Concentrated PV
Most of us are aware of standard solar panels that generate electricity from the sun but another way to increase the output from the photovoltaic systems is to supply concentrated light onto PV cells.
This can be done by using optical light collectors such as lenses or mirrors.
The CPV collects light from a larger area and concentrates it to a smaller area super efficient solar cell.
Concentration categories
In general, the CPV can be classified into three different categories which are: Low-concentration, Medium-concentration and High-concentration.
This concentration is expressed in the number of suns. E.g. 3x means that the intensity of the light that hits the photovoltaic material is 3 x times more than it would be without concentration.
What methods of concentration are used?
Concentrator technologies are either line or point and generally fall into the following categories: Fresnel Lens, Parabolic Mirrors and Reflectors
Fresnel Lens
The Fresnel lens was named after the French physicist. It comprises sections of glass with different angles, which reduces the weight and thickness in comparison to a normal lens.
Possible to achieve short focal length and large aperture while keeping the lens light.
Parabolic Mirrors
In the case of the Parabolic mirror method all incoming parallel light is reflected by the collector (the first mirror) through a focal point onto a second mirror.
This second mirror, which is much smaller, is also a parabolic mirror with the same focal point.
It reflects the light beams to the middle of the first parabolic mirror where it hits the super efficient solar cell.
Reflectors
In this case low concentration photovoltaic modules use mirrors to concentrate sunlight onto a standard solar cell. This technique lowers the reflection losses by effectively providing a second internal mirror.
What kinds of material are used for the panels?
There are both lower and higher efficiency CPV technologies that may use:
- Silicon
- CdTe ( Cadmium telluride )and
- CIGS (copper indium gallium selenide) cells
But the highest efficiencies are achieved with multi-junction cells with multiple p–n junctions made of different semiconductor materials.
Each material’s p-n junction will produce electric current in response to different wavelengths of light which allows the absorbance of a broader range of wavelengths, improving the cell‘s sunlight to electrical energy conversion efficiency.
What are the best locations for CPV?
The CPV can only use direct beam radiation and cannot use diffuse radiation (diffused from clouds and atmosphere) so these systems are best suited for areas with high direct normal irradiance.
For proper light concentration, sun tracking is needed to achieve high cell performance and tracking is especially critical for high concentration systems.
What about temperature?
As CPV systems produce increased temperature on the surface of the PV material, energy has to be distributed evenly over the cell area to avoid local overheating (hot spots) which can damage the PV material
Efficiency of the photovoltaic conversion is less at elevated temperatures and this happens with standard PV systems but this is substantially increased due to the much higher temperatures involved and therefore cooling may be beneficial.
This cooling can be active or passive.
For the CPV cells with low and medium concentration ratios, active cooling is not necessary, since the temperatures reached are moderate. The high-concentration cells require high-capacity heat sinks to avoid thermal destruction of the materials.
Cooling
With passive cooling the cell is placed on a cladded ceramic substrate with high thermal conductivity and the ceramic also provides electrical isolation.
Active Cooling, on the other hand, typically uses liquid metal as a cooling fluid, capable of cooling from 1,700°C to 100°C.
Concentration, materials, cooling, tracking
In the table below I have outlined the three different concentration categories, the PV material utilized and the tracking requirements.
What are the advantages of CPV?
The advantage is that these systems require less photovoltaic material to capture the same sunlight as non-concentrating pv and this makes the use of expensive multi-junction cells viable due to smaller space requirements.
Concentrating light, however, requires direct sunlight rather than diffuse light, limiting this technology to clear, sunny locations. It also means that, in most/all instances, tracking is required.
Conclusion
Concentrated PV increases the amount of usable light hitting a photovoltaic cell and depending on the level of concentration required, different cell materials are utilized.
Concentration can be achieved by a variety of methods; fresnel lens, parabolic mirrors, reflectors and the best locations have a high level of direct irradiation for the majority of time.
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About the author
Veli Markovic
CEC Designer
Veli has nearly two decades of experience in the renewable industry. He is passionate about providing people with valuable education and is highly regarded throughout the industry as an educator and operator.
