Renewable energy sources present particular challenges for the electricity grid. For example, the performance of hydro plants is diminished in dry seasons; good wind conditions don’t generally correlate to when electricity demand is at its highest; and, of course, solar power generating technologies depend on clear sunshine.
While weather prediction technologies are improving, they are still nowhere near 100% accurate. Of the two major intermittent renewables, solar and wind, solar PV more closely matches peak demand use. However, it trails off towards the middle of the afternoon and cannot meet the second daily peak in demand after 6 pm when people arrive home from work. Furthermore, cloud cover causes rapid variability in output, which needs to be covered from other generation sources.
For wind generation, winds tend to blow more strongly at night than during peak hours in the day. In addition, stronger winds usually occur in the winter time in the northern hemisphere, not in the summer months when demand is usually higher due to the use of air conditioning units. As with solar, sudden weather change, such as wind drops or gusts of wind, can change electricity generation quite rapidly.
Weather forecasting technology can help forecast solar insolation or wind speeds, but they have reported inaccuracies of around 10%. If the solar insolation or wind speeds are higher than predicted, more load following and peak capacity will be online than needed, leading to higher start-up and fuel costs than necessary. If solar insolation or wind speeds are lower than predicted, less capacity will be online and high cost ‘quick start’ capacity will be used. Other consequences could be electricity imports or demand management through paying customers to consume less electricity. Or, in the worst case, a blackout may occur.
In 2009, real-life examples include a sudden drop in wind speeds in Texas in December, which almost resulted in blackouts in western parts of the state. In the same month, strong winds in Spain resulted in excess electricity production and all of the electricity generated from wind was effectively ‘dumped’. The price of electricity generated from wind fell to minus figures.
An additional complication that needs to be considered is that some renewable energy sources may be close to load centres such as rooftop solar, small scale wind and some biomass projects and others further away (large scale wind, hydro, geothermal and large scale solar). The generation of electricity from various small energy sources at medium or low voltage and delivery into the distribution network is known as distributed generation (DG). This avoids the need for a high voltage transmission network, but makes grid balancing more challenging if intermittent, renewable energy sources are used.
For example, business and residential consumers can fit a solar panel to their rooftop and use it to generate electricity for self-consumption with the excess being fed and/or sold back to the utility. So, the electricity customer is both a consumer and producer of electricity. At the end of the month credit for electricity sold is taken off the charges for the customer for electricity purchased from the grid.