For this insight analysis, a statistically average day of hourly production for renewables in South Africa across all years (April 2012 – July 2017) and provinces was compiled. Furthermore, seasonal variations in hourly production were analysed.

Average day of hourly production

The figure above illustrates that wind power output is relatively stable throughout an average day, observing a peak around 17:00-19:00 (5pm-7pm). The wind production peak correlates fairly well with the demand peak, which appears around 19:00 (7pm) in winter and 20:00 (8pm) in summer according to data from Eskom [Eskom, 2014].

In order to understand this phenomenon, it is necessary to understand that:

1. The performance of PV depends on the angle of incidence – the lower the angle, the higher the performance, and

2. That majority of PV installations with single-axis tracking in South Africa are so-called horizontal single-axis systems, which tracks the sun from east to west. An illustration of PV tracking systems is included in the notes below.

A horizontal single-axis system tracking from east to west is gradually tilting the panel to minimize the angle of incidence. However, at mid-day, the horizontal single-axis PV panel is horizontal to the ground. This results in a higher angle of incidence, and thus lower production, than that of a PV panel without tracking, which is fixed tilted north towards the sun.

A tilted single-axis tracking system would be able to match the fixed tilted PV without tracking at mid-day. However, tilted single-axis tracking systems are more expensive than horizontal single-axis tracking systems, mainly as they cast more shadow and therefore require more land-use.

Seasonal variations

Significant seasonal variations are observed. For example, while wind power output is virtually constant on an average winter day, the afternoon peak is double than that of the morning output on an average summer day.

In the figure above, the statistical representation of hourly production has been normalized to the installed capacity in each hour, where the value 1,0 (one) represents maximum output at full capacity.

The term load factor here describes the actual production as a fraction of the theoretical production at full capacity. For example, if a 1 MW-peak PV installation in a given hour produces 0,75 MWh, the load factor in that hour is 0,75. Subsequently, load factors can be averaged for days, months, and years. Please refer to REDIS Update #3 for an analysis of annual load factors.

References and Further Readings

B. Sørensen, Renewable Energy: Its physics, engineering, use, environmental impacts, economy and planning aspects, 2004.

Eskom, State of the power system quarterly update, September 2014.

G. M. Masters, Renewable and Efficient Electric Power Systems, 2004.

We would like to extend our thanks and appreciation to Jarrad Wright and his colleagues from CSIR - Energy Centre for their assistance in this REDIS Insight.