Would you invest in a relatively new product that offered half of the performance in efficiency when for just minimally higher cost you could purchase a much higher efficiency product with over a half century worth of proven performance ?.
The discussion here pertains to thin film solar products versus crystalline solar products on residential roof tops. Year after year companies have come and gone that have tried to successfully commercialize thin film solar panels in the U.S. market. Many of those companies failed, leaving their customers with unenforceable warrantees.
In light of the developing market for solar, several companies are once again attempting to make thin film meet up to its promises of lower priced solar, but at what cost.
Well for one, efficiency. Many of the thin film amorphous solar products on the market today offer efficiency ratings that are quite inferior when compared to conventional mono and poly crystalline solar products. 6.3% for one of the more popular thin film amorphous a-Si solar panels that are being offered on today's market versus 13 to 14 % for conventional crystalline silicon solar products. If this reduced efficiency was reflected in the true cost to the consumer, then thin film might be something worthwhile for residential applications.
But consider the following:
1. Because of thin film's low efficiency ratings, a thin film
CdTe or amorphous a-Si solar panel can be up to twice the size of a typical
crystalline solar panel. Here's a graphic that was drawn to scale in Photoshop
which illustrates the difference in size of a 60 Watt thin film amorphous a-Si solar panel
shown in purple versus a 60 Watt crystalline
solar panel shown in yellow. Note how much
space the less efficient thin film solar
Part of the reason for this thin film a-Si solar panel's larger size is an anomaly know as the Staebler-Wronski effect whereby the conversion efficiency of a amorphous thin film solar panel has the tendency to degrade causing a drop in output of up to 20% when it is first exposed to sunlight. This drop in output requires that the thin film solar panel be intentionally manufactured to a temporary higher rating to make up for this loss once the solar panel has finally stabilized after about 6 weeks and has settled at its 60 Watt nominal rating.
2. This increase in size obviously translates into increased weight. So the load that is applied to a residential rooftop is significantly increased. For example a 1,920 Watt solar system consisting of thirty two of the 60 watt thin film solar panels referenced above weigh approximately 985.6 pounds whereas a high efficiency mono-crystalline system using only twelve 170 watt solar panels with an output of 2,040 watts weighs in at only 422.40 pounds. That's almost a half a ton on the roof of your home when installing a relatively small solar system consisting of thin film solar panels and we haven't even considered the added weight of the metal mounting racks that support the solar panels. A typical solar system for a residential application is about 4 kw so after adding in the weight of the mounting racks you're now talking about placing over one ton of weight on the roof of your home.
In the case of CdTe thin film solar panels, the CdTe deposition process and resultant crystal formation requires high temperatures for processing. As a result of this, CdTe must be manufactured in what is know as a "superstrate" arrangement in that sunlight must pass through some medium to get through to the active CdTe material. Glass is one of the only materials available that can survive these high temperatures and still be transparent enough to allow light to penetrate.
Because of it's inherent fragile nature, the glass which is used for this purpose must be very thick and heavy to survive the stresses that are encountered in the manufacturing process as well as in practical use. The increased processing of the glass used in CdTe solar technology can create the potential to increase stress in the glass "superstrate" which may lead to fractures after installation and of course adds to the weight load that a roof must carry in a residential application.
3. Increased weight means increased shipping costs for the same distance that the product is shipped, which of course raises your acquisition cost and the product's carbon footprint.
4. Along with the increased number of panels that are required, comes increased labor costs. Remember there are thirty two 60 Watt solar panels in this thin film solar system versus only twelve 170 Watt solar panels in the mono-crystalline solar system.
5. The increased number of solar panels in the thin film solar system means that a much larger metal mounting structure will be required to attach the panels to your roof which translates into a larger number of holes that will be needed to be drilled into your roof to mount the larger mounting structure which may increase the risk of roof leaks.
6. Several new companies that have appeared onto the thin film solar market are touting their product's reduced impact on the environment, but the CdTe ( Cadmium-Telluride ) technology that is used in many of these new thin film offerings contain the toxic, heavy metal, Cadmium. Cadmium and cadmium compounds are known to be human carcinogens which poses a concern for for health officials as well as policy makers. Of course direct exposure would be necessary but nonetheless the toxicity of Cadmium and Cadmium compounds do exist and must be addressed should breakage of a panel occur as well as disposal of this toxic material the end the the product's life cycle.
Thin film CdTe or amorphous solar may have it's applications in large multi-acre commercial "Solar Farm" installations, but on a residential roof top ?
Our opinion is this: Given the current state of the technology as well as the limited amount of time that current thin film CdTe ( Cadmium Telluride ) technology has been used in the field, both higher efficiency, poly-crystalline and mono-crystalline solar technologies posses significant time tested performance and reliability advantages that outweigh any possible cost saving that might be afforded by low efficiency thin film CdTe or amorphous a-Si solar products. Thin film solar, barring some major technological advancement is now and shall be for the near future one of the lowest efficiency performance options that the solar industry has to offer.
New CIGS Thin Film Technology Just Over The Horizon
If you are still interested in trying thin film or amorphous solar panels on the roof of your home, then we suggest that you wait a few more months. A relatively new, higher efficiency, thin film technology called CIGS ( Copper Indium Gallium Selenide ) is just about to reach wide spread commercialization here in the U.S.
CIGS solar Technology will offer higher efficiency than CdTe ( Cadmium Telluride ) or amorphous a-Si technology, it will be lighter in weight and it does not use the toxic heavy metal Cadmium.
CIGS Solar technology consists of a higher efficiency heterojunction system. The Copper Indium diSelenide material that is used in CIGS technology has an very high light absorption capacity that allows up to 99% of the light which strikes the material to be absorbed within the first micron of material depth. By adding minute amounts of Gallium, the light absorbing band gap is increased which allows the CIGS material to more closely match the Sun's light spectrum thereby improving the cell's efficiency and the voltage output.
Recently a team at the National Renewable Energy Laboratory reached a new world record efficiency rating of 19.9% by modifying the CIGS surface. Of course this result was created in a lab but the potential for much higher efficiency when compared to CdTe and a-Si technology does exist. CIGS solar offers an additional advantage in that it doesn't degrade like other thin film technologies, is cost effective to manufacture and can be deposited onto low cost, flexible substrates.