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The evaporation rate can be calculated as below:  
The evaporation rate can be calculated as below:  


:<math> Q = \frac{ S + \eta_{overall}\cdot A \cdot G}{2.3} \,</math>
:<math> Q = \frac{ S + \eta_{still}\cdot A \cdot G}{2.3} \,</math>


where
where
*'''Q''' is the daily output of distilled water (<math> \frac{litters}{day} \,</math>)
*'''Q''' is the daily output of distilled water (<math> \frac{litters}{day} \,</math>)
*'''<math> \eta_{overall} </math>''' is the efficiency of the still, as the fraction of the energy transfered to the water to the total absorbed solar energy.  
*'''<math> \eta_{still} </math>''' 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 [http://en.wikipedia.org/wiki/Insolation solar insolation]) (<math> \frac {MJ} {m^2} </math>). The typical solar insolation at the Earth's surface is approximately 1,000 <ref>[http://www.acrim.com/ Satellite observations of total solar irradiance]</ref> <ref>[http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant Solar constant ]</ref>  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.  
*'''G''' is the daily global solar irradiation (see [http://en.wikipedia.org/wiki/Insolation solar insolation]) (<math> \frac {MJ} {m^2} </math>). The typical solar insolation at the Earth's surface is approximately 1,000 <ref>[http://www.acrim.com/ Satellite observations of total solar irradiance]</ref> <ref>[http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant Solar constant ]</ref>  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.  
*'''S''' is the thermal energy obtained from the solar energy collector.
*'''S''' is the thermal energy obtained from the solar energy collector.

Revision as of 19:39, 31 March 2010

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Solar Water Disinfection System with Solar distillation

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 radiations with additional use of solar heating. Since SODIS is only ideal to disinfect small quantities of micro-biologically contaminated water of low turbidity, a solar heated distillation process is combined with the conventional SODIS. Contaminated water is first distilled by using a solar heated 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 since the water may still contain some pathogenic microorganisms . To address this problem, the distilled water is then contained in clean and transparent PET bottles or glass bottles and are exposed to the sun light 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]. It uses only solar energy and can be built using only recycling materials, thus, the system is environmentally sustainable.

Theory

Water distillation is a physical process that filter solid impurities out of fluid based on differences in the volatility. At a given temperature, substances with higher volatility (water in this case) vaporizes more readily than substances(solid impurities) with lower volatility. The water vapor would be directed to a cool region which would condense the water vapor back into 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 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 within water are eliminated if the water is heat heated to 50-60°C for one hour. In order to disinfect contaminated water for drinking, 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

Solar Energy Collector is a device that collects solar radiation and converts it into thermal energy for the SODIS and the solar distillation process. The solar energy collector is composed of columns of painted black pop aluminum can, a frame to hold the cans 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.

Solar Distillation system is similar to the conventional water distillation system. It is composed of a vaporizer that holds holds the water, a vapor collector 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 and at the bottom of the vaporizer, there are some serpentine gas channels where warm air flow 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 surface to concentrate the radiation can be used to improve the performance of the system.

The evaporation rate can be calculated as below:

where

  • Q is the daily output of distilled water ()
  • 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) (). The typical solar insolation at the Earth's surface is approximately 1,000 [6] [7] 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.
  • S is the thermal energy obtained from the solar energy collector.
  • A is the total fluid surface area.


Solar water disinfection system takes the distilled water from the solar distillation system and disinfects it to drinkable water with utilization of solar radiation. To improve the efficiency of the system, reflective surfaces can be used to intensify the solar radiation toward the contaminated water. The effectiveness of SODIS depends on the nature of the container, sunlight intensity and the operation condition.

Figure 1: SODIS with Solar Distillation Schematic Layout

Solar Distalltion System

Using solar heat evaporation ( Water Distillation)

Solar Water Disinfection System

Figures

Materials Selection

  • Paints
  • Transparent, smooth surface PET bottle
  • Concerns about PET bottle safety under UV light

Limitation

References

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