Most solar enthusiasts know that there are two main types of solar modules (commonly understood to be panels) in mass production today – monocrystalline and polycrystalline. For many years in the solar industry, these two types of panels have competed for the number one spot in the market. The current standard for the panel type being installed today is typically monocrystalline, which is largely because of the higher efficiency numbers and wattage outputs experienced during laboratory testing when compared to polycrystalline panels. On average, a monocrystalline panel has an efficiency of about 16-18%, whereas a polycrystalline panel with equivalent specifications has an efficiency around 14-16%. Monocrystalline panels have, historically, been more expensive. Because of the recent technological developments in the monocrystalline sector, however, the cost per watt between monocrystalline and polycrystalline is roughly identical. Looking exclusively at the production numbers and comparable costs noted previously, it is easy to see why most installers choose to install monocrystalline panels on the majority of their systems. But do these numbers tell the whole truth?
Typically, solar panels / modules installed in many residential and commercial applications are comprised of 60-72 cells per panel. The efficiencies and max power outputs of the modules are based on tests conducted in laboratories on the individual solar cells that make up each panel. Solar cells are tested with a highly-concentrated light placed directly above each solar cell. This light gives off 1000W/m2 of light directly on top of each solar, and special instruments are used to see how the cell performs in this (essentially ideal) condition. This is a very reasonable testing method and is, currently, the only standard for testing cells in modules today. If the sun was always directly on top of each cell in all of the panels in an installation, this testing method would be perfect, but that is not the case. The sun is in constant motion and is very rarely directly on top of each cell – which is where the internal textural differences between monocrystalline and polycrystalline solar cells comes into play in the system performance scenario.
Polycrystalline (left) vs Monocrystalline (right) solar cell texturing. (Image from RenewableEnergyWorld.com)
Monocrystalline and polycrystalline solar cells have very different internal textures, which impacts the cell’s performance under different lighting scenarios. Because of the way monocrystalline is made, cells comprised of it have a very uniform internal texture of pyramids, meaning that the cell works great when light shines directly on top of them because it optimizes the cell’s available surface patterns (see photo above). The overhead light passes directly through the pyramids and little light is wasted. That said, if light shines in from the side, however, the surface pattern available becomes less broad, making the cells less productive.
Polycrystalline has a much more random internal texturing, similar to rough sand paper (see photo above). This less uniform internal texture is much less ideal in overhead light, as noted previously, but is significantly better at catching more diffuse light – light that enters the cell from many different directions (i.e. the light that shines through a cloud). Hence, polycrystalline panels soak up much more light during non-ideal weather, as well as in the morning and evening when the sun is much lower in the sky.
Depending upon the climate in which the solar panels are to be installed, monocrystalline panels may not be the best choice, particularly if the panels will be fixed in position instead of on a tracker. We at Ecosolar and Electric are planning on doing some original research in the near future to compare the energy output of fixed monocrystalline and polycrystalline panels to see how their production values compare in our Southern Oregon climate.