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PostPosted: Mon May 06, 2013 9:27 pm 

Joined: Fri May 03, 2013 5:03 pm
Posts: 1
Hi there - first time poster, new-ish user...
I'm interested to know what users' confidence level is with estimated specific yields for systems that are heavily oversized on the DC size, that have a DC:AC ratio of, say, 1.4:1 or greater.
With crystalline modules cheaper than ever, this is becoming common practice, but there can't be much empirical evidence of how effective it is. Some inverter manufacturers are publishing white papers that utilise PVSyst simulations and are showing almost-linear increases in specific yield as DC-side installation increases.
The point at which the additional saturation losses outweigh the additional production elsewhere must eventually come, but the more immediate question is at what point do the incremental increases in production become too expensive to justify, even at $0.50/W or whatever the utility-scale price-point of PV modules is these days.
Can anyone at PVSyst (Andre?) comment on what, if any, empirical research is being done to verify the model's outputs?
I'm thinking of putting together a 'small' research project that attempts to quantify at least the incremental gains vs. increased saturation losses question, and would be interested to know if there are any specific parameters that PVSyst would want to be sure were covered in order for the findings to be considered useful for use in making any adjustments to the tool.
Thanks to any and all who respond!
Ian in Ontario.


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PostPosted: Tue Jun 18, 2013 1:29 am 

Joined: Thu Aug 02, 2012 3:38 am
Posts: 105
This would be interesting to know. I'm not sure that any of the following issues are being factored into the simulation when heavily overdriving inverters:

* Effect on MPPT tracking efficiency from operating in the steep slope area as the array voltage is increased to curtail power output
* Effect of inverter temperature derating when operating under continous high load and power curtailment
* Effect on PV module temperature due to constrained power output vs. harvesting maximum power

Sounds like a good paper topic for someone with an array they can play around with and collect data.


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PostPosted: Tue Jun 18, 2013 5:20 pm 

Joined: Fri Jun 14, 2013 12:06 am
Posts: 6
Very interesting question indeed and one that has been debated for a while now.

To add to what Marvin mentioned, I think that constantly operating the inverter at a higher voltage (yet within its limits), while shifting the MPPT seems to have unintended consequences that inverter manufacturers don't seem to be very clear about. I've hear a wide range of speculation ranging from DC:AC ratios not exceeding 1.12 to ones above 1.5, and some of the impacts particularly with instantaneous spikes in current when the inverter is constantly operating in the steeper part of the curve is still an open question.

On the other hand, with Marvin's second and third points, I'm a little confused, since correct MPPT tracking would imply that the energy from the modules was never realized. This would imply that there shouldn't be a temperature derate effect due to over-powering either at the inverter or the module, since the area under the IV curve is itself reduced, so the power is never produced. So, could you please clarify your points Marvin?


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PostPosted: Tue Jun 18, 2013 7:55 pm 

Joined: Thu Aug 02, 2012 3:38 am
Posts: 105
Relating to the second and third points, and I have no proof of this but it is an interesting speculation. Inverters monitor the internal temperature and will reduce the power production to prevent the internal temperature from exceeding a preset limit. My speculation is, have inverter manufactures anticipated the effect of the operation of the inverter at full power for longer periods of time and the effect that will have on internal temperature rise? For instance if the inverter manufacturer lists that the inverter can operate at full power at 40°C does that mean operating at full power for say 4 hours at 40°C or is there an assumption by the manufacturer that full power operation will only occur for maybe 30 minutes during any average day? The internal temperature rise over 30 minutes might be quite a bit lower than the rise over 4 hours. If there is an assumption of full power operation being of limited duration then as higher ratios are used and inverters are operating at full power for longer periods can we still assume that the inverter will not derate to limit the internal temperature at lower operating temperatures?

I am pretty sure I have read that there is a relationship between the power production of a PV array and the cell temperature. That relationship says that drawing power from the PV array reduces PV cell temperature because if the power is not drawn from the cell some of it has to be dissipated as heat. The result is that a PV module with no load would run hotter than a PV module that had a load. I can't put my finger on where I read this but I'm pretty sure that there is some basis for it. In this case it would mean that the PV array with power curtailed by the inverter would run hotter and as a result would produce less power. I'm not sure where the equilibrium would fall but it would be interesting to examine.


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PostPosted: Fri Jun 21, 2013 10:59 pm 
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Joined: Mon Apr 16, 2012 7:29 pm
Posts: 1680
For the first question, manufacturers define Pnom = "Nominal power", valid for any temperature conditions, and Pmax = "Maximum power", attainable in some conditions (not always well defined, usually an operating period).
To my understanding and after discussion with some manufacturers, this seems to correspond to a device internal temperature limitation:
Attachment:
File comment: Pnom and PMax according to temperature
Inverter_Pmax.png
Inverter_Pmax.png [ 2.94 KiB | Viewed 8819 times ]
Now for the Array temperature increase when power decreases: this is evident from the energy balance used for the evaluation of the module's temperature:
Tarray - Tamb = ( Alpha · Ginc · (1 - Effic)) / U

where Alpha = absorption coeff., Ginc = incident irradiance and Effic = module efficiency.
However with Alpha = 0.9, Ginc = 1000 W/m2, U = 29 W/m²K, the cell temperature increase is 26.4°C for effic = 15% and 31°C for effic = 0 (open circuit).

Yes the array temperature is slightly higher, but at this time the array doesn't have to produce anything !!! (or works at reduced power).


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