Training videos
Concentrated pv cell
Photovoltaic (PV) devices generate electricity directly from sunlight via an electronic process that occurs naturally in certain types of material, called semiconductors. Electrons in these materials are freed by solar energy and can be induced to travel through an electrical circuit, powering electrical devices or sending electricity to the grid.
PV devices can be used to power anything from small electronics such as calculators and road signs up to homes and large commercial businesses.
How does PV technology work?
Photons strike and ionize semiconductor material on the solar panel, causing outer electrons to break free of their atomic bonds. Due to the semiconductor structure, the electrons are forced in one direction creating a flow of electrical current. Solar cells are not 100% efficient in crystalline silicon solar cells, in part because only certain light within the spectrum can be absorbed. Some of the light spectrum is reflected, some is too weak to create electricity (infrared) and some (ultraviolet) creates heat energy instead of electricity. Diagram of a typical crystalline silicon solar cell. To make this type of cell, wafers of high-purity silicon are “doped” with various impurities and fused together. The resulting structure creates a pathway for electrical current within and between the solar cells.
Other Types of Photovoltaic Technology
In addition to crystalline silicon (c-Si), there are two other main types of PV technology:
- Thin-film PVis a fast-growing but small part of the commercial solar market. Many thin-film firms are start-ups developing experimental technologies. They are generally less efficient – but often cheaper – than c-Si modules.
- In the United States, concentrating PVarrays are found primarily in the desert Southwest. They use lenses and mirrors to reflect concentrated solar energy onto high-efficiency cells. They require direct sunlight and tracking systems to be most effective.
- Building-integrated photovoltaics serve as both the outer layer of a structure and generate electricity for on-site use or export to the grid. BIPV systems can provide savings in materials and electricity costs, reduce pollution, and add to the architectural appeal of a building.
History of Photovoltaic Technology
The PV effect was observed as early as 1839 by Alexandre Edmund Becquerel, and was the subject of scientific inquiry through the early twentieth century. In 1954, Bell Labs in the U.S. introduced the first solar PV device that produced a useable amount of electricity, and by 1958, solar cells were being used in a variety of small-scale scientific and commercial applications.
The energy crisis of the 1970s saw the beginning of major interest in using solar cells to produce electricity in homes and businesses, but prohibitive (nearly 30 times higher than the current price) made large-scale applications impractical.
Industry developments and research in the following years made PV devices more feasible and a cycle of increasing production and decreasing costs began which continues even today.
Costs of Solar Photovoltaics
Rapidly falling have made solar more affordable than ever. The average price of a completed PV system has dropped by 59 percent over the last decade.
For more information on the state of the solar PV market in the US, visit our solar industry data page.
Modern Photovoltaics
The cost of PV has dropped dramatically as the industry has scaled up manufacturing and incrementally improved the technology with new materials. Installation costs have come down too with more experienced and trained installers. Globally, the U.S. has the third largest market for PV installations, and is continuing to rapidly grow.
Most modern solar cells are made from either crystalline silicon or thin-film semiconductor material. Silicon cells are more efficient at converting sunlight to electricity, but generally have higher manufacturing costs. Thin-film materials typically have lower efficiencies, but can be simpler and less costly to manufacture. A specialized category of solar cells. called multi-junction or tandem cells. are used in applications requiring very low weight and very high efficiencies, such as satellites and military applications. All types of PV systems are widely used today in a variety of applications.
There are thousands of individual photovoltaic panel models available today from hundreds of companies. Compare solar panels by their efficiency, power output, warranties, and more on EnergySage.
High-power potential: the future of concentrated solar power
We speak to Hyperlight Energy to learn how concentrated solar power’s efficient and flexible characteristics could aid in the energy transition.
JP Casey
Solar power, alongside wind, is something of a poster child for renewable power, and with images of rooftop-mounted panels and swathes of undeveloped land covered in solar farms a mainstay of energy writing, it is easy to see why. Solar has enjoyed decades of consistent growth, with Our World In Data reporting that from the first recorded instance of solar power in 1983, to its most recent figures in 2020, global electricity consumption from solar sources passed 2,000TWh.
