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Concentrating Photovoltaics (CPV)

This contribution describes the status and challenges of low concentrating (LCPV) and high concentrating (HCPV) photovoltaics. The main research initiatives on the development high efficient, low cost, reliable CPV systems in Italy and in Europe are reported. Economical aspects as well as the National supporting program for the take off the CPV technology in the energy market are included.

pubblicato: 4 ottobre 2011

Concentrating Photovoltaics

DOWNLOAD PDF VERSION (1,7 Mb)

1. Introduction on the CPV technology

2. CPV technological development in Italy and abroad

3. Energy production by CPV

4. Economical Aspects: the indicators of the Strategic Research Agenda (SRA)

5. Market aspect: the feed in tariff

6. Conclusion Future challenges and development of CPV

7. Bibliography

Annex 1. CPV Standards and Specifications (pdf)

Annex 2. The CPV Companies (pdf)

 


Author: Gianluca Timò, Research about the Energy System - RSE S.p.A.
(Power Generation System Department - Concentrating Photovoltaic and Leds Head)
Via  Nino, Bixio, 39 - Piacenza (Italy)
gianluca.timo@rse-web.it


A breve verrà pubblicata la versione in italiano dello Speciale

 

 

1. Introduction on the CPV technology

The main concept behind the development of the CPV technology is very straightforward: concentrating the sunlight by optical devices such as lenses or mirrors reduces the area of expensive solar cells of a factor roughly equal to the concentration factor; in practice, the use of the photovoltaic material is replaced by the less expensive material used for the optical devices (glass, silicone, etc.) therefore the technology has great potential for a substantial cost reduction (see Fig.1 a).   

Figure 1.a) - Schematic of a PV concentrator: a lens is used to concentrate the sunlight onto a small solar cell.

Furthermore, the solar cells efficiency increases with the concentration factor, making the CPV the most efficient among the PV technologies. In particular, the best performances are obtained by using multi-junction (MJ) solar cells made by III-V elements of the period table, which were primarily designed for aerospace applications and powering satellites. As an example, with 35% efficient MJ solar cells produced from a wafer of 100 mm diameter, by concentrating the light 500 times, it is possible to get the same power obtained from 7 m2 of silicon solar cells having 18% conversion efficiency. Therefore if we covered a football field with silicon solar cells 18% efficient, we could produce the same amount of power just concentrating 500 times the sunlight over an area 891 times smaller, covered with 35% MJ cell (see Fig .1 b).

Figure 1.b) - By concentrating the sunlight 500 times on the red area in the football field, covered by 35% efficient MJ solar cell, we would produce the same power which we could get by covering the whole green football field with 18% efficient Si cells.

III-V MJ solar cells can reach high efficiency value since they can extract more energy from the range of wavelengths in sunlight (see Fig.2). This enables these cells to produce a significant increase in voltage, while simultaneously losing less energy to heat.

Figure 2 - Multijuction solar cell. The light is selectively absorbed in the different layers of the solar cell: UV light is absorbed in the top of the structure; visible light is absorbed in middle of the structure, while infrared light is absorbed in the bottom of the structure.

For this reason, concentrated photovoltaic systems are becoming a highly sought-after technological advance in the solar energy field.  Nowadays the research on CPV is also addressed to develop PV systems working even at higher concentrator factor (1000 times and higher); therefore, it is evident that the CPV technology has also a great potential to answer effectively to the problem related to the abundant but anyway limited amount of materials present on the earth needed for photovoltaic conversion. Differently from the flat plate PV technologies, CPV convert mainly the direct solar irradiation (DNI) and has the necessity to track the sun’s motion across the sky.  Although the diffuse light is lost, the DNI resource is often greater then the resources available to fixed flat-plate panel, just because the tracking allow capturing a higher value of the solar radiation.  Therefore the disadvantage of CPV technology of having a moving part, it is partially compensated by larger exposure of the cells to sunlight during the day. A large-scale commercialisation of CPV systems still have to take off, however,  several steps ahead have been perused in these last few years to face the main problems that had hindered a full deployment of this technology, namely:

  • The presence of a large variety of the CPV system typologies which had scattered the efforts in different technological paths, avoiding a fast development towards industrial application. Without pretending to be exhaustive, a list of the different CPV typologies concerned: point focus [1], line focus [2], dense array [3], RXI concentrator [4], micro-reflective dishes [5], parabolic mirror and spectral splitting systems [6, 7], V-trough concentrator [8].
  • A high cost in spite of the potentiality for a strong cost reduction.
  • The lack of international norm and standardisation on CPV.
  • The lack of initiatives for supporting the demonstration and industrialisation of CPV
  • The lack of bankability owing to the lack of information on the reliability to the CPV technology

