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Talk:Directory:Micro-Inverter System by Enphase Energy

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Discussion page for Directory:Micro-Inverter System by Enphase Energy

Enphase Energy has extended the concept of a micro-inverter into a complete system, which includes the micro-inverter, embedded powerline communications and Enlighten — a web-based performance monitoring and analysis tool. (Video)

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One of the first things I noticed after being in the solar industry for over a decade is that they are NOT the first commercially available microinverter. Many have tried and many have failed. Don't get me wrong, they have a chance here but it's slim. MTBF is a great estimate but from my experience in many industries including aerospace the real world is different and manufacturing is a whole new set of challenges. It is extremely difficult to have a highly reliable product with a low manufacturing cost. Looking at the 2nd generation product it looks very low quality but it's hard to say just by looking at the outside. One other way to look at this is as a entire system MTBF. If you take the number of inverters and put the new number of components into the equation all of the sudden the MTBF number changes. Using a central inverter the number stays the same. Just as if I have a single satellite in orbit it's extremely unlikely it will fail but if I put 5,000 satellites in orbit I know I am likely to see some failures even with an extremely high MTBF. One of the reasons is that I cannot easily control the quality of the components I put in. I can choose very high reliability components but it's expensive and not 100% reliable. Just my 2 cents......

This is a strange post.
"Reliability" is not a monolithic concept. In this case the issue is whether or not a failure is critical. A failure in a string inverter, or anywhere up the line, takes the system offline. A failure in a uInverter typically takes only that panel offline. There are some musing below about other failure modes, but it's not clear to me that these are either real or have ever happened.
It is clearly false that one cannot make a produce that is both inexpensive and highly reliable. There are few things more reliable than a transistor, which is the lowest cost non-free object on the planet. The cost of an inverter is increasingly dominated by the cost of the copper in the transformer cores, and eliminating those lowers cost AND improves reliability.
The rest is speculation. The post claims the 2nd generation product appears lower quality, but I see the exact opposite. YMMV.
But who cares? They've sold over 1/2 a million of them now, and real-world success trumps anyone's gut feeling. That's called "evidence based science". Maury Markowitz 04:40, 11 February 2011 (PST)

Competition from Big Player National Semiconductor

--Penny Gruber 07:45, 8 November 2008 (PST)

If I were Enphase I would be really worried. National Semiconductor has entered this area with a product called Solar Magic. National's devices are DC: They work with conventional inverters. They don't require a complete rewire of the install if the units fail and National leaves the business. How reliable National's design is I don't know. National is also being quiet about pricing just yet.

To be fair about the comments below concerning reliability, Enphase takes the (disputable) position that if one of their devices fails, only power from the one panel is lost. To Enphase's view, Enphase's scheme is redundant and fails gracefully one bit at a time. Enphase's argument against central inverters is that any failure tends to be 100%. For a homeowner with a 3kW solar array producing 17kWH / day, the cost of a grid tie system being down is less than $5. a day. The labor cost to have a contractor climb up on the roof to replace an Enphase inverter has to be in the $100's of dollars.

Different Presentation of Existing Products

--Penny Gruber 08:57, 10 October 2008 (PDT)

Microinverters have been around. What they can do well is better harvest energy from panels that mismatch due to aging and shading variations. However, Richard raises legitimate points concerning Enphase. What Enphase has done that is really new other than develop a nice computer display escapes me. The 95% efficiency number for a 240V output inverter isn't special. Higher efficiencies can be had. They cost money.

The Enphase inverters use a simple flyback design that is serviceable but is in no way notable. Per Enphase's literature, the first units could only handle 185W panels. Flybacks can be cost effective up to a couple hundred watts after which forward topologies become much more economical. Flybacks suffer from high switching noise and stresses they impose on the input and output capacitors. This is made worse by the fact that like lower cost central inverters the Enphase inverters use aluminum electrolytic capacitors, notorious for their poor reliability. Aluminum electrolytic capacitors: don't work well in cold, die quickly in heat, and ultimately just die from the electrolyte escaping past the rubber seals. It is largely why residential inverter warranties used to be just five years before regulatory mandates pushed them to ten. It is why high reliability commercial inverters and motor drives don't use them.

