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Assessing Rare Metal Availability Challenges for Solar Energy Technologies

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Solar energy is commonly seen as a future energy source with significant potential. Ruthenium, gallium, indium and several other rare elements are common and vital components of many solar energy technologies, including dye-sensitized solar cells, CIGS cells and various artificial photosynthesis approaches. This study surveys solar energy technologies and their reliance on rare metals such as indium, gallium, and ruthenium. Several of these rare materials do not occur as primary ores, and are found as byproducts associated with primary base metal ores. This will have an impact on future production trends and the availability for various applications. In addition, the geological reserves of many vital metals are scarce and severely limit the potential of certain solar energy technologies. It is the conclusion of this study that certain solar energy concepts are unrealistic in terms of achieving TW scales.
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Sustainability 2015, 7, 11818-11837; doi:10.3390/su70911818
sustainability
ISSN 2071-1050
www.mdpi.com/journal/sustainability
Article
Assessing Rare Metal Availability Challenges for Solar
Energy Technologies
Leena Grandell 1,2,* and Mikael Höök 2
1 VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Espoo 02044, Finland
2 Global Energy Systems, Department of Earth Sciences, University of Uppsala, Villavägen 16,
Uppsala 75121, Sweden; E-Mail: Mikael.hook@geo.uu.se
* Author to whom correspondence should be addressed; E-Mail: leena.grandell@vtt.fi;
Tel.: +358-207-225-700; Fax: +358-207-227-604.
Academic Editor: Andrew Kusiak
Received: 2 June 2015 / Accepted: 20 August 2015 / Published: 26 August 2015
Abstract: Solar energy is commonly seen as a future energy source with significant
potential. Ruthenium, gallium, indium and several other rare elements are common and
vital components of many solar energy technologies, including dye-sensitized solar cells,
CIGS cells and various artificial photosynthesis approaches. This study surveys solar energy
technologies and their reliance on rare metals such as indium, gallium, and ruthenium.
Several of these rare materials do not occur as primary ores, and are found as byproducts
associated with primary base metal ores. This will have an impact on future production
trends and the availability for various applications. In addition, the geological reserves of
many vital metals are scarce and severely limit the potential of certain solar energy
technologies. It is the conclusion of this study that certain solar energy concepts are
unrealistic in terms of achieving TW scales.
Keywords: solar energy; solar cells; rare metals; material constraints
1. Introduction
Continued oil dependence is environmentally, economically and socially unsustainable [1]. Peaking
of conventional oil production has been a topic of interest for more than 50 years [2]. Anthropogenic
emissions of greenhouse gases and potentially harmful climatic change are strongly connected to
future hydrocarbon combustion [3], so reducing fossil fuel use has been an integral part of climate
OPEN ACCESS
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negotiations. All this has resulted in renewed interest in alternative energy systems. IPCC states that
the present energy system is not sustainable and that the solar energy could become a significant
contributor to the energy infrastructure [4].
Solar energy is commonly seen as a future energy source with significant potential. The amount of
energy that the Earth receives from the sun in a single hour is many times greater than the combined
output of fossil energy. Harvesting this abundant solar influx could, in theory, supply mankind with all
the energy it demands for millions of years. However, Ion concluded that the supply potential of an
energy source is generally dependent on concentration [5]. Numerous inexhaustible energy sources
exist, but their practical significance is often hampered by low energy density. This applies to solar
energy as it arrives in dilute form (up to 2500 kWh/m2 annually depending on location) requiring
significant area in comparison with more concentrated energy sources such as coal or nuclear.
To mitigate the low energy concentration in solar rays, numerous technical solutions have been put
into practice while others are being developed. Photovoltaic solar cells of various types capable of
converting the solar rays directly to electricity are already in the market, while concentrating solar
power based on thermal cycles is another solution. Another possibility is artificial photosynthesis,
aiming at mimicking natural photosynthesis, which can convert solar energy to carbohydrates or even
hydrogen for easy storage and human consumption. New renewable energy forms (geothermal, solar
energy, wind) only account for roughly 1.1% of the primary energy consumed in the world [6]. IPCC
estimates that direct solar energy constitutes only 0.1% of the primary energy supply [7].
The path to a solar future is long, and significant amounts of work, research and development
remain before solar energy will be a major energy supplier. It is also necessary to investigate solar energy
feasibility using a life-cycle perspective. Power plant installations consume concrete, steel, plastics and
similar everyday materials that are available in relative abundance and can be easily produced. Other
materials are uncommon or even rare and can only be produced in small volumes or by complex measures.
Some of these rare materials, mainly metals, are essential parts in certain solar energy technologies.
Aim of This Study
Historically, the most important obstacle for solar energy has been high costs in relation to
competing energy sources. If economics are disregarded and future solar energy systems assumed to
achieve a globally significant scale, the underlying reliance on rare metals might appear as one limiting
factor. Ruthenium, gallium, indium and several other metals are essential components of certain solar
energy technologies, such as dye-sensitized cells, thin-film cells and other innovative solar energy
technologies. More general approaches have also raised the importance of rare metals for high
technology such as the CRM Innonet (Critical Raw Materials Innovation Network) financed by the
European Commission [8].
The infrequent occurrence of these rare materials makes it necessary to ask whether they could limit
the growth of future solar energy expansion plans. Some researchers have already considered material
constraints for future solar energy applications [9–12]. There are assessments of natural resource
requirements for renewable energy systems, but they often dismiss potential resource constraints on
inadequate grounds [13,14].
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In this study, geological endowment of important minerals and the required production methods for
obtaining usable products are discussed. Reserve and resource data were compiled from various
geological assessments, mainly from the United States Geological Survey [15]. Based on the findings,
rough estimates are calculated for possible electricity production based on respective PV technologies.
The findings are finally discussed from a sustainability perspective.
2. Solar Energy and Rare Metals
The resource base for solar energy can be regarded in practical terms as limitless. However, due to
the dilute nature of solar energy, only a small fraction of this energy flow can be transformed into a
form usable for society. A useful metaphor is the distinction between tank and tap. Although the tank
may be enormous, it is the size of the tap that matters for users. It is only incoming solar radiation that
can be transformed into useful energy that matters for society. Thus, electricity is the required output
from most solar energy systems.
