Concentrated photovoltaics

This Amonix system consists of thousands of small lenses, each focusing sunlight to ~500X higher intensity onto a tiny, high-efficiency multi-junction photovoltaic cell.[1] A Tesla Roadster is parked beneath for scale.
CPV modules on dual axis solar tracker in Golmud, China

Concentrator photovoltaics (CPV) is a photovoltaic technology that generates electricity from sunlight. Contrary to conventional photovoltaic systems, it uses lenses and curved mirrors to focus sunlight onto small, but highly efficient, multi-junction (MJ) solar cells. In addition, CPV systems often use solar trackers and sometimes a cooling system to further increase their efficiency.[2]:30 Ongoing research and development is rapidly improving their competitiveness in the utility-scale segment and in areas of high solar insolation.

Especially High concentrating photovoltaic (HCPV) systems, have the potential to become competitive in the near future. They possess the highest efficiency of all existing PV technologies, and a smaller photovoltaic array also reduces the balance of system costs. Currently, CPV is not used in the PV roof top segment and far less common than conventional PV systems. For regions with a high annual direct normal irradiance of 2000 kilowatt-hour (kWh) per square meter or more, the levelized cost of electricity is in the range of $0.08–$0.15 per kWh and installation cost for a 10-megawatt CPV power plant was identified to lie between €1.40–€2.20 per watt-peak (Wp).[3]:12

In 2013 CPV installations accounted for only 0.1% or 50 megawatts (MW) of the annual global PV market of nearly 39,000 MW.[2]:10[4][5]:21 However, by the end of 2014, cumulative installations already amounted to 330 MW.[3]:5 Commercial HCPV systems reached efficiencies of up to 42% with concentration levels above 400,[5]:26 and the International Energy Agency sees potential to increase the efficiency of this technology to 50% by the mid-2020s.[2]:28 As of December 2014, best lab cell efficiency for concentrator MJ-cells reached 46% (four or more junctions). Most CPV installations are located in China, the United States, South Africa, Italy and Spain.[3]:12

HCPV directly competes with concentrated solar power (CSP) as both technologies are suited best for areas with high direct normal irradiance, which are also known as the Sun Belt region in the United States and the Golden Banana in Southern Europe.[5]:26 CPV and CSP are often confused with one another, despite being intrinsically different technologies from the start: CPV uses the photovoltaic effect to directly generate electricity from sunlight, while CSP – often called concentrated solar thermal – uses the heat from the sun's radiation in order to make steam to drive a turbine, that then produces electricity using a generator. Currently, CSP is more common than CPV, especially in Spain and the United States, where concentrated solar thermal contributes a significant part on the overall electricity generation from solar power.[6]

History

Research into concentrator photovoltaics has taken place since the 1970s. Sandia National Laboratories in Albuquerque, New Mexico 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.

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 of providing uniform illumination of the cell. All these reasons make nonimaging optics the most suitable for CPV.

CPV Strengths CPV Weaknesses
High efficiencies for direct-normal irradiance HCPV cannot utilize diffuse radiation. LCPV can only utilize a fraction of diffuse radiation
Low temperature coefficients Tracking with sufficient accuracy and reliability is required
No cooling water required for passively cooled systems May require frequent cleaning to mitigate soiling losses, depending on the site
Additional use of waste heat possible for systems with active cooling possible (e.g.large mirror systems) Limited market – can only be used in regions with high DNI, cannot be easily installed on rooftops
Modular – kW to GW scale Strong cost decrease of competing technologies for electricity production
Increased and stable energy production throughout the day due to tracking Bankability and perception issues
Very low energy payback time New generation technologies, without a history of production (thus increased risk)
Potential double use of land e.g. for agriculture, low environmental impact Optical losses
High potential for cost reduction Lack of technology standardization
Opportunities for local manufacturing –
Smaller cell sizes could prevent large fluctuations in module price due to variations in semiconductor prices –
Greater potential for efficiency increase in the future compared to single-junction flat plate systems could lead to greater improvements in land area use, BOS costs, and BOP costs –
Source: Current Status of CPV report, January 2015.[3]:8 Table 2: Analysis of the strengths and weaknesses of CPV.

