Mighty Micros Make Their Mark: PV Inverter Scene Overhauled by Micros and the AC Module

Manufacturers and installers know only too well that these items must be made simpler, smarter, more integrated, more efficient and more reliable to reduce overall costs and speed up the rate of return for customers.

Some of the most significant recent technological advances are in the area of inverters, with several firms racing to market with micro-inverters, devices that convert direct current (DC) from a single solar module (panel) to alternating current (AC).

Drawbacks in Central Inverter Architecture

Shortcomings inherent to the central inverter architecture are creating opportunities for a range of new technologies, with a growing number of companies developing products and technology to generate more power from the PV panels already on the market.

One of them is Enphase, which claims to have been first to develop a commercially available micro-inverter. The company has shipped more than 300,000 units to date, grabbing 13% of the under-10 kW market in a remarkably short time.

Unlike a central, or string inverter, that aggregates and converts the power generated by an entire array of solar modules, a micro-inverter converts the DC power from a single solar module to AC. When connected to a central or string inverter, modules are typically connected in series; when they have micro-inverters, the modules are all connected in parallel.

Enphase, like others developing micro-inverters, claims there are numerous advantages, particularly for smaller — domestic and commercial — applications. The concept of panels delivering AC power has significant appeal for small-scale home project applications at lower voltage levels. But the technology is rapidly sizing-up, and the company, which recently completed a 223-kW build, is currently working on a further installation about twice that size.

Several producers are now promoting grid-tied PV micro-inverters, which ensure that the power supplied will be compliant with the grid code. It means that in geographic locations where buyback agreements are in place, installations with surplus power can sell it to the utility. In net-metering environments the meter turns forward during normal consumption, such as at night or in the day when local loads demand more than the PV system can supply, and backwards when PV production is greater than the load.

Micro-inverters are not new, but advances in electronic components have made them commercially viable for the first time, enabling manufacturers to penetrate lower-end markets while addressing some of the challenges associated with standard central inverters. A distributed approach to inverter technology reduces the effect of dust, debris and shade on the array. As the modules are installed in parallel via the AC connection only, any issues with a single module no longer disrupt the rest of the array as had occurred in previous designs using a central inverter. As a result, the solar installation is no longer subject to the 'Christmas light' effect in which shading or failure of a single module affected an entire string.

Smart Monitoring Closes the Convergence Loop

Manufacturers are also offering smart monitoring as part of their package to customers. The changes in the size and sophistication of these devices, and consequently the overall improvements in solar system performance, close the loop in the convergence of energy and information technologies, to great effect.

Raghu Belur, Enphase's vice president of marketing, explains how the firm came to develop its micro-inverter. "All entrepreneurs go looking for a problem; the harder the problem, the better it is. Our chief technical officer and co-founder, Martin Fornage, installed a solar system on his property and during that time recognised the limitations of a traditional inverter solution. Other people had worked on micros before but had been unable to make the technology commercially viable. Our approach, with its digital design and development of custom semiconductors made the micro-inverter commercially viable and attractive. We were able to address the issues of efficiency, reliability and cost for the first time," he says.

"Secondly, we didn't think of it simply as a piece of hardware, but instead as a system, which means embedding computing technology and monitoring for operations and maintenance purposes. And a lot of that comes from the founders' backgrounds. I came from Cisco Systems, which has a very system level approach to all its activities."

Micro-inverters can supply operational data to a web-connected mobile device (Source: Enphase)

Enphase started working on the idea in December 2005 and installed its first micros in August 2007. The firm's micro-inverters were made commercially available in June 2008 and growth since then has been rapid. In March 2010, Enphase secured $40 million in funding from equity partners led by Silicon Valley-based venture capital firm Bay Partners to fuel further expansion.

Enphase offers two micro-inverters — 190 W and 210 W — and to speed installation, the company is working on a system that will allow two modules to connect to a single inverter unit.

At the heart of the system, the Enphase Micro-inverter utilises advanced technologies to maximise energy harvest and increase reliability of solar systems. In addition, the unit turns each solar module into a 'smart' module by connecting it to the internet, thereby providing unprecedented visibility and analysis of solar system performance. The package includes the energy management unit (EMU), which allows solar performance data to be transmitted from the micro-inverters across a standard AC power line, eliminating the need for additional wiring and keeping installation costs and complexity to a minimum.