Some solar thermal technologies aim to use the heat of solar radiation for direct heating or for
powering conventional steam cycles. These systems generally rely on mirrors that concentrate solar
energy on a single point or a line. Fresnel lenses and parabolic troughs are simple and inexpensive
approaches that can achieve temperatures of 400–600 °C. Point focusing systems are more complex,
but can reach temperatures as high as around 1200 °C. Solar-powered Stirling engines [16], parabolic
trough systems [17], and concentrating solar power systems [18] have all been discussed more
comprehensively by others. The mirrors are plated with silver due to the high optical reflectivity of this
metal. Silver is not investigated in further detail in this study, but a recent analysis indicated that silver
could form a serious bottleneck for the construction of concentrated solar power on a large scale [19].
Photovoltaics (PV) or solar cells are alternative ways of harvesting solar energy by converting light
directly into electricity. Today, roughly 90% of the PV market is dependent on silicon [20]. Current
and foreseeable solar energy markets will probably be dominated by silicon technologies. Silicon-based
PV systems, forming the first generation of solar cells, will not be discussed in any detail since silicon
is a common material. However, silicon technologies commonly use silver as an electrode material and
this dependence is discussed in detail by Grandell and Thorenz [19].
Thin-film photovoltaics are referred to as second generation PV technologies. These involve several
approaches dependent on rare metals. Third generation photovoltaic technology has currently reached
a pre-market stage. Such technologies include dye-sensitised solar cells (DSSC), organic solar cells,
and other novel approaches.
2.1. Thin-Film Solar Cells
Thin-film cells consist of thin photoactive layers, typically in the range of 1–4 μm thick, leading to
a light-weight structure. A semiconducting material is deposited on a common material such as glass
or polymer. The need for semiconducting material is greatly reduced and could be up to 99% less
compared to c-Si based technology [21]. However, cost advantages from low material use are
somewhat offset by a lower electricity generation efficiency.
Silicon thin-films can be produced by chemical vapour deposition. Depending on the process, one can
obtain amorphous, microcrystalline or polycrystalline structures. The solar cells made from these
Sustainability 2015, 7 11821
materials tend to have lower energy conversion efficiency than bulk silicon, but the production
technology is very cost effective. The semiconducting material can be deposited on cheap materials,
and both flat and curved surfaces are possible. As a transparent conducting oxide, typically an
indium-tin-oxide (ITO) film with a thickness of 60 nm will be sputtered on the p-side of the
semiconductor [22]. Amorphous silicon suffers from optically induced conductivity changes that lead
to efficiency losses, resulting from the Staebler-Wronski effect [23], but this can be alleviated by
doping Ge into the structure. The efficiency of the cells is in the range of 11.6% [24].
Tellurium is classified as a critical metal [21], and is used in cadmium-telluride (CdTe) technology,
which is currently the most commercially successful thin-film application in the market. The band gap
of CdTe cells is 1.4 eV, which is very close to the ideal value of 1.5 eV [25]. Modules have achieved
17.5% efficiencies and the best reported cell efficiencies are as high as 20.4% [24].
CIGS cells are another successful thin-film technology based on a compound semiconductor made
of copper, indium, gallium, and selenide. Copper/indium/selenide (CIS) and copper/gallium/selenide
(CGS) form a solid solution with the chemical formula of Cu(InxGaz)Se2, where the value of x can
vary from 1 (pure copper indium selenide) to 0 (pure copper gallium selenide). The material is a
tetragonal chalcopyrite crystal structure, and a band gap can be varied between 1.04 eV (CIS) and
1.7 eV (CGS) through different material combinations [25]. Recently 15.7% module efficiency has
been reported [24]. Selenide can also be substituted by sulfur.
2.2. Dye-Sensitized Solar Cells
A dye-sensitized solar cell functions on a different principle than first and second generation
technologies. The incoming light is absorbed by a dye sensitizer that is anchored to the surface of a
mesoporous oxide film, typically TiO2. The dye gets excited by a photon, and the resulted electron is
injected into the conduction band of the film. The electrons diffuse to the anode and are conducted
over an external load to the cathode. The construction of the solar cell and its operation principle are
explained in detail by Gong [26].
The appeal of dye sensitized solar cells is that they rely on fairly abundant and inexpensive
materials. Manufacturing does not require elaborate equipment, and the simplicity of this type of solar
cell can potentially lead to good price/performance ratio. However, the most efficient cells generally
rely on dyes that are derived from rare metals.
The dye is essential for photovoltaic performance and needs to be carefully selected to fulfill several
technical requirements related to light absorption, ability to anchor the dye on the semiconducting
oxide, electron transfer properties between the dye and the semiconductor oxide and stability. Thus far,
the dyes are based on metal complexes of ruthenium. These dyes are superior to all other known dyes
in terms of light absorption. The highest performance achieved is 11.1%, exhibited by the black dye,
an atrithiocyanato-ruthenium complex [26]. Other approaches on an organic metal-free basis are being
developed [27].
2.3. Concentrating Photovoltaic Technology
The idea of a concentrating photovoltaic system is to generate concentrated illumination with the
help of systems of lenses or mirrors. The concentration factor can vary from 2 suns (low concentration)
Sustainability 2015, 7 11822
to 100 suns (medium concentration) or up to 1000 suns (high concentration). The concentrated solar
radiation is then directed to a small area of high-efficiency multijunction (MJ) solar cells.
Multijunction systems are currently the most proficient PV systems and can reach over 44%
efficiency [24]. The fundamental difference between multi-junction solar cells and c-Si solar cells is
that there are several junctions connected in series instead of one. This is to better cover the solar
spectrum. To achieve a working MJ cell, various suitable materials are placed in layers. Each layer is
optically in series, with the highest band gap material at the top. The first junction receives the entire
incoming spectrum. Photons above the band gap of the first junction are absorbed in the first layer.
Photons below the band gap of the first layer pass through to the lower layers to be absorbed there.
The thermodynamic Carnot cycle efficiency can be approached if all solar photons can be converted
to electricity. In theory, it can be shown that 59% efficiency can be achieved with four junction
devices [28] and as the number of junctions approaches infinity, the efficiency can reach as high as
68% [29]. However, it is difficult to construct such optoelectronically matched junctions, and thus
commercial devices are either tandem or triple-junction cells. Typical materials used in multijunction
cells are InGaP (band gap 1.67 eV) for top layers, GaInAs or GaAs (1.18 eV) for middle layers and Ge
(0.66 eV) as a bottom layer [30].
2.4. Emerging Solar Cell Technologies
There are various emerging solar cell technologies, still far from commercial markets. Organic
photovoltaics (OPV) are based on cheap, abundant, non-toxic materials and a high-speed roll-to-roll
manufacturing process. However, problems related to low conversion efficiency and instability over
time make it difficult to foresee the future potential of the technology. Other novel technologies still in
the fundamental development phase include quantum dots or wires, quantum wells, and super lattice
technologies [21].
