By Kasule Hannah Talinda*, Odongo Obbo
Hannah Kasule is a fourth-year student at Kyambogo University pursuing a Bachelor of Science in Chemical and Process Engineering. Her research interests focus on sustainable energy and water technologies, and their potential to accelerate economic and social development. For her undergraduate thesis, she designed and fabricated an inclined solar water distillation system to treat contaminated spring water.
Corresponding Author: Kasule Hannah Talinda
The rapid growth of Uganda’s population over the past two decades has placed significant strain on the current national water supply service. While some communities in Uganda use the main NWSC water supply, umbrella authorities, NGOs and other private sector organizations as their main water source, other residents depend mainly on groundwater springs to access clean water. However, due to factors like inadequate spring protection and poor solid waste management, some of these springs have become contaminated with physio-chemical and microbiological pollutants. This research project focussed on solar water distillation as an alternative method for treating contaminated spring water in Ugandan communities. A continuous flow, inclined solar water distillation system was designed and modelled using SolidWorks software, and fabricated using heat-strengthened glass, hardwood timber and mild steel as the main materials. The major methods used for fabrication were gas metal arc welding, and metal, wood, and glass panel cutting. The performance of the solar distiller was then analyzed by measuring hourly and daily distillate productivity, determination of optimum input flow rate, determination of performance efficiency, and physiochemical characterization of both the raw feed water and the distillate. From the experiments performed, it was concluded that the solar distiller has production efficiency of 36.88% and productivity of 4.71 litres/m2, which can be sufficiently scaled up to meet domestic water needs. The solar distiller temperature and inlet flow rate also have a direct influence on the productivity of the solar distiller, with high distillate volumes being observed at higher temperatures (75.5°C) and moderate flow rates (437ml/min).
The research project aimed at designing, fabricating and analyzing the performance of the designed and constructed solar distillation unit. It was carried out over the course of seven months, from August 2021 to March 2022. The study area for this project was a groundwater spring located in Masajja Village, Makindye, Ssabagabo Subcounty, Wakiso District shown in Figure 1, where the implementation and performance analysis of the solar distillation system was carried out.
The main objectives of this project were to design the solar distiller, fabricate it and then analyse its performance using three main parameters i.e., solar distiller temperature, inlet flow rate, physiochemical and bacteriological water quality.
This chapter covers the procedure that was followed while carrying out this research. It expounds on the sample collection, design and construction of the solar distiller, performance analysis and characterisation of the raw feedwater.
3.1 Sample collection
Field studies were conducted at the selected groundwater spring in Masajja Village to collect spring water samples. Sampling was done during both the dry and wet season to understand fully the influence of seasonal changes on the productivity and efficiency of the solar distiller. The sampling was done with the help of local authorities. Eleven samples of spring water, with a volume of 5 litres each, were obtained between 10th February 2022 and 9th March 2022 from the spring water outlet using plastic sample collection bottles.
3.2 Design of the solar distiller
3.2.1 Principles of operation
The conventional solar distiller consists of an airtight basin containing the contaminated water and enclosed by a transparent cover through which solar radiation is transmitted as shown in Figure 2.
Solar radiation is absorbed by the basin plate and heat is transferred to the water, increasing its temperature to evaporation temperature. Evaporation then occurs, and water vapour rises upward to the cooler inner surface of the transparent cover and condenses as pure water, which runs down along the sloped cover due to gravity. It is then collected in a trough at the lower edge of the cover. The cover is at sufficient slope so that the surface tension of water will cause it to run down to the trough without falling back into the basin. (Gnanadason, Kumar, & Sivaraman, 2014)
3.2.2 Material selection
a) Solar distiller container
Several materials were considered for use as the basin material such as aluminum, stainless steel and galvanized steel. Aluminum and stainless steel were first considered because of their resistance to corrosion, light weight, long life and ease of cleaning. However, because of their high cost per kilogram, a solar distiller made out of these materials would be relatively expensive to build (Anil & Tiwari, 2015). Galvanized mild steel was used for this project because of its high tensile strength, high thermal conductivity, ease of cleaning and relative low cost.
