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Within the EU-funded project HIPERION, significant progress has been made on the Insolight solar panel: a high-efficiency rooftop compatible micro-CPV device with integrated planar microtracking and diffuse collection via additional low-cost Si solar cells. The IES-UPM and other R&D centers in the consortium have been tasked with characterizing the various generation of this technology. In this work we discuss the evolution of the module design and present the latest results for “Gen1” solar panels showing significant improvement in performance in self-tracking compared to earlier results even in the face of a 5X increase in module area.

The hybrid modules developed by a Swiss startup, Insolight, consist of a micro-concentrator photovoltaic (micro- CPV) sub-module mounted on a non-concentrator Si backplane. Embedding the micro-CPV cells on top of the flat plate module complements the high efficiency of the CPV with the geographic versatility of the Si cells. Characterization of such modules should be adapted for the requirements and challenges posed by the component submodules. The top CPV layer utilizes the direct component of the incident radiation and the bottom Si layer utilizes the diffuse component. Therefore, it is necessary to test the modules under two lighting conditions. This paper demonstrates a novel solar simulator design for the indoor characterization of hybrid modules based on the collimated-light solar simulator for CPV modules developed by the Instituto de Energía Solar-UPM. The new simulator design adds provisions for testing Silicon submodules under uniform diffuse light by incorporating diffusive elements in the collimated-light path and reducing stray light. The existing CPV simulator at UPM has been refurbished to test the new design concepts. The I-V curve of a hybrid module has been characterized indoors using several configurations and compared to reference measurements obtained outdoors in order to optimize and validate the new setup.

Micro-concentrator photovoltaics (micro-CPV) consists of the reduction in size of the components of the conventional concentrator photovoltaic (CPV) technology, attaining equally high efficiencies and reducing material costs and manufacturing costs. In this publication we focus on the implementation of high throughput manufacturing methods for the interconnection of the solar cells. The goal is to enable large area interconnection of thousands of micro-solar cells with a low cost of 3€/m2 . A proof-of-concept was achieved for interconnection via directly printing onto the front contact pads. Prototypes using two different cell technologies where manufactured achieving good results. The highest achieved fill factor (FF) is of 84.3% at 200X with a short circuit current (Isc) of 2.4mA and open circuit voltage (Voc) of 3.41V

Glass molded optics have a wide range of applications in commercial industry such as for secondary optical elements in concentrator photovoltaics (CPV), optics for light emitting diodes (LEDs), etc… In micro-CPV the reduction in size of all the components demands a very good optical quality of the primary lenses to achieve high concentrations and high efficiencies without the need of secondary optics. Glass for molding is a good candidate due to benefits such as a high abbe number, great mechanical properties, low coefficient of thermal expansion (CTE), low thermal coefficient of the refractive index (dn/dT), proven reliability, and manufacturing by a straight forward compression molding process. In collaboration with the company Holophane S.A.S. in France, several arrays of plano-convex concentrator lenses were manufactured using a manual compression mold. These first prototypes have shown absolute optical efficiency over 80% at 178X using a 1mm2 multi-junction (MJ) solar cell. Further results and potential improvements are discussed.

Roll-to-roll nanoimprint lithography (R2R-NIL) is an enabling technology for the low-cost mass production of high-quality micro- and nano-sized optical elements. Particularly, the fabrication of Fresnel lenses using R2R-NIL is a promising approach to produce optical arrays for micro-concentrator photovoltaic modules. This work investigates the application of a continuous R2R imprinting process based on ultraviolet curing of transparent photopolymer resins (UV-NIL) to fabricate high-efficiency and low-cost Fresnel lenses. The morphological attributes and the related optical performance of the lenses fabricated using roll-to-roll UV-NIL on flexible PET sheets yielded optical efficiency values up to ∼ 69% at a concentration ratio of 178X, whereas a value of ∼ 77% was obtained for the UV-NIL batch processed on a flat rigid substrate. Further improvement of the optical efficiency has been achieved by adding moth-eye inspired antireflective (AR) features on the side opposite to the Fresnel motifs via a double-sided R2R UV-NIL process. The process developed paves the way for cost-effective mass production of high-efficiency Fresnel lenses for micro-concentrator photovoltaics.