2.5. Artificial Photosynthesis
Technologies aimed at mimicking photosynthesis are also a way of converting solar energy to
satisfy human needs. These approaches are commonly grouped in a field known as artificial
photosynthesis. They are not directly similar to photovoltaics, but also tend to rely on rare metals.
Natural photosynthesis uses light-harvesting complexes to collect incident photons, which
participate in chemical reactions to produce carbohydrates and oxygen from carbon dioxide. However,
natural photosynthesis observed in plants has very low efficiencies (typically ~1%) for biomass
production and this has stimulated great interest in creating an artificial counterpart with higher
efficiency [31]. Artificial photosynthesis could be used to convert and store solar energy as carbohydrates
or alternatively as hydrogen. In theory, this could solve many of the intermittency problems that are
related to more conventional forms of solar energy. The rare metal ruthenium is a key component in
many approaches and may be a limiting factor for implementation. Other platinum-group metals and
nickel might constitute alternatives [32].
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3. Occurrence of Rare Elements
Many of the rare metals used in solar cells occur in low concentration within the Earth’s crust. Most
do not occur as primary ores, and are only found as by-products associated with primary base metal
and precious-metal ores. This section briefly reviews the geological abundance of some rare elements
used in solar energy applications.
Cadmium is primarily extracted from zinc ores, mainly from sphalerite deposits. Cadmium has
chemical properties similar to zinc’s and can replace it in crystal lattices of certain ores. Sphalerite ore
contains 3%–11% zinc along with 0.0001%–0.2% cadmium and less than 0.0001%–0.01% indium,
copper, silver, iron, gold, germanium and thallium [12]. Carbonate-hosted sphalerite in Mississippi
Valley-type (MVT) deposits have high cadmium concentrations, while sedimentary exhalative (sedex)
occurrences have low concentrations [33]. Certain coals can also have relatively high cadmium
content, but they are all subeconomic for the moment [15].
The estimated abundance of indium is 0.1 ppm in the earth’s crust, making it the 69th most
abundant element [34]. However, indium is highly dispersed in nature and enriched deposits of
economic interest are rare. A comprehensive overview of indium and its mineralogy has been
conducted by Schwarz-Schampera and Herzig [35]. Indium is only recovered as a by-product from
zinc-sulfide ore mineral sphalerite [15]. Indium can also be found as trace element in deposits of other
base metals, but it is generally difficult to process and extract it economically.
Gallium can be found in low concentration in many ores. Burton et al. [36] investigated the
presence of gallium in 280 minerals and determined the crustal abundance to be 17 ppm, while
Emsley [34] gives an average concentration of 18 ppm. Andersson [9] notes that gallium is
approximately as abundant as copper but seldom forms any enriched mineralisation. In contrast,
copper is enriched by a factor of 200–800 in mined ores, while gallium rarely occurs in minable
concentrations. Such differences will have significant repercussions for production feasibility. Gallium
only seems to be concentrated in certain oxide minerals, primarily bauxite but also corundum and
magnetite [36]. Bauxite ores contain from 0.003% to 0.008% gallium [37]. World resources are
estimated to be 2 million tonnes in bauxite deposits and 6500 tons in zinc deposits [38]. Recent works
have also identified certain coals as potentially massive sources [39], although only a small part of the
gallium can be recovered in practice [15].
Germanium occurs primarily through silicate minerals in the earth’s crust due to ionic substitution
with the silicon ion. Typical concentrations are a few ppm. Moskalyk gives a mean concentration of
6.7 ppm [40], while Höll et al. states an average of 1.7 ppm [41]. The highest enrichments can be
found in non-silicate formations as zinc/copper-sulphides, primarily low-iron sphalerite, or in certain
coals [42]. In addition, fly ash from certain coals can contain as much as 1.6%–7.5% germanium [40],
and may be an important source if proper recovery methods are developed. Furthermore, high
concentrations have been commonly found in iron-nickel meteorites, and this suggests that major
shares of the earth’s germanium may reside in the planetary core [42]. A review of germanium and its
occurrence have been provided by Höll et al. [41].
Both selenium and tellurium are found in low concentrations in copper ores and commonly
recovered as side-products from copper refining [43]. Additionally, selenium occurs at concentrations
between 0.5 and 12 ppm in various coals, which equals a much larger resource base than the worlds
Sustainability 2015, 7 11824
copper ores (USGS, 2015) [15]. Yodovich and Ketris reviewed selenium in coal and pointed out that
coal ash has enriched selenium concentrations [44]. However, recovering selenium from coal does not
appear likely due to the high volatility of the material [12]. World selenium reserves are estimated to
be 120,000 tons derived from copper ores [15].
Tellurium is the 72nd most abundant element in the Earth’s crust, with 5 ppb. Some tellurium
minerals are found in nature such as calaverite, sylvanite or tellurite, but are not mined [34]). USGS
estimates the world tellurium reserves to be 24,000 tons based on identified copper ores [15], but also
mentions the possibility of recovering tellurium from certain gold-telluride or lead-zinc ores. Over
90% of tellurium is produced from anode slimes from copper refining, which can contain as much as
8% tellurium [34].
Ruthenium and platinum are rare elements that occur together with other platinum-group metals.
The largest platinum-group metal deposit is the Bushveld Complex in South Africa [15]. Nearly 90%
of the world’s known platinum reserves are located in South Africa [45], while other deposits can be
found in Russia, North America, and Zimbabwe, and only to a smaller degree in other countries [15].
Andersson highlighted how this dominance of a single country would make platinum group metal
supply sensitive to monopolistic behavior and geopolitical issues [9].
4. Production and Future Outlooks
Mining and processing of ore deposits requires mining, rock blasting, transportation, crushing,
milling, and different chemical procedures. The conversion form ore to a marketable commodity is
usually an energy intensive process. Rosa and Rosa gave a formula for the energy cost of mining and
refining a substance [46]:
J = C/gY (1)
where J is the energy cost of unit mass of the desired product, C the energy expenditure of mining,
milling and concentrating per unit mass of the actual ore, Y is the joint recovery rate or process yield
and g denotes the grade or mass fraction of the substance in the ore. From this it is evident that moving
in to low grade ores inescapably requires more energy input per unit mass unless technological
improvements can offset the disadvantages caused by lower ore grades. Rosa and Rosa and references
therein discuss this topic in more detail [46]. As a consequence, production of materials derived from
low concentration ores will be sensitive to future energy prices, especially when moving towards lower
and lower ore grades.
