8,727 research outputs found
Universal Compressed Sensing
In this paper, the problem of developing universal algorithms for compressed
sensing of stochastic processes is studied. First, R\'enyi's notion of
information dimension (ID) is generalized to analog stationary processes. This
provides a measure of complexity for such processes and is connected to the
number of measurements required for their accurate recovery. Then a minimum
entropy pursuit (MEP) optimization approach is proposed, and it is proven that
it can reliably recover any stationary process satisfying some mixing
constraints from sufficient number of randomized linear measurements, without
having any prior information about the distribution of the process. It is
proved that a Lagrangian-type approximation of the MEP optimization problem,
referred to as Lagrangian-MEP problem, is identical to a heuristic
implementable algorithm proposed by Baron et al. It is shown that for the right
choice of parameters the Lagrangian-MEP algorithm, in addition to having the
same asymptotic performance as MEP optimization, is also robust to the
measurement noise. For memoryless sources with a discrete-continuous mixture
distribution, the fundamental limits of the minimum number of required
measurements by a non-universal compressed sensing decoder is characterized by
Wu et al. For such sources, it is proved that there is no loss in universal
coding, and both the MEP and the Lagrangian-MEP asymptotically achieve the
optimal performance
A Minimum Relative Entropy Principle for Learning and Acting
This paper proposes a method to construct an adaptive agent that is universal
with respect to a given class of experts, where each expert is an agent that
has been designed specifically for a particular environment. This adaptive
control problem is formalized as the problem of minimizing the relative entropy
of the adaptive agent from the expert that is most suitable for the unknown
environment. If the agent is a passive observer, then the optimal solution is
the well-known Bayesian predictor. However, if the agent is active, then its
past actions need to be treated as causal interventions on the I/O stream
rather than normal probability conditions. Here it is shown that the solution
to this new variational problem is given by a stochastic controller called the
Bayesian control rule, which implements adaptive behavior as a mixture of
experts. Furthermore, it is shown that under mild assumptions, the Bayesian
control rule converges to the control law of the most suitable expert.Comment: 36 pages, 11 figure
Clustering of nonstationary data streams: a survey of fuzzy partitional methods
YesData streams have arisen as a relevant research topic during the past decade. They are realātime, incremental in nature, temporally ordered, massive, contain outliers, and the objects in a data stream may evolve over time (concept drift). Clustering is often one of the earliest and most important steps in the streaming data analysis workflow. A comprehensive literature is available about stream data clustering; however, less attention is devoted to the fuzzy clustering approach, even though the nonstationary nature of many data streams makes it especially appealing. This survey discusses relevant data stream clustering algorithms focusing mainly on fuzzy methods, including their treatment of outliers and concept drift and shift.Ministero dellāIstruzione, dellāUniversitĆ” e della Ricerca
On signal-noise decomposition of timeseries using the continuous wavelet transform: Application to sunspot index
We show that the continuous wavelet transform can provide a unique
decomposition of a timeseries in to 'signal-like' and 'noise-like' components:
From the overall wavelet spectrum two mutually independent skeleton spectra
can be extracted, allowing the separate detection and monitoring in even
non-stationary timeseries of the evolution of (a) both stable but also
transient, evolving periodicities, such as the output of low dimensional
dynamical systems and (b) scale-invariant structures, such as discontinuities,
self-similar structures or noise. An indicative application to the
monthly-averaged sunspot index reveals, apart from the well-known 11-year
periodicity, 3 of its harmonics, the 2-year periodicity (quasi-biennial
oscillation, QBO) and several more (some of which detected previously in
various solar, earth-solar connection and climate indices), here proposed being
just harmonics of the QBO, in all supporting the double-cycle solar magnetic
dynamo model (Benevolenskaya, 1998, 2000). The scale maximal spectrum reveals
the presence of 1/f fluctuations with timescales up to 1 year in the sunspot
number, indicating that the solar magnetic configurations involved in the
transient solar activity phenomena with those characteristic timescales are in
a self-organized-critical state (SOC), as previously proposed for the solar
flare occurence (Lu and Hamilton, 1991).Comment: 22 pages, 2 figure
Techno-economic and environmental evaluation of CO2 utilisation for fuel production. Synthesis of methanol and formic acid
The present report assesses the technological, economic and environmental performances for producing methanol and formic acid from carbon dioxide. Methanol and formic acid are well known chemicals that can be used in the future transport sector and as hydrogen carriers. This study evaluates the potential of methanol and formic acid synthesis from captured CO2 on (i) the net reduction of CO2 emissions and (ii) their economic competitiveness, in comparison with the benchmark conventional synthesis processes using fossil fuels as raw materials. We use a process system engineering approach to calculate the technological, economic and environmental key performance indicators. The boundaries of the study are set gate-to-gate the carbon dioxide utilisation (CDU) plant: this includes hydrogen production via an electrolyser, CO2 purification, CO2 compression and the CDU plant itself. The technologies are represented at the commercial scale of the existing fossil fuel plants. Through a financial analysis, the net present value for each one of the plants is used to evaluate the price of CO2 as raw material or the price of methanol and formic acid as products that would be needed to make the CO2-based processes financially attractive. In our market analysis (by year 2030), we evaluate the possible penetration ways of methanol and formic acid, thus accepting a growing demand of both products.
