• Energy Research

Abstract In this paper we present an extension of a stochastic model of groundwater contaminant transport, previously proposed by the authors, to account for the presence of colloids in the system. These have been shown experimentally to facilitate significantly the transport of contaminants. The stochastic model is based on the Kolmogorov–Dmitriev theory of branching stochastic processes and is capable of explicitly accounting for the individual interactions, possibly nonlinear and not at equilibrium, which occur during the transport process. We tackle the problem by means of a Monte Carlo method properly devised to account for the nonlinearities introduced by the presence of colloids. For the determination of the model parameters an approach is followed which is based on a comparison of the stochastic model equations with those of the classical advection-dispersion one. This has enabled us to establish relationships which link the transition rates to the classically measurable quantities.

Conversion efficiency and unit vibration are two important indexes in evaluating the stability of hydraulic generating systems (HGSs). Most of related studies have been carried out in the deterministic theory framework. As running times of HGS increased, understanding uncertainties and limitations of model parameters are important for accurate modeling and stability evaluation. In this study, first, we establish an integrated model of a HGS by proposing unbalanced hydraulic forces based on the Kutta-Zhoukowski assumption. Second, global sensitivity and parametric interactions for conversion efficiency and unit vibration are investigated based on this model. Finally, the novel unified model is verified with two conventional models. This integrated and accurate mathematical model is a major advance in the diagnosis and prediction of failures in hydropower operation.

Vulnerability and robustness are major concerns for future power grids. Malicious attacks and extreme weather conditions have the potential to trigger multiple components outages, cascading failures and large blackouts. Robust contingency identification procedures are necessary to improve power grids resilience and identify critical scenarios. This paper proposes a framework for advanced uncertainty quantification and vulnerability assessment of power grids. The framework allows critical failure scenarios to be identified and overcomes the limitations of current approaches by explicitly considering aleatory and epistemic sources of uncertainty modelled using probability boxes. The different effects of stochastic fluctuation of the power demand, imprecision in power grid parameters and uncertainty in the selection of the vulnerability model have been quantified. Spectral graph metrics for vulnerability are computed using different weights and are compared to power-flow-based cascading indices in ranking N-1 line failures and random N-k lines attacks. A rank correlation test is proposed for further comparison of the vulnerability metrics. The IEEE 24 nodes reliability test power network is selected as a representative case study and a detailed discussion of the results and findings is presented.

Abstract In a previous paper, the authors have presented a newly developed stochastic model of groundwater contaminant transport which is capable of accounting for the contribution of colloids flowing in the porous medium. The model is nonlinear and accounts explicitly for the exchange interactions among contaminants, colloids and the porous matrix, by means of properly introduced transition rates. The physical model is straightforward and intuitive, and it results in a system of differential equations whose dimensionality, in practical cases, is such to entail a Monte Carlo simulation to approach its solution. In this second part of the work, we explore in details the role played by those parameters which characterize the contaminant-colloid and the colloid-solid matrix interactions. A case study of Pu-239 transport inspired by a potential site for high level waste deposit at Mol (Belgium) is considered and the effects of colloid-facilitated transport is investigated under various conditions. Furthermore, an uncertainty analysis is performed to account for the uncertainties which in realistic situations affect the system and the transport phenomena. In this regard, a case study of Pu-239 transport inspired by the test site in Nevada has been considered.

Hydropower has the advantages of quickly responding to load variability, which overcomes the unpredictable and unstable variabilities of solar and wind power. Therefore, such power generation can be combined into a hydro-wind-photovoltaic complementary plant (HWPCP). However, hydropower units running at partial load are prone to suffer from hydraulic instabilities generated by a cavitating vortex rope, which may lead to power swings and high vibrations. Operation in these vibration zones may affect the operation and ultimately cause structural damage and affect the power plant. The problem of avoiding running hydropower units in the vibration zones is effectively addressed in this study. This is achieved by adopting a vibration avoidance strategy to determine a rational power distribution scheme for hydropower units. Multi-objective optimization is performed to maximize power generation, minimize output power fluctuations, and minimize the deviation between the power generation and the planned output. The power distribution strategies of hydropower units under 12 scenarios, composed of different inflow and weather conditions, are analyzed. The results indicate that the vibration avoidance strategy effectively avoids the operation of hydropower units in the vibration zones and ensures the operation of hydropower units in the non-vibration zone for more than 99.31% of the operation time. This study contributes to the identification of the relationship between conflicting objectives and provides operational strategies for the safe and stable operation of hydropower units. Peer Reviewed

This paper addresses the stability of a hydro-turbine governing system under hydraulic excitations. During the operation of a hydro-turbine, water hammer with different intensities occurs frequently, resulting in the stochastic change of the cross-sectional area (A) of the penstock. In this study, we first introduce a stochastic variable u to the cross-sectional area (A) of the penstock related to the intensity of water hammer. Using the Chebyshev polynomial approximation, the stochastic hydro-turbine governing model is simplified to its equivalent deterministic model, by which the dynamic characteristics of the stochastic hydro-turbine governing system can be obtained from numerical experiments. From comparisons based on an operational hydropower station, we verify that the stochastic model is suitable for describing the dynamic behaviors of the hydro-turbine governing system in full-scale applications. We also analyze the change laws of the dynamic variables under increasing stochastic intensity. Moreover, the differential coefficient with different values is used to study the stability of the system, and stability of the hydro-turbine flow with the increasing load disturbance is also presented. Finally, all of the above numerical results supply some basis for modeling efficiently the operation of large hydropower stations.

Coupling energy storage pumps with conventional hydropower plants is one of the most valuable methods to increase the consumption rate of renewable energy. There are few small-scale hybrid power systems configurating energy storage pumps in the world (e.g., Ikaria Island, Greece). However, whether the operating principle and configuration method are feasible and transferable for a large-scale renewable energy base is unclear. This study proposes specific operating principles and configuration method for a large-scale hybrid power system, demonstrating the feasibility by investigating typical scenarios of an engineering case in Qinghai Province, China. Assume the conventional hydropower plants, wind farms, and PV stations in the case are constructed and in operation completely. The configuration relationship between energy storage pump and hydropower is investigated by setting the unit of energy storage pump from 1 to 50, the per-kW investment cost from CNY5000/kW to CNY30000/kW under the constraint of individual capacity of 100 MW. Furthermore, the economic indicators of internal rate of return and dynamic payback period are introduced to evaluate the performance of the whole hybrid power system. The configuration consequence of 18–21 pumps shows a good relation with renewable energy. This study provides theoretical and technical support for planning relevant hybrid power station projects.