Mark, What do I tell my clients about soiling? 96% if they clean them once a year. If they clean them on the 4th of July, they'll do a percentage point or two better.
The Forest and the Trees: That's just the point. A bunch of trees makes a forest! A PV system can suffer from a score of small losses, each of which may be only a few %. Individually they may be small, but cumulatively they can add up to 10% or more. How do we know? Take a look at the DC-AC derate terms in PV Watts 2. By the way I am a firm believer in PVWatts 2; I used to think the tool was overly conservative, now I think it is spot on. I have read (and re-read) the Fronius White Paper. I find it unconvincing. Remember these are the same guys who in 2006 couldn't get MPP tracking correctly. For my more astute clients, 5% is significant. For our Company, we try to reduce every loss contribution to its practical minimum. For example we are conservative on (1) specifying wire gauges for minimum IR losses, (2) strongly encouraging our clients to trim their trees/bushes and keep them trimmed, (3) encouraging our clients to remember to clean their PV modules on schedule, (4) using the inverters with the highest efficiency ratings, (5) using PV modules with the tight tolerances. These five issues can easily add up to 10%. Heck, shading can be 10% or more on its own if you don't pay attention to it (e.g. because the utility company that disburses rebates doesn't require a shading analysis). - Peter Peter T. Parrish, Ph.D., President California Solar Engineering, Inc. 820 Cynthia Ave., Los Angeles, CA 90065 Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 CA Lic. 854779, NABCEP Cert. 031806-26 peter.parr...@calsolareng.com -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Mark Frye Sent: Monday, December 29, 2008 10:39 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] Inverter with two strings of different orientation Peter, What are you modelling for annualized soiling loss? What is the basis for that value, margin of error? With industry standards for soiling in the range 5 to 7%, are you not seeing the forrest for the trees, looking so closely at effects of multiple orientations of separate strings. Go with the Fronius White paper: one inverter, multiple strings, all modules in any given string with the same orientation. If you must, throw a number at the orientation loss, but how meaningful will it be in the overall picture? Mark Frye Berkeley Solar Electric Systems 303 Redbud Way Nevada City, CA 95959 (530) 401-8024 www.berkeleysolar.com -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Peter Parrish Sent: Monday, December 29, 2008 10:07 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] Inverter with two strings of different orientation Thanks Bob, these are really great reference articles. The equation for Vmp had four terms, three of which are temperature dependent. So I think we put that issue to bed. The reason that this issue is so important for us is this: if Vmp is temperature dependent, then there must be some loss when combining two strings together with different tilts/azimuths. How much, I don't know. Specifically, I have a client for whom we are designing a system with two strings with different tilts/azimuths (otherwise identical) 18deg/East and 15deg/South. The questions is, "Go with one inverter and wire the strings in parallel, or go with two inverters one for each string." Since the cost for two inverters is considerably more than one inverter of twice the capacity, I would like to go with the single inverter, if the losses are in the 1-2% range. I am also very skeptical about MMP tracking algorithms. I documented the "saw-toothing" on the Fronius IG-4000 input (July 2006), and we still have an outstanding malfunction with a Xantrex XW6048 with an output power oscillation. Right now I will be telling our clients that "using a single inverter -- with two or more subarrays of widely differing orientations (see above) -- is not recommended, because there may be significant losses (significantly more than 1-2%)". - Peter Peter T. Parrish, Ph.D., President California Solar Engineering, Inc. 820 Cynthia Ave., Los Angeles, CA 90065 Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 CA Lic. 854779, NABCEP Cert. 031806-26 peter.parr...@calsolareng.com _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
