Bob MacDonald, Skyline Solar, Mountain View, CA USA
Pre-engineered solar PV power systems are the next step as solar manufacturers leverage well-established manufacturing processes, infrastructure, and supply chains to ensure their systems are easy to implement, cost effective and scalable.
By the end of 2008, total world energy consumption was approximately 15TW [1] while the PV installed capacity was 16GW [2]. To make a significant contribution to worldwide energy demand, the industry needs to move from gigawatts of production to terawatts. The historic rate of growth for PV since 1975 has been 30% per year [3], and in the last decade the growth has been close to 40% [4]. The continuation of this exponential growth for several more decades, however, will create new challenges.
Many solar technologies will have trouble scaling to meet the needs of large commercial customers, industrial sites or utilities because these technologies were not designed with a systems-based approach. The reality is that there exists a "factory in the field" phenomenon that creates rework, waste and inefficiencies. End users are just beginning to see systems-level innovation to streamline the installation process and provide field-level scalability. Pre-engineered solar PV power systems are the next step as solar manufacturers leverage well-established manufacturing processes, infrastructure, and supply chains to ensure their systems are easy to implement, cost effective and scalable.
Pre-engineered high-gain PV system
An emerging class of pre-engineered, high-gain PV systems, are those that have a manufacturing strategy that relies only on the existing silicon solar cell, module, reflector and aluminum parts supply chain, as well as turnkey PV module production lines and metal fabrication industries that already exist at enormous scale. These systems are known as "high-gain" and they can also enable cost effective replacement of PV panels as cell efficiencies improve. As the industry scales, the carbon footprint and recyclability of the systems becomes ever more important. High-gain systems address these issues by getting dramatically more energy per pound of silicon than a traditional flat-plate PV system.
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| Figure 1. Market segmentation model. |
Pre-engineered tracked PV systems combine the best aspects of tracked PV and tracked CSP and are a leading solution for sunny climates. These systems are optimal over a wide range of applications from 100kW to multi-megawatt installations. The market segmentation for various technologies across system size and annual sunshine is graphically summarized in Fig. 1.
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Figure 2. Evolution from tracked PV and CSP to high-gain solar. |
Combining the best of PV and CSP
Skyline Solar's High Gain Solar (HGS) approach takes the next step beyond traditional PV and CSP by combining the best concepts from each industry while addressing their weaknesses, as seen in Fig. 2. High-gain architecture uses proven silicon cells, but uses them more efficiently by reducing the amount of silicon required per Watt. It also improves cooling to enable higher cell operating efficiency and tightly integrates racking and tracking functions for simpler deployment and lower operating costs.
The new technology takes advantage of long reflective troughs and single axis tracking that have been proven in the CSP industry over decades of operating experience. Unlike the heavy reflective structures (silver-coated glass mirrors) used in early CSP, the high-gain approach uses sheets of low cost reflective metal encased in oxide layers to ensure high durability.
CSP systems are best suited for central power plants requiring large up-front design and capital investments, whereas the high-gain approach is based on much smaller and more modular building blocks, which can be used in plants ranging from less than one hundred kilowatts to many megawatts with capital investment proportional with size.
Designing for scale
At the core of the high-gain approach is the separation of light collection from the energy conversion done by the PV portion of the system. The reflective portion of the system serves two functions; it provides structural support for the panels – similar to a traditional solar rack – and it collects and reflects light from a large aperture onto the much smaller surface area of HGS panels. Skyline Solar's system currently has a concentration factor of roughly seven times.
The HGS reflector rack components are made with metal extrusions, stamps and die casts available in industrial parks around the world. These reflector racks are produced from roll-to-roll sheets that are available in high volumes for lighting applications.
The manufacturing process follows an efficient path from rolled metal through stamping and robotic bonding and assembly. Manufacturing considerations that lower costs guide every facet of the high-gain design. This includes retaining integral fractions of standard cell and reflector widths, minimizing cuts, joints and machined features, and integrating racking and tracker support into the reflective rack.
High-gain systems significantly reduce parts per watt and unit operations per watt. Capital costs are an order of magnitude smaller and support the plausibility of a quick capacity ramp from megawatts to gigawatts. This emerging product design and commercialization strategy promise to dramatically lower levelized cost of energy (LCOE) and achieve grid parity more rapidly than other technologies at a substantially lower risk
Integrated tracking enables high gain arrays to run at peak output through most of the daylight hours. This means they deliver more energy during peak afternoon demand when utilities charge their highest rates.
Although other companies are starting to include these design elements in their products – most notably, single-axis tracked high efficiency silicon systems – the traditional paradigm of component-level optimization around flat panels has slowed the movement towards total system level optimization. For example, most panel vendors do not make trackers, and tracker vendors do not make panels. Panels are manufactured in the largest practical sizes, and trackers end up having generic (often over-engineered) designs that work for a range of commonly available panel sizes.
Each set of vendors is constrained to working within its own sphere of influence. Working within a silo, it's not possible to rethink multiple elements such as form factor, cooling and other untapped opportunities for higher gain. As an example, leading PV trackers typically have a range of ±45°. However, a high-gain system with an integrated approach can have a range of ±80° for greater energy capture throughout the day.