In this paper we present a comprehensive outdoor experimental characterization of the first modules assembled using the Achromatic Doublet on Glass (ADG) Fresnel lens technology. First, only the elementary units comprising one lens and one cell are investigated: the electrical performance is measured varying the cell-lens distance in order to identify the optimal focal distance. Second, a mono-module (module composed of one lens and one solar cell) has been assembled and installed on a two-axis tracker where it has been continuously measured between June and October 2018. Also, a mono-module including conventional Silicone on Glass (SoG) Fresnel lenses has been assembled and used as benchmark. Results demonstrated that the achromatic design of the ADG lenses used as Primary Optical Elements (POE) provides a significantly reduced temperature dependency of the module performance. The performance is maintained constant throughout the whole measurement period enhancing the energy output over time.

The possibility of using thermoplastic such as solar encapsulants as an alternative to silicone is shown for encapsulant-on-glass (EOG) lenses. This is an alternative with a respectable potential due to lower material cost, lower coefficient of thermal expansion (CTE), lower thermal coefficient of the refractive index (dn/dT) and a manufacturing process which shares many similarities to the silicone-on-glass (SOG) and flat-plane-PV fabrication. The cost of the optical component of a CPV module can be brought down with this new approach of fabricating Fresnel lenses. Measured efficiency results show optical efficiencies losses of 8% compared to state-of-the-art SOG lenses. This result is preliminary with improvements in lens design and manufacturing still pending.

CPV incorporates a wide variety of technologies and options. The application of these technologies, however, is not limited to CPV only. The basic approach of the standardization for CPV is thus expanding the scope to a broader range of technologies. The knowledge gained through our activities is contributing to standardizations in PV technologies like, e.g. standards for trackers. In this regard, the standardization activities for CPV is also moving to new fields of applications.

The first edition of the Technical Specification (TS) on Primary Optics for Concentrator Photovoltaic (CPV) Systems, IEC TS 62989:2018, has been published by the International Electrotechnical Commission (IEC), on March 8th, 2018. TS 62989 covers aspects related to the primary optics including: product identification, optical characteristics, mechanical characteristics, materials, (design) geometry, and visual appearance. This paper focuses on the key aspects of the norm, including optical performance, in order to promote the awareness and use of the standard. We describe the three methods intended to measure the key optical characteristics of the primary elements, i.e. focal spot size and optical efficiency, by using the encircled energy. The three methods are using different light sources, optical components and receiver sensors, but yielded very similar results in a round robin test. This justifies the continued use of all three methods. The use of a website (opticstests.pbworks.com) for documentation of discussions and references was novel to the standard development. The website proved to be useful for the introduction of new members of the group. The website helped to keep track of changes to the document as well as the required actions of the project team.The first edition of the Technical Specification (TS) on Primary Optics for Concentrator Photovoltaic (CPV) Systems, IEC TS 62989:2018, has been published by the International Electrotechnical Commission (IEC), on March 8th, 2018. TS 62989 covers aspects related to the primary optics including: product identification, optical characteristics, mechanical characteristics, materials, (design) geometry, and visual appearance. This paper focuses on the key aspects of the norm, including optical performance, in order to promote the awareness and use of the standard. We describe the three methods intended to measure the key optical characteristics of the primary elements, i.e. focal spot size and optical efficiency, by using the encircled energy. The three methods are using different light sources, optical components and receiver sensors, but yielded very similar results in a round robin test. This justifies the continued use of all three methods. The use of a website (opticstests.pbworks.com) for documentation ...

Micro concentrator photovoltaics (micro-CPV) is an unconventional approach for developing high-efficiency low-cost PV systems. The micrifying of cells and optics brings about an increase of efficiency with respect to classical CPV, at the expense of some fundamental challenges at mass production. The large costs linked to miniaturization under conventional serial-assembly processes raise the need for the development of parallel manufacturing technologies. In return, the tiny sizes involved allows exploring unconventional optical architectures or revisiting conventional concepts that were typically discarded because of large material consumption or high bulk absorption at classical CPV sizes.

