Identification and Development of Novel Optics for Concentrator Photovoltaic Applications

Concentrating photovoltaic (CPV) systems are a key step in expanding the use of solar energy. Solar cells can operate at increased efficiencies under higher solar concentration and replacing solar cells with optical devices to capture light is an effective method of decreasing the cost of a system without compromising the amount of solar energy absorbed. CPV systems are however still in a stage of development where new designs, methods and materials are still being created in order to reach a low levelled cost of energy comparable to standard silicon based photovoltaic (PV) systems. This work outlines the different types of concentration photovoltaic systems, their various design advantages and limitations, and noticeable trends. Comparisons on materials, optical efficiency and optical tolerance (acceptance angle) are made in the literature review as well as during theoretical and experimental investigations. The subject of surface structure and its implications on concentrator optics has been discussed in detail while highlighting the need for enhanced considerations towards material and hence the surface quality of optics. All of the findings presented contribute to the development of higher performance CPV technologies. Specifically high and ultrahigh concentrator designs and the accompanied need for high accuracy high quality optics has been supported. A simulation method has been presented which gives attention to surface scattering which can decrease the optical efficiency by 10-40% (absolute value) depending on the material and manufacturing method. New plastic optics and support structures have been proposed and experimentally tested including the use of a conjugate refractive-reflective homogeniser (CRRH). The CRRH uses a reflective outer casing to capture any light rays which have failed total internal reflection (TIR) due to non-ideal surface topography. The CRRH was theoretically simulated and found to improve the optical efficiency of a cassegrain concentrator by a maximum of 7.75%. A prototype was built and tested where the power output increase when utilising the CRRH was a promising 4.5%. The 3D printed support structure incorporated for the CRRH however melted under focused light, which reached temperatures of 226.3°C, when tested at the Indian Institute of Technology Madras in Chennai India. The need for further research into prototyping methods and materials for novel optics was also demonstrated as well as the advantages of broadening CPV technology into the fields of biomimicry. The cabbage white butterfly was proven to concentrate light onto its thorax using its highly reflective and lightweight wings in a basking V-shape not unlike V-trough concentrators. These wings were measured to have a unique structure consisting of ellipsoidal pterin beads aligned in ladder like structures on each wing scale which itself is then tiled in a roof like pattern on the wing. Such structures of a reflective material may be the answer to lightweight materials capable of increasing the power to weight ratio of CPV technology greatly. Experimental testing of the large cabbage white wings with a silicon solar cell confirmed a 17x greater power to weight ratio in comparison to the same set up with reflective film instead of the wings. An ultrahigh design was proposed taking into account manufacturing considerations and material options. The geometrical design was of 5800x of which an optical efficiency of either ~75% with state of the art optics should produce and effective concentration of ~4300x. Relatively standard quality optics on the other hand should give an optical efficiency of ~55% and concentration ratio ~3000x. A prototype of the system is hypothesised to fall between these two predictions. Ultrahigh designs can be realised if the design process is as comprehensive as possible, considering materials, surface structure, component combinations, anti-reflective coatings, manufacturing processes and alignment methods. Most of which have been addressed in this work and the accompanied articles. Higher concentration designs have been shown to have greater advantages in terms of the environmental impact, efficiency and cost effectiveness. But these benefits can only be realised if designs take into account the aforementioned factors. Most importantly surface structure plays a big role in the performance of ultrahigh concentrator photovoltaics. One of the breakthroughs for solar concentrator technology was the discovery of PMMA and its application for Fresnel lenses. It is hence not an unusual notion that further breakthroughs in the optics for concentrator photovoltaic applications will be largely due to the development of new materials for its purpose. In order to make the necessary leaps in solar concentrator optics to efficient cost effective PV technologies, future novel designs should consider not only novel geometries but also the effect of different materials and surface structures. There is still a vast potential for what materials and hence surface structures could be utilised for solar concentrator designs especially if inspiration is taken from biological structures already proven to manipulate light

