03 September 2014 | Posted by Zachary
Many of us think solar power is just one technology, or type of technology. In actuality, there are many types of solar power technology. Make sure you know what you're talking about at your next cocktail party by scrolling through the article below and coming articles in this series.
Note that this article doesn’t deal with solar hot water systems, passive solar heating or daylighting. It only deals with power generation technologies.
Furthermore, the main types of solar power technology are solar photovoltaics (PV) and concentrated solar thermal power (CSP), but this article is only discussing PV technologies. There are many different types of PV technologies and CSP technologies, so it seemed best to break these out into two articles.
The most common types of solar cells and solar panels on the market are crystalline silicon (c-Si) solar cells and panels. They account for over 90% of the global market. There are two main types of c-Si solar cells, and then there’s another type that isn’t widely on the market these days.
Monocrystalline silicon (aka mono-Si): Monocrystalline silicon solar cells use single-crystal silicon to convert sunlight into electricity. These solar cells are black or iridescent blue and somewhat rounded on the corners. This site explains how these solar cells are made and some of their key advantages and disadvantages. A top mono-Si solar cell and solar panel manufacturer is SunPower*.
Polycrystalline/Multicrystalline silicon (aka poly-Si): Polycrystalline (aka multicrystalline) silicon solar cells are similar, but less pure and typically less efficient. However, they’re also typically cheaper and are the type of solar cell most used around the world. Mono-Si solar cells are basically blue with a “metal flake” pattern on them. “Semiconductor grade (also solar grade) polycrystalline silicon is converted to ‘single crystal’ silicon – meaning that the randomly associated crystallites of silicon in ‘polycrystalline silicon’ are converted to a large ‘single’ crystal.... Polycrystalline silicon can be as much as 99.9999% pure,” Wikipedia states. Due to their low cost relative to efficiency, poly-Si solar cells and panels are the most commonly used solar cells and solar panels in the world, with many of the top solar manufacturers producing them. #1 solar panel producer Yingli Green Energy* produces both mono-Si and poly-Si solar products.
“The reason polycrystalline solar panels are less expensive than monocrystalline solar panels, is because of the way the silicon is made. Basically, the molten silicon is poured into a cast instead of being made into a single crystal,” Solar Facts and Advice states. “In the cast process, silicon pieces are melted in a ceramic crucible and then formed in a graphite mold to form an ingot. As the molten silicon is cooling a seed crystal of the desired crystal structure is introduced to facilitate formation. Although molding and using multiple silicon cells requires less silicon and reduces the manufacturing costs, it also reduces the efficiency of the solar panels.”
Thin-film solar cells are other types of solar cells that can be made in a variety of ways. The basic underlying characteristic of these is that a type of photovoltaic material is thinly added to a substrate. Different materials can be used with different effect. Wikipedia states:
Thin-film solar cells are usually categorized according to the photovoltaic material used:
* Amorphous silicon (a-Si) and other thin-film silicon (TF-Si)
* Cadmium telluride (CdTe)
* Copper indium gallium selenide (CIS or CIGS)
* Dye-sensitized solar cell (DSC) and other organic solar cells
Amorphous silicon (a-Si): This thin-film material is “is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates.” Because a-Si solar cells use about 1% of the silicon that is needed for typical c-Si solar cells, they save a lot on costs there. However, due to their inherent limitations, multiple layers have to be manufactured to capture different specific frequencies of light. In the end, that adds up to a cost that is higher per watt than conventional c-Si solar cells.
Nonetheless, thanks to the fact that a-Si can be deposited onto the substrate material at a relatively low temperature (as low as 75° Celsius), it can be used on some plastics, which allows for use in places where flexibility is needed or useful. Furthermore, where power needs aren’t great (for example, on calculators), the simpler manufacturing process can make this process more competitive.
Cadmium telluride (CdTe): Used by leading solar company First Solar, CdTe solar cells use cadmium and tellurium, usually with cadmium sulfide in the middle. Their low cost has led to their wide use, as has First Solar’s overall leadership as a solar power plant installer. These solar cells also do not see their efficiency as harmed by high heat as c-Si solar cells. It is the only type of solar power technology that beats c-Si solar on a cost basis in a significant number of markets and applications. “On a life cycle basis, CdTe PV has the smallest carbon footprint, lowest water use, and fastest energy payback time of all solar technologies,” Wikipedia writes.
