Keywords: solar energy materials; crystalline solar cells; thin films solar cells; applications of solar cells; photovoltaic leaf; solar tree; degradation rate; solar tree performance; optimal mounting of PV leaf
Photovoltaic (PV) systems allow consumers to generate useful electricity sustainably from incident sunlight using the PV devices at the load centers themselves. In addition, PV also offers numerous other benefits that typically fall under Sustainable Development Goal 7 (SDG7) of the United Nations Development Programme (UNDP), i.e., "ensuring affordable, reliable, sustainable, and modern energy for all" [[
Traditionally, we could see mostly open-mount photovoltaics (OMPV) and roof-mount photovoltaics (RMPV) power plants, and these are typically classified under two groups: off-grid and on-grid [[
The recent advances in PV installations include roof-integrated photovoltaics (RIPV) [[
The above-listed PV plants are briefly described in Table 1, which also presents considerations of the preferred solar cell technologies, the medium of installation (e.g., land, water, and X-integrated), and land footprints. Here, X-integrated refers to integrating solar PV cells onto the peripherals of the buildings, façades, canopies, vehicles, road, rail tracks, poles, etc. Among all the discussed PV plant types in Table 1, only a few are commercialized and used for power generation, and few others are still in the research stage. OMPV and SPVTs are installed on the land surface, and they are accountable for the land footprint [[
The current research on SPVTs focuses on the design aspects, energy performance, and influence of weather parameters on the performance. The design of a SPVT is mostly bio-inspired, and the SPVT is primarily used as an attraction in urban landscaping. Hence, the design is made more of a decorative tree where the solar PV cells are arranged as leaves to an edifice. This has resulted in layered structures, depending on the possibility and applicability of the number of layers [[
Table 1 Comparing different types of solar photovoltaic plant types based on the installation considering recent advances.
Photovoltaic Plant Type Installation Medium Brief Description Most Preferred Solar Cell Technology Land Footprint Reference Solar photovoltaic tree Land surface and the existing poles or towers Photovoltaics modules are mounted as leaves on tree-like structures Crystalline silicon and thin-film solar cells Very minimum land footprint [ Open mount Land surface Photovoltaics modules are installed on iron mounting structures that are laid on the ground surface with concrete support Mono and polycrystalline silicon Very high land footprint and depends on the plant capacity [ Roof mount Building outer peripherals In roof-mount, building-attached, and canopy-mount solar PV, the photovoltaic modules are attached to the building's outer peripherals using a rail-less or railed support structure (e.g., windows, roofs, façades, etc.) No direct land footprint but there exists indirect land footprint [ Building attached [ Canopy-mount solar Crystalline silicon, amorphous silicon, thin films like CdTe, CIGS, and flexible solar cells [ Roof integrated In roof-integrated, façade-integrated, and building-integrated PV, the photovoltaic modules are integrated into the outer building peripherals by replacing the building structures, such as windows, roofs, façades, etc. [ Façade integrated [ Building integrated [ Vehicle integrated or vehicle mount Vehicle outer peripherals Photovoltaics modules are installed or integrated into vehicle structures such as window glass, sunroof, etc. [ Road and rail integrated On-road and rail track infrastructure Photovoltaics modules are integrated into the road, rail tracks, and other infrastructure Crystalline silicon, amorphous silicon, thin-film, and flexible solar cells [ Pole mounted and integrated Outer peripherals of the street poles Photovoltaics modules are attached or integrated to the poles, e.g., streetlights [ Floating solar or Floatovoltaics Surface of the water body Photovoltaics modules are mounted onto the floating structures. Dual glass solar cells [ Underwater on-board solar Underwater at varying depths of water Photovoltaics modules are mounted or integrated onto the robot structures or underwater infrastructure peripherals Crystalline silicon, thin-film, and flexible solar cells [ Submerged [ Wavevoltaics Surface of the wave energy device or any floating buoy Photovoltaics modules are mounted onto the wave energy devices like a buoy Thin-film and other flexible solar cells [
However, most literature claims have not considered the critical factors that affect PV cell performance in the long run, such as orientation and tilt, the difference in height for each layer in SPVT design, wind and temperature effects, degradation rate, and cell technology. In this article, a brief study is presented to act as the best approach in selecting the best performing PV leaf for an SPVT design by considering the above-said vital factors.