For almost 30 years researchers have been working in developing concentrator PV components (see for example [9]) however there has been little commercial interest in CPV till the industry has started to consider high volume of manufacturing (> tens of megawatt per year) and, in parallel, solar cells efficiency values have grown over 40 % still maintaining a steep learning curve [10]. The lower initial capital costs opposed to conventional PV, the high volume production along with the high efficiency are the main ingredients for reaching low values of the levelized cost of energy (LCOE, the total energy produced by a system over a given time- typically 25 years- and its total cost over that period) [11].  A strong production increment and cost reduction is expected from the reference year 2008, in which the CPV industries surpassed 1 MW of installation: nowadays CPV companies have projects for tens of Megawatts and the expected yearly production of CPV system will be around 100 MEuro in 2020 with a price of turn key installed CPV system of 1.5 Euro/W [12].

The lack of international norms and standardisation on CPV that had hindered the market penetration of this technology has been strongly reduced. In 2007 the standard IEC 62108 on Concentrator photovoltaic (CPV) modules and assemblies – Design qualification and type approval, has been issued.  The standard IEC 62108 has been fundamental for the access of the CPV technology to National economical supporting program. In Italy, this program guarantees an economical support measure for 20 years, pushing the penetration of the CPV technology in the market. (see the chapter: Market aspect: the feed in tariff).  Other standard (IEC 62670 – Power rating, IEC 62688 – CPV safety standard) and technical specifications (Energy rating, Plant acceptance, Tracker specification, CPV cell specification and qualification) are under development (see for a better detail of their scope the annex 1). For a successful commercialisation of the CPV technology it is very essential that the work on norms andstandard on CPV will be concluded. The existence and acceptance of well-designed and robuststandards covering all aspects of Concentrator PV systems is in fact crucial to ensure a fair and transparent marketplace and to provide end-user confidence.

Since early 1991, Arizona Public Service (APS) has been installing and routinely operating Amonix high CPV systems to support the industrialisationof this technology [13]. In Europe, the most important initiative for supporting the demonstration and industrialisation has only started in July 2006 by ISFOC [14].  ISFOC is now executing a number of CPV power plants (up to 3 MW in total) incorporating different concentrator technologies.  The huge amount of data so far collected as greatly contributed to improve the CPV technology and made possible a benchmarking with other PV ones. In some cases, electrical utilities have been directly involved in the CPV development in order to facilitate the penetration of the technology in the energy production market [15]. Initiatives for supporting the demonstration and industrialisation of CPV have been also boosted by the supporting National programs which foreseen the feed- in tariff for CPV [16].

Owing to the initial lack of information on the reliability to the CPV technology, lack of bankability was a critical issue for CPV deployment and it required time to gain the confidence of the investors. Nowadays long term operation data can be available (see for example [17]), further the introduction of standard (IEC 62108), assuring that the CPV modules and assemblies are capable of withstanding prolonged exposure, has decreased the risk in the investment. The presence of the National economical supporting program, should definitively secure CPV project finance.  It is worthwhile to point out that an advantage of CPV technology with respect to the conventional flat plate technology, is that it requires a reduced capital investment to be set-up (at least an order of magnitude less) , this aspect can surely attract  investors, especially  looking at the grow of this technology predicted far into the future.   

Nowadays the CPV technology can be classified in two main sectors: low concentrating PV (LCVP) with concentrator factor below 100 and high-concentration PV (HCPV) with concentration factor from 300 up to 1000. Medium concentration range modules with concentration factor between 100 and 300 have mostly disappeared [12]. These numbers have not to be considered as rigid border lines to classify the CPV systems but they just give us a rough indication where most of the technological efforts have been focused. In the segment of the LCPV technology, it is also possible to find conventional flat-plate silicon or thin film modules with enhanced performance from mirrors on either side of the module. In this case, there is no a real technology innovation but only a different utilization of conventional modules which can perform better having a higher irradiation level hitting their surfaces. In this contribution this class of CPV module will not be considered. Most of the CPV systems can be also classified as “point  focus”,  and “dense arrays”. In the former, each solar cell on the CPV module has a dedicated optic (lens or mirror). “Dense array” systems are rarer, but still present on the market and are characterized by one unique optic, usually a mirror, which concentrates the light on a group of cell (cell array). Such a CPV system, in general, does require an active cooling, while point focus, generally, do not.  

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2. CPV technological development in Italy and abroad

CPV systems will get a strong penetration in the PV market as soon as they will reach a costwhich lie well under the cost of “flat module” photovoltaic system. The achievement of this goal must concern the minimisation of the cost/performance ratio of all the key components of a CPV system, namely:

  • solar cells
  • optic concentrator
  • module
  • tracker

CPV systems have gained increased interest thanks to the year-by-yeartechnological improvement related to the solar cell technology, which represents the “heart” of theCPV system, therefore a detailed description of the different development strategies regarding this component will receive a higher emphasis in this report with respect to the other CPV components.

