I am sure it protects us not only from lightning but also from alien attacks and EMP guns …
So I wrote about our lightning protection, installed together with our photovoltaic generator. Now our PV generator is operational for 11 months and we have encountered one alien attack, albeit by beneficial aliens.
The Sunny Baseline
This is the electrical output power of our generator – oriented partly south-east, partly south-west – for some selected nearly perfectly cloudless days last year. Even in darkest winter you could fit the 2kW peak that a water cooker or heat pump needs under the curve at noon. We can heat hot water once a day on a really sunny day but not provide enough energy for room heating (monthly statistics here).
Alien Spikes and an Extended Alien Attack
I was intrigued by very high and narrow spikes of output power immediately after clouds had passed by:
There are two possible explanations: 1) Increase in solar cell efficiency as the panels cool off while shadowed or 2) ‘focusing’ (refraction) of radiation by the edges of nearby clouds.
Such 4kW peaks lasting only a few seconds wide are not uncommon, but typically they do not show up in our standard logging, comprising 5-minute averages.
There was one notable exception this February: Power surged to more than 4kW which is significantly higher than the output on other sunny days in February. Actually, it was higher than the output on the best ever sunny day last May 11 and as high as the peaks on summer solstice (Aliens are green, of course):
Temperature effect and/or ‘focusing’?
On the alien attack day it was cloudy and warmer in the night than on the sunny reference day, February 6. At about 11:30 the sun was breaking through the clouds, hitting rather cool panels:
At that day, the sun was lingering right at the edge of clouds for some time, and global radiation was likely to be higher than expected due to the focusing effect.
The jump in global radiation at 11:30 is clearly visible in our measurements of radiation. But in addition panels had been heated up before by the peak in solar radiation and air temperature had risen, too. So the different effects cannot be disentangled easily .
Power drops by 0,44% of the rated power per decrease in °C of panel temperature. Our generator has 4,77kW, so power decreases by 21W/°C panel temperature.
At 11:30 power was by 1,3kW higher than power on the normal reference day – the theoretical equivalent of a panel temperature decrease by 62°C. I think I can safely attribute the initial surge in output power to the unusual peak in global radiation only.
At 12:30 output power is lower by 300W on the normal sunny day compared to the alien day. This can partly be attributed to the lower input radiation, and partly to a higher ambient temperature.
But only if input radiation is changing slowly, panel temperature has a simple, linear relationship with input temperature. The sun might be blocked for a very short period – shorter than our standard logging interval of 90s for radiation – and the surface of panels cools off intermittently. It is an interesting optimization problem: By just the right combination of blocking period and sunny period overall output could be maximized.
Re-visiting data from last hot August to add more dubious numbers
Panels’ performance was lower for higher ambient air temperatures …
… while global radiation over time was about the same. Actually the enveloping curve was the same, and there were even negative spikes at noon despite the better PV performance:
The difference in peak power was about 750W. The panel temperature difference to account for that would have to be about 36°. This is three times the measured difference in ambient temperature of 39°C – 27°C = 12°C. Is this plausible?
PV planners use a worst-case panel temperature of 75°C – for worst-case hot days like August 12, 2015.
Normal Operating Cell Temperature of panels is about 46°C. Normal conditions are: 20°C of ambient air, 800W/m2 solar radiation, and free-standing panels. One panel has an area of about 1,61m2; our generator with 18 panels has 29m2, so 800W/m2 translates to 23kW. Since the efficiency of solar panels is about 16%, 23kW of input generates about 3,7kW output power – about the average of the peak values of the two days in August. Our panels are attached to the roof and not free-standing – which is expected to result in a temperature increase of 10°C.
So we had been close to normal conditions at noon radiation-wise, and if we had been able to crank ambient temperature down to 20°C in August, panel temperature had been about 46°C + 10°C = 56°C.
I am boldly interpolating now, in order to estimate panel temperature on the ‘colder’ day in August:
|Air Temperature||Panel Temperature||Comment|
|20°C||56°C||Normal operating conditions, plus typical temperature increase for well-vented rooftop panels.|
|27°C||63°C||August 1. Measured ambient temperature, solar cell temperature interpolated.|
|39°C||75°C||August 12. Measured ambient temperature.
Panel temperature is an estimate for the worst case.
Under perfectly stable conditions panel temperature would have differed by 12°C, resulting in a difference of only ~ 250W (12°C * 21W/°C).