Still, solar power is not a one-size-fits-all practice – as evidenced by the difference between rooftop panels and utility-scale plants – and perhaps the greatest variance within the sector is between photovoltaic (PV) panels and concentrated solar power (CSP). Simply put, CSP uses mirrors to concentrate the sun’s rays to particular points on solar panels, dramatically improving the efficiency of the practice, at the cost of additional manufacturing complexity at the beginning of the construction process.
Environment Sustainability in Power: Quantum Dot Solar Cell Manufacturing
Hyperlight Energy
CSP Inc
Yet CSP has been historically under-utilised compared to its more conventional cousin, PV. University College London reported that by the end of 2014, there was more than 140GW of PV capacity installed around the world, compared to just 5GW of CSP. With high-profile failures and industrial caution urging investors to stay away from high-reward CSP practices, will the future of solar power remain dominated by PV systems?
CSP versus PV
One company pushing back against this imbalance is Hyperlight Energy, an American firm whose work includes the Hylux solar steam technology and that has already received a 5.4m grant from the California Energy Commission to develop CSP solutions in the state.
“For utility scale, [a solar photovoltaic panel] will go on many 100-acre or 100-hectare deployments,” explains Hyperlight CEO and founder John King, who remains keenly aware of the imbalance in the solar sector. “Then it connects directly to the grid and provides electric power, so the photovoltaic effect is native electric, photons moving electrons. CSP, over the last decade and a half, has not been as widely deployed.”
“It has advantages and disadvantages, [including] concentration, where you are using mirrors instead of PV panels, a silicon-based technology,” King continues, going into detail about the role of the mirrors in CSP. “Mirrors bounce the photons and aim them all on a common target, so that’s where you get concentrated, and that target gets very hot. I like to say it’s just like a kid using a magnifying glass to burn a hole in a leaf. When you concentrate sunlight, it’s very powerful.”
Despite what may or may not be common assumptions about the high start-up costs of CSP facilities, King is optimistic that the improved efficiency of the CSP process can yield dividends in the long-term. This is particularly significant considering the inherent inefficiencies associated with PV technology.
A combination of technological limitations and the inflexibility of a system that does not move as the sun moves has combined to create solar panels whose efficiency often hovers around 20%, with the most efficient panels for home use boasting efficiencies of just 22%.
“For PV panels, you’re just capturing the visible portion of the spectrum,” says King, who notes that the relative maturity of technologies such as mirrors further improves the economic efficiency of the entire CSP project. “About half the energy in sunlight is visible and about half is infrared, which is heat, and PV only gets the visible. When you’re bouncing photons, you bounce all over them.
“You have about twice the energy capture possible per unit of surface area, and the surface area you’re using to get the photons is cheaper.”
Scaling challenges
Of course, there are reasons for the relatively under-developed CSP sector, beginning with the logistical and manufacturing challenges associated with a technology predicated on tracking and responding to the sun’s movements.
“If you’re aiming and the sun moves, your reflection is off, so you have to move,” says King. “These giant reflectors are like aeroplane wings; a single gust of wind gives lift and will knock you off of alignment. So you have these very complicated, expensive metal structures to make sure that doesn’t happen, and so that eats into your cost advantage.
“Picture a 95m² mirror on a pedestal. That’s an enormous amount of lift, that’s the size of an aeroplane. You can lift jumbo jets in the air with that much lift, that’s how much lift we’re talking.”
There is also something of an unfavourable reputation tarnishing the idea of CSP, especially in the US, thanks to the publication of the ‘Concentrating Solar Power Best Practices Study’ in 2020. The report, led by the National Renewable Energy Lab, compiled feedback from representatives from around 80% of the world’s operating CSP plants, and found 1,008 technological and operational issues associated with these projects.
In fairness, the report’s introduction notes that its authors are confident “that future tower and trough plants can be built on time and within budget and will perform as expected”, but the range of problems identified will do little to encourage greater adoption of a technology that is already under-utilised. Similarly, the high-profile failure of the Crescent Dunes facility in the US state of Nevada, where leaks to molten salt tanks caused the facility’s production to drop to as low as 3% of its total capacity, raised questions about the economic and productive viability of large-scale CSP plants.
