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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.

Traditional Inverters

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.

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