Considering the material (galvanized mild steel) and the thickness of the basin, there was a necessity to insulate the solar distiller in order to ensure a high system operating temperature. Insulation on the walls and bottom of the distiller will minimize the energy losses of the unit. Wood was a good option for insulating the distiller because of its relatively low price and availability in rural areas, low thermal conductivity and high tensile strength. For an effective insulation, a thickness of 2cm was chosen in order to increase heat retention capacity.
The glazing cover of the solar distiller acts as the transmission medium for incoming solar radiation into the solar distiller. It is located on top of the distiller, where steam is condensed, clean water droplets are produced and then it is collected in the gutter collection container. There is varied research about glazing cover materials; it is common to use polyethylene, acrylic and tempered glass. However, these materials are expensive and would not be affordable by the residents of the study area. For this research project, ordinary glass with a thickness of 5mm was utilized because it is cheaper than other materials, has 50 years of life expectancy, has 86% of solar transmittance and 2% of infrared light transmittance (Mauricio, 2014).
3.2.3 Modelling of the solar distiller
The 3D modelling of the solar distiller was performed in SolidWorks. Modelled components included the main solar distiller container, inlet feed distribution pipe, inlet and outlet pipes, metal glass support, residual feedwater collection gutter, and distillate collection gutter.
|Component||Length (cm)||Width/Diameter (cm)||Thickness/Height (cm)|
|Solar distiller basin||50||50||10|
|Feedwater distribution pipe||48||5||0.02|
|Inlet and outlet pipes||15||2||0.02|
|Metal glass support||50||3||0.05|
|Feedwater collection gutter||48||5||0.03|
|Distillate collection gutter||48||5||0.03|
|Transparent glass cover||46||46||0.05|
|Hardwood timber insulation||50||50||2|
To obtain 3D models for the prototype, a SolidWorks part file was opened. A 2D sketch model of the metal basin was then drawn. Dimensions listed in Table 1 were then applied to the 2D sketch. The obtained basin sketch was then extruded and cut into a 3D model. The material specifications were then adjusted to mild steel. These steps were repeated for the transparent cover glass, hardwood timber insulation and mild steel support stands, along with their respective dimensions and color specifications. A SolidWorks assembly file was then used to load the part model files and combine them into a 3D model. Elevations of this model are shown in Figure 3.
3.3 Construction of the solar distiller
This section expounds on the procedure that was followed while constructing the distiller. It breaks the procedure into the material cost breakdown, fabrication and assembly of the components.
3.3.1 Cost breakdown of solar distiller components
The estimated financial cost for all the primary and secondary solar still components is broken down in Table 2:
|Galvanized metal sheet (1m2)||15,000|
|Polycarbonate sheet (1m2)||10,000|
|Polyvinyl pipe (1m length)||10,000|
|Plastic jerrycan with tap (5l)||10,000|
|Flexible plastic pipe (0.5m length)||15,000|
|Timber panels (1.5m2)||20,000|
3.3.2 Fabrication of solar distiller components
The solar distiller was fabricated in Katwe, Kampala District, Uganda. The basin, which is made out of 1.5mm thick mild steel sheets has an evaporating area of 0.25m2 for a basin length and width of 0.5m and 0.5m respectively. Four sheets were galvanized and then cut using a jigsaw, and welded together using a 110V-210V gas metal arc welder to form the metal basin.
To create the feed water distribution pipe, a mild steel pipe of inner diameter 5cm and thickness 0.05cm was cut to a length of 48cm using a jigsaw, and perforated with 8 circular holes of 1mm diameter using an electric hand drill. The holes were spaced 1cm apart to aid in controlling the flow rate and distribution of inlet feedwater from the feed tank. The feedwater pipe was then welded to the top panel of the metallic solar distiller basin to create what is seen in Figure 4.
To create the inlet and outlet pipes, mild steel pipes of inner diameter 2cm and thickness of 0.02 cm were cut to length of 15cm each using a jigsaw, and this process was repeated to make 3 identical pipes, which were welded to the inlet and outlet points. To create the distillate and residue collection gutters, one pipe with a length of 48cm and diameter of 2cm, were cut in half to create two pipes, which were welded to the top and bottom of the bottom metal panel of the solar distiller basin. All metal components were then coated with black, heat and water-resistant paint to increase heat absorption capacity, and sealed with a high temperature silicone sealant to prevent leakages coming up with what is seen in Figure 5.