“You’re chasing economies of scale,” says King, pointing out how the pursuit of large-scale production can lead to similarly large-scale issues. “So what everybody in the industry, except us, did was they would build a 500KW pilot design, and then go raise 100m, a billion dollars to then build a multi-100MW plant.”
“You’re going up 1,000x scale in one step. When that happens you have problems that manifest at 1,000x scale, and you have to fix it at 1000x.”
A flexible renewable future
CSP does offer one distinctive advantage over PV, however, and this is a benefit that also extends beyond most other forms of renewable power: its use of heat. In 2018, the International Energy Agency reported that heat makes up two-thirds of the energy demand of the world’s industries.
The production of this heat accounts for one-fifth of the world’s total energy consumption, meaning that securing a reliable and clean source of heat could make a significant difference to the world’s carbon footprint.
“For heavy industry, they actually need the heat,” says King, explaining how CSP’s FOCUS on generating heat, rather than electricity, grants it a unique position to provide power to industrial facilities. “Large sections of the economy, large sections of industry, need the heat, they don’t need electricity.
“So for the right kind of site, the right kind of end-use, we can come in and displace fuel, usually natural gas, burned for process heat, and then you don’t have to build that power plant.”
King also pointed towards the flexibility of CSP infrastructure as a potential benefit, not just for CSP but the energy industry as a whole. The need to construct CSP facilities alongside other power plants or energy facilities, and the fact that CSP produces both heat and electricity, means that CSP can work in tandem with a range of other energy sources, helping to both generate power on its own, and decarbonise other energy industries.
“I think in the future, one of the things that could happen, and that I hope does happen, is the infrastructure we’ve put in place, and if others put in place too, the oil patch, California, could be leveraged going forward,” explains King.
“You might get the mirrors in place to generate the steam, to go downhole and get the oil out. [Eventually] the well runs dry and then you’re out of oil; that’s probably going to happen on a timeframe where you still have a lot of useful life on the solar equipment.
“So I could see a world in which you say ‘well, the solar equipment is long since paid for, let’s build a power plant and keep it in service and do some green power type of thing’.”
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He also pointed to the work of Ørsted, which became an excellent example of decarbonising industry by shifting its business from 85% oil and gas to 85% offshore wind in a decade. With time to achieve the world’s
climate goals quickly running out, and many climate challenges taking on a complex, multi-faceted nature, perhaps the flexible elements of CSP could help accelerate its adoption in the future.
Sunny superpower: solar cells close in on 50% efficiency
For solar cells, efficiency really matters. This crucial metric determines how much energy can be harvested from rooftops and solar farms, with commercial solar panels made of silicon typically achieving an efficiency of 20%. For satellites, meanwhile, the efficiency defines the size and weight of the solar panels needed to power the spacecraft, which directly affects manufacturing and launch costs.
To make a really efficient device, it is tempting to pick a material that absorbs all the Sun’s radiation – from the high-energy rays in the ultraviolet, through to the visible, and out to the really long wavelengths in the infrared. That approach might lead you to build a cell out of a material like mercury telluride, which converts nearly all of the Sun’s incoming photons into current-generating electrons. But there is an enormous price to pay: each photon absorbed by this material only produces a tiny amount of energy, which means that the power generated by the device would be pitiful.
Hitting the sweet spot
A better tactic is to pick a semiconductor with an absorption profile that optimizes the trade-off between the energy generated by each captured photon and the fraction of sunlight absorbed by the cell. A material at this sweet spot is gallium arsenide (GaAs). Also used in smartphones to amplify radio-frequency signals and create laser-light for facial recognition, GaAs has long been one of the go-to materials for engineering high-efficiency solar cells. These cells are not perfect, however – even after minimizing material defects that degrade performance, the best solar cells made from GaAs still struggle to reach efficiencies beyond 25%.
Further gains come from stacking different semiconductors on top of one another, and carefully selecting a combination that efficiently harvests the Sun’s output. This well-trodden path has seen solar-cell efficiencies climb over several decades, along with the number of light-absorbing layers. Both hit a new high last year when a team from the National Renewable Energy Laboratory (NREL) in Golden, Colorado, unveiled a device with a record-breaking efficiency of 47.1% – tantalizingly close to the 50% milestone (Nature Energy 5 326). Until then, bragging rights had been held by structures with four absorbing layers, but the US researchers found that six is a “natural sweet spot”, according to team leader John Geisz.