Pre-engineered design simplifies installation
PV arrays have grown much larger for utility scale deployment, to speed installation and reduce theft. HGS rack assemblies go beyond increasing size; they also are pre-engineered for simplified construction in the field. First, they have 50% fewer parts than a traditional tracked PV system because they are designed with more pre-fabricated sub-assemblies. The reflective rack provides both the reflective surface and the structural rack function. Second, by engineering alignment of the structure into the design, installation time is reduced; this can be done by a tightly coupled linkage between the arrays.
As solar panels and components get larger to speed on-site installation, it is important to make sure that these sub-assemblies are also engineered for efficient packaging and transportation. For example, HGS racks are designed to stack compactly in standard shipping containers with minimal packaging enabling efficient shipping from factory to field. This systems thinking enables a significant reduction in packaging materials, construction waste and the associated cost of recycling this material. In a typical 10MW PV site, the cardboard waste alone can be 140 tons. Fortunately, this is typically recycled. However, as we grow PV deployments to utility scale, reducing the construction waste is an issue that deserves attention.
Upgradable design
An architecture that can be upgraded as PV efficiency improves would have significant benefits. This means that an investment in an infrastructure could be upgraded over time increasing the upside potential for generating more energy and revenue in the same space. This upgradability would also significantly extend the life of the solar power plant.
For systems such as high-gain types, the PV portion of the system is dramatically smaller than traditional silicon or thin film PV system, so existing systems in the field can be profitably upgraded to use the latest efficient PV technology when it becomes available. The other durable system components (footing, reflective racking, etc.) can be used for decades to come. With proper engineering for upgradability, this type of design can help move the industry beyond just building solar power plants toward creating a solar power infrastructure.
Carbon footprint and recyclability
Embodied energy and associated energy payback are key metrics used to judge the impact a technology has on the environment. Embodied energy is the total amount of energy required to build the system from raw materials through conversion to the final installed product. Energy payback is the period of time required for the system to generate that amount of energy embodied in it.
The typical energy payback time for a PV system has been estimated to be 2.5 years to 3.1 years in 2000 depending on whether thin film is used or mono crystalline silicon [5]. This payback time is significantly better than 30 years ago and improvements have been made in the last decade. However, if the industry continues to grow to meet the terawatts of energy that are needed, even a two year energy payback may be too much.
The environmental footprint created by high-gain systems is smaller relative to traditional flat plate system in two ways: they have lower embodied energy and faster payback than traditional PV. Since HGS is manufactured almost entirely out of recyclable metal, there is lower energy input up front. Additionally, the high-gain system uses substantially less silicon, which requires a high level of energy to manufacture, and it uses less glass and encapsulant, which are hard to recycle. In addition, a tracked system produces up to 30% more energy than an untracked system, so the additional energy production creates a faster energy payback time. One estimate of energy payback for a concentrating system has been estimated at between 0.7 and 1.3 years [6].
Recyclability should also be considered. At the end of its useful life, the majority of the metal content of high-gain systems can be recycled and re-used. In addition, the ability to upgrade just the PV portion of a system extends the life of the entire system and therefore reduces the embodied energy needed to replace the complete system.
Conclusion
As PV systems expand to meet the energy needs around the world, an integrated pre-engineered approach has several advantages including:
- Reduces capital investment and technology risk;
- Empowers an efficient, streamlined and swiftly scalable approach to manufacturing operations, supply chain logistics, and installation;
- Delivers customizable and scalable systems to meet a range of applications, without introducing new layers of system complexity and labor costs;
- Delivers an infrastructure that can be upgraded with newer technology and
- Reduces construction waste and overall carbon footprint
This type of design philosophy will help move the industry toward building a sustainable solar infrastructure on a global scale.
References
- Energy - Consumption "Consumption by fuel, 1965 - 2008," Statistical Review of World Energy 2009, BP, from information found at http://www.bp.com/statisticalreview
- Renewables Global Status Report: 2009 Update, Renewable Energy Policy Network for the 21st Century, p. 12, http://www.ren21.net/pdf/RE_GSR_2009_update.pdf.
- Green Mountain Engineering; Solar Scalability: Critical Metrics for Technology Assessment by Tyler Williams, November 23, 2009, http://blog.greenmountainengineering.com
- W. Hoffmann, L. Waldmann, Ch. 3, "PV Solar Electricity: From a Niche Market to One of the Most Important Mainstream Markets for Electricity," in High-Efficient Low-Cost Photovoltaics,
p. 29, 2009. - E. Alsema, "Energy Pay-back Time and CO2 Emissions of PV Systems," in Progress in Photovoltaics: Research And Applications, Ch. 8, pp. 17-25, 2000.
- G. Peharz, F. Dimroth, "Energy Payback Time of the High-concentration PV System FLATCON," in Progress in Photovoltaics: Research and Applications, vol. 13, pp. 627–634, 2005.
Bob MacDonald received a BSEE degree and MS and PhD degrees in physics from Brown U., and an MSEE from Stanford U. and is co-founder/CEO at Skyline Solar, 185 E. Dana St., Mountain View, CA 94041 USA; ph.: 650-864-9770; email bmacdonald@skyline-solar.com
