Polytechnic Institute of Porto
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Rear Optical Reflection and Passivation Using a Nanopatterned Metal/Dielectric Structure in Thin-Film Solar Cells
No full textCurrently, one of the main limitations in ultrathin Cu(In,Ga)Se 2 (CIGS) solar cells are the optical losses, since the absorber layer is thinner than the light optical path. Hence,light management, including rear optical reflection, and light trapping is needed. In this paper, we focus on increasing the rear optical reflection. For this, a novel structure based on having a metal interlayer in between the Mo rear contact and the rear passivation layer is presented. In total, eight different metallic interlayers are compared. For the whole series, the passivation layer is aluminum oxide (Al 2 O 3 ). The interlayers are used to enhance the reflectivity of the rear contact and thereby increasing the amount of light reflected back into the absorber. In order to understand the effects of the interlayer in the solar cell performance both from optical and/or electrical point of view, optical simulations were performed together with fabrication and electrical measurements. Optical simulations results are compared with current density-voltage (J-V) behavior and external quantum efficiency measurements. A detailed comparison between all the interlayers is done, in order to identify the material with the greatest potential to be used as a rear reflective layer for ultrathin CIGS solar cells and to establish fabrication challenges. The Ti-W alloy is a promising a rear reflective layer since it provides solar cells with light to power conversion efficiency values of 9.9%, which is 2.2% (abs) higher than the passivated ultrathin sample and 3.7% (abs) higher than the unpassivated ultrathin reference sample.info:eu-repo/semantics/publishedVersio
Promising bacterial genera for agricultural practices: An insight on plant growth-promoting properties and microbial safety aspects
No full textIn order to address the ever-increasing problem of the world's population food needs, the optimization of farming crops yield, the combat of iron deficiency in plants (chlorosis) and the elimination/reduction of crop pathogens are of key challenges to solve. Traditional ways of solving these problems are either unpractical on a large scale (e.g. use of manure) or are not environmental friendly (e.g. application of iron-synthetic fertilizers or indiscriminate use of pesticides). Therefore, the search for greener substitutes, such as the application of siderophores of bacterial source or the use of plant-growth promoting bacteria (PGPB), is presented as a very promising alternative to enhance yield of crops and performance. However, the use of microorganisms is not a risk-free solution and the potential biohazards associated with the utilization of bacteria in agriculture should be considered. The present work gives a current overview of the main mechanisms associated with the use of bacteria in the promotion of plant growth. The potentiality of several bacterial genera (Azotobacter, Azospirillum, Bacillus, Pantoea, Pseudomonas and Rhizobium) regarding to siderophore production capacity and other plant growth-promoting properties are presented. In addition, the field performance of these bacteria genera as well as the biosafety aspects related with their use for agricultural proposes are reviewed and discussed.Thisworkwas financed by the FEDER funds through the Operational
Competitiveness Factors Program - COMPETE and by National Funds
through FCT - Foundation for Science and Technology within the scope
of the project PTDC-AGR-TEC/0458/2014 – POCI-01-0145-FEDER-
016681.info:eu-repo/semantics/publishedVersio
An EPS@ISEP 2019 Project
No full textThis paper describes the journey of a multinational and
multidisciplinary team enrolled in the European Project Semester
(EPS) at the Instituto Superior de Engenharia do Porto (ISEP)
during the spring semester of 2019. The team embraced the idea of
repurposing coffee leftovers to cultivate oyster mushrooms and
benefited from the background diversity of the team members as
well as from newly acquired marketing, sustainability and design
ethics skills to consolidate and strengthen the overall feasibility of
the project. The project was set to design, develop and test grey
oyster mushroom growth kits with an automated monitoring
system, using coffee grounds as growing substrate and complying
with the applicable regulations and pre-defined requirements. The
ulterior aims of the project were to reconnect people with the food
they eat and to disseminate sustainable food production processes,
which are not only healthy but environmentally friendly. To
achieve these goals, the team developed a circular economy
business model where grey oyster mushroom growth kits reuse
coffee grounds as growing beds and food buckets as containers.
