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Chemicals used in Manufacturing Solar Panels


Whilst a household-sized set of solar panels can save approximately 1,000kg of carbon dioxide emissions every year into the atmosphere, they are sometimes criticized because their manufacturing process uses a lot of chemicals and energy. Just what is the environmental toll of building solar panels in the first place?

Solar panels which generate electricity are made of photovoltaic cells - light sensitive cells that produce a direct current. These photovoltaic cells are made of compound semiconductors – a large range of elements from the periodic table can be combined as compound semiconductors.

toxic waste
Photo credit: ThinkGeek.com


These elements used for semiconductors are found naturally in trace amounts in the the ground and in water. The quantity varies depending on the geology - they leak out from naturally occurring rocks. Indeed, some of these elements are vital for humans, such as zinc, aluminium and selenium, deficiencies in which can cause serious health problems.

However, some of the elements used in solar panels are much more harmful. Arsenic, cadmium, mercury, selenium (yes, the same as above, but in higher quantities it's harmful!) and antimony are all commonly used in their fabrication and they are all classified as pollutants by many countries, such as the USA.

Solar panels are certainly not the main source of these harmful elements. Extensive geochemical mapping has been done in detail across the UK. This has shown up many regional hot spots of potentially harmful chemicals, often demonstrating areas of high vehicle traffic density have high levels of these pollutants. Ethyl lead used to be added to petrol, and due to this legislation only being introduced well after the adoption of the car, certain areas already register high levels for this reason.

Another significant man-made cause of these pollutants is mining. Mine waters have a high level of acidity, which can cause arsenic and zinc to leak out of rocks and into the surface and ground water even long after mines are closed.

Cadmium, Mercury and Zinc

Shipham in Somerset, England, has an abnormally high level of cadmium. This is due to historical mining of lead and zinc started in medieval times. Cadmium is a substitute for zinc in the mineral sphalerite, which was something that would be discarded by mines as waste. Studies have found little evidence of health risks to the population of Shipham due to these deposits, although this may be due to most food being bought in supermarkets rather than being grown locally.

Mercury, most commonly found around your house in old-fashioned thermometers, has long been known as a harmful pollutant. In the 1950s, many neurological diseases were first found to be caused by it, such as Minamata disease – a syndrome which causes numbness in hands and feet, narrowed vision and damage to hearing and speech. Particularly severe cases involve insanity, paralysis, comas or even death within a few weeks of the disease's onset. It is caused by eating seafood contaminated with mercury.

The protagonists in mercury pollution are coal-burning power stations and the artisan gold-mining industry. This type of gold-mining is particularly harmful for human health – artisans, typically in countries such as Tanzania, mix mercury by hand with crushed ore to extract gold. This clearly exposes the individual to a very high level of mercury, with tools seldom used to do the mixing. The waste from this process is also usually just thrown onto the ground, which is then absorbed by water and therefore extending the reach of the pollution significantly.

You've probably heard of zinc being a useful vitamin in your diet. Indeed, it is essential as a trace element for basic biological functions. However, at high concentrations, it is a pollutant. Metal workers are the main victims of zinc poisoning – inhalation of zinc oxide fumes causes 'zinc chills' – violent shaking, although the long-term harm of this is unknown. Zinc may even reduce the uptake of other harmful elements, such as cadmium.

Aluminium, Gallium and Indium

Gallium and aluminium are very similar to each other chemically. Indeed, gallium is extracted from aluminium ore. 8% of the outer layer of the earth's crust is aluminium, meaning it's very abundant, especially in comparison to gallium which is much rarer.

Whilst there have been no conclusive studies linking gallium to harmful health effects, aluminium has, not without controversy, been linked to Alzheimer's. However, natural exposure to aluminium far outweighs human-caused pollution due to it's sheer abundance.

Finally, indium is a rarely occurring chemical in nature and, as with gallium, has not at this point, been tied conclusively to any health risks.

Arsenic, Antimony and Phosphorus

Arsenic is inexorably considered a poison – just 180mg is enough to kill a human. There are two principle types of arsenic – As+3 and As+5. Waste from solar panel production can contain high levels of soluble As+5, which fortunately is over 50 times less harmful than As+3. However, it is still known to be very harmful, and some As+3 is still released too.

Arsenic can actually be released into water as long as arsenic in sediments is reduced from As+5 to As+3 and unbound from certain other minerals. Indeed, millions of people across the world, particularly in India, receive a high dosage of arsenic from drinking water – wells using groundwater in certain areas can contain harmful levels. A large amount of exposure over several years has been proved to cause several types of cancer. Even in more economically developed countries, arsenic exposure has to be carefully avoided – the water produced by wells even in the USA has to be treated to reduce levels of this pollutant.

Antimony, similarly to arsenic, is highly toxic, but it has a relatively low bio-availability – the rate at which is is absorbed, meaning a larger dose is required to cause serious health-effects. Smelting, used in lead and zinc production, is the main human cause of this pollution. In the news, it has recently featured because of fears that it would leach from plastic water bottles, being ingested. Indeed, Japan has banned the use of this type of packaging, although it is still commonplace in the rest of the world.

Phosphorus is an essential element for life in its inorganic form. It is the second most abundant mineral in your body after calcium, making up 1% of your body-weight. Phosphorus is found in bones and teeth and in cell energy production. The main molecule in your body for storing energy, ATP, contains phosphorus. It also helps synthesize protein, and is used to make cell membranes. It certainly has its uses!

The element phosphorus can exist in several forms, such as red or white phosphorus. Indeed, white phosphorus is infamous for causing 'phossy jaw' – a condition that plagued Victorian match makers. This disease became common because women making matches were exposed to the chemical during the manufacturing process. This was later easily avoided by switching to red phosphorus, which is still harmful but only in larger doses.

Conclusion

Manufacturing solar panels, if carried out carefully, does not pose a great risk to factory workers. The elements in their waste could potentially have a severe environmental impact if they are not disposed off properly and according to local regulations. The semiconductor industry as a whole accounts for only a very small percentage overall of these pollutants, but it is definitely an important consideration for the industry as a whole, particular as it is currently undergoing massive growth.

Thanks to Keith Torrance and Helen Keenan from the University of Strathclyde, and Compound Semiconductor magazine for the facts and figures behind this article.
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