Solar cell research

From Wikipedia, the free encyclopedia

 

There are currently many research groups active in the field of photovoltaics in universities and research institutions around the world. This research can be categorized into three areas: making current technology solar cells cheaper and/or more efficient to effectively compete with other energy sources; developing new technologies based on new solar cell architectural designs; and developing new materials to serve as more efficient energy converters from light energy into electric current or light absorbers and charge carriers.

Silicon processing

One way of reducing the cost is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica, or silica sand. Processing silica (SiO₂) to produce silicon is a very high energy process - at current efficiencies, it takes one to two years for a conventional solar cell to generate as much energy as was used to make the silicon it contains. More energy efficient methods of synthesis are not only beneficial to the solar industry, but also to industries surrounding silicon technology as a whole. 

The current industrial production of silicon is via the reaction between carbon (charcoal) and silica at a temperature around 1700 °C. In this process, known as carbothermic reduction, each tonne of silicon (metallurgical grade, about 98% pure) is produced with the emission of about 1.5 tonnes of carbon dioxide. 

Solid silica can be directly converted (reduced) to pure silicon by electrolysis in a molten salt bath at a fairly mild temperature (800 to 900 °C). While this new process is in principle the same as the FFC Cambridge Process which was first discovered in late 1996, the interesting laboratory finding is that such electrolytic silicon is in the form of porous silicon which turns readily into a fine powder, with a particle size of a few micrometers, and may therefore offer new opportunities for development of solar cell technologies. 

Another approach is also to reduce the amount of silicon used and thus cost, is by micromachining wafers into very thin, virtually transparent layers that could be used as transparent architectural coverings. The technique involves taking a silicon wafer, typically 1 to 2 mm thick, and making a multitude of parallel, transverse slices across the wafer, creating a large number of slivers that have a thickness of 50 micrometres and a width equal to the thickness of the original wafer. These slices are rotated 90 degrees, so that the surfaces corresponding to the faces of the original wafer become the edges of the slivers. The result is to convert, for example, a 150 mm diameter, 2 mm-thick wafer having an exposed silicon surface area of about 175 cm² per side into about 1000 slivers having dimensions of 100 mm × 2 mm × 0.1 mm, yielding a total exposed silicon surface area of about 2000 cm² per side. As a result of this rotation, the electrical doping and contacts that were on the face of the wafer are located at the edges of the sliver, rather than at the front and rear as in the case of conventional wafer cells. This has the interesting effect of making the cell sensitive from both the front and rear of the cell (a property known as bifaciality). Using this technique, one silicon wafer is enough to build a 140 watt panel, compared to about 60 wafers needed for conventional modules of same power output.

Nanocrystalline solar cells

These structures make use of some of the same thin-film light absorbing materials but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections (and hence increase the probability of light absorption). Using nanocrystals allows one to design architectures on the length scale of nanometers, the typical exciton diffusion length. In particular, single-nanocrystal ('channel') devices, an array of single p-n junctions between the electrodes and separated by a period of about a diffusion length, represent a new architecture for solar cells and potentially high efficiency.

Thin-film processing

Thin-film photovoltaic cells can use less than 1% of the expensive raw material (silicon or other light absorbers) compared to wafer-based solar cells, leading to a significant price drop per Watt peak capacity. There are many research groups around the world actively researching different thin-film approaches and/or materials.

One particularly promising technology is crystalline silicon thin films on glass substrates. This technology combines the advantages of crystalline silicon as a solar cell material (abundance, non-toxicity, high efficiency, long-term stability) with the cost savings of using a thin-film approach.

Another interesting aspect of thin-film solar cells is the possibility to deposit the cells on all kind of materials, including flexible substrates (PET for example), which opens a new dimension for new applications.

Metamorphic multijunction solar cell

NREL compilation of best research solar cell efficiencies from 1976 to 2010

As of December 2014, the world record for solar cell efficiency at 46% was achieved by using multi-junction concentrator solar cells, developed from collaboration efforts of Soitec, CEA-Leti, France together with Fraunhofer ISE, Germany.

The National Renewable Energy Laboratory (NREL) won one of R&D Magazine's R&D 100 Awards for its Metamorphic Multijunction photovoltaic cell, an ultra-light and flexible cell that converts solar energy with record efficiency.

The ultra-light, highly efficient solar cell was developed at NREL and is being commercialized by Emcore Corp. of Albuquerque, N.M., in partnership with the Air Force Research Laboratories Space Vehicles Directorate at Kirtland Air Force Base in Albuquerque. 

It represents a new class of solar cells with clear advantages in performance, engineering design, operation and cost. For decades, conventional cells have featured wafers of semiconducting materials with similar crystalline structure. Their performance and cost effectiveness is constrained by growing the cells in an upright configuration. Meanwhile, the cells are rigid, heavy and thick with a bottom layer made of germanium. 

In the new method, the cell is grown upside down. These layers use high-energy materials with extremely high quality crystals, especially in the upper layers of the cell where most of the power is produced. Not all of the layers follow the lattice pattern of even atomic spacing. Instead, the cell includes a full range of atomic spacing, which allows for greater absorption and use of sunlight. The thick, rigid germanium layer is removed, reducing the cell's cost and 94% of its weight. By turning the conventional approach to cells on its head, the result is an ultra-light and flexible cell that also converts solar energy with record efficiency (40.8% under 326 suns concentration).

Polymer processing

The invention of conductive polymers (for which Alan Heeger, Alan G. MacDiarmid and Hideki Shirakawa were awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics. However, organic solar cells generally suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable. The bonds in the polymers, are always susceptible to breaking up when radiated with shorter wavelengths. Additionally, the conjugated double bond systems in the polymers which carry the charge, react more readily with light and oxygen. So most conductive polymers, being highly unsaturated and reactive, are highly sensitive to atmospheric moisture and oxidation, making commercial applications difficult.

