Micro-Inverters: Advantages and Disadvantages in Solar Photovoltaic Systems.

Micro-Inverters: Advantages and Disadvantages in Solar Photovoltaic Systems

The Laws of Thermodynamics in PV Systems

In every energy system, especially in solar photovoltaic (PV) systems, the laws of thermodynamics are ever present and lead to countless inefficiencies. Each solar PV system continuously contends against a vast barrage of inefficiencies, some of these are unavoidable and insurmountable, but a number of these inefficiencies are amendable, manageable, and novel solutions are continually being developed either in the form of new practices or with new technologies. Now, the factors causing these latter inefficiencies in solar PV systems are commonly referred to as the ‘derating factors’. These common derating factors, according the much trusted National Renewable Energy Laboratory’s (NREL) PVWatts, are shown in table below (Energy, 2011).

Component Derate Factors PVWatts Default Range
PV module nameplate DC rating 0.95 0.80–1.05
Inverter and transformer 0.92 0.88–0.98
Mismatch 0.98 0.97–0.995
Diodes and connections 0.995 0.99–0.997
DC wiring 0.98 0.97–0.99
AC wiring 0.99 0.98–0.993
Soiling 0.95 0.30–0.995
System availability 0.98 0.00–0.995
Shading 1.00 0.00–1.00
Sun-tracking 1.00 0.95–1.00
Age 1.00 0.70–1.00
Overall DC-to-AC derate factor 0.77 0.09999–0.96001

Given these parameters, it is in every solar PV system owners’, and in every PV manufacture and integrators, best interest to make their systems and products as energy efficient as possible, while respecting affordability. As the above table shows, the three major inefficiencies currently limiting the end-use power of a solar PV system, in order from highest to lowest, begins at the inverter/transformer, continues with soiling, and ends with PV module nameplate DC rating (Energy, 2011). However, this chart significantly underestimates the potential negative effects that shading has on a solar PV system because it simply assumes all solar PV systems suffer no shading (I will touch on this shading issue later).

The primary focus of this research paper is not on solving all the inefficiencies and derating factors affecting a solar PV system – such a task would be impossible especially when you’re up against the second law of thermodynamics (Entropy) – but rather the focus is presenting solutions that have recently been developed, which can begin to decrease the significance of the derating factors, while increasing the future adoption of solar PV technology. Specifically, this paper attempts to tackle the inefficiencies associated with the inverter, and as a consequence of presenting new solutions we furthermore manage to decrease the significance the other derating factors in the process.

First, we begin with a brief overview of solar PV systems, next we go into an in depth focus on the advantages and disadvantages that stem from configuring a solar PV system under the micro inverter model versus configuration under the central (string) inverter model. Finally, I touch on market trends and speculate on the industries future.

Both inverter models, as will be shown, provide advantages and disadvantages whether those revolve around certain inefficiencies, reliability factors, economics, or functionality characteristics will be made clear throughout the paper. What is clear today, given the state of technology, is the existence of definite irreconcilable tradeoffs between both models when configuring the system. However, before I can begin writing about central and micro-inverters, I think a brief and general discussion is in order of the components that go into a conventional solar PV system for a home or business.

Photovoltaic System: Configurations

Commercially available since the 1970s, photovoltaic (PV) technology converts energy from solar radiation directly into electricity using semiconductor materials. It has no mechanical moving parts, so it lasts for decades and requires only minimal maintenance. Photovoltaic projects range from small-scale projects for lighting and pumping water to large-scale projects for whole buildings and even utility-scale photovoltaic farms (Sciences, 2011).

The question now becomes: What in the world is a solar photovoltaic system? A photovoltaic (PV) system begins with the solar cell, this is where electrical current is actually produced, at the atomic level, and the solar cell is made up of a semi-conductor material such as silicon, then it is placed into a framed module and covered with a later of protection. Then the solar module is commonly flush mounted onto a roof or it’s pole or bleacher mounted on the ground. The module, or the array (group of modules), is located – preferably – on a southern exposure (orientation), and is positioned at an angle (tilt), relatively close to or slightly below your locations latitude, this will give it access to the greatest amount of sunlight. However, their exists different strategics, other than capturing the greatest amount of annual radiation possible. Specific needs or strategies include maximizing power (for shaving peak demand), maximizing return on investment for areas with a time-of-use utility incentive, or maximizing space (the use of solar trackers would be ideal for the maximization of energy captured per unit area).

Once sunlight, or what are actually photons, strike the module, a percentage of that sunlight creates an electrical flow in the form of direct current (DC). That DC current flows out of the solar cells, into the conductors, then out the module, through a junction box and out of the array through the wiring. It then briefly flows through the combiner box or a DC disconnect, and then enters either a charge controller if you have batteries, then to a second DC disconnect box, and then finally into the inverter. Or if no charge controller/battery bank is present, the current may simply flow directly into the inverter input after passing through at least one, but sometimes two, DC disconnect boxes. The inverter transforms the direct current (DC), which by National Electric Code (NEC) cannot exceed 600 volts in residential or commercial properties, into alternating current (AC). From here the new AC power travels out the inverter wire through one or two AC disconnect then immediately into your buildings AC power distribution, or load, center. From there the AC is either used to power your electrical loads or sent to the utility grid, possibly earning the owner a credit or financial profit.

There are various applications for solar cells and PV systems, but for the sake of simplicity and length I will only mention the various configurations of PV systems based on the type of PV inverter chosen. There exist three mainstream subcategories of inverter PV system configurations: the stand-alone/off-grid inverter system, the (straight) utility-interactive inverter system, and the bimodal/grid-tied with battery backup inverter system.

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Direct-tied PV System

But before we go into inverter system, and as a side note, not every solar PV system requires the use of an inverter. The primary example is the solar PV direct system. It’s the simplest configuration available using PV and it’s very popular and has been around for many decades. Think of a solar calculator, solar attic fan, solar powered construction or traffic signs. These products have been around since the late 1970’s and consumers find them reliable and a useful application for solar power. The PV module directly feeds the loads with DC power, and so correct sizing is critical.