An optical characterization technique is introduced for the measurement of the spectrally-resolved optical efficiency of concentrators that employ multi-junction cells. The method takes into account the optical coupling between any secondary optic and its multi-junction cell receiver and allows the latter to be used as light detector for the first time. A series of band pass filters are used to isolate the optical performance of different narrow spectral bands throughout the spectrum of interest. The subcell with a response in each narrow band is used as a light sensor. A high-intensity filtered broad band light is cast on the receiver surroundings to avoid the other subcells to limit the current. The technique presented has been key to identify solarization effects in molded soda-lime glass secondary optics exposed to outdoor operation during several months.

Fresnel lenses used as primary optics in concentrating photovoltaic modules may show warping produced by lens manufacturing or module assembly (e.g., stress during molding or weight load) or due to stress during operation (e.g., mismatch of thermal expansion between different materials). To quantify this problem, a simple method called “checkerboard method” is presented. The proposed method identifies shape errors on the front surface of primary lenses by analyzing the Fresnel reflections. This paper also deals with the quantification of the effects these curvatures have on their optical performance and on the electrical performance of concentrating modules incorporating them. This method can be used to perform quality control of Fresnel lenses in scenarios of high volume production.

In the frame of the European project SOPHIA, a spectral network based on component (also called isotypes) cells has been created. Among the members of this project, several spectral sensors based on component cells and collimating tubes, so-called spectroheliometers, were installed in the last years, allowing the collection of minute-resolution spectral data useful for CPV systems characterization across Europe. The use of spectroheliometers has been proved useful to establish the necessary spectral conditions to perform power rating of CPV modules and systems. If enough data in a given period of time is collected, ideally a year, it is possible to characterize spectrally the place where measurements are taken, in the same way that hours of annual irradiation can be estimated using a pyrheliometer.

Thermal characterization of Concentrating Photovoltaics (CPV) modules and arrays is needed to determine their performance and modelling of energy forecast. Module-ambient thermal resistance is easily obtained from its definition but the cell-module thermal resistant needs to be estimated from indirect procedures, two of them are presented in this paper. In addition, an equivalent parameter is defined, the Concentrator Nominal Operating Module/Cell Temperature (CNOMT/CNOCT), the temperature at Concentrator Standard Operating Conditions (CSOC). Definitions and expression to relate (CNOMT/CNOCT) to thermal resistances are presented, plus several examples of estimations from real operating arrays.

Air Mass and atmosphere components (basically aerosol (AOD) and precipitable water (PW)) define the absorption of the sunlight that arrive to Earth. Radiative models such as SMARTS or MODTRAN use these parameters to generate an equivalent spectrum. However, complex and expensive instruments (as AERONET network devices) are needed to obtain AOD and PW. On the other hand, the use of isotype cells is a convenient way to characterize spectrally a place for CPV considering that they provide the photocurrent of the different internal subcells individually. Crossing data from AERONET station and a Tri-band Spectroheliometer, a model that correlates Spectral Mismatch Ratios and atmospheric parameters is proposed. Considering the amount of stations of AERONET network, this model may be used to estimate the spectral influence on energy performance of CPV systems close to all the stations worldwide.

A method based on experimental data is proposed to optimize the energy harvesting of a silicone-on-glass Fresnel-lens based CPV system. It takes into account the spectral variations along the year in a particular location as well as the thermal and spectral sensitivities of the optics and solar cell. In addition, different alternatives to tune the top/middle subcells current ratio in a CPV module are analyzed and their capacity to maximize the annually produced energy is quantified.

In order to complement ISFOC’s characterization capabilities, a Helios 3198 CPV Solar Simulator was installed in summer 2010. This Solar Simulator, based on a parabolic mirror and a high-intensity, small area Xenon flash lamp was developed by the Instituto de Energia Solar in Madrid [1] and is manufactured and distributed by Soldaduras Avanzadas [2]. This simulator is used not only for R&D purposes, but as a quality control tool for incoming modules that are to be installed in ISFOC’s CPV plants. In this paper we will discuss the results of recent measurements of close to 5000 modules, the entire production of modules corresponding to a small CPV power plant (200 kWp). We scrutinize the resultant data for signs of drift in the measurements, and analyze the light quality before and after, to check for changes in spectrum or spatial uniformity.