Overall, it is clear that the most difficult challenges with SPVTs lie with the structure design, the solar cell technology selection for a leaf, and energy performance. Moreover, academics, researchers, and the industry workforce are working together to address these challenges to facilitate the deployment of SPVTs into sectors such as energy, built environments, and others wherever it is applicable. Therefore, this paper aims to provide the most practical solution for a typical multilayered SPVT, and the following objectives are undertaken in this study:
- A framework with a performance prioritization approach (PPA) is proposed to report the performance of a multilayered SPVT intending to select an efficient PV leaf design.
- A three-layered SPVT (3-L SPVT) that has nine leaves—where the upper layer has only one solar PV leaf, the middle and bottom layers have four solar PV leaves each—is simulated, and lifetime energy performance is evaluated for three different PV cell technologies, namely crystalline silicon (c-Si), copper indium gallium selenide (CIGS), and cadmium telluride (CdTe). While evaluating the 3-L SPVT's performance, power conversion efficiency, thermal regulation, and degradation rate are considered.
- An analysis of the investigated results is carried out, and at the same time the best performing solar PV leaf for a three-layered SPVT is identified among the c-Si, CIGS, and CdTe PV technologies.
This paper is structured into six sections. In Section 2, a brief description of the proposed 3-L SPVT is given. Section 3 provides the performance modeling. Section 4 describes the proposed framework, i.e., the performance prioritization approach. Performance results of the 3-L SPVT with different solar cell type PV leaves and the related discussion are provided in Section 5. The conclusions are drawn and presented in Section 6.
This section provides a brief description of the design of the proposed SPVT. The layered structure of the SPVT was modelled with a provision for mounting different solar cells as PV leaves at fixed tilt angles and was designed using SolidWorks (which is a computer-aided design and computer-aided engineering computer program) modeling.
In the SPVT, the primary components include an edifice (generally called a central tower whose shapes can be varied, but in this study it is of a circular pole), branches, PV leaf holders for each branch, and the PV leaves. In the proposed SPVT, a total of 9 branches are considered and arranged in 3 layers.
The principal purpose of using a layered structure is to capture the incoming sunlight from the sun in various directions. The PV modules are well suited for converting the received beam of solar radiation from all directions into useful energy in such designs. Further, by having adjustments in the PV-leaf tilt and azimuthal angles, the energy outputs can be optimized. Overall, the PV leaves in the layered structure allows the SPVT to follow the respective installed site's sun path. Hence, at first, all the components of the SPVT are designed separately and later assembled to form the SPVT using the assembly function in SolidWorks. In Figure 2, the details about the SPVT are shown. The designed 3-L SPVT is shown in Figure 2a. It can be seen that the SPVT structure contains a long edifice, which is generally referred to as the trunk for SPVT. The trunk is arranged with 3 layers, where the bottom (5 m above the ground) and the middle layer (7 m above the ground) have 4 strips commonly known as branches, each branch elevating to one of the eight directions (i.e., north, northeast, east, southeast, south, southwest, west, northwest). The upper layer (9 m above the ground) has only one branch facing the open sky. Overall, the designed 3-L SPVT has 9 branches, and each branch holds a PV leaf at a fixed tilt angle as depicted in Table 2.
The arrangement of the PV leaf on each branch of the SPVT, along with its direction, is shown in Figure 2b. A PV leaf is arranged on each branch of the SPVT, as the physical constraints related to shadowing were also considered. Here, the PV branches are placed to avoid such shadowing, and hence in each layer, a 2 m height difference is considered; the PV leaves are mounted in different orientations, having exactly a 45° angle apart in a way that no panel is oriented to same direction, irrespective of the layer. Finally, these branches are exposed to the sunlight and ultimately generate electricity.
However, the generated electricity potentials would vary upon the chosen solar cell technology; in addition, the exposed weather conditions would have an impact on the overall performance. To understand the variation in electricity potentials for the chosen solar cell technologies used as the PV leaf for the SPVT, this study investigates the energy performance and lifecycle-based emissions. The solar cell technologies considered for the PV leaf are shown in Figure 2c–e, and these include the crystalline silicon (c-Si), copper indium gallium selenide (CIGS), and cadmium telluride (CdTe).
The installation configuration of the PV leaves in each layer and the number of PV leaves used in each layer, along with the mounted tilt angle, is shown in Table 2. Here, the chosen tilt angle is as per the studied location.
In this section, the proposed SPVT performance modeling based on the energy and sustainability indicators is discussed. The detailed modeling of the weather-parameter-influenced energy outputs and the lifecycle-based emissions for the three solar cell technologies used in the 3-L SPVT are presented in Section 3.1 and Section 3.2, respectively.