The rare materials used in several solar technologies chiefly occur as byproducts of base metal ores.
Platinum is an exception; PGMs are mined as well as by-products and primary products. As a result,
future production of those materials is intrinsically linked to the base metal production. This
relationship makes it challenging to significantly increase production of by-products without
increasing the production of the main product. Fthenakis have also highlighted the opposite relation
and how an increasing production of base metals unavoidable generates by-products, such as cadmium,
that either must be put to constructive use or disposed of in an environmentally acceptable way [43].
Sustainability 2015, 7 11825
4.1. Base Metal Production
About 90% of all zinc production is accomplished by the electrolytic process, while 10% rely on
older pyrometallurgical treatment. For lead production, after sintering, lead is usually smelted in a blast
furnace. Smelting frees the metal from the oxidized form. About half of lead originates from recycled
sources [47]. Copper production is mainly (80%) done by pyrometallurgical processing of sulphide
ores, with the remainder being hydrometallurgical treatment of oxide ores. Fthenakis et al. provide a
comprehensive overview of copper and zinc production and their flow schemes [12]. Treatment of
various residues is the main feedstock for recovering numerous other metals, such as indium or
cadmium, as by-products.
World production of copper, lead, and zinc can be seen in Figure 1. More than half of the present
world mine production of lead comes from China [15]. In addition, 58% of the global zinc mine
production originates from China, Australia and Peru. Nearly 55% of present world copper production
originates from Chile (31% alone), USA, Peru, and China. Global production of base metals is not
evenly distributed, intrinsically affecting the recovery and supply of by-products.
Figure 1. World production of zinc, lead and copper from 1900 to 2014 [15].
A similar situation can be seen for bauxite mining and aluminium production. Bauxite is converted
via Bayer process to alumina, an aluminium oxide, which is further electrolysed to obtain pure metal.
World production of bauxite and aluminum has increased significantly after 1950 (Figure 2). Australia
and China presently account for roughly 55% of the world bauxite production, and China alone also
accounts for 47% of global aluminum production [15].
World production of base metals is unevenly distributed with significant concentration in a few
countries, resembling the situation for world supply of fossil fuels [48,49]. Occurrences have been
identified in all over the world, but many of them are sub-economic or otherwise unattractive deposits.
However, it should be specifically noted that geological abundance has little to do with the likelihood
of significant future production, as actual recovery must be both technically and socioeconomically
feasible. As a consequence, seemingly abundant but dilute formations may never be attractive for
mining, while scarce but highly concentrated deposits can be attractive to exploit.
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Figure 2. World production of aluminum and bauxite from 1900 to 2014 [15].
4.2. Recovery of By-Products
Hartman finds that significant shares of the gallium reserves will not be produced in any foreseeable
time, simply because they are a by-product of bauxite mining and have to be primarily governed by
future aluminum demand [50]. Gallium is extracted from bauxite in conjunction with aluminum oxide
based on the Bayer process [37]. The second recovery method involves electrolytic processes with
mercury, allowing gallium extraction after addition of caustic soda. Despite environmental challenges
surrounding mercury, this method is employed many countries. The last recovery method is based on
chelating agents and addition of diluted acids, eventually making gallium recoverable by direct
electrolysis. Moskalyk has provided a more comprehensive overview of the production methods and
the worldwide suppliers of gallium [37]. World gallium production for the last 100 years can be seen
in Figure 3. Gallium is produced by a small number of producers and world primary production is
currently in the order of 400 tons, with additional gallium derived from recycling of scrap electronics
containing GaAs.
Figure 3. World production of gallium, germanium and tellurium from 1900 to present.
Tellurium production began in the 1930s. Important countries such as China and USA are
not included in the tellurium production figures, which therefore underestimate reality.
Gallium and germanium have been increasing for the last decades [15].
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Germanium production usually consists of two stages, where the first step creates a concentrate and
the second is the actual recovery. Hydrometallurgical processes using precipitation are generally
preferred. In comparison, thermal and pyrometallurgical processes have inherent complications with
the volatility of germanium oxides and sulphides and their environmental challenges. Moskalyk
compiled a review of worldwide germanium production and suppliers [40]. Historical production of
germanium since the late 1950s can be seen in Figure 3.
More than 90% of the world’s tellurium is recovered from anode slimes collected from electrolytic
copper refining, and the remainder is a by-product of lead refineries and from bismuth, copper, and
lead ores [15]. Anode slimes are primarily treated to recover gold, silver, platinum, palladium and
rhodium, while selenium and tellurium are of secondary priority [12]. The actual percentage of
tellurium recovery from anode slimes is variable and depends on concentration. Recovery is done by
cementation with copper to form copper telluride. This is further processed to a sodium telluride
solution with caustic soda and air. In the next step pure tellurium metal or tellurium oxide are produced
for solar cell applications. Fthenakis et al. have compiled a more detailed overview of tellurium
production [12]. Important tellurium producing countries are Japan, Russia, and Canada [15].
Cadmium production originates from smelting of zinc and lead-zinc ores. The cadmium sponge,
a by-product from precipitating zinc sulphate solution at the zinc smelter is almost pure cadmium
(more than 99% purity) and is used as the main feedstock in cadmium recovery facilities [43]. Fumes
and dust from zinc sinter plants are also important feedstock for further purification. Comprehensive
overview of cadmium recovery processes have been made by Safarzadeh et al. [51]. Commonly,
cadmium is seen as a highly toxic metal and cadmium disposal is connected to environmental hazards.
Thus, recovering cadmium from primary and secondary sources is of great importance [51]. China and
South Korea are the largest producers and account for roughly half of world production, followed by
Kazakhstan, Canada, Japan and Mexico [15]. Additionally, recycling of Ni-Cd batteries is also a
source for cadmium.
Indium production is similar to cadmium and recovery is chiefly done as a by-product from
collected dust, fumes, and other residues from zinc refining. Advantages in indium recovery processing
have now increased, and extraction rates have reached 75% of the treatable residues [52]. Many details
on the actual production technology are proprietary, but some general recovery methods based on
standard methods and general information from producers have been compiled by Fthenakis et al. [12].
More discussions on indium production and worldwide suppliers have been conducted by Alfantazi
and Moskalyk [52] and Fthenakis et al. [12]. More than 50% of the world’s primary indium production
originates from China [15].