The transparent glass cover was created by cutting a 5mm thick glass panel according to the dimensions of the solar distiller (50cm by 50cm), and an allowance of 5mm was added to enable the cover to comfortably fit on top of the basin. An additional support consisting of a metal panel 50cm long and 3cm wide was welded to the middle of the basin, in order to reduce the possibility of crack formation and propagation in the glass panel, which would reduce its transmissivity.
Hardwood timber insulation was created by cutting a 1cm thick wooden panel using a circular hand saw, according to the metal basin dimensions (50cm by 50cm). It was then coated with a water-resistant sealant, in order to prevent rotting from constant contact with water during the experiment. In order to incline the solar distiller at an angle equal to the latitude of the study area, two square metal pipes of length 50cm and two square metal pipes of length 35cm were painted with black water-resistant paint, and welded to the top and bottom corners of the solar distiller respectively.
3.3.3 Assembly of solar distillation system
In order to facilitate the solar water distillation process, various secondary components were added to the fabricated solar distiller basin to create what is seen in Figure 6
A 10-litre plastic tank with an outlet tap was used as the feedwater tank. It was covered with 0.5mm thick black polythene bag in order to increase the temperature of the feedwater, thus leading to more vapor production within the solar distiller. A blue feedwater absorption pad made of 100% cotton was sealed to the solar distiller basin surface, in order to increase the residence time of the feedwater from the distribution pipe, thus increasing the evaporation rate within the distiller.
Transparent seal tape was also placed on all sides of the transparent glass cover, as well as all the inlet and outlet pipes, to prevent any leakage of water vapor from the solar distiller basin. Finally, to improve the condensation rate, insulation cover made of newspapers was attached to the gutter collection area at the bottom of the distiller basin, in order to prevent double vaporisation of the condensed distillate.
3.4 Performance analysis of the solar distiller
In order to determine the efficiency and productivity of the solar distiller, 11 samples of distillate from the groundwater spring in Masajja Village were distilled over the course of 11 days, using the constructed solar distillation system.
3.4.1 Measurement of hourly and daily solar distiller productivity
The collected sample from the study area was collected at 8:00am, and then transported to the solar distillation unit to start the experiment. The water was then allowed to sediment for 1 hour, from 9:00am-10:00am, in order to reduce the amount of particulate matter in the sample.
From 10:00am to 5:00pm, the amount of distillate in ml, collected every hour was measured using a measuring cylinder. The corresponding temperature inside the solar distiller was also measured every hour using a digital thermometer in °C. At the end of the day, the total volume of distillate collected was also measured. Distillate volume and solar distiller temperature were used to determine the productivity and efficiency of the solar distiller, from the equations below.
Where PD is the solar distiller productivity in kg/m2, VT the daily volume of distillate produced in m3, ρ the density of water in kg/m3 and A the cross-sectional area of the transparent glass cover in m2 (Mbadinga, 2015).
Where ŋ is the solar distiller efficiency, m the mass of distillate produced in kg, hfg the latent heat of vaporization of water in kJ/kg (taken as 2.26 × 106 kJ/kg) and the solar radiation intensity of the study area in kJ/m2, obtained from (Solar GIS, 2021).
3.4.2 Variation of bi-hourly inlet feed flow rate
Since the solar distiller was a continuous flow system, it was necessary to determine the optimum inlet flow rate that would result in the highest value of solar distiller productivity. The feed tank was first calibrated using a measuring cylinder and a timer. Next, the inlet feed flow rate was held at a constant value of 113ml/min, and the volume of distillate collected every 30 minutes, during the peak solar radiation intensity hours of the day, from 12pm to 2pm, was recorded. The inlet feed flow rate was then varied from the lowest flow rate value (113ml/min) to highest flow rate value (623ml/min), in intervals of 30 minutes from 12pm to 2pm. The corresponding volume of distillate produced every 30 minutes was then recorded.
3.5 Physicochemical and bacteriological characterisation of the raw feedwater and distillate
This chapter covers the results that were obtained and discussions that give their rationale to the research objectives.
