A full electrical system study for a large refinery with major expansion is reported. The expansion added two new gas turbine generators (GTG) as well as one new steam turbine generator (STG) to the ...system. A gas-insulated switchgear (GIS) lineup was installed at the 22 kV main substation with a new current limiting reactor to tie the main buses. Four of the substations are new as part of the expansion to increase the total number of substations to fourteen. Due to the new generators, a complete system study to size equipment, adjust protection settings, verify operating conditions and ensure system stability was carried out. The system study scope includes load flow, short circuit, protection and coordination, motor starting, and transient stability. The project lasted and covered the entire construction stages from For Review (FR) and For Design (FD) to For Construction (FC). The detailed system study helped the construction contractor and refinery owners to validate system design, confirm operation procedures and finalize protection schemes under both normal and abnormal conditions. Further, this paper shares experiences gained from performing and manage a complete system study project for a industrial facility in after front-end engineering design (FEED) phase in terms of validating system models, selecting study scenarios, leveraging computer software features, analyzing study results and proposing recommendations.
Understanding the Fundamentals of Empty-Car Routing in Ridesharing Systems
How to efficiently route empty-cars in ridesharing systems? In this paper “Empty-car Routing in Ridesharing Systems,” A. ...Braverman, J.G. Dai, X. Liu, and L. Ying introduce a novel model based on closed queueing networks and propose an optimization framework to optimize empty-car routing for maximizing system-wide utility functions. We propose a fluid-based optimal routing policy by solving the optimization problem in a large market regime. We establish both process-level and steady-state convergence of the closed queueing network to the fluid-limit and prove the optimal network utility obtained from the fluid-based optimization is an upper bound on the utility in the finite car system for any routing policy under which the closed queueing network has a stationary distribution. This upper bound is achieved asymptotically under the fluid-based optimal routing policy.
This paper considers a closed queueing network model of ridesharing systems, such as Didi Chuxing, Lyft, and Uber. We focus on empty-car routing, a mechanism by which we control car flow in the network to optimize system-wide utility functions, for example, the availability of empty cars when a passenger arrives. We establish both process-level and steady-state convergence of the queueing network to a fluid limit in a large market regime where demand for rides and supply of cars tend to infinity and use this limit to study a fluid-based optimization problem. We prove that the optimal network utility obtained from the fluid-based optimization is an upper bound on the utility in the finite car system for any routing policy, both static and dynamic, under which the closed queueing network has a stationary distribution. This upper bound is achieved asymptotically under the fluid-based optimal routing policy. Simulation results with real-world data released by Didi Chuxing demonstrate the benefit of using the fluid-based optimal routing policy compared with various other policies.
Problem definition
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Inpatient beds are usually grouped into several wards, and each ward is assigned to serve patients from certain “primary” specialties. However, when a patient waits excessively ...long before a primary bed becomes available, hospital managers have the option to assign her to a nonprimary bed. although it is undesirable. Deciding when to use such “overflow” is difficult in real time and under uncertainty.
Relevance
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To aid the decision making, we model hospital inpatient flow as a multiclass, multipool parallel-server queueing system and formulate the overflow decision problem as a discrete-time, infinite-horizon average cost Markov decision process (MDP). The MDP incorporates many realistic and important features such as patient arrival and discharge patterns depending on time of day.
Methodology
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To overcome the curse-of-dimensionality of this formulated MDP, we resort to approximate dynamic programming (ADP). A critical part in designing an ADP algorithm is to choose appropriate basis functions to approximate the relative value function. Using a novel combination of fluid control and single-pool approximation, we develop analytical forms to approximate the relative value functions at midnight, which then guides the choice of the basis functions for all other times of day.
Results
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We demonstrate, via numerical experiments in realistic hospital settings, that our proposed ADP algorithm is remarkably effective in finding good overflow policies. These ADP policies can significantly improve system performance over some commonly used overflow strategies—for example, in a baseline scenario, the ADP policy achieves a congestion level similar to that achieved by a complete bed sharing policy, while reduces the overflow proportion by 20%.
