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.
|
|
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:
- double-fed asynchronous generators (DFAG)
- conventional asynchronous generators (CAG)
- conventional synchronous generators (CSG)
- permanent magnet synchronous generators (PMSG)
- high-temperature superconductor generators (HTSG)
The amount of copper in each of these generator types is summarized here.
| 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:
| 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.