Solar Cells

The real hope on HCPV is indeed related to the steep learning curve of the MJ solar cells: while one-sun crystalline silicon cells in the last twenty years have gained hardly few percent points in efficiency (from 20 to 24%), III-V based MJ solar cells have more thandouble their starting values (from 18 to 40%). It is pretty obvious that the enhancement of solar cellefficiency will allow to produce more power and the unit cost per watt will be reduced. However, isquite important to recognise that high efficiency MJ solar cells obtained at low concentration factorare not useful for commercial purpose. It is worthwhile to point out that 40% efficient solar cells working at 240 suns have an estimated cost per watt 28% higher than the cost per watt of 25% efficient MJ cell working at 500 suns! When it comesto the competitiveness of CPV technology, it is not sufficient to have excellent or even record resultswithout taking in due accounts the concentration level. This consideration brings us to address thetechnology efforts to obtain high efficiency solar cells at high concentration levels. Anotherimportant factor very often forgotten, but well known to industrial operator, essential for a largecommercialisation of CPV systems, is the solar cell process yield. It depends mainly by:

  • Technology level and types of equipment’s selected by the solar cell manufacturer
  • Level of industrialisation (experience gained in the solar cell manufacturing process)

This parameter is particularly important for III-V based MJ concentrating cells, where one of the keyelement is to obtain on the whole wafer “good working” tunnel junctions, that is: tunnel junctions with peak currents equal or higher than the solar cell short circuit current [18]. As the level o concentration increases higher peak tunnel currents are required, and this target is not easy to fulfil when several A/cm2 have to cross the cell without important resistive losses. It is reallystraightforward to recognise that there is no economical advantage in getting high efficiency MJ solarcells, when the process yield is not high enough: an yield drop from 95% to 65% introduces a cellCost/W increase of about 46%; consequently 25% efficient solar cells, obtained with a process yieldof 95%, show a Cost/W smaller than 35% efficient solar cells, obtained with a process yield of 65%. Therefore, it is theoverall product of cell efficiency, concentrator factor and cell process yield that has to be maximised to judge the improvement on CPV cell technology.  Finally, it could desirable, as a further step, a cell cost reduction getting the same efficiency at the same concentration factor and yield. This is possible, for example, by considering cell structures grown on cheaper substrates and/or by using less amount of material. In [19] a new CPV Cell economical performance index has been defined to compare the competitiveness of the different solar cell technologies.

Several different approaches are under development to increase the cell efficiency value and reduce the costs, some of them are summarized in table 1

Record efficiency of 42,3 % has been reached with the BFC approach,The Inverted methamorphoic approach (IMM) grows an “upside-down” tandem with a lattice-matched InGaP cell first then a GaAs middle cell and then a lattice mismatched InGaAs bottom cell last. Dislocations in the InGaAs thread upward during growth, so that the top and middle cells can remain defect-free.  The cell wafers are bonded to a carrier substrate and an epitaxial liftoff removes the GaAs wafer.  The BFG cell seeks to implement a similar InGaP/GaAs/InGaAs cell stack without using complex wafer-bonding and epitaxial liftoff steps. This process, however, uses 625 mm thick GaAs substrates.  Semprius Microcell-based HCPV are very small cell with 600 µm edge, this allow a better dissipation of the heat.  RSE has been developing epitaxial germanium to grow 4 J solar cells with  III-V/-IV superlattices. The goal is to get 1 eV materials replacing the N-based III-V materials which present very low diffusion length. QuantaSol has introduced Quantun well in conventional InGaP/InGaAs/Ge solar cells to enhance the infrared response. A similar goal can be reached by using the quantum dot (QD) approach of Cyrium. QDs are grown considering the Stranski-Krastanov growth mode. Dots are elastically strained and free of dislocations (Dot Diameter =200-300A, Dot height = 5-10A, Dot density=100-150mm2). Since the bottom Ge cells produces an excess of current with respect to InGaP top cell and to InGaAs middle cells, and contribute with low voltage, this material is replaced by InGaAs with the IMM approach by EMCORE and NREL and by simpler metamorphic approach of ISE. Methamorphic approaches suffer of the presence of strong wafer bending during the growth due to the deposition of materials with different lattice constant, which can introduce some issues on the wafer yield. The efficiency increases by increasing the number of junctions, so approaches to manufacture 5-6 junction solar cells are under development as well, by Spectrolab, for example. It has been reported that the advantage to increase the number of exploitable number of junctions can have a limit due to the increased spectral sensitivity of the MJ device when the number of junction is higher then 9 [31].

