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<JUN, Issue, 2012>
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Component & Power

Harvesting Maximum Solar Power

The photovoltaic industry is striving to become competitive with traditional energy production. Distributed solar power harvesting systems accelerate achievement of this goal. In traditional systems, the inverter performs Maximum Power Point Tracking (MPPT) for all modules, therefore working at an average point compatible for all, leading to lower energy production. Other mismatch contributors increase the losses. In distributed systems, such as SolarEdge?, MPPT is performed at module level. This, together with the SolarEdge fixed string voltage, has multiple benefits: maximum module power output regardless of other modules?performance; design simplicity and flexibility; module-level monitoring and advanced safety and anti-theft features.

By Hannah Mann,Amit Rosner

PowerBoxes are connected to each module, performing Maximum PowerPoint Tracking, and monitoring each modules performance in real time

                                                                                                       

 

The Strive for Energy Efficiency
In 1763, at the beginning of the industrial revolution, James Watt, the famous inventor, was asked to review the only working steam engine available during those times of mounting energy demands. He modified it to provide rotary motion. The new innovation used dramatically less coal, was cheaper to run and was more suitable for driving factory machinery, accelerating the revolution.
The main challenge faced by the solar Photovoltaic (PV) industry today is becoming truly competitive with traditional forms of energy production, independently of government incentives. To reach this goal the industry will need to leap in efficiency while reducing the overall cost of energy.
This article will first shed light on some of the limitations of PV installations based on traditional inverters. As will be shown, the standard approach is prone to inherent energy loss and other shortcomings, making way for optimization technologies to step in. The article then exemplifies the advantages of distributed power harvesting systems and introduces the unique SolarEdge solution. 
Small PV installations are composed of dozens to a few hundred solar modules. An installation is typically divided into parallel strings, each consisting of several modules (typically 10-15) in a series, and connected to an inverter. The inverter performs two tasks: first, it converts the harvested energy from Direct Current (DC) into Alternating Current (AC), the prevailing type of current in the electrical grid. Second, it performs Maximum Power Point Tracking (MPPT) for all the modules. MPPT is a continuous process in which the current that would yield the maximum power is sought--a delicate fine-tuning task.
It remains questionable whether a single device can effectively handle the MPP tracking of multiple modules. In fact, power derating of roughly 25% in comparison with the modules name-plate capacity is commonly assumed.

 

The Missing 25%
Panels in todays market are not completely uniform, and they vary in their output power and electric characteristics by an accepted standard of about 3%. This phenomenon is called panel mismatch and its causes are built in the module manufacturing process. As a result, each module requires a slightly different MPP current to provide its maximum power. But inverters cannot set the optimal current for every module individually, since the same current has to flow
through all of them. The inverter is bound to select an average current, typically leading to sub-optimal performance of the stronger panels.
Take the mismatch problem a step further. What happens if shadows are cast over a panel by a nearby tree, chimney or clouds? Such real-life conditions amplify the mismatch scenario. Shaded panels can produce less energy and therefore require lower currents. When a partially shaded module is connected to non-shaded modules, the inverter has to lower the overall current to a value suitable for the most shaded module, or to bypass that module altogether. Due to the modules interdependency, in both cases, the energy loss is disproportionate to the shaded area.

 

Distributed Power Harvesting:
The Quiet Revolution

As the industry becomes more aware of traditional system drawbacks, a new technological approach is emerging in an effort to retrieve lost energy. Distributed power harvesting systems address the inherent limitations of traditional PV systems. As the name implies, distributed systems remove the MPPT function from the inverter and transfer it to each module. The benefit of the individual MPP trackers, also called Power Optimizers, is that modules become self-governing entities, able to locate and manage their own maximum power point. Each panel is able to deliver its maximal energy output at all times independently of other panels in the string. The chain is no longer as strong as its weakest link. If a panel is shaded, it is still contributing to the arrays output instead of lowering it.

The SolarEdge System Approach
While all distributed systems share the ability to retrieve energy lost through module mismatch and shading, other shortcomings of traditional inverters are left unattended. In order to analyze the advantages of a holistic system approach rather than partial solution, the following section will review the SolarEdge distributed power harvesting system.  
From a birds view perspective, the SolarEdge optimization stretches all the way from flexible design and safer installation, to optimizing each modules power output and ensuring constantly efficient inverter, and to remote monitoring for advanced maintenance and operation. The solution is comprised of PowerBoxes, which are per-module Power Optimizers, DC-AC inverters tailor-made to work with the PowerBoxes, and a module-level monitoring Web portal. The end-to-end topology allows installers and system owners unique possibilities.

