Concentrated photovoltaics
Encyclopedia
Concentrated photovoltaic (CPV) technology uses optics
such as lenses
to concentrate a large amount of sunlight onto a small area of solar photovoltaic
materials to generate electricity
. Unlike traditional, more conventional flat panel systems, CPV systems are often much less expensive to produce, because the concentration allows for the production of a much smaller area of solar cells.
According to the Photoelectric effect
solar cells make electrical energy that is proportional to the amount of light energy that falls on them. By putting 10 times the amount of light on a solar panel it is possible to make 10 times the amount of electricity. The problem is the cell efficiency goes down as the temperature goes up. This means that to keep the increase in output, you need to keep the temperatures down. The second problem is the cost of concentrator and tracking system can defeat the cost saving from having fewer solar panels
in Livermore, California was the site for most of the early work, with the first modern photovoltaic concentrating system produced there late in the decade. Their first system was a linear-trough concentrator system that used a point focus acrylic Fresnel lens
focusing on water-cooled silicon cells and two axis tracking. Ramón Areces' system, also developed in the late 1970s, used hybrid silicone-glass Fresnel lenses, while cooling of silicon cells was achieved with a passive heat sink.
The design of photovoltaic concentrators introduces a very specific optical design problem, with features that makes it different from any other optical design. It has to be efficient, suitable for mass production, capable of high concentration, insensitive to manufacturing and mounting inaccuracies, and capable to provide uniform illumination of the cell. All these reasons make nonimaging optics
the most suitable for CPV.
. Except for very low concentrations, active solar tracking is also necessary.
Semiconductor
properties allow solar cells to operate more efficiently in concentrated light, as long as the cell Junction temperature
is kept cool by suitable heat sink
s. Expected future efficiencies of multijunction photovoltaic cell
are nearly 50%.
Also crucial to the efficiency (and cost) of a CPV system is the concentrating optic since it collects and concentrates sunlight onto the solar cell. For a given concentration, nonimaging optics combine the widest possible acceptance angles
with high efficiency and, therefore, are the most appropriate for use in solar concentration. For very low concentrations, the wide acceptance angles of nonimaging optics avoid the need for active solar tracking. For medium and high concentrations, a wide acceptance angle can be seen as a measure of how tolerant the optic is to imperfections in the whole system. It is vital to start with a wide acceptance angle since it must be able to accommodate tracking errors, movements of the system due to wind, imperfectly manufactured optics, imperfectly assembled components, finite stiffness of the supporting structure or its deformation due to aging, among other factors. All of these reduce the initial acceptance angle and, after they are all factored in, the system must still be able to capture the finite angular aperture of sunlight.
/Watt
, whereas silicon flat panels that are commonly sold are 3 to 5 USD/Watt (not including any associated power systems or installation charges). CPV could reach grid parity
in 2011.
).
is low enough that the cells do not need to be actively cooled. The laws of optics dictate that a solar collector
with a low concentration ratio can have a high acceptance angle and thus in some instances does not require active solar tracking.
s or more. The solar cells require high-capacity heat sinks to prevent thermal destruction and to manage temperature related performance losses. Multijunction solar cells are currently favored over silicon
as they are more efficient and have a lower temperature coefficient (less loss in efficiency with an increase in temperature). The efficiency of both cell types rises with increased concentration; multijunction efficiency rises faster . Multijunction solar cells, originally designed for non-concentrating space-based satellites, have been re-designed due to the high-current density encountered with CPV (typically 8 A/cm2 at 500 suns). Though the cost of multijunction solar cells is roughly 100 times that of silicon cells of the same area, the small cell area employed makes the relative costs of cells in each system comparable and the system economics favor the multijunction cells. Multijunction cell efficiency has now reached 41% in production cells.
