Predicting Benzene Concentration Using Machine Learning and Time Series Algorithms

[EN] Benzene is a pollutant which is very harmful to our health, so models are necessary to predict its concentration and relationship with other air pollutants. The data collected by eight stations in Madrid (Spain) over nine years were analyzed using the following regression-based machine learning models: multivariate linear regression (MLR), multivariate adaptive regression splines (MARS), multilayer perceptron neural network (MLP), support vector machines (SVM), autoregressive integrated moving-average (ARIMA) and vector autoregressive moving-average (VARMA) models. Benzene concentration predictions were made from the concentration of four environmental pollutants: nitrogen dioxide (NO2), nitrogen oxides (NOx), particulate matter (PM10) and toluene (C7H8), and the performance measures of the model were studied from the proposed models. In general, regression-based machine learning models are more effective at predicting than time series models.S

Technical feasibility of lined mining tunnels in closed coal mines as underground reservoirs of compressed air energy storage systems

[EN] In this paper, four mining levels in a closed coal mine in the Asturian Central Coal Basin (NW Spain) have been selected as a case study to investigate the technical feasibility of underground compressed air energy storage systems. First, in order to determine the suitable level and type of concrete lining, a numerical model has been established to analyze the geomechanical performance considering air pressures of 6, 10, 20 and 25 MPa and three different embodiments of concrete lining. Then, another numerical model is used to study the coupled thermo-mechanical performance at level 3, considering 100 operation cycles between 6 and 10 MPa air pressure using a circular concrete lining with a 20 mm thick sealing layer. The results obtained indicate that the deformations are lower at levels 1 and 3, where the shales are located at the top of the coal seam. Deformations and tensile stresses are significantly reduced when a circular concrete lining is used. As the thermal analysis shows, temperature fluctuations are restricted to sealing layer and concrete lining and do not reach the rock mass itself. Therefore, negligible deformations are produced by the effect of temperature compared to the effect of air pressure. Maximum tensile stress and total displacements during the operation occur at the top of the mining drift and reach 9.5 MPa and 3.6 mm, respectively. A technical feasibility can be achieved using a circular concrete lining with a suitable reinforcement system.S

Degradation Monitoring of Photovoltaic Plants: Advanced GIS Applications

In order to evaluate a photovoltaic (PV) plant performance, payback time, profitability and environmental impact, an analysis must be made of plant maintenance needs, module and wiring degradation, mismatches and dust effects and PV cell defects and faults. Although a wide range of studies can be found that show the theory and laboratory testing of how these circumstances may affect PV production, very few studies in the field have covered or quantified real degradation effects and faults using a systematic procedure. The authors have therefore reviewed the conditions of PV plants operating in Southern Europe, examining the most frequently found faults and types of degradation, and they look at how novel technologies, such as geographic information system (GIS) applications, can help maintainers, owners, and promoters to supervise and locate damaged PV modules and monitor their evolution and impact on plant working conditions. GIS applications in this field allow the organization of a geo-referenced database of the system, locating and supervising the thirds of each PV cell in the power plant. With this information, investors and maintainers can exert increased control on the PV plant performance and conduct better preventive maintenance measures. The examples given demonstrate that these sorts of applications can be applied both to large PV plants and to domestic installations

Material Behavior and Fatigue Assessment of Old Steel Bridges of the Spanish Conventional Rail Network

[EN] This work presented salient features of the steel behavior of seven metallic bridges close to, or over, 100 years old, among the Spanish conventional rail network as well as the results of a fatigue life expectancy study. A preliminary study of the properties of the constituent materials obtained from the bridges samples was carried out followed by dynamic fatigue tests under service representative loads. Due to the steelmaking techniques in the late 19th and early 20th centuries, disperse fatigue behavior results were obtained. However, the wide safety margins with which these bridges were designed, as well as the mechanical properties of the steel (relatively good mechanical resistance but with low ductility), seem to guarantee a long fatigue life. This estimate decreases sharply with increasing loads.S

Thermodynamic Analysis of Compressed Air Energy Storage (CAES) Reservoirs in Abandoned Mines Using Different Sealing Layers