Managerial implications
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We quantify the trade-off between the overflow proportion and congestion from implementing ADP policies under a variety of system conditions and generate useful insights. The plotted efficient frontiers allow managers to observe various performance measures in different parameter regimes, and the ADP policies provide managers with operational strategies to achieve the desired performance.
Circulating microRNAs (miRNAs) have been implicated as novel biomarkers for gastric cancer (GC) diagnosis. However, the mixture of GC subtypes may have led to the inconsistent circulating miRNA ...profiles, and the clinical performance of circulating miRNAs has not yet been evaluated independently on early detection of GC.
A four-phase study was designed with a total of 160 cancer-free controls, 124 patients with gastric non-cardia adenocarcinoma (GNCA) and 36 patients diagnosed gastric cardia adenocarcinoma (GCA). In the discovery phase, we screened the miRNA expression profile in plasma of 40 GNCA patients (stage I) and 40 matched controls by TaqMan low density array (TLDA) chips with pooled samples. Differentially expressed miRNAs were further validated in individual sample using quantitative reverse-transcriptase PCR (qRT-PCR) in the training phase. Subsequently, in an independent validation phase, the identified miRNAs were evaluated in 48 GNCA patients (stage I) and 102 matched controls. Finally, the identified miRNAs were further assessed in an external validation phase including advanced GNCA and GCA patients. Additionally, the expression levels of identified miRNAs were measured in the media of BGC823 and MGC803 cell lines.
Five miRNAs (miR-16, miR-25, miR-92a, miR-451 and miR-486-5p) showed consistently elevated levels in plasma of the GC patients as compared with controls, and were identified to be potential markers for GNCA with area under the receiver operating characteristic (ROC) curves (AUCs) ranging from 0.850 to 0.925 and 0.694 to 0.790 in the training and validation phases, respectively. The five-miRNA panel presented a high diagnostic accuracy for the early-stage GNCA (AUCs=0.989 and 0.812 for the training and validation phases, respectively). Three miRNAs (miR-16, miR-25 and miR-92a) were excreted into the culture media of GC cell lines.
The five-miRNA panel in plasma may serve as a potential non-invasive biomarker in detecting the early-stage GC.
Summary
Biodiversity is responsible for the provision of many ecosystem services; human well‐being is based on these services, and consequently on biodiversity. In soil, earthworms represent the ...largest component of the animal biomass and are commonly termed ‘ecosystem engineers’. This review considers the contribution of earthworms to ecosystem services through pedogenesis, development of soil structure, water regulation, nutrient cycling, primary production, climate regulation, pollution remediation and cultural services. Although there has been much research into the role of earthworms in soil ecology, this review demonstrates substantial gaps in our knowledge related in particular to difficulties in identifying the effects of species, land use and climate. The review aims to assist people involved in all aspects of land management, including conservation, agriculture, mining or other industries, to obtain a broad knowledge of earthworms and ecosystem services.
Hemifacial microsomia (HFM) is a common congenital malformation of the craniofacial region. There are 3 possible pathogenic models of HFM—vascular abnormality and hemorrhage in the craniofacial ...region, damage to Meckel’s cartilage, and the abnormal development of cranial neural crest cells—and the most plausible hypothesis is the vascular abnormality and hemorrhage model. These 3 models are interrelated, and none of them is completely concordant with all the variable manifestations of HFM. External environmental factors (e.g., thalidomide, triazene, retinoic acid, and vasoactive medications), maternal intrinsic factors (e.g., maternal diabetes), and genetic factors (e.g., the recently reported mutations in OTX2, PLCD3, and MYT1) may lead to HFM through ≥1 of these pathogenic processes. Whole genome sequencing to identify additional pathogenic variants, biological functional studies to understand the exact molecular mechanisms, and additional animal model and clinical studies with large stratified samples to elucidate the pathogenesis of HFM will be necessary. Small-molecule drugs, as well as CRISPR/CAS9-based genetic interventions, for the prevention and treatment of HFM may also be a future research hotspot.