Belur says installers and owners of the company's micro-inverter systems gain from dramatically simplified design, installation and management of their solar energy systems. As a result, he says, this new approach will help accelerate the broad adoption of solar technology by increasing the return on investment of residential and commercial solar systems. Customers are seeing energy harvest increases of 5%-25% and balance-of-system (BOS) savings of 13%-15%, he says.

The micro-inverter represents a win-win solution, he adds. "It is having an impact on various industry stakeholders. Installers benefit from the dramatically simplified design and installation. The reduced footprint of the micro-inverter appeals to residential customers concerned about aesthetics, while business owners are interested in increased reliability and visibility of their system. All benefit from the opportunity for improved return on investment based on increased energy production, increased reliability and balance-of-system savings.

"In addition, Enphase Micro-inverters have an unprecedented mean time between failures (MTBF) of 119 years as shown in a study by Relex, an expert in reliability engineering and testing." The company is to launch generation four of the product in the coming months, says Belur. This will aim for still greater efficiency, higher reliability and more power.

Currently, Belur says the Enphase micro-inverter comes at a cost of roughly 20 cents per watt more than a comparable central or string inverter. "But you have to take into account the overall system savings," he explains. "Overall, customers are seeing a 15% reduction in balance of system, giving cost parity with a central inverter system. Then you have to consider the total lifecycle costs of ownership internal rate of return. When you do that you see a 20% bump in your IRR over the entire system, compared to a central inverter system."

Enhanced Fire Safety

A further advantage of micros, says Belur, is that of enhanced fire safety. "Traditional systems deal with high-voltage DC, which is very prone to arc fault. And arc faults start fires. In the case of the micro you eliminate the high-voltage DC completely. You're only dealing with very low-voltage DC and all the AC is in a protected circuit — it goes through a breaker. So one could argue that micro-inverters are inherently safer compared to central inverter systems."

The Enphase foray into micro-inverters has reignited interest in the sector, with several companies about to offer commercially available devices. One of them is Sparq Systems, which is producing a high-durability, lightweight micro-inverter for the North American market, to be launched in the third quarter of 2010.

"This will be the first of its kind in the market that will for the first time meet the same reliability as PV modules," claims CEO Dr Praveen Jain. "It is a lightweight, high-density design, and perfectly suited to the integrated AC panel. The biggest advantages of micro-inverters are in the small installations below 10 kW.

"It is easy to justify micro-inverters for the 10 kW-and-under market, but when you look at the large farms — solar farms — there are studies which have been done, and those studies have shown that if you use micro-inverters, you can gain maybe 5% additional energy harvesting. It has come down to a trade-off between how big an increase in energy harvesting you are getting and cost. When we see the cost of micro-inverters come down to a level where they can compete with centralised inverters or the string inverters, then micro-inverters will be equally useful as any other centralised inverter in large installations. In my opinion, this will happen in three-to-five years' time, maximum. A lot of module manufacturers are saying that they want to produce integrated AC panels. When that happens, there will be no place for the large inverters."

SunSil is one company on the brink of pushing the technology further. In July 2010 it launched its 300 Solar PV AC module at the InterSolar Show in Munich. SunSil's AC modules are expected to commence shipping in the first quarter of 2011.

The company claims the new module will be significantly more cost-effective to install than current PV systems and provide a much quicker return on investment for customers due to its higher energy yields, of up to 30%. According to the manufacturer, the SunSil product is a complete PV 'smart module' where every panel is a complete system capturing the sun's power and providing an output of 230 V AC. SunSil's unique design of microinverter operates in the region of 300 W, which means that high-specification, yet standard, off-the-shelf components can be used. Crucially, the company aims to offer a warranty for its smart modules, including the micro-inverter, of 25 years.