Nanoparticle processing

Experimental non-silicon solar panels can be made of quantum heterostructures, e.g. carbon nanotubes or quantum dots, embedded in conductive polymers or mesoporous metal oxides. In addition, thin films of many of these materials on conventional silicon solar cells can increase the optical coupling efficiency into the silicon cell, thus boosting the overall efficiency. By varying the size of the quantum dots, the cells can be tuned to absorb different wavelengths. Although the research is still in its infancy, quantum dot modified photovoltaics may be able to achieve up to 42% energy conversion efficiency due to multiple exciton generation (MEG).

MIT researchers have found a way of using a virus to improve solar cell efficiency by a third.

Transparent conductors

Many new solar cells use transparent thin films that are also conductors of electrical charge. The dominant conductive thin films used in research now are transparent conductive oxides (abbreviated "TCO"), and include fluorine-doped tin oxide (SnO₂:F, or "FTO"), doped zinc oxide (e.g.: ZnO:Al), and indium tin oxide (abbreviated "ITO"). These conductive films are also used in the LCD industry for flat panel displays. The dual function of a TCO allows light to pass through a substrate window to the active light-absorbing material beneath, and also serves as an ohmic contact to transport photogenerated charge carriers away from that light-absorbing material. The present TCO materials are effective for research, but perhaps are not yet optimized for large-scale photovoltaic production. They require very special deposition conditions at high vacuum, they can sometimes suffer from poor mechanical strength, and most have poor transmittance in the infrared portion of the spectrum (e.g.: ITO thin films can also be used as infrared filters in airplane windows). These factors make large-scale manufacturing more costly. 

 

A relatively new area has emerged using carbon nanotube networks as a transparent conductor for organic solar cells. Nanotube networks are flexible and can be deposited on surfaces a variety of ways. With some treatment, nanotube films can be highly transparent in the infrared, possibly enabling efficient low-bandgap solar cells. Nanotube networks are p-type conductors, whereas traditional transparent conductors are exclusively n-type. The availability of a p-type transparent conductor could lead to new cell designs that simplify manufacturing and improve efficiency.

Silicon wafer-based solar cells

Despite the numerous attempts at making better solar cells by using new and exotic materials, the reality is that the photovoltaics market is still dominated by silicon wafer-based solar cells (first-generation solar cells). This means that most solar cell manufacturers are currently equipped to produce this type of solar cells. Consequently, a large body of research is being done all over the world to manufacture silicon wafer-based solar cells at lower cost and to increase the conversion efficiencies without an exorbitant increase in production cost. The ultimate goal for both wafer-based and alternative photovoltaic concepts is to produce solar electricity at a cost comparable to currently market-dominant coal, natural gas, and nuclear power in order to make it the leading primary energy source. To achieve this it may be necessary to reduce the cost of installed solar systems from currently about US$1.80 (for bulk Si technologies) to about US$0.50 per Watt peak power. Since a major part of the final cost of a traditional bulk silicon module is related to the high cost of solar grade polysilicon feedstock (about US$0.4/Watt peak) there exists substantial drive to make Si solar cells thinner (material savings) or to make solar cells from cheaper upgraded metallurgical silicon (so called "dirty Si"). 

IBM has a semiconductor wafer reclamation process that uses a specialized pattern removal technique to repurpose scrap semiconductor wafers to a form used to manufacture silicon-based solar panels. The new process was recently awarded the “2007 Most Valuable Pollution Prevention Award” from The National Pollution Prevention Roundtable (NPPR).

Infrared solar cells

Researchers at Idaho National Laboratory, along with partners at Lightwave Power Inc. in Cambridge, MA and Patrick Pinhero of the University of Missouri, have devised an inexpensive way to produce plastic sheets containing billions of nanoantennas that collect heat energy generated by the sun and other sources, which garnered two 2007 Nano50 awards. The company ceased operations in 2010. While methods to convert the energy into usable electricity still need to be developed, the sheets could one day be manufactured as lightweight "skins" that power everything from hybrid cars to computers and iPods with higher efficiency than traditional solar cells. The nanoantennas target mid-infrared rays, which the Earth continuously radiates as heat after absorbing energy from the sun during the day; also double-sided nanoantenna sheets can harvest energy from different parts of the Sun's spectrum. In contrast, traditional solar cells can only use visible light, rendering them idle after dark.

UV solar cells

Japan's National Institute of Advanced Industrial Science and Technology (AIST) has succeeded in developing a transparent solar cell that uses ultraviolet (UV) light to generate electricity but allows visible light to pass through it. Most conventional solar cells use visible and infrared light to generate electricity. Used to replace conventional window glass, the installation surface area could be large, leading to potential uses that take advantage of the combined functions of power generation, lighting and temperature control.

This transparent, UV-absorbing system was achieved by using an organic-inorganic heterostructure made of the p-type semiconducting polymer PEDOT:PSS film deposited on a Nb-doped strontium titanate substrate. PEDOT:PSS is easily fabricated into thin films due to its stability in air and its solubility in water. These solar cells are only activated in the UV region and result in a relatively high quantum yield of 16% electron/photon. Future work in this technology involves replacing the strontium titanate substrate with a strontium titanate film deposited on a glass substrate in order to achieve a low-cost, large-area manufacture.

Since then, other methods have been discovered to include the UV wavelengths in solar cell power generation. Some companies report using nano-phosphors as a transparent coating to turn UV light into visible light. Others have reported extending the absorption range of single-junction photovoltaic cells by doping a wide band gap transparent semiconductor such as GaN with a transition metal such as manganese.

Flexible solar cell research

Flexible solar cell research is a research-level technology, an example of which was created at the Massachusetts Institute of Technology in which solar cells are manufactured by depositing photovoltaic material on flexible substrates, such as ordinary paper, using chemical vapor deposition technology. The technology for manufacturing solar cells on paper was developed by a group of researchers from the Massachusetts Institute of Technology with support from the National Science Foundation and the Eni-MIT Alliance Solar Frontiers Program.

