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Solar Energy

Author: Centre for Renewable and Sustainable Energy Studies - Stellenbosch University

( Article Type: Sustainable Development )


Solar energy is radiation emitted by the sun. The light takes around 8 min 20 seconds to reach the earth’s surface.


How Solar Energy Works

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 mainly reduced by 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, 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
  • For solar thermal, radiation is converted into heat. 

When a PV panel is exposed to light (not necessarily sunlight), the energy in the incident photons is converted to electrical energy by means of the photo-electric effect.

A standard PV panel comprises 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 of PV is storage.


Solar Technology

PV is the fastest growing power generation technology in the world.

PV in South Africa is represented by South African Photovoltaic Industrial Association (SAPVIA).

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.

The Southern African Solar Thermal Training and Demonstration Initiative (SOLTRAIN) see is currently facilitating knowledge transfer and promoting the adoption of this technology within the country.

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.


Storing Solar Energy

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.


Did you know?

CSP in South Africa is represented by the Southern African Solar Thermal and Electricity Association (SASTELA).