Helping National Grid manage the sudden growth in solar power

By Daniel Drew

In Britain it had been a year without summer. Wet spring had merged imperceptibly into bleak autumn. For months the sky had remained a depthless grey. Sometimes it rained, but mostly it was just dull, a land without shadows. It was like living inside Tupperware”.

Bill Bryson, The Lost Continent: Travels in small town America

Given the reputation of the Great British weather, it is perhaps surprising that the UK now has more solar panels than France, Spain and Australia. Some of these installations generate hot water (solar thermal) but the vast majority harness the photoelectric effect to generate electricity, known as solar photovoltaic cells, or solar PV (potentially confusing initials for a meteorologist). Since January 2014 the installed capacity of solar PV has increased dramatically from 2.8 GW to 10.7 GW (as of July 2016), nearly all of which benefit from feed-in tariffs which guarantee a set income for each kWh of solar generation.

While the increased proportion of renewable generation is beneficial to reducing the carbon intensity of UK electricity, it does present a challenge to National Grid, the system operator responsible for ensuring supply equals demand throughout the day. Solar PV generation is highly variable over a range of temporal scales. Clearly there is a well understood seasonal and diurnal pattern, but also higher frequency variability due to clouds. National Grid is however used to dealing with variable generation, over the last 15 years the capacity of wind power in the UK has steadily increased to 14.5 GW (as of January 2016). During this time, National Grid has been working on research projects (including several with the University of Reading) to develop a detailed understanding of the variability and predictability of UK wind power.

Solar generation presents a new challenge. Whereas wind capacity is located in a relatively small number of very large wind farms, solar PV capacity is distributed across a large number of small installations- typically on the rooftops of buildings. The installations are generally very small, therefore the owners are under no obligation to provide National Grid with electricity generation data or even inform them of the existence of the panels. Solar generation is therefore ‘seen’ by National Grid simply as a reduction in the electricity demand. Proportionally this reduction can be quite large, particularly on a clear weekend day in summer when the electricity demand is typically only 25 to 30 GW.

To maintain the previously accurate predictions of electricity demand, an understanding of the variability and predictability of solar generation is required. Our project uses state-of-the-art meteorological datasets to address questions such as; how changeable is UK solar power? How extreme can solar power swings get? Are they correlated with swings in wind power? How well are extreme events forecast?

Find out more about our project at www.met.reading.ac.uk/~energymet/

2016 09 22 Daniel Drew - Fig 1 demand_schematic

Illustration of the reduction in GB electricity demand as observed by National Grid due to solar PV generation on a typical summer weekday.

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