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Posted on February 24, 2009 by  & 

Environmental issues with energy harvesting

The environmental benefits of energy harvesting are proving to be greater and more widespread than originally realised. Most importantly, the runaway use of small batteries is leading to uncontrolled disposal of poisons such as lithium and highly alkaline electrolytes and exposure of children to these chemicals as they are used in everyday items such as toys and disposables such as talking gift cards. In industry, replacing the increasingly vast number of batteries is extremely expensive in both labor cost and materials and energy harvesting is increasingly the only way forward.
Different priorities
North America leads the world in energy harvesting in aerospace and military applications, from piezoelectric vibration harvesting for sensors in helicopters to all those photovoltaic panels on satellites. Europe leads in industrial applications, using thermoelectrics, electrodynamics, photovoltaics and piezoelectrics. In the UK, the Freeplay wind-up radios for the Third World need no battery. (Most wind up torches etc from East Asia have a battery that stays in for life but this adds cost and limits life). Freeplay radios also use photovoltaics.
East Asia leads in consumer applications, wristwatches with photovoltaics and electrodynamics being particularly successful as well as the huge number of calculators, toys and so on with photovoltaics and bicycles with dynamos. Next they will learn how to prevent flat batteries in all the four billion mobile phones worldwide. Many people have two and, in East Asia, replacing them every six months is commonplace. In addition, much work is also directed towards affordable, compact renewable power for those troublesome laptops. The first offerings are underwhelming. As the ergonomics are improved, it is interesting that the Massachusetts Institute of Technology One Laptop Per Child project for Africa has concluded that a ripcord is better than a crank for generating the electricity. Another step forward will be tightly rolled photovoltaics for portable electronics that pulls out and springs back as required.
An elephant in the room
Energy harvesting for small electronic and electrical products can clearly become a business of tens of billions of dollars yearly. However, there is an elephant in the room because some energy harvesting devices employ toxic or rare materials subject to price hikes. Most thermoelectrics use bismuth as bismuth telluride. Some proposed alternatives use lead. Most piezoelectrics use lead as lead zirconate titanate. The most efficient, lightest weight solar cells on all those satellites employ arsenic as gallium arsenide, and arsenic may be used as a dopant in nanosilicon inks for photovoltaics. Other forms of the new photovoltaics variously employ highly corrosive electrolyte (dye sensitised solar cells DSSC). In production, carcinogens are often used to make polymers employed as substrates and as the semiconductor in organic photovoltaics. Cadmium is used as cadmium telluride semiconductor in that form of photovoltaics and it is an element also used in copper indium gallium diselenide photovoltaics. Here it is in the form of cadmium sulfide buffer layers.
The good news concerning the poisonous elements is that they are so tightly bound in compounds such as cadmium telluride and cadmium selenide that they are highly unlikely to be released in use or disposal. They are also encapsulated. Cadmium telluride photovoltaics, based on by products of zinc and copper mining, uses only 1-2% of the amount of semiconductor used by traditional photovoltaics and its lower resulting cost and better temperature tolerance has already resulted in billions of dollars of orders being placed. The European Commission Joint Research Center concluded, "...CdTe used in PV is in an environmental stable form that does not leak into the environment during normal use or unforeseen accidents, and therefore can be considered the environmental safest current use of cadmium." First Solar PV Modules constitute one of the safest manners of deploying cadmium mining waste. They are even classified as "waste for recovery" and non-hazardous in accordance with the German Waste Code, European Waste Legislation and U.S. Environmental Protection Agency standards. Indeed, First Solar is able to argue that it binds up toxic waste to replace power stations and batteries and it is therefore doubly environmental. Huge orders from France and the USA have resulted. Arsenic dopant in semiconductors is only at trace levels.
However, regulatory authorities will not allow all of the elements we have mentioned to be used in the human body - where energy harvesting is starting to be used for sensing, drug delivery and life support - or in items that children may chew. Yet we need energy harvesting on medical disposables, e-labels, e-packaging, toys, electronic greeting cards and much more in our homes.
