Micro-Inverters

Micro-Inverters – The New Kid on the Block

Given that there are no moving parts in photovoltaic (PV) solar systems, they don’t require any maintenance except for periodic cleaning of the glass surfaces. The photovoltaic solar collectors simply don’t fail. Here in Colorado, we have PV modules that were installed 30 years ago that are still sitting on rooftops silently generating power today. That said, the PV modules being manufactured today are anticipated to last even longer, due to superior designs and manufacturing processes. If a PV module does fail, it usually happens when it’s brand new; usually this happens because a solder joint connecting adjacent cells fails. Although this is a very rare occurrence, when it does occur, we call this kind of failure “infant mortality” because it is most likely to be discovered when the modules are fresh out of the box. If a module survives shipment from the factory to a distributor, then subsequent transportation to the install site where it’s hauled to the rooftop and bolted in place, all without solder-joint failure, it’s almost certain to perform without failure for years to come. Most PV module manufacturers provide their products with a warrantee that says they will still produce at least 80% of their initial rated output after 25 years. That’s a pretty strong warranty, but it only applies to the solar modules. That leaves the other main component of the system – the inverter.

Inverters have traditionally been the weak point in any PV solar system. Mean (average) failure rate has been roughly 5 years. In this industry, two-year warranties were the norm in the late 1990s. Fortunately, five-year warranties are more common now, and some manufacturers offer 10 year warranties. System designers and installers usually inform their clients that a standard string inverter (from SMA, Fronius, etc) will need to be replaced in 10-15 years, and thus that replacement cost should be included in the return-on-investment (ROI) calculations for the system.

Planning to replace a key system component every 10-15 years isn’t the ideal scenario. Furthermore, that still leaves us with an entire system that’s ready to fail (shut down and stop producing energy) on a moment’s notice when that single inverter does fail.

Central String Inverters

PV solar modules generate DC power, which is either used directly by DC lighting and appliances in off-grid or dedicated field applications, or is fed into a central “string” inverter, which is typically located adjacent to the service panel and electric meter on the building. A recent NREL study claims research has shown that string inverters need to be replaced every 5-10 years, while PV modules typically last 25 years or more.

Central “string” inverter type PV systems are laid out such that a group or “string” of modules are wired in series, much like old-fashioned Christmas tree lights. Thus, the term “string”, because this string or series of modules generates a high voltage output which is fed into the central string inverter for conversion to AC power. Modern central inverters then use a Maximum Power Point Tracking (MPPT) algorithm to determine the optimal output for the overall system. Unfortunately, this means that the output of the whole system may be only as strong as the worst performing module. If there’s one bad solder connection, one dirty cell, or one partially-shaded module in the entire string, the whole system is compromised. Further, just like Christmas tree lights, it’s nearly impossible to find a problem in the string without testing every individual part.

Micro-Inverters

All of this is changing with the advent of micro-inverters. These devices mount outside as part of the PV array itself, typically directly on the PV mounting rails, underneath the modules, where they are attached to each module, one-to-one. With micro-inverters, the DC power that leaves each module is converted directly to AC power right at that point. This improves overall system efficiency due to the elimination of DC voltage drops across the string. The Christmas light problem is also solved – each module/micro-inverter pair is able to develop its own MPPT optimal performance profile, and each can be tracked independently through monitoring software. Micro-inverters get as much energy out of each module as it is able to produce, so partial shading is no longer a problem. If shading impacts one module, it has zero affect on the rest of the array. Further, if one micro-inverter fails, the rest of the system still functions normally, the system owner or the installing company are notified (e.g. email), and it’s a relatively small replacement cost to swap out that micro-inverter, compared with a central inverter that takes the entire system down when it fails.

The Enphase Micro-inverter

Micro-inverters are also designed to be much more reliable. Enphase, the leader in this segment of the industry, credits four things for this advancement:

  1. Micro-inverters process relatively small amounts of power at low DC voltages which allows them to incorporate more components on the semiconductor chip, rather than soldering together a bunch of analog electronics.
  2. Because of the small amount of power processing that’s occuring, the temperature rise inside each micro-inverter is lower. In fact, they utilize passive cooling rather than fan cooling, which is common in traditional string inverters.
  3. The NEMA 6 enclosure used to encase each Enphase inverter is air, water, dust, and insect tight. You could drop one in the ocean, with the cables sticking out, and connect it to a module….no problems. In contrast, traditional inverters are like desktop computers with cooling fans actively pulling dust and humidity into the enclosure while they attempt to keep the components from overheating.
  4. Potted design. The enclosure is filled with “an encapsulating compound” which improves heat dissipation and provides component protection. The “potting” material is much like an epoxy resin…nothing can permeate it so as to get into the electronics.

Because of their design requirements, traditional string inverters use large electrolytic capacitors in their circuitry. These are notorious for their short operating life, especially in field applications. Micro-inverters use these components also, but they utilize a more durable design. Here’s some material taken from an Enphase Reliability Study on Electrolytic Capacitors:

“For traditional power converters, an acceptable useful life of capacitors is as low as 2000 hours at 85°C. Out of desire to increase the reliability of its inverters, Enphase Micro-inverters use capacitors rated from 4000 to 10000h at 105°C. The capacitor lifetime is very sensitive to temperature as its useful life doubles for every 10°C temperature drop.”

Enphase also uses designs that leverage the use of parallel capacitors – they use two devices connected together in parallel instead of one larger device. If / when one capacitor fails, the quality of the current wave degrades because it gets a little more ripple in it, but it’s not catastrophic to the inverter. This redundant design strategy also facilitates longer useful life for these inverters.

Since micro-inverters are a relatively new development in the industry, there’s not much lifespan data available yet. In an Enphase white paper, they compare their Mean Time Between Failures (MTBF) determined from accelerated lifecycle testing to other electronics:

Sun Microsystems: “The concept of MTBF is often confused with a component’s expected useful life. In fact, these concepts are not the same. For example, a battery may have a useful life of four hours and have an MTBF of 100,000 hours. These figures indicate that in a population of 100,000 batteries there will be approximately one battery failure every hour during its four hour lifespan.”

Currently Enphase’s product specification is an MTBF of 330 years, or more than 2.8 million hours. The company claims it has a 600-year MTBF goal, which, when achieved, would make integrating micro-inverters with solar panels at the factory the default industry design. When we reach that point (which Arise predicts will occur within the next 10 years), solar modules with integrated micro-inverters will be truly plug and play devices. Meanwhile, even with 330 year MTBFs, the current micro-inverters are highly reliable. Couple this with their increased performance over string inverters, and far superior reporting capability (down to the module level), and it’s no wonder they’re enabling the most advanced PV solutions being installed in the industry today. Watt-for-watt, micro-inverters do cost more than string inverters, but we believe the increased cost is a smart investment given the advantages they deliver.

There are at least a dozen companies working on micro-inverters, but Enphase is clearly the industry leader. This is why Arise has partnered with Enphase, and why we offer our clients PV solar solutions that include an upgrade to Enphase micro-inverters, while remaining cost competitive with solutions other companies propose that are based on traditional string inverters.