Solar energy is radiation emitted by the sun; the light takes around 8 min 20 seconds to reach the earth’s surface. Our sun is the origin of almost all renewable energy forms (excluding geo-thermal). Coal, which is exploited as fuel source for over 90% of electricity generated within South Africa, is a form of ’stored’ solar energy.
Stellenbosch University’s Department of Mechanical and Mechatronic Engineering (SU-M&M) is establishing itself in the field of solar energy. This is evident with the launch of the solar thermal energy research group STERG - the first South African academic group of its kind. The Centre for Renewable and Sustainable Energy Studies (CRSES) continues to assist various institutions in the mapping of the Southern African solar resource.
The intensity of the sun’s rays reaching the outer surface of the Earth’s atmosphere is approximately 1 360 W/m2. This is known as the solar constant and fluctuates with the Earth’s varying distance from the sun. The intensity of solar energy at the earth’s surface is reduced by, amongst others, absorption, scattering and reflection.
Sunlight illuminating a material can either be reflected, absorbed, pass through the material (transmitted) or a combination thereof. For the application of solar energy, ultimately, the mechanism of interest is absorption. The harnessing of solar energy can be divided into two groups:
- solar electric, for instance photovoltaic (PV)
- solar thermal, for instance domestic solar water heater (SWH)
Solar radiation is converted into a ’useful’ form by means of technologies; for solar electric, directly to direct current (DC) electricity and for solar thermal, radiation is converted into heat. These technologies have received much attention in South Africa with the announcement of the Renewable Energy Feed-In Tariff (REFIT) in 2009.
When a PV panel is exposed to light (not necessarily sunlight), the energy in the incident photons are converted to electrical energy by means of the photo-electric effect. A standard PV panel comprises of modules, which are made up of cells. These cells are made from semi-conductor materials and are passive devices with no moving parts. Furthermore, PV technology is modular. Therefore, this technology is receiving extensive attention/research in the Renewable Energy field. A limitation with PV is storage.
PV is the fastest growing power generation technology in the world with currently 35 GW of global installed capacity (approximately ¾ of South Africa’s generation capacity). PV in South Africa is represented by SAPVIA (South African Photovoltaic Industrial Association). Solar thermal technologies range from low temperature pool heating systems to high temperature central receivers. For low temperature applications (below around 100°C) concentration of sunlight is not required, as with SWH systems.
For SWH systems there are two main types of collectors, namely flat plate and evacuated tubes collectors. Southern African Solar Thermal Training and Demonstration Initiative SOLTRAIN is currently facilitating knowledge transfer and promoting the adoption of this technology within the country. This project funded the new SWH testing facilities at SU-M&M. South Africa is the newest member of the International Energy Agency’s Solar Heating and Cooling Programme (EIA-SHC). This programme is an international collobroative effort to accelerate relevant technology development. There is considerable potential for industrial solar heating and cooling within South Africa.
To obtain higher operating temperatures, concentration of solar radiation is employed. This is achieved with reflectors, typically mirrors. There are four main types of commercial concentrated solar thermal power (CSP) technologies, namely parabolic trough (PT), liner Fresnel (LF), central receiver (CR) and parabolic dish. The former two are line focus technologies and the latter two are point focus.
Storage is a concern for solar electricity generation, especially in the South African context with the country’s 18:00 to 21:00 electricity demand peak. This peak occurs on the fringes of sunlight hours. Fortunately, heat can be efficiently stored with CSP plants and dispatched when required. Additionally, the power plant’s power block (heat engine and generator) is better utilized, thereby reducing the cost of electricity. Uninterrupted power generation for a day was achieved in June 2011 by Torresol Energy’s central receiver power plant in Seville, Spain, namely Gemasolar. Molten salt thermal storage is utilized.
There are 34 operational CSP plants in the world – a total capacity of around 1GW (approximately 2% of South Africa’s generation capacity). A further 15 GW capacity is under construction. CSP in South Africa is represented by SASTELA (Southern African Solar Thermal and Electricity Assoication).