Solar and the longest day
We’re approaching the longest day in the Northern hemisphere. But will it be the best day for solar? The answer may not be as you’d expect. Jan Muller, Director of Asset Management, explains.
Solar really likes a combination of low temperatures and high levels of irradiance. It still works in cloudy conditions, but performs best on a clear, bright day; the cooler the better, perhaps contrary to your expectations.
The perfect environment for solar is up in space – which is very cold at minus 270 degrees Celsius with no clouds.
Does that mean colder countries get better results than warmer climes? It’s certainly the case that systems built in southern Spain experience higher losses due to temperature than those in the UK when measured on a percentage basis. However, there is a reason that Spain and regions closer to the Equator are warmer – the sun’s light doesn’t travel as far to get to Spain as it does to get to the UK, meaning there is more energy in the light when it arrives; so as well as being warmer, there’s higher irradiance or light energy on arrival.
The increased temperature losses in Spain are compensated for by this higher light energy level (irradiance) and an increase in the number of clear daylight hours across the year. At midsummer, a UK solar project might be producing close to 75% of a project in southern Spain, but in January, it’s more like 25% – and that’s because the project in Spain, due to its latitude, gets higher irradiance levels, slightly more hours of daylight and the weather is somewhat better than the UK! (An interesting fact is that the annual total hours of sunlight are the same for every point on earth provided that the hours between sunrise and sunset are counted regardless of cloud cover.)
The way we design the solar projects also affects how well they perform throughout the year. And that’s a judgement call (supported by some pretty sophisticated modelling). If you space out a ground mount solar project so that each row of panels is a good distance from the next, the panels will shade each other less during those crisp, high-yielding days in April when the sun is low – but that could cost more in terms of the overall project as you are building a lower power system in the space provided and that may be inefficient.
Another major design consideration is inverter matching. If you put in enough inverters to capture every unit of electricity on the highest-yielding days of the year, your project is over designed for the winter months and therefore commercially inefficient at that time. It is usually more cost-effective to lose a bit of production and undersize the inverter power compared to module power by about 20-25%.
Solar is a predictable resource but with climate change the trend is towards higher temperatures during summer. It varies from year to year, but not as much as you might think – with an annual range of approximately +/-7% looking at long-term historic weather data. Solar projects are designed for the long term and over a 30-year period, the variation is smoothed out. This gives us confidence in our modelling particularly as our models are supported by the massive amount of data we collect on existing projects. (In true Matrix style, our SCADA team can sit in an office and analyse thousands of datapoints and pick out potential improvements such as which modules in which row could do with a clean.)
So what has been the best yielding day for Solarcentury systems in the UK so far this year?
The 25th May; the bank holiday.
Following a mini heatwave in the previous weeks, we experienced a cloudy weekend just prior to the bank holiday. This allowed the systems to cool down completely. By the 25th it was a sunny again with temperatures in the low 20s.
Perfect for us humans as well as the technology!