Moving forward, first there is the stand-alone system inverters that receive DC power from the batteries and operate independently of the PV array and the utility grid. Next, there are the utility-interactive inverters that are connected to, and work in parallel with, the electric utility grid. The DC coming into the inverter is transformed and interfaced with the utility grid AC power, making it consistent and synchronous. Therefore, if your PV system is not producing enough electricity to power all your loads then the utility grid electricity is supplemented into your home or business to energize the remaining demand.

Stand-Alone/Off-Grid PV System

On the other hand, if your PV system is supplying more electricity than what is currently demanded by your loads, at that point the surplus electricity is sent into the utility grid. From there your surplus theoretically provides your neighbors with electricity. In most cases, this state of affairs would be earning you credits from the utility company, assuming you have a utility-interactive PV system and your local utility has a net metering policy.

Straight Utility-Interactive/Straight Grid-Tied PV System

However, a major drawback of the utility-interactive inverter is that it is completely reliant on the availability of the utility grid, meaning that if for any reason your utility provider goes offline (power outage) then your utility-interactive inverter will shut off all power production from your solar array for safety reasons (anti-islanding protection). This drawback, of the utility-interactive inverter being completely reliant on the functioning of the utility grid, has helped lead to the invention of an inverter that can function without the grid. With that we have the introduction of a new inverter into the market, referred to as a bimodal inverter.

Utility-Interactive/Grid-tied w/ Battery Back-up PV System

Bimodal inverters, or utility-interactive inverter with battery backup, are unique in that they combine both the characteristics of both stand-alone and utility-interactive inverters. It can operate in either stand-alone or interactive mode, though not simultaneously (Dunlop, 2010). If a grid outage occurs, the inverter automatically shuts-off connection with the utility and switches operations from the battery bank to continue feeding power to the loads as if the grid never disappeared. And that’s the catch, unlike a straight utility-interactive system, a bimodal system needs extra components (batteries and a charge controller) to function. Usually this type of inverter device is popular where backup power supply is required for critical loads such as computers, refrigerators, water pumps, medical equipment, etc.

The above paragraphs provided a simplified explanation of how a typical solar photovoltaic system works in a home or business installation, and it had a general overview of the available inverter system configuration options. In truth there are many more complexities, differences, and even several more configurations when it comes to a PV system, or any other renewable energy system. However, what is common among all photovoltaic energy system is that not only do they produce clean electrons, but those electrons are flowing in a direct current (DC) and in order to be useful to the average residential, commercial, or industrial property, the electric current (essentially the movement of the electrons) needs to be transformed from DC into AC. One may ask: is this extra step in the process of transforming electricity necessary? Yes, the vast majority of our electrical loads function on AC power, not to mention that the entire electricity grid in the United States and the world; all power plants produce and export strictly AC power. The only exception in foregoing the purchase and installation of an inverter is if your electricity loads/appliances can function under DC power, which is rare because DC electronics are expensive and hard to find.

However, the story of the inverter does not end here.

Enter Solar Micro-Inverter

Through years of technological advancements; from the first conceptual undertaking (who knows how many years ago) to the developmental undertaking in 1991 by Ascension Technology, to the first working prototype in 1992 by ZSW labs in Germany. All the way to the solar micro inverters 1996 market introduction by NKF Kabel B.V. (OK4-100), and finally to the first two decades of the 21st Century (Katz). There now exists a new, fourth type, of inverting device – the solar micro-inverter.  This new inverter, sometimes referred to as the AC module inverter (if it is factory integrated into the module), or the solar micro-inverter, is both conceptually and physically a radical departure from the standard central inverter model (Katz).

In more detail, the AC module is essentially a normal solar panel with a micro-inverter imbedded into the internal wiring of the panel at the factory, making it a true AC module; however in most instances the micro-inverter component is purchased separately by the end-users. Regardless, whenever a micro-inverter is utilized that configuration necessarily becomes a strict utility-interactive configuration.

This new inverter has opened up the market, creating a new category of inverter to compete with the central, string inverter. Both are expected to compete neck and neck for top market share, and many speculate that eventually both models may end up dominating different sectors/niches: micro-inverters dominating residential installations; both inverters sharing the market in commercial; central, string inverters dominating larger solar farm and utility-scale PV power plant projects.

The micro-inverter performs the all of the various functions of a central inverter, except it performs these tasks on an individual, single module basis.  As most micro-inverters are separate from the module itself, they require installation on the racking system directly behind the module, and require a simple “plug and play” connection, virtually eliminating complicated DC circuit and wiring requirements. However, this market is rapidly changing; two examples of this change include EnPhase Energy, the leading micro-inverter manufacturer, and SolarBridge another major micro inverter manufacture. Both are now working with solar module manufacturers, to have their products integrated at the factory level. Westinghouse Solar already went to market with Emphases’ micro-inverters factory assembled back in 2010; more partnerships with other manufactures are planned. Next, SunPower went to market with an AC module partnering with Solar Bridge’s line of micro inverters. Overall, despite their small size, micro-inverters include many (and a few extra) of the important and sophisticated features of central, string inverters. These include solid-state design, anti-islanding, sine wave output, and maximum power point tracking.

As to the commonalities that both these inverter models share, easily the first feature is that they both have zero moving parts to their internal workings – solid state devices.

The solid-state inverter used in PV systems employ the latest in power electronics to produce AC power from a DC power source that is either a PV array or a battery bank. Inverters can use different circuit designs, switching devices, and control methods to affect the output waveform properties. Such properties affect the efficiency and quality of the AC power. As complete power conditioning units, inverters may also include functions for battery charging, monitoring, system control, and maximum power point tracking (Dunlop, 2010).

The second of these shared features includes anti-islanding protection, meaning that if the utility experiences an outage, the inverter automatically shuts down operations, stops supplying power to the grid, because it sensed irregular or insufficient voltage or frequency. The main reason behind the anti-islanding protection is in preventing electrocution hazards to any line workers attempting to restore power to the utility.

The third important shared feature is the sine wave output. The sine wave, being a pure wave, “is a period waveform the value of which various over time according to the trigonometric sine function, what is important about the sine wave is that is the same waveform used by the utility grid” (Dunlop, 2010).

The fourth important feature that most central inverters and all micro-inverters have is the maximum power point tracker technology.

