As considered experimental results, the effect of BD settlement on RFP and NFP designed structure has been represented by an output electrical study (i.e., power loss, ΔP%) at the different TA (β). Afterward, these obtained results were also explained by the transmittance study (i.e., transmittance losses, ΔT%) of BD deposition patterns (settlement of BD on the flat glass cover space, i.e., “dropping pattern”) categorized corresponding to three different inclined regions (i.e., a region TR I, TR II, and TR III).
5.1 The effect of BD deposition on RFP and NFP design structure on solar power output (%) with different TA (β°)
The effect of BDs contamination on the RFP and NFP designed structure has been determined in terms of average power losses (ΔP%) at different TA (β°). The obtained results show a significant reduction in output power loss (%) due to BD phenomena onto the set of eleven TA (0° to 90°) for both designs (i.e., RFP and NFP). The particular observed results and study can be analyzed into three specific TRs corresponding to different inclinations, i.e., a TR I (0° to 25°), TR II (25° to 60°), and TR III (60° to 90°) as indicated in Fig.16.
In TR I (0° to 25°), it is pointed out that a critical reduction in output power has been observed for both types (i.e., RFP and NFP structure) of contaminated glass cover at the inclination 0° (i.e., close to horizontal way) due to great deposition of BD material rather than other tilt configuration listed as in Table 3. Moreover, it is also fairly pointed out that almost the same average power losses have been detected for both design structure, i.e., 19.21% for RFP and 19.17 for NFP. Because, in this horizontal position of glass cover, birds can easily move/walk over entire the surface area in the same manner on the both plate structures. Now with TR II (25° to 60°), a significant change in average power loss has been noticed, i.e., 19.21 to 9.77% for RFP and 19.17 to 2.67% of the NFP glass surface across the border of TR I / TR II i.e., β° (~ 25) which is termed as “threshold tilt angle (βthr)”as shown in Fig. 16. In TR II, a 9.77% reduction in the RFP design has been measured while 2.67 % for NFP designs structure. In contrast, it is well noticed that a suitable improvement (2.67 %) in output power has been detected out for NFP design structure as compared to the RFP design (9.77%). Finally, a very good progress has been observed 2.67% to 0.316% for NFP glass cover across the boundary of TR II/TR III. Meanwhile, a great power loss has been again progressing 9.77% to 14.68 for the RFP glass plate which is explained by the tendency of bird’s sitting/movement at the upper side of the common RFP surface. Because, in RFP, it widely noted in nature that birds have always a common habit to walk/sit along the upper straight side (i.e., towards the slop up side) and the traditional aluminium frame (L-shape) of PV module offers a suitable space where they can simply grip the metal frame edge at any inclination (β°) as shown in Fig. 10b and 17.
5.2 Bird movement (BD process) on NFP designed glass surface at different inclinations (β)
It was definitely seen that the different obtained “BD patterns” of the RFP and NFP glass plates at different slops β described as the tendency of bird’s movement over the front glass surface of PV module. In this regarding, it is also commonly noticed that birds have the general tenacity to easy walk on the flat surface and move towards the slope up side. Moreover, they also like to sit where they can easily grip the object by their claws (Isaksson 2008). A significant difference in technical characteristics (i.e., output power and transmittance) has been noticed in between both design structures (RFP and NFP) which was explained by the BD deposition phenomenon on both types of glass covers. Hence, outcome regarding the phenomenon of BD contamination over NFP and RFP design is directly associated with the walking/sitting tendency of bird on the surface which is characterized as follows:
5.2.1 Close to horizontal plane (small inclination) / TR I (0° ≤ β ≤ 25°):
In this tilt region, an utmost BD stores on the entire PV glass surface of the RFP and NFP designs. In this regarding, it is widely noticed that birds are capable to simply move over the entire glazing surface without any restrain and hence, resulting the random deposition of BDs in the same manner on the complete glass cover of the RFP and NFP designs. In this situation (TR I), same response of birds has been noticed corresponding to both glass cover (RFP and NFP). Hence, an almost relatively the similar reduction in average output power loss (ΔP%) has been evaluated 19.21% and 19.17% for RFP and NFP designs respectively in TR I. A BDs deposition phenomena and their corresponding exposed NFP design glass plate is illustrated in Fig. 18.
5.2.2 Above-threshold inclination (moderate inclination) / TR II (25° ≤ β ≤ 60°):
As inclination changes from TR I to II (i.e., across the boundary, β : 25°), now the BD accumulation starts to fall rapidly due to confinement of bird’s movement (i.e., a restriction of bird’s movement) on the RFP and NFP glass collectors. A significant fall in average output power loss from 19.21 to 9.77% and 19.17 to 2.67% respectively for the RFP and NFP design glass surface has been observed across the first boundary of the TR I/TR II. As result, It is clearly observed that a good improvement in output power production has been achieved by employing the NFP rather than RFP design at the moderate inclination (region II) where, power loss cuts 9.77% (RFP) to 2.67% (NFP) respectively as depicted in Table 3. Therefore, the main advantage by employing NFP design is, a well improvement “72.67%” in output power reduction has been achieved in this moderate inclination (i.e., TR II).
