3DPV technology is a new approach for achieving optimum solar energy that will yield a cost- effective, more reliable and more economically friendly alternative energy source. In 3DPV systems, the absorbers have multiple orientations that allow effective capturing of off-peak sunlight, and the re- absorption of reflected light within the 3D structure [11, 13]. It utilizes 3D nature of structures such as the spherical or cubic system, etc., to absorb power in the entire volume of that material. Hence, power is measured in Watts per unit volume as against per area measurement as is the case in the planar or two- dimensional (2D) system. Furthermore, the impact of height in system efficiency for the 3DPV has been found to be remarkable [4, 51, 52].
The plot of energy obtained in a day by the Genetic Algorithm (GA) optimized 3DPV structures compared with that of a flat panel under the same conditions was found to be much higher as shown in Figure 2-8: The insertion in the figure illustrates the power generated during the day for the flat panel as against what was obtained in the 3DPV structure at height of 10 meter [11, 53].
21 Figure 2-8: Energy plot comparison between GA optimized 3DPV structures and a flat panel [11]
These are major advantages of 3DPV over the planar arrangement. These are in contrast to the popular flat design of PV systems [54, 55].
2.6.1 Review of literatures on 3DPV structure
Various research findings have been made and are still on-going on solar energy generation.
Research has proved that 3DPV structures are able to increase the generated energy density (energy per footprint area, Wh/m2) in a linear proportion to the configuration height, for a given day and location [10, 56]. Some of these assertions on 3DPV technology are as stated below:
2.6.1.1 Fibonacci PV module (FPM)
The mathematical work of Leonard of Pisa, popularly known as Fibonacci, played a major role in utilizing the nature advantage of the abundant intake of sunlight energy. Fibonacci revealed through his analytical numbers that the leaf grows vertically upwards the stem in the direction of the sun so as to absorb more sunlight energy for its photosynthesis and this growth follows a spiral arrangement called ‘phyllotaxis’
[57]. Another research group, carried out a test on a 3DPV module whose configuration was based on Fibonacci numbers. The results of simulations on the test device revealed that the power generation characteristics of the solar cells depend on the shape and spacing of the solar cells for the most effective use of sunlight energy.
Some other researchers carried out some work on 3DPV technology based on Fibonacci’s phyllotaxis:
1. The absorbers and reflected method - By using self-supporting 3D shapes, taller and more complex shapes - such as the open parallelepiped, cubes, trapezium and ridged tower for increased energy density that could enable the use of cheaper thin film material [10].
2. The Fibonacci PV module (FPM) - B y u sing Fibonacci numbers to obtain the shape and spacing arrangement in a 3DPV structure for most effective solar radiation conversion into optimized output solar energy [8].
22 3. The Fibonacci PV module (FPM) - By using Fibonacci numbers to increase the number of
solar module stages (height) in the FPM for volumetric energy output [51].
4. 3D nanopillar-based cell modules - By using new device structures and materials processing, such as embedded 3D single-crystalline n-CdS nanopillars in polycrystalline thin films of p- CdTe to facilitate high absorption of light and efficient collection of the carriers for acceptable efficiencies [55].
2.6.1.2 Spherical solar technology (SST)
Accurate 3D technology has been found to enable innovative and improved device design which can result in overall cost-effectiveness, improved material processing and system utilization. Of particular interest is the spherical Si solar technology (SSST) which has been found to be attractive because it uses low-cost Si feedstock and the fabrication process is found simple and inexpensive. Similarly, new innovation in PV installation exploits the use of cheaper, thin film materials on 3D shapes self-supporting structures to facilitate increased energy density generation. Hence, utilizing solar energy in three dimensions can open new avenues towards large-scale power generation [11, 54, 58].
2.6.1.3 3D Nanopillar-based cell modules.
