G is the daily global solar irradiation (see solar insolation) (MJ/m^2). The typical solar insolation at the Earth's surface is approximately 1,000 [7] [8] watts per square meter for a surface perpendicular to the Sun's rays at sea level on a clear day. Based on the assumption of 5 hours of sunlight per day, the daily solar irradiation is approximately 18 MJ/m^2.
  • A is the still surface area (perpendicular to the sunlight).
  • S is the thermal energy obtained from the solar energy collector. It can be calculated using Enthalpy:
ΔH = :HfinalHinitial = Cp x (T2 - T2)

where

  • ΔH  is the enthalpy change.
  • Hfinal is the final enthalpy of the system, expressed in MJ.
  • Hinitial is the initial enthalpy of the system, expressed in MJ.
  • is the mass flow rate out of the air flow (kg/s).
  • T2 is the flow outlet temperature of the solar energy collector in Kelvin scale.
  • T1 is the flow inlet temperature of the solar energy collector in Kelvin scale.

Solar water disinfection system

To improve the efficiency of solar water disinfection system , reflective surfaces can be used to intensify the solar radiation toward the contaminated water. Another way to improve the system performance is to increase the fluid temperature. According to study, if water temperatures exceed 50°C, one hour of exposure is sufficient to obtain safe drinking water. This is when the solar energy comes in place. A portion of the thermal energy collected from the solar energy collector is directed to the heat up the bottled distilled water.

Figure 4:Solar water disinfection system

Solar Energy Collector Construction

The construction of the solar energy collector starts with preparing the aluminum cans. The construction process is shown as follow:

Step 1: Cut the pattern shown in Figure 5 at the bottom the can.

Figure 5: Cutting pattern at the bottom of the can

Step 2: Twist the cut-outs in one direction to form a set of vanes. The vanes at the bottom of the can are used to induce swirls to the flow, improving the heat convection process as air flow through the columns.

Figure 6: Demonstration of twisting the pattern in one direction
Figure 7: Finished pattern at the bottom of the can
Figure 8: Finished pattern at the bottom of the can (2)


Step 3: Cut the top off the can.

Figure 9: Demonstration of cutting the top and the bottom of the can
Figure 10: Demonstration of cutting the top and the bottom of the can (2)

Step 4: Paint the outer surface of the can in black weather(UV) resistant paint.

Step 5: Glue all the cans into columns. Size of the column can be varied for different needs.

Figure 11: Demonstration of can column

Step 6: House all the columns. The frame can have multiple inlets/outlets and can be made out of wood or metal.

Figure 12: The main frame of solar energy collector
Figure 13: The top frame of solar energy collector
Figure 14: Solar energy collector assembly

The dimension of the model shown below is based on 2 inches by 4 inches lumber and 355ml aluminum pop can.

Figure 15: Solar energy collector 2D drawing (in mm)

If available, a water sealed transparent case can be added to the design to protect it from weather. The solar energy collector should be oriented toward south and at an angle of 22-70 degrees [9] above the horizon to accommodate the sun's path. The temperature gain is approximately 10-20 [10] degrees Celsius above ambient temperature with a 240 cans system. A more detailed walk-through of the manufacturing process can be found here.

Solar Distillation System Construction

The solar distillation system is consisted of 4 main parts: vaporizer, condenser, water collector and channel manifold. The design shown below is for concept demonstration. It can be manufactured with metal sheet and can be resized for different needs. Thermal conductive materials such as aluminum, cooper or zinc are recommended for manufacture to maximize the thermal conductivity of the system.


Vaporizer is used to contain and vaporized contaminated water. After all the water has been vaporized, residuals in the vaporizer must be removed before loading another tank of contaminated water.

Figure 16: Vaporizer for the Solar distillation system


Condenser is designed to condense steam back to liquid water. Attention should taken when designing the slope of the roof. If the slope of the roof is too small, water condensation may not able to stream down to the edge of the condenser. The condenser is supported by 4 screwed on stands at two opposite walls and it should be made as thin as possible for the sufficient heat exchange ability.