Instituto de Energía Solar is following an original approach based on the introduction of “extra” energy levels in the band gap of single junction solar cells. The overall result is similar of that of having a multijunction device with the capacity to absorb a wider spectrum range. While silicon solar cells seem to have reached the maximum reachable efficiency limit, the utilization of silicon quantum dots can open the path to a real step efficiency enhancement. The main idea is to exploit the quantum confinement effect which arises by forcing the electron in a low dimensional space, in this case, new extra energy levels are introduced in the energy gap with respect the bulk material.  By modulating the size of quantum dots is possible to introduce different energy levels, therefore a silicon oxide matrix containing Si quantum dots with different dimension is equivalent to a multijunction device, with the usual advantages that this device owns.

Conventional triple junction InGaP/InGaAs/Ge devices have reached considerable high efficiency values (> 37%) and have the advantage to allow getting high process yield. In total the number of companies involved in the epitaxial growth of MJ solar cells is around twenty.  A more complete list can be found in [32]. LCPV usually adopt the silicon cell technology which is cheaper than the III-V one. The structure of the device is slightly modified with respect to the one sun device; a special attention is given to the grid design in order to decrease the series resistance. Si solar cells can be also used in CPV systems which use the splitting of the solar spectrum (see for example [33]). Some investors see the LCPV silicon-based technology a less risky product then HCPV MJ-based, since the silicon technology is more mature than the III-V MJ one, further, as it will be reported hereafter, low accuracy trackers can be installed for LCPV.  In spite of the maturity of the Si technology, the number of the CPV Si solar cell producers seems much lower than the companies involved in epitaxial growth of MJ solar cells. Also referring to the S. Kurz report [32], it is possible to count just four Si CPV solar cell producers (Sunergy. NaREC, BP Solar. Q-cells). This can also explain why some LCPV system suppliers have switched their technologies from Si based to III-V MJ based: this is the case of Amonix who introduced the III-V technology in 2008 and Abengoa Solar Sa who introduced the III-V technology in 2010.

Optics concentrator

The optical system is a fundamental element of PV concentration systems.  Different kinds of optics are under development and comprise refractive and reflective elements. An ideal concentrating optic for HCPV should have the following characteristic:

  • High concentration
  • Low cost
  • High optical efficiency in the whole wavelength range
  • Adequate Irradiation uniformity in the whole MJ wavelength range
  • Long term durability
  • High acceptance angle

A high acceptance angle is required to:

  • decrease tracker structure stiffness
  • relax optic accuracy
  • relax tolerances in module assembling and field installation
  • reduce mismatch losses

The same criteria can be used to select an adequate concentrator for LCPV systems, removing the need to maximize the concentration factor.  A figure of merit useful for designing a lens for concentrating solar power is the product of the concentration factor times the acceptance angle, which can be expressed as

CAP = (Cg)1/2sinα

where Cg is the geometrical concentration factor, while α is the angle of acceptance, defined as the angle of incidence in which the concentrator collects 90% of the power outside of its optical axis. For a given architecture of the optical concentrator, the CAP is almost constant, so it should be noted that as the concentration factor increases, the angle of acceptance decreases. This explains why HCPV needs tracker high higher accuracy.   The highest values ​​of CAP are around 0.85. Another important parameter is the uniformity of illumination: usually, the Fresnel lenses produce a Gaussian profile light on the solar cell surface. In this case, the lens produces a concentration factor different from point to point and this causes degradation in the solar cell power output, because there are some regions of the solar cell device where the photovoltaic current exceeds the peak current of the tunnel diodes.The optical efficiency is a key parameter to get high efficiency CPV module.  As a matter of fact, between the CPV cell efficiency value and the module efficiency value there is a difference of around 10 points in efficiency:  for example, module efficiency values are around 23%, when the starting MJ cells efficiency values are around 33%. So far, Fresnel lenses made of silicone have been applied in Point focus CPV modules for their lowcost [34].

At high concentration level (500 X) they have demonstrated to produce not uniform spatial light distribution in the focal plane. Light intensity profile is far form being flat and furthermore, chromatic aberration and temperate dependent refractive index can introduce possible mismatch current problems on MJ solar cells. At high concentration the acceptance angle of the lens can be quite reduced (±0.47°) and the tracking requirements are more severe. This in turn produces an increase in the CPV system cost.  In order to bring the CPV cost at competitive level, secondary opticselements, (SOE) have been introduced in the recent years [35]. They are used as a second stage optic under the primary lens and can homogenise the incident light spectrum over the solar cell surface, as well as to increase the acceptance angle.  Apart Fresnel lens, otherinteresting optical solutions, are still under the CPV community interest.  One of these is the prismatic/hybrid lens optic under development in the European APOLLON Project by ENEA/CRP [22] (see figure 3).

