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Skyline Solar's HGS arrays offer the high scalability characteristic of thin-film arrays, together with the high efficiency of silicon-based systems, which combine to make possible large-scale, affordable energy.
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The challenges of manufacturing silicon-based photovoltaic systems large enough to supply utility-scale needs at an affordable price has sparked a number of solutions that circumvent the use of silicon entirely, like indium-gallium thin film products, or try to minimize the use of silicon by maximizing the efficiency of the silicon components. Solar installations that use innovations like high-efficiency cells, solar tracking, convection cooling or reflectors to get the most watts out of every photon are known as "high-gain" solar (HGS) arrays.
Skyline Solar, founded in 2007 in Mountain View, California, will release its first-generation high-gain systems onto the market later this year, following a successful 27kW demonstration project built for the Silicon Valley Transportation Authority. The company specializes in high-gain arrays that reduce the total amount of silicon needed to produce a given wattage, maintaining the high efficiency of a silicon solar array using relatively cheap metal reflectors to reduce the amount of silicon by 90%. Given that peak demand on energy systems reaches into multiple terawatts, and that silicon's price and availability fluctuate widely, a highly scalable, silicon-miser system like Skyline's seems like an ideal way to meet these energy needs affordably. At Intersolar North America 2009, GreenMomentum had the opportunity to speak with Tim Keating, Skyline's VP of Marketing and Operations, about the advantages and promises offered by Skyline's products.
Skyline Solar's HGS arrays offer the high scalability characteristic of thin-film arrays, together with the high efficiency of silicon-based systems, which combine to make possible large-scale, affordable energy; as Keating puts it, "we're all about extreme low costs." Because they rely on efficient, pre-engineered systems composed of existing equipment rather than specialized equipment assembled in the field, HGS arrays can be constructed using local manufacturing and labor sources. In fact, the metal reflectors that make up the bulk of the arrays can be rolled out, stamped, and assembled using legacy auto-manufacturing equipment, making this a particularly promising and highly scalable option for making use of decommissioned auto plants.
The largest part of Skyline's design is the reflector, which is composed of a sheet of aluminum bent into a parabolic cross-section and lined with a reflective coating; the development of the solar-thermal industry has led to the creation of many different materials that can be used for this purpose. In order to minimize the heat stress the silicon cell suffers, the reflector does not focus the heat onto a single point, but onto a 30-mm wide strip containing the encapsulated cells. The reflector also serves as a frame, minimizing the weight and cost of the system. The environmental impact of the whole array is minimized through the predominant use of easily recyclable metal rather than the silicon and encapsulants that dominate traditional silicon solar arrays; each of Skyline's panels uses 6 cells rather than the typical 72 cells.
Racks of these reflectors measuring around 6m by 2m, assembled inexpensively in local manufacturing facilities, are deployed with an integrated single-axis tracking system. This integrated, pre-engineered system, as well as the combination of racking and tracking functionality in a single structure, drastically reduces costs and simplifies installation compared to double-axis tracking and even traditional single-axis tracking. This provides the higher solar power output characteristic of tracked systems, while lowering the cost of installation and maintenance. Vertically-oriented air gaps behind each panel couple to metal heat sinks at the edges of the array to dissipate heat and improve the array's efficiency. The reflectors are designed to minimize self-shadowing at all tracking positions. Even the posts that support the racks are pre-engineered for proper footing in a variety of soil types; they are pre-wired and pre-strung to minimize the time that must be spent by electricians in the field, which represents another source of cost savings.
While the reduced installation time and complexity of Skyline's pre-engineered systems offer significant cost savings compared to what Keating calls "stick-built" custom systems, the pre-engineered systems can offer even higher capacity factors. The complexities of custom-designed systems installed piece by piece in the field out of specialty equipment are such that, as he puts it, "you have to have a brain the size of a planet" to engineer these complex systems properly for maximum efficiency. Complex, custom systems provide plenty of room for things to go wrong, and even slight misalignments of individual components can mean drastic losses of efficiency while the problem is detected.
Skyline's systems-level approach allows them to design high-efficiency components that are pre-assembled to avoid the loss of efficiency that results from component misalignment. The high-gain system makes the most of each silicon cell, whereas systems that use high-performance silicon PV cells and don't multiply their output through solar concentrators can be seen as "wasting" the cells. Skyline's approach is to get the most out of each cell in the system:"because we're taking a systems viewpoint, we can stretch silicon capacity, so we can take a 6GW-capacity system and turn it into a 60-80GW system." This high-capacity, highly scalable approach, coupled with its ease of manufacturing and installation using local labor and resources, make Skyline's high-gain solar arrays a product to watch for when they become available at the year's end.
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