In this section, the design of the solar stills, the preliminary test, and the experimental procedures are discussed. In this study, two prototypes namely Model A and Model B have been used to carry out the experiments with four different configurations as listed in Table 3.1. Model A was firstly designed and fabricated by performing preliminary tests. Model B with a similar design but with a smaller size was constructed based on the preliminary results obtained.
Figure 3.1 illustrates the overall flow chart of the research.
Figure 3.1: Overall flow chart of the research
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Such configuration aims to understand and identify the effects of Fresnel Lens (FRL) and Paraffin Wax (PCM) towards the efficiency and performance of the passive solar still. The reason of utilizing two prototypes with similar design but different aspect ratios is to obtain a consistent and repeatable result.
Table 3.1: Solar Still Configurations Experimental Setup Details
Conventional Solar Still Double Slope Single Basin Passive Solar Still without any approach or modification.
FRL Solar Still Double Slope Single Basin Passive Solar Still associated with Fresnel Lens only. The Fresnel Lens concentrates the solar radiation on the basin to heat up the water rapidly.
PCM Solar Still Double Slope Single Basin Passive Solar Still with PCM tubes inside the basin. PCM acted as thermal storage and alternate thermal source under different circumstances.
Modified Solar Still Double Slope Single Basin Passive Solar Still with the combined effect of FRL and PCM.
38 3.2 Design of the Solar Still
The study aims to construct a solar still that is viable in the rural region while at the same time study the performance of FRL and PCM. The solar still was expected to have low cost, simple design and portable. Thus, direct passive solar still is the ideal choice for the research.
Single effect double slope basin type solar still is chosen as the experimental prototype for the research. The solar still has a simple working mechanism that makes it highly adaptive to both FRL and PCM. Moreover, basin type solar still has relatively lesser variables affecting its performance. This makes the effect of FRL and PCM more significant when coupled with the solar still. At the same time, the solar still should also have strong durability so that least maintenance is required.
Figure 3.2 demonstrates the CAD design of the solar still. The solar still consists of 3 major parts, which is the roof structure, basin, and the outer base.
The roof structure was made with aluminium bars as the supporting frame. Two transparent acrylic sheets were attached on the slopes with the aids of silicone glue. Other sides of the roof were covered with the smaller acrylic board as well.
The gaps between components were sealed to make sure it is airtight so that the leakage of water vapour from the solar still to ambient can be minimised during the desalination process. Two-point Fresnel Lens was mounted above the slope with aluminium bars as shown in Figure 3.2. The focal point of the Fresnel Lens was subjected on the basin and thus heat up the saline water in it.
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Figure 3.2: CAD drawing of double slope single basin solar still
A black-painted stainless steel basin was used as the solar receiver while holding the saline water at the same time. The outer layer of the base is made of wood, while the inner layer is covered with polystyrene to enhance the thermal insulation. As the solar still constantly receives the heat energy from solar radiation, saline water in the basin will evaporate. Water vapour would condense on the cooler surface beneath the acrylic cover and drip down eventually.
Drainage made of PVC pipes were installed at the bottom edge of the acrylic wall to collect the condensates. An outlet connecting all the pipes will channel the condensates collected into a detachable bottle. The desalination process is illustrated in Figure 3.3.
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Figure 3.3: Desalination process
A crucial factor in designing slope solar still is the slope angle of the transparent cover. The slope angle of passive solar still decides the solar power received by the absorber (Loss due to reflectivity of the slope). Theoretically, the reflectivity of the slope would be minimal when the solar ray is perpendicular to the slope (Belessiotis et al., 2016).
The research was conducted in Universiti Tunku Abdul Rahman (UTAR) Sungai Long Campus, Kuala Lumpur from April 2019 to September 2019. The elevation angle of the sun was examined to identify the best slope angle for the transparent condensing cover of the still. Figure 3.4 shows the solar path diagram and the specific elevation angle of the sun in a day at Kuala Lumpur. Solar flux is at its strongest magnitude within 10 a.m. to 2 p.m. Within that time, the elevation angle of the sun ranged from 55˚ to 60˚. Thus, the slope of the prototype was made 30˚ from the horizontal, which the solar ray could pass through the slope perpendicularly. Figure 3.5 shows the schematic drawing of the slope angle of the prototype.
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Figure 3.4: Solar Path Diagram and Sun Chart
Figure 3.5: Schematic Drawing of the slope angle of the prototype
A prototype (Model A) was constructed based on the design considerations mentioned earlier, which are simple mechanism, least maintenance and highly adaptive to enhancement features. The present design of the prototype still poses the limitation of inefficient and low productivity as it does not undergo any modification yet. Moreover, leakage and small degree of thermal deformation were expected for the very first prototype. Figure 3.6 shows
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the photograph of prototype setup for the preliminary test. Temperature range of the experimental location together with PCM performance test was carried out with the prototype, while the results were used to improve the design of the solar stills.
A prototype (Model A) was constructed based on the design considerations mentioned early. Figure 3.6 shows the photograph of prototype setup for the preliminary test. Temperature range of the experimental location together with PCM performance test was carried out with the prototype, while the results were used to improve the design of the solar stills.
Figure 3.6: Photograph of prototype solar still for preliminary test