A maximum power point tracker (MPPT) is a device or circuit that uses electronics to continuously adjust the load on a PV device under changing temperature and irradiance conditions to keep it operating at its maximum power point. Since they are connected directly to the array, all interactive inverters include MPPT circuits (Dunlop, 2010).

The graph below displays the traditional crystalline PV system IV (Amperage/Voltage) curve, depicting the normal range voltage and amperage production of a solar module. First, let’s go over the basics then the actual maximum power point.  Power is the product of voltage (V) multiplied by current/intensity (A/I) – P=V*I.  Mono or polycrystalline, essentially all crystalline solar cells change their voltage and current output based on a number of factors, the two primary factors being irradiance (intensity of light) and cell temperature.

Depending on how hot or cold the solar cell is proportionally affects the voltage; heat in these types of solar cells is analogous to heat in any consumer power electronic, it should be avoided because it will lead to inefficiencies. For instance, just like when heat slows downs the processing speed of your computer, which may lead to a system shutdown if the fan cannot provide sufficient cooling, in addition the overheating of a mono or polycrystalline solar cell will significantly lower its total voltage, though slightly increase its amperage, but regardless the overall total power output is decreased. The opposite is true in cold temperatures (<25°C/77°F) – voltage will increase, and if irradiance is maintained or even lower than STP parameters, the module nameplate rating still can be exceeded – a 200 watt module could be putting out 230 watts.

I-V Curve

This heat/inefficiency factor is overlooked by many in the PV industry, and many find it counter-intuitive that solar panels in San Francisco, CA or New Haven, NY are able to harvest more power out of there modules than from the exact same modules located in parts of Arizona (Llorens, 2008).

In the end, the maximum power of a module is just the highest amount of power (P) possible you can achieve at that particular point in time, by multiplying it’s voltage (V) by its current (I). Now the function of the MPPT device or circuit is to find that particular point where the amps and voltage will combine to produce the most power at that particular point in time. Now, given that irradiance is inherently unstable, the MPPT is continuous readjusting the point of max power production – always searching for the ideal resting spot on the IV curve. A central inverter will perform the MPPT for the entire array, while the micro-inverter will perform it for the individual panel.

In conclusion, these four commonly shared features – solid-state, anti-islanding, sine wave frequency, and MPPT – are just a few of the numerous, yet the most important, features that central and micro-inverters share amongst each other (Dunlop, 2010). However, with a more in-depth analysis into both technologies, the surface similarities begin to give way to stark differences, presenting the end-user with clear advantages, and disadvantages to both inverter options.

The remainder of the research paper will be tasked with presenting an ordered description of the general advantages and disadvantages of a solar micro-inverter PV system to that of a traditional central inverter PV system.

Advantages of Micro-Inverters Compared to that of Central Inverters

Productivity: Probably the most cited advantage, especially by the manufactures themselves, of the micro-inverter technology is its claim of increasing the amount of energy production and power output from each module compared to the energy and power production levels of central, string inverters. Micro-inverter technology claims to press out anywhere from 5% to 25% more power from an entire PV system under similar environmental conditions, than what would otherwise be captured under a central inverter configuration. This is consequence of both the micro-inverters ability perform several operations on a per solar module scale while simplifying system design, all of which stems from the unique ability of the micro-inverter to transform DC to AC power at the point of power/electrical production (near the solar cell location). Presented below are the prominent reasons why micro-inverters will increase the amount of end-use power a PV system user can utilize in their home or business.

The first reason follows from the micro-inverters ability to use the maximize power point tracking (MPPT) function on an individual per solar module basis, leading to greater precision. When compared to a central, string inverter, which is limited in pinpointing the maximum power point on just the whole solar array or series string, micro-inverters decentralize this task, improving MPPT’s accuracy by distributing this task to every module.  Therefore, if a single module is operating at a different power point (due to a number of reasons, for example it’s orientation, it’s tilt, or shading or debris factors) the micro-inverters MPPT can quickly and accurately adjust, capture the modules unique power signature and mix it in with all the other power signatures of all the other modules in that particular branch. This technological advantage becomes clearer when you understand it relative to a central, string inverter.

The central inverter can only use its inverting, including its MPPT, capabilities on the aggregate solar array or series string level. The vast majority of inverters have only one MPP algorithm/tracker; however a few manufactures are have introduced inverters with multiple MPP trackers placed on each string input; inverters have anywhere from two to six inputs depending on its size. The standard central inverter converts the DC to AC power at a single power point, regardless of the number of strings, while the other deploys MPPT on the each string input. Regardless, of which two inverters are chosen, several insurmountable problems and limitations could present themselves which a micro inverter system would otherwise solve. The first limitation being that all the strings – the entire array – should be facing the same direction/exposure (azimuth) and the same tilt (Llorens, 2008). Now with the previously mentioned MPPT string inverters, the various strings could be facing different directions and angles, but each module on the string should be nonetheless identical in both tilt and direction. In both cases, if these parameters are breached, power production is negatively affected because the MPPT will find and function at power point of the lowest producing module or string. Meaning that if one panel in the array, or the string, is shaded, the MPPT will be limited to the lowest producing one. Therefore long term energy production is hindered, negatively affecting the economic payback schedule (Katz).

As already noted, the micro-inverter converts the DC power from a single solar module to AC power. When connected to a central or string inverter, modules are typically connected in series, however with micro-inverters, the modules are all connected in parallel (or what is termed a ‘branch’). The end result being that the previous central and string inverter limitations are overcome; each module on each branch can have its own unique direction and tilt, without ever effecting the power production of all other modules on that branch – problem solved!

This presents us with the second reason for productivity increase with the micro-inverter, and that’s its ability to solve the “Christmas light effect”; when one solar module goes out or is affected, they all go out or are affected. What this means, in terms of productivity, is that when a solar module in a normal series connection is negatively affected – say due to shading, internal cell malfunction, or physical damage – the entire string suffers. Therefore, the amount of power (volts and amps) coming out of string is limited by the lowest performing panel on that string – this could range from a few power percentage losses to complete power failure/loss. For example, shading of as little as one tenth of the entire surface array or string in a typical central or string inverter PV system can, in some circumstances, lead to a system-wide power loss of as much as one half. If this is a recurring problem then lone term energy harvest by the PV system will obviously suffer, maybe then will a solar micro inverter we well worth the purchase.