Modelling technique and analysis of porous anti-reflective coatings for reducing wide angle reflectance of thin-film solar cells

Bio-inspired anti-reflective (AR) coatings with porous graded refractive index structures are known to considerably reduce the reflectance of light at optical interfaces, however, research is lacking for thin-film cell application. Ray Tracing software coupled with the Effective Medium Theory were used to simulate the reflectance of nanostructured coatings placed above a thin-film system. The most optimal coating was paraboloid-shaped, with 300 nm nipple heights and spacings of 15%. The non-zero refractive index 'step' aids light trapping and energy absorption. This coating reduced reflectance in the λ = 300–800 nm range by an average of 2.665% and 11.36% at 0∘ and 80∘ incident light, respectively, whilst increasing annual energy output by 4.39% and 5.39% for standard UK roof and vertical window tilts, respectively. Significant wide angle reflectance capabilities are demonstrated at specifically λ = 300 nm and 80∘ incident light, with a reflectance reduction of 19.192%. There are now many promising manufacturing techniques for these porous nanostructures, such as AR or wavelength filtering coatings for photovoltaics. Further understanding of the exact parameters needed to replicate these nanostructures must be explored to proceed

Optics for concentrating photovoltaics: Trends, limits and opportunities for materials and design

Concentrating photovoltaic (CPV) systems are a key step in expanding the use of solar energy. Solar cells can operate at increased efficiencies under higher solar concentration and replacing solar cells with optical devices to capture light is an effective method of decreasing the cost of a system without compromising the amount of solar energy absorbed. However, CPV systems are still in a stage of development where new designs, methods and materials are still being created in order to reach a low levelled cost of energy comparable to standard silicon based PV systems. This article outlines the different types of concentration photovoltaic systems, their various design advantages and limitations, and noticeable trends. This will include comparisons on materials used, optical efficiency and optical tolerance (acceptance angle). As well as reviewing the recent development in the most commonly used and most established designs such as the Fresnel lens and parabolic trough/dish, novel optics and materials are also suggested. The aim of this review is to provide the reader with an understanding of the many types of solar concentrators and their reported advantages and disadvantages. This review should aid the development of solar concentrator optics by highlighting the successful trends and emphasising the importance of novel designs and materials in need of further research. There is a vast opportunity for solar concentrator designs to expand into other scientific fields and take advantage of these developed resources. Solar concentrator technologies have many layers and factors to be considered when designing. This review attempts to simplify and categorise these layers and stresses the significance of comparing as many of the applicable factors as possible when choosing the right design for an application.From this review, it has been ascertained that higher concentration levels are being achieved and will likely continue to increase as high performance high concentration designs are developed. Fresnel lenses have been identified as having a greater optical tolerance than reflective parabolic concentrators but more complex homogenisers are being developed for both system types which improve multiple performance factors. Trends towards higher performance solar concentrator designs include the use of micro-patterned structures and attention to detailed design such as tailoring secondary optics to primary optics and vice-versa. There is still a vast potential for what materials and surface structures could be utilised for solar concentrator designs especially if inspiration is taken from biological structures already proven to manipulate light in nature

Theoretical Investigation of the Temperature Limits of an Actively Cooled High Concentration Photovoltaic System

Concentrator photovoltaics have several advantages over flat plate systems. However, the increase in solar concentration usually leads to an increase in the solar cell temperature, which decreases the performance of the system. Therefore, in this paper, we investigate the performance and temperature limits of a high concentration photovoltaic Thermal system (HCPVT) based on a 1 cm2 multi-junction solar cell subjected to a concentration ratio from 500× to 2000× by using three different types of cooling fluids (water, ethylene glycol and water mixture (60:40), and syltherm oil 800). The results show that, for this configuration, the maximum volumetric temperature of the solar cell did not exceed the manufacturer’s recommended limit for the tested fluids. At 2000× the lowest solar cell temperature obtained by using water was 93.5 °C, while it reached as high as 109 °C by using syltherm oil 800, which is almost equal to the maximum operating limit provided by the manufacturer (110 °C). Overall, the best performance in terms of temperature distribution, thermal, and electrical efficiency was achieved by using water, while the highest outlet temperature was obtained by using syltherm oil 800