Figure 3 - ENEA/CRP hybrid lens.In the peripheral areas, the prisms are replaced by the Fresnel grooves in order to reduce the optical losses.

Very recent results show an optical efficiency ranging from 80% to 82% (with maximum value of 82,4%) in function of different molding parameters. In the attempt to increase the acceptance angle and the concentration factor new solutions have been recently presented.  It is worth while mention the FK concentrator (Cg= 624, acceptance angle=±1.25°), the F-RXI concentrator (Cg= 2300, acceptance angle = ±1.02°), the XXR concentrator (Cg= 2070, acceptance angle = ±0.85°) produced by LPI [36] . Considering reflective optics Solfocus has proprietary Aplanatic design which allow getting: Cg= 850, acceptance angle = ±0.85°. The optical system  includes a primary mirror to capture sunlight, along with a secondary mirror and non-imaging optical system to concentrate sunlight 650 times onto high-efficiency III-V solar cells [37]. It is interesting to compare the performances of the optics designed for HCPV with those designed for LCPV.  Silicon CPV, for example adopts prismatic lenses with Cg = 120, and acceptance angle = ±1° It seems that the formers (used for HCPV) are reaching acceptance angle comparable with that obtained on LCPV optics, therefore the cost difference between LCPV tracking systems with respect to HCPV ones, can be in prospective reduced.  Dense array systems use big mirrors to concentrate the light on group of solar cells. Few data are available on these concentrators. In general these kinds of optics do not suffer of thechromatic aberration but are very much sensible to the shape error.   (Mirror based) Spectrum splitting system are also under development [38], however, owing the complexity of the optics they are, nowadays, not economically competitive with respect to the point focus ones.

Modules

The development and improvement of high-throughput module assembly techniques that will allow increasing the industrialisation level of CPV technology is a common objective for the LCPV and HCPV technologies.  By all means in case of HCPV, a high automation level is required in order to decrease the alignment error between the cell and the optic, which can introduce mismatch losses between the solar cells installed in the module.  The production lines have to foresee solar cell measurement and sorting systems, which takes the cells after cutting, measures each single cell, sorts the cells according to their IV characteristic and then prepares the cells onto a carrier band for the next manufacturing step.  Pick and place systems, used also in the Light emitting diode sector, are used for the precise mounting of the receivers in the focus of the Fresnel lenses (see Fig.4).

Figure 4 - Automatic positioning of solar cells on receivers

The higher is the level of automation, the higher is the competitiveness of the CPV technology, and the lower is the risk to suffer from the competition with countries having a lower labour cost. From this point of view, the CPV technology offers a unique opportunity to the European industry to be competitive world wide. Extremely important is the thermal management of the module, in order to keep the operating temperature as low as possible. However, in general, even in case of point focus module operating at 700 X, with a proper module design any active cooling is required, since the cells can be kept, just by the passive cooling, around 50-70°C. These values are very near to the operating temperature values of the flat-plat modules. The challenge for the CPV technology to be a winner into the energy market is indeed related to the future efforts addressed towards a strong automation of the module production lines.  A list with the different module typologies and conversion efficiency is reported in annex 2 (see also http://www.enf.cn/database/panels-cpv-p.html).

Tracker

This component has been initially overlooked in the earlier technology CPV development but it has subsequently recognised that, apart the race to get higher conversion efficiency, the performances of the CPV systems were strictly related to the reliability and precision of the tracking, and therefore, adequate efforts had also to be strongly addressed to improve the technology of this component in order to maintain high the CPV system output power.  An inaccurate sun tracking can rise for several reasons:

  • Defect in the tracking electronic control: so far open loop methods and close loop methods or a mixture of both have been developed for tracking the sun. The first ones are based on astronomical calculation of the sun position. However, owing the refractive local-depend nature of the atmosphere, these methods do not allow reaching the precision requested in HCPV systems and are usually utilised along with the close loop ones. The close loop methods are based on:
  • Tracking accuracy sensors, usually PV devices or position sensitive devices
  • Modules Power Feedback tracking.