The energy output of an SPVT usually depends on the numerous components used in the energy conversion process. For example, the use of the inverter device, i.e., direct current–alternating current (DC-AC), will enable us to harness AC energy outputs from the SPVT. However, the PV modules are primarily designed to produce DC energy outputs. Hence, in the proposed 3-L SPVT, only DC energy outputs were considered for the performance. When modeling the energy output of an entire SPVT, we first modeled the energy outputs for each PV leaf individually. The energy output of a single PV leaf in the proposed 3-L SPVT design is evaluated using Equation (
(
where
As per Equation (
However, the data related to weather parameters such as the plane of PV leaf irradiance, reference temperatures, and wind speeds need to be obtained from the meteorological stations. Here, the required weather data were obtained from the Indian Meteorological Department (IMD) [[
(
In the NOCT model, the cell temperature of the open-circuited cells in a PV module is estimated by assuming the plane of irradiance, reference temperature, and wind speed as 800 W/m
Using Equations (
(
where
In the SPVT, each PV leaf is mounted at different heights, and the wind speeds as per the mounted height are evaluated using Equations (
(
(
where
Using the above-discussed methodology as per Equations (
(
where
The annual energy outputs of the proposed 3-L SPVT are calculated by summing up the produced energy daily using Equation (
(
where
In any PV applications, the overall performance is affected by the degradation, and this will be specific to the PV technology and operating conditions. Even in the case of the SPVT, each PV leaf would experience the issue of degradation; hence, it is essential to estimate the degradation-influenced energy outputs.
The degradation-influenced annual energy outputs are referred to as the effective annual energy output (i.e.,
(
where
The lifetime energy output from the SPVT (i.e.,
(
The proposed 3-L SPVT system's sustainable performance is assessed based on the CO
The LCEA of various types of solar cells has been well studied in the literature. Peng and Yang (2013) conducted a detailed review highlighting the energy payback and greenhouse gas emission of different solar cells. The emission data related to the balance of the system used in various PV applications were also highlighted [[
A performance prioritization approach (PPA) based framework was proposed to select the best performing solar cell technology for a PV leaf used in the 3-L SPVT. The PPA framework is developed based on energy and sustainability performance indicators. In this approach, under the energy performance indicators, degradation-influenced lifetime energy outputs from the 3-L SPVT were evaluated by considering all the critical parameters. These critical parameters include solar irradiance, wind speed, ambient temperature, and module or cell temperature.
Likewise, under the sustainability performance indicators, lifecycle-based CO
The ranking criteria are based on the maximum energy outputs and minimum emissions throughout the considered lifetime of the 3-L SPVT system. The criteria for ranking is represented mathematically using Equation (
(
where
The
The weather parameters impact is the most difficult challenge with the PV systems. Unlike other PV applications, the SPVT performance is also influenced by the weather parameters such as solar irradiance, wind speeds, ambient, and cell temperatures. In the proposed 3-L SPVT, the PV leaves are arranged with varying installation configurations; hence, the structural design will inform us regarding the possible performance variations with the exposed weather parameters.
The monthly average of daily solar irradiance incident on each PV leaf in the three layers was analyzed for the 3-L SPVT and shown in Figure 5.
The range of the monthly average of daily solar irradiance incident on different PV leaves varied between 2.88 and 7.06 kWh/m
The yearly average daily incident solar irradiance on the solar PV leaf mounted in the open sky configuration in the upper layer of the SPVT is 5.64 kWh/m
The effect of wind speed and temperatures was considered while evaluating the energy outputs of each PV leaf. The sheer wind speeds at each solar PV leaf were estimated by considering the reference height's collected reference wind speeds. Figure 6 shows that wind speeds and temperatures are varied every month.
From Figure 6, it is understood that the sheer wind speeds at each PV leaf, which are based on its orientation and height at which they are mounted, are varied. The reference wind speed at 50 m is varied in the range of 2.29 to 4.07 m/s. The lowest observed reference wind speed is in October, and the highest is in May and June. The wind speeds at the upper layer (height = 9 m), middle layer (height = 7 m), and bottom layer (height = 5 m) are estimated using the power law described in Equations (
Using the solar PV leaf modeling approach presented in Section 3.1, each PV leaf's energy outputs were quantified based on their mounting conditions. In Table 5, Table 6 and Table 7, the monthly average of daily energy outputs of c-Si, CIGS, CdTe PV leaves of 3-L SPVT is presented.
