The International Energy Agency (IEA), from a global point of view, described solar heating and cooling technologies as “the sleeping giant of renewable energy potential” [1
]. The IEA’s roadmap projects that by 2050, the installed solar water heaters’ (SWH) capacity could reach about 3200 GWth
with 7.2 EJ produced annually [2
]. Solar energy is utilised to generate electricity, hot water and space heating. A brief policy report for South Africa on solar water heating technologies affirms three ways of converting harvested solar energy: electricity, hot water and space heating [3
]. Furthermore, the report shows that solar water heaters and solar air collectors convert incident irradiance hot water and space heating, respectively, while photovoltaic modules convert solar irradiance to electricity.
The development of a technology for hot water generation ensures the sun’s irradiance is adequately utilised [4
]. Hirbodi et al. [5
] simulated and analysed the techno-economic performance of two solar heating technologies. In their work, two concentrating solar power collectors’ (CSP) efficiency reduced significantly under low irradiance. The authors also showed that utilising solar technologies reduces greenhouse gas emissions considerably while saving about 193 × 106
of natural gas in south-central Iran. Lim et al. [6
] evaluated the performance of passive concentrator and reflective systems in Malaysia. Their work showed that partial shading impacts the performance of the systems as extremely cloudy conditions characterise the study area. Hence, solar thermal equipment performs better in summer than in winter and is attributed to solar irradiation [7
A study in Ethiopia by Endale [8
] showed that implementing SWH could reduce wide-scale deforestation for heating water. Furthermore, the study revealed that about 1480 GWh of electric energy could be saved by using SWH. Roberts and Forbes [9
] reported that the majority of the solar heating industries are for water heating for domestic use. Their work showed that the prevalent type of collector used is the flat plate. However, environmental conditions and seasons limit the hot water production of flat plate and evacuated tube collectors, as water may freeze on frigid days for passive systems [10
]. Furthermore, the collector’s pipe diameter, absorber, orientation, size and storage tank’s capacity influences the performance of solar water heating systems [11
A review of the performance of various absorber designs for a fluid-based solar collector was conducted by Abdullah et al. [12
]. Their study showed that the spiral flow absorber performed best. The overall energy produced by the system increased by 3.5%. Investigations by Duffie et al. [13
] revealed that the ideal pipe diameter to minimise heat losses is 19–25 mm. Their findings show that the pipe diameters used in their study did not impact the hot water generated. However, the flow condition was faster for the 25 mm pipe and slower for the 19 mm pipe. Smaller tube diameters (4–25 mm) offer several advantages, as seen in Tanase et al. [14
]. However, the collector’s performance may be significantly reduced if the pipes corrode [10
Salgado-Conrado and Lopez-Montelongo [15
] reported that 65% of the electricity used in the domestic sector is for water heating. As illustrated in Figure 1
, the domestic sector in South Africa accounts for 27% of electricity demand, with 40–60% of the electricity consumed in the domestic sector attributed to water heating [16
]. The agricultural sector has the least energy demand of 2% while the industrial sector is the highest with 36%.
Part of the efforts made by the South African government in encouraging the use of solar water heating systems is the massive rollout of SHW in the low-cost housing sector in 2008. The programme was halted due to the enormous failure of the installed SWH systems [17
]. Thobejane et al. [17
] noted that the collapse of the installed SWH is majorly a social problem as most installers had inadequate training and expertise. The authors recommended that the government provide quality training and ensures that only accredited installers handle solar water heater installation. Netshiozwi [18
] revealed that the failures resulted from inadequate community sensitisation, faulty installation and poor craftsmanship. Hence, it has impacted the ability of the national power utility to reach its load reduction target. The author recommended that responsible agencies should intensify the efforts towards bridging the educational divide in the country as the failure of SWH is a social problem. About thirteen years later, the programme, with an estimated 50–70% savings on energy consumption, remains in the shadows, unlike other renewable energy technologies.
In recent years, consumer energy demand in South Africa has put an enormous burden on the already struggling coal-fired national utility. Hence, there is a need to provide alternative means of providing hot water on demand. South Africa, at the moment, has the most significant carbon footprint in Africa [19
]. The South African government has made much effort to encourage renewable and sustainable energy technologies to combat climate change and reduce its carbon emission as a party of the Kyoto Protocol [20
]. The report by the International Energy Agency [21
] shows the commitment of the South African government in the renewable sector with an investment of USD 140 million (ZAR 2 billion) in credit facilities. Jain and Jain [22
] showed that South Africa has enormous potential for using renewable energy technologies. It boasts of receiving annually about 2500 sunshine hours, with daily average irradiation levels in the range of 4.5–6.5 kWh/m2
. Despite these potentials, the use of electricity for domestic water heating is still prevalent in the country [23
]. The situation is further compounded by the inability of the national utility, Eskom, to service its existing customers. Hence, South Africa is plagued with incessant load shedding, as reported by Apeh et al. [24
Solar water heaters’ performance is usually monitored through parameters such as irradiance, inlet and outlet water temperatures, ambient temperature, wind speed and direction. An experiment carried out by Lizama-Tzec et al. [25
] on the electrodeposition of selected coatings on three flat plate collectors revealed that the system’s thermal efficiency was determined by measuring the incident irradiance and the water temperature of the collector. In agreement with the parameters measured by the authors Lizama-Tzec et al. [25
], Budea and Bǎdescu [26
] worked on improving the performance of solar collectors for producing hot water. This was done through the measurement of global irradiance and water temperature. Furthermore, Roberts [27
] showed that irradiance, inlet water temperature and ambient temperature are vital parameters for determining the merit for absorbers used in solar water heating systems.
The massive failures of solar water heating systems installed in the low-cost housing sector in South Africa have left many wondering about the efficacy of these systems as a result of these failures and the unavailability of a detailed report about the causes of these failures. The foregoing pose a significant barrier to the utilisation of SWH in South Africa. Hence, the study seeks to unravel the durability of solar water heating systems through a comprehensive analysis of the performance of flat plate and evacuated tube solar heating systems and their usage profiles. The installed systems will provide hot water in the low-cost housing sector in South Africa, thereby reducing the country’s carbon footprint and reducing demand on the ailing national power plant.
This study presented the relevance of evaluating the performance of flat plate and evacuated tube solar water heating systems and their usage profiles. The results over five days, characterised by varying sky conditions, reveal that the available insolation greatly influenced FP and ET systems’ performance. A typical clear day with a maximum irradiance of 1050 W/m2 produced 62.77 °C and 69.63 °C hot water for the FP and ET systems, respectively. However, a cloudy day with a maximum of 400 W/m2 irradiance corresponds to 24.84 °C and 28.32 °C of hot water produced for the FP and ET, respectively. Further results reveal that the FP and ET systems’ efficiencies on a clear day were 73% and 84%, respectively. However, FP had an efficiency of 66% on a cloudy day, while the ET’s efficiency was 75%. Furthermore, the hot water usage profile conducted on both systems shows that their performance on a clear and cloudy day can adequately provide hot water for the domestic sector.
Sensitivity analyses on the hot water production by both of the systems show that the FP is more sensitive to irradiance while the ET is more sensitive to ambient temperature. Additionally, a cost and payback period carried out revealed that the payback periods for the FP and ET are 3 and 3.8 years, respectively. The research demonstrates that solar water heating systems are viable and that the failures encountered in the mass installation were due to inexperienced installers. Hence, the widespread adoption of these technologies in South Africa will ensure a greener future as well as reduce the demand on the strained national utility.