The designed growth kits include a controlled fruiting chamber
with an integrated monitoring system. This allows easy domestic
cultivation, monitoring through a smart phone. Moreover, the
proposed solution contemplates information sharing on the
mushroom cultivation process, monitoring system and recipes as
well as the maintenance of a dedicated discussion forum. Tests
have been conducted to test the concept, cultivation process,
monitoring system and fruiting chamber from the incubation of
mycelium all the way to the harvesting. Results show the feasibility
of creating a business based on the devised conceptinfo:eu-repo/semantics/publishedVersio
Ultra-Reliable Low Latency based on Retransmission and Spatial Diversity in slowly fading channels with co-channel interference
No full textThis paper presents the analysis of the statistics of latency and information theoretic capacity of an adaptive link with retransmission-spatial diversity in a scenario with co-channel interference. The paper focuses specifically on the delay of the wireless transmission component, measured from the instant a packet at the head of the queue is first transmitted until it is correctly received by the destination (considering retransmissions). The objective is to evaluate the ability of temporal and spatial diversity tools to achieve ultra-low values of latency as desired in future 5G and machine-to-machine (M2M) networks with real-time requirements. It is assumed that the source transmits information towards the destination in a Rayleigh fading spatially correlated channel. In case the instantaneous signal-to-interference-plus-noise (SINR) ratio has not surpassed a predetermined reception threshold, then the source engages in a persistent retransmission protocol. All the copies of the original transmission and subsequent retransmissions are stored in memory and processed at the destination using maximum ratio combining (MRC) to obtain a more reliable copy of the signal (a scheme also called retransmission diversity). The retransmission scheme stops once the instantaneous post-processing SINR achieves the desired target threshold. This persistent retransmission scheme can also be regarded as a security mechanism against interference jamming attacks. Since retransmissions are assumed to take place in a short time interval in order to achieve very low values of latency, they are modelled with statistical temporal correlation, which is explicitly introduced in the embedded Gaussian channel distribution model. Results suggest that retransmission diversity can provide good latency results in moderate to high values of SINR. However, at low SINR, a combination with other diversity sources will be necessary to achieve the desired target value.info:eu-repo/semantics/publishedVersio
Experimental evaluation of the two-ray model for near-shore WiFi-based network systems design
No full textIn the design of shore-to-shore and shore-to-vessel wireless links, the impact of the ray reflected on the surface is often neglected. It adds that, in some coastal areas, the geometry of the reflection changes over time due to tides. When choosing an antenna height for an inshore node, often the largest possible height is used, but this approach can lead to signal degradation. The two-ray model is the most fundamental path loss model to account for the contribution of the reflected ray. We carried out experimental measurements at the shores of a freshwater body to verify that the two-ray model can predict the major trends of the path loss experienced by a 2.4 GHz over-water wireless link. We focus on short-to-medium distance links, with antennas installed a few meters above surface. We observed considerable consistency between measurements and model estimates, leading us to conclude that the two-ray model may bring benefits when applied to the network design of over-water links affected by tidal variations, which is our end-goal.info:eu-repo/semantics/publishedVersio
Response time analysis of multiframe mixed-criticality systems with arbitrary deadlines
No full textThe well-known model of Vestal aims to avoid excessive pessimism in the quantifcation of the processing requirements of mixed-criticality systems, while still
guaranteeing the timeliness of higher-criticality functions. This can bring important
savings in system costs, and indirectly help meet size, weight and power constraints.
This efciency is promoted via the use of multiple worst-case execution time
(WCET) estimates for the same task, with each such estimate characterized by a
confdence associated with a diferent criticality level. However, even this approach
can be very pessimistic when the WCET of successive instances of the same task
can vary greatly according to a known pattern, as in MP3 and MPEG codecs or
the processing of ADVB video streams. In this paper, we present a schedulability
analysis for the new multiframe mixed-criticality model, which allows tasks to have
multiple, periodically repeating, WCETs in the same mode of operation. Our work
extends both the analysis techniques for Static Mixed-Criticality scheduling (SMC)
and Adaptive Mixed-Criticality scheduling (AMC), on one hand, and the schedulability analysis for multiframe task systems on the other. A constrained-deadline
model is initially targeted, and then extended to the more general, but also more
complex, arbitrary-deadline scenario. The corresponding optimal priority assignment for our schedulability analysis is also identifed. Our proposed worst-case
response time (WCRT) analysis for multiframe mixed-criticality systems is considerably less pessimistic than applying the static and adaptive mixed-criticality scheduling tests oblivious to the WCET variation patterns. Experimental evaluation with
synthetic task sets demonstrates up to 20% and 31.4% higher scheduling success
ratio (in absolute terms) for constrained-deadline analyses and arbitrary-deadline
analyses, respectively, when compared to the best of their corresponding frame-oblivious tests.info:eu-repo/semantics/publishedVersio
