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Solar water purification system with solar heating
Note this project covers two technologies to purify water, which should be broken into two - as distilled water does not need further purification.
- 1 Introduction
- 2 Principle
- 3 Design
- 4 System Construction
- 5 Limitation
- 6 References
Solar water purification system is a water purification system at household level based on solar radiation treatment and water distillation with additional use of solar heating. It is a combination of two water purification processes, the Solar Water Disinfection System (SODIS)and the solar distillation process. Since SODIS, initiated by Professor Aftim Acra, is only ideal to disinfect small quantities of low turbidity, micro-biologically contaminated water, a solar heated still is added to the system to address the issue of heavily contaminated water( such as sea water, water with high turbidity and water contaminated by heavy metal or pathogenic microorganisms).
For the cases where low turbidity water is not available, contaminated water will be distilled to drinking water using the solar heated still to remove any non-volatile solid impurities such as salts, sediment, heavy metals and microorganisms . Water from some wells or rivulets may be visibly clear (turbidity of less than 30 Nephelometric Turbidity Units), but it may not be drinkable since the water may still contain pathogenic microorganisms. To solve this problem, the contaminated water would be contained in clean, transparent Polyethylene terephthalate (PET) bottles and are exposed to the sunlight for a certain amount time (depending on the intensity of the sunlight) allowing the solar radiation to deactivate any waterborne pathogens in the contaminated water. Solar water disinfection is an effective way to disinfect drinking water as it is recommended by World Health Organization. The solar water purification system uses only solar energy and can be built using recycling materials, thus, the system is environmentally sustainable.
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, pathogenic microorganisms and heavy metals behind. However, the distilled water may not be suitable for drinking since it may still contain some volatile organic compounds. 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% 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 PET bottles for 6 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.
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 low turbidity, micro-biologically contaminated water and disinfects it to drinkable water with utilization of solar radiation. The process can be summarized in Figure 1. Insulated or thermal resistive piping system is used to connect all three systems and the piping system should be as short as possible to minimized the heat losses. For the water transportation, Polyvinyl chloride(PVC) piping is recommended due to its sufficient chemical resistance.
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 house 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 needed 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. Note that the total height of the column is not equal to the sum of exact height of each can since aluminum cans are designed to fit on top of each other with use of groove.
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 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.
The evaporation rate can be calculated as below :
- heat of vaporization is heat of vaporization of water = 2.27 MJ/L 
- 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. The typical efficiency for single basin solar stills approach 60  percent.
- is the efficiency of the flow channel manifold, as the fraction of the energy transfered to the water to the energy collected from the solar energy collector.
- 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   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):
- 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).
- Cp is the specific heat of air (MJ/kg/K).
- 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.
A simple calculation can be done as follow:
- Daily hours of sunlight = 5 hours/day = 5 hours/day x 3600 sec/hour = 18,000 sec/day
- = = 60%
- Daily global solar irradiation (G) = 1.0 kW
- The solar energy obtained from the solar collector (S)= 1.2 kW based on the Model RA 240 SOLAR MAX by Consolair.In
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 water.
Required Materials and Tools
To construct this system, following materials and tools are required.
- Thermal conductive metal (such as aluminum, cooper or zinc) sheet for component bodies
- Aluminum cans
- Clear PET bottles (water bottles)
- PVC piping system (parts depend on the size and the layout of the system)
- Nails or screws (sizes depend on the size of the lumber)
- Measuring tape
- Metal sheet cutter
- Hand saw
- Silicone glue
- Exacto knife
- Electric or hand drill
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.
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.
Step 3: Cut the top off the can.
Step 4: Paint the outer surface of the can in black. Indoor paints may crack if they are exposed to heat and UV, therefore, it is recommended to use weather/UV resistant paint.
Step 5: Glue all the cans into columns. Size of the column can be varied for different needs. Silicone glue is recommended since other silicone/latex or pure latex glues require a long time for the fumes to goes away.
Step 6: House all the columns. The frame can have multiple inlets/outlets and can be made out of wood or metal.
The dimension of the model shown below is based on 2 inches by 4 inches lumber and 355ml aluminum pop can.
If available, a 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  above the horizon to accommodate the sun's path. The temperature gain is approximately 10-20  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.
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.
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.
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.
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.
The solar powered distillation system is assembled as follow:
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.
Step 2: Cut the cylindrical tube open to obtain an aluminum sheet.
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.
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.
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.
The solar disinfection system is recommended to be made from thermal conductive materials and can be assembled as below:
Water sources must be tested for the Physical and chemical water quality(turbidity, oxygen and color) and microbiological water quality (pathogenic microorganisms) before treatment since SODIS can not change any chemical quality of the water and the solar still is not capable of filtering volatile organic compounds.
Adding minerals to distillated water
Distillation has the ability to remove almost everything from the water, except some volatile organic compounds who volatilized more easily than the water and carried away with the vapor to condense again in the distillate water. Because everything else is left out, mineral salts are absent from distillated water. A minimum level of mineral salts is necessary for water to be proper for human consumption. Hence mineral salts should be added to distillated water before drinking, which can be done by letting blocks of granit rest in the water for a short while. Precisely detailed process and references on the needed concentrations of mineral salts in the water to make it drinkable are needed.
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. To identify PET bottles from PVC bottles, if PVC is burnt, the smell of the smoke is pungent, whereas the smell of PET is sweet.
Experiment shows that ordinary window glass of 2mm thickness transmits almost no UV-A light  due to its iron oxide content.In other words, glass (except Pyrex, Corex, Vycor, Quartz) bottles can not be used for SODIS.
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.
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%. It is recommended to use 1-2L PET bottle for the SODIS.
The SODIS does not work satisfactorily during lengthy periods of rain. On these days, it is recommend collecting rainwater.
- Solar Still Basics by Solaqua
- Solar Disinfection System Manual by EAWAG and SANEC,Page 15
- Waterborne pathogens,WHO
- World Health Organization
- Volatile Organic Compounds in Consumer and Commercial Products, definition of VOCs
- Solar Disinfection System Manual by EAWAG and SANEC,Page 11
- Solar Disinfection System Manual by EAWAG and SANEC, Page I
- Solar distillation by Practicalaction.org
- Water vapor
- Still operation by Solaqua
- Satellite observations of total solar irradiance
- Solar constant by Physikalisch-Meteorologisches Observatorium Davos World Radiation Center
- Consolair Solar Inc.
- RA 240 SOLAR MAX model specification by Consolair Solar Inc.
- Solar Disinfection System Manual by EAWAG and SANEC,Page 16
- Sommer B. et al. (1997): SODIS – An Emerging Water Treatment Process, J. Water SRT – Aqua 46, pp. 127-137.