NZEB (Net zero energy building) is regarded as an integrated solution to address problems of energy-saving, environmental protection, and CO2 emission reduction in the building section. NZEB could be ...even possible with electricity production if enough renewable energy could be used. Moreover, various building-service systems with renewable energy sources have been widely considered for potential applications in NZEB. All of these new features extend the technical boundary of the conventional energy-efficient buildings, attach a more profound implication to the sustainable development of building technology, and therefore pose a challenge to evaluation works on NZEB performance.
This paper presents a guided tour on NZEB evaluation through literature-research. An overview about definitions and energy-efficient measures of NZEB is presented so that the research object and technology boundary can be clarified for NZEB evaluation. Then, a summary of widely-used research method, tool and performance indicator in evaluation is provided for the methodology part. This part also includes a discussion on the application of LCA (life cycle assessment) in NZEB evaluation and LCA's role in promoting a well-defined NZEB. Finally, potential progress in NZEB evaluation with possible development trends is highlighted in terms of energy storage, load match and smart grid.
•Definitions and typical energy-efficient measures of net zero energy building are reviewed.•Research methods, tools and performance indicators for evaluation methodology are summarized.•Possible development trends of net zero energy building are discussed.
One key factor contributing to emergency department (ED) overcrowding is prolonged waiting time for admission to inpatient wards, also known as ED boarding time. To gain insights into reducing this ...waiting time, we study operations in the inpatient wards and their interface with the ED. We focus on understanding the effect of inpatient discharge policies and other operational policies on the time-of-day waiting time performance, such as the fraction of patients waiting longer than six hours in the ED before being admitted. Based on an empirical study at a Singaporean hospital, we propose a novel stochastic processing network with the following characteristics to model inpatient operations: (1) A patient's service time in the inpatient wards depends on that patient's admission and discharge times and length of stay. The service times capture a two-time-scale phenomenon and are not independent and identically distributed. (2) Pre-and post-allocation delays model the extra amount of waiting caused by secondary bottlenecks other than bed unavailability, such as nurse shortage. (3) Patients waiting for a bed can overflow to a nonprimary ward when the waiting time reaches a threshold, where the threshold is time dependent. We show, via simulation studies, that our model is able to capture the inpatient flow dynamics at hourly resolution and can evaluate the impact of operational policies on both the daily and time-of-day waiting time performance. In particular, our model predicts that implementing a hypothetical policy can eliminate excessive waiting for those patients who request beds in mornings. This policy incorporates the following components: a discharge distribution with the first discharge peak between 8 A.M. and 9 A.M. and 26% of patients discharging before noon, and constant-mean allocation delays throughout the day. The insights gained from our model can help hospital managers to choose among different policies to implement depending on the choice of objective, such as to reduce the peak waiting in the morning or to reduce daily waiting time statistics.
Much like higher-order Taylor expansions allow one to approximate functions to a higher degree of accuracy, we demonstrate that, by accounting for higher-order terms in the Taylor expansion of a ...Markov process generator, one can derive novel diffusion approximations that achieve a higher degree of accuracy compared with the classical ones used in the literature over the last 50 years.
We derive and analyze new diffusion approximations of stationary distributions of Markov chains that are based on second- and higher-order terms in the expansion of the Markov chain generator. Our approximations achieve a higher degree of accuracy compared with diffusion approximations widely used for the last 50 years while retaining a similar computational complexity. To support our approximations, we present a combination of theoretical and numerical results across three different models. Our approximations are derived recursively through Stein/Poisson equations, and the theoretical results are proved using Stein’s method.
Funding:
X. Fang is partially supported by Hong Kong RGC Grants 24301617, 14302418, and 14304917, a CUHK direct grant, and a CUHK start-up grant. J. G. Dai is partially supported by NSF Grant CMMI-1537795.
Supplemental Material:
The e-companion is available at
https://doi.org/10.1287/opre.2022.2362
.
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