I have noticed the impact of traversing clouds on solar power output: Immediately after a cloud has passed, power surges to a record value. This can be attributed to the focusing effect of the surrounding clouds and/or cooling of the panels. Comparing data for cloudless days in May and June, I noticed a degradation of power – most likely due to higher ambient temperatures in June.
We had a record-breaking summer here; so I wondered if I could prove this effect, using data taken at extremely hot days. There is no sensor on the roof to measure temperature and radiation directly at the panels, but we take data taken every 90 seconds for:
- Ambient air temperature
- Global radiation on a vertical plane, at the position of the solar thermal collector used with the heat pump system.
I was looking for the following:
- Two (nearly) cloudless days, in order to rule out the impact of shadowing at different times of the days.
- These days should not be separated by too many other days, to rule out the effect of the changing daily path of the sun.
- Ideally, air temperature should be very different on these days but global radiation should be the the same.
I found such days: August 1 and August 12 2015:
August 12 was a record-breaking day with a maximum temperature of 39,5°C. August 1 was one of the ‘cool’ but still perfectly sunny days in August. The ‘cold day’ resulted in a much higher PV output, despite similar input in terms of radiation. For cross-checking I have also included August 30: Still fairly hot, but showing a rather high PV output, at a slightly higher input energy.
August 2015 in detail:
Overlaying the detailed curves for temperature and power output over time for the three interesting days:
The three curves are stacked ‘in reverse order’:
The higher the ambient air temperature, the lower the output power.
Note that the effect of temperature can more than compensate for the actually higher radiation for the middle curve (August 30).
I have used global radiation on a vertical plane as an indicator of radiation, not claiming that it is related to the radiation that would be measured on the roof – or on a horizontal plane, as it is usually done – in a simple way. We measure radiation at the position of our ribbed pipe collector that serves as a heat source for the heat pump; it is oriented vertically so that it resembles the orientation of that collector and allows us for using these data as input for our simulations of the performance of the heat pump system.
Our house casts a shadow on the solar collector and this sensor on the afternoon; therefore data show a cut-off in the afternoon:
Yet, if you compare two cloudless days where the sun traversed about the same path (thus days close in the calendar) you can conclude that solar radiation everywhere – including the position on the roof – was the same if these oddly shaped curves are alike.
This plot shows that the curves for these two days that differed a lot in output and temperature, August 1 and 12, were really similar. Actually, the cooler day with higher PV output, August 1, even showed the lower solar radiation due to some spikes. Since the PV inverter only logs every 5 minutes whereas our system’s monitoring logs every 1,5 minutes those spikes might have been averaged out in the PV power curves. August 30 clearly showed higher radiation which can account for the higher output energy. But – as shown above – the higher solar power could not compensate for the higher ambient temperature.
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Logging setup:
- Temperature and solar radiation have been measured using sensors attached to universal control UVR1611 by Technische Alternative and C.M.I. as data logger, logging interval 90 seconds. Temperature sensor – PT1000, radiation sensor. Log files are exported daily to CSV files using Winsol.
- PV output power has been measured by Datamanager 2.0 included with PV inverter Fronius Symo 4.5-3-M, logging interval 5 minutes, logged to USB stick.
- CSV log files are imported into Microsoft SQL Server 2014 for analysis and consolidation. Plots are created with Microsoft Excel as front end to SQL Server.
elkemental Force 
Interesting. Thermal resistive effects in the circuit, perhaps? Nah–too simplistic, I suppose. I love the new name on your site, by the way :-) On a related note I’ve been talking about your other system a lot lately. In a few years we expect our electricity costs to start climbing sharply and our current lazy electric only method of heating houses will have to end. Systems such as yours will make a lot more sense here.
The current-voltage curve of solar panels is the the photo current minus the leakage current of the photodiode, which contains exp(U/kT)). Current is about constant and then decreases steeply at a characteristic voltage. With increasing temperatures, curves are shifted to slightly higher currents, but smaller open-circuit voltages. The optimum power – I*U at the ‘corner’ of this curve – decreases because it is determined by the more significant decrease of voltage. I have omitted that explanation deliberately and saved it for a potential future post :-)
Thanks for talking about our system :-) I have been reading about the history of heat pumps recently, and how the adoption of heat pumps and related technical innovations in specific countries had been determined mainly by prices of fuels or shortage of traditional fuel as wood – politics and economy seems to be more important than climate. If people start considering heat pumps as 1:1 heating gets too expensive it will be interesting how the market for air heat pumps will develop in comparison to a systems as ours that requires installation of the heat source. If electricity is still rather cheap (compared to our middle European prices), a performance factor of 2-3 might still be considered sufficient.