Silver that is employed in energy harvesting is both a precious metal and a biocide. Indium is more abundant than silver. Unfortunately, the huge amounts needed for so called CIGS photovoltaics and for transparent electrodes on much of the new printed electronics, including all other forms of flexible, low cost photovoltaics, means that further huge price hikes are possible. It is only produced as a by product of zinc mining. Similar concerns have been expressed about tellurium used in CdTe solar cells and in thermoelectrics.
Another aspect will come to the fore as energy harvesting reaches more consumer products. Recycling of packaging can be inhibited even by non toxic substances. For example, parts per million of metals can color glass when it is recycled, making it relatively useless.
Fortunately, about 500 organisations, half of them academic, have major programs to develop improved energy harvesting and there is massive market potential for what is available already, with no risk to humans. In addition, 650 organisations are developing photovoltaics beyond silicon that can be used for both energy harvesting and general production of power. Some is transparent and very low cost and tightly rollable and very wide area versions are in prospect a few off which even use heat as well as light. They may be followed by biodegradable, stretchable and even edible versions.
IDTechEx forecasts that, despite the concerns about certain materials in certain forms of energy harvesting, the consumer applications will remain in the ascendant over the next ten years, even moving to toys, labels and packaging at the end of that period. Industrial, military and aerospace applications will grow, with industrial applications becoming particularly important and widespread from wireless sensor networks to building controls. The number of energy harvesting devices will therefore grow as follows.
Figure: Global market for energy harvesting devices for small electronic and electrical equipment
Source "Energy Harvesting and Storage for Electronic Devices 2009-2019" IDTechEx.
This will be assisted by the advent of energy harvesting with more affordable, acceptable materials. For example, in stark contrast to traditional silicon, organic photovoltaics has the advantage of working optimally with only 15 nanometers thickness of semiconductor and it can be printed at high speed, which can keep cost well down. The thinness and flexibility opens up ten times the market potential. It can even turn a broader spectrum of light into electricity than the conventional forms of photovoltaics.
Lifetime is a problem, with barrier layers to extend life being rather expensive as yet. At least they consist of harmless inorganic oxides and nitrides alternating with harmless polymers. This will be solved within a few years with energy harvesting for disposable, low cost products of limited life being feasible in a few years with little or no use of rare elements and no toxic materials. Use of the new electroactive polymers, zinc oxide and organic piezoelectrics, carbon nanowire semiconductors, harmless compounds as quantum dot semiconductors and other options also promise further safety and environmental dividends.
Meanwhile, energy harvesting is providing at least ten years longer life than batteries used on their own. Indeed, batteries in electronic products rarely last longer than two years and there are many applications where they are thrown away in weeks or months. With 15 to 20 years life frequently offered for all the leading forms of energy harvesting, the environmental gain in saving disposal and - rarely even offered - recycling is considerable. Avoiding recharging batteries every few months also saves on the cost and pollution involved in visiting devices. The current state of play is shown below, though the actual figures are contentious. Those technologies with no moving parts are shown in red.
Figure: Performance of the favourite energy harvesting technologies.
Source "Energy Harvesting and Storage for Electronic Devices 2009-2019" IDTechEx.
There is too little work on biodegradable energy harvesting devices for disposable products. There is a strong move towards printing which means much less material is used. For example, with photovoltaics, Dye Sensitised Solar Cells DSSC, Copper Indium Gallium Arsenide and organic photovoltaics are all printed by ink jet or something similar by some suppliers. Piezoelectrics are sometimes screen printed and the best performing thermoelectric are in thin film form at about five to twenty micrometers thick. Commendably, substrates used when printing energy harvesting devices cause no chemical pollution. In the main, they consist of polyester or polyethylene naphthalate film or stainless steel foil where high temperatures are involved. However, for medical, consumer and other disposable products such as e-labels and e-packaging there should be more attention paid to biodegradable forms and the small number of developers using paper should be encouraged.
Top Image: Freeplay Foundation wind up or solar powered radio. (Source: Freeplay Foundation)

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Posted on: February 24, 2009

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