Find out how well solar PV works in bad weather
Posted by Chris Jardine on 19 March 2013 at 9:11 am
Snow in March? Last summer was dreadful, and seemed to rain constantly. Seems like we’ve had bad weather for ages now, and a period of prolonged sunshine seems eons ago. So we might expect the performance of solar PV systems to be equally poor over the same period of time. But just how bad has it been?
To look at this in more detail I’ve looked at a long-term dataset of PV output (my house as it happens!) since February 2009 and looked at the variability in annual generation. The graph below shows annual variability in output compared to the overall average. Each point is the previous year’s generation, so the value on 1/2/2010 is the year’s output between 1/2/2009-1/2/2010 and so on.
We can see the variability in solar radiation over the last 3 years. In last March (remember that? It was really hot and sunny!) the year’s output between March 2011 and March 2012 was the highest we have seen - up 6% on the average. However, after our dreadful summer last year, the period August 2011 to August 2012 was some 5% down on the average.
So we can see there is a spread of output, based on natural weather variability over the past three years. In fact, this spread is remarkably narrow; with the very best weather and very worst within plus or minus 6% from the average. Intuitively, it’s felt darker and more miserable than this over the last year, but this isn’t reflected in PV output. Which shows that what PV experts have been saying all along is true – even in cloudy weather there is still a substantial amount of solar energy that can be converted into electricity.
This actually makes solar PV output really rather constant – certainly more so than wind output. This is reassuring for householders purchasing systems – the worst case scenario is that your income is just 6% down on average. For developers of large scale projects, or if you are thinking of investing in a community-based solar scheme, a knowledge of this spread of output provides useful high/low scenarios for financial modelling.
So here’s hoping we get back to long term average weather over 2013. Or, heaven forbid, above it. Either way we can be confident about the performance of PV systems.
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About the author: Chris Jardine is technical director of Joju Solar and teaches on the MSc course in Environmental Change and Management at Oxford University.
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