As photovoltaic technology increasingly penetrates mainstream markets rooftop solar PV system integration based on series configured arrays are becoming antiquated due to new system design architectures enabled by new inverter technologies.

Power Optimization Approaches for Trouble-free Project Execution and Superior System Performance

Brent Harris | Sustainable Energy Technologies


When an industry grows as quickly as the photovoltaic industry has been – averaging greater than 30% per year for the past decade – it is easy to lose perspective. New markets emerge swiftly and can vanish even quicker. Suppliers and customers shift from aloof to desperate in a matter of months. At this rate of growth, roughly a quarter of the workforce has yet to achieve their first anniversary in the industry, and more than half of the workforce has less than three years of experience. In such an environment, companies can’t help but cling to proven techniques and seek stability wherever they can in order to stay focused on the more clearly dynamic facets of their business. Before long, as the industry matures, these ‘classic’ approaches can become counterproductive, eventually creating enough distress to drive innovation that promotes a shift in practice. For example, rooftop solar PV system integration based on series configured arrays, which was originally developed in order to enable high inverter efficiency and reliability, achieves this at the expense of requiring detailed custom design, perfect operating conditions, and high operating voltages. As photovoltaic technology increasingly penetrates mainstream markets where such trade-offs are not acceptable, these classic techniques are becoming antiquated due to new system design architectures enabled by new inverter technologies that are proving their worth.

Parallel System Designs
The classic approach to system integration prescribed by inverter designs popularized in the early part of the past decade requires stringing modules in a series or “Christmas light like” fashion and in doing so, increases the voltage and the vulnerability of system performance to less-than-perfect conditions. By connecting each module together this way, if one module outputs less energy due to shading, debris or other ‘real world’ factors, the entire series of modules operates less efficiently, similar to the way in which a string of Christmas lights will go out when one bulb is blown. Parallel wiring connects every solar module to the system independently, so each may operate at its maximum potential, ensuring maximum power yield for the overall system.

Under the classical approach, ‘real world’ challenges are usually greeted first with disbelief and then dismay by the system designer whose tools of the trade are not working the way they used to. As the cost of photovoltaic equipment drops and as the industry becomes increasingly dominated by production based incentives, sophisticated structural analysis and fancy racking techniques become unaffordable. Lower equipment costs also drive designs toward greater coverage of the rooftop. In all but the most ideal cases, these considerations preclude rooftop solar PV systems from having the optimized system layout – south in orientation, at latitude tilt, and free from shading all year round – that is described in training guides.

Conversely, a parallel system design can accommodate such constraints without a second thought, allowing a standardized approach to meet a variety of requirements without incurring large upfront design costs or sacrificing performance.

In addition to the ease of project execution and the more reliable performance offered by parallel system configurations, there is a safety benefit associated with keeping the system DC voltages to a minimum. Unlike the basic AC system that permeates the building, it is not possible to turn off a solar power system at the source. This may sound trivial, but it means that first responders and others who may come in contact with the system wiring cannot rely on their standard technique of disconnecting power to the building in order to ensure their safety. While it is not possible to eliminate this risk, it can be minimized by keeping the voltage as low as possible. Parallel systems have an inherently low operating voltage since the system operating voltage is fixed to the voltage of the individual photovoltaic module - whereas in series arrangements the operating voltages can reach levels as high as 600V in North America and 1000V in Europe. The risk can also be minimized by limiting the ingress of DC circuits into the building. This is achieved by locating the inverters near the array on the roof. In effect, the photovoltaic system becomes a self-contained building appliance, like a rooftop HVAC unit, with only AC circuits connecting it with the rest of the building.

Recently introduced micro-inverters and low voltage inverters are now enabling parallel system design that has been standard for off-grid systems for decades but which is incompatible with classic high voltage inverters. Each of these new technologies offer similar benefits in system performance, design flexibility, and safety but achieve parallel system integration in a different way. It is important to examine your needs and the application of the system before deciding on which technology to use.

Micro Inverters
Several companies currently offer a fully distributed power generation option called micro-inverters. A separate micro-inverter is connected to each solar module in the system and delivers the energy from its own module to the AC grid independently. In this case, the parallel integration is accomplished exclusively in the AC circuit. This configuration achieves the highest level of modularity as each solar module acts as its own complete photovoltaic system. Taken to the extreme, this technology could allow the inverters to be mounted directly onto the module during the manufacturing process, creating an easy turn-key solution – an AC module – which can be marketed direct to consumers at home improvement stores, or online direct from the manufacturer. However, this business model has yet to be proven and micro inverters are not the right fit for every project. Micro inverters are most advantageous when used in smaller residential or commercial projects of a few kilowatts. With projects of this size the additional cost of micro inverters is more easily counterbalanced by time savings during the design and installation process.

Low-Voltage Inverters
Commercial building owners, installers of large scale systems and roofers and other trades may want to consider another, less costly approach to achieve parallel architecture: systems that connect the modules in parallel on the DC circuit prior to converting the energy to AC electricity. This type of parallel system solution requires a low voltage inverter that can efficiently convert the output of the system to grid voltage. This approach offers the same benefits of maximized power yield and enhanced safety as micro inverters with the added benefit of being scalable to large commercial rooftop installations while maintaining the installed cost maintenance paradigm that owners and installers of these larger systems require.

SUNERGY low-voltage inverters installed in a parallel wiring architecture in Medicine Hat, Alberta

The key to lower cost achieved by such a system is to avoid adding electronics to each module. This is enabled by an important characteristic of PV module behaviour: variations in module performance affect primarily the output current and not the operating voltage. This means that the output of modules connected in parallel is decoupled, allowing them to operate as individual generators, thereby decreasing the impact of shading, debris, non-optimized system layout and other real world factors, when compared with series arranged arrays. These benefits are well known in off-grid systems, but it is only recently that new inverter technology has allowed the approach to be employed in grid tied systems.

As the industry progresses further, reducing costs and penetrating deeper into the mainstream, we will also start to see more BIPV products coming onto the market. BIPV applications encounter the same concerns as mainstream rooftop applications – only more so. In this case, we are talking about a truly standardized building appliance, and the architects and builders really take over. The system must be able to efficiently convert whatever sunlight it can get and no matter where it is installed. Parallel approaches clearly shine in these applications, but again each of the approaches will be better suited to different applications. For smaller, more accessible BIPV products – like factory integrated photovoltaic awnings – a micro inverter could be a very convenient option. However, in the context of larger, more integrated systems – such as shingled roofs, facades, or semitransparent skylights or windows – it is difficult to imagine how a multitude of micro inverters buried into the structure of the building could be easily serviced. In these cases the low voltage inverter approach is the natural fit.

As photovoltaic technology moves into mainstream rooftop and BIPV applications, standardized systems will become the only way to achieve acceptable installed cost, and systems that incorporate parallel system architecture will be the only ones to offer reliability of performance and flexibility of installation in a variety of applications. These parallel systems will also be the only ones to deliver an inherent level of safety no matter where they are installed. There are currently two different approaches that deliver on all three of these counts, and the decision will ultimately come down to cost and maintenance requirements, and which technology offers the best value for the application. After careful evaluation of your project requirements, you will know which technology is right for your application; either way it is out with the fancy classics and in with safe, standardized, reliable performance!

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