From Table 5, it is noted that the c-Si PV-leaf based 3-L SPVT can generate electricity that varied approximately between 0.29 and 0.75 kWh. The c-SI PV leaf in the upper layer that is oriented to the open sky generates monthly average daily energy outputs that are ranged between 0.50 and 0.75 kWh. In the middle layer, the observed range of variation of the monthly average of daily energy outputs for the c-Si PV leaves mounted in NE, SE, SW, and NW are 0.34–0.69, 0.50–0.70, 0.51–0.75, and 0.37–0.74 kWh, respectively. Similarly, in the bottom layer, the observed range of variation of the monthly average of daily energy outputs for the c-Si PV leaves mounted in N, E, S, and W are 0.29–0.72, 0.45–0.68, 0.51–0.75, and 0.50–0.74 kWh, respectively.
From Table 6, it is understood that the monthly electricity productions from the CIGS PV-leaf based 3-L SPVT are varied between 0.24 and 0.61 kWh. The monthly average daily energy outputs of the CIGS PV leaf in the upper layer oriented to open sky are varied between 0.41 and 0.61 kWh. In the middle layer, the observed range of variation in the monthly average of daily energy outputs for the CIGS PV leaves mounted in NE, SE, SW, NW are 0.28–0.56, 0.41–0.57, 0.41–0.61, and 0.30–0.60 kWh, respectively. Similarly, in the bottom layer, the CIGS PV leaves mounted in N, E, S, and W orientations generate monthly average of daily energy outputs whose range is varied at 0.24–0.59, 0.37–0.55, 0.40–0.61, and 0.41–0.0.61 kWh, respectively.
Similar to the other two types of PV leaves, the energy outputs of CdTe are observed. From Table 7, it is noted that the CdTe PV-leaf based 3-L SPVT can generate electricity that varied approximately between 0.30 and 0.73 kWh. The CdTe PV leaf in the upper layer oriented to the open sky generates monthly average daily energy outputs whose range is between 0.49 and 0.73 kWh. In the middle layer, the observed range of variation of the monthly average of daily energy outputs for the CdTe PV leaves mounted in NE, SE, SW, and NW are 0.34–0.67, 0.49–0.69, 0.50–0.73, and 0.37–0.72 kWh, respectively. Similarly, in the bottom layer, the observed range of variation of the monthly average of daily energy outputs for the CdTe PV leaves mounted in N, E, S, and W are 0.30–0.70, 0.45–0.66, 0.48–0.73, and 0.50–0.72 kWh, respectively.
In Figure 7, the correlation between the incident solar irradiance and the daily energy outputs is shown for the three solar cell technology based 3-L SPVTs. Pearson's correlation coefficient (R
The monthly energy outputs for each PV leaf are evaluated using the daily generated energy outputs as per the investigated solar cell technologies (c-Si, CIGS, and CdTe) are evaluated, as seen in Figure 8. From Figure 8, it is understood that the three solar technologies demonstrated deviation in terms of energy generation for the same weather inputs and by using the same SPVT structure. Upon investigating the c-Si solar cell technology based 3-L SPVT, it is understood that the c-Si PV leaf oriented to open sky in the upper layer generates a monthly sum of energy outputs that vary between 15.50 and 22.96 kWh. In the middle layer, the observed range of variation of the monthly sum of energy outputs for the c-Si PV leaves mounted in NE, SE, SW, and NW are 10.62–21.34 kWh, 15.50–21.83, 15.80–23.17, and 11.49–22.99 kWh, respectively. Similarly, in the bottom layer, the observed range of variation of the monthly sum of energy outputs for the c-Si PV leaves mounted in N, E, S, and W are between 9.24–22.41, 14.03–20.62, 15.39–23.30, and 15.54–22.72 kWh, respectively.
Similarly, the CIGS-based 3-L SPVT reveals that the PV leaf oriented to the open sky in the upper layer generates a monthly sum of energy outputs that vary between 12.61 to 18.62 kWh. In the middle layer, the observed range of variation of the monthly sum of energy outputs for the CIGS PV leaves mounted in NE, SE, SW, NW are 8.64–17.31, 12.58–17.73, 12.82–18.82, and 9.34–18.65 kWh, respectively. Similarly, in the bottom layer, the observed range of variation of the monthly sum of energy outputs for the CIGS PV leaves mounted in N, E, S, and W are 7.52–18.17, 11.41–16.72, 12.49–18.84, and 12.64–18.43 kWh, respectively. Likewise, the analysis of the CdTe-based 3-L SPVT reveals that the PV leaf oriented to open sky in the upper layer generates a monthly sum of energy outputs that vary between 15.34 and 22.29 kWh. In the middle layer, the observed range of variation of the monthly sum of energy outputs for the CdTe PV leaves mounted in NE, SE, SW, and NW is 10. 51–20.72, 15.11–21.36, 15.41–22.67, and 11.37–22.32 kWh, respectively. Similarly, in the bottom layer, the observed range of variation of the monthly sum of energy outputs for the CdTe PV leaves mounted in N, E, S, and W are 9.14–21.75, 13.88–20.01, 15.01–22.71, and 15.37–22.05 kWh, respectively.
