Assessment and performance analysis of roof-mounted crystalline stand-alone photovoltaic (SAPV) system at selected sites in South Africa – Bulletin of the National Research Centre

The global energy demand is on the increase due to population, industrialisation, and urbanisation growth. Globally, urbanisation is expected to extend to all the regions by 2050 and the growth will be faster in Asia and Africa than in other regions. An urbanisation rise of 56% and 64% has been projected for Africa and Asia, respectively (UN 2014). For several years, fossil fuels, such as petrol, coal, diesel, and natural gas, have provided the needed energy to meet the identified growths, and in 2018, about 64.2% of the global electricity was generated from fossil fuels (BP 2019). The energy sources for generating electricity are shown in Fig. 1. Today’s large economies, such as the USA and Germany, were built and driven by fossil fuels, and this makes fossil fuels efficient economic development drivers (Ebhota 2019a; Ebhota and Tabakov 2020). However, this comes with negative health and environmental consequences. Studies have linked climate change, global warming, and greenhouse gas (GHG) emissions to the burning of fossil fuels for electricity generation (Baz et al. 2021; Srivastava et al. 2021; Vohra et al. 2021). Among the components of GHG, carbon dioxide (CO₂) contributes the most to global warming, thereby becoming one of the critical challenges confronting humanity. Simultaneously, there is continuous depletion of fossil fuel reserves, increase and fluctuation of prices, subsidy challenges, and increasing awareness of ecological, green, or sustainable consumerism. Despite these deleterious consequences, man is not deterred from fossil fuel usage, as fossil-based energy is still heavily being consumed year in and year out. Growth rates of 1.5% and 2.9% were reported for 2007–2017 and 2018, respectively (BP 2019). The global annual fossil fuel consumption and gas growth rates were reported to rise by 1.6% and 1.5%, respectively; the rise is mostly from developing regions, such as sub-Saharan Africa and Asia (BP 2019; WorldBank 2021). This is due to energy poverty in developing countries, especially in rural areas, where biomass (charcoal and firewood) is the main source of energy for cooking (Ebhota 2019b).

Sources of global electricity generation (BP 2019)

To address the challenges emanating from the use of fossil fuels, the United Nations (UN), under the sustainable development goals (SDGs), came up with a universal clarion call that targets the reduction in CO2 emissions (UN, 2015, 2020; Ebhota 2021). One of the goals of SDGs is the provision of clean, adequate, and affordable energy for all by 2030. The human existence will be threatened if there is no energy to support socio-economic activities. It implies that the reduction or elimination of fossil fuels is only possible if there are alternative energy supplies. Several renewable energy sources, such as solar, geothermal, tidal, biofuels, nuclear, hydro, and wind, have been identified and are being exploited as alternatives to fossil fuels. The deployment of renewable energy technologies has been challenging because of their intermittency in supply, low energy densities, and uneven spread of resources. Solar energy is the commonest renewable energy and its potential is region dependent. As a result, research and development activities have been focussed on solar efficiency, optimisation, energy storage, and cost. Prices of solar photovoltaic (PV) parts, especially PV modules, have witnessed a drastic decline of over 50% in recent years (U. S. 2015); however, the conversion efficiency of most commercially available crystalline silicon PV panels is from 15 to 20%, while concentrated multifunction and other new cells are between 40 and 50% (Kammen and Sunter , 2017, 2016, 2020; NREL ).

The exploitation of solar PV energy is limited by several factors, which include location, low energy densities, and low conversion efficiency. Site location conditions, such as season, length of sunshine, time of the day and year, speed of the wind, topography, relative humidity (RH), temperature, and many other environmental factors determine solar PV potential and performance. The estimation and understanding of these factors play an invaluable role in the accurate assessment of solar PV potential and performance prediction of a solar PV system. This implies that inaccurate assumptions of solar resources and sizing of PV systems often lead to either system’s under-sizing or over-sizing, erratic power supply, and higher cost.

The previously published solar PV-related studies of the regions under consideration were mainly focussed on the econo-technical feasibility assessment (Semelane et al. 2021; Stanley et al. 2021; Mutombo and Numbi 2019; Zawilska and Brooks 2011) and the significance (Conway et al. 2019; Kumar et al. 2019; Meyer and Overen 2021) of the solar PV system. Studies on the assessment of solar PV potential and systems performance across the nine provinces of South Africa could not be found. Again, a comparative analysis of solar PV potential and systems performance across the nine provinces of South Africa could not also be found. Therefore, this study aims to assess the solar PV potential and predict the performance of PV systems in selected locations across the nine provinces of South Africa. Computer modelling of a hypothetical PV system of a 10 kWp installed capacity of tilted rooftop monocrystalline PV modules will be used in this assessment and performance prediction study. This study with the aid of computer modelling will proffer answers to the following research questions—what is the range of the yearly average:

1. 1.

