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Solar cells are deployed in applications ranging from multi-megawatt utility installations to solar-powered consumer wristwatches. But some of the most interesting applications design engineers are working on today include portable power for remote monitoring and control systems, fixed power station for energy production, mobile robotics and solar charging stations.

There’s no doubt that electrical performance is an important factor when selecting a solar cell or panel. However, the form factor, packaging, construction, and other mechanical design considerations are also key factors in choosing the appropriate solar panel for any application.

For example, consider a solar panel used in an outdoor application without significant space or size constraints, such as a typical roof-top solar panel system connected to the power grid. First, the designer needs to understand the basic application requirements – the power output needed for the climate, solar angle and light available for energy conversion. These will help determine the best cell material and type. However, the environment, its potential impact on the panel’s useful life and the mechanical design for solid mounting, may also factor into the design configuration.

Basic panel construction for large area, low-cost applications is similar across many cell types (refer to first Solar Tech-Zone installment for a primer on cell types and materials). The basic requirements are:

• Rigid back plate for cell support and mounting

• Transparent front cover

• Encapsulation to seal out moisture

Broadly speaking, silicon cells for relatively large panels are either amorphous silicon (a-Si) or crystalline. These can be single-, poly-, or multi-crystal forms. While a-Si panels are created by coating a large piece of substrate glass with a continuous layer (sometimes referred to as a “thin film”) of a-Si, crystal silicon devices are manufactured into various shapes of cell. These individual cells are then assembled into 2D arrays to form individual panels.

There are similarities between the construction methods for both types of devices. Although there is research into coating other substrates with a-Si, the standard is to use heavy, robust glass plates. There are a number of reasons for this, not least of which is that thin film photovoltaic a-Si producers re-use most of the manufacturing equipment and know-how of the high-volume LCD display industry.

Many cell array panels using crystal silicon technology use glass substrates and front covers. The glass substrate is an integral part of both the cell and panel production. As a result, there are constraints on the substrate material considering the relatively high temperature of a-Si deposition to produce high quality solar cell material.

When it comes to a-Si, there is a reason “panel” and “pane” seem so similar. As you might imagine, the physical design and production of large panels are similar to glass windows. Just like a window, the glass shape, strength and clarity are only the beginning. The transparency needs to be held in place in such a way that the wind and rain (and sleet and snow for some of us) keep to one side. Furthermore, the framing design must maximize the area that lets light through.

Perhaps the best way to understand the basics of panel construction is to build one. Educational value aside, there’s little point in building something you can’t deploy to a real energy conversion application. Fortunately, Digi-Key offers an excellent solution.

The Parallax 33000-ND is a 30-W solar panel kit that can be assembled in the simplest home workshop with a modicum of care. In fact, reading through the datasheet for the Parallax 33000-ND provided on the Digi-Key web site explains the basics and many important details necessary to understand solar panel construction.

Figure 1: The Parallax 33000-ND solar panel kit provides everything you need to construct your own 30-W solar panel suitable for a wide range of applications.

All the budding solar panel assembler requires aside from the kit are: a 30 to 40-W soldering iron and solder, multimeter, scissors, x-acto knife, masking tape, ruler or measuring tape, hot glue gun and glue, felt-tipped pen, safety glasses and a reasonably large enough work surface that is clean and flat.

The main mechanical components of the panel are the milled base panel and the front cover, both comprising polycarbonate. Naturally, the key electrical components are the 13 2.5-W solar cells. The bill-of-materials also includes four feet of double-bus wire, 25 feet of tinned tabbing wire, a flux pen (this is your friend according to the video and instructions), polycarbonate cement, work gloves, and 6-inch square plywood “workbenches” to provide a perfectly flat surface for tabbing individual cells.

Since the online materials provided by Parallax are both comprehensive and easily digestable, we won’t cover the panel assembly details here. Standard tabbing and bus connections will be well-known to anyone who reviews these materials, while someone who puts together one of the panel kits will have mastered the same technology used by several commercial manufacturers.

Figure 2: Adding the tabbing wires on one of the solar cells in the Parallax 33000-ND panel kit.

The Parallax 33000-ND panel casing differs from commercial panel construction. High volume assembly plants choose different materials for the substrate and covers, based on cost and the ability to automate processes. On the other hand, the Parallax kit was designed to allow reasonably skilled persons to assemble the components.

The Parallax 33000-ND starts with a milled polycarbonate base panel designed to allow straightforward placement of individual cells with convenient routing for tab and bus wires. Polycarbonate cement bonds the cover panel to the base thereby welding the two pieces together. Cementing the two polycarbonate panel pieces together forms a tight seal against the environment while providing a stiff and durable shell to the entire assembly.

Figure 3: The Parallax 33000-ND includes a milled polycarbonate base panel shown here with bus wiring in place prior to installing the cells.

Polycarbonate is favorable when compared against other panel casing options. It has excellent mechanical properties, including very high impact strength, compared to other plastics. There are cheaper alternatives for high volume assembly plants, but the polycarbonate shell of the Parallax 33000-ND allows simplified construction for the novice while offering s solid platform that will allow the finished panel to be deployed to harsh outdoor environments. Furthermore, the polycarbonate is clear as well as structural, maintaining transparency right to the panel edge unlike alternatives that add a metal frame to the assembly.

Although the final mechanical assembly differs from panels from other manufacturers, the Parallax 33000-ND kit offers two key advantages.

First, the Parallax kit relies on the high strength of the polycarbonate shell for rigidity. This provides even the hobbyist with an extremely robust and durable solar panel capable of offering significant power output for off-grid applications. Second, despite using alternative assembly technology, the Parallax 33000-ND provides serious insight into the relative merits of mechanical designs should the kit assembler ever need to procure commercially manufactured panels in the future. Even if you never need to put your understanding of panel construction to further use, assembling the Parallax 33000-ND kit provides a working 30 W panel capable of 6-V output (12 cells X 0.5 V).

Figure 4: A completed Parallax 33000-ND solar panel ready to deploy.

If the instruction tutorials provided by Parallax spark your interest, but you have another application in mind that does not require the polycarbonate panel substrate, there is another option. Digi-Key offers the same set of cells without the assembly hardware under part number 750-00036-ND.

If you don’t have time to build the Parallax 33000-ND solar panel kit, a quick review of the assembly instructions provides a very good appreciation for mechanical design requirements for a solar cell array. If you decide to take on this worthy challenge, you will get a first hand look at the fragility of the cells and why mechanical design is a critical piece of the puzzle. The Parallax 33000-ND includes a spare cell in case you break one during assembly. The Parallax kit instructions also includes details for cell repair should the single spare prove inadequate.