Considering that the three technologies have different efficiencies and temperature coefficients, in order to perform a meaningful comparison, the energy outputs were normalized to the corresponding value of the crystalline PV leaf as plotted in Figure 9. It can be seen that the crystalline PV leaf and the CdTe PV leaf show quite a similar energy performance with differences up to 3% during spring. The CIGS PV leaf underperforms with a difference in the energy output of 19% with respect to the c-Si PV leaf.
Based on the monthly sum of energy produced by each PV leaf, the annual energy outputs are evaluated, and they are discussed in Section 5.2.1 and Section 5.2.2, while considering the effect of PV leaf mounting aspects and the possible degradation rate for each solar cell technology.
This section discusses the effect of orientation, the layered structure, and the solar cell technology on the 3-L SPVT system's annual energy outputs. Figure 10 shows the yearly energy outputs of each PV leaf. From Figure 10, it is understood that the effect of orientation on the energy outputs is noticeable, and the same was thoroughly analyzed in Section 5.2, as the daily variations were also considered. Here, 9 PV leaves installed in different orientations are analyzed. Among these 9, the solar PV leaves oriented in the southwest (i.e., in the middle layer) and south (i.e., in the bottom layer) directions produced the maximum energies annually, and the minimum was by the PV leaf mounted in the north direction for the three different solar cell technologies.
The observed difference between the maximum energy-producing PV leaf to minimum energy-producing PV leaf is 23.88%, 23.98%, and 24.05% for c-Si, CIGS, and CdTe solar cell technology based SPVTs, respectively. In addition, we observed that the difference in energy production between the middle and bottom layers is quite negligible, which is less than 0.025%.
Upon comparing the solar cell technologies, it was observed that the c-Si technology based PV leaves perform well, followed by CdTe and CIGS. Regarding the annual energy outputs, the SPVT with c-Si, CIGS, and CdTe produces 1893.09, 1537.41, and 1853.14 kWh, respectively.
Degradation is one of the most affecting parameters on SPVT performance, and this is possible due to many factors (e.g., faults, breakdown, rust, and other risks) [[
Using the data shown in Table 8, the degradation influenced energies are estimated for the 3-L SPVT system lifetime. The considered lifetime is 25 years. In Table 9 the lifetime energy outputs with and without considering the degradation for the c-Si, CIGS, and CdTe solar PV-leaf based 3-L SPVT systems are given.
In Figure 11, the degradation influenced lifetime energy outputs are illustrated for the c-Si, CIGS, and CdTe solar PV-leaf based 3-L SPVT system.
From Figure 11, it is understood that the first year's observed energy outputs are 1893.09, 1537.41, and 1853.14 kWh for the c-Si, CIGS, and CdTe solar cell technology based SPVTs, respectively. Upon considering the degradation, the CdTe PV-leaf based 3-L SPVT was observed to perform well, followed by c-Si and CIGS, which is in line with the literature [[
It can be seen from Figure 12 that the normalized value dropped to 0.82, 0.52, and 0.84 from 1 in the case of the c-Si, CIGS, and CdTe PV-leaf based 3-L SPVTs, respectively. The CdTe PV-leaf based 3-L SPVT shows a favorable energy performance due to its lower degradation rates when compared to other two as per the current scenarios. The observed difference between the c-Si and CdTe is almost negligible.