   Global horizontal irradiation (GHI) among the sites?

2. 2.

   Direct normal irradiation (DNI) among the sites?

3. 3.

   Diffuse horizontal irradiation (DIF) among the sites?

4. 4.

   Global tilted irradiation (GTI) among the sites?

5. 5.

   Temperature (TEMP) among the sites?

6. 6.

   Specific PV power output (PVOUT specific) among the sites?

7. 7.

   Total PV power output (PVOUT total) among the sites?

8. 8.

   Performance ratio (PR) among the sites?

To address the set objectives satisfactorily, the paper was expressed in four sections (“Background, Methods, Results, and Discussion”), in addition to the background in “Background” section. “Methods” section presents the methods, “Results” section presents the simulation results, and “Discussion” section discusses the results as they connect to the existing literature. Finally, critical points and responses to the research questions are highlighted in “Conclusions” section, the concluding section.

Solar radiation and PV system

Solar radiation is the emission of energy from the sun, which travels through space as either waves or particles, and can take the form of heat, light, or sound. These waves or particles can be categorised into four types: alpha, beta, neutrons, and electromagnetic waves. This study focuses on electromagnetic radiation. Electromagnetic solar radiation has been described as a phenomenon by which the sun emits energy in the form of a wave at the speed of light. This electromagnetic radiation emitted by the sun is often called solar radiation or just sunlight. Incidentally, solar radiation is the most abundant renewable energy resource in nature that can alternatively replace fossil fuels. The sun releases radiation at a rate of 3.72 × 10²⁰ MW, and the earth’s surface receives only about 40% of this energy (Lumen 2021; Iqbal 1983). The energy received from the sun is called incoming solar radiation or insolation. A variety of technologies have been developed and deployed to capture and convert solar radiation into vital forms of energy, such as heat and electricity. Solar PV technology is used to convert the received sunlight or radiation into useable electricity. Light photons release energy to the electrons once the PV cells in the solar panels absorb light and this drives the electrons to flow through the material as a current (Wikipedia 2018). The PV system consists mainly of a solar PV panel, charge controller, inverters, battery, and distribution board, as shown in Fig. 2.

Main components of a stand-alone solar PV system

The PV system is either off-grid (with battery) or grid-connected system (without battery). The solar PV panel receives the sun’s irradiance and produces a direct current (DC) power from it. The DC power generated, which is the product of voltage and current, is linked to the inverter that converts the DC voltages to alternating current (AC) voltages. The meter coupled with the fuse box, helps the grid to receive the AC output and to inject the electricity safely and efficiently into the grid. The meter could be twin meters or a single bidirectional meter, used to record the net amount of electricity taken from or given to the grid. Generally, the grid-connected PV system is made up of solar PV panel arrays, a solar inverter, electrical panel, array mounting racks, cabling, meters, combiner box, surge protection, disconnects (array DC disconnect, inverter DC disconnect, inverter AC disconnect, exterior AC disconnect), and grounding equipment and other electrical accessories. The off-grid or stand-alone solar PV systems are usually not connected to a grid, but to a bank of batteries that store the power generated by the solar modules, and are then linked to the electrical loads.

Classification of solar radiation

Solar radiation is absorbed and/or deflected by the extra-terrestrial bodies along its path or diffused through them. The amount of radiation absorbed per unit of a horizontal area is called insolation, and the extent of absorption depends on the solar zenith angle and distance of the earth from the sun.

Three components of radiation are formed as it gets to the earth’s surface, as shown in Fig. 3

• Direct radiation The unobstructed direct radiation from the sun that is intercepted by the earth

• Diffuse radiation Indirect radiation sent to the earth’s surface from all directions. The radiation is dispersed and reflected on the earth’s surface by solid bodies, and atmospheric constituents, such as molecules, dust particles, and clouds.

• Reflected radiation A radiation reflected off its path as it hits on surface features along its line of travel.

Components of total/global radiation (direct, diffused, and reflected radiations)

Direct radiation is the largest among these components, followed by diffuse radiation while reflected radiation is the least. However, reflected radiation of locations surrounding highly reflective surfaces, such as snow and glass, may be high. The addition of these three forms of radiation is called total or global solar radiation (Cotfas, 2021; ESRI-ArcGIS 2021).