3.5.1 Measurement of pH
pH measurements were conducted using a pH meter. The electrode tip was rinsed in de-ionised water. Then, the electrode tip was fully immersed in a beaker containing the sample solution. The fluctuating readings were observed for a while until they became steady. The pH reading was then recorded.
3.5.2 Measurement of conductivity, TDS, salinity and temperature
Values of conductivity, total dissolved solids, salinity and temperature were recorded using a conductivity meter shown in Figure 7.
After the calibration process, the electrode tip was fully immersed in a beaker containing the sample solution, and left for a few minutes until steady values were obtained. The respective values of conductivity in uS/cm, totally dissolved solids in ppm, salinity in psu and temperature in °C were recorded.
3.5.3 Measurement of turbidity
The turbidity value of the samples was conducted using a turbidity meter shown in Figure 8.
The turbidity meter was first calibrated, and once the values had stabilized, a cuvette was filled to the black mark with sample water and placed in the meter. The “CAL” button was then pressed. Once the value stabilized, the turbidity of the sample was then recorded in NTU.
3.5.4 Measurement of total and faecal coliforms
Fifteen test tubes containing 10ml each, of single strength McConkey broth culture medium and E. coli medium were prepared. Dilution series of 1, 1/10 and 1/100 were then created using both sample volumes and buffered dilution water, to make a total of 15 tubes. The tubes were then placed on a rack in an incubator for 48 hours at 37 °C. After the 48 hours elapsed, the tubes were removed from the incubator and observed for growth. Those tubes that showed turbidity/gas production were regarded as positive. The number of positive tubes at each dilution series was recorded, and the corresponding total coliforms were determined from the Most Probable Number Table. Positive tubes were then inoculated in E. coli medium and incubated for another 24 hours at 44 °C to determine the faecal coliforms.
4.0 RESULTS AND DISCUSSION
This chapter covers the results that were obtained and discussions that give their rationale to the research objectives.
4.1 Hourly volume of distillate produced from 10:00am to 5:00pm
The hourly volume of distillate collected by the solar distiller from 10:00am to 5:00pm on 12th February 2022, and the corresponding solar distiller temperature, can be seen in Figure 9.
From the graph, it was observed that the increase in distillate volume was directly proportional to the increase in solar distiller temperature, which is in agreement with the findings of (Abu-Arabi et.al., 2012). From this analysis, it can be concluded that the amount of distillate produced directly depends on the temperature of the solar distiller. This is because higher temperatures increase the kinetic energy of the incoming feedwater molecules, thus leading to an increase in evaporation rate.
4.2 Daily solar distiller productivity
The daily volume of distillate collected by the solar distiller for 11 days spanning 10th February 2022 and 9th March 2022 from 10:00am to 5:00pm, and the corresponding solar distiller temperature, can be seen in Figure 10.
It can be observed that the daily solar distiller productivity was higher during the first 6 experiment days in the month of February, and lower during the last 5 experiment days in the month of March, with the lowest productivity being recorded as 3.77kg/m3 on 5th March 2022. This data agrees with the conclusion that variation in monthly and seasonal weather conditions will affect the productivity of the solar distiller, since according to (Spark, 2021), the dry season in Wakiso District, when solar radiation intensity and thus ambient temperature is highest, ends in late February of every year, and the wet season begins in early March, where the solar radiation intensity is much lower.
4.3 Bi-hourly variation of feed inlet flow ratefrom 12:00pm to 2:00pm
It can be observed from Figure 11 that when the inlet feed flow rate was varied, the amount of distillate produced rose exponentially at a much higher rate than the amount of distillate produced at a constant inlet feed flow rate. The highest distillate volume observed was 390ml at 1:00pm, for an inlet feed flow rate of 473ml/min, which is higher than the distillate produced for the constant flow rate by 90ml. However, as the flow rate was increased from 473ml/min, the amount of distillate produced began to drop moderately, reaching the lowest volume of 55ml at a maximum inlet feed flow rate of 673ml/min. This can be attributed to the fact that an increase in inlet feed flow rate decreased the residence time of the feed water in the basin, thus decreasing the evaporation rates.
4.4 Daily solar distiller production efficiency
The daily solar distiller efficiency from 10th February 2022 to 9th March 2022 was calculated from Equation 1.2.