Mined platinum group metal (PGM) ores are initially crushed and grinded before being
concentrated in a froth flotation process. Addition of water, air, and chemicals created a froth
containing the PGM metals and is collected. Following the matte-smelting process, high purity platinum is
refined through a series of hydrometallurgical processes [45]. Ruthenium is recovered as a byproduct
during platinum-group metal refining. This is mainly done through insolubility in aqua regia, which
leaves a solid residue that can be refined to obtain pure ruthenium, osmium, and other commonly
associated metals. Solvent extraction has been described as a method [53], although very little details
are available for ruthenium refining methods presently in use. Figure 4 shows the production of
indium, selenium and PGMs.
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Figure 4. World production of indium, selenium and platinum group metals (including
platinum and ruthenium) for 1900 to present. Selenium (data does not include production
in USA) production peaked in 1990 and has stabilized somewhat lower than the peak level
for more than a decade. Indium shows a rapid increase over the last decades [15].
4.3. Competition from Non-Solar Energy Sectors
Many of the critical metals discussed here also have important uses other than solar energy
applications. Therefore, it can be argued that the assumption that all the available reserves or
production of the rare materials would go to solar energy pursuits is unrealistic. In reasonable cases,
only a share of the metal flows would be available for solar energy solutions. How large this share will
become is a complicated question and will be affected by several factors, such as how the metals’
intensity in solar applications and the competing markets will evolve. What are the perspectives for
substitution, substituting materials or substituting technologies and approaches both in the solar sector
and the competing markets?
More than 80% of the world’s cadmium is used to make rechargeable batteries, but other important
uses are for pigments, coatings, and platings, stabilizers for plastics, alloys and photovoltaics.
However, due to environmental and health concerns significant effort has been made to replace
cadmium with other less toxic substances [15].
Gallium has been described as a backbone of the electronics industry and constitutes an important
component in semiconductors, diodes and laser systems. Gallium arsenide for semiconductor
applications makes up 95% of global gallium consumption [37]. Only some 2% of the produced GaAs
is consumed by photovoltaic industry, whereas other uses include electric circuits, laser technology,
diodes, and LED lights [22].
The photovoltaic industry is the most important end-use segment of tellurium with a 40% market
share. It is followed by thermoelectric modules, which function as a small heat pump and are based on
semiconducting materials. Other uses include metallurgy and the rubber industry [54]. Currently
photovoltaics form a niche market for selenium, whereas 40% of selenium is consumed for the production
Sustainability 2015, 7 11829
of electrolytic manganese, which is a key material component for alkaline and litium-ion batteries.
Other uses for selenium are found in the glass industry, agriculture, pigments and metallurgy [54].
About 90% of indium flows in the production of ITO (indium-tin-oxide), which is a transparent,
conducting foil used in flat display panels and thin-film coatings. Other end-uses include solders,
cryogenics, and special alloys. The electrical industry, including photovoltaics, is responsible for only
3% of the global indium consumption [55].
The electronics industry, also including solar applications, consumes 15% of the global germanium
markets. Important competing applications are found in infrared optics, fiber optics, and
polymerization catalysts (together 70% market share), whereas smaller market segments are found in
phosphors, metallurgy and chemotherapy [56].
Ruthenium is used for creating wear-resistant electrical contacts, thick film chip resistors, and for
various catalyst applications. The electrical industry is the most important ruthenium consuming
sector, with a market share of over 60% [57]. Currently almost no ruthenium is used in the
photovoltaics and solar energy industries.
In summary, many of the materials used here will be subjected to competition regarding usage.
In some places it is possible to switch to substitutes, but likely several sectors will continue to rely on
the same rare metals that several solar energy technologies are built around. The kind of financial
repercussions this will bring should be investigated more closely and taken into account in any holistic
study of economic long-term feasibility.
4.4. Recycling of Scarce and Rare Metals
Valero and Valero point out that there is no substitute for metals if they are irretrievably dispersed
by human use [58]. Therefore, recycling is an important factor for making the best possible use of
produced metals and should be encouraged. To some extent, recycled material can also help with
balancing production from mining by alleviating mismatches in supply and demand. However,
recycling does not increase recoverable volumes. It is only a way to reuse some of the already mined
materials again and prevent them from being locked up in scrap heaps or waste disposals. It is
important to remember that recycling is only something that makes the use of materials more
sustainable while it is incapable of removing intrinsic limits caused by recoverable volumes. Some of
the metals discussed here are already extensively recycled or reused—gallium in particular, as the
world primary gallium production capacity in 2011 was estimated to be between 260 and 320 tons,
while the recycling capacity was 198 tons [15].
This analysis uses the amount of known global metals reserves or resources as bases and calculates
the maximum PV electricity production, which can be achieved with the given amount of metal.
One can thus argue that this approach intrinsically includes recycling with the very optimistic
assumption of a 100% recycling rate.
5. Material Consumption of Solar Technologies
Harvest solar energy is often seen as abundant, rich, and lasting supply of energy without any
practical constraints. That is not entirely true, as the conversion technologies are dependent on raw
material inputs necessary for construction. Solar energy technologies harvest renewable energy, but
Sustainability 2015, 7 11830
there are no such things as renewable power plants. Material availability or production bottlenecks
may lead to significant constraints for the necessary building components for solar energy
technologies. This section explores whether scarcity of certain key materials may provide an upper
limit for some selected solar energy technologies. Similar studies were performed by Andersson and
others [9,59]. No good material consumption estimates could be found for artificial photosynthesis
approaches, but it is expected to be at a similar magnitude as the other solar energy technologies.
Material requirement per square meter for solar energy is a key property, as the incoming energy
must be harvested over large areas. Table 1 gives some estimated material consumptions for relevant
technologies. These consumption figures are based on a 100% material utilization [9,22], which is
optimistic because it entirely ignores process losses. However, this optimistic assumption may
compensate for some of the potential reductions in material requirements since year 2000.
Table 1. Estimated material consumption for selected solar energy technologies [9,22,60].
Technology Material Material Consumption (g/m2) Source
CdTe Cadmium 6.9 DOE [60]
Tellurium 7.8 DOE [60]
CIGS Indium 2.9 Angerer et al., & Andersson [9,22]
Gallium 0.53 Angerer et al., & Andersson [9,22]
Selenium 4.8 Angerer et al., & Andersson [9,22]
aSiGe Germanium 0.44 Andersson [9]
Indium 0,38 Angerer et al. [22]
Grätzel Ruthenium 0.1 Angerer et al. [22]
Solar insolation can be as high as over 2000 kWh/m2 per annum at excellent sites like the desert
areas of Sahara or in Australia, where clouds are virtually nonexistent. For comparison the global
average insolation value is 1700 kWh/m2. The average value for Central Europe and Northern Europe
is in the range of 1000 kWh/m2. The last two columns in Table 2 give the annual electricity production
of the respective solar technology, assuming that 50% or 100% of the respective world material
reserves are devoted to solar cell fabrication. For comparison, the present global primary energy
demand is over 13,371 million tons of oil equivalents (MTOE) [6].
