Editor’s Note: GreenVolts, the company featured here, has suspended most of its operations since the print publication of this article. The company says a “change in direction” from a major investor has limited its access to funding. GreenVolts says a small team will remain to provide customer support and seek potential buyers of the system.
In the burgeoning solar market, many customers are finding that a complete solar system can be a difficult puzzle to piece together. While new solar deals, planned projects and installations are making headlines, actual commissionings are fewer and farther between.
While solar energy has been around for years, the industry is still in its infancy. Most traditional photovoltaic (PV) systems are still composed of parts sourced from different suppliers, including solar panels, inverters, trackers and monitoring systems. Because these parts are not designed to work specifically with one another, the overall system performance suffers. Many customers are turning to a new solution: the total solar system.
More and more companies are beginning to offer a complete, fully integrated system-design solution that essentially allows buying the entire puzzle, as opposed to separate pieces with jagged and conflicting edges. The conventional approach to building a solar-power plant typically includes installating modules, mounts or trackers, and inverters designed and manufactured from different suppliers, followed by having engineering and construction firms provide custom, aftermarket integration. With a system approach, all components can be designed to work optimally together, producing a higher energy yield and eliminating costly and time-consuming integration. In addition, much of the system is pre-assembled, enabling a modular and scalable approach to power-plant design and construction.
One such system, produced by GreenVolts, uses concentrated photovoltaic (CPV) technology, taking advantage of module efficiency that is 1.5 to 3 times that of traditional silicon or thin-film photovoltaics. In their design, the sunlight passes through primary and secondary optics, which deliver an industry-leading 1,300 times concentration onto the cell. The smaller cells, in turn, enable more effective heat dissipation, helping the cell produce maximum power.
The primary optic has 24 individual Fresnel lenses (one for each cell in the module) based on silicon-on-glass (SOG) technology. These optics are made by stamping the optical pattern on a highly transmissive sheet of glass. The silicon side is inside the module, and the tempered glass is exposed to the elements.
The secondary optic is a molded glass element with a domed top to extend the acceptance angle and redirect the light in parallel rays for full and even coverage across the cell. Higher concentration ratios have been a challenge in the industry, as they require much more precise and accurate solar tracking — the margin for error is smaller — to focus the light for maximum power. The wide acceptance angle of this optic, integrated in a design together with the module and tracker, was critical in overcoming this challenge.
The light is converted to DC power by a multi-junction (MJ) solar cell made from III-V materials. In contrast to traditional silicon or thin-film cells, the MJ cell is able to use the entire spectrum of light, thereby yielding more energy. These also have a low-temperature coefficient of power relative to other photovoltaic technologies, so they maintain higher efficiency in hot climates.
Together, these optical elements and the MJ cell form the power unit, which is approximately 11 watts, and the 24-cell module is rated at 300W at STC at 1000W/m2.
Building utility-scale power plants requires logistics, storage and assembly that comprise a significant portion of the overall project cost. Total solar system providers can address this issue by developing modular systems, which are largely assembled at the manufacturing location, as opposed to the traditional approach of assembling and installing materials on-site. For example, GreenVolts preassembles eight modules into a system element called a paddle, which is approximately 2,400W. The paddle is a structural element that has been pre-aligned to better than 0.1 degrees at the factory, installing directly to the tracker with four bolts and two snap-together electrical connectors. This model improves performance, while minimizing installation costs, compared to a multi-component, multi-vendor project.
To maximize efficiencies, CPV requires two-axis tracking. An ideal tracker comes with a tilt and roll design, which breaks up the mechanical structure into smaller sections that can easily ship and quickly install on site. Another key aspect of these tracking systems is that they are divided into multiple smaller sections (16m2 area per grouping). This keeps the modules lower to the ground where there is less wind effect, minimizing the large sail effect of larger trackers. The result is a much higher wind operating range, up to 15 m/s when compared with multivendor systems. Using a feedback loop only reachable in a fully integrated design, the inverters operate in conjunction with roll-and-tilt motors to ensure each axis has been positioned to maximize power. This feedback loop and control results in approximately 10% greater power production than other high concentration tracking systems.
A complete system approach also allows deviation from the industry standard centralized inverter, and to develop highly integrated, distributed inverter architecture, with inverters built into each array. This is a key advantage compared to other multifaceted CPV systems that use a more conventional centralized inverter approach, where the combination of cell and optical mismatch can lead to a significantly reduced energy harvest. In addition, since there are electronics required at each array for motion control, power monitoring, fusing, and disconnects, adding inverters at the array level is not significant in terms of additional cost. The benefit, however, of an integrated inverter at the array level is significant. An integrated inverter can substantially reduce cost for the BOS components. The DC string wiring for the array can be pre-manufactured in high volume, reducing costs and simplifying installation. As the DC and AC disconnects are built directly into the array electronics, the engineering design for the DC portion of the power plant is completely eliminated.
GreenVolts’s inverter design has been optimized for CPV technology. The high Voc (open circuit voltage) characteristics of MJ solar cells enable single-stage power conversion. This eliminates a significant number of components, further increasing system reliability. Four parallel strings with eight modules (or 192 solar cells) generate up to 600V DC on both the north and south sides of the array. The strings are connected such that +600V and -600V are connected at the inverter in a bipolar configuration and synthesized from 1200V DC to 480V three-phase AC. This configuration simplifies the inverter design since a DC boost is not required.
Another advantage of the distributed approach is that the power electronics are much smaller and all electronic subsystems are integrated into a single electronic enclosure. The design is PCB-based and leverages high-volume electronics technology as opposed to high power, heavy copper, bus-bar based designs in central inverters. The enclosure is divided into two sections — a sealed NEMA-4 section, which is not exposed to air, and a NEMA-3 section that is. All the active electronics are housed in the NEMA-4 section. Heat sinks and enclosed components such inductors and thin-film capacitors are in the NEMA-3 section. There is also an industrial grade CPU for communications and control along with an 802.11 wireless communication module and antenna.
Operations and maintenance is a key aspect of any utility-scale power plant, but unfortunately, capturing statistics and managing systems can be challenging when dealing with multiple components from different vendors. In a system approach, solar-plan management software can also deeply integrate across the system, providing much better monitoring, diagnosis, reporting and control. These types of applications are much more than power monitoring — they include asset management for power plant equipment, alerting and messaging for component and system failures, and debugging and analysis tools to remotely diagnose and repair issues. They can also include intuitive graphical user interfaces with highly granular string monitoring and reporting tools.
With the continued pressure for solar providers to reduce the overall cost of energy, the integrated solar system approach offers fresh opportunities for innovations and optimization. As the industry continues to evolve, we expect more to embrace this method, which can provide increased efficiency and durability, a greater energy yield and simplified maintenance. SPW
By: Wayne Miller, senior vice president of products and field operations of GreenVolts in Fremont, Calif.