3D solar cells

Three-dimensional solar cells that capture nearly all of the light that strikes them and could boost the efficiency of photovoltaic systems while reducing their size, weight and mechanical complexity. The new 3D solar cells, created at the Georgia Tech Research Institute, capture photons from sunlight using an array of miniature “tower” structures that resemble high-rise buildings in a city street grid. Solar3D, Inc. plans to commercialize such 3D cells, but its technology is currently patent-pending.

Luminescent solar concentrator

Luminescent solar concentrators convert sunlight or other sources of light into preferred frequencies; they concentrate the output for conversion into desirable forms of power, such as electricity. They rely on luminescence, typically fluorescence, in media such as liquids, glasses, or plastics treated with a suitable coating or dopant. The structures are configured to direct the output from a large input area onto a small converter, where the concentrated energy generates photoelectricity. The objective is to collect light over a large area at low cost; luminescent concentrator panels can be made cheaply from materials such as glasses or plastics, while photovoltaic cells are high-precision, high-technology devices, and accordingly expensive to construct in large sizes. 

Research is in progress at universities such as Radboud University Nijmegen and Delft University of Technology. For example, at Massachusetts Institute of Technology researchers have developed approaches for conversion of windows into sunlight concentrators for generation of electricity. They paint a mixture of dyes onto a pane of glass or plastic. The dyes absorb sunlight and re-emit it as fluorescence within the glass, where it is confined by internal reflection, emerging at the edges of the glass, where it encounters solar cells optimized for conversion of such concentrated sunlight. The concentration factor is about 40, and the optical design yields a solar concentrator that unlike lens-based concentrators, need not be directed accurately at the sun, and can produce output even from diffuse light. Covalent Solar is working on commercialization of the process.

Metamaterials

Metamaterials are heterogeneous materials employing the juxtaposition of many microscopic elements, giving rise to properties not seen in ordinary solids. Using these, it may become possible to fashion solar cells that are excellent absorbers over a narrow range of wavelengths. High absorption in the microwave regime has been demonstrated, but not yet in the 300-1100-nm wavelength regime.

Photovoltaic thermal hybrid

Some systems combine photovoltaic with thermal solar, with the advantage that the thermal solar part carries heat away and cools the photovoltaic cells. Keeping temperature down lowers the resistance and improves the cell efficiency.

Penta-based photovoltaics

Pentacene-based photovoltaics are claimed to improve the energy-efficiency ratio to up to 95%, effectively doubling the efficience of today's most efficient techniques.

Intermediate band

Intermediate band photovoltaics in solar cell research provides methods for exceeding the Shockley–Queisser limit on the efficiency of a cell. It introduces an intermediate band (IB) energy level in between the valence and conduction bands. Theoretically, introducing an IB allows two photons with energy less than the bandgap to excite an electron from the valence band to the conduction band. This increases the induced photocurrent and thereby efficiency. 

Luque and Marti first derived a theoretical limit for an IB device with one midgap energy level using detailed balance. They assumed no carriers were collected at the IB and that the device was under full concentration. They found the maximum efficiency to be 63.2%, for a bandgap of 1.95eV with the IB 0.71eV from either the valence or conduction band. Under one sun illumination the limiting efficiency is 47%. 

Copper in renewable energy

From Wikipedia, the free encyclopedia

Renewable energy sources such as solar, wind, tidal, hydro, biomass, and geothermal have become significant sectors of the energy market. The rapid growth of these sources in the 21st century has been prompted by increasing costs of fossil fuels as well as their environmental impact issues that significantly lowered their use.

Copper plays an important role in these renewable energy systems. In fact, copper usage averages up to five times more in renewable energy systems than in traditional power generation, such as fossil fuel and nuclear. Since copper is an excellent thermal and electrical conductor among the engineering metals (second only to silver), power systems that utilize copper generate and transmit energy with high efficiency and with minimum environmental impacts.

When choosing electrical conductors, facility planners and engineers factor capital investment costs of the materials against operational savings due to their electrical energy efficiencies over their useful lives, plus maintenance costs. Copper often fairs well in these calculations. One pertinent factor, called "copper usage intensity,” is a measure of the number of pounds of copper necessary to install one megawatt of new power-generating capacity.

When planning for a new renewable power facility, engineers and product specifiers seek to avoid supply shortages of selected conductor materials. According to the United States Geological Survey, in-ground copper reserves have increased more than 700% since 1950, from almost 100 million tonnes to 720 million tonnes today, despite the fact that world refined usage has more than tripled in the last 50 years. Copper resources are estimated to exceed 5,000 million tonnes. Bolstering the annual supply is the fact that more than 30 percent of copper installed during the last decade came from recycled sources.

Regarding the sustainability of renewable energy systems, it is worthy to note that in addition to copper's high electrical and thermal conductivity, its recycling rate is higher than any other metal.
This article discusses the role of copper in various renewable energy generation systems.

Overview of copper usage in renewable energy generation

Copper plays a larger role in renewable energy generation than in conventional thermal power plants in terms of tonnage of copper per unit of installed power. The copper usage intensity of renewable energy systems is four to six times higher than in fossil fuel or nuclear plants. So for example, while conventional power requires approximately 1 tonne of copper per installed megawatt (MW), renewable technologies such as wind and solar require four to six times more copper per installed MW. This is because copper is spread over much larger land areas, particularly in solar and wind energy power plants, and there is a need for long runs of power and grounding cables to connect components that are widely dispersed, including to energy storage systems and to the main electrical grid.

Wind and solar photovoltaic energy systems have the highest copper content of all renewable energy technologies. A single wind farm can contain between 4 million and 15 million pounds of copper. A photovoltaic solar power plant contains approximately 5.5 tons of copper per megawatt of power generation. A single 660-kW turbine is estimated to contain some 800 pounds of copper.

The total amount of copper used in renewable-based and distributed electricity generation in 2011 was estimated to be 272 kilotonnes (kt). Cumulative copper use through 2011 was estimated to be 1,071 kt.