A more comprehensive study would naturally use more realistic assumptions about solar hours
related to geographical location into account than in this study. However, we do not believe that such
details would change the overall picture that material constraints pose a challenge for moving solar
technology from its present small scale (134 GWp installed capacity by the end of 2013, resulting in
~14 Mtoe globally) to production of globally significant amounts of energy [61]. Even though the
consumption of rare materials is only a few grams per square meter, the diffuse influx of solar energy
requires large areas to provide significant energy amounts. This results in considerable material use
that could possibly surpass production capacity and resource availability for rapid growth rates.
Available reserves and resources were mostly taken from the USGS where available [15]. Reserve
(or resource) data on some metals did not allow the USGS to make estimates compatible with their
standards. In such cases, reserve estimates were taken from other sources: ruthenium, germanium [34],
indium [62], gallium [38], and germanium [63].
Sustainability 2015, 7 11831
Table 2. Potential contribution to future world energy supply constrained by available
reserves and resources. Three cases with 10%, 50% and 100% diversions to solar energy
applications were considered. For comparison, world primary energy consumption in 2014
was slightly more than 13,000 Mtoe, final energy consumption 4700 Mtoe and electricity
consumption 1600 Mtoe [6].
Technology Material
Global
Production Reserves
Constrained
Contribution Resources
Constrained
Contribution
10% 50% 100% 10% 50% 100%
tons tons Mtoe Mtoe Mtoe tons Mtoe Mtoe Mtoe
CdTe Cd 22,000
500,000
(2014) 190 930 1860
1,200,000
(2009) 450 2230 4470
Te ~500
24,000
(2015) 8 40 80
CIGS In 820 11,000 9 40 90 65,000 50 260 510
Ga 440 6500 30 140 280 N/A
Se ~2000 120,000 60 290 570
Se in coal
deposits
aSiGe or
aSi/nc/Si Ge 165 N/A 27,000 100 520 1040
In 820 11,000 50 250 490 65,000 290 1450 2900
Grätzel Ru 12 5000 60 320 640 N/A
Table 2 shows the results of the analysis in a matrix with respect to global reserves—and when
possible to global resources—and with three different resource allocations to the solar sector, namely
10%, 50% and 100%. Depending on competing end-uses for the critical metals, different resource
allocations seem reasonable. Global reserves reflect those deposits, which can be mined with current
technology economically. Thus, figures related to reserves show a minimum level of how much solar
energy can be produced with the technologies in question. Global resources can be understood as an
upper limit. The estimations are very uncertain, and for some metals, even missing, and therefore
estimations based on resources should be viewed critically.
6. Discussion
For CdTe the constraining metal is tellurium. Currently 40% of the annual tellurium markets are
consumed in the photovoltaic industry. The USGS does not give any resource estimation for reasons of
data accuracy, and therefore the estimation used in the analysis refers to global tellurium reserves.
In this case and assuming 50% market share, electricity production from CdTe panels would be limited
to 40 Mtoe annually. However the reserve figure considers only tellurium from the anode slimes of
copper refining with a currently relatively low recovery rate of approximately 40%. Fthenakis argues
that the recovery rate could technically be as high as the recovery rate for copper in the electrolytic
refining process, 80%. Even higher rates, such as 95% for gold, would technically be possible [64].
The question is more economical in nature, i.e., whether the price of tellurium is a sufficient incentive
for higher recovery rates. In addition to copper mines, other geological reserves for tellurium exist, such
as by-product in lead-zinc ores, primary tellurium mines, ocean crusts and sour oil and gas [65].
Sustainability 2015, 7 11832
However, no resource estimation exists for these additional sources and therefore they are excluded
from the analysis. Also the material intensity has a potential for remarkable improvements by a factor of
four as shown by Woodhouse et al.: the efficiency can be almost doubled while, at the same time, the
active layer thickness can be cut to 1 μm. It is however, not yet clear to what extent this potential will
become reality for commercial applications [66]. In the optimistic case, this would allow more than
300 Mtoe or 3500 TWh of annual electricity production. This is comparable with the cumulative
capacity of 0.9–1.8 TWp until 2050 modelled by Fthenakis [64].
Grätzel cells are constrained by the availability of ruthenium, which is currently used mostly in the
electrical industry. Even if half of the known reserves were devoted to solar cell production, only some
300 Mtoe could be annually produced.
CIGS technology is constrained by both indium and gallium. Indium is consumed currently to 90%
for ITO production. Even if all available indium resources were to be used in the solar industry—an
unrealistic assumption—a maximum of some 500 Mtoe as annual production seems plausible.
Another technology dependent on indium is based on amorphous silicon. The dependency on
germanium can be avoided by a tandem structure, which also has a stabilizing effect on the efficiency
of the module. Thus, the constraining metal is indium. ITO films are also used beside solar energy in
various other application areas such as flat panel displays, plasma displays or touch panels. Therefore,
the upper limit for electricity produced by amorphous silicon seems to be in the range of some
hundreds of Mtoe annually.
Silicon is the second most abundant element in the Earth’s crust, making up approximately one
fourth of it when measured by mass. However, Grandell and Thorenz foresaw a problem with scaling
up silicon technologies due to material constraints from silver, commonly used as an electrode
material, and estimated the upper limit to be some 13,000 TWh annual electricity production or 1000
Mtoe [19]. This estimate is based on a very low silver content (0.82 g/m2), which already reflects a
technical approach to reduce silver consumption, such as the “wrap through technology” or
substitution of silver with copper, both of which are currently in development stage. Indium currently
used in ITO could possibly be replaced by FTO (fluorine doped tin oxide) and AZO (aluminium doped
zinc oxid).
The above mentioned figures can be compared with world primary energy consumption (13,000 Mtoe),
world final energy consumption (4700 Mtoe) or world electricity production (1600 Mtoe). All figures
refer to the year 2014 [6]. The world energy sector is expected to experience a shift away from fuels
towards electricity due to climate concerns and energy security questions. Currently one third of the
global final energy consumption is due to the traffic sector, mainly consisting of oil consumption.
