Amongst all the components that make up a solar photovoltaic system (or solar PV system), solar module is perhaps the most ubiquitous to many people. Unfortunately, it is the component that is often overlooked when making decision about component selection. Solar module selection is not usually given a well-thought-out consideration by many intending photovoltaic (PV) users because they assume that what is installed on a neighbor’s roof should be a benchmark for their personal energy needs.
In this article, we attempt to list and explain some factors that suggest why such a simplistic decision can hardly guarantee satisfactory Solar PV solution and therefore, it is a good idea to select your solar panel (the common name for solar module) based on your carefully estimated kilo Watts hour (kWh) consumption, prevailing climatic conditions, shadow free space (SFS) available in your location, the orientation of your roof and the type of solar cell technology.
The most important and perhaps difficult stage in sizing an off-grid PV system is providing the daily electricity consumption in a carefully worked out and breakdown format (James, 2008:317; Falk, et al, 2010:185). When carefully done by an expert, there is more guarantee that the output from the PV system will balance with the solar input while taking into consideration losses in the system. After evaluating the daily average energy consumption as accurately as possible, then follows the sizing (using the solar resource available in the planned site), of the solar array and storage capacities (Mertens, 2014:199). Thus, radiation condition in a planned site is one major limiting factor that determines how well solar energy can be converted to electricity.
Almost all energy needs for life survival on earth depend on radiation from the sun (Mertens, 2014:21; Messenger & Ventre, 2010:21). Similarly, photovoltaic utilization also depends on the availability of sunlight. Hence, the optimization of the performance of photovoltaic systems depends on the knowledge of the properties of sunlight (Messenger & Ventre, 2010:21). In addition, it is important to have adequate understanding of the spectral composition of the sun and the effects of the atmosphere on the radiation from the sun before making a choice of solar cell technology to be used in the optimal conversion of sunlight to electricity (Messenger & Ventre, 2010:23). Furthermore, the local weather conditions (cloud, fog and smog cover) play a major influence in the performance of any PV installation (Blazev, 2013:125). Therefore, to achieve a properly designed solar PV system, solar professionals should be very familiar with “global radiation” (Blazev, 2013:7) and local weather conditions.
Site survey and shadow analysis are important tasks when designing stand-alone PV systems to determine the shadow free space (SFS) available for solar PV utilisation. The SFS is the percentage of roof space with solar potential. This is necessary because not every space on your roof has optimal solar potential. Thus, before attempting to select a solar module, it is necessary that the designer should visit the site and identify the constraints and solar potentials.
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The common factors that can be influenced by an installer (or a user/designer) in order to improve the performance of a PV array are location, tilt angle and orientation. For every solar module, more light means more output energy, but this can further be enhanced when shading is eliminated, the module is optimally oriented, and it is mounted at an appropriate angle of attack to the horizontal. Unlike some solar thermal collectors, a PV module’s output is almost linearly dependent on the incident radiation, so even small enhancements in incident radiation can make a positive big difference (Ross & Royal, 1999:25).
Optimum location: The caveat is to avoid shading as much as possible (The Renewable Energy Hub). Shading is one of the most detrimental factors to be put into check when designing a system and when calculating the available solar radiation. Every module in a solar array must be evenly and equally illuminated – shading of the solar array decreases its output greatly (Ross & Royal, 1999:25). Merely shading one cell in a module will reduce the output of the module very significantly (Ross & Royal, 1999:26). To minimize shading, the solar array should be located as high above the ground and as far away from trees and other shading objects as possible (Ross & Royal, 1999:25). Solar Arrays consisting of a field of sub-arrays arranged in parallel must be spaced such that the front rows will not shade the rows behind them (Ross & Royal, 1999:25). However, when shadows cannot be avoided, the array must be enlarged so that it still produces the required amount of electricity (Ross & Royal, 1999:26). This increases the cost and leads to underutilization of the system. Hence, other techniques have been recommended to proffer good design that can mitigate the effect of shading, where it is inevitable, without unnecessarily oversizing the array (Falk, et al, 2010:126). They are discussed below.
Optimum direction: For fixed arrays, the optimum surface azimuth angle of the array is usually due south in the northern hemisphere and due north in the southern hemisphere, i.e., the array should face the equator.
Optimum tilt angle: Several practical installations have shown that mounting array on a pitched roof maximises area more than array on flat roof (Falk, et al, 2010:122). Designers should try to explore the feasibility of seasonal adjustment of the array tilt angle as a means of optimising system performance (Messenger & Ventre, 2010:300).
PV module construction may be done using any of the following foundation material: crystalline-silicon (c-Si), polycrystalline-silicon (pc-Si) and recently exploited thin films of amorphous-silicon (a-Si). The c-Si modules are the most highly efficient, but they are also the most exorbitantly expensive because they require a comprehensive and energy-intensive procedures to produce them (Enteria Napolean and Aliakbar Akbarzadeh, 2014). Therefore, in selecting a solar module, it is up to the investor to decide which parameter to trade off, efficiency or cost.
Enteria Napolean and Aliakbar Akbarzadeh (2014). Solar Energy Sciences and Engineering Applications 2014, Taylor & Francis Group, London, UK
Ross M. & Royal J. (1999). Photovoltaic in Cold Climates. Published by James & James (Science Publishers) Ltd
Messenger, R. A., & Ventre, J. (2010). Photovoltaic Systems Engineering, Abingdon.
Falk, A., Durschner, C., & Remmers, K. H. (2013). Photovoltaics for professionals: solar electric systems marketing, design and installation. Routledge.
Blazev, A. S. (2013). Photovoltaics for commercial and utilities power generation. Lulu Press, Inc.
Mertens, K. (2014). Photovoltaics: fundamentals, technology, and practice. John Wiley & Sons.
James, J. (2006). Planning and installing Photovoltaic Systems, a Guide for installers, architects and engineers.