White butterflies as solar photovoltaic concentrators

Man’s harvesting of photovoltaic energy requires the deployment of extensive arrays of solar panels. To improve both the gathering of thermal and photovoltaic energy from the sun we have examined the concept of biomimicry in white butterflies of the family Pieridae. We tested the hypothesis that the V-shaped posture of basking white butterflies mimics the V-trough concentrator which is designed to increase solar input to photovoltaic cells. These solar concentrators improve harvesting efficiency but are both heavy and bulky, severely limiting their deployment. Here, we show that the attachment of butterfly wings to a solar cell increases its output power by 42.3%, proving that the wings are indeed highly reflective. Importantly and relative to current concentrators, the wings improve the power to weight ratio of the overall structure 17-fold, vastly expanding their potential application. Moreover, a single mono-layer of scale cells removed from the butterflies’ wings maintained this high reflectivity showing that a single layer of scale cell-like structures can also form a useful coating. As predicted, the wings increased the temperature of the butterflies’ thorax dramatically, showing that the V-shaped basking posture of white butterflies has indeed evolved to increase the temperature of their flight muscles prior to take-off

Conjugate refractive–reflective homogeniser in a 500x Cassegrain concentrator: design and limits

In this study, we present the conjugate refractive reflective homogeniser (CRRH) to be used in a 500× Cassegrain photovoltaic concentrator. The CRRH is a dielectric crossed v-trough lined with a reflective film whilst maintaining an air gap between them. This air gap between the two surfaces helps in trapping the scattered light from the refractive geometry and ensures both total internal reflection and standard reflection of the escaped rays. A 10–42% drop in optical efficiency has been shown to occur due to varying the surface roughness of the homogeniser in these ray trace simulations for the Cassegrain setup. The CRRH increased the overall optical efficiency by a maximum of 7.75% in comparison with that of a standard refractive homogeniser simulated within the same concentrator system. The acceptance angle and flux distribution of these homogenisers was also investigated. The simple shape of the CRRH ensures easy manufacturing and produces a relatively uniform irradiance distribution on the receiver. The theoretical benefit of the CRRH is also validated via practical measurements. Further research is required but a 6.7% power increase was measured under a 1000 W/m2 solar simulator at normal incidence for the experimental test

Prototype fabrication and experimental investigation of a conjugate refractive reflective homogeniser in a cassegrain concentrator

The conjugate refractive reflective homogeniser (CRRH) is experimentally tested within a cassegrain concentrator of geometrical concentration ratio 500× and its power output compared to the theoretical predictions of a 7.76% increase. I–V traces are taken at various angles of incidence and experimental results showed a maximum of 4.5% increase in power output using the CRRH instead of its purely refractive counterpart. The CRRH utilises both total internal reflection (TIR) within its core refractive medium (sylguard) and an outer reflective film (with an air gap between) to direct more rays towards the receiver. The reflective film captures scattered refracted light which is caused by non-ideal surface finishes of the refractive medium. The CRRH prototype utilises a 3D printed support which is thermally tested, withstanding temperatures of up to 60 °C but deforming at >100 °C. A maximum temperature of 226.3 °C was reached within the closed system at the focal spot of the concentrated light. The material properties are presented, in particular the transmittance of sylguard 184 is shown to be dependent on thickness but not significantly on temperature.Utilising both TIR and standard reflection can be applied to other geometries other than the homogeniser presented here. This could be a simple but effective method to increase the power of many concentrator photovoltaics

Perspectives and Future Directions Concerning Fresh, Whole Foods in Montana School Nutrition Programs