Copper usage in renewable energy generation

	Installed power in 2011	Cumulative installed power to 2011	Copper use in 2011	Cumulative copper use to 2011
	Gigawatts (GW)	Gigawatts (GW)	Kilotons (kt)	Kilotons (kt)
Photovoltaics	30	70	150	350
Solar thermal electricity	0.46	1.76	2	7
Wind	40	238	120	714
Total for all three technologies			272	1071

Copper conductors are used in major electrical renewable energy components, such as turbines, generators, transformers, inverters, electrical cables, power electronics, and information cable. Copper usage is approximately the same in turbines/generators, transformers/inverters, and cables. Much less copper is used in power electronics. 

Solar thermal heating and cooling energy systems rely on copper for their thermal energy efficiency benefits. Copper is also used as a special corrosion-resistant material in renewable energy systems in wet, humid, and saline corrosive environments. 

Copper is a sustainable material that is 100% recyclable. The recycling rate of copper is higher than any other metal. At the end of the useful life of the renewable energy power plant or its electrical or thermal components, the copper can be recycled with no loss of its beneficial properties.

Solar photovoltaic power generation

Of the 20,000 TWh of power consumed globally in a single year, approximately 90 TWh are generated from solar PV systems. While this is only a very small percentage of global energy consumption (0.6% of total installed electricity generating capacity worldwide), it is nevertheless sufficient to power the needs of more than 10 million people living at the standard of living in a developed country. 

Various overlapping statistics regarding the growth of solar PVs have been cited. Solar PVs have been cited to have a 40% annual growth rate, which may grow even faster as the cost of the technology continues to decline. Another source cites operating capacity to have increased by an average of 58% annually from year end-2006 through 2011. Installed capacity estimates to 2020 suggest a rapid rise in solar PV generation, growing by a factor of five between 2010 and 2020.

Household PV systems are able to generate their own electricity and use the electrical grid for support and reliability. 

For these reasons, policy initiatives are taking place to enhance the deployment of solar photovoltaic energy installations. This would boost the steady expansion of PV markets by reducing the competitiveness gap of PVs compared to fossil fuel technologies. The goal at this point is to reach grid parity, where the cost of producing energy from rooftop panels over the course of their 25-year lifetime equates to the cost of retail electricity generated by conventional sources. This achievement has already been accomplished in some regions.

Copper in photovoltaic power systems

There is eleven to forty times more copper per unit of generation in photovoltaic systems than in conventional fossil fuel plants. The usage of copper in photovoltaic systems averages around 4-5 tonnes per MW or higher if conductive ribbon strips that connect individual PV cells are considered.

Copper is used in: 1) small wires that interconnect photovoltaic modules; 2) earthing grids in electrode earth pegs, horizontal plates, naked cables, and wires; 3) DC cables that connect photovoltaic modules to inverters; 4) low-voltage AC cables that connect inverters to metering systems and protection cabinets; 5) high-voltage AC cables; 6) communication cables; 7) inverters/power electronics; 8) ribbons; and 9) transformer windings.

Copper used in photovoltaic systems in 2011 was estimated to be 150 kt. Cumulative copper usage in photovoltaic systems through 2011 was estimated to be 350 kt.

Photovoltaic system configurations

Solar photovoltaic (PV) systems are highly scalable, ranging from small rooftop systems to large photovoltaic power station with capacities of hundreds of megawatts. In residential systems, copper intensity appears to be linearly scalable with the capacity of the electrical generation system. Residential and community-based systems generally range in capacity from 10 kW to 1 MW. 

PV cells are grouped together in solar modules. These modules are connected to panels and then into PV arrays. In grid-connected photovoltaic power system, arrays can form sub-fields from which electricity is collected and transported towards the grid connection.

Copper solar cables connect modules (module cable), arrays (array cable), and sub-fields (field cable). Whether a system is connected to the grid or not, electricity collected from the PV cells needs to be converted from DC to AC and stepped up in voltage. This is done by solar inverters which contain copper windings, as well as with copper-containing power electronics.

Solar cells

The photovoltaic industry uses several different semiconducting materials for the production of solar cells and often groups them into first and second generation technologies, while the third generation includes a number of emerging technologies that are still in the research and development phase. Solar cells typically convert 20% of incident sunlight into electricity, allowing the generation of 100 - 150 kWh per square meter of panel per year.

Conventional first-generation crystalline silicon (c-Si) technology includes monocrystalline silicon and polycrystalline silicon. In order to reduce costs of this wafer-based technology, copper-contacted silicon solar cells are emerging as an important alternative to silver as the preferred conductor material. Challenges with solar cell metallization lie in the creation of a homogenous and qualitatively high-value layer between silicon and copper to serves as a barrier against copper diffusion into the semiconductor. Copper-based front-side metallization in silicon solar cells is a significant step towards lower cost.

The second-generation technology includes thin film solar cells. Despite having a slightly lower conversion efficiency than conventional PV technology, the overall cost-per-watt is still lower. Commercially significant thin film technologies include copper indium gallium selenide solar cells (CIGS) and cadmium telluride photovoltaics (CdTe), while amorphous silicon (a-Si) and micromorphous silicon (m-Si) tandem cells are slowly being outcompeted in recent years. 

CIGS, which is actually copper (indium-gallium) diselenide, or Cu(InGa)Se₂, differs from silicon in that it is a heterojunction semiconductor. It has the highest solar energy conversion efficiency (~20%) among thin film materials. Because CIGS strongly absorbs sunlight, a much thinner film is required than with other semiconductor materials. 

A photovoltaic cell manufacturing process has been developed that makes it possible to print CIGS semi-conductors. This technology has the potential to reduce the price per solar watt delivered. 

While copper is one of the components in CIGS solar cells, the copper content of the cell is actually small: about 50 kg of copper per MW of capacity.

Mono-dispersed copper sulfide nanocrystals are being researched as alternatives to conventional single crystals and thin films for photovoltaic devices. This technology, which is still in its infancy, has potential for dye-sensitized solar cells, all-inorganic solar cells, and hybrid nano-crystal-polymer composite solar cells.