 

  • Flexure and torsion of the frame structure where module are loaded
  • Inaccurate in-fields module installation
  • Thermal dilatations of the materials: undesired misalignment can arise under the numerous temperature cycles undergone by the CPV system
  • Mechanical deformation: wind effects

The sum of the tracking inaccuracy given by the above mentioned factors must be lower than the optical acceptance angle of the CPV system, otherwise the solar cells will not be properly illuminated and a strong current (power) mismatch factor will arise, penalising the power output of the CPV system. Of course the solutions for obtaining an accurate and reliable tracking should not be expensive and material/energy consuming, otherwise unaffordable penalty on the competitiveness of the CPV technology and a strong environmental impact would be introduced.  Indeed if we have a look at the environmental impact of the CPV technology, the tracker, along with the module, has a considerable weight on the energy pay back time [39].  Different strategies are under development to improve the tracking accuracy:

  • Sensorless and Power –optimized [40]
  • Integrated closed loop with four sensors [41]
  • Intelligent sun tracking with hybrid approach [42]
  • Realisation of “intelligent concentrating modules” (ICMs) with Internally Integrated Position Sensitive Detectors and Maximum Power Point Tracking Devices [22].

While in case of HCPV, a two axis tracking is required; in case of LCPV a single axis tracking can be sufficient.  This introduces a cost advantage for LCPV with respect to HCPV systems. However, as observed in the previous chapter, owing to the day by day increase of the acceptance angle of the HCPV optics, the precision and rigidity of the HCPV trackers will be in the future a bit relaxed and the costs of these components subsequently reduced.  

Italian companies on CPV: industry 2015

“Industria 2015” is an Italian program which funds industrial initiatives: in particular the initiative “SCOOP” collects 16 partners, coordinated by ENEL, with the scope to: 

  • Maximize synergies among industries, research centers and universities
  • Manufacture low cost and high performance CPV systems competitive inPV market
  • Develop pre-standardized testing procedures for both components andassemblies
  • Build accreditated test laboratory for CPV

The project has started in 2009 and will end in 2012.

 Figure 5 - The SCOOP program foresees the development of Solar cells, optical systems and Inverters, on 4 different CPV production lines, which will develop four different typologies of CPV systems.

The European Projects

Three main projects are running on CPV activities in the frame of FP7: APOLLON, NACIR and APIS. Hereafter, the objectives of the projects, the partners with their role are reported

APOLLON (2008-2013) Multi-APprOach for high efficiency integrated and inteLLigent CONcentrating PV modules (Systems) Project Cost: 11.8 MEuro

Objective: The main objective of APOLLON is to develop High concentration Point Focus and Dense Array systems (MBS3) based on monolithic and discrete MJ technology with a final target cost of 2 Euro/W. This ambitious objective is foreseen to be reached after five years ofresearch and technological activities concerning all the different components of the Concentrating Photovoltaic (CPV) systems.

Partners (jpg)

NACIR (NEW APPLICATIONS FOR CPV'S: A FAST WAY TO IMPROVE RELIABILITY AND TECHNOLOGY PROGRESS)2009-2012

Objectives: The main goal of this proposal is to join together the owners of the most advanced CPV technology, with respect to the state of the art, in order to research from its leading position new applications for CPV systems. In addition to opening up new markets, it will unveil possible sources of failure in new environments outside Europe, in order to assure component reliability. The proposed project will also try to improve the current technology of the industrial partners (ISOFOTON and SOITEC) by accelerating the learning curve that CPV must follow in order to reach the competitive market, and lowering the cost under the current flat panel PV significantly within 3-4 years. 

Partners (jpg)

ASPIS (Active Solar Panel Initiative) (2009-2011).  Project cost 3.76 M Euro

Objectives: The main objective of the S&T program is the development of methods and technologies leading to a working prototype of the first-generation Active Solar panels. The prototype will use silicon cells designed and produced by a partner in the consortium and uni-directional Parallactic Tracking. With encapsulated optics, concentrating the sun by a factor of 10x, the overall system height won’t exceed manageable limits for rooftop applications.

Partners (jpg)

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3. Energy production by CPV

The energy yield is location dependent, therefore is useful to check the performance of different CPV systems in locations characterized by different DNI.  Some examples of CPV energy production are reported and benchmarked with the flat plate technology.

Silicon CPV has analysed the energy gain of a LCPV system 120 x -Si based over a fixed system in function of different location characterized by different DNI [43]. The location where: Newmarket (UK), Sinarcas (Spain), Hattar (Pakistan), Laayoune ( Marocco). The energy gain (+0.8%) of CPV with respect to flat-plate, comes into play in location with DNI=1683 KWh/m2year (Spain). At this value of DNI, the energy yield is 1482 KWh/KWp and reaches 1870 KWh/KWp in location with DNI= 2549 KWh/m2year (Marocco).

Concentrix (now Soitec) tested Gen I modules at ISFOC site at Puertollano in 2008-2009[44]. At this location , ISFOC measured accumulated DNI energy of 1974 kWh/m2, the cumulated specific yield of the CPV systems were >2000 kWh/kWinstalled.