Copper indium gallium selenide (CIGS): Deposited on glass or plastic, CIGS solar cells absorb light very strongly, which allows for a smaller amount of the photovoltaic material to be deposited on the substrate. The thin layer makes CIGS a better fit for use on flexible materials and for certain unique applications. Despite lower costs and decent efficiency, CIGS solar cells haven’t been able to compete with conventional solar PV following major cost drops related to changes in supply and demand, nor with CdTe solar products.
Organic solar cells (OSCs): “An organic solar cell or plastic solar cell is a type of polymer solar cell that uses organic electronics, a branch of electronics that deals with conductive organic polymers or small organic molecules, for light absorption and charge transport to produce electricity from sunlight by the photovoltaic effect,” Wikipedia writes. “The plastic used in organic solar cells has low production costs in high volumes. Combined with the flexibility of organic molecules, organic solar cells are potentially cost-effective for photovoltaic applications. Molecular engineering (e.g. changing the length and functional group of polymers) can change the energy gap, which allows chemical change in these materials. The optical absorption coefficient of organic molecules is high, so a large amount of light can be absorbed with a small amount of materials. The main disadvantages associated with organic photovoltaic cells are low efficiency, low stability and low strength compared to inorganic photovoltaic cells.”
Multi-junction solar cells: Conventional solar cells have one junction. That junction is where electric current is generated. Multi-junction solar cells use layers of different semiconductor materials in order to capture different wavelengths of light and boost the cell’s efficiency. Whereas the theoretical maximum efficiency for a single-junction solar cell is 34%, the theoretical maximum efficiency for a multi-junction solar cell is 87%. A similar story exists in the labs, where single-junction solar cells have reached an efficiency record of about 29% and multi-junction solar cells have reached an efficiency record of 44.7%.
In the end, though, the matter for commercial use is cost relative to performance. Due to their greater manufacturing needs and costs, multi-junction solar cells don’t compete with conventional single-junction solar cells. However, they are used in certain niche applications (like in outer space). There’s also the hope or expectation that they might work quite well with concentrating photovoltaic technology (see below) and the general idea of combining different layers of different semiconductor materials could be useful for thin-film solar panels, which can have simpler manufacturing processes than conventional c-Si but suffer from lower efficiency.
There are other very niche types of solar cells as well, or types of solar cells that just keep moving along in the lab but not commercially. For example, dye-sensitized solar cells, quantum dot solar cells, and plasmonic solar cells.
Opposite from thin-film solar cells, concentrated photovoltaics (CPV) are much more efficient than conventional c-Si solar cells. CPV uses optics (curved lenses, mirrors) to focus more light on photovoltaic cells. As with most of the other PV options above (which capture only a tiny portion of the PV market), market share (or lack thereof) comes down to cost.
“To get the sunlight focused on the small PV area, CPV systems require spending extra money on concentrating optics (lenses or mirrors) and sometimes solar trackers, and cooling systems. Because of these extra costs, CPV is far less common today than non-concentrated photovoltaics. However, ongoing research and development is trying to improve CPV technology and lower costs.”
The cooling mentioned there is important, and is one of the drawbacks of CPV. Concentrating light also concentrates heat, and heat doesn’t play well with solar PV material, so some mechanism of cooling the solar cells is typically needed in CPV systems.
But cost is coming down for CPV, and there is a lot of potential for it to compete with conventional PV on that critical metric. The cost of a solar collector could be much lower than the cost of extra solar PV material needed for the same extra output.
Another downside is the mismatch between CPV and diffuse light. “Diffuse light, which occurs in cloudy and overcast conditions, cannot be concentrated. To reach their maximum efficiency, CPV systems must be located in areas that receive plentiful direct sunlight.” That weakness is harder to get around, but for sunny locations, CPV may have a bright future.
Image Credits: SunPower; unknown; Panasonic x 2; First Solar; National Renewable Energy Laboratory; Adjwilley; Fraunhofer Institute for Solar Energy Systems; National Renewable Energy Laboratory; Cogenra.
*Full Disclosure: I own stock in SunPower and Yingli Green Energy.