Cables

Solar generation systems cover large areas. There are many connections among modules and arrays, and connections among arrays in sub-fields and linkages to the network. Solar cables are used for wiring solar power plants. The amount of cabling involved can be substantial. Typical diameters of copper cables used are 4–6 mm² for module cable, 6–10 mm² for array cable, and 30–50 mm² for field cable.

Energy efficiency and system design considerations

Energy efficiency and renewable energy are twin pillars of a sustainable energy future. However, there is little linking of these pillars despite their potential synergies. The more efficiently energy services are delivered, the faster renewable energy can become an effective and significant contributor of primary energy. The more energy is obtained from renewable sources, the less fossil fuel energy is required to provide that same energy demand. This linkage of renewable energy with energy efficiency relies in part on the electrical energy efficiency benefits of copper. 

Increasing the diameter of a copper cable increases its electrical energy efficiency. Thicker cables reduce resistive (I²R) loss, which affects lifetime profitability of PV system investments. Complex cost evaluations, factoring extra costs for materials, the amount of solar radiation directed towards solar modules per year (accounting for diurnal and seasonal variations, subsidies, tariffs, payback periods, etc.) are necessary to determine whether higher initial investments for thicker cables are justified. 

Depending upon circumstances, some conductors in PV systems can be specified with either copper or aluminum. As with other electrical conducting systems, there are advantages to each. Copper is the preferred material when high electrical conductivity characteristics and flexibility of the cable are of paramount importance. Also, copper is more suitable for small roof facilities, in smaller cable trays, and when ducting in steel or plastic pipes.

Cable ducting is not needed in smaller power facilities where copper cables are less than 25mm². Without duct work, installation costs are lower with copper than with aluminum.

Data communications networks rely on copper, optical fiber, and/or radio links. Each material has its advantages and disadvantages. Copper is more reliable than radio links. Signal attenuation with copper wires and cables can be resolved with signal amplifiers.

Concentrating solar thermal power

The Sun’s solar energy can also be harnessed for its heat. When the Sun’s energy heats a fluid in a closed system, its pressure and temperature rise. When introduced to a turbine, the fluid expands, turning the turbine and producing electrical power.

Concentrating solar power (CSP), also known as solar thermal electricity (STE), uses arrays of mirrors that concentrate the sun’s rays to temperatures between 400⁰C -1000⁰C. Electrical power is produced when the concentrated light is converted to heat, which drives a heat engine (usually a steam turbine) connected to an electrical power generator. 

CSP facilities can produce large-scale power and hold much promise in areas with plenty of sunshine and clear skies. Poised to make Sun-powered grids a reality, CSP is currently capable of providing power and dispatchability on a scale similar to that of fossil fuel or nuclear electrical power plants. 

The electrical output of CSP facilities match shifting daily demand for electricity in places where air conditioning systems are spreading. When backed by thermal storage facilities and combustible fuel, CSP offers utilities electricity that can be dispatched when required, enabling it to be used for base, shoulder and peak loads.

Industry groups have estimated that the technology could generate a quarter of the world’s electricity needs by 2050. For this reason, plans for future CSP facilities are ambitious. A timeline of CSP deployment around the world is available. Total installed power is forecasted to increase exponentially through 2025, creating as much as 130,000 jobs.

In 2010, Spain, the world leader of CSP technology, was constructing or planning to build some 50 large CSP plants. That nation has a total installed base of 1581 MW of power plus an additional 774 MW nearing completion for installation. Other countries in southern Europe also have CSP facilities, as do countries in emerging markets, such as Chile, India, Morocco, Saudi Arabia, South Africa, and the United Arab Emirates.

Unlike wind energy, photovoltaics, and most distributed power, the main advantage of CSP is its thermal storage capability and hybridization possibilities. Storage systems range from 4 hours in the most typical plants to more than 20 hours when base load is required. This can complement variable generation of other renewable power sources. 

CSP systems are sometimes combined with fossil fueled steam turbine generation, but interest is growing in pure CSP technology. Further information on concentrating solar power is available from the Global Solar Thermal Energy Council.

Copper in concentrating solar thermal power facilities

A CSP system consists of: 1) a concentrator or collector containing mirrors that reflect solar radiation and deliver it to the receiver; 2) a receiver that absorbs concentrated sunlight and transfers heat energy to a working fluid (usually a mineral oil, or more rarely, molten salts, metals, steam or air); 3) a transport and storage system that passes the fluid from the receiver to the power conversion system; and 4) a steam turbine that converts thermal power to electricity on demand. 

Copper is used in field power cables, grounding networks, and motors for tracking and pumping fluids, as well as in the main generator and high voltage transformers. Typically, there is about 200 tonnes copper for a 50 MW power plant.

It has been estimated that copper usage in concentrated solar thermal power plants was 2 kt in 2011. Cumulative copper usage in these plants through 2011 was estimated to be 7 kt.

There are four main types of CSP technologies, each containing a different amount of copper: parabolic trough plants, tower plants, distributed linear absorber systems including linear Fresnel plants, and dish Stirling plants. The use of copper in these plants is described here.

Parabolic trough plants

Parabolic trough plants are the most common CSP technology, representing about 94% of power installed in Spain. These plants collect solar energy in parabolic trough concentrators with linear collector tubes. The heat transfer fluids are typically synthetic oil that circulates through tubes at inlet outlet/temperatures of 300 °C to 400 °C. The typical storage capacity of a 50 MW facility is 7 hours at nominal power. A plant of this size and storage capacity can generate 160 GWh/year in a region like Spain. 

In parabolic trough plants, copper is specified in the solar collector field (power cables, signals, earthing, electrical motors); steam cycle (water pumps, condenser fans, cabling to consumption points, control signal and sensors, motors), electricity generators (alternator, transformer), and storage systems (circulating pumps, cabling to consumption points). A 50 MW plant with 7.5 hours of storage contains approximately 196 tonnes of copper, of which 131,500 kg are in cables and 64,700 kg are in various equipment (generators, transformers, mirrors, and motors). This translates to about 3.9 tonnes/MW, or, in other terms, 1.2 tonnes/GWh/year. A plant of the same size without storage can have 20% less copper in the solar field and 10% less in the electronic equipment. A 100 MW plant will have 30% less relative copper content per MW in the solar field and 10% less in electronic equipment.

