New paper: wind and solar generation scenarios

We conducted a study to understand wind and solar variability by the time of day and time of year. The study indicates the behaviour of solar and wind generation throughout New Zealand based on the weather data.

On renewable behaviour and firming

The recent growth in wind and other renewable energy sources will likely continue. These technologies have many positive characteristics but also present challenges due to their intermittent nature. Such variation requires other generation sources to be available in case of a sudden drop in the renewable generation - a process known as firming. By understanding the behaviour of the wind and solar sources, we can understand the scale of the firming requirement. Sufficient firming is critical for transitioning to a low-emission electricity system to ensure reliability.

Wind and solar showed some similarities – but also important differences

The study consists of two scenarios, the first scenario models wind and solar sites for the entire country, using data from local weather stations. The study simulates a wind turbine or solar photovoltaic array as if they existed alongside those weather stations. The second scenario selects the top-performing sites from the original set.

If wind turbines existed in all weather station locations (scenario one), we estimate that wind generation would be below 10 percent of total capacity at around 5 percent of the time. In other words, at about 5 percent of the time, firming would be required for 90 percent of the installed wind capacity (assuming that demand was sufficient to support this generation). Additionally, only around 9 percent of the modelled wind generation was above 50 percent of total capacity.

For solar power, if solar photovoltaic arrays existed at all sites (scenario one), we estimate that generation would be below 10 percent of capacity around 60 percent of the time and above 50 percent of total capacity about 14 percent of the time (considering all 24 hours of the day).

Both wind and solar tended to generate more power during the daytime and warm seasons. Wind generation peaked later in the day (around 4pm), whilst solar tended to peak earlier (around 2pm). We found spring to be the windiest season and summer – as expected – the peak season for solar generation. The results suggest that greater firming would be required in winter when demand is highest and (both) wind and solar generation are lowest. Also, since most renewable generation occurs off-peak, it is less likely to contribute to peak demand, increasing the requirement for firming.

Wind generation showed the most dramatic changes in scenario two (selected sites) compared to scenario one (whole country), with a significant increase in the average capacity factors.

Final disclaimer

We must emphasize that this study is an indication of the behaviour of solar and wind generation throughout New Zealand based on weather data available to the Electricity Authority - a purely theoretical exercise not to be confused with a feasibility study.