A multi-stage down-draft evaporative cool tower for semi-enclosed spaces: Experiments with a water spraying system

A novel down-draft evaporative cool tower (DECT) was developed which incorporates a secondary air inlet and a complex longitudinal section that comprises two partly overlapping cones. The objective of this design is to conserve water while providing equal or superior cooling to conventional DECTs, in which dry ambient air is drawn in through a single inlet at the top and cooler moist air is delivered at the bottom.The complex section and the addition of a secondary air intake near the middle required a sophisticated water spraying system. In addition to maximizing the cooling potential, the spraying system was designed to limit spray drift beyond the base of the tower, to reduce maintenance costs (especially due to clogging of the sprayers) and to minimize pumping energy. Analysis shows that maximum cooling may be obtained either by employing a very fine spray, requiring the introduction of a relatively small volume of water, or by spraying a larger volume of coarser drops. However, spraying fine drops requires more pumping power, finer nozzles are more likely to clog and small drops of water aggravate the problem of drift near the tower base. If full evaporation of the water spray is not required and excess water is collected for reuse, the second option is thus preferable. In addition to the theoretical analysis, the paper presents experimental findings on temperature reduction, water consumption and cooling output of an 8-m high prototype tower constructed at Sede-Boqer, Israel.     

Engineering Carbon Nanotube Forest Superstructure for Robust Thermal Desalination Membranes

Desalination by membrane distillation (MD) using low‐grade or waste heat provides a potential route for sustainable water supply. Nonwetting, porous membranes that provide a selective pathway for water vapor over nonvolatile salt are at the core of MD desalination. Conventional water‐repelling MD membranes (i.e., hydrophobic and superhydrophobic membranes) fail to ensure long‐term desalination performance due to pore wetting and surface fouling. To address these challenges, a defect‐free carbon nanotube forest (CNTF) is engineered in situ on a porous electrospun silica fiber substrate. The engineered CNTF forms an ultrarough and porous interface structure, allowing outstanding wetting resistance against water in air and oil underwater. As a result of this antiwetting property, the composite CNTF membrane displays a stable water vapor flux and a near complete salt rejection (>99.9%) in the desalination of highly saline water containing low surface tension contaminants. The antimicrobial property of the composite CNTF membrane imparted by the unique forest‐like architecture and the oxidative effect of carbon nanotubes (CNTs) are further demonstrated. The results exemplify an effective strategy for engineering CNT architecture to elucidate the structure–property–performance relationship of the nanocomposite membranes and to guide the design of robust thermal desalination membranes.     

Heating experiments with a radiative cooling system

A hybrid space-cooling system for hot arid zones previously investigated by the authors was based on the nocturnal radiative cooling of water circulated through flat plate radiators. Preliminary investigations also indicated that the same system, with no modifications to the physical set-up, could provide a significant proportion of the winter heating requirements of buildings exposed to these climatic conditions, where summers are hot yet winters are frequently cold enough to justify the installation of heating systems.

The heat output of the system averaged 370 W/m² of collector under the sunny but cool conditions typical of Sde-Boker winters. However, on windy and overcast days the thermostat control prevented water circulation and the system was inoperative. The primary factors determining the heat output were the intensity of global solar radiation incident on the collectors, wind speed and the temperature difference between the water in the roof pond and the ambient air. An expression was derived linking these parameters, which may be used to predict the heat output of the specific system with a high degree of accuracy, and thus define the climatic conditions where such a system may be of value.     

Green building standards as socio-technical actors in municipal environmental policy

Methods for the environmental assessment and certification of ‘green’ buildings are increasingly being adopted by local governments and other entities as a means of meeting environmental, and often energy-related, goals. Literature on building environmental assessment has examined how these tools are interpreted by stakeholders in design and construction, but less attention has been paid to their interpretation and usage as objects of policy – despite this being a channel through which they have a potentially huge impact. Based on a case study of the emergence of green building in Israel, and drawing on socio-technical literatures, this paper explores the meanings attributed to building environmental assessment in the policy context. It finds that these meanings include a platform for divergent environmental goals and a proxy measure for greenhouse gas abatement. The analysis suggests that for policy-makers, the significance of green building lies not in its constituting a set of environmental benchmarks but as a standard that gains currency as a black-boxed policy object. An open discussion is needed on the various logics driving the use of green building tools in public policy, and what they hope to achieve.     

Cell type regulation in response to expression of ricin A in Dictyostelium

Expression of ricin A in either prespore or prestalk cells of Dictyostelium discoideum results in cell-autonomous lethality. Strains expressing the toxic gene under the control of a prestalk-specific regulatory region fail to culminate or form stalks, but form spores normally. Strains expressing ricin A under the control of a prespore-specific regulatory region form neither spores nor stalks. Regulation of the cell types results in conversion of prestalk cells to prespore cells when the prespore cells are poisoned. The newly converted cells then express ricin A and die. In contrast, we could not detect any significant conversion of prespore cells to prestalk cells when the prestalk cells are poisoned under our experimental conditions. This regulation of cell types suggests that the tendency of prestalk cells to regulate and become prespore cells is inhibited by the already established prespore cells. It appears that prespore cells control prestalk cell regulation by producing an inhibitor of prespore differentiation to which they themselves are insensitive.     

Scattering of singular beams by subwavelength objects

In recent years, there has been a mounting interest in better methods of measuring nanoscale objects, especially in fields such as nanotechnology, biomedicine, cleantech, and microelectronics. Conventional methods have proved insufficient, due to the classical diffraction limit or slow and complicated measuring procedures. The purpose of this paper is to explore the special characteristics of singular beams with respect to the investigation of subwavelength objects. Singular beams are light beams that contain one or more singularities in their physical parameters, such as phase or polarization. We focus on the three-dimensional interaction between electromagnetic waves and subwavelength objects to extract information about the object from the scattered light patterns.     

The G-JF Thermostat for Accurate Configurational Sampling in Soft-Matter Simulations

We implement the statistically sound G-JF thermostat for Langevin dynamics simulations into the ESPResSo molecular package for large-scale simulations of soft-matter systems. The implemented integration method is tested against the integrator currently used by the molecular package in simulations of a fluid bilayer membrane. While the latter exhibits deviations in the sampling statistics that increase with the integration time step dt, the former reproduces near-correct configurational statistics for all dt within the stability range of the simulations. We conclude that, with very modest revisions to existing codes, one can significantly improve the performance of statistical sampling using Langevin thermostats.     

A multi-stage down-draft evaporative cool tower for semi-enclosed spaces: Aerodynamic performance

A multi-stage down-draft evaporative cool tower (DECT) was developed as an improvement to a previously developed single-stage design. The new tower incorporates a secondary air inlet, added to reduce the water consumption required to produce a desired cooling output in a tower of given maximum cross-section and primary inlet geometry. The secondary air, which may be drawn from the interior space being chilled, is cooled by evaporation in the lower section of the tower. This paper reports on the results of experiments conducted to establish the aerodynamic performance of the design prior to installation of a water spraying system. Design of the water spraying system and experiments on cooling performance are discussed in a companion paper.