Copper quantities also vary according to design. The solar field of a typical 50 MW power plant with 7 hours of storage capacity consists of 150 loops and 600 motors, while a similar plant without storage uses 100 loops and 400 motors. Motorized valves for mass flow control in the loops rely on more copper. Mirrors use a small amount of copper to provide galvanic corrosion protection to the reflective silver layer. Changes in the size of the plants, size of collectors, efficiencies of heat transfer fluids will also affect material volumes.

Tower plants

Tower plants, also called central tower power plants, may become the preferred CSP technology in the future. They collect solar energy concentrated by the heliostat field in a central receiver mounted at the top of the tower. Each heliostat tracks the Sun along two axes (azimuth and elevation). Therefore, two motors per unit are required. 

Copper is required in the heliostat field (power cables, signal, earthing, motors), receiver (trace heating, signal cables), storage system (circulating pumps, cabling to consumption points), electricity generation (alternator, transformer), steam cycle (water pumps, condenser fans), cabling to consumption points, control signal and sensors, and motors.

A 50 MW solar tower facility with 7.5 hours of storage uses about 219 tonnes of copper. This translates to 4.4 tonnes of copper/MW, or, in other terms, 1.4 tonnes/GWh/year. Of this amount, cables account for approximately 154,720 kg. Electronic equipment, such as generators, transformers, and motors, account for approximately 64,620 kg of copper. A 100 MW plant has slightly more copper per MW in the solar field because the efficiency of the heliostat field diminishes with the size. A 100 MW plant will have somewhat less copper per MW in process equipment.

Linear Fresnel plants

Linear Fresnel plants use linear reflectors to concentrate the Sun’s rays in an absorber tube similar to parabolic trough plants. Since the concentration factor is less than in parabolic trough plants, the temperature of the heat transfer fluid is lower. This is why most plants use saturated steam as the working fluid in both the solar field and the turbine.

A 50 MW linear Fresnel power plant requires about 1,960 tracking motors. The power required for each motor is much lower than the parabolic trough plant. A 50 MW lineal Fresnel plant without storage will contain about 127 tonnes of copper. This translates to 2.6 tonnes of copper/MW, or in other terms, 1.3 tonnes of copper/GWh/year. Of this amount, 69,960 kg of copper are in cables from process area, solar field, earthing and lightning protection and controls. Another 57,300 kg of copper is in equipment (transformers, generators, motors, mirrors, pumps, fans).

Dish Stirling plants

These plants are an emerging technology that has potential as a solution for decentralized applications. The technology does not require water for cooling in the conversion cycle. These plants are non-dispatchable. Energy production ceases when clouds pass overhead. Research is being conducted on advanced storage and hybridization systems. 

The largest dish Sterling installation has a total power of 1.5 MW. Relatively more copper is needed in the solar field than other CSP technologies because electricity is actually generated there. Based on existing 1.5 MW plants, the copper content is 4 tonnes/MW, or, in other terms, 2.2 tonnes of copper/GWh/year. A 1.5 MW power plant has some 6,060 kg of copper in cables, induction generators, drives, field and grid transformers, earthing and lightning protection.

Solar water heaters (solar domestic hot water systems)

Solar water heaters can be a cost-effective way to generate hot water for homes. They can be used in any climate. The fuel they use, sunshine, is free.

Solar hot water collectors are used by more than 200 million households as well as many public and commercial buildings worldwide. The total installed capacity of solar thermal heating and cooling units in 2010 was 185 GW-thermal.

Solar heating capacity increased by an estimated 27% in 2011 to reach approximately 232 GWth, excluding unglazed swimming pool heating. Most solar thermal is used for water heating, but solar space heating and cooling are gaining ground, particularly in Europe.

There are two types of solar water heating systems: active, which have circulating pumps and controls, and passive, which don't. Passive solar techniques do not require working electrical or mechanical elements. They include the selection of materials with favorable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the Sun.

Copper is an important component of solar thermal heating and cooling systems because of its high heat conductivity, resistance to atmospheric and water corrosion, sealing and joining by soldering, and mechanical strength. Copper is used both in receivers and primary circuits (pipes and heat exchangers for water tanks). For the absorber plate, aluminum is sometimes used as it is cheaper, yet when combined with copper piping, there may be problems in regards to allow the absorber plate to transfer its heat to the piping suitably. An alternative material that is currently used is PEX-AL-PEX but there may be similar problems with the heat transfer between the absorber plate and the pipes as well. One way around this is to simply use the same material for both the piping and the absorber plate. This material can be copper off course but also aluminum or PEX-AL-PEX. 

Three types of solar thermal collectors are used for residential applications: flat plate collectors, integral collector-storage, and solar thermal collector: Evacuated tube collectors; They can be direct circulation (i.e., heats water and brings it directly to the home for use) or indirect circulation (i.e., pumps heat a transfer fluid through a heat exchanger, which then heats water that flows into the home) systems.

In an evacuated tube solar hot water heater with an indirect circulation system, evacuated tubes contain a glass outer tube and metal absorber tube attached to a fin. Solar thermal energy is absorbed within the evacuated tubes and is converted into usable concentrated heat. Copper heat pipes transfer thermal energy from within the solar tube into a copper header. A thermal transfer fluid (water or glycol mixture) is pumped through the copper header. As the solution circulates through the copper header, the temperature rises. The evacuated glass tubes have a double layer. The outer layer is fully transparent to allow solar energy to pass through unimpeded. The inner layer is treated with a selective optical coating that absorbs energy without reflection. The inner and outer layers are fused at the end, leaving an empty space between the inner and outer layers. All air is pumped out of the space between the two layers (evacuation process), thereby creating the thermos effect which stops conductive and convective transfer of heat that might otherwise escape into the atmosphere. Heat loss is further reduced by the low-emissivity of the glass that is used. Inside the glass tube is the copper heat pipe. It is a sealed hollow copper tube that contains a small amount of proprietary liquid, which under low pressure boils at a very low temperature. Other components include a solar heat exchanger tank and a solar pumping station, with pumps and controllers.

