Supporters of renewable energy argue that wind- and solar-generated electricity can form the basis of a secure, affordable, low carbon energy supply for the UK and EU, despite the inherently variable and intermittent nature of these sources.

• This paper first contributes to the debate by estimating the output of a model UK fleet of solar farms rated at 8.4GW, by using ten years of half-hourly aviation weather reports as a data source.

• The key findings for such a solar fleet are that:
- It has a capacity factor of just 9% when the panels are new, and so generates less than a tenth of its nominal output over the course of a year.
- It produces hardly any power in winter when demand is highest.
- Power output is severely intermittent, lying below 10% of installed capacity for 5,790 hours a year and exceeding 60% for only 7.

• The claim that a mixture of solar and wind generation can smooth out this intermittency is found untrue. Under this compounded system, power output falls below 10% of installed capacity 97 times a year for periods of between 6 and 141 hours.

• This study evaluates three other proposed solutions to the intermittency problem, and comes to the following conclusions:
- Pumped storage is currently the best and most cost-effective solution for large-scale energy storage, but would be enormously expensive on such a huge scale and have a severe environmental impact, even in the unlikely event that enough UK sites could be identified.
- Battery storage on this scale is likely to be even more expensive and batteries would require frequent replacement.
- Interconnectors to a northern European renewable-energy grid would be ineffective, because both solar and wind resources vary with time across the region in much the same way as for the UK.

• With the energy storage technology available for the foreseeable future, no combination of wind and solar energy with backup storage would be suitable to supply a significant proportion of grid electricity without full conventional backup being available.

• However, intermittent renewable energy could have a useful role to play if it was used primarily for domestic space and water heating. An option which has been successfully implemented in New Zealand in the past.

Read the full paper here.