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Passive air samplers (polyurethane foam disks) were deployed on an altitudinal transect in the rural Italian Alps to investigate the potential influence of forest cover on air concentrations. Samplers were exposed overtwo periods, each of several weeks, either in clearings or in forests. In the first period, there was high leaf coverage (high leaf area index, LAI); in the second, the LAI was low after the autumnal leaf fall. PCBs sequestered in the PUF generally declined with altitude, for example, in the clearings PCBs-28, 52, 90/101, 118, and 138, all showed statistically significant declines (p < 0.05). The mass of HCB sequestered increased with altitude, evidence of cold condensation. Ratios of the forest:clearing concentrations were calculated; this ratio expresses the filtering ability of forests to deplete air concentrations compared to the adjacent clearings. During the high LAI sampling period, these depletion factors ranged between 0.93 and 0.54 and were inversely correlated with temperature-corrected log K0A. This relationship was notobserved during the low LAI sampling period. The depletion factors were normalized using the LAI to give a density independent depletion factor (DIDF). The slopes of the correlations with K0A were comparable for broadleaf or coniferous forests at different altitudes, suggesting that leaf surfaces determine the exchanges with air. Broadleaf forests at 1000 and 1400 m showed similar behavior, while a conifer forest at 1800 m gave depletion factors which were higher by about a factor of 2. It is suggested that DIDF can be used in regional environmental fate models to estimate the contribution of forests to contaminant fate.

Capítulos en libros This paper addresses the vulnerability analysis of the electric grid under terrorist threat. This problem is formulated as a mixed-integer nonlinear bilevel program. In the upper-level optimization, the terrorist agent maximizes the damage caused in the power system, which is measured in terms of the level of system load shed. On the other hand, in the lower-level optimization, the system operator minimizes the damage by means of an optimal operation of the power system. The distinctive modeling feature introduced in this paper is that, among the different corrective actions available, the system operator has the capability to modify the network topology. Due to its nonconvexity and nonlinearity, the resulting bilevel programming problem cannot be equivalently transformed into a standard one-level optimization problem. Therefore, this paper proposes a new approach based on Benders decomposition within a restart framework. Some numerical results obtained by the proposed algorithm are provided and compared with those published, based on the IEEE Reliability Test System. info:eu-repo/semantics/publishedVersion

Abstract A simple, transient model for the characterization of the dynamic thermal performance of solar thermal collectors was developed and experimentally validated. The proposed model equation is linear with respect to the input parameters and does not require any treatment for ordinary differential equations (ODEs). The temperature distribution in the fluid flowing inside the collector is described by means of the piston flow and finite increment concepts. The dynamic effect, for a given flow rate, is expressed by the heat transport time and is based on the effective thermal capacity of the collector. The results reveal that the characteristic parameters involved in the model agree reasonably well with the experimental variables obtained from standard steady-state measurements. After a calibration process the model can well predict the thermal performance of a solar thermal collector, for a specific weather data set.

Artículos en revistas The electrical systems of the Central American countries are linked by 230 kV ac weak border interconnections forming two separated subsystems. The first one includes Guatemala and El Salvador, and the other one comprises the systems of Honduras, Nicaragua, Costa Rica and Panamá. As a consequence, unrestricted energy exchanges among all countries are not possible. Since 1987, the following utilities have been promoting the SIEPAC project: INDE from Guatemala, CEL from EL Salvador, ENEE from Honduras, INE from Nicaragua, ICE from Costa Rica, IRHE from Panamá, together with the ENDESA Group from Spain. The SIEPAC project consists of a 1678 km long ac power transmission line that would link the six electrical systems of the region through several power substations installed close to the highest demand national centers and six control centers of energy (one for each country) to allow coordinated operation of the interconnection. This project also consideres a set of complementary assets into some countries and the border transmission line between El Salvador and Honduras, and various other equipment. The power transmission line will greately reinforce the actual border interconnections, which hace a reduced capacity of exchange and technical problems associated with the stability of a weak longitudinal system. On the other hand, economic savings for the region would be achieved, coming from a higher coordination level in the operation and planning of their systems. info:eu-repo/semantics/publishedVersion

Capítulos en libros In this paper we compare two cutting-edge timeperiod aggregation methodologies for power system models that consider both renewables and storage technologies: the chronological time-period clustering; and, the enhanced representative period approach. Such methodologies are used in order to reduce the computational burden of highly complex optimization models while not compromising the quality of the results. With this paper, we identify which method works best, and under which conditions, in order to reproduce the outcomes of the hourly benchmark model. info:eu-repo/semantics/publishedVersion

Thermostatic loads are expected to support the transition to low-carbon systems by providing an array of ancillary services. The delivery of energy-intensive services requires the thermostatic appliances to payback the deployed thermal energy via extra power consumption at a later time. This is made available by conventional generators providing contingency reserve. Previous works acknowledged the interplay between secondary response and contingency reserve. However, the structure of the payback period was fixed and did not fully consider the capabilities of generators providing contingency reserve, e.g., modelling only spinning generators while neglecting standing units. This paper introduces a novel methodology that allows to optimally coordinate the energy recovery of thermostatic loads and the allocation of contingency reserve among spinning and standing units. To do so, the commitment times and structures of secondary response and contingency reserve can be optimally chosen depending on system needs and current availabilities at each time step. The model is applied to the Great Britain network, which is connected to the power system in Continental Europe via a high-voltage direct-current link. Results illustrate the techno-economic advantages stemming from the proposed methodology.

Abstract The effect of the geometrical parameters of interdigitated batteries, including the number, thickness and the length of the digits, on the delivered battery capacity was investigated in order to optimize battery performance. This optimization was carried out in two dimensions maintaining the area of the different components constant and based on the Doyle/Fuller/Newman theoretical model that describes the electrochemical and ionic transport processes within separator and electrodes. It is observed that the delivered capacity of the battery increases with increasing number of digits as well as by increasing the thickness and the length of the digits, which is related to the smaller path for lithium ion movement during the intercalation/deintercalation processes. Thus, an optimized interdigitated geometry with eight digits shows a delivered capacity of 323 Ah·m − 2 at 350 °C in comparison with 0.7 Ah·m − 2 for a conventional geometry.

Artículos en revistas This paper proposes a model to carry out analysis of storage facilities operation including a transmission network. The model represents short-term storage operation in an approxi-mated way that reduces computational requirements, which makes it suitable for medium and long-term operational planning in power systems with a high level of renewable energy penetra-tion. In the proposed model, we cluster hourly data using the so-called system-states framework developed in previous work. Within this framework, non-consecutive similar time periods are grouped, while chronological information is represented by a tran-sition matrix among states. We extend the system-state framework from a single-bus system to a transmission network. We define and analyze two alternative sets of representative variables for clustering hours to obtain system states when the transmission network is considered. This extension of the system states framework allows us to evaluate the impact of transmission congestions in medium- and long-term planning models in a rea-sonable computation time. A case study shows that the proposed model is 235 times faster than an hourly approach, used as benchmark, whereas the overall system cost is approximated with less than 2% error. The overall charging/discharging trends are similar enough to those of the hourly model, being hydro storage better approximated than fast-ramping batteries. Besides, for the analyzed case study, it is shown how congestion in the transmission network in fact improves the accuracy of the proposed approach. info:eu-repo/semantics/publishedVersion