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The Essential Differences of String Inverters, Microinverters, and Power Optimizers
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Since a solar array generates Direct Current (DC), it needs to be converted to Alternating Current (AC) for normal house-whole use. One of the key decisions to make (besides the solar panel itself) is the choice of which type of inverter to use. The differences between the types of inverters can be confusing. This article, therefore, briefly discusses the difference between the current three (3) essential types from which to choose. They include:
- a) String Inverters (also referred to as centralized inverters)
- b) Microinverters (also referred to as ‘Module Level Power Electronics’ or MLPE)
- c) Power Optimizers – a hybrid of a) and b) (also referred to as ‘Module Level Power Electronics’ or MLPE)
Although MLPE technologies are rapidly gaining market share, string inverters are the most commonly deployed and time tested option on the global market to date. They are suitable for installations where individual strings of panels can be installed perfectly on a single plane without shading during any part of the day.
For string inverter systems, individual solar panels connected into series strings with each other, deliver accumulated DC voltage to a single inverter that transforms the Direct Current from the entire PV array into grid-compliant AC power that is fed into the power grid. Although string inverters have statistically less component failures than other types of inverters, their main shortcoming is that a single shaded or under performing module in a string can have an adverse impact on the entire system. In other words, the maximum output performance of the string is defined by the poorest performing panel. For instance, if a single PV module is partially shaded and loses 20% of its output, every module in that string can become limited to the same loss.
String inverters that use a technique known as maximum power point tracking (MPPT) in order to optimize PV output by adjusting applied loads can best use the available power at particular levels of available insolation. Because the effects of shading, snow covering, and module defects can cause variations in the output of an individual module, the inverter will change MMPT settings causing a divergence from an inverter’s optimal performance. If a string inverter has multiple MMPT capabilities, the operating point with the highest performance can be found using more of the energy supply from the PV modules under such shading-obstruction conditions. Because string inverters can have a limited selection of power ratings, the power rating of the solar modules have to be matched with the power rating of the string inverter.
An alternate type of inverter to a string inverter is the microinverter which is suitable for installations where one or more panels may be shaded or installed on different planes and/or different directions. Microinverters convert the DC output of a single PV module into grid-compliant AC power. These are actually small DC to AC inverters rated to handle the power output of a single panel. Each solar PV module has its own microinverter, with no need for a separate central inverter.
Microinverters are connected in parallel with each other, and the AC power travels upstream through an ordinary branch circuit and then to a service panel. This type of microinverter system is a combination of multiple microinverters all along the branch circuits converting DC to AC power, all injecting their individual current supply. This individual parallel AC output structure as opposed to the DC series structure of a string inverter system has the advantage of isolating each panel. Reducing or losing the output from a single panel does not disproportionately affect the output from the entire array. Each microinverter is able to maintain optimum power by performing MPPT for its own individual module and allows more flexibility in module arrays. The failure of a single panel or inverter in this type of system therefore will have minimal impact on overall system performance. Microinverters also allow monitoring the power production of each individual module whereas string inverters allow monitoring of only the entire array. Drawbacks of microinverters include higher initial cost, higher failure rates, and labor intensive replacement when they are located under the solar panels mounted on a roof.
Power optimizers are similar to microinverters in that they allow optimizing and monitoring the power production of each individual module as well as offer many of the same benefits as microinverters by mitigating shading issues. Similar to microinverters, they are located or integrated at each solar panel. Unlike microinverters, power optimizers can start MPPT at lower voltages, meaning they track a module’s Maximum Power Point even under severe shading. They are connected to a single string inverter. This configuration reduces the amount of electronics on the roof (compared to microinverters) and takes advantage of string inverter cost and efficiency. With DC optimizers, each module output is optimized separately prior to sending it to the string inverter for conversion into AC power. This results in a higher overall efficiency than that of the DC output from each module directly to a single string inverter. Cost may also be a consideration in that one DC optimizer can be used for two solar modules versus one microinverter for each solar module. Power optimizers appear to have fewer parts, simpler rooftop wiring, and higher durability than microinverters, which are more complex.
For shaded or differently pitched roofs, microinverters and power optimizers will generally produce more power than a single string inverter. However, harsh weather conditions are more likely to affect multiple MLPEs versus one string inverter, causing more service and installation repairs for failed components. Since the technology and design of inverters is continuously improving, the use and cost of string inverters versus microinverters versus power optimizers should be discussed with the solar energy installer or dealer for each particular application.
In the immediate future, the Rapid Shutdown Requirements per the 2014 National Electrical Code, Article 690.12, and the additional changes suggested for the 2017 NEC requirements will have an effect on the complexity of inverters. This is discussed in another blog available from Elsevier Publishing and the author’s website at http://www.russellhplante.com.
Acknowledgement: Appreciation is given to Geoff Sparrow, Director of Engineering for ReVision Energy, for his suggestions and contributions to this article.
Russell H. Plante is the author of Solar Energy: Photovoltaics and Domestic Hot Water – A Technical & Economic Guide for Project Planners, Builders, and Property Owners
- Provides a fundamental understanding of solar DHW and photovoltaic systems
- Uses clear guidelines to evaluate solar DHW and photovoltaic systems’ value as a long-term investment vs traditional power and heat generation methods
- Discusses cost and operating expenses relative to investment and return on capital which will be beneficial to project planners, installers, energy managers, builders and property owners
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