Multivariate time series clustering and forecasting for building energy analysis: Application to weather data quality control
No full textIn recent years, several tools for building energy analysis and simulation have been developed to assist in increasing building energy performance, harvesting its computing capabilities for a reliable and accurate energy performance prediction. To perform this analysis, energy tools typically require crucial data regarding the building's surrounding environment, which is acquired from neighbouring weather stations. However, these stations often experience hardware malfunctions, resulting in either erroneous or missing data. Traditionally, these values are rectified through empirical and geostatistical methods, which, while reflecting several decades of practice, may prove to be inadequate when considering a purely data-driven approach. To this end, the present study introduces a machine learning methodology proposing the application of regression algorithms to rectify the erroneous values in datasets, and the clustering of weather stations, based on their recorded climatic conditions, to enhance the regression models. A shape-based approach for clustering time series of different climatic parameters and weather stations is pursued, using the k-medoids algorithm alongside dynamic time warping as the similarity measure. Both Artificial Neural Networks (ANN) and Support Vector Regression (SVR) models are evaluated as exemplary regression algorithms, with different sets of predictors. Mean Squared Error is used as the performance metric. A data set of different climatic parameters from southeastern Brazil was used, with air temperature being chosen as the response variable, given its importance in energy consumption. Results indicate that a machine learning approach to the problem is indeed viable. ANN slightly outperforms SVR in the prediction of the studied weather variable.Building energy analysisThis work was partially financially supported by UID/ECI/04708/
2019 – CONSTRUCT –Instituto de I&D em Estruturas e Construções and
UIDB/04234/2020 – CISTER Research Unit, both funded by national
funds through the FCT/MCTES (PIDDAC).info:eu-repo/semantics/publishedVersio
Kinetics of Optical Properties of Colorectal Muscle During Optical Clearing
No full textIn this paper, we describe a simple and indirect method to evaluate the kinetics of the optical properties for biological tissues under optical clearing treatments. We use the theoretical formalism in this method to process experimental data obtained from colorectal muscle samples to evaluate and characterize the dehydration and refractive index matching mechanisms.info:eu-repo/semantics/publishedVersio
From Code to Weakly Hard Constraints: A Pragmatic End-to-End Toolchain for Timed C
No full textRTSS 2019 originally postponed from December 2019 (Hong-Kong) to February 2020 (York, UK) was cancelled.Complex real-time systems are traditionally developed in several disjoint steps: (i) decomposition of applications into sets of recurrent tasks, (ii) worst-case execution time estimation, and (iii) schedulability analysis. Each step is already in itself complex and error-prone, and the composition of all three poses a nontrivial integration problem. In particular, it is challenging to obtain an end-to-end analysis of timing properties of the whole system due to practical differences between the interfaces of tools for extracting task models, execution time analysis, and schedulability tests. To address this problem, we propose a seamless and pragmatic end-to-end compilation and timing analysis toolchain, where source programs are written in a real-time extension of C, called Timed C. The toolchain automatically translates timing primitives into executable code, measures execution times, and verifies temporal correctness using an extended schedulability test for non-preemptive generalized multiframe task sets. Novel aspects of our approach are: (i) both soft and firm tasks can be expressed at the programming language level and stated timing requirements are automatically verified by the schedulability test, and (ii) the schedulability test outputs per-job response-time information that enables a new approach to sensitivity analysis. Specifically, we perform a weakly hard sensitivity analysis that determines the worst-case execution time margins for the strongest still-satisfied (M;K) constraint, where M = m_1 + … + m_N denotes the number of deadline misses across the entire task set, and K = {k_1; … ; k_N} is the set of windows of interest of the different tasks. The toolchain is implemented as a source-to-source compiler, freely available as open source, and conveniently distributed as a Docker container.info:eu-repo/semantics/publishedVersio
Multi-Level Preemption in TSN: Feasibility and Requirements Analysis
No full textTo overcome the limitation of strictly non-preemptive frame transmission in Ethernet networks, the IEEE 802.1Qbu standard was introduced. This standard specifies a one-level frame preemption paradigm wherein, depending on their priority levels, frames are grouped into two categories: namely, the “express frames” and the “preemptable frames”. These two categories are given with the interpretation that (1) only express frames can preempt preemptable frames; and (2) two frames belonging to the same category cannot preempt each other. While this approach partially solves the problem, some preemptable frames can still suffer long blocking periods, irrespective of their individual priority levels. Indeed, there are frames that do not fall into the express frames category, but nevertheless have firm timing requirements that can only be met if they can benefit from preempting lower priority frames. To ameliorate the condition of such frames, we propose a multi-level preemption paradigm. Specifically, we expose the limitations of the one-level preemption approach experimentally; and we present the feasibility and implementation requirements of the multi-level preemption scheme in details.info:eu-repo/semantics/publishedVersio