Efficiency

Main article: Solar cell efficiency
Reported records of solar cell efficiency since 1975. As of December 2014, best lab cell efficiency reached 46% (for ⊡ multi-junction concentrator, 4+ junctions).

All CPV systems have a concentrating optic and a solar cell. Except for very low concentrations, active solar tracking is also necessary. Low concentration systems often have a simple booster reflector, which can increase solar electric output by over 30% from that of non-concentrator PV systems.[7][8]

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 sinks. Efficiency of multi-junction photovoltaic cells developed in research is upward of 44% today, with the potential to approach 50% in the coming years.[9]

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[10][11] 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.

Grid parity

Grid parity refers to the cost of solar/wind watts produced compared to watts available from the electrical utility grid. Grid parity is achieved when renewable energy watts are monetarily equal to watts produced on the grid (from coal, hydro, etc.).

Compared to conventional flat panel solar cells, CPV might be advantageous because the solar collector is less expensive than an equivalent area of solar cells. However CPV hardware (solar collector and tracker) is nearing US$1 per watt, whereas silicon flat panels that are commonly sold are now below $1 per watt (not including any associated power systems or installation charges).

Types

CPV systems are categorized according to the amount of their solar concentration, measured in "suns" (the square of the magnification).

Low concentration PV (LCPV)

An example of a Low Concentration PV Cell's surface, showing the glass lensing

Low concentration PV are systems with a solar concentration of 2–100 suns.[12] For economic reasons, conventional or modified silicon solar cells are typically used, and, at these concentrations, the heat flux is low enough that the cells do not need to be actively cooled. There is now modeling and experimental evidence that standard solar modules do not need any modification, tracking or cooling if the concentration level is low and yet still have increased output of 35% or more.[13] 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.

Medium concentration PV

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 1000 suns or more.[9] The solar cells require high-capacity heat sinks to prevent thermal destruction and to manage temperature related electrical performance and life expectancy losses. To further exacerbate the concentrated cooling design, the heat sink must be passive, otherwise the power required for active cooling will reduce the overall efficiency and economy. Multi-junction solar cells are currently favored over single junction cells, 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; multi-junction efficiency rises faster. Multi-junction 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 multi-junction 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 multi-junction cells. Multi-junction cell efficiency has now reached 44% in production cells.

The 44% 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 temperature 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 44% multi-junction cell might deliver a DC efficiency around 36%. Under similar conditions, a silicon-cell module would deliver an efficiency of less than 18%.

When high concentration is needed (500–1000 times), as occurs in the case of high efficiency multi-junction 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. To this end, improvements in sunshape modelling at the system design stage may lead to higher system efficiencies.[14]

Luminescent solar concentrators

A new emerging type of concentrators which are still at the research stage are Luminescent solar concentrators, 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 trackers.

Installations

Concentrator photovoltaics technology has established its presence in the solar industry in the past few years. The first CPV power plant that exceeded 1 MW-level was commissioned in Spain in 2006. By the end of 2014, the fast-growing number CPV power plants around the world accounted for a total installed capacity of 330 MW.[3]:5,10

Cumulative CPV Installations in MW by country by November 2014[3]:12
Yearly Installed CPV Capacity in MW from 2002 to 2013.[3]:10

List of large CPV systems

The largest CPV power plant currently in operation is of 80 MWp capacity located in Golmud, China, hosted by Suncore Photovoltaics.[15]

Power station Capacity (MWp) Location Ref
Golmud 2 79.83 in Golmud/Qinghai province/China [16]
Golmud 1 57.96 in Golmud/Qinghai province/China [17]
Alamosa Solar Project 35.28 in Alamosa, Colorado/San Luis Valley/United States [18]
Source: The CPV Consortium[19]