In the future this will be to a large degree electricity consumption. Additionally, the rising economies
in the developing world are another factor stressing the need for more electricity production. If we
assume that 50% of the currently known global resources of Te and 10% of the resources of Ru and In
are devoted to the solar industry, we could generate 500 Mtoe, or in the most optimistic case, 800 Mtoe
of solar electricity annually. Additionally c-Si technology provides more potential for PV electricity
generation, but the technology is constrained by silver dependence and it remains to be seen to which
degree new approaches with decreased silver content will enter the market.
If a future global energy system based on solar energy is sought, it is vital to consider
material challenges or alternatively focus on other technological pathways than those explored here. A
Sustainability 2015, 7 11833
practical path for future research is use of alternative and more abundant materials if solar energy is to
become a sustainable backbone of the global energy system. Todorov et al. showed that thin-films
based on the abundant copper-zinc-tinchalcogenide kesterites (Cu2ZnSnS4 and Cu2ZnSnSe4) could
reach over 9.6% conversion efficiency [67]. The selenium usage in these cells could in theory be
entirely replaced by sulphur, creating a thin-film cell only relying on abundant materials. For certain
other technologies, such as dye-sensitized cells, it would be fairly easy to replace scarce or rare
materials with more abundant ones. Organic dyes that do not required noble metal complexes have
been discussed by Hara et al. [68].
7. Conclusions
When summarizing several promising solar energy technologies, it was found that they rely on
several critical metals as important components. Many technologies are likely going to face constraints
in material supply if scaled to the TW level (Table 2). Material questions are an important factor for
the development of several future solar energy technologies. Without a holistic treatment of required
material questions, solar energy production outlooks should be regarded with sound skepticism.
Increasing demand for scarce materials may become a factor of importance in the future. Many of the
unusual materials are key ingredients to various technologies, including several of the more promising
solar energy applications.
There are prospects for reducing material requirements by significant amounts for CIGS and CdTe
by utilizing even thinner films and advanced light trapping technologies [9,66]. Large scale
development of the studied solar technologies would likely require either substantial reductions in
material intensity, technical advancements in electricity generation efficiency or increased world
mineral reserves as well as significant increases in mine production.
These results points to obstacles for certain solar technologies when it comes to reaching a TW
scale. Indium, tellurium, germanium and ruthenium are in potentially tight supply. Research and
development paths that aim to circumvent the dependence on rare materials are generally encouraged
from a longer perspective. Additionally, the constraints imposed by nature on critical metals may
direct solar energy research to usage of other materials in the long run. Solar energy technologies that
do not require rare elements are the only feasible technology for large-scale implementation. CdTe,
CIGS, a-Si and ruthenium-based Grätzel cells will all be limited by material availability and only able
to provide small shares of the present world energy consumption (Table 2). It is important to use
CIGS, CdTe and the other technologies discussed in this study as bridges to alternative and less limited
solar energy applications.
Acknowledgments
Financial support for the work has been provided by the Fortum Foundation.
Author Contributions
Mikael Höök started with data collection which was finalized and analysed by Leena Grandell.
The writing of the article was done by both authors.
Sustainability 2015, 7 11834
Conflicts of Interest
The authors declare no conflict of interest.
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(http://creativecommons.org/licenses/by/4.0/).
... Different aspects of resource scarcity were pointed out already by Meadows et al. (1974), and more recently by Bardi (2013) and in general terms for fossil fuels, materials, and metals. Goe and Gaustad (2014), Grandell and Höök (2015), and Nassar et al. (2015) addressed the needs for photovoltaics and semiconductors specifically. These studies presented different types of supply assessments and expressed worries about potential scarcity in the future for germanium. ...
... The germanium markets in general were discussed by Naumov (2007). Germanium demand and supply for photovoltaics was discussed by: Fthenakis (2009), Candelise et al. (2011, Moss et al. (2011Moss et al. ( ), Öhrlund (2011, Goe and Gaustad (2014), Grandell and Höök (2015), Graedel et al., (2015a, b), Guberman (2013aGuberman ( , b, 2014Guberman ( , 2015, Davidsson and Höök (2017), Almosni et al. (2018), Alcaniz et al. (2019), Eilu et al. (2021), Simandl et al. (2023), andLuceno-Sancez et al. (2019). An increased germanium demand from the photovoltaic sector can be expected in the future, considering the political initiatives going on and the need to phase out fossil fuels. ...
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The WORLD7 model was used to assess the sustainability of production and market supply of germanium. The model incorporates dynamic market dynamics, mass balance, and endogenous dynamic market prices based on supply and demand dynamics. The results suggest that there will be germanium scarcity in the near future, and a price increase is predicted. Future demand for germanium for the photovoltaic technologies can only partially be met. The total global extractable potential for germanium was estimated to be about 342,000 tons in 2022 from a geological presence of about 5.5 million tons. The major obstacle for germanium supply is the opportunity for extraction from mother metals, the availability of the required infrastructure, and low extraction yields. Germanium is extracted as a secondary metal from zinc and fly-ash today, but potential new sources are lead, copper, nickel refining residuals, and Bayer liquid from bauxite processing. The maximum germanium production rate was estimated to be about 1250 ton/year. The actual 2023 global extraction rate is about 210 ton/year. With respect to supply sustainability, germanium may suffer from a scarcity of supply and limit the application of key technologies in the future. The supply per person peaks in 2053 and declines to 2020 level by 2200. A doubling of demand above business-as-usual would imply germanium shortages in the market. The recycling rate for germanium is far too low for a circular society, and the supply situation may be significantly improved if the recycling rate can be increased substantially. The implications for the EU imaginaries indicate that four policy pathway approaches would be necessary to address the scarcity of germanium: regulation and innovation, investment in local solutions, market-driven adaptations, and community engagement/conservation.
... Recovery of precious metals and REEs are essential and serves manifold objectives of human development. For example, geological reserves of REEs are scarce, though they are significant for shifting to renewable sources of energy like wind and solar, as 171 kg of REE is required per MW of electricity produced by a wind turbine (Grandell & Höök, 2015;Navarro & Zhao, 2014). During the virgin mining of metals and REEs, a significant quantity of solid and liquid waste is produced, which imposes an environmental burden (Edahbi et al., 2019), also, the leaching of heavy metals from e-waste piled up in dumpsites causes additional risk to human health and environment (Murthy & Ramakrishna, 2022). ...
... For instance, solar energy systems based on photovoltaics commonly require silicon, silver, aluminum, and copper as base materials. 11,12 Frequently enlisted materials for wind turbines include steel (turbine tower), fiberglass/carbon fiber (blades), and rareearth elements such as neodymium (generator magnets). 13,14 Finally, batteries built to function as energy storage systems often use lithium, cobalt, and nickel for optimal performance and durability of electrodes. ...