As mentioned, the micro-inverter commonly solves this and other problems through a distributed approach to inverter technology that helps reduce the effects of dust, debris, and shading on the array, and reduces other more serious factors that would otherwise negatively affect the entire PV system, such as module mismatch, balance of system (BOS) equipment defects,  or inverter failure. Furthermore, the micro-inverter continues to solve these problems through parallel connections, and from single panel AC power production, allowing MPPT to be performed on an individual solar panel basis, rather than being determined by the array’s lowest performing panel’s power point intersection.

The last third and fourth prominent reasons why micro-inverters increase any PV systems productivity are because of their slight advantage in both conversion efficiencies and wiring configuration. First, the average conversion efficiency of an individual consumer micro-inverter is 95.5% with a range of 93 to 98%, which is slightly better than traditional central, string inverters with 92% average efficiency in the conversion process, with a range of 88 to 98% (Energy, 2011).

Turning the focus to the BOS characteristics of a micro-inverter PV installation, we noticed that all DC wiring components are replaced with AC wiring; AC wiring is much more efficient in carrying electricity thus leading to higher productivity out of the PV system (this wiring advantage is further elaborated on in the “Simplified Installation and System Design” section below). The DC wiring requirements and regulations, enforced by the National Electrical Code (NEC) – which is a set of rules widely adopted by many jurisdictions expounding on safe practices for the installation of electrical equipment – are not only highly technical and restrictive, can be bypassed entirely when utilizing the micro inverter (Dunlop, 2010). Lastly, DC wiring is inherently prone to higher voltages drops compared to AC wiring.

Voltage drop can be an issue in any electrical system, but it is particularly important to address in PV systems – voltage is important for starting and keeping an inverter on and running. The PV output circuit may have a relatively low voltage and the conductor runs can be long, increasing voltage drop. Excessive voltage drop can also affect the proper operation of charge controllers, batteries, loads, and other devices that require specified voltages (Dunlop, 2010).

In the end, the longer the DC current travels the more power is lost as waste heat through the wires, also factors such as the wire thickness, and the material used can all lead to unwanted voltage drop, adding to larger derating factors, not to mention higher total costs and lower energy harvest. Furthermore, the inverter itself has to use a percentage of its input electricity in order for it to perform its intended function. Overall, the AC wiring scheme a micro inverter creates will lead to a simple configuration resulting in more power and energy being utilized by the end-user.

Reliability/Availability: Micro-inverters are more reliable and available, meaning they experience less down time, compared to the average central, string inverter. In a central inverter based PV system all the module output wires, after being strung in series and parallel at a combiner box, are then connected into the single, central inverter input. Now if for some reason there was a partial malfunction or complete inverter failure, it would necessarily follow that the entire PV system would shutdown, though the modules would still be producing power, that power however would be left unconverted and unused (unless there was a DC lead off wire/sub-panel that powered DC electronics).  Rather, under the micro-inverter model each module would be self-sufficient and a single inverter failure affects only that solar module – not the branch, not the entire solar array or PV system, electricity would still be powering your loads or running your meter backwards! The micro inverted system has multiple points of failure compared to the central inverter and not every micro inverter will fail at the same instance. Some may view the increase of failure points as a disadvantage, but it’s important consider that each failure point has minimal impact over system reliability and availability. Would you rather have less failure points but with grave consequences or more failure points with minimal consequences suffered by the system? In summary, the end user of a micro inverted PV system will experience longer system up time and greater mean time between inverter failures all because of it greater reliability coupled with a greater energy harvest potential (Llorens, 2008).

Lifespan/Warranty: Today the life of solar micro inverters is much closer to the lifespan of the typical solar module power warranty, and this trend is continuing through annual innovations.  The typical solar cell and module will theoretically a lifetime, as long as the crystalline material is protected and intact. Therefore, what accounts for the vast majority of the decreased power production of a solar cell and module over the decades is a result of the breakdown of the all the components that go into it a module except for the crystalline cell: materials such as all the different metals, the protective glass, and adhesives. Nobody really knows how long a solar module, under a mild climate, will last.  Most estimates are largely based on the environmental conditions, for example solar modules that were manufactured and installed in the 1970’s are still producing clean electrons today. Of course, their efficiencies have lowered over the years, but they are still churning out clean electrons using 1970’s technology!

Today several micro-inverters manufacturers have placed 10-year warranties on their 2nd-generation micro-inverters, 15-20-year warranties on their 3rd generation micro-inverters. Two major manufacturer of micro-inverters, EnPhase Energy and Sparq Systems Inc., have in the past year announced 25-year warranties on their new line of micro-inverters (3rd and 4th generation), many other micro inverter manufactures are catching up or have already reach the 25-year warranty mark (Jain, 2010). Compared to the micro-inverter, the typical central, string inverter typically comes with a 10-year warranty, yet the overall industry numbers do range from as low as 5-years to the highest set at 20-years under a extended warranty option.  It’s clear from this assessment that micro-inverters not only have a clear edge with respect to the durability of their equipment, but given the fact that the micro-inverter technology has only been commercially at market for a tiny fraction of the time (2007) relative to how long the central, string inverter has been at market. Given these facts, still the micro-inverter has managed to advance and surpass central inverters in the category of lifespan, reliability, and warranty is a testament to the technologies superiority and potential (Llorens, 2008).

Simplified Installation and System Design: Installing a micro-inverter PV system makes the design and installation easier and faster than ever before, and leads to lower costs compared to central, string inverter installations – though there is still legitimate dispute over this claim.

The micro-inverter installs quickly using the existing PV racking system, so there is no worry about installing a bully central, string inverter, while simultaneously finding an indoor space that is well ventilated and temperately cool enough to house the inverter. Micro-inverters simply install behind each module, or pairs of modules (when using a dual micro inverter), and are well protected against most environmental hazards. To ensure proper installation, most manufactures include all the necessary hardware in the form of nuts and bolts, and to ensure protection cable clips are included.

Furthermore, they connect at AC branch circuits, eliminating the need for complex string design, sizing, possible conduits runs, DC combiners boxes, DC disconnects, high voltage fuses or circuit breakers. This simplification and installation advantage opens the door too more do-it-yourself folks and homeowners to forgo costly installation costs and undertake either a faction or the entire PV system installation process themselves.

