Assessment of energy gain in SPV Tree structure with bifacial PV modules in Rajasthan

With the rising use of renewable energy generation sources, there is a need for dependable and sustainable measuring techniques to monitor and measure the performance of the energy sources. Existing guidelines and standards for measurement of performance of PV modules are specified only for monofacial PV modules, in which the measurement of irradiance falling on the PV module is irrespective of the ground-clearance or mounting height up to a certain extent. With reference to techno-commercial aspects, the pitch in a ground-mounted power plant is set based on how optimum energy generation can be obtained with minimum land utilization. In case of bifacial PV modules, generation gain can be achieved by varying the ground-clearance/mounting height of the modules as the energy generation depends on the irradiance on both front as well as back side of the module. In this research study, the authors have compared field measurements of energy gained by varying the ground-clearance of bifacial PV modules by using bifacial PV modules in a novel structure of SPV Tree. The simulation and practical results show that the energy gain increases up to 3.5% in Marigold SPV Tree structure as the mounting height is 4 m, contrary to a conventional ground-mounted structure with mounting height 0.5 m from the ground.


Introduction
For optimized output by utilizing least land area for installation, a sustainable model like SPV Tree and SPV Artefact/tower structure can be beneficial [1].SPV Tree is a compact system designed to produce electricity, making use of a single or multiple number of PV modules, a charge controller, may be a battery bank for storage and an inverter circuitry to supply electrical loads, in case of off-grid system [3 -4].Conventional microgrids face huge challenges due to the intermittent nature of renewable energy resources and the lack of effective energy management system [10].SPV Tree is an elegant and uniquely designed super-structure with PV modules installed to generate power for lighting, remote power, and feeding-tariff applications [5].The advantages of SPV Tree models over conventional ground-mounted plants are shown in research study by Vyas et. al. [1] and the study was done for SPV Tree with monofacial PV modules.To continue the research study further, the authors in this research study have compared the energy generation gain in SPV Tree with bifacial PV modules to that of conventional ground-mounted structure with bifacial PV modules.In extreme shortage of space and to obtain high power to land ratio (PLR) in an aesthetically pleasing structure, a Solar artefact structure has been proposed by Vyas et.al. [2].
As no significant research study has been conducted to study the improvement of bifaciality of PV modules in SPV Tree structures, this study focusses on assessment of energy generation gain due to usage of bifacial PV modules in SPV Tree structure by comparing the energy generation gain at different ground-clearance.The system has been simulated in PVSyst software and the energy gain due to increase in mounting height of the bifacial PV modules has been studied.The authors have also studied the effect of mounting height on performance of bifacial PV modules in an experimental setup on field.

Research Study
In urban landscapes, the SPV Trees prove to be the most beneficial renewable energy source as it occupies very less space for installation [3].In the study by Cao et.al. [4], the authors have proposed the use of organic solar cells as a power generation source in the SPV Tree structure, however, the organic solar cells are not widely used in market due to its less robust stability as compared to silicon based crystalline solar cells.Vyas et.al. [1] have described that usage of SPV Tree structure helps in generating more energy from less land area.

Figure-1: Effect of clearance height on energy yield [7]
The energy generation can be optimized further by using bifacial PV modules in the SPV Tree structure.Hugo Sánchez, et al. [7] has described a comparative analysis on the effect of ground clearance, albedo, tilt, and azimuth angle in bifacial PV energy estimation using various algorithms, concluding that accuracy of view factor models is higher as compared to ray tracing models.Hugo Sánchez, et al. [7] describes that usage of estimation model also affects the computation of rear-side irradiance.Variables that affect energy production in bifacial models are Albedo, Rear/front-side irradiance ratio, clearance height, tilt angle, module mounting height, azimuth.The effect of clearance/mounting height on energy yield in case of different simulation models is as shown in Fig. -1.In PVSyst software, bifacial PV modules are characterized by "bifaciality factor" i.e., the ratio of the nominal efficiency at the rear side, with respect to the nominal efficiency of the front side [6].The irradiance on rear side of a PV module is characterized by the amount of irradiance emitted back towards the rear side of PV module from the ground, which is commonly known as "albedo" [6].In the research paper Deline, et.al. [8], the authors have compared indoor power measurement of bifacial modules with outdoor power measurements and single-diode theory.The energy generation from bifacial modules in a relative comparison with monofacial modules depends mostly on the deployment scenario and environmental conditions [8].However, there can be a further optimization in the energy yield of bifacial modules by variation in various parameters out of which, ground clearance has been assessed in this research study.A noteworthy hypothesis in PVSyst is that the light re-emitted from the ground is of same intensity irrespective of the direction of the light [6].The effect of clearance height is significant in affecting the energy yield.As the mounting height increases, so does the energy yield.
Vyas et.al. [1] have described how energy generation can be done using minimized land coverage by using SPV Tree structures.The SPV Tree structures have reduced land coverage ratio (LCR) and increased power-to-land occupancy ratio (PLR) as compared to the conventional ground-mounted structure.The benefit of using SPV Tree structures is that the land area beneath the SPV Tree can be used for other purposes, after excluding the land area occupied by the base/foundation of the structure [1].Also, as seen in Fig. -1, as ground clearance increases, the energy yield increases due to increase in the bifacial gain in the PV modules.As the SPV Tree structures in Vyas et.al. [1] have ground clearance of 4 m, the use of bifacial PV modules can be further beneficial in increasing the energy yield.As the structure has higher mounting height, this feature can be made use of by installing bifacial modules as their bifacial gain increases at 4 m height, as compared to the mounting them at 0.5 m height in conventional ground-mounted structure.In this study, the authors aim to assess the energy gain that shall be obtained by use of bifacial PV modules in SPV Tree structure.To fulfil the aim, the authors have simulated SPV system in PVSyst software at various ground-clearance to compare the generation gain and get the graphical results depicting the energy gain due to increase in mounting height.The results of energy yield at 0.5 m shall be similar to the scenario of using bifacial PV modules in conventional ground-mounted structure and at 4 m shall be similar to scenario of installing bifacial PV modules in SPV Tree (structure proposed in Vyas et.al. [1]) and along with it, a small study has been carried out on field by capturing the variation in irradiation values due to variation in ground clearance/mounting height.