Getting this far has not been easy, because it is far from trivial to create layered structures from different materials. High-efficiency solar cells are formed by epitaxy, a process in which material is grown on a crystalline substrate, one atomic layer at a time. Such epitaxial growth can produce the high-quality crystal structures needed for an efficient solar cell, but only if the atomic spacing of each material within the stack is very similar. This condition, known as lattice matching, restricts the palette of suitable materials: silicon cannot be used, for example, because it is not blessed with a family of alloys with similar atomic spacing.
Devices with multiple materials – referred to as multi-junction cells – have traditionally been based on GaAs, the record-breaking material for a single-junction device. A common architecture is a triple-junction cell comprising three compound semiconductors: a low-energy indium gallium arsenide (InGaAs) sub-cell, a medium-energy sub-cell of GaAs and a high-energy sub-cell of indium gallium phosphide (InGaP). In these multi-junction cells, current flows perpendicularly through all the absorbing layers, which are joined in series. With this electrical configuration, the thickness of every sub-cell must be chosen so that all generate exactly the same current – otherwise any excess flow of electrons would be wasted, reducing the overall efficiency.
Bending the rules
Key to the success of NREL’s device are three InGaAs sub-cells that excel at absorbing light in the infrared, which contains a significant proportion of the Sun’s radiation. Achieving strong absorption at these long wavelengths requires InGaAs compositions with a significantly different atomic spacing to that of the substrate. Additionally, their device has been designed with intermediate transparent layers made from InGaP or AlGaInAs to keep material imperfections in check. Grading the composition of these buffer layers enables a steady increase in lattice constant, thereby providing a strong foundation for local lattice-matched growth of sub-cells that are not riddled with strain-induced defects.
The NREL team, which has pioneered this approach, advocates the so-called “inverted variant” structure. With this architecture, the highest energy cell is grown first, followed by those of decreasing energy, so that the cells lattice-matched to the substrate precede the growth of graded layers. This approach improves the quality of the device, while the fabrication process also results in the removal of the substrate – a step that could trim costs by enabling the substrate to be reused.
One other technique that can further boost solar-cell efficiency is to FOCUS sunlight on the cells, either with mirrors or lenses. The intensity of light on a solar cell is usually measured in “suns”, where one sun is roughly equivalent to 1 kW/m 2. Concentrated sunlight increases the ratio of the current produced when the device is illuminated compared to when it is in the dark, thereby boosting the output voltage and increasing the efficiency. The gain is considerable: the NREL device achieves a maximum efficiency of just 39.2% when tweaked to optimize efficiency without any concentration, a long way short of the 47.1% record.
When Geisz and colleagues assessed how the performance of their six-junction cell varies with concentration, they found that peak efficiency occurs at 143 suns. Nevertheless, the device still produces a very impressive 44.9% efficiency at 1116 suns, which would generate a large amount of power from a very small device. As a comparison, a record-breaking cell operating at 500 suns could deliver the same power as a commercial solar panel from just one-thousandth of the chip area. At such high concentrations, however, steps must be taken to prevent the cell from overheating and diminishing performance.
Just over a decade ago, this approach to generating power from high-efficiency cells spawned a concentrating photovoltaic (CPV) industry, with a clutch of start-up firms producing systems that tracked the position of the Sun to maximize the energy that could be harvested from focusing sunlight on triple-junction cells. Unfortunately, this fledgling industry came up against the unforeseeable double whammy of a global financial crisis and a flooding of the market with incredibly cheap silicon panels produced by Chinese suppliers. The result was that so few CPV systems were deployed that even on a sunny day when all operate at their peak, their global output totals less than one-tenth of the power of a typical UK nuclear power station.
Extra-terrestrial encounters
Far greater commercial success for makers of multi-junction cells has come from powering satellites, most recently buoyed by the rollout of satellite broadband by companies such as OneWeb and Starlink. The key advantage here is that high-efficiency cells can drive down the costs of making and launching each satellite. As well as reducing the number of cells needed to power the spacecraft, higher efficiencies shrink both the size and weight of the solar panels that form the “wings” of the satellite. While launch costs have plummeted over the last few decades, satellite operators can still expect to pay almost 3000 per kilogram to get their spacecraft into orbit – and thousands of satellites are due to be deployed over the next few years.