It can be seen from Figure 12 that daily solar distiller efficiencies varied between 47.8% on 12th February 2022, and 30.2% on 5th March, with an average production efficiency of 35.99% over the course of the 11 days. This relatively low efficiency can be explained by the high heat losses from the sides of the metal solar distiller basin, as well as the evaporative leakage from the area between the edges of the glass cover and the sides of the solar distiller basin.
4.5 Physicochemical and bacteriological analysis of feed water and solar distillate
From the Table 3, the first observation is that none of the major water quality parameters above were within the permissible limits of the East African Potable Water Standard (EAC, 2017). The spring feed water was had an average pH of 4.24, conductivity of 2538 uS/cm and TDS value of1728 ppm. These values are relatively high when compared to East African Potable Water Standards of 6.5-8.5, 1500uS/cm and 700ppm for pH, conductivity and total dissolved solids respectively. These high values imply that the spring may be inadequately protected from environmental contaminants like agricultural runoff, stormwater runoff, and decomposing organic matter.
|Total Dissolved Solids (ppm)||1728||1731||1724||1759||1734||1728.00|
|Faecal Coliforms (MPN/100ml)||70||90||50||70||90||74.00|
From the Table 4, the first observation is that most of the major water quality parameters above were within the permissible limits of the East African Potable Water Standard (EAC, 2017). The distillate produced was neutral with an average pH of 7.01, low conductivity and TDS values of 466.80 uS/cm and 61.20 ppm respectively, and an average turbidity value of 1.63 NTU, which confirmed that most of the physical and chemical contaminants had been successfully removed.
|Total Dissolved Solids (ppm)||53||67||63||49||74||61.20|
|Faecal Coliforms (MPN/100ml)||2||2||4||2||2||2.40|
However, the faecal coliform parameter value was above the permissible limits of the East African Standard, which states that faecal coliforms should be absent in both potable and drinking water (EAC, 2017). This can be attributed to the fact that the distillation process requires more than one evaporation condensation cycle for less volatile solutes like faecal coliforms to fully separate from the feedwater (ChemLibre, 2021).
From the results above, the following conclusions can be drawn:
The experiments conducted confirmed the expected influence of solar radiation intensity and climatic conditions of the location on the amount of distillate produced by the solar distiller. In the specific study area for this project, Masajja Village, higher distillate productivities will be obtained in the dry season, and lower productivities will be obtained in the wetter seasons.
High distillate productivities will be observed at a moderate flow rate of 437ml/min. Lower distillate amounts will be observed at very high flow rates, since the residence time of the feedwater in the inclined solar distiller basin will be much lower.
The distillate produced from the solar distiller was determined to have a high quality, with most of the physico-chemical parameters adhering to East African Potable Water Quality Standards. However, although 96.7% of the faecal coliforms were removed from the distillate, final faecal coliform values are still above the permissible limit of the East African Standard.
Lastly, the solar distiller has an average efficiency of 36.88% and a daily average productivity of 4.7litres/m2. These values have been confirmed to be competitive when compared to results of similar research in the field. These values can be attributed to the unique design and geometry of the solar distiller basin, as well as the installation of basic secondary heating and cooling components.
In an attempt to further improve the performance of the solar distiller, more advanced secondary heating and cooling components e.g., external condensers, double-glass covers, solar heaters, etc. can be added to the feed tank and distillate collection containers. This could potentially improve the evaporation and condensation rates in the solar distiller, as well as the removal efficiency of faecal coliforms.
A secondary re-mineralising system can be added to the distillate outlet point in order to replenish the distillate with minerals lost from the evaporation process.
Possibilities of scaling up the solar distiller to meet community needs can also be explored. This can be done by integrating more unit operations into the solar distiller in order to improve its effectiveness e.g., a steeped evaporation tray to reduce the gap between the evaporation and condensing surface.
Further research can be performed using alternative materials e.g. softwood timber as a more affordable form of insulation, and galvanized iron steel as a more corrosion resistant heat transfer medium. Corresponding effects on performance efficiency can also be investigated.
Lastly, a thermodynamic analysis of the solar distiller may be performed using computational simulation and modelling, in order to determine the optimum design parameters for maximizing solar distiller productivity.
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