ENERGYHARVESTING
into, or attached to, each PV module. This architecture is illustrated in Figure 3. Each micro-inverter converts the DC from each module to AC, ready for direct connection to the grid. An example of a micro-inverter is shown in Figure 4.
The micro-inverter architecture allows each module to be an independent, standalone, solar AC grid connect system with its own optimized energy production output. Micro-inverters mean faster, simpler and safer plug-and-play installation. There are no high voltage DC circuits to handle and installation time and costs are cut by 15% to 25%. Further, the need for a large, hard-to-install, central inverter, which is the single most common point of failure in conventional systems, is completely eliminated. Installations are flexible and scalable, and modules can be located on any plane, or on multiple planes within a single system.
Micro-inverters deliver between 5% and 15% more power from the installation and such systems are intrinsically safer. By eliminating the central string inverter – or several of them over the life of the system - the additional cost of adding a micro- inverter to each module is mitigated. In fact, it has been calculated that using reliable micro-inverters – those with a realistic operating life in excess of 25 years – enables the cost-per-harvested Watt of PV solar systems to be cut by up to 25%, compared with conventional systems. This dramatic improvement is in stark contrast to the small, incremental advances being made in the efficiency of PV modules over the last few years.
Equally important from the installer perspective, micro-inverter PV systems allow monitoring down to the level of individual modules. Any faults that do arise are easily located, reducing maintenance costs. In fact, these monitoring systems can even be web enabled, so the status of the system can checked from anywhere in the world where there is Internet access. The monitoring systems also give users real-time and historical data on the performance of their system.
Micro-inverters may not yet be as efficient as large central inverters, but because the overall yield from PV-systems based on micro-inverter architectures is so much higher, it is important not to be misled by headline efficiency figures. Efficiency is important but it’s only part of the story. Micro- inverters represent one of the fastest growing parts of the solar-electric industry. The solar inverter
* According to Greentech Media, Dec 2009.
market was valued around $2.4 billion in 2009 and is growing fast*.
The reliability question
Micro-inverters are not a new idea. The challenge has been to design these products to be reliable in real-world conditions. Temperatures can easily reach +85 degrees C behind a solar module and high temperatures are not good for electronic devices. A rule of thumb used by electronics designers is that every 10 degrees C rise in temperature will halve the mean time between failures (MTBF) of an electronic system.
The MTBF figure is a guide to the predicted failure rate during the so-called “useful life” period of a product. It’s a statistical calculation based on the predicted failure rates of the individual components within the product.
Figure 3: A
However, the concept of MTBF is widely misunderstood, and often misrepresented in marketing material. The inference made by some suppliers in the solar industry is that a high MTBF supports an expectation of a long life for their
micro-inverter based PV system reduces the harvested cost-per-Watt by up to 25% over the life of the system
Figure 2: A PV solar installation with DC-DC power optimizers
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www.solar-pv-management.com Issue II 2010
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