Flexibility/Scalability: Another advantage of the solar micro-inverter design is the potential for PV system installations to be expanded over time – its scalability. An initial set of solar modules can be installed and additional modules added as energy needs and budgets grow without requiring the replacement of a large centralized, string inverter. Let’s say you, the homeowner (who wants to 100% solar electric), currently has a PV system that meets all your electricity needs on an annual basis, but well before you installed the PV system you had decided you wanted to build a new section onto your home three years from now, but you don’t want to oversize your current PV system as of the initial installation because you do not want to overproduce with your solar PV system because your utility won’t cut you a check for excess production, just credits that typically expire at the end of the year.

Consequently, due to the nature of the traditional one sized central, string inverters you are encouraged, being that it is the most reasonable course of action, in purchasing an inverter with the capability of handling your current production capacity, the main reason being that string inverters work better when input energy is equal to, or fairly near (75 to 125%) of the inverters power converting rating. Therefore, over-sizing your inverter, under the thought of adding 2000 DC watts over a few years, would cause your inverter to function at suboptimal efficiency. Unless you decide to pay for a whole new inverter when your PV expansion comes, you are stuck with a lackluster over-sized inverter. Once again, ideally, for any PV system the power coming into the inverter should closely approximate the inverting converting capabilities and power rating. For example, a 2kW rated inverter should have anywhere from 1.75 to 2.10kW DC of power being pumped into the inverter input circuit, in order for the inverter to function at its optimal level.

However, the micro-inverter application eliminates this limitation in all forms, allowing the PV owner the freedom to just slap on another solar module with an attached micro-inverter – roof and/or yard space is your only limiting factor at this point.  If at all possible you’d want to get your PV system sized right the first time to avoid having to increase the racking system and the number of roof penetrations, but the micro-inverter route is still far more scalable and flexible than a string inverter.

Furthermore, with central, string inverters you have to adhere to a strictly controlled string design and an upgrade or panel expansion would require a number of new things, from a different and larger inverter to a newly designed string configuration to accommodate any and all new modules.

Anywhere from 1 to 17 to 24-modules under the micro-inverter application can be linked in parallel per AC branch circuit depending on the modules nameplate rating and the micro inverter manufacture specifications. That does not mean you can only have only 17 to 24 modules maximum, rather you can have literally any number between 1-24modules on the AC branch circuit, from just one branch to however many your electrical service will accommodate – usually a typical residential home is limited to accepting/handling up to 200 amps at 240 volts (Llorens, 2008).

A major problem the majority of PV system owners and integrators continue to run into is the problem of module mismatch. Mismatches within a PV system primarily stem from mismatches between a solar module nameplate rating – just like any product, no two modules are exactly alike when they come off the assembly line.  Whether it’s a mismatching in wattage, panel efficiencies, or manufacture, mismatching will lead to power production losses, due to a lower IV curve, that on first glance may be perceived as a miniscule and acceptable loss, but over the longer haul these losses amount to a substantive loss of energy production and money in the bank. Therefore, when you’re in a situation in which your particular type of installed solar module model is discontinued, or worst yet, the manufacture went under (think Evergreen Solar August 11, 2011 filing for Chapter 11 bankruptcy), you can easily switch over to another model or manufacture without any power losses, assuming you connect on a micro-inverter to the back of it. The reason this is possible is because, once again, all the energy and power coming out of that particular panel is autonomous from all other modules, and it’s house-ready load-friendly AC.

Safety: Safety is another category the micro-inverter has an advantage under.  Under this application, your PV system is no longer the traditionally based DC-to-AC-based system, rather now it is strictly an AC-based PV system.  In an AC-based system each module is a utility-interactive unto-it-self, and thereby shuts down each and every module if there’s a fault in the wiring system or a utility outage. In contrast, DC wiring in a conventional solar PV system is energized at high voltages as long as the sun is shining, and as a result, a malfunctioning fault or connection point can create continuous arc faults, increasing the potential for fires or electrocution.  This reality has lead the National Electric Code to mandate that all DC or AC based PV systems come standard with some sort of arc fault detection and interruption device to prevent these dangers. This new regulation has a price tag attached to it, either stemming from manufactures having integrate this new feature into their product or with installers who have to manually add this new feature into each installation. However, even as the costs of both inverter systems have slightly increased as a result, the micro inverter system has virtually no DC wiring – just the two wires coming of the module. Therefore, will less DC wiring, and less DC wiring at a lower voltage, you therefore have a safer solar PV system configuration resulting in a lower risk for fires or electrocutions (Larsen, 2010).

Once again, with the transition to a AC micro-inverter based PV system, high-voltage (600 volts) strings of DC power coupled with a complex, thick, and expensive DC wiring and string design on your rooftop is eliminated – replaced with household safe AC that is traveling through a simpler designed and thinner sized wire. All of these advantages slightly reduce system costs while dramatically reducing the risks of electrocution and fire.

Monitoring: EnPhase Energy, seen below, was the first company to introduce both the hardware and software necessary for monitoring the PV array. Soon other micro-inverter manufacturers adopted this new feature, and today it is considered a standard feature with any micro inverter system. The monitoring capabilities allow the end-user to access data from each and every micro-inverter, or the solar array as a whole. Things like power production/conversion for the hour, day, month, or year are displayed; with the data continuously being updated every 5 minute interval. If there is a problem with a particular panel, caused by either shading or internal malfunctions, the monitoring hardware will pick it up and relay the information to the software program available to you on your PC or smart phone. As of today, EnPhase Energy, and other micro-inverter companies, have these PV monitoring applications available for your iPhone or Android smart phone. Furthermore, for added assistance, you can set your monitoring software to automatically contact either the manufacture of the micro inverter, your solar installer/maintenance provider, or yourself via email or phone call, if a problem develops within your PV array.

