823 research outputs found
The role of biomass in the renewable energy system
Europe is striving for zero carbon electricity production by 2050 in order to avoid dangerous climate change. To meet this target a large variety of options is being explored. Biomass is such an option and should be given serious consideration. In this paper the potential role of biomass in a NW-European electricity mix is analyzed. The situation in NW-Europe is unique since it is a region which is a fore runner in renewable technology promotion but also an area with little sun, almost no potential for hydro and a lot of wind. This will result in a substantial need for non-intermittent low-carbon options such as biomass. The benefits and issues related to biomass are discussed in detail from both an environmental and an economic perspective. The former will focus on the life cycle of a biomass pellet supply chain, from the growth of the trees down to the burning of the pellets on site. The latter will provide detailed insights on the levelized cost of electricity for biomass and the role of biomass as a grid stabilizer in high intermittent scenarios. During the discussion, biomass will be compared to other competing electricity technologies to have a full understanding of its advantages and drawbacks. We find that biomass can play a very important role in the future low carbon electricity mix, the main bottleneck being the supply of large amounts of sustainably produced feedstock
Rotational dynamics of entangled polymers
Some recent results on the rotational dynamics of polymers are reviewed and
extended. We focus here on the relaxation of a polymer, either flexible or
semiflexible, initially wrapped around a rigid rod. We also study the steady
polymer rotation generated by a constant torque on the rod. The interplay of
frictional and entropic forces leads to a complex dynamical behavior
characterized by non-trivial universal exponents. The results are based on
extensive simulations of polymers undergoing Rouse dynamics and on an
analytical approach using force balance and scaling arguments. The analytical
results are in general in good agreement with the simulations, showing how a
simplified approach can correctly capture the complex dynamical behavior of
rotating polymers.Comment: 13 pages; 7 figures; proceedings of the International Workshop on
"Brownian Motion in Confined Geometries", Max Planck Institute for the
Physics of Complex Systems in Dresden from 17 - 21 March 2014; to appear in
EPJ-S
Torque-Induced Rotational Dynamics in Polymers: Torsional Blobs and Thinning
By using the blob theory and computer simulations, we investigate the
properties of a linear polymer performing a stationary rotational motion around
a long impenetrable rod. In particular, in the simulations the rotation is
induced by a torque applied to the end of the polymer that is tethered to the
rod. Three different regimes are found, in close analogy with the case of
polymers pulled by a constant force at one end. For low torques the polymer
rotates maintaining its equilibrium conformation. At intermediate torques the
polymer assumes a trumpet shape, being composed by blobs of increasing size. At
even larger torques the polymer is partially wrapped around the rod. We derive
several scaling relations between various quantities as angular velocity,
elongation and torque. The analytical predictions match the simulation data
well. Interestingly, we find a "thinning" regime where the torque has a very
weak (logarithmic) dependence on the angular velocity. We discuss the origin of
this behavior, which has no counterpart in polymers pulled by an applied force.Comment: 30 pages, 8 figures, 1 TOC figure; video abstract at
https://youtu.be/LwicoSkh3m
Flow competition between hepatic arterial and portal venous flow during hypothermic machine perfusion preservation of porcine livers
Hypothermic machine perfusion (HMP) is regarded as a better preservation method for donor livers than cold storage. During HMP, livers are perfused through the inlet blood vessels, namely the hepatic artery (HA) and the portal vein (PO. In previous HMP feasibility studies of porcine and human livers, we observed that the PV flow decreased while the HA flow increased. This flow competition restored either spontaneously or by lowering the HA pressure (P-HA). Since this phenomenon had never been observed before and because it affects the HMP stability, it is essential to gain more insight into the determinants of flow competition. To this end, we investigated the influence of the HMP boundary conditions on liver flows during controlled experiments. This paper presents the flow effects induced by increasing P-HA and by obstructing the outlet blood vessel, which is the vena cava inferior (VCI).
Flow competition was evoked by increasing P-HA to 55-70 mmHg, as well as by obstructing the VCI. Remarkably, a severe obstruction resulted in a repetitive and alternating tradeoff between the HA and PV flows. These phenomena could be related to intra-sinusoidal pressure alterations. Consequently, a higher P-HA is most likely transmitted to the sinusoidal level. This increased sinusoidal pressure reduces the pressure drop between the PV and the sinusoids, leading to a decreased PV perfusion. Flow competition has not been encountered or evoked under physiological conditions and should be taken into account for the design of liver HMP protocols. Nevertheless, more research is necessary to determine the optimal parameters for stable HMP
Analyzing the human liver vascular architecture by combining vascular corrosion casting and micro-CT scanning: a feasibility study
Although a full understanding of the hepatic circulation is one of the keys to successfully perform liver surgery and to elucidate liver pathology, relatively little is known about the functional organization of the liver vasculature. Therefore, we materialized and visualized the human hepatic vasculature at different scales, and performed a morphological analysis by combining vascular corrosion casting with novel micro-computer tomography (CT) and image analysis techniques. A human liver vascular corrosion cast was obtained by simultaneous resin injection in the hepatic artery (HA) and portal vein (PV). A high resolution (110 mu m) micro-CT scan of the total cast allowed gathering detailed macrovascular data. Subsequently, a mesocirculation sample (starting at generation 5; 88 x 68 x 80 mm(3)) and a microcirculation sample (terminal vessels including sinusoids; 2.0 x 1.5 x 1.7 mm(3)) were dissected and imaged at a 71-mu m and 2.6-mu m resolution, respectively. Segmentations and 3D reconstructions allowed quantifying the macro- and mesoscale branching topology, and geometrical features of HA, PV and hepatic venous trees up to 13 generations (radii ranging from 13.2 mm to 80 mu m; lengths from 74.4 mm to 0.74 mm), as well as microvascular characteristics (mean sinusoidal radius of 6.63 mu m). Combining corrosion casting and micro-CT imaging allows quantifying the branching topology and geometrical features of hepatic trees using a multiscale approach from the macro- down to the microcirculation. This may lead to novel insights into liver circulation, such as internal blood flow distributions and anatomical consequences of pathologies (e.g. cirrhosis)
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