Time for a Change: Micro-inverters Improve Performance of Solar Systems
There are many challenges in current solar technology. These challenges include the inefficiencies in photon-to-electron conversion, system reliability, and the difficulties in installing and managing solar installations. The majority of current research and development funding is looking to address the first issue by focusing on improvements in solar module technology and manufacturing. While this is important to drive down the cost of solar modules, there are additional opportunities for improvement in solar system performance. Re-thinking the role inverters can play in solar installations is one way to address the challenges associated with energy harvest, reliability and management of solar systems.
Inverters perform two key functions — converting the DC power from the solar modules into grid quality AC power and performing Maximum Power Point Tracking (MPPT) on the solar array. MPPT is the algorithm that extracts the maximum amount of power from the solar array.
Traditional centralized inverters manage all the solar modules in an array as an aggregate — a single source of energy. But there are limitations in using a single MPPT for multiple solar modules. Non-uniform changes in temperature, irradiance and shading create complex heterogeneous current-voltage curves, making it difficult for an MPPT algorithm to operate efficiently. This can result in the conversion of less than the maximum power available from the solar modules. An associated issue is that a traditional inverter represents a single point of failure in the solar system. A final limitation of traditional inverters includes noise pollution, space constraints and aesthetic issues. The larger the solar power system, the larger the inverter that is required, sometimes requiring a separate facility that must be constructed, powered, cooled and maintained.
The Micro-inverter Systems Approach
The idea of a micro-inverter has existed for many years — using multiple small inverters instead of a large, centralized inverter to distribute the power conversion of a solar installation. Early designs met with limited success because they failed to achieve the efficiency, reliability and price point required to achieve regulatory and commercial viability. More recent designs such as one from from Enphase have taken these lessons to heart, and micro-inverters are being developed and tested to meet the industry regulations and customer requirements. What is emerging is a micro-inverter that produces more energy, is more reliable and less expensive to install and maintain than traditional inverters.
Harvest Gains, Reliability, Design and Installation
Micro-inverter technology offers significant technological advances. The first is per-module MPPT, which ensures energy harvest is maximized. With traditional inverters, the performance of the entire array is degraded if one or more modules becomes dusty, covered in debris or shaded. Micro-inverters enable each module to perform independently within the solar array. This benefits the system owner by maximizing energy harvest since degraded performance from one solar module will not prevent the rest of the modules from producing their maximum energy. The per-module micro-inverter also eliminates the problem of reduced energy harvest due to module mismatch. Tests have shown an increase in energy harvest in the order of 5 to 25%.
As with the migration from a central mainframe to personal computers in the information technology industry, the distributed nature of the micro-inverter system significantly alters the landscape of the solar industry. A centralized inverter failure in a traditional design renders the entire solar array useless until a replacement can be ordered and installed. With a micro-inverter approach, a single inverter failure only affects the module to which it's attached, and contributes to an insignificant degradation in total system output. This means that micro-inverters can be replaced at the convenience of the installer or during a regularly scheduled maintenance visit. In addition, no specialized personnel, equipment or tools are required to replace a micro-inverter. This distributed approach can achieve system availability of greater than 99.8 percent — a key factor for commercial systems.
The micro-inverter approach also promises to increase design flexibility and lower installation costs. Micro-inverters are connected in AC branch circuits eliminating the need for expensive DC combiner boxes, disconnects etc. This means that installers no longer have to deal with restrictions of string design or with the headache of locating and installing a large traditional inverter. Individual solar modules no longer have to be "matched" because each module is an independent energy producer with its own MPPT. Use of micro-inverters eliminates the hassle of working around varying module ratings and standardizes the installation process. The per-module inverter approach means that installers have the flexibility to install solar modules on any available roof surface without the restriction of coplanarity.
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. The tool is able to compare modules and if it notices an energy harvest issue, it uses built-in algorithms to analyze the cause of the issue and whether the owner or installer needs to be notified.
With a systems approach to solar generation that delivers greater productivity, higher reliability and smart management, micro-inverter systems significantly improve the performance of solar systems.
Raghu Belur is a co-founder and vice president of marketing at Enphase Energy Inc. He has 15 years of experience in engineering and engineering management. He began his career at the Indian Institute of Science where he worked on a team developing an alternative energy gasification system. He was an early engineer at Cerent Corporation where he was responsible for the development of the optical 2.5Gig interface for the Cerent-454 ADM. Cerent was acquired by Cisco Systems in 1999. At Cisco, Raghu was managing the team developing the 10Gig interface products. He co-founded Enphase Energy Inc in March of 2006. Raghu has a MSEE from Texas A&M University.
From Jonathan Cole:
Excellent, Raghu! I have been trying to promote this approach for decades. Finally it is getting done. Another incredible advantage to this approach is longevity. Small inverters (and charge controllers) at 90%+ efficiency dissipate hardly any heat because the load of the entire array is distributed into such small packets. This means the long-term heating issues that degrade components in larger inverters are no longer an issue. It means that passive heat dissipation such as radiative cooling and micro heat pumps are viable instead of having to rely on fan driven systems which are a point of failure in a system made to last 15 years or more. This amounts to longer life and lower cost.
Since such mini inverter modules can be plugged into the back of a solar panel, much as you might plug a nightlight into a receptacle, replacement takes minutes. This then means that advanced technology upgrades are viable which is important over the life of a solar electric system since it may be as log as 40 or 50 years.
