Rebekah, My understanding was that PVsyst modeled inverter nominal power not maximum power. I believe this helps to compensate for inverter temperature limitations. For instance, depending on inverter location and power ratio, I would suppose that there may be instances where the simulation could clip potential max power production. This may balance those instances where inverter temp derating would result in production loss.
I would be curious to see how subhourly data could affect a simulation. Admittedly not something I have thought about. Matt Matthew Partymiller Solar Energy Solutions LLC (877) 312-7456 m...@solar-energy-solutions.com On Tue, August 26, 2014 7:10 pm, August Goers wrote: > Hi Rebekah, > > > > > This is where checking simulation data versus real output data is key. > Our > company has hundreds of monitored systems but very few with undersized > inverters. So, I donât have many data points that I can study. > Generally > our systems meet or beat our simulated as-built production estimates > within an amazingly close margin. I think your point about inverter > heating and derating is valid for inverters which are significantly > undersized. You seem to have a strong opinion that simulators generally > underestimate clipping. Do you have any particular sites where you learned > this from? > > > > Best, August > > > > > *From:* RE-wrenches [mailto:re-wrenches-boun...@lists.re-wrenches.org] > *On > Behalf Of *Rebekah Hren > *Sent:* Tuesday, August 26, 2014 10:54 AM > *To:* RE-wrenches > *Subject:* Re: [RE-wrenches] Inverters Maximum Input Ratings > > > > > One of the problems with relying on simulators (PVsyst, Helioscope, > whatever) to estimate clipping is that they rely on hourly averaged > irradiance from TMY files (with very few exceptions), which may not > predict clipping, especially for areas with variable climates. > > > > Thinking about partly cloudy days in NC where I live and install systems, > an average hourly value predicts no or a very low percentage clipping > for systems with even a fairly high dc to ac ratio (by high I mean > anything over about 125%). Without subhourly meteo data, I would be > skeptical of simulators estimates of clipping. I believe they are > generally underestimated, which can lead to a false financial analysis of > the most cost effective dc-ac ratio. > > > > Also, I do not think most (any?) of the simulators have the capability to > take inverter internal temperature related power derating (another form > of power clipping) into account in the simulations, which could lead to > overestimated generation. I realize hot inverter temperatures may not > coincide with the colder temps that usually create clipping, but the > higher the ratio goes the more often we see an overlap in hot days/high > temps/clipping. > > > > Cheers, > > > Rebekah Hren > > > > > -- > > > Tel: 336.266.8800 > http://o2energies.com/ > Project Engineer > NABCEP Certified PV Installation Professional⢠091209-85 > NC Licensed Electrical Contractor > > > > > > > On Tue, Aug 26, 2014 at 11:30 AM, August Goers <aug...@luminalt.com> > wrote: > > > Hi Jason and All, > > > > > Good topic. As far as I know, simple energy estimation programs like > PVWatts donât take inverter size into account when estimating annual > production but more robust simulation programs like PVsyst do. I think it > is a matter of really being able to trust your simulator to tell the > designer how sizing the inverter will affect annual production. Maybe > others can chime in on this. > > > > Weâve run different scenarios through PVsyst in house to see how under > sizing inverters affects the overall annual production. It is surprising > for our area (SF Bay Area) that we can really push an inverter with very > little reduction in annual simulated production. Iâm talking about > putting maybe 5 kW (or even a little more) of PV on a 3.8 kW inverter or > thereabouts. Array orientation also factors in. To Issacâs point, if > keeping the inverter small reduces the initial installation cost > significantly while possibly only reducing annual production by a little > bit then I feel it is a good design. > > > > All that said, weâre still fairly conservative with pushing our > inverters. Iâve found that it is very hard to go back to a client after > the salesperson has made the sale and try to reduce the inverter size. In > other words, we need to nail the inverter size up front during the sales > process or the client will oftentimes feel taken advantage of later. It is > hard to explain why we are only giving a client a 3.8 kW inverter when we > are calling their system 5 kW DC. > > > > To Isaacâs questions: most of our clients wouldnât notice about mild > to moderate clipping during peak periods. However, some would. This is why > it is important to setup proper expectations about how we designed the > system up front and do our homework to assure that the clipping is > accurately estimated and factored in to the production estimates. If we > can provide clear logical reasoning for sizing the inverter the way we did > then we shouldnât have any problem. I have no idea how array sizing > might affect inverter longevity. > > > > I think the general points made by others about the declining costs of > the modules themselves and increased cost of BOS components means that > these types of discussion are very valid. > > > > Best, August > > > > > *From:* RE-wrenches [mailto:re-wrenches-boun...