An emission analysis of the 3-L SPVT was carried out by considering the lifecycle-based CO
The emissions from the 3-L SPVT system with three different solar PV leaf technologies were estimated by considering the case without degradation. The main reason for considering this is to quantify the actual emissions associated with the system. If degradation is accounted, the emissions associated with the lost energy due to degradation are simply ignored, but the total embodied emissions for the actual possible amount of energy will remain. Hence, the emissions are calculated for the 3-L SPVT without considering degradation. Emissions for c-Si PV leaves are observed as 1,118,786.22 gCO
In this section, the PV leaf selection based on the energy and sustainability indicators is discussed. The selection was based on the PPA under which the main criteria are maximum energy output and minimum emission (see Table 11). We observed that when degradation is considered, the conventional c-Si tends to perform more or less similarly to the CdTe. The c-Si PV-leaf based 3-L SPVT generated approximately 0.21% lower energy outputs than the CdTe PV-leaf based 3-L SPVT. The lifetime energy outputs of an SPVT with c-Si, CIGS, and CdTe are 43,049.48, 30,963.96, and 43,141.49 kWh, respectively. The emissions released from the 3-L SPVT with CdTe PV leaves are a lot lower when compared to the other two. Comparing specifically with the c-Si, the CdTe performs far better under the sustainability category. The energy and sustainability indicator-based ranking comparison between the three PV technologies confirms that the use of CdTe solar cells would be most beneficial for SPVT applications.
Overall, this study presents a brief analysis of the selection of solar cell technology for PV leaves used in the SPVT application. To understand the selection procedure, a multilayered case was proposed and investigated. In addition, an investigation of three different PV technologies was carried out, and the following conclusions were drawn:
- c-Si PV cells perform better when all the factors that affect performance are taken into account; however, this is found to be true for only a few years.
- When the DR is considered, the CdTe cells are observed to perform better for SPVT applications due to its lower degradation rates.
- It was observed that the PV cell degradation rate plays a crucial role in identifying the best performing PV technology for SPVTs.
- The CdTe solar PV leaves produced lower CO
2 emissions when compared to the other two. - In addition, the benefits associated with CdTe cells, such as a flexible structure, a ultrathin glass structure, and low-cost manufacturing, make them the best acceptable PV leaves for a SPVT design.
This study only focused on a direct current analysis; there is greater scope for conducting an alternating current analysis. Limiting this work to DC scope is to understand the sustainable selection process for PV leaves. On the other side, the considered case has 9 leaves, whose energy outputs vary with approximately a 23% difference; hence, feeding all the inputs to the power converter might have an impact. Therefore, we suggest the use of string inverters for each PV leaf depending upon its size. In most cases, the large-size SPVT systems might need such concepts.
Graph: Figure 1 Visual representation of different types of solar photovoltaic plants based on the installation and mounting area. SPVT = solar photovoltaic tree; OMPV = open-mount photovoltaics; RMPV = roof-mount photovoltaics; BAPV = building attached photovoltaics; CMSPV = canopy-mounted solar photovoltaics; RIPV = roof-integrated photovoltaics; FIPV = façade-integrated photovoltaics; BIPV = building-integrated photovoltaics; VAPV = vehicle attached photovoltaics; VIPV = vehicle-integrated photovoltaics; RoAPV = road-attached photovoltaics; RoIPV = road-integrated photovoltaics; LoMPV = locomotive-mount photovoltaics; RaTIPV = rail track integrated photovoltaics; FPV = floating photovoltaics or floatovoltaics; FSPV = floating solar photovoltaics; SPV = submerged photovoltaics; UobSPV = underwater on-board solar photovoltaics; PMPV = pole mounted-photovoltaics; PIPV = pole-integrated photovoltaics.
Graph: Figure 2 The schematic structure of the solar photovoltaics tree (SPVT): (a) Three-layered SPVT; (b) SPVT showing the PV leaf arrangement; (c) c-Si PV cells; (d) CIGS PV cells; (e) CdTe PV cells.
Graph: Figure 3 The lifecycle stages of the typical photovoltaic module and the system.
Graph: Figure 4 A framework based on a performance prioritization approach for selecting the photovoltaic leaf with maximum lifetime energy outputs and minimum lifetime emissions.
Graph: Figure 5 Incident solar irradiance on the photovoltaic leaves in the proposed three-layered SPVT (3-L SPVT) system. Data source: Indian Meteorological Department (IMD).
Graph: Figure 6 The observed wind speeds are the height variation of the three different layers of the proposed 3-L SPVT system and the ambient temperatures.
Graph: Figure 7 Relationship between the energy outputs and the incident solar irradiance (a). c-Si PV-leaf based 3-L SPVT; (b) CIGS PV-leaf based 3-L SPVT; (c) CdTe PV-leaf based 3-L SPVT.
Graph: energies-13-06439-g007b.tif
Graph: Figure 8 Comparison of monthly energy outputs of a solar photovoltaic tree in three different layers for the three different c-Si, CIGS, and CdTe solar cell technologies.
Graph: Figure 9 Comparison of normalized monthly energy outputs of a solar photovoltaic tree in three different layers for the three different c-Si, CIGS, and CdTe solar cell technologies.