I have a friend who has been operating a 300 watt inverter continuously for 15 years without a failure. Packaging all components into individual panels is the way to go. Now when we include energy storage such as the lithium titanate batteries being offered by Altairnano, we come up with the ultimate self charging uninterruptible power supply. Take it to mass production, and the energy problem is on its way to being solved.
Comments From Ben Gatti
Spreading the heat components around, makes it more expensive to manage the heat not less. When the heat is close, you can use fans, heat-sinks, and temperature sensors to control heat, with active heat control, one can use the full capacity of the components.
If many large inverters where more cost effective than larger ones, there would be no market and a negative price curve for larger inverters. In Fact, inverters at the high end of the power continuum are about half the cost per watt as those at the low end.
Having serviced residential and commercial low-voltage systems in the past, I am somewhat skeptical of the savings long-term. A single point of failure can be very easy to diagnose; while finding a problem somewhere on a high, hot, and sloped roof, at the end of a long ladder, is more dangerous and more expensive.
On flat commercial roofs, you can get away with equipment spaghetti, but on a sloped roof; I would prefer as little as possible up there, and as much as possible mounted in a cool place at eye level, but that's the been-there-done-that part of me talking.
I do like the idea of better MPPT efficiency, and fully scalable systems; But micro-inverters will need to be fully integrated into the Panel before the pros outweigh the cons.
Reply from Jonathan Cole:
Ben, You clearly have not looked at the instrumentation demo on EnPhase's website. With this advance in instrumentation there is no longer any diagnosis necessary. You know what you need too fix before you even go up on the roof. And while economies of scale may now favor large inverters, that is only because all of these products are not yet in the true realm of advanced automated mass production. When these things are being manufactured in the hundreds of millions of units, I guarantee you that they will be dirt cheap
Ben, I think you may be misinterpreting the way such a system would work. It is true that with a single inverter you would not have to go up on the roof. On the other hand with the micro-inverter system, with each panel putting out AC and running in parallel, if one fails, you won't need to go on the roof until a convenient time. Because even if 15% of your panels fail, you are not out of business. But with a single inverter, failure is a complete power outage. Since small inverters will be more reliable because they don't have mechanical heat dissipation, there should be much less need for repair. The ideal is to have inverters that can last the life of the PV and even to be economically upgradable, since electronic technology continues to improve over the 30-50 year life of the PV.
When the inverter and charge controller are integrated with the panels it means that a single AC wire can come from the entire array which seems like much fewer connections and much less wire cost to me. And the data for the instrumentation can be multiplexed on the electrical signal so that no additional wiring is required for complete AC/DC instrumentation.
As far as connecting batteries into such a system, they are connected between the charge controller and the inverter, just as in every un-integrated independent system - like the one I have been living on for 26 years. I am sure that EnPhase is probably targeting the grid-tied market, but it is only a relatively small step to integrate storage when the economic issues are supportive.
Cheaper per watt has to include the maintenance and installation costs, including wiring, and enclosures for weather protection, etc. Even in Hawaii we have these costs and they are significant. We need an integrated product that can be installed in a morning and supply sophisticated data reports, querying, alarms and instrumentation to have user-friendly, 30-50 year installations with high reliability.
This awaits visionary investors.
Comment From Ben Gatti:
I appreciate your points - I do, But I think you've failed to make the case for three key points:
1. That a transistor sitting on a roof will run cooler than in the moderate temperatures of a shaded garage. Try this experiment, park your car in a garage, then park it in the hot sun; use the vinyl seat test or some other scientific method. Trust me that roof is a hotter environment.
2. A Single failure won't cause a problem. This I think is a maybe. Ever had a Mosfet fail? It shorts. Not sure what would happen if a few Mosfets short over; maybe not much. But saying doesn't make it so.
3. How do you include batteries in a one-panel, one inverter system? If I understand you correctly, You'd have batteries behind each panel, connected between the "Charge Controller" and the inverter. That would mean climbing the roof to replace the batteries. Now you have to admit that's a bit of a tricky wicket.
Reply from Jonathan Cole:
Actually the development of panel mounted inverters already goes back quite a way. The first that I heard of was an Australian firm. They did not have the right design and went under. Lately, Exceltech has come out with this product on the market.
Before you get too sure that this is a bad idea, you might want to look a little closer. Actually underneath a PV panel remains remarkably cool. One quarter inch from the back of my Kyocera panels in Hawaii on a full-sun summer day at noon (we have very strong sun), it is the same as ambient temperature. If you put your hand back there it does not feel any warmer than the air. Most of the heat dissipated from most solar panels rises (because heat rises) and does not come out the back of the panel. This is why here in Hawaii, large hotels are covering there roof with PV, because they generate electricity and lower their cooling costs at the same time. PV on a roof is a heat interceptor and dissipates nearly all of it upward. As far as being able to put a battery pack back there as well, you might want to check out
Integrated PV/electronics/storage is the future in my opinion. The fact is, that without storage there is a limit to how much time-variant solar energy can be utilized by the grid. Of course it might put a lot of old style installers out of business, but only if they refuse to see the writing on the wall, because once these products are in mass production, the demand will be enormous. So it might only take a few hours for the installation of the entire system, but there will be millions of systems to install instead of tens of thousands.
Don't forget that 100 years ago, every automobile was a custom, one of a kind integration of a hodgepodge of parts. Mass production is what changed things and it will be no different with integrated PV/electronics/storage systems