Concentrated photovoltaics

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This Amonix system consists of thousands of small lenses, each focusing sunlight to ~500X higher intensity onto a tiny, high-efficiency multijunction photovoltaic cell.[1] A Tesla Roadster is parked beneath for scale.


Concentrated 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. Despite the need for additional concentrating optics, and the usage of more expensive MJ-cells, CPV systems, and 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. CPV systems often use solar trackers and sometimes a cooling systems to increase further more their efficiency. They are currently more expensive and far less common than conventional PV systems. However, ongoing research and development is rapidly improving their competitiveness.

CPV also competes with concentrated solar thermal. CPV turns the sun's radiation directly into electricity, while solar thermal uses the heat from the sun's radiation to make steam, which turns turbines, which then produce electricity. Solar thermal is far more common than CPV, although the two technologies are sometimes combined.

History[edit]

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.[citation needed] 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.[citation needed]

Challenges[edit]

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.

Efficiency[edit]

Main article: Solar cell efficiency
Reported records of solar cell efficiency since 1975

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.[2][3]

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 multijunction photovoltaic cells developed in research is upward of 44% today, with the potential to approach 50% in the coming years.[4]

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[5][6] 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[edit]

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,[citation needed] whereas silicon flat panels that are commonly sold are now below $1 per watt (not including any associated power systems or installation charges).

Types[edit]

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

Low concentration PV (LCPV)[edit]

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.[7] 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. 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[edit]

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.

HCPV Cell from Fotrousi Electronics.jpg

High concentration photovoltaics (HCPV)[edit]

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.[4] 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. Multijunction 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; multijunction efficiency rises faster[citation needed] . 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 44% 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 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% multijunction 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 (500-1000x), 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. To this end, improvements in sunshape modelling at the system design stage may lead to higher system efficiencies [[8]].

Luminescent solar concentrators[edit]

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.

Concentrated photovoltaics and thermal[edit]

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.

Unlike CSP systems which may be designed to function at temperatures in excess of several hundred degrees, the maximum operating temperatures (Tmax) of CPVT systems are limited to less than 125-150°C on account of the efficiency and reliability behavior of their multi-junction PV cells (i.e. the cells are fabricated from a layering of III-V semiconductor materials having intrinsic lifetimes with an Arrhenius-type temperature dependence.) Additional extrinsic factors, such as those imposed by frequent system thermal cycling and by seemingly minor variations/defects in receiver fabrication, will further reduce the Tmax compatible with long system life. Since the concentration of thermal work available is proportional to system size and Tmax, CPVT systems may become economically viable to augment the energy supply for lower demand thermal applications. The heat may be employed in district heating, water heating and air conditioning, desalination or process heat.

In developing a first generation of CPVT technologies, performance certification testing standards (e.g. IEC 62108) include stress conditions that may be useful to uncover some infant and early life (<1 year) failure modes at the system, module, and sub-component levels. However, such standardized tests are generally neither designed nor sufficient to evaluate comprehensive long-term (10-25 plus year) reliability under a broader range of actual operating conditions. Long-life performance of these complex systems is therefore assessed and improved through aggressive product development programs which are guided by the results of accelerated component aging, system field testing, and failure analysis.

The long-life performance of a first generation of CPVT products now being deployed by several startup ventures appears promising but remains publicly undemonstrated. Despite their impressive headline efficiencies sometimes exceeding 70% (~35% electric, ~40% thermal), such claimed performances are achievable only within a narrow range of system operating paramters. Ultimately, the market acceptance and economic value of CPVT products will be judged in comparison to potentially more flexible and durable systems that combine CSP, CPV, PV, and other mature/maturing technologies.

CPVT systems are currently in production in Europe,[9] with Zenith Solar developing CPVT systems with a claimed efficiency of 72%.[10] The US company Cogenra [11] has installed CPVT systems.

This 240mm diameter by 80mm 1,000 suns concentrated photovoltaic (CPV) heat sink design thermal animation, was created using high resolution CFD analysis, and shows temperature contoured heat sink surface and flow trajectories, predicted using a CFD analysis package, courtesy of NCI.

See also[edit]

References[edit]

  1. ^ 500X concentration ratio is claimed at Amonix website.
  2. ^ 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,
  3. ^ 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
  4. ^ a b S. Kurtz. "Opportunities and Challenges for Development of a Mature Concentrating Photovoltaic Power Industry". www.nrel.gov. p. 5 (PDF: p. 8). Retrieved 2012-02-08. 
  5. ^ Julio Chaves, Introduction to Nonimaging Optics, CRC Press, 2008 ISBN 978-1420054293
  6. ^ Roland Winston et al.,, Nonimaging Optics, Academic Press, 2004 ISBN 978-0127597515
  7. ^ A Strategic Research Agenda for Photovoltaic Solar Energy Technology Photovoltaic technology platform
  8. ^ Cole, IR, Betts, TR, Gottschalg, R (2012) Solar profiles and spectral modeling for CPV simulations, IEEE Journal of Photovoltaics, 2(1), pp.62-67, ISSN: 2156-3381. DOI: 10.1109/JPHOTOV.2011.2177445.
  9. ^ "Researchers Explore Hybrid Concentrated Solar Energy System". Renewable Energy World. Retrieved 2009-01-23. 
  10. ^ "Zenith Solar Projects - Yavne". zenithsolar.com. 2011. Retrieved May 14, 2011. 
  11. ^ "Cogenra Product Applications". 

External links[edit]