Even considering higher panel temperatures at the hotter day or a non-linear relationship between air temperature and panel temperature will not easily give you the 35° of temperature difference required to explain the observed difference of 750W.
I think we see aliens at work again:
At about 10:45 global radiation for the cooler day, August 1, starts to fluctuate – most likely even more wildly than we see with the 90s interval. Before 10:45, the difference in output power for the two days is actually more like 200-300W – so in line with my haphazard estimate for steady-state conditions.
Then at noon the ‘focusing’ effect could have kicked in, and panel surface temperature might haved fluctuated between 27°C air temperature minimum and the estimated 63°C. Both of these effects could result in the required additional increase of a few 100W.
Since ‘focusing’ is actually refraction by particles in the thinned out edges of clouds, I wonder if the effect could also be caused by barely visible variations of the density of mist in the sky as I remember the hot period in August 2015 as sweltry and a bit hazy, rather than partly cloudy.
I think it is likely that both beneficial effects – temperature and ‘focusing’ – will always be observed in unison. On February 11 I had the chance to see the effect of focusing only (or traces of an alien spaceship that just exited a worm-hole) for about half an hour.
On temperature dependence of PV output power:
- Current-voltage curve of a solar cell – containing a temperature-dependent exponential function.
- The effect of temperature in depth.
On the ‘focusing’ effect:
- Can You Get More than 100% Solar Energy?
Note especially this comment – describing refraction, and pointing out that refraction of light can ‘focus’ light that would otherwise have been scattered back into space. This commentator also proposes different mechanism for short spikes in power and increase of power during extended periods (such as I observed on February 11).
- Edge-of-Cloud Effect
Source for the 10°C higher temperature of rooftop panels versus free-standing ones: German link, p.3: Ambient air + 20°C versus air + 30°C
10 Comments Add yours
I was thinking at first that it may have been a triggered release of stored energy–a discharging “ersatz” capacitor of some sort but I figure your explanation makes much mire sense.
I am the first to believe that some electronic component behaves in an odd way, but in this case I think I can prove it does not :-) I have plotted only the AC output power of the whole generator here (after the inverter) but the DC input powers from both strings of panels on each roof show the same behaviour.
I am in the process of catching up now–just re-read the two previous posts as I need continuity. I had another thought as I read this: I wonder to the manufacturers of the panels know about teh cool data and analysis you do? It would not surprise me that you’re ahead of them in many regards since your “lab” is the real world.
Thanks, Maurice – wow, my WordPress statistics is dominated by the Canadian flag ;-) ! The PV inverter has local logging but is also connected to the vendor’s cloud. So these guys have (some of) my data :-)
This reminds me of walking through pockets of air which are startlingly different temperatures than other places in the surrounding area. Something like a small cooling current in a very narrow stream. I have to read again and comment in a mode that doesn’t require the phone keypad.
There can be weird instabilities in fluids / gases where adjacent ‘cells’ develop that don’t mix – at least not for some time. But I think although convection is important in describing the steady-state temperature of solar panels correctly it cannot account for such very short peaks in output power as the transport of air is always slower than changes due to changes in irradiance.
Localised air convection due to drop in temp. You may have enough panels for this to have an effect. Just a thought.
Convection is for sure important in cooling or heating the panels. It’s hard to make an estimate as you get into a tangle of complex formulas quickly which are based on lots of half-empirical parameters – even for simple cases, like steady-state convection near a wall.
My hunch is that it is too slow a process to make any contribution to the very fast (1 second) peaks. If temperature is important here, then I think those are rather caused by cooling off a surface layer of the solar cells as this happens instantaneously in contrast to moving parcels of air.
The extended (~30-60 minutes) ‘alien’ change in power was about 1,3 kW high, compared to the normal sunny baseline. If panel temperature (caused by whatever mechanism of heat transfer) would have been accountable for that, temperature suddenly would have to decrease by about 63°C, starting from a stable panel temperature about 20-30°C higher than ambient air temperature before. In winter this would have meant cooling below zero – which is very unlikely as water on the panels would freeze before they would cool down further.
As the weird phenomenon happened in mid-winter I had thought about a mechanism involving hoarfrost: Ambient temperature was 1,5°C and given the precision of sensors and small variations of temperature with position there might have been a thin layer of ice on the panels. Then the sun suddenly started melting the ice and there might have been a short period of time where 0°C cold solar cells were hit by the full power beam as the layer of ice was already translucent. However, it’s hard to explain how that would have lasted for half an hour.
I prefer to blame gremlins, they’ve been around a bit longer