EnPhase Energy Enlighten Mobile Monitoring Software

In contrast, the end user of an average central inverter monitoring system unable to neither monitor a particular modules power production/conversion nor zero-in a point of failure in the PV array. Rather the software will simply alert you of an existing problem, while leaving you guessing as to which string or panel has the issue. It’s kind of like the check engine light found on the average vehicle, which informs you a problem exists, but doesn’t tell much else without professional assistance. However, there are solutions to fix this monitoring problem found under the central inverter modal, but most central inverts do not come standard with any monitoring hardware or software – a separate purchase is necessary for this feature.

Moving on, soon after micro inverters began to be accepted by the industry in the late 2000’s, several new companies have come out with DC-DC power optimizers. These devices are similar to micro-inverters in that they apply the maximum power point tracking on each module, they allow the user to monitor and track all the various system and module diagnostics through the hardware and software, and however they are not inverting devices, so the modules power still requires the presence of a central inverter at the end of the string/array.

Reduction in Life-cycle Costs: There exists immediate, intermediate, and long-term cost savings associated with the solar micro-inverter installations. The immediate advantage, or cost savings, the micro-inverter application brings to any PV system includes no longer having to pay for DC wires, DC wire installation, string design, and all the other necessary DC components, such as DC disconnects or redundant fusing requirements (Llorens, 2008). A few indirect cost saving (or avoidance’s) include forgoing having to designate an ideal space for the big central inverters, whether that is no longer having to find a space inside having proper ventilation, or a shaded area outside for installation. No more heavy, two person, lifting is required with the light weight micro inverters (<5Ibs) – possible injuries are here avoided.

Over the intermediate life of a micro-inverter versus central, string inverted PV system, the life-cycle cost advantage lies with two things: avoided inverter failures and avoided cost due to possible expansion. Firstly, the avoided cost of system failure, or replacing an inverter every 10 to15-years with a central, string inverter versus every 15 to 25-years with either a 2nd, 3rd, or 4th generation micro-inverters will save equipment costs, installation costs, and electricity costs (one micro-inverter failure will not shut down power production in the entire system as would be the case with a central, string inverter failure). Secondly, if the time comes for an array expansion, especially a slow expansion of one or modules every year, micro-inverters can be purchased individually to accommodate your changing needs and growing energy needs, without having to buy a whole new larger and more expensive central, string inverter.

Taking a long-term perspective, a micro-inverter based PV system has been field tested by both the industry and independent testers and analysts to demonstrate that this technology does allow each module, and therefore your overall PV system, to produce more kWh monthly, annually, and over its entire lifetime compared to the central, string inverter system, even though the upfront costs of the micro-inverter system are higher, payback is guaranteed sooner and many times over because more energy is harvested out of each day, month, and over the lifetime of the products and system.

Disadvantages of Micro-Inverters Compared to Central, String Inverters

Upfront Cost: The primary counter force against the micro-inverter based PV system in the vast majority of residential, and a few commercial scale installations is the higher upfront costs/investment. Traditional central, string inverters range between 35 to 50-cents per watt and this estimate depends on the size, technological capabilities, and the brand name of the inverter. Contrast this with the micro-inverter, which brings a cost range of between 75 to 90-cents per watt; again depending on size, technology, and brand name. According to another source, as of October 2010, a central inverter costs approximately 40 cents per watt, whereas a micro-inverter costs approximately 52 cents per watt. Overall, this cost difference is viewed as the primary disadvantage of micro-inverters (Abhishek, 2011). Take note that these higher micro inverter costs are coming down every quarter due to economies of scale and the overall growth of the solar industry.

Placement: Micro-inverters are placed on the panel racking system directly underneath their allotted solar modules.  A small faction within the solar industry makes the argument that the micro-inverter location causes problems. The argument goes: The micro-inverter, being placed near, or on, the hottest part of the PV system, the module, will suffer overheating on hot days and since it is exposed to all the other forces like damage causing weather conditions or even damage from creatures of mother nature, its proper functioning will be affected, therefore the chances of the micro-inverter under performing or even failing is vastly increased.

Realistically, micro-inverters are exposed to more heat and other weather conditions than a central, string inverter would ever be, this includes a combination of heat radiating off the back of the solar module, and heat radiating off from the roof, also rain, snow, high winds, debris, and animals can each play a factor. There is no doubting, the micro-inverter has to contend against many more elements than its counterpart the central inverter.

On the other hand, many other solar industry experts argue against the micro-inverter naysayer’s. They argue that given that the micro-inverter is protected and shaded by the solar module, with plenty of ventilation due to space most racking systems provide it, the inverter will remain safe and performing at optimal levels. Still, the placement of the micro-inverter, compared with the conventional central, string inverters, leaves it exposed to much more extreme and perilous environmental conditions. This could lead to replacing micro-inverters on a more frequent basis, even though most warranties would cover these costs.

Installation: The initial and actual installation of the micro-inverter over a central, string inverter is commonly argued as being fairly equivalent, though a reasonable argument can be made that the micro-inverter presents a few extra steps, therefore many argue this is a disadvantage.  For instance, there may be ten or twenty micro-inverters instead of one inverter to be installed; yet to counter this is the foregoing of string designs, DC wiring, and DC components with the micro-inverter.  Regardless, under the micro-inverter model your maintenance fees may eventually either surpass that of a central inverter or become the largest part of your cost over the long term. This is because you have more points of failure; one small failure may lead you to call the maintenance company, which is always followed by a fee.  Furthermore, depending where your solar array is (on the roof), and where the failed micro-inverter is (surrounded by modules on each side) your fees could have a large range – becoming unpredictable at times (Larsen, 2010).

Limited Application: As of this writing, all utility-scale PV installations that have exceeded 1MW in size, have not utilized the micro-inverter technology.  Essentially, micro-inverters lack both market demand and are perceived by many in the industry as failing to add much value in these large utility-scale PV projects compared to central, string inverters. Under further analysis it will be clear that the latter is causing the former. But in the end, this “perceived” disadvantage simply could disappear once someone decides to use solar micro inverters for their utility scale project – actually collecting hard data to put disputes to rest.

