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Different Types of Solar Panels

There’s a lot more to selecting a solar panel than just output. With several cell technologies to choose from, you need to balance factors like cost, performance and durability to find a good fit for your system. Here’s what you need to know to pick the right type of panel for your power generation needs.

solar panel

How Solar Panels Fit into a Portable Generation System

A solar cell is made from two or more layers of material with opposing electrical charges. To get an electric charge, materials are “doped” with materials that add or remove electrons. In a typical silicon cell, one layer is doped with phosphorous, adding electrons to create a negative charge. The opposing layer is doped with boron, reducing the number of electrons, creating a positive charge.

When exposed to sunlight, photons knock free electrons off of atoms inside these layers, generating direct current (DC) electricity. Thin wires inside the cell connect to the panel's electrical network, sending the charge to terminals that connect to the rest of the power system.

Understanding Solar Panel Ratings

There are four main factors to consider when choosing a solar panel.

  • Efficiency: This is the amount of sunlight a panel can utilize to generate electricity. All things being equal, a panel with 20% efficient cells will produce twice as much electricity as a panel with 10% efficient cells. Using efficient cells decreases the size and number of panels needed to deliver the required amount of power for your system.
  • Cost: While there are only a few materials used to make solar cells, manufacturing techniques vary widely. This has a major impact on panel pricing. A higher price doesn’t necessarily mean a panel will perform better.
  • Thermal performance: Most common technologies perform well in normal climate conditions, but output can drop off in extreme heat and cold.
  • Shade performance: Solar cells need a certain amount of light to kick off electrical reactions. The lower this threshold is, the better the cells will work in low light conditions. This extends how long they can generate power, and it makes them less sensitive to changes in the sun’s position.

Ratings are defined by Standard Test Conditions (STC.) This is the panel’s output at 77°F (25°C) with an air mass spectrum of 1.5 and 1,000 watts of sun light per square meter, or about 10.7 square feet.

Air mass spectrum is the type of sunlight that reaches a panel after passing through the atmosphere. Seasons are caused by the Earth tilting on its access. This change in angle forces light to pass through more or less atmosphere. If the sunlight is coming in at a sharp angle, as it would close to the poles or in winter, there’s more air mass, decreasing available light. If the panel is close to the equator during the summer, there’s less air mass, increasing light. Altitude also has an effect, but it’s not big enough to be noticeable in ground-based applications.

Some panels have a temperature coefficient rating. This is the expected loss in output by temperature. Let’s say you’re using your solar generator on a hot summer day. It’s 95°F (35°C) outside, and your 100 watt panel has a temperature coefficient of 0.5%/°C. On average, the panel surface will be 15°C higher than the ambient temperature, so the cells are 50°C, 25 degrees more than STC. 0.5% X 25 = 12.5%. The panel’s output is 12.5% less, or 87.5 watts.

Currently, there are three manufacturing processes and four cell chemistries on the market. Each type of cell has advantages and disadvantages.

solar panel  reflect

Monocrystalline Silicon

These panels are made by slicing a uniform silicon crystal into wafers. This creates a panel with a series of circular sections, each with a uniform surface. These average 15-20% efficiency, making them the most powerful cells on the market. They’re also the longest lasting cells. It’s common for manufacturers to warranty stationary panels for 25 years.

Manufacturing starts by making silicon ingots using the Czochralski process. A small seed crystal is dipped into molten silicon, growing it into a larger crystal. The resulting ingot is polished and sliced. This process is energy intensive and makes a significant amount of waste, so it’s the most expensive way to make a solar cell.

These panels are less efficient in cold weather. If the panel is partially shaded or covered in dirt or snow, the entire circuit will produce less electricity. Just one shaded cell can drop output by 20%. This makes it important to keep the panel clean and in full sunlight to maintain performance.

Polycrystalline Silicon

Also called multi-crystalline cells, these are made by pouring raw silicon into molds, then cutting the resulting ingots into square wafers. Multiple crystals form during the cooling process, creating a streaked, speckled appearance. Almost all of the silicon ends up in the finished product, cutting manufacturing costs. Durability is close to that of monocrystaline cells, but they’re less than half the price.

These panels are only 13-16% efficient, so more cells are needed for a given output. They’re more sensitive to heat than monocrystalline panels, but they’re less sensitive to shade.

Thin-Film Solar Cells

Also called TFSC or amorphous cells, these panels are made by depositing thin layers of photovoltaic materials on a substrate. No matter the material, this is the most cost effective manufacturing process. The resulting panels can be solid or flexible, and they aren’t affected as much by high temperatures and shade as other cells. Panels made from these cells are dark brown, gray or black.

Thin-film cells are only 7-13% efficient with most cells averaging an output of 9%. They also degrade faster than other cell technologies. However, their flexibility makes them less susceptible to damage, and some panels can be folded for storage. This makes these cells popular in portable applications.

There are three thin film chemistries on the market today: cadmium telluride (CdTe,) amorphous silicone (a-Si) and copper indium gallium selenide (CIS/CIGS.)

  • Cadmium Telluride (CdTe) – This is the most popular thin film technology, and it’s the only real competitor to crystalline solar panels in terms of energy density. Current commercial products average an efficiency of 9-11% with experimental products reaching 14%. These cells also work better in natural sunlight than any other cell technology. However, cadmium is a highly toxic material, causing concerns about pollution from manufacturing and disposal.
  • Amorphous Silicon (a-Si) - This type of cell is used in low voltage devices like calculators. A single a-Si cell doesn’t produce much electricity, but a 6-8% efficient cell can be built by stacking several layers of silicon. These cells need a small fraction of the silicon used in other cell manufacturing methods, but multi-layer cells are expensive to produce. These panels are also subject to the Staebler-Wronski effect. As they’re used, the chemical makeup of the cells changes, decreasing their output. After 6 months or so of continuous use, the cell chemistry stabilizes and output remains about the same for the rest of the panel’s life.
  • Copper Indium Gallium Selenide - Like CdTe, CIS/CIGS panels have a lot of potential, and they use less cadmium in manufacturing. Currently, commercially available panels are 10-12% efficient. In the future, this could be the most efficient technology with lab-made cells reaching 20% efficiency.

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