The 41% value given above is for a specific set of conditions known as "standard test conditions". These include a specific spectrum, an incident optical power of 850 W/m², and a cell temperate of 25°C. In a concentrating system, the cell will typically operate under conditions of variable spectrum, lower optical power, and higher temperature. The optics needed to concentrate the light have limited efficiency themselves, in the range of 75-90%. Taking these factors into account, a solar module incorporating a 40% multijunction cell might deliver a DC efficiency around 30%. Under similar conditions, a silicon-cell module would deliver an efficiency of less than 18%.
When high concentration is needed (500-1000x), as occurs in the case of high efficiency multijunction solar cells, it is likely that it will be crucial for commercial success at the system level to achieve such concentration with a sufficient acceptance angle. This allows tolerance in mass production of all components, relaxes the module assembling and system installation, and decreasing the cost of structural elements. Since the main goal of CPV is to make solar energy inexpensive, there can be used only a few surfaces. Decreasing the number of elements and achieving high acceptance angle, can be relaxed optical and mechanical requirements, such as accuracy of the optical surfaces profiles, the module assembling, the installation, the supporting structure, etc.
, they are composed of luminescent plates either totally impregnated by luminescent species or fluorescent thin films on transparent plates. They absorb solar light
which is converted to fluorescence
guided to plate edges where it emerges in a concentrated form. The concentration factor is directly proportional to the plate surface and inversely proportional to the plate edges. Such arrangement allows to use small amounts of solar cells as a result of concentration of fluorescent light. The fluorescent concentrator is able to concentrate both direct and diffuse light which is especially important on cloudy days. They also don't need expensive Solar tracker
s.
or micro cogeneration technology used in concentrated photovoltaics that produces both electricity and heat in the same module. The heat may be employed in district heating
, water heating
and air conditioning, desalination
or process heat.
CPVT systems are currently in production in Europe, with Zenith Solar
developing CPVT systems with a claimed efficiency of 72%.
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
such as lenses
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
to concentrate a large amount of sunlight onto a small area of solar photovoltaic
Photovoltaics
Photovoltaics is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material...
materials to generate electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
. Unlike traditional, more conventional flat panel systems, CPV systems are often much less expensive to produce, because the concentration allows for the production of a much smaller area of solar cells.
According to the Photoelectric effect
Photoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
solar cells make electrical energy that is proportional to the amount of light energy that falls on them. By putting 10 times the amount of light on a solar panel it is possible to make 10 times the amount of electricity. The problem is the cell efficiency goes down as the temperature goes up. This means that to keep the increase in output, you need to keep the temperatures down. The second problem is the cost of concentrator and tracking system can defeat the cost saving from having fewer solar panels
History
Research into concentrated photovoltaics has taken place since the 1970s. Sandia National LaboratoriesSandia National Laboratories
The Sandia National Laboratories, managed and operated by the Sandia Corporation , are two major United States Department of Energy research and development national laboratories....
in Livermore, California was the site for most of the early work, with the first modern photovoltaic concentrating system produced there late in the decade. Their first system was a linear-trough concentrator system that used a point focus acrylic Fresnel lens
Fresnel lens
A Fresnel lens is a type of lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses.The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design...
focusing on water-cooled silicon cells and two axis tracking. Ramón Areces' system, also developed in the late 1970s, used hybrid silicone-glass Fresnel lenses, while cooling of silicon cells was achieved with a passive heat sink.
Challenges
CPV systems operate most efficiently in concentrated sunlight, as long as the solar cell is kept cool through use of heat sinks. Diffuse light, which occurs in cloudy and overcast conditions, cannot be concentrated. To reach their maximum efficiency, CPV systems must be located in areas that receive plentiful direct sunlight.The design of photovoltaic concentrators introduces a very specific optical design problem, with features that makes it different from any other optical design. It has to be efficient, suitable for mass production, capable of high concentration, insensitive to manufacturing and mounting inaccuracies, and capable to provide uniform illumination of the cell. All these reasons make nonimaging optics
Nonimaging optics
Nonimaging optics is the branch of optics concerned with the optimal transfer of light radiation between a source and a target...
the most suitable for CPV.