For a solar cell in space, the crucial metric is the value at the end of its lifetime – after the device has been bombarded by radiation
However, for a solar cell in space, the crucial metric is not the initial efficiency but the value at the end of its intended lifetime after the device has been bombarded by radiation. Compound semiconductors hold up to this battering far better than those made from silicon. Early studies showed that the difference in efficiency of compound semiconductors rises with age from 25% to 40–60%, which ensured the dominance of triple-junction cells for space applications. Even so, the efficiencies of the best commercial cells for satellites remain limited to around 30–33%. This is partly because the solar spectrum beyond our atmosphere has a stronger contribution in the ultraviolet, where it is much harder to make an efficient cell, and partly because there are no concentrating optics to FOCUS sunlight onto the cell.
To drive down the watts-per-kilogram of solar power in space, a US team working on a project known as MOSAIC (micro-scale optimized solar-cell arrays with integrated concentration) has been making a compelling case for CPV in space. The team points out that it should be relatively easy to orientate the solar panels on a satellite to maximize power generation with lenses in front of the cells shielding them from radiation. Concentrations must be limited to no more than around 100 suns, however, because cells in space cannot be cooled by convection, only by heat dissipation through radiation and conduction.
For CPV to have a chance of succeeding in space, the large and heavy solar modules used in early terrestrial systems must be replaced with a significantly slimmed-down successor. Technology pioneered by project partner Semprius, a now defunct CPV system maker, excels in this regard. The firm developed a process that uses a rubber stamp to parallel-print vast arrays of tiny cells, each one subsequently capped by a small lens.
The best results have come from stacking a dual-junction GaAs-based cell on top of an InP-based triple-junction cell separated by a very thin dielectric polymer. Current cannot pass through this polymer film, so separate electrical connections are made to extract the current from each cell independently. While this doubles the number of electrical connections, it eliminates the need for current matching between the two devices. Lifting this restriction gives greater freedom to the design, potentially enabling this approach to challenge the efficiency of NREL’s record-breaking device under high concentrations. Operating at 92 suns under illumination which mimics that in space, the team’s latest device, still to be fully optimized, has an efficiency of 35.5%.
Towards 50%
The NREL researchers know what they need to do to break the 50% barrier. The goal they are chasing is to cut the resistance in their device by a factor of 10 to a value similar to that found in their three- and four-junction cousins. They are also well aware of the need to bring down the cost of producing such complex multi-junction cells.
Also chasing the 50% efficiency milestone is a team led by Mircea Guina from Tampere University of Technology in Finland. Guina and colleagues are pursuing lattice-matched designs with up to eight junctions, including as many as four from an exotic material system known as dilute nitrides – a combination of the traditional mix of indium, gallium, arsenic and antimonide, plus a few per cent of nitrogen.
Dilute nitrides are notoriously difficult to grow. Back in the 1990s, German electronics powerhouse Infineon developed lasers based on this material, but they were never a commercial success. recently, Stanford University spin-off Solar Junction showcased the potential of this material in solar cells. Although the start-up went to the wall when CPV flopped, devices produced by the company grabbed the record for solar efficiency in 2011 and raised it again in 2012 with triple-junction designs. Guina and co-workers are well positioned to take their technology further. They have made progress in producing all four of the dilute nitride sub-cells needed to produce record-breaking devices, and their efforts are now focused on optimizing the high-energy junction. The team’s work has been delayed due to the COVID-19 pandemic, but Guina believes that the approach could break the 50% barrier, possibly raising the bar as high as 54%.
There is still a question of impetus, however. The lack of commercial interest in terrestrial CPV may well encourage Guina to change direction and FOCUS on chasing the record for space cells with no concentration. Much of today’s multi-junction solar-cell research is not focusing on power generation here on Earth, so while that 50% milestone is tantalizingly close, it might not be broken anytime soon.
Richard Stevenson is editor of Compound Semiconductor magazine, e-mail richardstevenson@zoho.com