Graph: Figure 10 Comparing the solar PV leaves' annual energy outputs based on their installation orientation: (a) c-Si solar cell technology; (b) CIGS solar cell technology; (c) CdTe solar cell technology.
Graph: Figure 11 Comparison of degradation influenced energy outputs of a 3-L SPVT for the three different PV technologies.
Graph: Figure 12 Comparison of normalized degradation influenced energy outputs of a 3-L SPVT for the three different PV technologies.
Graph: Figure 13 Lifecycle-based CO2 emissions per the produced electricity by the 3-L SPVT system with and without degradation.
Table 2 Installation configuration of photovoltaic leaves in the three-layered solar photovoltaic tree.
Solar Tree PV Leaf Layer Number of PV Leaves in a Layer PV Leaf Orientation Tilt Angle (°) Three-layer design Upper layer 1 PV Open sky 0 Middle layer 4 PV Northeast 25.4358 PV Southeast PV Southwest PV Northwest Bottom layer 4 PV North 25.4358 PV East PV South PV West
Table 3 Power performance modeling specification of the chosen solar cell technologies as a photovoltaic leaf for the solar photovoltaic tree [[
Solar Cell Technology Efficiency (%) Area (m2) Temperature Coefficient (%/°C) Crystalline silicon (c-Si) 14.90 0.72 −0.47 Copper indium gallium selenide (CIGS) 12.10 −0.45 Cadmium telluride (CdTe) 14.60 −0.34
Table 4 Faiman coefficients for solar cell technologies used as a photovoltaic leaf [[
Solar Cell Technology Faiman Coefficients for Different Solar Cell Technologies Crystalline silicon (c-Si) 30.02 6.28 Copper indium gallium selenide (CIGS) 22.19 4.09 Cadmium telluride (CdTe) 23.37 5.44
Table 5 Monthly average of daily energy outputs of c-Si photovoltaic leaves in the three different layers of the solar photovoltaic tree.
Month Monthly Average of Daily Energy Outputs of C-Si Photovoltaic Leaf (kWh) Upper Layer Middle Layer Bottom Layer P P P P P P P P P January 0.53 0.37 0.60 0.64 0.40 0.32 0.48 0.67 0.53 February 0.63 0.47 0.68 0.72 0.50 0.44 0.57 0.74 0.63 March 0.71 0.58 0.70 0.75 0.62 0.57 0.64 0.75 0.69 April 0.75 0.66 0.69 0.74 0.71 0.68 0.68 0.72 0.74 May 0.74 0.69 0.65 0.70 0.74 0.72 0.66 0.67 0.73 June 0.64 0.62 0.56 0.58 0.64 0.64 0.59 0.56 0.62 July 0.55 0.53 0.50 0.51 0.54 0.54 0.51 0.49 0.53 August 0.54 0.50 0.50 0.52 0.52 0.51 0.50 0.51 0.52 September 0.60 0.51 0.59 0.62 0.55 0.51 0.55 0.61 0.59 October 0.56 0.45 0.59 0.61 0.46 0.42 0.51 0.62 0.54 November 0.54 0.39 0.61 0.64 0.41 0.34 0.49 0.67 0.53 December 0.50 0.34 0.57 0.61 0.37 0.29 0.45 0.64 0.50
Table 6 Monthly average of daily energy outputs of CIGS photovoltaic leaves in the three different layers of the solar photovoltaic tree.
Month Monthly Average of Daily Energy Outputs of CIGS Photovoltaic Leaf (kWh) Upper Layer Middle Layer Bottom Layer P P P P P P P P P January 0.43 0.30 0.49 0.52 0.32 0.26 0.39 0.55 0.43 February 0.51 0.38 0.55 0.59 0.41 0.36 0.47 0.60 0.51 March 0.57 0.47 0.57 0.61 0.51 0.47 0.52 0.61 0.57 April 0.61 0.54 0.57 0.60 0.58 0.55 0.55 0.59 0.61 May 0.60 0.56 0.53 0.57 0.60 0.59 0.54 0.54 0.60 June 0.52 0.50 0.46 0.47 0.52 0.52 0.48 0.45 0.50 July 0.45 0.43 0.41 0.41 0.44 0.44 0.42 0.40 0.43 August 0.44 0.41 0.41 0.42 0.42 0.41 0.41 0.41 0.42 September 0.49 0.42 0.48 0.51 0.45 0.42 0.44 0.50 0.48 October 0.45 0.36 0.48 0.49 0.38 0.34 0.42 0.51 0.44 November 0.44 0.32 0.50 0.52 0.33 0.28 0.40 0.54 0.43 December 0.41 0.28 0.47 0.50 0.30 0.24 0.37 0.52 0.41
Table 7 Monthly average of daily energy outputs of CdTe photovoltaic leaves in the three different layers of the solar photovoltaic tree.
