Riaan Meyer, managing director of GeoSUN Africa – a Stellenbosch University spin-off company, is an expert in solar resource assessment and has been working in this field since 2006. In this industry insight, Meyer briefly discusses the key factors when identifying an optimal site for the development of a solar photovoltaic (PV) plant.
If you search Google for a solar map of South Africa and spend some time surveying the internet, you will find at least eight different solar maps for South Africa.
They all have the same colour spread. Red and orange highlight the Northern Cape, green and blue highlight the coastal areas and yellow for the remainder.
But which map should you use? And how accurate are they?
This is the dilemma faced by many solar developers. Firstly, you should look at the fine print. Is it a GHI (Global Horizontal Irradiance) or a DNI (Direct Normal Irradiance) map? Both maps are solar maps, but for different solar components.
A GHI map, pictured above, is the most common solar map available for any particular country or region. It depicts the long term annual average of solar irradiation incident on a horizontal surface. This map should be used by solar photovoltaic (PV) developers and those who wish to install a solar water heater.
DNI maps, pictured below, are less common and apply to concentrated solar technologies. It represents the annual direct, or beam, component of the sun on a plane normal to the rays of the sun.
DNI maps are used by Concentrated Solar (Thermal) Power (CSP) or Concentrating Photovoltaic (CPV) developers. Direct irradiation is the component of the sun that will cast a shadow but also the component that can be reflected with a mirror or be concentrated by a lens.
On an overcast day, the DNI is typically zero, but the GHI could be about 30% of its maximum values. The result is that in overcast conditions a CPV plant will shut down, while a PV plant will still produce around 30% of its rated capacity.
Secondly, the number of years on which a solar map is based, is of interest. Solar maps represent long term annual averages. As annual rainfall numbers vary from year to year, so does the annual solar irradiation. A good solar map takes 10 or more years of data into account.
It is important to know what data was used to develop the solar map. Most modern day solar maps are based on satellite derived data. Good solar maps also incorporate accurate ground measured solar data. Ground data is used to validate the satellite models at various locations.
GHI and DNI solar map created for RSA
Recently, an accuracy enhanced GHI and DNI solar map for South Africa has been created and released into the public domain.
The mapping project was done by the University of Stellenbosch with funding from the German Development Cooperation (GIZ).
The map was based on ground measurements that was run by GeoSUN Africa and SolarGIS satellite derived data, which was modelled by GeoModel Solar. The data, of 14 ground measurement stations, was obtained from various locations, of which the majority can be obtained from www.sauran.net.
To view the above maps in a high resolution digital format, as well as the Google Earth and GIS layers, visit: http://geosun.co.za/solar-maps/
Meyer consults developers and operators of solar power plants, investors, financial and governmental institutions. Meyer remains in close collaboration with the University of Stellenbosch and is involved in teaching post graduate modules in renewable and specifically solar energy.