Concentrator photovoltaics

Research into concentrator photovoltaics has taken place since the mid 1970s, initially spurred on by the energy shock from a mideast oil embargo. Sandia National Laboratories in Albuquerque, New Mexico was the site for most of the early work, with the first modern-like 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. Cell cooling with a passive heat sink and use of silicone-on-glass Fresnel lenses was demonstrated in 1979 by the Ramón Areces Project at the Institute of Solar Energy of the Technical University of Madrid. The 350 kW SOLERAS project in Saudi Arabia—the largest until many years later—was constructed by Sandia/Martin Marietta in 1981.[8][9]

Research and development continued through the 1980s and 1990s without significant industry interest. Improvements in cell efficiency were soon recognized as essential to making the technology economical. However the improvements to Si-based cell technologies used by both concentrators and flat PV failed to favor the system-level economics of CPV. The introduction of III-V Multi-junction solar cells starting in the early 2000s has since provided a clear differentiator. MJ cell efficiencies have improved from 34% (3-junctions) to 46% (4-junctions) at research-scale production levels.[3]^:14 A substantial number of multi-MW CPV projects have also been commissioned worldwide since 2010.[10]

Modern CPV systems operate most efficiently in highly concentrated sunlight (i.e. concentration levels equivalent to hundreds of suns), as long as the solar cell is kept cool through the use of heat sinks. Diffuse light, which occurs in cloudy and overcast conditions, cannot be highly concentrated using conventional optical components only (i.e. macroscopic lenses and mirrors). Filtered light, which occurs in hazy or polluted conditions, has spectral variations which produce mismatches between the electrical currents generated within the series-connected junctions of spectrally "tuned" multi-junction (MJ) photovoltaic cells.[11] These CPV features lead to rapid decreases in power output when atmospheric conditions are less than ideal.

To produce equal or greater energy per rated watt than conventional PV systems, CPV systems must be located in areas that receive plentiful direct sunlight. This is typically specified as average DNI (Direct Normal Irradiance) greater than 5.5-6 kWh/m²/day or 2000kWh/m²/yr. Otherwise, evaluations of annualized DNI vs. GNI/GHI (Global Normal Irradiance and Global Horizontal Irradiance) irradiance data have concluded that conventional PV should still perform better over time than presently available CPV technology in most regions of the world (see for example [12]).

International CPV-x Conference - Historical Participation Statistics. Data Source - CPV-x Proceedings

CPV research and development has been pursued in over 20 countries for more than a decade. The annual CPV-x conference series has served as a primary networking and exchange forum between university, government lab, and industry participants. Government agencies have also continued to encourage a number of specific technology thrusts.

ARPA-E announced a first round of R&D funding in late 2015 for the MOSAIC Program (Microscale Optimized Solar-cell Arrays with Integrated Concentration) to further combat the location and expense challenges of existing CPV technology. As stated in the program description: "MOSAIC projects are grouped into three categories: complete systems that cost effectively integrate micro-CPV for regions such as sunny areas of the U.S. southwest that have high Direct Normal Irradiance (DNI) solar radiation; complete systems that apply to regions, such as areas of the U.S. Northeast and Midwest, that have low DNI solar radiation or high diffuse solar radiation; and concepts that seek partial solutions to technology challenges."[13]

In Europe the CPVMATCH Program (Concentrating PhotoVoltaic Modules using Advanced Technologies and Cells for Highest efficiencies) aims "to bring practical performance of HCPV modules closer to theoretical limits". Efficiency goals achievable by 2019 are identified as 48% for cells and 40% for modules at >800x concentration.[14] A 41.4% module efficiency was announced at the end of 2018.[15]

The Australian Renewable Energy Agency (ARENA) extended its support in 2017 for further commercialization of the HCPV technology developed by Raygen.[16] Their 250 kW dense array receivers are the most powerful CPV receivers thus far created, with demonstrated PV efficiency of 40.4% and include usable heat co-generation.[17]

A low concentrating solar device that includes its own internal tracker, is in development by ISP Solar which will enhance the efficiency of solar cell at low cost.[18]

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

According to theory, semiconductor properties allow solar cells to operate more efficiently in concentrated light than they do under a nominal level of solar irradiance. This is because, along with a proportional increase in the generated current, there also occurs a logarithmic enhancement in operating voltage, in response to the higher illumination.[19]