Concentrated photovoltaics and thermal

Concentrated photovoltaics and thermal (CPVT), also sometimes called combined heat and power solar (CHAPS), is a cogeneration or micro cogeneration technology used in concentrated photovoltaics that produces both electricity and heat in the same module. CPVT at 100-1000 suns utilizes similar components as CPV, including dual-axis tracking and multi-junction photovoltaic cells. A fluid actively transports the collected heat and simultaneously cools the integrated thermal+photovoltaic receivers. Typically, an array of receivers and a heat exchanger operate within a closed thermal loop. To maintain efficient overall operation and avoid damage from thermal runaway, the demand for heat from the secondary side of the exchanger must be insured to remain consistently high. Under such optimal operating conditions, collection efficiencies exceeding 70% (~35% electric, ~40% thermal) are anticipated.

Unlike CSP and other CHP systems which may be designed to function at temperatures in excess of several hundred degrees, the maximum operating temperatures (Tmax cell) of CPVT systems are limited to less than approximately 100-125 °C on account of the reliability limitation of their multi-junction PV cells. (i.e. the cells are fabricated from a layering of thin-film III-V semiconductor materials having intrinsic lifetimes that rapidly decrease with an Arrhenius-type temperature dependence.) Additional extrinsic factors, such as those imposed by the frequent system thermal cycling and/or by seemingly minor variations in receiver heat-transfer performance, will further reduce the Tmax compatible with long system life. Since the concentration of thermal work available is proportional to Tmax, CPVT systems may become economical to augment the energy supply for lower temperature applications with constant high demand. The heat may be employed in district heating, water heating and air conditioning, desalination or process heat. For applications having lower or intermittent heat demand, the system may be augmented with a switchable heat dump to the external environment in order to maintain reliable electrical output, despite the resulting reduction in net operating efficiency.

Due to their lesser standardization and added complexities in comparison to zero and low-concentration PV systems, demonstrating long-life performance will be a key engineering challenge for the first generations of CPV and CPVT technologies. Performance certification testing standards (e.g. IEC 62108, UL 8703, IEC 62789, IEC 62670) include stress conditions that may be useful to uncover some predominantly infant and early life (<1-2 year) failure modes at the system, module, and sub-component levels. However, such standardized tests - as typically performed on only a small sampling of units - are generally incapable to evaluate comprehensive long-term (10-25+ plus year) lifetimes for each unique CPVT system design and application under its broader range of actual operating conditions. Long-life performance of these complex systems is therefore assessed in the field, and is improved through aggressive product development cycles which are guided by the results of accelerated component/system aging, enhanced performance monitoring diagnostics, and failure analysis. Significant growth in the deployment of CPV and CPVT can be anticipated once the long-term performance and reliability concerns are better addressed to build confidence in system bankability.[20]

The economics of CPVT are improving, along with those of some competing CHP options. Their application is presently optimal for niche markets having all of the following tactical characteristics.
- High solar direct normal incidence (DNI),
- Tight space constraints for placement of a solar collector array,
- High and constant demand for low temperature heat,
- High cost of grid electricity, and
- Access to backup sources of power or cost-efficient storage.
Access to a power purchase agreement (PPA), government assistance programs, and innovative financing schemes are also helping potential manufacturers and users to mitigate the risks of early technology adoption.

A first generation of CPVT products ranging from low (LCPVT) to high (HCPVT) concentration are now being deployed by several startup ventures.
- HCPVT (~1000x concentration) systems utilizing primary lens concentrators have been developed by Solergy with a claimed collection efficiency up to 75%.[21]
- HCPVT systems (~1000x concentration) utilizing reflective dish concentrators have been developed by Zenith Solar-Suncore Photovoltaics with a claimed 72-75% efficiency.[22] [23]
- HCPVT (~2000x concentration) systems utilizing similar reflective dish concentrators and an optimized cell cooling technology have been developed in the "dsolar" collaboration between Airlight Energy and IBM Research with a claimed 80% efficiency.[24] [25]
- HCPVT systems (~1000x concentration) utilizing a reflective dish/receiver array technology pioneered by SolFocus have been assembled by Brightleaf Power with a claimed 70% efficiency.[26]
- LCPVT systems at ~14x concentration using silicon cells with dense interconnects have been assembled by Cogenra.[27]

This 240 x 80 mm 1,000 suns CPV heat sink design thermal animation, was created using high resolution CFD analysis, and shows temperature contoured heat sink surface and flow trajectories as predicted.