Overall, depending on the specific conditions of each case: source of feedstock CO2, source of H2 and/or source of electricity, amount of electricity needed and price of electricity, price of the product; the CDU plant may be directly profitable and contribute at different levels to decrease CO2 emissions. The capacity of the CDU plant depends on the available renewable electricity that is used to power it, rather than on the demand of the product. Under specific conditions, the business model becomes feasible.JRC.F.6-Energy Technology Policy Outloo
Design, development and testing of SOEC-based Power-to-Gas systems for conversion and storage of RES into synthetic methane
International and national initiatives are promoting the worldwide transition of energy systems towards power production mixes increasingly based on Renewable Energy Sources (RES). The integration of large shares of RES into the actual electricity infrastructure is representing a challenge for the power grids due to the fluctuating characteristics of RES. The adoption of long-term, large-scale Electric Energy Storage (EES) is envisaged as the key-option for promoting the integration of RES in the electricity sector by overcoming the issue of temporal and spatial decoupling of electricity supply and demand. Among the several EES options, one of the most promising is the conversion of energy from the electrical into the chemical form through the synthesis of H2 and synthetic natural gas (SNG) in Power-to-Gas (P2G) systems based on the electrolysis of water (and also CO2) in Solid Oxide Cells (SOCs). The application of SOC technology in P2G solutions shows attractiveness for the high efficiency of high-temperature electrolysis and the flexibility of SOCs that can operate reversibly as electrolyzers or fuel cells (rSOC) and can directly perform the electrochemical conversion of CO2 and H2O to syngas by co-electrolysis. The capability of reversible operation also allows the application of SOC-based systems to Power-to-Power (P2P) concepts designed for deferred electricity production.
This dissertation is focused on the investigation of electricity storage using Power-to-Gas/Power systems based on SOCs. The aim of this Thesis has been the investigation of the thermo-electrochemical behavior of SOCs integrated P2G/P2P systems, with the purpose to identify the system configuration and the operating conditions that ensure the most efficient electricity-to-SNG (P2G) or electricity-to-electricity (P2P) conversion within the thermal limits imposed by state-of-the art SOC materials. To this purpose, a detailed thermo-electrochemical model of an SOC has been developed at cell level, validated on experimental data, extended at stack level and coupled with models of the main P2G/P2P components for the system analysis. Model validation was performed through the characterization of planar commercial SOCs in the reversible operation as electrolyzers (SOEC) and fuel cells (SOFC) with H2/H2O and CO/CO2 fuel mixtures at different reactant fractions and temperatures. The physical consistency of electrode kinetic parameters evaluated from the model was verified with the support of literature studies.
The investigation of SOC-based P2P and P2G solutions was performed using the models developed. Three different configurations were analyzed and simulated: 1) hydrogen-based P2P with rSOC, 2) SOEC-based electricity storage into hydrogen with subsequent SNG production by methanation with CO2 and 3) electricity storage by co-electrolysis of water and carbon dioxide with SOEC for syngas production and subsequent upgrading to SNG by methanation.
