Auxetic structure for increased power output of strain vibration energy harvester (article)

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordAll data created during this research are in ORE at: https://doi.org/10.24378/exe.703This paper develops an auxetic (negative Poisson’s ratio) piezoelectric energy harvester (APEH) to increase the power output when harnessing strain energy. The APEH consists of a piezoelectric element bonded to an auxetic substrate. The auxetic substrate concentrates the stress and strain into the piezoelectric element’s region and introduces auxetic behaviour in the piezoelectric element, both of which increase the electric power output. A finite element model was developed to optimise the design and verify the mechanism of the power increase. Three APEHs were manufactured and characterised. Their performance was compared with two equivalent strain energy harvesters with plain substrates. Experimental results show that the APEHs, excited by sinusoidal strains peak to-peak of 250 με at 10 Hz, are able to produce electric power of up to 191.1 µW, which is 14.4 times of the peak power produced by the plain harvesters (13.4 µW). The power gain factor is constant between samples as the amplitude and frequency of their applied strains are varied. The model and experimental results are in good agreement, once accounting for the imperfect bonding of the epoxy using the spring constant of the Thin Elastic Layers on the modelled epoxy surfaces.We acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1

Cold gas accretion in galaxies

Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: 1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by HI cloud complexes, tails and filaments. It may be regarded as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material infall as the stellar streams observed in the halos of our galaxy and M31. 2) Considerable amounts of extra-planar HI have been found in nearby spiral galaxies. While a large fraction of this gas is produced by galactic fountains, it is likely that a part of it is of extragalactic origin. 3) Spirals are known to have extended and warped outer layers of HI. It is not clear how these have formed, and how and for how long the warps can be sustained. Gas infall has been proposed as the origin. 4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean ``visible'' accretion rate of cold gas in galaxies of at least 0.2 Msol/yr. In order to reach the accretion rates needed to sustain the observed star formation (~1 Msol/yr), additional infall of large amounts of gas from the IGM seems to be required.Comment: To appear in Astronomy & Astrophysics Reviews. 34 pages. Full-resolution version available at http://www.astron.nl/~oosterlo/accretionRevie

Shedding Light on the Galaxy Luminosity Function

From as early as the 1930s, astronomers have tried to quantify the statistical nature of the evolution and large-scale structure of galaxies by studying their luminosity distribution as a function of redshift - known as the galaxy luminosity function (LF). Accurately constructing the LF remains a popular and yet tricky pursuit in modern observational cosmology where the presence of observational selection effects due to e.g. detection thresholds in apparent magnitude, colour, surface brightness or some combination thereof can render any given galaxy survey incomplete and thus introduce bias into the LF. Over the last seventy years there have been numerous sophisticated statistical approaches devised to tackle these issues; all have advantages -- but not one is perfect. This review takes a broad historical look at the key statistical tools that have been developed over this period, discussing their relative merits and highlighting any significant extensions and modifications. In addition, the more generalised methods that have emerged within the last few years are examined. These methods propose a more rigorous statistical framework within which to determine the LF compared to some of the more traditional methods. I also look at how photometric redshift estimations are being incorporated into the LF methodology as well as considering the construction of bivariate LFs. Finally, I review the ongoing development of completeness estimators which test some of the fundamental assumptions going into LF estimators and can be powerful probes of any residual systematic effects inherent magnitude-redshift data.Comment: 95 pages, 23 figures, 3 tables. Now published in The Astronomy & Astrophysics Review. This version: bring in line with A&AR format requirements, also minor typo corrections made, additional citations and higher rez images adde

Energy Harvesting in Smart Cities

This is the author accepted manuscript. The final version is available from Springer via the DOI in this recordSmart cities rely on a plethora of sensors at various locations in urban environments to collect data so that the living standard can be improved using the information gathered. Wired sensors that have limited flexibility and high installation costs are less attractive. Wireless solutions are flexible and low cost to install but their requirement of regular battery replacement introduces a high maintenance cost. By endowing wireless sensors with energy harvesting capabilities to harvest energy from the environment such that they can be energy self-sufficient, the maintenance cost associated with battery replacement can be eliminated, which is a more sustainable and environmentally friendly approach for the realization of smart cities. This chapter reviews the core elements of an energy harvesting powered wireless sensor system, from the energy harvesters that harvest the energy to the power management circuits that convert the harvested energy into a form that is usable by the wireless sensors, and finally the wireless sensors that collect and transmit data. Kinetic energy is abundant in urban environments due to the dynamism in cities that comes from high human activities. Therefore, this chapter focuses on kinetic energy sources that are available in urban environments and their associated energy harvesters. For the power management circuit, particular attention will be on its key subsystems to achieve a high performance circuit. Finally, the features that make a wireless communication technology suitable for the applications of smart cities with the available energy sources will be reviewed with some example applications given