At Puertollano ISOF has carried out a comparison among the productivity of 100 KWp of CPV, Fix PV and track PV, having a cumulated production of DNI, @ 850 W/m2, equal to 2000 KWh/m2. The energy production of HCPV CPV system was the highest with a value of 2000 kWh/kWp [45]. 

Semprius compared the energy production of its HCPV plant with a CdTe plant on a surface of 80 acre at Las Vegas. The former resulted in energy production of 43 GWhac/yr (75 W HCPV modules, 2,600 kWhac/kWdc energy yield) while the second resulted in a energy production of 23 GWhac/yr (75 W CdTe Modules, 1,850 kWhac/kWdc energy yield, Las Vegas [46].  

The output characteristics of tracking type concentrator photovoltaic (HCPV), (Daido Steel Co., Ltd) tracking type silicon solar cell, and fixed silicon solar cell were also compared in Japanese meteorological condition (University of Miyazaki) [47] The average uncorrected conversion efficiency around 15% was observed for the 14 kW CPV system throughout the period. The average uncorrected conversion efficiency of the 50 kW C-Si system was around 10%. However, the conversion efficiency of CPV system was not stable. The conversion efficiency of CPV module using multi-junction solar cell was influenced by the spectrum of irradiated light, therefore, the conversion efficiency of CPV system was more unstable than that of C-Si system.  

Skyline (LCPV) reported in its white paper that the Skyline ’s performance advantage begins over fixed cSi and thin film in locations where DNI is 5.5 kWh/m2/day. [48]

Emcore third generation concentrating photovoltaic (CPV) modules were evaluated in the low latitude location of Kihei, Hawaii (daily energy of 5.8 kWh/m2) [49]. For comparison, the best available monocrystalline silicon flat panel modules (SunPower)  were included in both dual-axis tracked and fixed mount configurations, The daily DC uncorrected efficiency value for the CPV modules averaged over the six-month performance period was 25.9% compared to 17% (dual-axis tracked ) and to 16% for the flat panels. Higher daily energy was obtained from CPV modules than tracked flat panels when daily direct solar insolation was greater than 5 kWh/m2and more than fixed mount flat panel when direct insolation was greater than 3 kWh/m2.

As far as the energy production is concerned, both LCPV and HCPV are competitive with respect tracked conventional PV when the daily energy is  5-6 kWh/m2and with the fixed mount  flat plate technology, when the daily energy is > 3 kWh/m2. The best energetic yields values are as high as 2000 kWh/kWinstalled for of annual DNI around 1900 kWh/KWp. Therefore CPV technology can be already successfully installed in all world regions with Direct Solar Irradiance grater than 1800 KWh/m2, looking at Fig.6 it is possible to claim that there is indeed a huge market available to be conquered by CPV.

Figure 6 - DNI energy world map

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4. Economical Aspects: the indicators of the Strategic Research Agenda (SRA)

Several cost analysis have been reported and differ in relation to the system efficiency, volume of production and region of installation.  

Hereafter, in Fig.7 and Fig.8, it is reported, as an example, the cost analysis carried out by Silicon CPV [43] on LCPV and HCPV systems to be installed in Spain, considering a 50 MW volume production.  

Figure 7 - Assumption: Si cell 20 % efficiency, concentration factor =120 x, Volume production = 50  MW ->  Total cost/W= 2 Euro

Figure 8. Assumption: MJ cell 40 % efficiency, concentration factor = 960 x, Volume production = 50 MW , Total cost/W= 1.5 Euro.

In both the examples the tracker has the most important cost, however in the case of HCPV the cost percentage is slightly less than in the case of LCPV, because it is possible to get more energy from the former system. On the contrary, the inverter has a higher weight on HCPV with respect to LCPV; new design has to be conceived for HCPV systems. Finally, module assembly also plays an important role in the system cost pie.  Automation of the Module assembling lines is then mandatory to cut down this important cost voice.  The cost targets to be reach for each CPV component in order to increase the competitiveness of the CPV technology has been included in the first edition of the SRA, published in 2007 [50]. This data were used as input for the definition of the Seventh Framework Programme for Research of the European Union and also to facilitate a further coordination of research programmes in and between Member States. The CPV technology has not only followed the trend expected by this first edition but it has even grown faster than foreseen, in particular if we look at the evolution of the MJ cells technology, whose efficiency values have widely surpassed the targets reported in the first SRA. It is possible to check this evolution by comparing the strategic objectives on CPV reported in the first edition of SRA with those reported in the last edition of 2011 [51] (see Tab.2).

Table 2 - Comparison among the research priorities and cost targets contained in the SRAs 2007 and 2001.