Month Monthly Average of Daily Energy Outputs of Cdte Photovoltaic Leaf (kWh) Upper Layer Middle Layer Bottom Layer P P P P P P P P P January 0.52 0.36 0.60 0.64 0.39 0.32 0.48 0.67 0.52 February 0.62 0.47 0.67 0.71 0.50 0.43 0.56 0.73 0.62 March 0.70 0.57 0.69 0.73 0.61 0.56 0.63 0.73 0.68 April 0.73 0.65 0.68 0.72 0.69 0.66 0.66 0.70 0.72 May 0.72 0.67 0.63 0.68 0.72 0.70 0.65 0.65 0.71 June 0.62 0.60 0.55 0.57 0.62 0.62 0.57 0.54 0.60 July 0.54 0.52 0.49 0.50 0.53 0.53 0.50 0.48 0.52 August 0.53 0.49 0.49 0.52 0.51 0.50 0.49 0.50 0.51 September 0.59 0.50 0.57 0.61 0.54 0.50 0.54 0.60 0.58 October 0.55 0.44 0.57 0.60 0.46 0.41 0.52 0.61 0.53 November 0.53 0.38 0.60 0.63 0.41 0.34 0.49 0.66 0.52 December 0.49 0.34 0.57 0.61 0.37 0.30 0.45 0.64 0.50
Table 8 The degradation rate of the chosen solar cell technologies as a photovoltaic leaf for the solar photovoltaic tree.
Solar Cell Technology Degradation Rate (%/Year) Reference Crystalline silicon (c-Si) 0.80 [ Copper indium gallium selenide (CIGS) 1.86 Cadmium telluride (CdTe) 0.60
Table 9 Lifetime energy outputs from the 3-L SPVT using the c-Si, CIGS, and CdTe solar PV leaves.
Solar Cell Technology Lifetime Energy Outputs (kWh) Without Degradation With Degradation Crystalline silicon (c-Si) 47,325.98 43,049.48 Copper indium gallium selenide (CIGS) 38,435.34 30,963.96 Cadmium telluride (CdTe) 46,328.66 43,141.49
Table 10 Lifecycle greenhouse gas emissions for the chosen solar cell technologies as a photovoltaic leaf for the solar photovoltaic tree.
Solar Photovoltaic Cell Technology Lifecycle-Based CO2 Emissions (gCO2-eq/kWh) Reference Solar PV Leaf + Mounting Structure Crystalline silicon (c-Si) 23.64 [ Copper indium gallium selenide (CIGS) 24.47 Cadmium telluride (CdTe) 16.94
Table 11 Selection of solar cell technologies for SPVT leaves based on energy and sustainability indicator.
3-L SPVT Degradation-Influenced Lifetime Energy Outputs (kWh) Lifecycle-Based CO2 Emissions (tCO2-eq) Rank Energy Sustainability Overall c-Si PV leaf 43,049.48 1.12 2 3 2 CIGS PV leaf 30,963.96 0.94 3 2 3 CdTe PV leaf 43,141.49 0.79 1 1 1
Conceptualization, N.M.K. and S.S.C.; data curation, N.M.K. and M.M.; formal analysis, N.M.K.; funding acquisition (Only APC), R.M.E. and N.D.; investigation, N.M.K. and S.S.C.; methodology, N.M.K. and S.S.C.; resources, S.S.C.; software, S.S.C.; supervision, S.S.C.; validation, N.M.K.; visualization, N.M.K.; writing—original draft, N.M.K.; writing—review and editing, N.M.K. and S.S.C. All authors have read and agreed to the published version of the manuscript.
The contribution of the School of Electrical and Computer Engineering, National Technical University of Athens was funded by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 799835
The authors declare no conflict of interest.
The approach used for evaluating the energy and sustainability indicators is shown in Algorithm A1. Based on the obtained indicators, the solar cell technology that produces the maximum lifetime energy and minimum lifecycle emission is selected using the approach presented in Algorithm A2 and A3.
By Nallapaneni Manoj Kumar; Shauhrat S. Chopra; Maria Malvoni; Rajvikram Madurai Elavarasan and Narottam Das
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