Efficiency
All CPV systems have a concentrating optic and a solar cellSolar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
. Except for very low concentrations, active solar tracking is also necessary.
Semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
properties allow solar cells to operate more efficiently in concentrated light, as long as the cell Junction temperature
Junction temperature
Junction temperature is the highest temperature of the actual semiconductor in an electronic device. In operation it is higher than case temperature and the temperature of the part's exterior...
is kept cool by suitable heat sink
Heat sink
A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems and the radiator in a car...
s. Expected future efficiencies of multijunction photovoltaic cell
Multijunction photovoltaic cell
Multi-junction solar cells or tandem cells are solar cells containing several p-n junctions. Each junction is tuned to a different wavelength of light, reducing one of the largest inherent sources of losses, and thereby increasing efficiency...
are nearly 50%.
Also crucial to the efficiency (and cost) of a CPV system is the concentrating optic since it collects and concentrates sunlight onto the solar cell. For a given concentration, nonimaging optics combine the widest possible acceptance angles
Acceptance angle (solar concentrator)
Acceptance angle is the maximum angle at which incoming sunlight can be captured by a solar concentrator. Its value depends on the concentration of the optic and the refractive index in which the receiver is immersed...
with high efficiency and, therefore, are the most appropriate for use in solar concentration. For very low concentrations, the wide acceptance angles of nonimaging optics avoid the need for active solar tracking. For medium and high concentrations, a wide acceptance angle can be seen as a measure of how tolerant the optic is to imperfections in the whole system. It is vital to start with a wide acceptance angle since it must be able to accommodate tracking errors, movements of the system due to wind, imperfectly manufactured optics, imperfectly assembled components, finite stiffness of the supporting structure or its deformation due to aging, among other factors. All of these reduce the initial acceptance angle and, after they are all factored in, the system must still be able to capture the finite angular aperture of sunlight.
Grid parity
Compared to conventional flat panel solar cells, CPV is advantageous because the solar collector is less expensive than an equivalent area of solar cells. CPV hardware (solar collector and tracker) is nearing $1 USDUnited States dollar
The United States dollar , also referred to as the American dollar, is the official currency of the United States of America. It is divided into 100 smaller units called cents or pennies....
/Watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
, whereas silicon flat panels that are commonly sold are 3 to 5 USD/Watt (not including any associated power systems or installation charges). CPV could reach grid parity
Grid parity
Grid parity is the point at which alternative means of generating electricity is at least as cheap as grid power.For solar energy, it is achieved first in areas with abundant sun and high costs for electricity such as in California, Hawaii, Spain and Japan. Many solar power advocates predict that...
in 2011.
Types
CPV systems are categorized according to the amount of their solar concentration, measured in "suns" (the square of the magnificationMagnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...
).
Low concentration CPV (LCPV)
Low concentration CPV are systems with a solar concentration of 2-100 suns. For economic reasons, conventional or modified silicon solar cells are typically used, and, at these concentrations, the heat fluxFlux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
is low enough that the cells do not need to be actively cooled. The laws of optics dictate that a solar collector
Solar collector
-See also:*Solar thermal collector*Solar water heating*Solar air heating*Photovoltaic module*Renewable heat*Concentrating solar power...
with a low concentration ratio can have a high acceptance angle and thus in some instances does not require active solar tracking.
Medium concentration CPV
From concentrations of 100 to 300 suns, the CPV systems require two-axes solar tracking and cooling (whether passive or active), which makes them more complex.High concentration photovoltaics (HCPV)
High concentration photovoltaics (HCPV) systems employ concentrating optics consisting of dish reflectors or fresnel lenses that concentrate sunlight to intensities of 100 sunSun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
s or more. The solar cells require high-capacity heat sinks to prevent thermal destruction and to manage temperature related performance losses. Multijunction solar cells are currently favored over silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
as they are more efficient and have a lower temperature coefficient (less loss in efficiency with an increase in temperature). The efficiency of both cell types rises with increased concentration; multijunction efficiency rises faster . Multijunction solar cells, originally designed for non-concentrating space-based satellites, have been re-designed due to the high-current density encountered with CPV (typically 8 A/cm2 at 500 suns). Though the cost of multijunction solar cells is roughly 100 times that of silicon cells of the same area, the small cell area employed makes the relative costs of cells in each system comparable and the system economics favor the multijunction cells. Multijunction cell efficiency has now reached 41% in production cells.
