RegGuheert said:
I've already provided links that explain all the details for long service life, including the independent analysis of the capacitors, but since it is still not clear, I will put the calculation for the capacitor life directly into this thread:
<snip>
That is exactly what design for reliability is all about. You derate your components to achieve the reliability you need to meet the market objectives.
All that said, Nichicon has a general note that says not to estimate a life longer than 15 years. Perhaps that is how long they feel the seals will last. It is likely the reason for the 15-year warranty on the M190s.
As noted, there are other stresses involved over a long life, such as thermal cycling. Those are the types of things that can be wrung out in accelerated life tests. The life of the capacitor as well as the life of the unit as a whole have been verified with accelerated life tests.
Can (or have?) you provided proof of independent life tests for Enphase's inverters?
As for the rest, thanks for taking the time, but you could have saved the typing by simply linking to Enphase's marketing paper on capacitor life again. This is not an independent source of info, however. Yes, I said marketing because that's what all of their public website material is.
"Proprietary and Confidential" - really? This is another ploy to provide the appearance of something special or secret. If the doc really was "proprietary and confidential" it would not be on their public website - it would on an internal server or on WikiLeaks. Sorry. This is a message manipulation technique. Why would a company resort to such a tactic? They tell us why in their paper:
However, for micro-inverters to succeed in practical applications and be accepted by the marketplace, the reliability of micro-inverters must demonstrably exceed that of traditional string inverters.
Emphasis mine. (No, I'm not beating on Enphase here - other manufacturers do it too.)
Enphase MUST address capacitor life in some way because the rest of the industry has "raised the BS flag" about their performance claims. One must ask themselves how Enphase managed to hire the only engineers in the world that can turn design upside down. They must have hired the geniuses because the rest of industry - including engineers that designed some of the most reliable units on the streets - cannot understand what Enphase did. Red flag number one.
The paper includes plenty of numbers and other info that appears to be backed by cap manufacturers design documents. But there's a subject that's missing - electrolyte. Under the best conditions, electrolyte is consumed in the capacitor. Seal degrade and electrolyte leaks. Caps have a pressure release valve and electrolyte and gasses vent if the device overheats or operates out of limits. Nichicon, in their electrolytic cap design paper, after working through temperature, ripple, and other life factors, they include this statement:
Typically, fifteen years is generally considered to be the maximum for the estimated life obtained by the above formula.
Why does the component manufacture say the calcs should still be limited to a 15 year max, yet Enphase says the caps will last 25 years? Red flag two. Hmmm...maybe they don't, because:
Because several electrolytic capacitors are paralleled in the micro-inverter, and because capacitors normally fail open, there is in fact very little impact on the micro-inverter if a capacitor fails. The main impact is that MPPT efficiency is slightly reduced because there is now more ripple voltage on the PV bus.
Ok. How much is 'slightly'? Enphase took six pages to tell us that caps are important, and to suggest that they did their homework, yet finish by telling us that caps aren't really that important, and that, essentially, since the cap should fail open and since it's unlikely the end user will notice the loss of efficiency, accepting the probability that caps will fail after 15 years shouldn't negatively affect warranty return rates.
Never mind - the whole capacitor lifespan thing is a diversion.
The paper presents a 'paper exercise' that uses climate data from NREL. The numbers in this paper were NOT generated by instrumenting an inverter and putting it on a roof. Why not? Red flag three.
Marketing.
RegGuheert said:
Please note that a string inverter and an off-grid inverter are two different things. You seem to use them interchangeably.
No. They're still inverters. What brought me in to the inverter discussion from the start is precisely because 'inverters' and inverter reliability numbers are used interchangeably. I came here to highlight that all inverter numbers are NOT the same. Enphase and other microinverter companies MARKETING departments want to lump all the numbers together - the goal of marketing is to provide the worst case info for their competition and provide the best case for their products. It's clear that much of the dialogue here is based on sales-speak not tech fact.
The equipment that interfaces with the panels in an off-grid system is the charge controller. They 'only' have to do DC-DC. They're known to last much longer than their warranty as well - like the 30+ year example referenced earlier.
RegGuheert said:
Note that I do not. As you have noted, and DesertDenisen has confirmed, string inverters can actually achieve relatively long lives and decent MTBFs, but they are unable to derate their input capacitors to the degree that microinverters can due to the high voltages, as already discussed.
Except here as in other places, you do!
By "high voltages" I assume you mean the devices that accept PV inputs up to 600VDC? That's fine! But not all central inverters run to those levels. Seems to me that again, the brush is a bit wide. Seems we could extract lifetime numbers for the central inverters that keep a wider voltage margin. But lumping all inverters together does provide better marketing for the microinverter folks, doesn't it?
There are three major inverter types - string, central, and micro. Micros are one per panel, strings handle one string of PV, and central handle multiple strings. But how do we know, since an inverter that can accept inputs from, say, six strings, is likely still combining them internally into a single input?
http://www.pv-system-tech.com/fileadmin/user_upload/material2012/InSyst2012_Screen.pdf
I agree completely that "all" inverter reliability numbers should not be used in blanket statements because the majority of inverters in the field are not grid connected - they're used in cars (EV, hybrid), they're used in ships, trains, RVs and off-grid buildings. None of those are directly connected to PV inputs as grid-tied inverters are. The off-grid analog is the charge controller, not the inverter. And we already know that charge controllers last longer than the commonly used "5-10 years" for inverters...
You agree that central inverters CAN
can actually achieve relatively long lives and decent MTBFs
in SPITE of not being able to derate caps to magic Enphase levels. Good.