Obviously with the high costs associated with any large scale PV project, a huge amount of time and money is invested in the planning and design stages. Therefore, the vast majority of these utility-scale PV installations are planned so perfectly that most of all the possible negative contributors to power harvesting, such as shading, tilt, or orientation, or any other disadvantages, are entirely eliminated or largely minimized. Therefore, under the perfect site conditions, that these installation operate under, the advantages of the micro-inverter fade and are now equitable to the central, string inverter – remember micro-inverters perform at their best if shading is a major issue for the array; in this case it is not. In these types of projects, the decision on what category of inverter to utilize no longer rests on performance advantages, but rather is based on the up-fronts costs – so bids can be won. As was previous stated, overall micro-inverters are more expensive than your typical central, string inverter system.

Furthermore, as the derating factor numbers have it, a central inverter and micro-inverters, under perfect conditions – as would be the case in most utility-scale installations – perform under similar efficiencies – 94 to 96% – again this fades any stark differences between the inverter models in utility scale installations. However, if the installation location presented problems of shading or debris (like sand from a desert), micro-inverters, whose most important attribute is their ability to minimize power loss due to these exact variables, would produce a lot more energy than a conventional inverter, therefore making bring them back into serious consideration as a cost competitive option.

Market Trends and Future Thoughts

        In my opinion there is just a hand full, among the several dozen or more, of solar micro inverter companies that I believe will both capture major market share and lead the future in this emerging technology. Several of the few contain some aspect of their technology that help to differentiate themselves from the larger market, while oddly enough one of the few has adopted features that are currently not in high demand, but which are nonetheless very forward looking.

The first company to look out for – yes you guessed it – EnPhase Energy. Being the largest and the first company to make these inverters a popular and viable alternative since their inception in 2006, they have been the major pioneer in the features of the micro inverter. They have helped to advance key features in hardware (materials) and durability (warranty), software and communication features, customer service and technical support, and aesthetics all while making their product easier to install. Now, being the biggest in the pack has its advantages – customer trust, experience, and large manufacturing capability. Therefore, I see EnPhase continuing to lead the pack in for the next few years, but unless they reexamine their use of the electrolytic capacitors, and move to phase this component out, EnPhase could possibly be hampered down both in recalls and a marketing strategy to play down industry and public concerns of such components.

On the brighter side, in the past two years, EnPhase has partnered with module manufactures such as Westinghouse Solar, Upsolar, and Hanwha Solar One in order to integrate their micro inverter into their modules on the factory floor (Korosec, 2011). Next, we are seeing many mounting and racking systems being designed to be compatible with micro inverters. For example, Zep Solar mounting and grounding systems have teamed up with EnPhase, and both have products out that help to synergize their products (Korosec, 2011). Finally, EnPhase Energy, now in their 6 years of business, announced this last September that it had shipped its 1,000,000th AC micro inverter unit. As of late 2011, EnPhase claims to have about 15 to 20% of market share for US residential inverter installations below 10 kW (McCabe, 2011).

Next, we have Enecsys Micro Inverters, a company founded by a graduate of Cambridge in the United Kingdom. Encesys Micro Inverters is an exact replica of EnPhase Energy, offering pretty much everything the latter has except for the made in USA mark. However, what gives Encesys an edge over EnPhase is, “a unique and patented energy storage technique that enables the use of thin film capacitors instead of less reliable electrolytic capacitors and the elimination of opto-couplers” (Encesys MI, 2011).

SolarBridge’s Pantheon AC Module

This company has been a big player in the European micro inverter market (which EnPhase is just now entering) for quite some time, and as of 2010 they have entered full force in the North American market opening an office in both California and Ontario. This company, once its production is ramped up, would be the ideal replacement to fill an EnPhase Energy void, if that day ever comes.

Another company making headlines is SolarBridge Technologies. SolarBridge Technologies is based in Austin, Texas, with an R&D office in Champaign, Illinois (Green World Invester, 2011). They sell their Pantheon micro inverter with a 25-year warranty. In the second half of 2011, heavy weight technology leader and module manufacture SunPower announced and launched a series of AC solar modules utilizing SolarBridge’s line of micro inverter technology. Many industry insiders assumed SunPower would partner with EnPhase Energy; however SunPower cited serious worries of any micro inverter using electrolytic capacitors. Such capacitors have a tendency to leak or prematurely fail. SolarBridge’s engineers, similar to Encesys, managed to engineer a line of micro inverters without these electrolytic capacitors. However, unlike most manufactures, SolarBridge’s takes a different approach by only selling their units to module manufactures, to be embedded at the factory – this will definitely limit their market penetration and share because they can’t sell to the everyday consumer. AU Optronics (AUO), a module manufacture originating from Taiwan, also uses SolarBridge’s line of micro inverters to complete their AC modules.

Power-One, one of the largest power supply manufacturers of renewable energy and energy-efficient power conversion and power management solutions in the world and recently the world’s second largest provider of solar inverters, has entered the solar micro inverter game. In October 2011, Power-One launched its Aurora Micro-0.3 300W micro inverter as well as its Aurora OPTI-0.3 DC/DC power optimizer. This micro inverter is simply immaculate in its engineering and features, however, it’s one disadvantage is its lack of applicability. The current situation is that the vast majority of modules manufactures do not have a 300 watt or plus module in their product line, so this micro inverter will have trouble selling. To counter this, Power-One is expected to launch Aurora MICRO-0.25, a 250W micro inverter in first quarter 2012.

Aurora Micro-Inverter

Regardless, since Power-One is the only one making micro inverters capable of handling these high wattage modules coupled with the growing consumer trend of seeking out and purchasing ever higher wattage modules – which saves on space and installation costs – Power-One has strategically placed itself for future success in capturing this future market. As an already successful international company it has both the money and patience to invest and wait for the market to come to them.

The final solar micro inverter player any solar industry enthusiastic should take notice of is whom I consider to be the sleeping giant in the room: SMA Solar Technology AG, the market leader in a $7 billion dollar photovoltaic inverter market. In September 2009, SMA acquired Dutch company OKE-Services (OK4U) one of the first makers of the module inverters (micro-inverters) technologies with over fifteen years of experience (Cleanergy, 2009). Speculations and rumors spread, but it was obvious that SMA wanted to ticker and toy around with this technology and see if it do it better. Finally in September of 2011, SMA announced that it would be unveiling its first micro inverter, a 240 watt unit, at Solar Energy International in Dallas the following month. This micro inverter is set to reach the commercial market by June 2012.