[EN] Million cubic meters from abandoned mines worldwide could be used as subsurface reservoirs for large scale energy storage systems, such as adiabatic compressed air energy storage (A-CAES). In this paper, analytical and three-dimensional CFD numerical models have been conducted to analyze the thermodynamic performance of the A-CAES reservoirs in abandoned mines during air charging and discharging processes. Unlike other research works, in which the heat transfer coefficient is considered constant during the operation time, in the present investigation a correlation based on both unsteady Reynolds and Rayleigh numbers is employed for the heat transfer coefficient in this type of application. A tunnel with a 35 cm thick concrete lining, 200 m3 of useful volume and typical operating pressures from 5 to 8 MPa were considered. Fiber-reinforced plastic (FRP) and steel were employed as sealing layers in the simulations around the fluid. Finally, the model also considers a 2.5 m thick sandstone rock mass around the concrete lining. The results obtained show significant heat flux between the pressurized air and the sealing layer and between the sealing layer and concrete lining. However, no temperature fluctuation was observed in the rock mass. The air temperature fluctuations are reduced when steel sealing layer is employed. The thermal energy balance through the sealing layer for 30 cycles, considering air mass flow rates of 0.22 kg s−1 (charge) and −0.45 kg s−1 (discharge), reached 1056 and 907 kWh for FRP and steel, respectively. In general, good agreements between analytical and numerical simulations were obtained.S

Development of a GIS Tool for High Precision PV Degradation Monitoring and Supervision: Feasibility Analysis in Large and Small PV Plants

[EN] It is well known that working photovoltaic (PV) plants show several maintenance needs due to wiring and module degradation, mismatches, dust, and PV cell defects and faults. There are a wide range of theoretical studies as well as some laboratory tests that show how these circumstances may affect the PV production. Thus, it is mandatory to evaluate the whole PV plant performance and, then, its payback time, profitability, and environmental impact or carbon footprint. However, very few studies include a systematic procedure to quantify and supervise the real degradation effects and fault impacts on the field. In this paper, the authors first conducted a brief review of the most frequent PV faults and the degradation that can be found under real conditions of operation of PV plants. Then, they proposed and developed an innovative Geographic Information System (GIS) application to locate and supervise them. The designed tool was applied to both a large PV plant of 108 kWp and a small PV plant of 9 kWp installed on a home rooftop. For the large PV plant, 24 strings of PV modules were modelized and introduced into the GIS application and every module in the power plant was studied including voltage, current, power, series and parallel resistances, fill factor, normalized PV curve to standard test conditions (STC), thermography and visual analysis. For the small PV installation three strings of PV panels were studied identically. It must be noted that PV modules in this case included power optimizers. The precision of the study enabled the researchers to locate and supervise up to a third part of every PV cell in the system, which can be adequately georeferenced. The developed tool allows both the researchers and the investors to increase control of the PV plant performance, to lead to better planning of maintenance actuations, and to evaluate several PV module replacement strategies in a preventive maintenance program. The PV faults found include hot spots, snail tracks, ethylene vinyl acetate (EVA) discoloration, PV cell fractures, busbar discoloration, bubbles and Si discoloration.SILaboratorio de Inspección Técnica de la Escuela de Minas (LITEM

Treatment of Soil Contaminated by Mining Activities to Prevent Contamination by Encapsulation in Ceramic Construction Materials

[EN] Mining is an essential activity for obtaining materials necessary for the well-being and development of society. However, this activity produces important environmental impacts that must be controlled. More specifically, there are different soils near new or abandoned mining productions that have been contaminated with potentially toxic elements, and currently represent an important environmental problem. In this research, a contaminated soil from the mining district of Linares was studied for its use as a raw material for the conforming of ceramic materials, bricks, dedicated to construction. Firstly, the contaminated soil was chemically and physically characterized in order to evaluate its suitability. Subsequently, different families of samples were conformed with different percentages of clay and contaminated soil. Finally, the conformed ceramics were physically and mechanically characterized to examine the variation produced in the ceramic material by the incorporation of the contaminated soil. In addition, in this research, leachate tests were performed according to the TCLP method determining whether encapsulation of potentially toxic elements in the soil occurs. The results showed that all families of ceramic materials have acceptable physical properties, with a soil percentage of less than 80% being acceptable to obtain adequate mechanical properties and a maximum of 70% of contaminated soil to obtain acceptable leachate according to EPA regulations. Therefore, the maximum percentage of contaminated soil that can be incorporated into the ceramic material is 70% in order to comply with all standards. Consequently, this research not only avoids the contamination that contaminated soil can produce, but also valorizes this element as a raw material for new materials, avoiding the extraction of clay and reducing the environmental impact.S