Simulation study
A Solar plant is assumed to be in Rajasthan state in India, and the energy yield of the SPV system at different mounting heights is compared.The system mounted at 0.5 m ground clearance resembles a conventional ground-mounted plant and the system at 4 m ground clearance resembles Marigold SPV Tree setup.The Marigold SPV Tree model has been used in this research study, from the previous research in Vyas et.al.
[1] as this model is mechanically easy to produce and install on field.

Field Study
Along with the simulation study in PVSyst, the on-field study was carried out in Rajasthan, at the same location for which the PVSyst simulation was done.A pyranometer of Kipp & Zonen make and SMP10 model was used in this study.The pyranometer was mounted on the back side of a single PV module and was placed on land with white gravel, to simulate albedo of 0.28 (similar to that considered in the PVSyst simulation).The pyranometer setup was placed with tilt angle of 20° and azimuth of the PV module was 0° and was first placed at 0.5 m ground clearance (which resembles PV module mounted in conventional ground-mounted structure) and the GII of rear side was noted down for a day.In another pyranometer setup like the former case, all conditions were kept similar except the ground clearance, which was kept at 4 m (resembling PV module mounted in SPV Tree structure).The rear side GII was noted down from both the pyranometer setups for a single day and the GII gain was compared.

Results
In urban landscapes, the SPV Trees prove to be the most beneficial renewable energy source as it occupies very less space.Hence, this innovative system can be used in many ways to promote the use of renewable energy.As a result of the field study conducted, there was a considerable gain observed in energy generation from a system at 4 m ground clearance (similar to SPV Tree structure) than a system at 0.5 m ground clearance (similar to conventional ground-mounted system).In the simulation study, PVSyst simulation was done for SPV system with bifacial modules at different ground clearance, and the energy generation was found to increase with increase in ground clearance by every 0.5 m interval, which can be seen in Fig. -4.It can be seen in Fig .-4 that for a system at 0.5 m ground clearance (similar to conventional ground-mounted structure), the specific yield is 1978.19kWh/kWp/year, whereas the specific yield for a system mounted with 4 m ground clearance (similar to SPV Tree structure) is 2048.11kWh/kWp/year, as obtained from the PVSyst simulation results.

Conclusion
In order to take the research study proposed by the authors in Vyas et.al.
[1], the research study proposed here as a part of current research concludes that by the use of bifacial PV modules in SPV Tree structure, we can get significant energy gain of ~3% (as seen in the results of simulation study in Fig. -4) and as seen in the field experiment done, we have seen a significant gain in irradiation at 4 m ground clearance as compared to that at 0.5 m ground-clearance (refer Table -1).Hence, it can be concluded that the use of bifacial PV modules can help in further optimizing the return of investment (ROI) of installing an SPV Tree structure by getting higher energy gain.

Fig. - 3
shows the pictorial representation of conventional ground-mounted structure with ground clearance of 0.5 m, and Fig. -2 shows the pictorial representation of Marigold SPV Tree structure with ground clearance of 4 m [1].The pictorial representations of both Fig. -2 and 3 have been designed in Sketchup Pro software.The SPV system is simulated in PVSyst at ground clearance of 0.5 m, 1 m, 1.5 m, 2 m, 2.5 m, 3 m, 3.5 m and 4 m respectively.The variation in ground clearance affects the bifacial gain, which results in energy gain with increase in ground clearance.In this study, the system is simulated considering the following conditions: x DC capacity: 224.64 kWp (using 540 Wp bifacial monocrystalline modules with 70% bifaciality) x AC capacity: 200 kW (using inverter of 200 kW capacity) x Location: Rajasthan, India x PV module tilt angle: 20° x Azimuth angle: 0° x GHI: 2060.10 kWh/m 2 x GII (front side): 2265.10 kWh/m 2 x GII (rear side, in conventional ground-mounted structure): 165 kWh/m 2 x GII (rear side, in SPV Tree structure at 4 m ground clearance): 305 kWh/m2 x Albedo: 0.28 (white gravel)Here, as the ground clearance increases, the bifacial gain increases, resulting into higher energy generation.

Figure- 4 :
Figure-4: Gain in specific yield at various ground clearances.
Further, in the second scenario of field study, the rear side irradiance was measured in single PV module arrangement on February 20, 2023.The radiation values recorded on front side as well as rear side of both PV module systems (one at 0.5 m ground clearance and another at 4 m ground clearance) are as shown in Table -1.In the table, all parameters were recorded for a single day on February 20, 2023.