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Renewable sources produced close to one-third of the world’s electricity in 2023. However, a limited but growing body of research suggests rapid renewable energy development is leading to conflict and resource exploitation in energy-transitioning communities. Such injustices are attributable to the extractivist nature of renewable energy development, where raw materials, also known as Clean Energy Technology Materials (CETMs), are in limited quantities and often concentrated in resource-constrained zones in the Global South. In this perspective, we call for an urgent need for energy justice considerations in CETM’s supply chain. We used demand projection data from 2020 to 2040 to look into the effects of important CETMs like nickel, cobalt, and lithium on distributive justice. We also examined the potential of these effects to tackle systemic injustices such as conflict, labor exploitation, and transactional colonialism. Next, we analyzed global mining production data from the United States Geological Survey using a CETM life cycle lens and found that increasing demand for these materials is exacerbating restorative injustices, particularly in the Global South. Finally, building on the above evidence, we called for the creation of multi-stakeholder partnerships and the establishment of fair trade standards across the critical CETM supply chain. Graphical abstract Highlights Here, we analyzed the projected demand growth for selected clean energy technology materials by 2040 relative to 2020 levels using data from the International Energy Agency, visualized their global mining production using data from the United States Geological Survey, explained how the demand for these materials is exacerbating certain injustices, and recommended multi-stakeholder partnerships across the supply chain of these materials. Discussion The rapid growth of renewable energy technologies is creating injustices throughout the supply chain of clean energy technology materials (CETM). A lack of any energy justice framework across CETMs’ extraction, processing, decommissioning, and recycling is exacerbating restorative injustices, especially in the Global South. By examining the projected demands and geospatial patterns for the extraction of minerals, metals, and other materials essential for clean energy technology development, the inequities faced by impoverished, marginalized, and Indigenous communities become apparent. We argue that if coffee can have fair trade standards across its supply chain, why can’t we have similar considerations for the CETMs? There is a need to include transparency in the sustainability, ethics, and energy efficiency of CETM extraction and processing through global partnerships across its supply chain.
... 12 In is a rare and noble metal, making it unsuitable for large-scale energy storage. 13 The plating/stripping behavior of Ni anode experiences sluggish kinetics, resulting in high potential polarization. 14,15 The redox potentials of the Cu 2+ /Cu (+0.34 V vs. SHE) and Bi 3+ /Bi (+0.32 V vs. SHE) couples are too high for practical use in batteries. ...
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Aqueous metal batteries have the potential to revolutionize the next-generation energy storage infrastructures due to their high energy density, high safety and low cost. However, two major issues of dendrite growth and corrosion reactions in metal anodes have hindered the deployment of this technology. To address these issues, we report an ideal candidate: aqueous cadmium-metal battery (ACB). The metal cadmium (Cd) anode not only shows a high specific capacity (476.5 mAh g -1 ) but also offers suitable redox potential (-0.4 V versus standard hydrogen electrode). Additionally, we introduce this ACB operating with a low-cost chloride electrolyte composed of CdCl 2 and NH 4 Cl in water. The inclusion of NH 4 Cl reconstructs the hydrogen bond network of aqueous electrolyte and forms tetrachlorocomplex ([CdCl 4 ] 2- ), which facilitate ultrafast reaction kinetics in ACBs and endow dendrite-free/corrosion-resistant capabilities in Cd anodes. Consequently, the tailored electrolyte achieves a convincing Coulombic efficiency (99.93%) for Cd plating/stripping behavior at a high anode utilization of 55.5%, making it suitable for practical applications. More importantly, the ACBs demonstrate outstanding compatibility paired with coordination-type, intercalation-type and capacitance-type cathodes, exhibiting excellent high-/low-rate and long-term rechargeable capabilities. On a practical note, the high-load ACB with a low negative-to-positive capacity ratio of 1.91 delivers an impressive lifespan of 800 cycles. In summary, our work suggests a practical aqueous battery capable of supporting robust energy storage infrastructures.
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The gallium resources were assessed and used as input to long term simulations using the WORLD7 model. The content of gallium in different mother ores has been estimated to be about 14.7 million ton of gallium. Much of this is not accessible because of low extraction yields, about 610,000 ton gallium appear to be extractable (4%) with present practices. The gallium content in all source metal refining residuals is about 51,000 ton/yr, but only a production of 1,374 ton/yr appear as the maximum with present technology and conditions. The actual gallium production was about 450 ton/yr in 2023. The gallium price is very sensitive to increases in demand, and production is not very likely to be able to rapidly increase. The simulations show that soft gallium scarcity sets in after 2028 and physical scarcity will occur about 2060. Better gallium extraction and recycling yields may push the scarcity date forward to 2100. In the long term, only 60% of the gallium demand to photovoltaic technology can be satisfied. To really improve the situation and prevent scarcity, extractive access, gallium extraction yields and recycling yields must be significantly improved to better than 50%. At present the overall yields are 7-15%. Increasing both extraction yields and recycling yields can reduce the shortage. The long term sustainable extraction is under Business-as-Usual about 300 ton gallium per year, about 67% of the present production. Doubling present extraction and recycling yields may increase this to 460 ton per year. This poses a major challenge to future plans for an energy transition, where under Business-As-Usual (BAU), such a transition will remain a fair fantasy. The four EEA imaginaries, Ecotopia, The Great Decoupling, Unity in Adversity, and Technocracy for the Common Good, offer different policy pathways for managing future gallium scarcity through varying degrees of technological advancement, resource conservation, and avoidance strategy.
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Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined, and new entries since June 2012 are reviewed.
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This Intergovernmental Panel on Climate Change Special Report (IPCC-SRREN) assesses the potential role of renewable energy in the mitigation of climate change. It covers the six most important renewable energy sources - bioenergy, solar, geothermal, hydropower, ocean and wind energy - as well as their integration into present and future energy systems. It considers the environmental and social consequences associated with the deployment of these technologies, and presents strategies to overcome technical as well as non-technical obstacles to their application and diffusion. SRREN brings a broad spectrum of technology-specific experts together with scientists studying energy systems as a whole. Prepared following strict IPCC procedures, it presents an impartial assessment of the current state of knowledge: it is policy relevant but not policy prescriptive. SRREN is an invaluable assessment of the potential role of renewable energy for the mitigation of climate change for policymakers, the private sector, and academic researchers.
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