A Method of Pruning and Random Replacing of Known Values for Comparing Missing Data Imputation Models for Incomplete Air Quality Time Series

[EN] The data obtained from air quality monitoring stations, which are used to carry out studies using data mining techniques, present the problem of missing values. This paper describes a research work on missing data imputation. Among the most common methods, the method that best imputes values to the available data set is analysed. It uses an algorithm that randomly replaces all known values in a dataset once with imputed values and compares them with the actual known values, forming several subsets. Data from seven stations in the Silesian region (Poland) were analyzed for hourly concentrations of four pollutants: nitrogen dioxide (NO2), nitrogen oxides (NOx), particles of 10 μm or less (PM10) and sulphur dioxide (SO2) for five years. Imputations were performed using linear imputation (LI), predictive mean matching (PMM), random forest (RF), k-nearest neighbours (k-NN) and imputation by Kalman smoothing on structural time series (Kalman) methods and performance evaluations were performed. Once the comparison method was validated, it was determine that, in general, Kalman structural smoothing and the linear imputation methods best fitted the imputed values to the data pattern. It was observed that each imputation method behaves in an analogous way for the different stations The variables with the best results are NO2 and SO2. The UMI method is the worst imputer for missing values in the data sets.S

Thin-film PV modules early degradation analysis: a case study on CIGS

[EN]Solar Photovoltaic (PV) energy generation will be one of the main energy generation technologies in the inner future thanks to the recent and significant reduction of involved costs. One of the most promising and lowest cost technologies is the thin-film PV modules technology. Their currently lower performance than single-Si PV modules is compensated by their lower costs and more aesthetical display, which make them especially attractive for building integration. However, because of the novelty of this technology, very few studies have been conducted to analyse its aging and degradation, which has a significant impact on the operation and maintenance needs and, in the end, in its Levelized Cost of Energy (LCOE). Thus, in this work, a systematic review on this technology and on its more frequent faults and degradation effects is conducted first. Then, a set of Copper Indium Gallium Selenide (CIGS) PV modules has been tested under real outdoor conditions and its degradation has been analysed after 3 years operation. A new procedure for faults location and quantification has been established for this sort of PV modules; and early degradation and aging data are presented and analysed. Results show a significant early degradation of this technology and a fast expansion of the PV faults.S

Concentration, Propagation and Dilution of Toxic Gases in Underground Excavations under Different Ventilation Modes

[EN] The drill-and-blast method is widely used for the excavation of hard rock tunnels. Toxic gases such as carbon monoxide and nitrogen oxides are released immediately after blasting by the detonation of explosives. To provide a safe working environment, the concentration of noxious gases must be reduced below the threshold limit value according to health and safety regulations. In this paper, one-dimensional mathematical models and three-dimensional CFD numerical simulations were conducted to analyze the concentration, propagation and dilution of the blasting fumes under different operating conditions. Forced, exhaust and mixed ventilation modes were compared to determine the safe re-entry times after blasting in a 200 m-long tunnel excavated using the top-heading-and-benching method. Based on the numerical simulations, carbon monoxide was the most critical gas, as it required a longer ventilation time to reduce its concentration below the threshold limit value. The safe re-entry time reached 480 s under the typical forced ventilation mode, but was reduced to 155 s when a mixed ventilation system was used after blasting, reducing the operating costs. The reduction of the re-entry time represents a significant improvement in the excavation cycle. In addition, the results obtained show that 1D models can be used to preliminary analyze the migration of toxic gases. However, to reliably determine the safe re-entry times, 3D numerical models should be developed. Finally, to verify the accuracy of the CFD results, field measurements were carried out in a railway tunnel using gas sensors. In general, good agreements were obtained between the 3D numerical simulations and the measured values.S