To be explicit, consider the power (P) generated by a solar cell under "one-sun" illumination at the earth's surface, which corresponds to a peak solar irradiance Q=1000 Watts/m².[20] The cell power can be expressed as a function of the open-circuit voltage (V_oc), the short-circuit current (I_sc), and the fill factor (FF) of the cell's characteristic current–voltage (I-V) curve:[21]

${\displaystyle P=I_{\mathrm {sc} }\times V_{\mathrm {oc} }\times FF.}$

Upon increased illumination of the cell at "χ-suns", corresponding to concentration (χ) and irradiance (χQ), there can be similarly expressed:

${\displaystyle P_{\chi }=I_{\mathrm {\chi sc} }\times V_{\mathrm {\chi oc} }\times FF_{\chi }}$

where, as shown by reference:[19]

${\displaystyle I_{\mathrm {\chi sc} }=\chi \times I_{\mathrm {sc} }\quad }$ and ${\displaystyle \quad V_{\mathrm {\chi oc} }=V_{\mathrm {oc} }+{kT \over q}\ln(\chi ).}$

Note that the unitless fill factor for a "high quality" solar cell typically ranges 0.75-0.9 and can, in practice, depend primarily on the equivalent shunt and series resistances for the particular cell construction.[22] For concentrator applications, FF and FF_χ should then have similar values that are both near unity, corresponding to high shunt resistance and very low series resistance (<1 milliohm).[23]

The efficiencies of a cell of area (A) under one-sun and χ-suns are defined as:[24]

${\displaystyle \eta ={P \over QA}\quad }$ and ${\displaystyle \quad \eta _{\chi }={P_{\chi } \over \chi QA}.}$

The efficiency under concentration is then given in terms of χ and the cell characteristics as:[19]

${\displaystyle \eta _{\chi }=\eta \times {P_{\chi } \over \chi P}=\eta \times \left({1+{kT \over q}}{\ln(\chi ) \over V_{\mathrm {oc} }}\right)\times \left({FF_{\chi } \over FF}\right),}$

where the term kT/q is the voltage (called the thermal voltage) of a thermalized population of electrons - such as that flowing through a solar cell's p-n junction - and has a value of about 25.85 mV at room temperature (300 K).[25]

The efficiency enhancement of η_χ relative to η is listed in the following table for a set of typical open-circuit voltages that roughly represent different cell technologies. The table shows that the enhancement can be as much as 20-30% at χ = 1000 concentration. The calculation assumes FF_χ/FF=1; an assumption which is clarified in the following discussion.

In practice, the higher current densities and temperatures which arise under sunlight concentration may be challenging to prevent from degrading the cell's I-V properties or, worse, causing permanent physical damage. Such effects can reduce the ratio FF_χ/FF by an even larger percentage below unity than the tabulated values shown above. To prevent irreversible damage, the rise in cell operating temperature under concentration must be controlled with the use of a suitable heat sink. Additionally, the cell design itself must incorporate features that reduce recombination and the contact, electrode, and busbar resistances to levels that accommodate the target concentration and resulting current density. These features include thin, low-defect semiconductor layers; thick, low-resistivity electrode & busbar materials; and small (typically <1 cm²) cell sizes.[26]

Including such features, the best thin film multi-junction photovoltaic cells developed for terrestrial CPV applications achieve reliable operation at concentrations as high as 500-1000 suns (i.e. irradiances of 50-100 Watts/cm²).[27][28] As of year 2014, their efficiencies are upwards of 44% (three junctions), with the potential to approach 50% (four or more junctions) in the coming years .[29] The theoretical limiting efficiency under concentration approaches 65% for 5 junctions, which is a likely practical maximum.[30]

All CPV systems have a solar cell and a concentrating optic. Optical sunlight concentrators for CPV introduce a very specific design problem, with features that make them different from most other optical designs. They have 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[31][32] the most suitable for CPV.

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.

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.[33] For economic reasons, conventional or modified silicon solar cells are typically used. The heat flux is typically low enough that the cells do not need to be actively cooled. For standard solar modules, there is also modeling and experimental evidence that no tracking or cooling modifications are needed if the concentration level is low [34]

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.[35][36] Experimental results from such LCPV systems in Canada resulted in energy gains over 40% for prismatic glass and 45% for traditional crystalline silicon PV modules.[37]

Medium concentration PV

From concentrations of 100 to 300 suns, the CPV systems require two-axis solar tracking and cooling (whether passive or active), which makes them more complex.

