An experimental study was carried out on solar distillation on the roof of the office building of Bangladesh Centre for Advanced Studies (BCAS). The solar still with a dimension of 3m × 1.5m having a glass cover of 5mm thickness and 20 deg. angle with the horizon was used. The study was carried out during the period December 2011 to April 2012. The highest yield was 8.1 liters per day giving an efficiency of 36.4% based on average incident solar energy. The total cost of the solar still was BDT 23,945 (US $300.00). This technology is suitable for getting pure drinking water anywhere in Bangladesh free of fuel cost. BCAS will provide the technological services on demand.
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Bangladesh is a thickly populated country and inhabited by more than 160 million people.
Diarrheal diseases constitute a major health problem in this country killing over hundred thousand children in each year (water.org/country/bangladesh). Thousands of adults also suffer from this disease. This disease has close biological and socio economic links to the problems of malnutrition, maternal health and ineffective labour force. Although work on solar distillation is in progress in some organizations (Khanam, 2014; Hossain, 2012), tangible results are yet to be reportd.
To add to this, arsenic in high concentrations was discovered in the ground water of western Bangladesh. Long term intake of arsenic-contaminated water leads to a number of health problems, particularly skin disorders and internal cancers.
Under these circumstances, people are constrained to drink surface water which is contaminated with pollutants and disease vectors. Solar distillation of surface water/polluted water/brackish water/arsenic-contaminated water yields clean water which is 100% free of pollutants and disease vectors.
This report is concerned with the design, construction and operation of a low cost solar water distillation plant (solar still) which is suitable for rural/urban/coastal use for preparation of pure drinking water.
What is solar distillation?
Solar distillation is a kind of distillation process in which heat energy for distillation is supplied by the sun and not by fossil or biomass fuels. Since solar energy is available everywhere and involves no cost for fuel, solar still is expected to be cheap and applicable where sunshine is available. Bangladesh is situated between 20° 34´ and 26° 38´ north latitude and, therefore the country is quite suitable for exploitation of solar energy. (Daniels, 1977)
The process is simple. In an appropriately designed plant, solar energy heats up water which vaporizes from the distillation tank and condenses in a place separate from the distillation tank. The diagrams given in the following section show the salient design which needs to be followed for effective solar distillation.
The experimental designs are shown in schematic diagrams (Fig.1, Fig.2 & Fig. 3) and the actual operations in photos-1 and 2.
Photo 1: Solar water distillation plant (2m × 1.5m) with steel tray and glass cover.
Photo 2: Solar water distillation plant (2m × 1.5m) with steel tray and glass cover (performance being examined by the project associates).
Saline water was taken in the tank (Fig. 3) covered by a glass sheet of 5mm thickness. The bottom of the tank is painted black for maximum absorption of solar radiation. Solar rays with small wave lengths pass through the glass sheet and get absorbed at the black surface of the tank and converted into rays of long wave lengths (heat) which can not get out through the glass sheet, and thus the tank containing brackish water gets increasingly heated giving water vapor leaving the contaminants in the tank. The vapors coming in contract with the cooler glass surface become converted to water and slides down the inner side of the glass sheet to be collected as pure distilled water.
The effectiveness of distillation depends on the following parameters.
- Solar insolation
- Glass thickness
- Ambient temperature
- Wind speed
- Water depth in the tank
- Contaminant concentration
- Construction materials
- Replacing glass by other materials
- Tilt angle with the horizon
The effects of these parameters will be described in series in future communications.
Cost of the solar still:
1. Solar still with glass cover:
- i. Dimension
Length 2.0 meter, Breadth 1.5 meter, Height 0.2 meter and 20º angle with the horizon
- ii. Cost:
Glass: BDT 4,400
Steel (MS) structure tray: BDT 17,195
Others (including pipe, polluted water delivery system, storage tank, etc):
Total: BDT 23,945 (US $ 300.00)
As stated above, the solar still has a dimension of 2m by 1.5m i.e an area of 3 sq.m. The glass sheet used had a thickness of 5 mm. 24-hourly data were collected during the period December 2011 to April 2012. Daily (24hour) yield of distilled water varied from 2.1 liters to 8.1 liters showing how significant the influence of parameters is. Hossain, (2012) got 1.2 liter/m2 in an experiment at BUET, Dhaka although the influence of parameters have been amply described in global literature (Malik, 1982), the yield is very much site specific (ambient condition) and design specific. These aspects will be discussed in future communications from this institute.
Greatest amount of solar energy is available between two broad bands encircling the earth between 15° and 35° latitude north and south (Daniels, 1977). Fortunately, Bangladesh is situated between 20° 43′ and 26° 38′north latitude and as such the country is in a very favorable posited in respect of the utilization of solar energy. Annual amount of radiation varies from 1575 to 1840 kWh/m2 which is 50% - 100% higher than those in Europe (Eusuf, 1997).
Taking a conservative value of 1700 kWh/year, daily average solar insolation is 4.66kWh i.e 16.77 MJ.
The efficiency of a solar still is given by the equation (Neil, 2002)
E = Efficiency (%)
Q = Amount of distilled water collected per day (liter/day)
2.26 = Latent heat of vaporization of water (MJ/liter)
G = Daily global solar radiation (MJ/m2)
A = Aperture area of the still (m2).
Putting the maximum yield of 8.1 liter/ day and other parameters as given above, the maximum efficacy during the December to April is estimated at 36.4% which is above the usual efficiency of 25-30% (Neil, 2002)
As stated above, details of experimental results will be given in future communications. It is however evidently clear from this short communication that solar distillation for drinking water is a feasible proposition. As it uses solar energy, it is environmentally friendly and can be used anywhere in the world where solar energy is available. This technology is especially suitable for Bangladesh. It can be immediately disseminated where water is saline and where underground water is contaminated with arsenic. BCAS will provide the technological services on demand.
- Daniels F, Direct Use of the Sun’s Energy, Ballantine Books, New York, 1977, P.30.
- Eusuf, 1997, Prospects and Problems of Solar Energy in Bangladesh: Implementation Stage of Solar Systems, BCAS, Dhaka.
- Huq, Rahman and Eusuf, 2005, Prospects of Solar PV Systems in Bangladesh (in Solar Photovoltaic Systems in Bangladesh: Experiences and Opportunities The University Press Limited, 2005) ed: M. Eusuf.
- Khanam, M., 2014, IFRD, BCSIR, Private Communication
- Malik A S et al., “Solar Distillation”- Pargamon Press,1982
- Neil N., 2002, Practical Action
- Hossain, Z., 2012, Civil Engineering Department, BUET