To understand just how bad the problem with aluminum electrolytic capacitors is, consider that at 12 hours average sunlight per day a single year is just shy of 5000 power on hours. That's the entire rated operating life of top grade aluminum electrolytic capacitors. The way that an inverter manufacturer extends the life of their capacitors is to operate them below their rated temperature. Placing the capacitors on the roof is hardly a way to keep them nice and cool. The same NEMA6 case that seals out water also makes it hard to remove heat from the capacitors.

Enphase promotes the idea of application to large commercial installs. Time will tell on that one. An article in Enphase's local newspaper featured installer interviews, and the commercial installers were quite wary of so many points of failure and maintenance. [1]

"Dan Thompson, president of SPG Solar in San Rafael, said he's heard the theory behind micro-inverters, but isn't sure whether the promised efficiency will be worth the additional cost.
On small residential systems with partial shade, for example, they may make perfect sense, but the benefit isn't as clear to him on large installations, Thompson said.
"If I'm installing 5,700 panels, personally I don't want 5,700 inverters," Thompson said."

Enphase's claim of no single point of failure is also subject to considerable question. Enphase's inverters all tie in parallel to shared AC wiring, and they do it in a daisy chain fashion. An ouput stage short or partial short circuit affects the entire bus. In order to isolate such a failure Enphase has put non-replaceable fuses on their circuit board. The idea is that if their output section shorts, power from the other inverters and/or the utility will blow the fuses. Then after they are blown the rest of the inverters will supply power again. To make that concept work, Enphase has to insure that the same stress that is high enough to blow the fuses in the failed unit doesn't substantially weaken the fuses in the surviving ones. How well Enphase have done in this respect begs Richard's point about how real-life field reliability can be very different than calculated MTBF. Fuses are relatively fragile parts that traditionally exhibit high failure rates.

I also agree with Richard in his caution against new entrants, particularly where as with Enphase the wiring is very different from and incompatible with established solutions. In the best case Enphase prospers as a company and supports their product for the entire 40-50 year life of the PV install. But should Enphase not realize their aspirations, it is the consumer who bears the risk. They will suffer if Enphase falters or for any other reason does not supply compatible replacement parts for the next 50 years. The consumer would face the unpalatable choices of either watching helplessly as their installation fails piece by piece, or go through an expensive labor intensive replacement of the wiring and installation of a traditional inverter. That's a painful episode adopters of now discontinued microinverters have already lived through. People who install using traditional inverters don't face this problem as if their inverter fails there are many manufacturers of compatible products. If the original manufacturer disappears there are others the consumer can buy a replacement from.

Microinverters offer better energy harvest under certain conditions than central inverters. But, microinverters come with a host of issues that in their 15 year history have so far prevented widespread adoption. I don't see where Enphase have effectively addressed the key issues, actual longevity topping that list. I for one would never put aluminum electrolytic capacitors on my roof.

Raghu responds to Richard's micro-inverter qualms - ("Not a Good Idea" Below)

Tuesday, September 30, 2008 12:15 PM


Sterling,

Here is the response to Richard's concerns.

1. Lets start with power. We fully understand the reason for the module vendors driving towards higher power. As higher power modules enter the market, we will address them. Higher power modules do not represent a technological barrier for us. The driver is to reduce BOS costs. In commercial installations, with the Enphase micro-inverter sytem, BOS and labor costs are further reduced because you are eliminating the need for expensive DC components such as DC combiner boxes, DC disconnects etc. Furthermore, you do not have to pour concrete, crane a large inverter or build a hut or chain link fence around an inverter, all of these lead to BOS and labor savings of about 17%.