Figure 17: Condenser for the Solar distillation system
Figure 18: Condenser for the Solar distillation system (2)
Figure 19: Condenser for the Solar distillation system (drawing)


Water collector is used to collect condensed water as they drip down from the edge of the condenser. For that, the dimension of the groove should be adjusted so that the position of the groove is under the edge of the condenser. The water collector is designed to sit on the vaporizer and to provide support to the condenser. To secure the water collector, a protrusion that fits the inner dimensions of the vaporizer is added to the bottom of the water collector.


Figure 20: Water collector of the solar distillation system
Figure 21: Water collector of the solar distillation system (bottom view)
Figure 22: Water collector of the solar distillation system (drawing in mm)



A cut out is placed on the wall of the water collector that is tangential to the groove bed for water drainage. The still should be placed on a horizontal surface if possible. Distilled water then would be collected for the solar disinfection process.

Figure 23: Drainage design of the water groove


The flow channel manifold is designed to transfer heat from the flow to the vaporizer, to raise the fluid temperature, thus, speed up the vaporization process. The channel layout can be varied depends on the flow speed. The serpentine design can provide a larger exchange area comparing to the straight design. However, the heat convection powered air flow may not able to flow through the serpentine channel due to losses. For low flow speed, the straight design layout is recommended.

Figure 24: Flow channel designs for the solar distillation system


The solar powered distillation system is assembled as follow:

Figure 25: Solar distillation system assembly

Solar Water Disinfection System Construction

To improve the effectiveness of the solar disinfection, reflective surface such as mirror or fine finished metallic surface can be used to concentrate radiation onto the contaminated water. Some radiation reflection ideas can be found here. One of the simple ways to reflect the radiations is to use the body of a aluminum pop can. The steps are as follow:

Step 1: Cut the top and the bottom of a aluminum pop can off to obtain the cylindrical tube.

Figure 26: Demonstration of cutting the body off the can

Step 2: Cut the cylindrical tube open to obtain an aluminum sheet.

Figure 27: Demonstration of obtaining aluminum sheet from pop can

Step 3: Resize the dimension of the sheet according to the size of the PET bottle. The sheet should be wide enough just to cover half of the bottle as shown. Due to the original shape of the can, the aluminium sheet would fit right onto the pet bottle.

Figure 28: Demonstration of fitting the aluminum can reflective surface to a PET bottle

The solar disinfection system can be assembled with two components: the bottle holder (Figure 28) and the heat distributer (Figure 29). The bottle holder can help to reflect the radiation if it is made of reflective material.

Figure 29:The bottle holder for the SODIS

Holes at the bottom of the the bottle holder and the heat distributer are designed to allow hot air flow from the solar energy collector to flow through, raising the fluid temperature.

Figure 30: The heat distributer for the SODIS

The solar disinfection system is recommended to be made from thermal conductive materials and can be assembled as below:

Figure 31: SODIS tray drawing
Figure 32: Heat distributer drawing
Figure 33: SODIS assembly

Limitations

Water sources

Water sources for the SODIS must be tested for the contaminations before treatment/drink since SODIS can not change any chemical quality of the water.

PET bottles

PET bottles should be used over PolyVinylChloride(PVC) bottles. UV-stabiliser is added to plastic bottles to increase their stability or to protect the content from oxidation. The use of bottles made from PET instead of PVC is recommended as PET contains much less additives than bottles made from PVC, which allow more UV radiation to transmit. Plastic bottles should be cleaned before use and any old or scratched bottles should be replaced.

Health concerns about PET bottles

Reports from around the world regarding substances in PET bottles that cause cancer may contaminate the content. According to WHO, a number of research institutions tested the scientific accuracy of these reports and carried out their own analyses of the materials. Studies have been produced for the following substances: antimony, adipates,phthalates,acetaldehydes and formaldehydes. These studies show that when the SODIS method is applied correctly with PET bottles, there is no danger to human health. The treated water should be kept in the bottle and drunk directly from the bottle, or poured into a cup or glass immediately before it is drunk. In this way, it can eliminate the possibility of the treated water becoming contaminated again.

Bottle sizes

The containers used for SODIS should not exceed a water depth of 10cm. UV radiation is reduced at increasing water depth. At a water depth of 10 cm, UV-A radiation is reduced to 50%[11].

Rain

The SODIS does not work satisfactorily during lengthy periods of rain. On these days, it is recommend collecting rainwater.

References

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