An abrupt fall in observed power loss (%) is explained by a different sitting/walking behavior of birds on the front glass cover. In this condition, a few BDs deposition localized only on the upper side of the front glass surface. In this contrast, it is usually seen that birds have more chance to sit at the top edge (i.e., birds try to sit at the tip of the plate) of the NFP glass plate, i.e., above the angle of “β° (~25)” (Threshold inclination), birds starts to slip down to the glazing surface and they try to move towards the “upper side” (i.e., towards the slope upside) of the plate as expected by their habit (i.e., birds are always likly to sit at the apex of the object). As result, the falling faecal mterial is only concentrated (localized) at the top edge of the angled sides as depicted in Fig. 19.
Meanwhile, in case of common RFP glass cover, the birds are confined to move along the complete upper straight edge of the PV plate. Hence the obtained dropping pattern consequently appears as a strip-like shape (2–5 inch approx.) on the upper side of PV surface rather than NFP design and the result is that more sunlight is blocked on the upper side of the RFP glass design of module as illustrated in Fig. 17.
Moreover, in the scenario of NFP structure, a significant improvement is seen due to its specific design and construction in comparison to the RFP design. In NFP design, the bird’s movement is to be more restricted in comparison to sitting behaviour on the RFP design. In the following, due to specific structure of NFP glass surface, birds are only capable to sit at the top of the plate (top of the isosceles triangle due to slope made by internal angles(θ) of two equal sides i.e., minimum θ ~ 45°) at the moderate inclination. Consequently, birds will contaminate only the small area (~16×22 cm2) of flat glass cover at the top of the plate as shown in Fig. 19. Hence, its result to a considerable improvement in output power production has been detected in this TR II. This small contaminated area will be same for any size of the NFP glass cover.
5.2.3 Close to vertical (at high inclination) / TR III (60° ≤ β ≤ 90°):
In the study, the most important part is the high inclination (60° ≤ β ≤ 90°/close to vertical) region. In this tilt part, once again more BD settlement takes place onto the RFP photovoltaic surface in which BDs slip down and traverse a long distance due to gravitational effects. Concerning this, BD covers the complete glass surface of the PV system in a specific settlement pattern (i.e., vertically long strip like shape) and its result is that power loss again increases from 9.77 to 14.69% in the case of common RFP glass cover at the high inclination.
But in case of NFP collector, a very insignificant amount (i.e., almost nil BDs) of BD deposition has been traced out on glass surface due to its specific designing and construction during the experimental period. Consequently, a very small average power loss ~ 0.316% has been recorded at the high inclination (i.e., TR III) for a particular plate design (NFP). Hence, the novel design surface works best effectively at the high inclination, i.e., particularly in the region III (60° ≤ β ≤ 90°) due to its explicit design of front glass plate rater than common RFP design surface of PV module.
According to this specific design, internal angles (θ°) of two equal sides, i.e., minimum ~ 45°, offers a well slopes on both equal sides of isosceles triangle which is able to restrict the bird’s movement only on the top edge of NFP glass plate at high inclination. In addition to this, one sees that a strip around ~5 cm on the both upper equal sides of an isosceles triangle is free from back support for maintaining only up to the thickness of the glass (i.e., blade like strip). It is made especially for avoiding the gripping of a bird’s claw to upper slanted edges of the glass plate design as shown in Fig. 20. As a result, with the help of this explicit novel design structure, birds are not able to sit at the tip of the thin glass plate, i.e., the perching birds have been strongly restricted in this TR III (i.e., close to vertical).
It is also commonly known that the sun is lowest in the sky during the winter and higher in summer and energy dropping on a PV module surface can considerably be improved by a suitable change in the plate inclination (β°). It is, therefore, the monthly optimum tilt angle of a PV module is maximum in December (winter) and minimum in May-July (end of summer) in WR as shown in Table 4 (Agarwal et al. 2012; Jamil et al. 2016). According to observed behaviour and study concerning the NFP design, it works more successfully at the high inclination β° (> 60). As mentioned above, the optimum tilt angle is maximum in winter. Asd above mentioned, the particular design (NFP) will be most appropriate in winter season (i.e., also critical BD settlement period in a particular zone) at higher slop to be honest.
The reduction in average transmittance ΔT (%) is measured across to three different inclined regions: TR I (0° ≤ β ≤25°) range from 23.6% to 42.6% (RFP), & 20.7% to 42.4% (NFP); (b) TR II (25° ≤ β ≤ 60°) range from 21.1% to 24.9% (RFP) & 4.1% to 7.1% (NFP); and (c) TR III (60° ≤ β ≤ 90°) 29.5% to 37% (RFP) & 0.5% to 0.8% (NFP) corresponding to both glass plates of PV module respectively as shown in Fig. 21.
A substantial difference in PV losses (%) and transmittance losses (%) for RFP and NFP designs with the inclination (β°) confirms the difference in the effect of BDs on both types of flat glass covers of photovoltaic modules and proves the utility of NFP design. Finally, it can be concluded from the above observation and study of the front NFP glass collector, it works well at the high inclination especially. Hence, the optimal inclination for NFP design β° has the minimum power loss, “0.316%” which is corresponding to TR III (60° ≤ β ≤ 90°) definately. The obtained results of this research work will be helpful for reducing the impact of BDs accumulation on the front glass cover of PV module in the solar field.