ABSTRACT Purpose/Objectives To meet new USDA school meal standards, school nutrition programs may need to transition from a "heat and serve" meal preparation approach to increased scratch cooking and use of fresh, whole foods. This study aims to assess the attitudes, motivations, and barriers for Montana school nutrition professionals and key stakeholders regarding the use of whole, fresh food in school nutrition programs. Methods The researchers conducted a survey of Montana school nutrition program staff (n=103) and semi-structured interviews with key stakeholders (n=12) including current and former school nutrition program staff (n=9), AmeriCorps FoodCorps service members (n=2), and a state level Farm to Cafeteria director (n=1). Survey responses were analyzed for statistically significant differences in responses between school nutrition programs based on size. Interviews were transcribed and coded to identify prevalent themes. Results: Study participants identified numerous benefits to utilizing fresh, whole foods including increased ability to meet USDA standards. A number of barriers and challenges were also identified including lack of staff training, time limitations, food cost, and inadequate equipment. Applications to Child Nutrition Professionals Training and professional development specific to the needs of the school nutrition program may address some barriers to utilizing fresh, whole foods and increasing adherence to National School Lunch Program and School Breakfast Program standards. However, changes in institution, community, and federal policies are necessary to facilitate broad adoption of scratch cooking and use of fresh, whole foods in school nutrition programs

Nanopatterned indium tin oxide as a selective coating for solar thermal applications

Indium tin oxide (ITO) coatings have been proposed to reduce thermal emission losses for solar thermal applications. Unfortunately, ITO also has a large amount of free charge carriers (∼1 × 1020 per cm3), which absorb sunlight. To address this issue, we propose a nano-patterned ITO-coated quartz exhibiting both anti-reflectivity (to maximize solar transmission) and low emissivity (to minimize long wavelengths radiative losses). A record small-size nanosphere (∼60 nm) etch mask was prepared via double self-assembly, followed by dry etching and characterisation. In parallel, alternative nanopattern geometries were modelled using the Lumerical FDTD software to optimise short wavelength transmission without diminishing the inherently low emissivity of unetched ITO. It was found that an inverted moth's eye pattern (height = 250 nm and spacing = 80 nm) gave the best results at various solar concentrations (1 sun @ 100 °C, 10 suns @ 400 °C, and 100 suns @ 600 °C), resulting in ∼7% improvement in the solar weighted transmission as well as a similar boost in the overall efficiency factor for selectivity. It was concluded that if the proposed deposition/etching processes can be cost-effectively scaled in a continuous process, it would provide a net performance boost for most solar thermal technologies

Theoretical investigation considering manufacturing errors of a high concentrating photovoltaic of cassegrain design and its experimental validation

A compact high concentrating photovoltaic module based on cassegrain optics is presented; consisting of a primary parabolic reflector, secondary inverse parabolic reflector and a third stage homogeniser. The effect of parabolic curvatures, reflector separation distance and the homogeniser’s height and width on the acceptance angle has been investigated for optimisation. Simulated optical efficiencies of 84.82–81.89% over a range of ±1° tracking error and 55.49% at a tracking error of ±1.5° were obtained. The final singular module measures 169 mm in height and 230 mm in width (not including structural components such as cover glass). The primary reflector dish has a focal length of 200 mm and is a focal with the secondary inverse reflector which has a focal length of 70 mm. The transparent homogenising optic has a height of 70 mm, an entry aperture of 30 × 30 mm and an output aperture of 10 × 10 mm to match the solar cell. This study includes an analysis of the optical efficiency, acceptance angle, irradiance distribution and component errors for this type of concentrator. In particular material stability and the surface error of the homogeniser proved to be detrimental in theoretical and experimental testing – reducing the optical efficiency to ∼40%. This study proves the importance of material choice and simulating optical surface quality, not simply assuming ideal conditions. In the experimental testing, the acceptance angle followed simulation results as did the optical efficiency of the primary and secondary reflectors. The optical efficiency of the system against increasing solar misalignment angles is given for the theoretical and experimental work carried out