A method is presented for estimating the photocurrent of the subcells of a multijunction cell within a concentrator, through the measurement of the cell’s short-circuit current under a large sweep of different spectral conditions. The spectrum is monitored by means of a set of component cells matched to the subcells of the concentrator cell. The method allows the calculation of the current-matching or J-ratio of the concentrator cell under any spectrum, and also the identification of losses due to the non-uniformity of the spectrum throughout the solar cell area. For illustration purposes, the method is applied to a concentrator composed of a Fresnel lens that concentrates light over a triple-junction cell 300 times smaller.

Results of several experimental tests performed to analyze the potential of CPV modules based on a single reflective stage and a fluid dielectric are summarized in this paper. Special emphasis is put on analyzing outdoor performance. The improved thermal design maintains the temperature difference between the solar cell and the ambient below 37°C during outdoor operation. In addition, the system performance under different spectra is analyzed experimentally determining the limiting subcell within the multijunction solar cell under variable spectral conditions.

When refractive optical systems are used to concentrate light onto high efficiency solar cells chromatic aberration creates irradiance distributions over the cells which are neither spatially nor spectrally uniform. Multijunction solar cells are the photovoltaic device with the highest conversion efficiency. However, their efficiency under a concentration system can be significantly lower than the measured efficiency under uniform irradiance and reference conditions. In this paper the irradiance distribution created by a Fresnel lens is studied by ray-tracing simulations and it is characterized using a CCD camera and adequate filters. The effects of the non uniform irradiance distribution on the solar cell performance are analyzed by measuring the IV curve of an elementary unit of the system when illuminated by a solar simulator. IV curves for different lens to cell distances are measured and compared to allow a better understanding of the system.

Manufactures of Concentration Photovoltaic (CPV) modules share a need for indoor testing, implying an appropriate artificial light source. This source must be highly spatially uniform and have a collimation angle similar to that of the sun. In this work, we report the results of a detailed sensitivity study of these two parameters, in order to provide a quantitative basis for setting the requirements for future simulators and in international standards.

In hybrid Silicone‐on‐glass Fresnel lenses, an optical silicone is molded onto a glass substrate and forms the Fresnel structure. These lenses offer a cost effective solution as a primary optical element in point‐focus concentrator photovoltaic modules, as well as performance advantages. However, these lenses have a high performance variation with temperature due both to the change in index of refraction of silicone as well as to deformations in the facets caused by coefficient of thermal expansion (CTE) mismatch. In this study we perform measurements of the light flux at the focal plane of a family of SOG lenses, varying temperature and lens‐to‐receiver distances. The effect of varying silicone cure temperature and the depth of the silicone between the lens and the glass substrate on temperature dependence was investigated. A preliminary computer model of this behavior is presented.

Two characterization techniques based on a CCD camera have been developed: an inverse illumination method based on cell light emission and a lens deformation analysis method. Its application to a specific CPV technology is presented.

FluidReflex is a novel concentrator concept that has been designed to achieve a low cost concentration photovoltaic (CPV) system over 1000× by using reflective optics and a fluid dielectric. In this paper, results regarding the characterization of the concentrator Optical Transfer Function (OTF) are presented. The reflectance and transmittance of the different materials that the concentrator is composed of have been measured using a spectrometer. In addition, a method using component cells (isotypes) has been used to experimentally determine which of the subcells within the multijunction (MJ) solar cell is limiting the current when it is illuminated by the concentrator.

Solar simulators for characterization of concentrator photovoltaic modules (CPV) are now commercially available, providing an alternative to outdoor performance characterization with clear advantages in terms of repeatability, availability, control of operating conditions, and cost. Nevertheless, optical concentration implies particular characteristics that introduce the need of new performance figures and characterization methods. CPV solar simulators have demonstrated characterization capabilities beyond simple current–voltage (I–V) curve acquisition. This paper summarizes other indoor performance tests developed at the Instituto de Energía Solar (IES-UPM) that have proven to be useful for the optical, mechanical, and spectral characterization and optimization of a CPV module.