The 41% value given above is for a specific set of conditions known as "standard test conditions". These include a specific spectrum, an incident optical power of 850 W/m², and a cell temperate of 25°C. In a concentrating system, the cell will typically operate under conditions of variable spectrum, lower optical power, and higher temperature. The optics needed to concentrate the light have limited efficiency themselves, in the range of 75-90%. Taking these factors into account, a solar module incorporating a 40% multijunction cell might deliver a DC efficiency around 30%. Under similar conditions, a silicon-cell module would deliver an efficiency of less than 18%.
When high concentration is needed (500-1000x), as occurs in the case of high efficiency multijunction solar cells, it is likely that it will be crucial for commercial success at the system level to achieve such concentration with a sufficient acceptance angle. This allows tolerance in mass production of all components, relaxes the module assembling and system installation, and decreasing the cost of structural elements. Since the main goal of CPV is to make solar energy inexpensive, there can be used only a few surfaces. Decreasing the number of elements and achieving high acceptance angle, can be relaxed optical and mechanical requirements, such as accuracy of the optical surfaces profiles, the module assembling, the installation, the supporting structure, etc.
Luminescent solar concentrators
A new emerging type of concentrators which are still at the research stage are Luminescent solar concentratorsLuminescent solar concentrators
A luminescent solar concentrator is a device to generate solar power.It uses luminescent plates either impregnated by luminescent substances or fluorescent thin films on transparent plates. The plates absorb solar light which is converted to fluorescence guided to plate edges where it emerges in a...
, they are composed of luminescent plates either totally impregnated by luminescent species or fluorescent thin films on transparent plates. They absorb solar light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
which is converted to fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
guided to plate edges where it emerges in a concentrated form. The concentration factor is directly proportional to the plate surface and inversely proportional to the plate edges. Such arrangement allows to use small amounts of solar cells as a result of concentration of fluorescent light. The fluorescent concentrator is able to concentrate both direct and diffuse light which is especially important on cloudy days. They also don't need expensive Solar tracker
Solar tracker
A solar tracker is a generic term used to describe devices that orient various payloads toward the sun. Payloads can be photovoltaic panels, reflectors, lenses or other optical devices....
s.
Concentrated photovoltaics and thermal
Concentrated photovoltaics and thermal (CPVT), also sometimes called combined heat and power solar (CHAPS), is a cogenerationCogeneration
Cogeneration is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat....
or micro cogeneration technology used in concentrated photovoltaics that produces both electricity and heat in the same module. The heat may be employed in district heating
District heating
District heating is a system for distributing heat generated in a centralized location for residential and commercial heating requirements such as space heating and water heating...
, water heating
Water heating
Water heating is a thermodynamic process using an energy source to heat water above its initial temperature. Typical domestic uses of hot water are for cooking, cleaning, bathing, and space heating...
and air conditioning, desalination
Desalination
Desalination, desalinization, or desalinisation refers to any of several processes that remove some amount of salt and other minerals from saline water...
or process heat.
CPVT systems are currently in production in Europe, with Zenith Solar
Zenith Solar
Zenith Solar is an Israeli solar energy company based in Ness Ziona. It was founded in 2006 by Roy Segev and Bob Whelen, who is also its chief executive....
developing CPVT systems with a claimed efficiency of 72%.
See also
- Concentrated solar power (CSP)
- Photovoltaic thermal hybrid solar collector#PV/T concentrator (CPVT) (CPVT)
External links
- System Cost Data, NREL