This makes SMA the only manufacturer in the world with a product portfolio which includes all existing inverter technologies for operating photovoltaic systems of any size and with optimal technical system configuration. SMA customers will continue to profit from highest quality standards, a global sales and service network and short delivery times (Cleanergy, 2009).

SMA Micro-Inverter

SMA’s global reach and reputation will help piggy back its line of micro inverter in capturing a good size of the market.

The previous five solar micro inverter manufactures, I believe will either continue leading the pack as a few already are, or in the case of Power-One and SMA, will emerge in status, continue innovating, and help to consolidate and mature the market. Next, I would like to briefly present just two potential setbacks the industry may face in the near future.

Currently, we are seeing a growing number of manufactures offering a 25-year warranty, matching module warranty with the inverter is a very good marketing strategy. Yet, I don’t see any manufactures in the near future offering anything longer, such as 30-year warranties. However, I would go as far as saying that the 25-year warranty may be an exaggeration on the part of several of these manufactures. Many made the 25-year warranty leap of faith just to stay competitive in the market, so their claims sound baseless.

As a consequence we could see a reduction back to a 15 or 20-year warranty period.  Even with 20-year warranties, one still has a very long mean time between failures (MTBF). Numbers were presented by various manufactures, in the 400 to 500 year ranges (McCabe, 2011). However, with the high operating temperatures of these inverters due to heat exposure from the sun and the hot to cold thermal cycling in many locations, these MTBF’s are highly questionable. Hopefully, the solar micro inverter industry will become savvier in estimating product lifespan, while adjusting its marketing of MTBF’s in the future. If by chance a few thousand of the already millions of micro inverters already in use begin to fail before their specified warranty is surpassed then we could be a huge shakeup, both the companies involved and the technology reputation as a whole will suffer great damage – prudence is require here, but it’s easier said than done.

Another setback to beware of is the problem of copy cats. These micro technologies are easily duplicated. Many in the developing nations have their electrical engineers working tirelessly in reverse engineering this technology. People are already reporting about seeing Enphase knock-offs from China, everything the same, except the very important aspect of quality (McCabe, 2011). The industry should come to consensus about how to properly label and deal with any rogue micro inverter manufactures, while being cautious not to disparage legitimate competitors.

Beyond these two theoretical setbacks, the industry as a whole is expected to undergo the well known symphony of massive influx of companies, growth, bust and contraction, consolidation, and hopefully maturity, but hopefully not monopoly. Overall, I cannot find a counter argument to this statement: Over the next decade the inverter world is set to immensely transform. Furthermore, according to the folks at EnPhase Energy, the micro inverter will be 11% of all worldwide solar inverters by 2014, which demands that those in the solar industry need to pay serious attention to this technology and all other market trends which are reshaping the PV industry.

Bibliography

Abhishek, P. (2011, April 14). Solar Central Grid Tie Inverters vs. Micro inverters – Advantages and Disadvantages. Retrieved from Green World Investor: http://www.greenworldinvestor.com/2011/04/14/solar-central-grid-tie-inverters-vs-microinverters-advantages-and-disadvantages/

Cleanergy. (2009, September 9). SMA Solar Technology AG Acquires Micro-inverter Technolog. Retrieved from Cleanergy.org: http://cleanergy.blogspot.com/2009/09/sma-solar-technology-ag-enters-micro.html

Dunlop, J. P. (2010). Photovoltaic Systems. Orland Park: American Technical Publishers, Inc.

Encesys MI. (2011). Technology Overview. Retrieved from Encesys Micro Inverters: http://www.enecsys.com/technology/index.php

Energy, U. D. (2011). NREL: PVWatts. Retrieved October 1, 2011, from http://www.nrel.gov/rredc/pvwatts/grid.html

Green World Invester. (2011, April 14). Solar Micro Inverter Market – Small Size but Huge Growth Ahead,Solar Microinverter Companies. Retrieved from Green World Invester: http://www.greenworldinvestor.com/2011/04/14/solar-micro-inverter-market-small-size-but-huge-growth-aheadsolar-microinverter-companies/

Hahn, D. (2008, August 23). Microinverters: A major advancement in solar energy technology. Retrieved from Solar Power Rocks: http://solarpowerrocks.com/solar-technology/microinverters-%E2%80%93-a-major-advancement-in-solar-energy-technology/

Jain, P. (2010, July 15). Photovoltaic micro-inverters: Advantages and challenges for rooftop PV systems. Retrieved from Solar Server: http://www.solarserver.com/solar-magazine/solar-interviews/solar-interviews/photovoltaic-micro-inverters-advantages-and-challenges-for-rooftop-pv-systems.html

Katz, D. (n.d.). Micro-Inverters and AC Modules. Retrieved from AEE Solar: av.conferencearchives.com/pdfs/091001/33.1308.pdf

Korosec, K. (2011, October 18). Trade Watch: Micro Inverters Invase Solar. Retrieved from Smart Planet: http://www.smartplanet.com/blog/intelligent-energy/trend-watch-microinverters-invade-solar/9707

Larsen, K. (2010, June 23). Micro-inverters vs. string inverters for solar PV. Retrieved from Renewable Energy Focus: http://www.renewableenergyfocus.com/view/10472/micro-inverters- vs-string-inverters-for-solar-pv/

Llorens, D. (2008, September 9). Micro Inverters (My Turn)- the boring technical stuff. Retrieved from Solar Power Rocks: http://solarpowerrocks.com/solar-trends/microinverters-my-turn-the-boring-technical-stuff/

McCabe, J. (2011, May 1). PV Micro Inverters and Optimizers not just for lazy designers. Retrieved from Alt Energy Stocks: http://www.altenergystocks.com/archives/2011/05/pv_micro_inverters_and_optimizers_not_just_for_lazy_designers.html

Sciences, N. I. (2011, August 5). Photovoltaic’s | Whole Building Design Guide. Retrieved from The Whole Building Design Guide: http://www.wbdg.org/resources/photovoltaics.php

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2 thoughts on “Micro-Inverters: Advantages and Disadvantages in Solar Photovoltaic Systems.

  1. With the introduction of a Do-It-Yourself (DIY) solar panel kits, it has become easier to install solar panels by an individual with no training in this field. These DIY kits can be used in households where energy requirement is less.

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