Grid parity. The “grid parity” is defined as the point where the cost of generating electricity through solar systems equal the average price of generating electricity by means of conventional methods which dip into fossil fuel, gas, or other non-renewable resources. In the last European Photovoltaic Conference held in Hamburg (26thPVSEC, 5-6 September 2011), R King of Spectrolab in his presentation, titled :”Solar cell generation over 40% Efficiency”, has traced the level’s lines related to the average daily irradiation whose value allows obtaining  a  CPV system cost of generating electricity equal to 0.14 $/kWh.  The possibility to extend the fulfilment of the grid parity  even in region with moderate average daily radiation energy, it is mainly related to the CPV system efficiency, which depends, of course, by the conversion efficiency of the solar cells and of the optical efficiency of the lens.  The evaluation of the conditions for obtaining the grid parity for CPV systems are reported in table 3.

Table 3 - Determination of the values of the CPV system parameters and of the average daily radiation energy to get a CPV system cost –of- generating -electricity equal to 0.14 $/kWh. 

The evaluation of R.King agrees also with the prediction ofEduardo Collado (Director Tecnico ASIF). Reported in Figure 9.

Figure 9 - LCOE in function of Average daily radiation energy.FromEduardo Collado, “Sector PV en España, papel que juega la CPV en este contexto”3rd Concentrated Photovoltaic summit EuropeSeville November 18th, 2010 

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5. Market aspect: the feed in tariff

In Italy the Energy production in 2009 has been around 292000 GWh. PV has only contributed for the 0.2 %.  (PV = 676 GWh).CPV can play a key role in the transition towards a more sustainable national energy system; in fact our Country is characterized in many of its parts of an amount of DNI higher than 1500 kWh/m2 (see Fig.10).

Figure 10. DNI energy distribution In Italy  

According to table 3, this should allow reaching the grid parity as soon as the system efficiency will reach efficiency 35.6%.  Therefore the CPV market is expected to grow and this will happen faster if propersupporting National programs will take place. It is worth noting that in Italy several industries are investing on CPV, therefore, the feed-in tariff programs will have a positive economical effect directly on the industrialization level of our Country.  

Italy started to support the CPV technology with the III “conto energia”. However this initiative did not find national CPV companies ready to jump in, since CPV suppliers still had to certify their systems.  The “incubation period” should have reached its end and possibly the “IV conto energia” issued in 20’11 will see the request of feed of tariff coming from a consistent number of national companies. The CPV feed in tariff values considered in the “IV conto energia” are applied for 20 years and changes with the time according to the following tables:

Until December 31, 2012, are eligible to access the feed in tariff CPV modules and assemblies not yet certified according to IEC 62108, but with certification process already started and, at the same time, having already successfully passed the essential tests reported in the guide CEI 82-25, in order to ensure compliance with the minimum technical requirements for safety and quality of the product. In this case the GSE requires the manufacturer's declaration that the product is under IEC 62108 process certification. The declaration must be supported by certificates issued by an accredited laboratory attesting the fulfillment of the minimum technical requirements for safety and quality of the product specified in IEC Guide 82-25. This laboratory must be accredited EA (European Accreditation Agreement) or shall have established mutual recognition agreements with EA or ILAC.

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6. Conclusion Future challenges and development of CPV 

Increase of solar cells efficiency value is always a constant objective of future research. Demonstration of the CPV technology in different world region is important to optimise the efficiency of the overall system.  It is worth noting that few companies in Europe are able to supply high efficiency MJ solar cells, for this reason, CPV firms can be forced to ask for American products once they will start increasing the production capacity. Therefore, it is important to increase the presence of solar cells manufactures in Europe to increase competitiveness and security of supply. Complete process automation for CPV system production is mandatory to decrease cost and industry survival against competition with Countries with low man power cost. The enhancement of the solar cell efficiency at value around 50 %, of the optical efficiency to value higher than 80%, of the acceptance angle to values higher then 1°, maintaining high the concentration level ( > 800 x)  will allow to the CPV technology to be competitive with the conventional methods to produce electricity based fossil fuel, gas, or other non-renewable resources in large part of our Country.

Research of how to reduce the soiling effect is another important subject to maintain high the CPV system yield. Another interesting challenge is the integration of the CPV technology with other energy production sources. The “dual use on land” can allow producing CPV energy where, for example, bio-fuel is produced. Therefore CPV can be profitably integrated with other form of energy production and a wider utilization of this technology can be foreseen. CPV technology nowadays reached solar cells efficiency values higher than 40%, module efficiency values of 25-30% and system efficiency values around 25%, its use will increase considerably, thanks to the technology improvements, cost reduction and feed in tariff. It is foreseen for 2015 a total installed CPV production per year of 1.8 GW [52]. 

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7. Bibliography (pdf)

 

Annex 1. CPV Standards and Specifications under development (pdf)

Annex 2. The CPV Companies (pdf)