Wind

Wind power is the conversion of wind energy into a useful form of energy, such as using wind turbines to make electricity, windmills for mechanical power, windpumps for water pumping or drainage, or sails to propel ships. In a wind turbine, the wind's kinetic energy is converted into mechanical energy to drive a generator, which in turn generates electricity.

Wind energy is one of the fastest growing energy technologies. Wind power capacity increased from a very small base of around 0.6 GW in 1996 to around 160 GW in 2009. It has also been reported that wind power capacity increased by 20% in 2011 to approximately 238 GW by 2012. This was the largest addition in capacity of any of the renewable energy technologies. It is anticipated that the growth of wind energy will continue to rise dramatically. Moderate estimates for global capacity by 2020 are 711 GW.

Some 50 countries operated wind power facilities in 2010.

Traditionally, wind power has been generated on land. But higher wind speeds are available offshore compared to land. Technologies are being improved to exploit the potential of wind power in offshore environments. The offshore wind power market is expanding with the use of larger turbines and installations farther from shore.

Offshore installation, as yet, is a comparatively small market, probably accounting for little more than 10% of installation globally. The location of new wind farms increasingly will be offshore, especially in Europe. Offshore wind farms are normally much larger, often with over 100 turbines with ratings up to 3 MW and above per turbine. The harsh environment means that the individual components need to be more rugged and corrosion protected than their onshore components. Increasingly long connections to shore with subsea MV and HV cables are required at this time. The need for corrosion protection favors copper nickel cladding as the preferred alloy for the towers. 

Wind power installations vary in scale and type. Large wind farm installations linked to the electrical grid are at one end of the spectrum. These may be located either onshore or offshore. At the other end of the spectrum are small individual turbines that provide electricity to individual premises or electricity-using installations. These are often in rural and grid-isolated sites.

The basic components of a wind power system consist of a tower with rotating blades containing an electricity generator and a transformer to step up voltage for electricity transmission to a substation on the grid. Cabling and electronics are also important components.

Copper in wind power generation

Copper is an important conductor in wind power generation. Wind farms can contain several hundred-thousand feet of copper weighing between 4 million to 15 million pounds, mostly in wiring, cable, tubing, generators and step-up transformers.

Copper usage intensity is high because turbines in wind generation farms are spread over large areas. In land-based wind farms, copper intensity can range between 5,600 and 14,900 pounds per MW, depending on whether the step-up transformers have copper or aluminum conductors. In the off-shore environment, copper intensity is much higher: approximately 21,000 pounds per MW, which includes submarine cables to shore. In both onshore and offshore environments, additional copper cabling is used to connect wind farms to main electrical grids.

The amount of copper used for wind energy systems in 2011 was estimated to be 120 kt. The cumulative amount of copper installed through 2011 was estimated to be 714 kt.

For wind farms with three-stage gearbox doubly fed 3 MW induction generators, approximately 2.7 t per MW is needed with standard windmills. For windmills with LV/MV transformers in the nacelle, 1.85 t per MW is needed.

Copper is primarily used in coil windings in the stator and rotor portions of generators (which convert mechanical energy into electrical energy), in high voltage and low voltage cable conductors including the vertical electrical cable that connects the nacelle to the base of the wind turbine, in the coils of transformers (which steps up low voltage AC to high voltage AC compatible with the grid), in gearboxes (which convert the slow revolutions per minute of the rotor blades to faster rpms) and in wind farm electrical grounding systems. Copper may also be used in the nacelle (the housing of the wind turbine that rests on the tower containing all the main components), auxiliary motors (motors used to rotate the nacelle as well as control the angle of the rotor blades), cooling circuits (cooling configuration for the entire drive train), and power electronics (which enable the wind turbine systems to perform like a power plant).

In the coils of wind generators, electric current suffers from losses that are proportional to the resistance of the wire that carries the current. This resistance, called copper losses, causes energy to be lost by heating up the wire. In wind power systems, this resistance can be reduced with thicker copper wire and with a cooling system for the generator, if required.

Copper in generators

Either copper or aluminum conductors can be specified for generator cables. Copper has the higher electrical conductivity and therefore the higher electrical energy efficiency. It is also selected for its safety and reliability. The main consideration for specifying aluminum is its lower capital cost. Over time, this benefit is offset by higher energy losses over years of power transmission. Deciding which conductor to use is determined during a project's planning phase when utility teams discuss these matters with turbine and cable manufacturers. 

Regarding copper, its weight in a generator will vary according to the type of generator, power rating, and configuration. Its weight has an almost linear relationship to the power rating. 

Generators in direct-drive wind turbines contain more copper, as the generator itself is bigger due to the absence of a gearbox.

A generator in a direct drive configuration could be 3.5 times to 6 times heavier than in a geared configuration, depending on the type of generator.

Five different types of generator technologies are used in wind generation:

1. double-fed asynchronous generators (DFAG)
2. conventional asynchronous generators (CAG)
3. conventional synchronous generators (CSG)
4. permanent magnet synchronous generators (PMSG)
5. high-temperature superconductor generators (HTSG)

The amount of copper in each of these generator types is summarized here. 

Copper in wind turbine generator technologies in multi-megawatt wind power plants

Technology	Average copper content (kg/MW)	Notes
Double-fed asynchronous generator (DFAG)	650	Geared; most common wind generator in Europe (70% in 2009; strong demand until 2015, then neutral as high cost of maintenance and servicing and need for power correction equipment for grid compliance will make these less popular in next ten years.
Conventional asynchronous generators (CAG)	390	Geared; neutral demand until 2015; will become negligible by 2020.
Conventional synchronous generators (CSG)	330–4000	Geared and direct; will become much more popular by 2020.
Permanent magnet synchronous generators (PMSG)	600–2150	Market expected to develop by 2015.
High-temperature superconductor generators (HTSG)	325	Nascent stage of development. It is expected that these machines will attain more power than other WTGs. Offshore could be the most suitable niche application.