In recent years, tremendous progress has been made in developing PV that can, potentially, be mass deployed. An example of this is in the use of 3D Nanopillar-based cell modules, with the aim to reduce solar cells cost by using novel device structures and materials processing for obtaining acceptable efficiencies. This enables the highly regular, single-crystalline nanopillar arrays of optically active semiconductors to be directly grown on aluminium substrates which are then configured as solar-cell modules. An example is a PV structure that incorporates three-dimensional, single-crystalline n-CdS nanopillars that is embedded in polycrystalline thin films of p-CdTe to enable high absorption of light and efficient collection of the carriers. Various other experiments and modelling have demonstrated the potency of this approach for enabling highly versatile solar modules on both rigid and flexible substrates.
These display enhanced carrier collection efficiency that arises from the geometric configuration of the nanopillars [59].
2.6.1.4 Solar energy generation by 3D method, using Fibonacci PV module (FPM).
In Fibonacci PV Module, (FPM), the aim is to maximize solar power generation per installation area. Low-cost solar cells (Thin-film with conversion efficiency of 10%) were assembled in modules in 3D structures. The PV modules were arranged in the shape of a tree based of Fibonacci’s sequence [51].
Eight solar cells make up a single stage in the module. As the number of stages increases, the total power generated also increases accordingly. For a single-stage FPM, the maximum power generated is about 75% [15], thus a two stage or three stage FPM maximum power output doubles of triples in values accordingly. In addition, the amount of generated power increases with increases in solar altitude. Thus a FPM will yield more power generation per installation area than a conventional plane module.
2.6.1.5 Three-dimensional modelling and simulation of P-N junction spherical silicon solar cells Another new promising technology for PV energy conversion is the Three-dimensional simulated spherical p-n junction spherical silicon solar cells (SST). The simulation is based on models
23 using finite-difference method. It has been proved that the efficiency of a spherical solar cell is slightly lower than a conventional planar device but this is being compensated-for in terms of cost advantage.
Furthermore, the materials being used are low-cost based, the device design gives optimal performance and the device processing technology is quite simple and affordable.
The spherical solar technology involves the use of tiny inexpensive Si spheres which are used as feed stock in the form of irregular shaped particles. These are melted and solidified into minute single crystalline spheres. The impurities are segregated to the outer layer to be removed, thereby improving their quality and purity and making them ideal for use for low-cost PV modules. Accurate Three- Dimensional (3D) modelling is necessary for advanced device design and material processing optimization [54].
The Three-Dimensional numerical approach has been used to overcome the modelling challenges. Simulation results in device quantum efficiency and the model is found to be useful in device design and simulation and in process optimization.
2.6.1.6 Three-dimensional nanopillar-array PV on low-cost and flexible substrates
In this technology, solar-cell modules are configured by growing optically active, highly regular single-crystalline nanopillar arrays of semiconductors directly on aluminium substrate [59]. The geometric configuration of the nanopillars enables highly versatile solar modules on the substrates achieved improved efficiency of the carrier charges. This method was reported not to be cost-effective.
When nanowires are grown non-epitaxially on amorphous substrates, their random orientation on the growth could also limit the explored device structure. This is reported to be an improvement on the hitherto-common approach being used for the coating of the epitaxial growth of thin films by using single-crystalline substrate as the template.
The success of the technology is in the ability to produce high density, single-crystalline nanopillar arrays on an amorphous substrate with fine geometrical control. The reliance on epitaxial growth from single-crystalline substrates has been overcome. Reduced reflectivity from CdS nanopillar arrays was observed for small inter-pillar distance indicating that the light absorption properties in 3D nanopillar-based cell modules could be improved while enhancing the carrier collection[59].
In spite of the various technological improvements being made so far on solar power generation, its effect on costs has only marginally improved. The cost is still comparatively higher than conventional energy technologies. Furthermore, the effect of the high variation in solar panel electrical parameters such as output voltages, currents and powers due to environmental conditions such as temperature and solar irradiance are not favourable to solar power generation [13, 21]. Hence, it is paramount to decide on the feasibility or otherwise of installing a solar power plant at a location for a particular project before actually investing in constructing the plant. There will otherwise be a high risk of project abandonment or unsustainability. In order to avoid this risk, there is the need to carry out pre-analysis of the solar plant by carrying out modelling and simulation of the solar panels and solar cells.