But this is all a huge red herring. Why? Because the central inverter world, both off-grid and large scale grid tied, sees capacitors as a maintenance item and designs for repairability. Enphase, on the other hand, is willing to accept the loss of efficiency when the caps fail in their non-serviceable assemblies. And, again, it's NOT about warranty, as Enphase's primary customers - folks like us and our parents - are not at ALL likely to notice the efficiency deterioration from years 12 thru 25 either because they're not looking, or because it's lost in the noise of panel and wiring deterioration and is thus deniable by Enphase.
RegGuheert said:
Most off-grid inverters and charge controllers continue to use fans for cooling and that fact is reflected in their short warranty periods.
Yes, many do use fans. Not all do, however. Outback has VFX...models (vented/fan cooled) as well as FX... (sealed, not fan cooled). They both live long lives in the field. And they both have the same warranty length.
Outback's cast aluminum case and heat transfer process is highlighted as a 'thing that works well' in Sandia and NREL reports - was adopted by microinverter companies. I wonder who invented that lumpy black case?
http://www.midnitesolar.com/pages/frontPage/nwHistory/history.php
Additionally, the commonly used off-grid equipment is more than an inverter - my Outback VFX3524, for example, is an inverter, a battery charger, generator controller, and transfer switch. This equipment is doing more than 'just' invert.
Grid-tied equipment uses a combination of cooling methods as well - convection, forced air, and liquid are all used.
RegGuheert said:
The Sandia report on a working group meeting for 100kW and larger inverters has no bearing on a discussion of inverters for home PV systems. True, they have no sway with component manufactures since their volumes are low. Enphase has lots of sway over their component manufacturers since their volumes are high.
Yes, you're correct, that particular working group does not reflect the entire industry. But that's not the source for my comment. That fact came from a number of other working groups hosted by Sandia and NREL that DO reflect the entire inverter spectrum - including automotive. While your faith in the company is admirable, there is absolutely no way that Enphase's ~12% of residential generation, has more pull with component manufacturers that the companies serving the remaining 88% of the residential market, plus commercial, plus industrial scale, plus mobile inverters, plus the automotive/hybrid/EV sector have.
RegGuheert said:
Enphase is unable to hide the failures of inverter with their current public websites. That is why I am able to obtain failure data from there.
Two points. First, from a tech perspective, they are the ONLY entity that can filter the data they display because they are the only entity that has full control of the data. It would be amazingly easy for them to filter the data. Second, they likely won't have to filter as moving to a pay system and removal of publically-accessible data allows them to fragment/hide/mask any failures that appear as their systems age much better - and they're paid to do it! Brilliant marketing!
RegGuheert said:
That won't last too much longer, but I already have enough data to see several things:
1) MTBF over a wide range of climates at 20 locations with over 500 inverters installed is quite high. MTBF outside the desert is significantly higher than Enphase's projections.
2) There are some relatively early failures in Phoenix and Tuscon. We have no way to kow if these are random failures or wear-out. With two failures in a single system with only 10 inverters, I suspect that this installation may experience temperatures much hotter than the product rating and some interconnects are failing, but there is really no way for us to know.
Sure - when all the data available is good, it can mean all the systems are good, or it could mean only the good systems are represented. Do you know? Do you have ANY way to confirm that you're looking at a full dataset?
RegGuheert said:
This is much more field operational data than I have seen for any other inverter type. I will note that no such public data has been produced for sting inverters or off-grid inverters. As stated, the manufacturers likely do not have good data for their products due to their inability to track failures.
I cannot agree with this from at least two directions. First, for you to suggest that manufacturers don't have failure data, one would have to accept that warranties are not claimed, that vendors are not tracking returns, and that more than 40 years of info just doesn't exist. Sorry, no. Secondly, the data you suggest does not exist does - but it takes more work to find. Much less in the internet age, however - but I agree, it's not as easy as sitting on the couch and looking over an internet feed. How do we find reliability info? With info from manufacturers (limited to what they disclose publicly), from conversations at meetings, conferences, etc. (some public, some not...), from equipment vendors, from installers, and from end-users. The community does a good job of providing the info you seek - here are two examples:
Solar-Guppy and others were working with Xantrex when they released their GT grid-tie inverters. Xantrex didn't have a good rep and wanted to show they'd improved. There was some improvement, but not all was rosy:
http://sgtechnology.dyndns.org/forum/solar-powered-gridtie-inverters-f19.html
Note the topic headings for the GTs: "smoked", "weird issues", "communications problems" etc.
(This was feedback from a small group actively beta-testing an inverter - it wasn't the typical internet enthusiast crowd.)
On the other hand, there are a number of inverter manufacturers that stand head and shoulders above the rest as reflected by this (this is not a sole source for this message by a long shot):
We have been selling Exeltech inverters for over 15 years, and we have never had one single report of any failure of any type. In fact, we have never even heard of one going bad that was not grossly abused (like hooking the battery terminals to AC power..).
http://www.solar-electric.com/exsiwain.html
The information you seek is available, but one has to look for it.
Enough of all this. Enphase has the warranty they have. Time will tell if they or their equipment is in the game for the long haul. But we've been here before.
Our job, seems to me, is to get whatever we want from our equipment. I prefer equipment that can be serviced, because there is nothing created by man that lasts forever. I'm not a fan of disposable tech. It doesn't matter, though, because Enphase doesn't serve my segment of the market anyway.
All equipment has a design regime, zones of acceptable and unacceptable performance, strengths and weaknesses.
http://solarbridgetech.com/wp-content/uploads/2011/05/SLB_E_Design_Reliability.pdf
We have a choice - we can either learn about the solutions and make our choice, or we can work to find an honest sales rep and hope they have the integrity to tell us that their produce is not the best fit for us if that's the right answer. It can be more difficult to find an honest sales rep than to find failure data for a Chinese inverter on eBay...
edit...spelling, typos, quote tags...