The performance of the P2P system was thoroughly assessed by analyzing the effects of rSOC stack operating parameters (inlet gas temperature, oxidant-to-fuel ratio, oxidant recirculation rate, cell current) and system configurations (pressurized/ambient rSOC operation, air/oxygen as oxidant/sweep fluid) on stack and system efficiency. The analysis allowed to identify the most efficient configuration of the P2P system, and to select the feasible operating currents (i.e., the currents included within the limits given by the physical thermal constraints of SOC materials) for which the highest roundtrip efficiency is achieved. Pressurized rSOC operation (10 bar) with pure oxygen as oxidant/sweep gas and full recirculation of the oxidant flow ensured the highest charging and discharging effectiveness, with a system roundtrip efficiency of 72% when the stack is operating at the maximum efficiency currents (-1.3 A/cm2 in SOEC and 0.3 A/cm2 in SOFC). A dynamic analysis was performed on the rSOC to determine the characteristic times of the thermal response of an SRU coupled with variable loads. The analysis showed that the SOEC is intrinsically more suitable to work with variable loads thanks to the balance between reaction endothermicity and losses exothermicity that reduces the magnitude and the rate of temperature fluctuations originated by current variations. A case study was presented to show the application of P2P with fluctuating RES. In the case study, the sizing of an rSOC-based P2P system designed for the minimization of the imbalance (i.e., the difference between effective and forecasted electricity production) of a 1 MW grid-connected wind farm was performed. An optimal number of cells was found, for which the imbalance is reduced by 77 %. The estimated roundtrip efficiency of the optimal-size P2P system coupled with the wind farm was 54 %.
The P2G systems analyzed are composed by three main sections: a hydrogen/syngas production and storage section based on an SOEC stack; a methanation section based on chemical reactors; and an SNG conditioning section for the upgrading of the produced SNG to grid-injection quality. The design and operating conditions of the SOEC section were selected following the results of the analysis performed on the P2P system, and the SNG production section was designed on the basis of a commercial methanation process based on catalytic reactors. The plant efficiency evaluated by simulations was 65.4% for the H2-based P2G and 65.5% for the co-electrolysis based P2G without considering cogeneration or thermal integration between plant sections. Even if the efficiencies were similar for the two P2G configurations, the storage capacity of the H2-based P2G plant was higher, because of the higher operating current achieved by the SOEC stack. The results suggested that even if the co-electrolysis based P2G system presents a slightly higher efficiency, the choice of a H2-based P2G option can ensure a better exploitation of the installed capacity, and also eliminates the risks of carbon-deposition in the stack related to the use of carbon containing mixtures and of stack poisoning related to contaminants potentially present in CO2 streams (e.g., hydrogen sulphide). A case study assessing the effect of H2S poisoning of the SOEC stack on the P2G system performance was also presented.
The results presented in this Thesis demonstrated that hydrogen-based P2P with rSOCs is the most efficient solution for local RES storage among the different SOC-based EES options investigated. The high values of roundtrip efficiency achieved demonstrated the competitiveness of rSOC-based P2P also with other large-scale EES options (PHS, CAES). The hydrogen-based P2P is however constrained to on-site applications due to the lack of a hydrogen transport infrastructure, while P2G solutions offer the possibility of transferring the electricity stored in the SNG form through the existing natural gas infrastructure, and also allow the direct use of SNG in already existing technologies (i.e., for mobility, heating, etc.), providing the technological bridge for transferring RES power to other markets different from the electrical one
RecuperaciĆ³n de hidrĆ³geno de corrientes gaseosas residuales de origen industrial para su aplicaciĆ³n en pilas de combustible
RESUMEN: La necesidad de promover una economĆa circular aprovechando los recursos da lugar a la importancia de la recuperaciĆ³n de corrientes gaseosas residuales que contienen hidrĆ³geno. Por ello, el objetivo global de la tesis es contribuir en la recuperaciĆ³n de mezclas de gases residuales de origen industrial investigando principalmente dos tipos de tecnologĆas de separaciĆ³n de hidrĆ³geno: membranas polimĆ©ricas y procesos de adsorciĆ³n por cambio de presiĆ³n (PSA, por sus siglas en inglĆ©s), con objeto de obtener hidrĆ³geno de elevada pureza. Para conseguir tal objetivo general, la investigaciĆ³n se ha centrado en proporcionar cuestiones metodolĆ³gicas relacionadas con la recuperaciĆ³n de hidrĆ³geno; evaluar el comportamiento de membranas comerciales polimĆ©ricas selectivas de hidrĆ³geno usando mezclas de gases multi-componentes; y producir hidrĆ³geno con la calidad apta para alimentar pilas de combustible a partir