General correlations among geometry, orientation and thermal performance of natural convective micro-finned heat sinks
The interest in micro-technologies has increased in the last decades, because of the low volumes and high performance granted by their application. Micro-fins can find application in several fields, such as power electronics, concentrating photovoltaics and LED. Although micro-technologies have been widely applied in cooling, there is still a lack of knowledge on the thermal behavior of micro-finned heat sinks under natural convective conditions. In the present study, the correspondences between fin geometries and heat transfer coefficients, as well as the effects of the orientation, are experimentally investigated using silicon micro-finned heat sinks with different geometries. The heat sinks are made of 5 cm × 5 cm squared silicon wafer and the fin height ranges between 0.6 mm and 0.8 mm, the spacing between 0.2 mm and 0.8 mm and the thickness between 0.2 and 0.8 mm. Power loads higher than those considered in previous works are studied. The experimental setup is validated using a software simulation and the Nusselt number correlation available in literature. The influence of the fin thickness on this parameter is analyzed and a modified correlation is proposed. Also, the effect of the radiative heat exchange on the overall heat transfer is considered and commented. An analysis of the uncertainty is conducted and reported too.
Enhancing ultra-high CPV passive cooling using least-material finned heat sinks
Ultra-high concentrating photovoltaic (CPV) systems aim to increase the cost-competiveness of CPV by increasing the concentrations over 2000 suns. In this work, the design of a heat sink for ultra-high concentrating photovoltaic (CPV) applications is presented. For the first time, the least-material approach, widely used in electronics to maximize the thermal dissipation while minimizing the weight of the heat sink, has been applied in CPV. This method has the potential to further decrease the cost of this technology and to keep the multijunction cell within the operative temperature range. The designing procedure is described in the paper and the results of a thermal simulation are shown to prove the reliability of the solution. A prediction of the costs is also reported: a cost of 0.151$/Wp is expected for a passive least-material heat sink developed for 4000x applications.
Thermal effectiveness and mass usage of horizontal micro-fins under natural convection
In recent times, the micro-technologies have gained prominence in various engineering applications. The micro-technologies are already in use for cooling purposes in several systems, but the information on the thermal performance of micro-fins under natural convective heat transfer conditions is yet limited. The correlations between heat transfer coefficients and fin geometry have already been investigated, but are not sufficient to optimize the design of the micro-finned arrays. For this reason, the present investigation gives an overview of micro-fins behavior taking into account, for the first time, different heat sink metrics: the fin effectiveness and the mass specific heat transfer coefficient. The results of an original experimental investigation are merged with the data available in literature. Natural convective micro-fins are able to achieve overall fin effectivenesses higher than 1.1. Even if not always beneficial in terms of heat transfer, micro-fins are found always positive in terms of the material usage. In this light, micro-fins can be considered advantageous in those applications that require a minimized weight of the heat sinks. Moreover, a limited effect due to the orientation is observed.
Design of a 16-cell densely-packed receiver for high concentrating photovoltaic applications
A novel densely packed receiver for concentrating photovoltaics has been designed to fit a 125× primary and a 4× secondary reflective optics. It can allocate 16 1cm2-sized high concentrating solar cells and is expected to work at about 300 Wp, with a short-circuit current of 6.6 A and an open circuit voltage of 50.72 V. In the light of a preliminary thermal simulation, an aluminum-based insulated metal substrate has been use as baseplate. The original outline of the conductive copper layer has been developed to minimize the Joule losses, by reducing the number of interconnections between the cells in series. Slightly oversized Schottky diodes have been applied for bypassing purposes and the whole design fits the IPC-2221 requirements. A fullscale thermal simulation has been implemented to prove the reliability of an insulated metal substrate in CPV application, even if compared to the widely-used direct bonded copper board. The Joule heating phenomenon has been analytically calculated first, to understand the effect on the electrical power output, and then simulate, to predict the consequences on the thermal management of the board. The outcomes of the present research will be used to optimize the design of a novel actively cooled 144-cell receiver for high concentrating photovoltaic applications. © 2014 The Authors.
Design and production of a 2.5 kWe insulated metal substrate-based densely packed CPV assembly
The original design of a new 144-cell concentrating photovoltaic assembly is presented in this paper. It is conceived to work under 500 suns and to generate about 2.5 kWe. An insulated metal substrate was selected as baseplate, in order to get the best compromise between costs and thermal performances. It is based on a 2mm thick aluminum plate, which is in charge of removing the heat as quick as possible. The copper pattern and thickness has been designed accordingly to the IPC Generic Standard on Printed Board Design and to the restrictions of fit a reflective 125x primary optics and a 4x secondary refractive optics. The original outline of the conductive copper layer has been developed to minimize Joule losses by reducing the number of interconnections between the cells in series. Multijunction solar cells and Schottky bypass diodes have been soldered onto the board as surface mounted components. All the fabrication processes are described. This board represents a novelty for the innovative pattern of the conductive layer, which can be easily adapted to be coupled with different optics geometries and to allocate a different number of cells. The use of an IMS as baseplate will give an experimental contribution to the debate about the exploitability of this kind of substrates in CPV. This board is being characterized indoor and outdoor: the results will be used to improve the design and the reliability of the future receivers.