2. We are especially well suited for large commercial installations. This has to do with system availability. In a traditional system, the inverter is a single point of system failure. The best one can expect for traditional inverters uptime is in the low 90s%. With a traditional inverter, it is a red alert because when that inverter is down, your entire array is rendered useless. With an Enphase Micro-inverter system, because there is no single point of system failure, a loss of an inverter or module has a minimal impact on the overall energy production. Replacing an micro-inverter is part of routine maintenance. As is common practice in large commercial installations, with quarterly maintenance (washing modules etc.), with a micro-inverter system, one can expect greater than 99.8% system availability. This is enabled by our communications technology that is built into our inverters. The O&M operator will know the precise location of the failed module or inverter before even being on the site. Stocking a few "spare" inverters becomes feasible. We have conducted extensive bottoms up cash flow analysis and have shown IRR gains of greater than 30% over the IRR with traditional systems. The numbers are very compelling and that is the reason there is tremendous amount of interest from large installers. We have a number of commercial installations already online.

3. It appears that the bulk of Richards concern is around reliability. Achieving 95% efficiency is absolutely non-trivial and vitally important for reliability. Our MTBF study was based on a standard called Telcordia SR332 Issue 1/2. This is a standard that is extensive used in the Telecom industry which is where a number of the Enphase team comes from. This standard impresses real world FIT rates for components in the model rather than relying on those supplied by vendors. We are in the process of corroborating our computed MTBF and empirical MTBF and thus far they are converging. As I had mentioned in the previous e-mail, we have done extensive accelerated life testing in the lab and test every single inverter before they are shipped. With regards to replacement - our inverters are not attached to the module, they do not physically touch the module. They are mounted on the racking. Obviously, replacing a broken inverter may require you to lift a couple of modules. This is a very important point, we as an industry need to make sure that we follow certain installation best practices. As we all know, the output power of crystalline modules are very sensitive to temperature. Typical modules will experience about 0.5% per deg C reduction in output power. When there are a numerous rows of modules installed, there is a reduction in air flow that causes a problem called the "chimney" effect that can result in a temperature delta of about 10 degrees from the top row to the bottom row. 10 degrees corresponds to a reduction of 5% in output power. With a traditional inverter this is a real problem since this is equivalent to module mis-match and with a single MPPT, there is a big reduction in energy harvest. This problem is obviously eliminated with a micro-inverter system. In systems installed on flat roof surface or ground mounts, using an angled mount, replacing an inverter takes a few minutes and the whole issue is moot. As I mentioned earlier, the software tells you exactly where the problem is and more importantly, because it is not a catastrophic failure, you can replace it at your convenience. Furthermore, Enphase offers an energy production guarantee that covers you for lost energy production.

4. Every startup with new technology face similar challenges to us and all great companies started as a startup! There are typically two segments of customers, the early adopters who embrace new technology that delivers tangible benefits (greater energy harvest, greater reliability and lower cost) and those that are more conservative and wait. We understand this and are very respectful of both these points of view. Just because a company has been around for a long time does not guarantee that their product is by default going to be more reliable. Our team is very experienced and we have built startups into large companies and understand the challenges. We are well capitalized with over $20M raised in venture financing and hope to be around for a long time.

Regards, Raghu

Not a Good Idea

Sept. 29, 2008 These are at best a marginal new entrant and not an innovative, ground-breaking product. He totally ignored the installation and O&M challenges. 215 Wp is still too small - the trend in the module industry is for larger 250 to 315+ Wp modules because larger modules have less racking and installation costs and one needs fewer of these modules to generate the same output as smaller modules. These are particularly poorly suited for commercial 3 phase installations or any large installation. Expected mean time between failure is not the same as proven MTBF. These have very little field data - particularly for units that have been continually operating for more than a year or two in real-world conditions -- 2,5 million hours is the same as ~285 units running for one year. Furthermore, given the unfortunate history of way too many inverter startups failing because of quality problems that occur several years after installation but before the expected MTBF and the resulting liabilities they created for solar installers and project investors, I am extremely wary of buying from ANY inverter startup. From a financial point of view, their warranties are worthless because there is a very significant risk that the company will go bankrupt before the end of the warranty period and the company has insufficient insurance or reserves to support their warranty.

Richard George

Umm, you are aware that (1) you use an inverter that's smaller than the panel, and (2) the 2nd gen product is 215 watts. Right? In 2011, the company is no longer a startup, and has grabbed a major chunk of the market. As I said, you can argue all you want, but success is success. Maury Markowitz 04:43, 11 February 2011 (PST)

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