de una corriente gaseosa residual de origen industrial mediante un proceso de PSA.ABSTRACT: The need to promote the circular economy by upcycling the resources leads to a great relevance of hydrogen-containing waste gas streams recovery. Thus, the overall objective of this thesis is the contribution to the recovery of hydrogen from industrial waste gas mixtures by investigating two different separation technologies: polymeric membranes and pressure swing adsorption (PSA) processes, in order to obtain high-purity hydrogen. To achieve such general goal, the research has been centered in providing methodological issues to surplus hydrogen recovery; evaluating the performance of commercial hydrogen-selective polymeric membranes using multicomponent gas mixtures; and producing fuel cell grade hydrogen from an industrial waste gas stream via PSA process
California Methanol Assessment; Volume II, Technical Report
A joint effort by the Jet Propulsion Laboratory and the California Institute of Technology Division of Chemistry and Chemical Engineering has brought together sponsors from both the public and private sectors for an analysis of the prospects for methanol use as a fuel in California, primarily for the transportation and stationary application sectors. Increasing optimism in 1982 for a slower rise in oil prices and a more realistic understanding of the costs of methanol production have had a negative effect on methanol viability in the near term (before the year 2000). Methanol was determined to have some promise in the transportation sector, but is not forecasted for large-scale use until beyond the year 2000. Similarly, while alternative use of methanol can have a positive effect on air quality (reducing NOx, SOx, and other emissions), a best case estimate is for less than 4% reduction in peak ozone by 2000 at realistic neat methanol vehicle adoption rates. Methanol is not likely to be a viable fuel in the stationary application sector because it cannot compete economically with conventional fuels except in very limited cases. On the production end, it was determined that methanol produced from natural gas will continue to dominate supply options through the year 2000, and the present and planned industry capacity is somewhat in excess of all projected needs. Nonsubsidized coal-based methanol cannot compete with conventional feedstocks using current technology, but coal-based methanol has promise in the long term (after the year 2000), providing that industry is willing to take the technical and market risks and that government agencies will help facilitate the environment for methanol.
Given that the prospects for viable major markets (stationary applications and neat fuel in passenger cars) are unlikely in the 1980s and early 1990s, the next steps for methanol are in further experimentation and research of production and utilization technologies, expanded use as an octane enhancer, and selected fleet implementation. In the view of the study, it is not advantageous at this time to establish policies within California that attempt to expand methanol use rapidly as a neat fuel for passenger cars or to induce electric utility use of methanol on a widespread basis
A computational approach to flame hole dynamics
Turbulent diffusion flames at low strain rates sustain a spatially continuous flame surface. However, at high strains, which may be localized in a flow or not, the flame can be quenched due to the increased heat loss away from the reaction zone. These quenched regions are sometimes called flame holes. Flame holes reduce the efficiency of combustion, can increase the production of certain pollutants (e.g. carbon monoxide, soot) as well as limit the overall stability of the flame. We present a numerical algorithm for the calculation of the dynamics of flame holes in diffusion flames. The key element is the solution of an evolution equation defined on a general moving surface. The low-dimensional manifold (the surface) can evolve in time and it is defined implicitly as an iso-level set of an associated Cartesian scalar field. An important property of the method described here is that the surface coordinates or parameterization does not need to be determined explicitly; instead, the numerical method employs an embedding technique where the evolution equation is extended to the Cartesian space, where well-known and efficient numerical methods can be used. In our application of this method, the field defined on the surface represents the chemical activity state of a turbulent diffusion flame. We present a formulation that describes the formation, propagation, and growth of flames holes using edge-flame modeling in laminar and turbulent diffusion flames. This problem is solved using a high-order finite-volume WENO method and a new extension algorithm defined in terms of propagation PDEs. The complete algorithm is demonstrated by tracking the dynamics of flame holes in a turbulent reacting shear layer. The method is also implemented in a generalized unstructured low-Mach number fluid solver (Sandia's SIERRA low Mach Module ``Nalu") and applied to simulate local extinction in a piloted jet diffusion flame configuration
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