@lists.re-wrenches.org] > *On > Behalf Of *Isaac Opalinsky > > > > *Sent:* Tuesday, August 26, 2014 6:11 AM > *To:* RE-wrenches > > > *Subject:* Re: [RE-wrenches] Inverters Maximum Input Ratings > > > > > Jason, > > > > > This seems to be a regular topic of discussion in our training classes > not only for microinverters, but also for string inverters. Especially > for 3.8 kW units that are optimal for backfeeding 100A service panels and > 7.6 kW > units that are optimal for 200A panels. A slightly bigger array can give > a higher total yield, maybe some power clipping, without the additional > cost of a supply-side connection. As long as you stay below the maximum > VOC and > ISC, there isnât a safety issue. > > > > > So it really just boils down to economics and the overall value > proposition for the customer, which makes it hard to provide a blanket > recommendation. Weâve been training people for years to model PV system > performance to determine an acceptable DC/AC ratio on a project-by-project > basis. > > > > The inverter manufacturers pretty much all claim that there is no concern > about overworking or shortening the useful life of their inverters since > limiting operating power limits the operating temperature as well, but > that leaves me with two questions: > > 1. Does anyone have any evidence that high DC/AC ratios does/does > not shorten the life of the inverter? > > 2. If there is a small amount of power clipping (say <1% total > annual energy), are many customers likely to notice/care? > > 3. If they do notice, does the customer service aspect of having to > defend a design decision outweigh the potential economic benefits of a > smaller inverter? > > > > *Isaac Opalinsky *| Technical Trainer | *SunPower Corporation* > > > Desk 443-569-3476 | Cell 443-277-6286 > > > > > *From:* RE-wrenches [mailto:re-wrenches-boun...@lists.re-wrenches.org > <re-wrenches-boun...@lists.re-wrenches.org>] *On Behalf Of *Jason > Szumlanski > *Sent:* Monday, August 25, 2014 4:41 PM > *To:* RE-wrenches > *Subject:* [RE-wrenches] Inverters Maximum Input Ratings > > > > > I had a tough customer recently that grilled me on how we can put a 270W > solar module on a 215W inverter. Fortunately, Enphase has a wonderful > white paper on the subject. However, it got me thinking... Enphase has > demonstrated that higher output panels in many climates (hot SW Florida > included) can benefit from modules that far exceed the inverter rating, > and even exceed the inverter's "recommended input" rating. Enphase has > shown that 270W+ modules can show energy harvest on the M215 where it > makes sense to "oversize" the module. > > > > I also received a similar query from a rather uninformed plan reviewer in > an area AHJ along similar lines. Fortunately I was within the > "recommended > input" rating on the spec sheet of 270W with a 265W module, but I wonder > what would happen if I had paired the M215 with a 280W module on my > plans, which are becoming readily available now in 60 cell modules with > 300W > modules on the near horizon. I'm pretty sure my plan would have been > kicked back for exceeding the manufacturer's recommendation. > > > > My question, which applies to string inverters and microinverters, is how > much is too much, what would happen if you paired an array that far > exceeded the rating, and how do inverter manufacturers determine the > recommended and/or maximum rating of the connected module or array? Also, > why do some manufacturers have a simple recommendation while others have > a "maximum" rating? > > > > > Jason Szumlanski > > > âFafco Solarâ > > > > > _______________________________________________ > List sponsored by Redwood Alliance > > > List Address: RE-wrenches@lists.re-wrenches.org > > > Change listserver email address & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > > List-Archive: > http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.htm > l > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out or update participant bios: > www.members.re-wrenches.org > > > > > > This message, together with any attachments, is intended only for the > named recipient(s). It may contain confidential or privileged > information that may be exempt from disclosure under applicable law. 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Thank you. > _______________________________________________ > List sponsored by Redwood Alliance > > > List Address: RE-wrenches@lists.re-wrenches.org > > > Change listserver email address & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > > List-Archive: > http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.ht > ml > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out or update participant bios: > www.members.re-wrenches.org > > _______________________________________________ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