Opportunities and challenges in micro- and nano-technologies for concentrating photovoltaic cooling: A review
Concentrating photovoltaic technology is one of the fastest growing solar energy technologies achieving electrical conversion efficiency in excess of 43%. The operating temperature of a solar cell strongly influences the performance of a photovoltaic system reducing its efficiency with a negative temperature coefficient. Thus, cooling systems represent a very important aspect in concentrating photovoltaic applications. This work presents an overview of micro- and nano-technologies applicable to passive CPV cooling and associated manufacturing technologies (such as monolithic applications). Among the different technologies, carbon nano-tubes and high-conductive coating are the most promising technologies to offer the best CPV cooling performance. A critical assessment of the technological review has also been made. © 2012 Elsevier Ltd.
Small-Volume Fabrication of a 144-Cell Assembly for High-Concentrating Photovoltaic Receivers
Concentrating photovoltaic (CPV) is a solution that is gaining attention worldwide as a potential global player in the future energy market. Despite the impressive development in terms of CPV cell efficiency recorded in the last few years, a lack of information on the module's manufacturing is still registered among the documents available in literature. This work describes the challenges faced to fabricate a densely packed cell assembly for 500× CPV applications. The reasons behind the choice of components, materials, and processes are highlighted, and all the solutions applied to overtake the problems experienced after the prototype's production are reported. This article explains all the stages required to achieve a successful fabrication, proven by the results of quality tests and experimental investigations conducted on the prototype. The reliability of the components and the interconnectors is successfully assessed through standard mechanical destructive tests, and an indoor characterization is conducted to investigate the electrical performance. The fabricated cell assembly shows a fill factor as high as 84%, which proves the low series resistance and the lack of mismatches. The outputs are compared with those of commercial assemblies. A cost breakdown is reported and commented: a cost of 0.79/Wp has been required to fabricate each of the cell assembly described in this paper. This value has been found to be positively affected by the economy of scale: a larger number of assemblies produced would have reduced it by 17%.
Optimization of the least-material approach for passive Ultra-High CPV cooling
The attention around Ultra-High CPV is increasing year by year, because of the potential cost-cutting achievable by increasing the concentration ratio. In these systems, an adequate thermal management becomes particularly important: cooling systems are required to be compact, reliable and efficient. In this work, the geometry of a passive heat sink for a 4000x CPV application is optimized to reduce the volume and the costs, limiting the effects on the thermal performance. The same approach is used to model the behavior of a UHCPV heat sink in New Orleans, LA (USA): the hourly cell temperatures are presented in the paper.
Modelling photovoltaic soiling losses through optical characterization
The accumulation of soiling on photovoltaic (PV) modules affects PV systems worldwide. Soiling consists of mineral dust, soot particles, aerosols, pollen, fungi and/or other contaminants that deposit on the surface of PV modules. Soiling absorbs, scatters, and reflects a fraction of the incoming sunlight, reducing the intensity that reaches the active part of the solar cell. Here, we report on the comparison of naturally accumulated soiling on coupons of PV glass soiled at seven locations worldwide. The spectral hemispherical transmittance was measured. It was found that natural soiling disproportionately impacts the blue and ultraviolet (UV) portions of the spectrum compared to the visible and infrared (IR). Also, the general shape of the transmittance spectra was similar at all the studied sites and could adequately be described by a modified form of the Ångström turbidity equation. In addition, the distribution of particles sizes was found to follow the IEST-STD-CC 1246E cleanliness standard. The fractional coverage of the glass surface by particles could be determined directly or indirectly and, as expected, has a linear correlation with the transmittance. It thus becomes feasible to estimate the optical consequences of the soiling of PV modules from the particle size distribution and the cleanliness value.
Plate micro-fins in natural convection: An opportunity for passive concentrating photovoltaic cooling
The raise in temperature is a non-negligible issue for concentrating photovoltaics (CPV), where the sunlight is concentrated up to thousands of times and a large amount of heat is collected on the solar cells. Micro-fins have been identified as one of the most promising solution for CPV cooling: despite its potentials, the number of publications on this subject is still limited. The present paper resumes the state-of-the-art of the research on micro-fins, in order to identify the most convenient fin geometry for CPV applications. The results of the investigation conducted in this work show that, compared to a conventional heat sink, micro-fins can improve the thermal performance and, at the same time, lower the weight of a system. For this reason, they are particularly beneficial for tracked systems, such as CPV, where a reduced weight means a reduced load for the tracker. The heat transfer coefficients measured through an experimental setup are used to predict the performance of a micro-finned CPV system in natural convection: an optimized fin array is found able to enhance the mass specific power up to 50% compared to an unfinned surface.