 

String Flexibility and Space Utilization
System owners and installers alike seek to best utilize all the sunlit space on a roof, but when using traditional inverters solar designers are bound by complicated design constraints. The number of modules which can actually be added in a string is flexible only to a certain extent: the string must include enough modules to reach the inverters minimum permissible voltage and yet not exceed the maximum permissible voltage either. If the installer wants to connect to an inverter more modules than allowed in a single string, which is typically the case, multiple strings can be connected in parallel. Planning then becomes even trickier: parallel strings must all be of identical length, placed on the same facet and have identical inclination.
On commercial roofs with irregular shapes and constructions, it is even more difficult to cover all space with a single type of Lego block--string length. Indeed, on typical roofs an average of only 75% of sunlit space is utilized. If installers want more flexibility, they have to use more inverters at additional cost.
SolarEdge dramatically simplifies site design. In addition to optimizing each module? performance independently, the PowerBoxes use DC to DC conversion to keep the string voltage at a fixed, optimal value. Unlike traditional systems in which the string voltage varies with the number of panels, shading, and temperature, in a SolarEdge system, the fixed string voltage guarantees optimal efficiency of DC to AC inversion by the inverter, regardless of these factors. This renders the design limitations of traditional PV systems irrelevant, allowing maximum flexibility in the design of PV arrays and full utilization of the roof area. Strings need not match in length and can be fitted on differing roof facets, and multiple module types and inclinations can be used. In addition, string length can be much longer, so fewer strings (and related connection boxes, fuses and breakers) are needed, reducing the cost of installation.

 

Operation and Maintenance
Large PV installations can consist of thousands of modules. Identifying a weak panel, or even a completely disconnected string, is like finding a needle in a haystack. Traditional PV systems do not provide analytical information which is detailed enough to make in-depth statements about system performance. Inverter-based monitoring only measures the total inverter output, so neither the system owner nor the installer can tell if all the panels are operating properly. If a system owner believes that his installation is not yielding the amount of energy it should, the installer has to send a technician to the site to manually search for the source, with few, if any, clues at all. On-site troubleshooting becomes more time-consuming as the installation size increases.
The SolarEdge monitoring Web portal presents real-time performance data at module-level for accurate remote maintenance. It accurately detects failures affecting the performance of individual modules, strings or inverters. For example, shaded or faulty modules are located and presented on a visual site map as demonstrated in Figure 1. Automatic alerts are sent whenever intervention is recommended, so problems can be detected and handled early, further increasing system availability and energy yield. Since comprehensive troubleshooting is now done from the office, maintenance providers can proactively offer customers improved services, while reducing their operation and maintenance costs.

 

Safety
PV systems are generally considered reliable and safe for system owners. However, it is important to remember that DC voltage is generated by PV panels as long as the sun is shining, even when disconnected from an inverter. Typical modules have voltage of several tens of volts, so even just a few modules connected in series produce a hazardous voltage. During installation and on-site maintenance touching an exposed contact can be life threatening. Likewise, PV installations pose a risk to firefighters while extinguishing fires on PV roofs.
The elimination of safety risks is one of the major advantages of the SolarEdge system. The PowerBoxes automatically shut down each modules voltage and power as long as the string hasn? been connected to a switched-on inverter, and whenever the grid AC voltage has been disconnected. As a further precaution the PowerBoxes can shut down automatically when a certain temperature is exceeded, eliminating the need for special procedures before tackling a fire. As a result, PV modules do not pose any risk to technical teams and firefighter, even when exposed to light. The industry demands, rightly so, higher safety standards, and law makers are expected to follow suit with new regulations by which all industry players will need to abide. With built-in module safety, SolarEdge systems are better suited for future insurance and regulatory requirements.

 

Theft Prevention
Facing a growing trend of panel theft, owners of large sites often employ costly prevention solutions including fences and security cameras. In addition to detecting theft attempts, SolarEdge further enhances site protection by immobilizing stolen modules. An embedded PowerBox can digitally lock the module to which it is attached when removed from site, making it of no use to the thief.


The development of power optimizing technology is a big leap on the path to parity between the cost-efficiency of clean energy and the cost of electricity produced from fossil fuels. It is an innovative technology based on a simple and proven technique, that of module-level maximum power point tracking. The last few years have seen the emergence of distributed inverter architectures of various sorts, with more and more companies entering the field. Increasing recognition of the benefits of these technologies is driving their market acceptance, estimated by Greentech Media Research to challenge and eventually eclipse the business of centralized inverters, and gain the majority of the market share by 2014/5. The SolarEdge end-to-end solution delivers a unique combination of Power Optimizers, fixed-string voltage inverters, and module-level monitoring. It provides more power from any given PV system installation, eliminates design constraints, provides complete panel-level and whole-system visibility for monitoring and maintenance, removes all safety hazards and offers unique theft deterrence mechanisms. SolarEdge products are commercially available and distributed worldwide. 

 

Hanna Mann is a marketing associate and Amit Rosner is Director of Marketing at SolarEdge Technologies (www. solaredge.com). SolarEdge provides distributed PV power harvesting and module-level monitoring solutions for maximum energy at lower cost.

 

For more information, please send your e-mails to pved@infothe.com.

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