Direct-drive configurations of the synchronous type machines contain the most copper. Conventional synchronous generators (CSG) direct-drive machines have the highest per-unit copper content. The share of CSGs will increase from 2009 to 2020, especially for direct drive machines. DFAGs accounted for the most unit sales in 2009.

The variation in the copper content of CSG generators depends upon whether they are coupled with single-stage (heavier) or three-stage (lighter) gearboxes. Similarly, the difference in copper content in PMSG generators depends on whether the turbines are medium speed, which are heavier, or high-speed turbines, which are lighter.

There is increasing demand for synchronous machines and direct-drive configurations. CSG direct and geared DFAGs will lead the demand for copper. The highest growth in demand is expected to be the direct PMSGs, which is forecast to account for 7.7% of the total demand for copper in wind power systems in 2015. However, since permanent magnets that contain the rare earth element neodymium may not be able to escalate globally, direct drive synchronous magnet (DDSM) designs may be more promising. The amount of copper required for a 3 MW DDSM generator is 12.6 t.

Locations with high-speed turbulent winds are better suited for variable-speed wind turbine generators with full-scale power converters due to the greater reliability and availability they offer in such conditions. Of the variable-speed wind turbine options, PMSGs could be preferred over DFAGs in such locations. In conditions with low wind speed and turbulence, DFAGs could be preferred to PMSGs.

Generally, PMSGs deal better with grid-related faults and they could eventually offer higher efficiency, reliability, and availability than geared counterparts. This could be achieved by reducing the number of mechanical components in their design. Currently, however, geared wind turbine generators have been more thoroughly field-tested and are less expensive due to the greater volumes produced.

The current trend is for PMSG hybrid installations with a single-stage or two-stage gearbox. The most recent wind turbine generator by Vestas is geared drive. The most recent wind turbine generator by Siemens is a hybrid. Over the medium term, if the cost of power electronics continues to decrease, direct-drive PMSG are expected to become more attractive. High-temperature superconductors (HTSG) technology is currently under development. It is expected that these machines will be able to attain more power than other wind turbine generators. If the offshore market follows the trend of larger unit machines, offshore could be the most suitable niche for HTSGs.

Copper in other components

For a 2 MW turbine system, the following amounts of copper were estimated for components other than the generator: 

Copper Content by other Component Types, 2 MW turbine

Component	Average Cu content (kg)
Auxiliary motors (pitch and yaw drives)	75
Other parts of the nacelle	<50 span="">
Vertical cables	1500
Power electronics (converter)	150
Cooling circuits	<10 span="">
Earthing and lightning protection	750

Cabling is the second largest copper-containing component after the generator. A wind tower system with the transformer next to the generator will have medium-voltage (MV) power cables running from the top to the bottom of the tower, then to a collection point for a number of wind towers and on to the grid substation, or direct to the substation. The tower assembly will incorporate wire harnesses and control/signal cables, while low-voltage (LV) power cables are required to power the working parts throughout the system.

For a 2 MW wind turbine, the vertical cable could range from 1,000-1,500 kg of copper, depending upon its type. Copper is the dominant material in underground cables.

Copper in grounding systems

Copper is vital to the electrical grounding system for wind turbine farms. Grounding systems can either be all-copper (solid or stranded copper wires and copper bus bars) often with an American gauge rating of 4/0 but perhaps as large as 250 thousands of circular mils or copper-clad steel, a lower cost alternative.

Turbine masts attract lightning strikes, so they require lightning protection systems. When lightning strikes a turbine blade, current passes along the blade, through the blade hub in the nacelle (gearbox/ generator enclosure) and down the mast to a grounding system. The blade incorporates a large cross-section copper conductor that runs along its length and allows current to pass along the blade without deleterious heating effects. The nacelle is protected by a lightning conductor, often copper. The grounding system, at the base of the mast, consists of a thick copper ring conductor bonded to the base or located within a meter of the base. The ring is attached to two diametrically opposed points on the mast base. Copper leads extend outward from the ring and connect to copper grounding electrodes. The grounding rings at turbines on wind farms are inter-connected, providing a networked system with an extremely small aggregate resistance.

Solid copper wire has been traditionally deployed for grounding and lightning equipment due to its excellent electrical conductivity. However, manufacturers are moving towards less expensive bi-metal copper clad or aluminum grounding wires and cables. Copper-plating wire is being explored. Current disadvantages of copper plated wire include lower conductivity, size, weight, flexibility and current carrying capability.

Copper in other equipment

After generators and cable, minor amounts of copper are used in the remaining equipment. In yaw and pitch auxiliary motors, the yaw drive uses a combination of induction motors and multi-stage planetary gearboxes with minor amounts of copper. Power electronics have minimal amounts of copper compared to other equipment. As turbine capacities increase, converter ratings also increase from low voltage (<1 a="" full="" have="" href="https://en.wikipedia.org/wiki/Electric_power_conversion" kv="" medium="" most="" title="Electric power conversion" to="" turbines="" voltage="" wind="">power converters

, which have the same power rating as the generator, except the DFAG that has a power converter that is 30% of the rating of the generator. Finally, minor amounts of copper are used in air/oil and water cooled circuits on gearboxes or generators.

Class 5 copper power cabling is exclusively used from the generator through the loop and tower interior wall. This is due to its ability to withstand the stress from 15,000 torsion cycles for 20 years of service life.

Superconducting materials are being tested within and outside of wind turbines. They offer higher electrical efficiencies, the ability to carry higher currents, and lighter weights. These materials are, however, much more expensive than copper at this time.

Copper in offshore wind farms

The amount of copper in offshore wind farms increases with the distance to the coast. Copper usage in offshore windmills is on the order of 10.5 t per MW. The Borkum 2 offshore wind farm in Denmark uses 5,800 t for a 400 MW, 200 kilometer connection to the external grid, or approximately 14.5 t of copper per MW. The Horns wind farm uses 8.75 tons of copper per MW to transmit 160 MW 21 kilometers to the grid.