SunSil's micro-inverter operates in the region of 300 W, meaning standard off-the-shelf components can be used (Source: SunSil)

Another benefit of using standard parts, says SunSil, is that circuitry can be assembled on conventional pick-and-place lines, in volumes that keep costs down. Software on the microprocessor gives access to operational data for improved performance and smart grid compliance, which, says the firm, leads to an overall efficiency of the smart module of 95%. When combined with SunSil's unique Dynamic MicroCell Optimisation technology, the module's optimal energy gathering is up to 30% better than conventional panels, the company maintains.

Designing Out the Points of Failure and Stress

"We looked at every aspect of the design of our microinverter to ensure that it would have a very long life," explains Erik Hansen, SunSil's chief executive officer. "Mortgage companies are starting to look at PV installations as part of the house so they expect them to have long functional lives. Properly made electronics can keep functioning for decades - the weak spots are keyboards, switches, batteries and displays and there are none of these in our micro-inverters. We have designed out all the usual points of failure and stress and built in quality so we can warrantee them for 25 years."

Yet another company to go down the micro route is Direct Grid Technologies, which recently announced a new series of OEM grid-tie micro-inverters that can mate with mono-crystalline and polycrystalline modules. Its DGM-S250 has an output of 250 W, while the DGM-S460 offers an unprecedented 460 W. Like the preceding DGA Series, the DGM Series uses Direct Grid's closed-loop MOSFET planar architecture, which offers high power and excellent thermal management, adding to efficiency.

"All of our micro-inverters are inherently designed with flexibility in mind. We now have a continuum of products for any wattage between 250 W and 460 W for our OEM customers," says Frank Cooper, Direct Grid president. "Additionally, unlike some other micro-inverters on the market, the system auto-tunes to any single phase grid it is connected to, so we can handle grid frequencies of 50 Hz or 60 Hz right out of the box."

The SMART DGM Series also includes 'Echelon' communication so each micro-inverter in a network can communicate with a remote access node. Power data, temperature, diagnostics and a unique identification code are collected, and network communication can also deter theft, says Cooper. The resulting data can be graphically presented to end users for easy system monitoring.

Micro-inverters Stuck at Smaller Systems?

But the emergent micro industry has its sceptics. Some believe micro-inverters are destined to languish at the smaller end of the solar market. One is Chris Edgette, a director of Strategen Consulting. "It's going to be a case of watching systems and seeing how they develop. We're going to see a spectrum. Certain modules perform better with certain systems. Smaller systems are going to have different issues to larger systems, while utility-scale systems are different again. In the medium term we're not going to see a domination of any one technology. We're going to see integrators looking closely to see what technology is appropriate."

Michael Lamb, vice president of business development for the start-up eIQ Energy, says central inverters can get smarter in response to perceived failings. His company uses maximum power point tracking (MPPT) technology to make central inverters more responsive to module-level problems in a solar array and to increase a system's power output. eIQ makes each panel perform a DC boost onto a bus, using a distributed 'vBoost' module - a small DC-to-DC converter that attaches to one or more panels in an array and provides MPPT while also stepping up panel output voltage to a constant level and creating a bus architecture.

The company claims that its architecture enables the connection of unprecedented numbers of panels on a single cable run - up to more than 100 thin-film panels. "We fix the voltage for maximum efficiency. We can put way more panels on a cable. And with the central inverter we have a single point of maintenance. If you maintain that properly, you can answer some of those issues that are important, including system availability and uptime," says Lamb.

Leo Casey, chief technology officer for Satcon, believes that central inverters will always be a more bankable option, being the superior cost-effective approach to developing large-scale solar systems. "If we look at the development of big inverters around the world, the break point for large electronic systems appears somewhere between 50 kW and 100 kW. There's an incredible cost advantage in making a 500 kW or 1 MW inverter. It's almost an order of magnitude cheaper than making a smaller inverter."

Satcon says its next-generation inverter design features best-in-class efficiency (98.5%) combined with three extreme climate packages to provide the highest levels of system performance and uptime and the utility scale solar industry's widest thermal operating range. "The breakpoint is 5-100 kW for micro-inverters. Above that it is difficult to see how they can compete with big cost advantage per watt at 1 MW level," says Casey.

Chris Webb is an independent energy writer.