A 10×10 mm HCPV solar cell

The higher capital costs, lesser standardization, and added engineering & operational complexities (in comparison to zero and low-concentration PV technologies) make long-life performance a critical demonstration goal for the first generations of CPV technologies. Performance certification standards (UL 3703,UL 8703, IEC 62108, IEC 62670, IEC 62789, and IEC 62817) include stress testing conditions that may be useful to uncover some predominantly infant and early life (<1–2 year) failure modes at the system, tracker, module, receiver, and other sub-component levels. [39] However, such standardized tests – as typically performed on only a small sampling of units – are generally incapable to evaluate comprehensive long-term lifetimes (10 to 25 or more years) for each unique system design and application under its broader range of actual - and occasionally unanticipated - operating conditions. Reliability 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, performance monitoring diagnostics, and failure analysis. [40] Significant growth in the deployment of CPV can be anticipated once the concerns are better addressed to build confidence in system bankability. [41] [42]

Concentrator photovoltaics technology established its presence in the solar industry during the period 2006 to 2015. The first HCPV power plant that exceeded 1 MW-level was commissioned in Spain in 2006. By the end of 2015, the number of CPV power plants (including both LCPV and HCPV) around the world accounted for a total installed capacity of 350 MW. Field data collected over six years is also starting to benchmark the prospects for long-term system reliability.[50]

The emerging CPV segment has comprised ~0.1% of the fast-growing utility market for PV installations over the decade up to 2017. Unfortunately, following a rapid drop in traditional flat-panel PV prices, the near term outlook for CPV industry growth has faded as signaled by closure of the largest HCPV manufacturing facilities: including those of Suncore, Soitec, Amonix, and SolFocus. [51] [52] [53] [54] [55] [56] [57] [58] The higher cost and complexity of maintaining the precision HCPV dual-axis trackers has also been reported in some instances to be especially challenging.[59][45] Nevertheless, the growth outlook for the PV industry as a whole continues to be strong, thus providing continued optimism that CPV technology will eventually demonstrate its place. [3][5]

Cumulative CPV Installations in MW by country by November 2014[3]^:12

Yearly Installed CPV Capacity in MW from 2002 to 2015.[3][5]

Yearly Installed PV Capacity in GW from 2002 to 2015.[5]

Concentrator photovoltaics and thermal (CPVT), also sometimes called combined heat and power solar (CHAPS) or hybrid thermal CPV, is a cogeneration or micro cogeneration technology used in the field of concentrator photovoltaics that produces usable heat and electricity within the same system. CPVT at high concentrations of over 100 suns (HCPVT) utilizes similar components as HCPV, including dual-axis tracking and multi-junction photovoltaic cells. A fluid actively cools the integrated thermal–photovoltaic receiver, and simultaneously transports the collected heat.

Typically, one or more 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 consistently high. Under such optimal operating conditions, collection efficiencies exceeding 70% (up to ~35% electric, ~40% thermal for HCPVT) are anticipated. Net operating efficiencies may be substantially lower depending on how well a system is engineered to match the demands of the particular thermal application.

The maximum temperature of CPVT systems is typically too low (below 80-90 °C) to alone power a boiler for additional steam-based cogeneration of electricity. Such systems may be economical to power lower temperature applications having a constant high heat 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, a system may be augmented with a switchable heat dump to the external environment in order to maintain reliable electrical output and safeguard cell life, despite the resulting reduction in net operating efficiency.

HCPVT active cooling enables the use of much higher power thermal–photovoltaic receiver units, generating typically 1–100 kilowatts electric, as compared to HCPV systems that mostly rely upon passive cooling of single ~20W cells. Such high-power receivers utilize dense arrays of cells mounted on a high-efficiency heat sink.[82] Minimizing the number of individual receiver units is a simplification that should ultimately yield improvement in the overall balance of system costs, manufacturability, maintainability/upgradeability, and reliability.[83]

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.

• Concentrated solar power (CSP)
• Luminescent solar concentrator
• Concentrated photovoltaic thermal hybrid solar collectors (CPVT)

• System Cost Data, NREL
• CPV Consortium, List of CPV Projets