See also

References

  1. ↑ 500x concentration ratio is claimed at Amonix website.
  2. ↑ 2.0 2.1 2.2 http://www.iea.org (2014). "Technology Roadmap: Solar Photovoltaic Energy" (PDF). IEA. Archived from the original on 7 October 2014. Retrieved 7 October 2014.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Fraunhofer ISE and NREL (January 2015). "Current Status of Concentrator Photovoltaic (CPV) Technology" (PDF). Archived from the original on 25 April 2015. Retrieved 25 April 2015.
  4. ↑ "Snapshot of Global PV 1992-2013" (PDF). http://www.iea-pvps.org/''. International Energy Agency - Photovoltaic Power Systems Programme. 2014. Archived from the original on 5 April 2014.
  5. ↑ 5.0 5.1 5.2 "Photovoltaics Report" (PDF). Fraunhofer ISE. 28 July 2014. Archived from the original on 31 August 2014. Retrieved 31 August 2014.
  6. ↑ PV-insider.com How CPV trumps CSP in high DNI locations, 14 February 2012
  7. ↑ Rob Andrews, Nabeil Alazzam, and Joshua M. Pearce, “Model of Loss Mechanisms for Low Optical Concentration on Solar Photovoltaic Arrays with Planar Reflectors”, 40th American Solar Energy Society National Solar Conference Proceedings, pp. 446-453 (2011).free and open access,
  8. ↑ Andrews, Rob W.; Pollard, Andrew; Pearce, Joshua M., "Photovoltaic system performance enhancement with non-tracking planar concentrators: Experimental results and BDRF based modelling," Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, pp.0229,0234, 16–21 June 2013. doi: 10.1109/PVSC.2013.6744136
  9. ↑ 9.0 9.1 S. Kurtz. "Opportunities and Challenges for Development of a Mature Concentrating Photovoltaic Power Industry" (PDF). www.nrel.gov. p. 5 (PDF: p. 8). Retrieved 2012-02-08.
  10. ↑ Julio Chaves, Introduction to Nonimaging Optics, CRC Press, 2008 ISBN 978-1420054293
  11. ↑ Roland Winston et al.,, Nonimaging Optics, Academic Press, 2004 ISBN 978-0127597515
  12. ↑ A Strategic Research Agenda for Photovoltaic Solar Energy Technology Photovoltaic technology platform
  13. ↑ Andrews, Rob W.; Pollard, Andrew; Pearce, Joshua M., "Photovoltaic system performance enhancement with non-tracking planar concentrators: Experimental results and BDRF based modelling," Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, pp.0229,0234, 16–21 June 2013. doi: 10.1109/PVSC.2013.6744136
  14. ↑ Cole, IR, Betts, TR, Gottschalg, R (2012), "Solar profiles and spectral modeling for CPV simulations", IEEE Journal of Photovoltaics, 2(1): 62–67, doi:10.1109/JPHOTOV.2011.2177445, ISSN 2156-3381
  15. ↑ Eric Wesoff, "Soitec, SunPower and Suncore: The Last CPV Vendors Standing , 29 October 2014
  16. ↑ http://cpvconsortium.org/projects/21
  17. ↑ http://cpvconsortium.org/projects/20
  18. ↑ http://cpvconsortium.org/projects/24
  19. ↑ http://cpvconsortium.org/locations
  20. ↑ Concentrated Photovoltaics Update 2014, GlobalData Market Research Report
  21. ↑ "Solergy Cogen CPV".
  22. ↑ "Zenith Solar Projects - Yavne". zenithsolar.com. 2011. Retrieved May 14, 2011.
  23. ↑ "Suncore".
  24. ↑ "Airlight Energy".
  25. ↑ "dsolar".
  26. ↑ "Brightleaf Power".
  27. ↑ "Cogenra".

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