Check this out Elke–reflectivity may also be a factor acting in there besides temperature: http://phys.org/news/2015-11-technology-metal-wires-solar-cells.html
Thanks, Maurice – a really interesting article that makes me feel nostalgic: It was been quite a while since I investigated electrical properties of laser-patterned micro-structures ….
The effect described in the article – shadowing the semiconductor by the contacts reduces the effective area, so this would already be included in the rated power of the panels … which is a measure of area and efficiency combined: 1 kilowatt peak is equivalent to panel(s) that deliver exactly 1kW at standardized conditions (temperature, incidence, spectrum of radiation) – so if contacts shadow about 10% the area 1kWp corresponds to a larger area but you still pay per kilowatt peak anywhere.
Re reflectivity of silicon in general: In a simulation of PV output power I once factored in the dependence of reflectivity on the angle of incidence (making an assumption on polarization ( https://en.wikipedia.org/wiki/Fresnel_equations#Formulas ) but the effect was not very pronounced, compared to the impact of ambient temperature.
That’s a neat correlation to know. Thanks.
Thanks, Joseph! I am sure you’d like the explanation, too! Neat math. I might cover that in another posting, some day!
Maybe solar panel efficiency could be improved with a capillaric water evaporation cooling system, similar to the system many plants have developed
There are also combined solar thermal / photovoltaic units – cooling the photovoltaic panels while harvesting heat. I think there are many technical viable options; finally it is an economic decision: Will the increase in efficiency outweigh the costs of cooling the panels, e.g. transporting water to the solar panels when the air is otherwise dry? (Including investment costs such as piping…)
Billions, if not trillions of dollars are being poured into the research of fusion power and quantum computing, both of which have been proven highly elusive due to many unforeseen technical problems. With inventions, the idea needs to come first. Economic viability, to my mind, is a secondary concern
Solar panels are an affordable established product for a mass market – and they are economical for consumers today; so I’d rather compare any improvement made to solar panels to other innovations that are ready to be shipped to consumers, but still just a bit too expensive to meet the bar of people’s personal economic assessment. Consider e.g. the fuel cell for homes (plus electrolyzer) – companies have done research since many years and the product as such does work, but there are still cheaper alternatives as people joke that since 10 years the go-to-market and breakthrough is always announced for the next year. I have seen many innovations in renewable energy being developed and marketed, but the product finally was not economically viable which made companies or the technology vanish again – as there are always competing technologies, equally mature, solving the same issue at a lower price.
I also believe that the next big innovation related to PV will rather be the mass production of affordable batteries, rather than improvement to the panels. There would be existing (but not yet cheap) alternative ways to improve performance, e.g. by layered structures of different materials, each with a different bad gap so that the incoming solar radiation would be utilized better. But the investment in panels pays off today already anyway, the investment in panels + batteries does not. Mitigating the performance drop due to heat is nice-to-have, as you only have this issue in summer when a reasonably sized system will have to sell energy to the utility. In winter, when you need the energy much more, the effect works to your advantage anyway. What really would help is to store energy for the night – which is technically possible (and easy for the end user) just not economically viable. Technologies do exist, and Tesla has not invented a completely new type of battery. They are rather making history for having disrupted a market by large-scale production of affordable units.
Flying over your article this sounds for me like. Cloudy –> low temperature lower electrical resistance in the PV –> more energy. Need for a supra PV ;-).
There are two effects, the lower temperature is one of them. But it is the dependence of the I-V curve of solar cells that matter, not so much the temperature dependence of losses in DC and AC wiring. The other one is the focusing effect of clouds, see e.g. this article: http://cliffmass.blogspot.co.at/2013/06/can-you-get-more-than-100-solar-energy.html
Note that you still need the radiation of course, so you see the positive power spikes immediately after a cloud has passed (either because the panels are still a bit cooler or because radiation is reflected by this cloud that has just passed), not while the cloud is still shadowing the panels. I’ve also described this in more detail in this article, showing also the spikes: https://elkement.wordpress.com/2015/06/17/solar-power-some-data-for-the-first-month/