University of Twente Student Theses


Optimizing a liquid-based energy conversion system

Bos, H.D. (2013) Optimizing a liquid-based energy conversion system.

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Abstract:In this thesis a theoretical multidisciplinary framework to describe and optimize the efficiency of a liquid-based energy conversion system is proposed. The energy conversion system that is described was developed by Yanbo Xie, who developed and improved a system related to the work of Duffin and Saykally in 2008. The system consists of a jetting micropore, that accelerates uid holding a net charge to high velocity and then converts the velocity to electric energy. We will describe this technique as 'ballistic electrokinetic energy conversion' which is derived from streaming potential in microfluidics, but for which can be shown that the mechanism is radically different. Measurements show conversion efficiencies from pressure mechnical energy to electrical energy of over 30%. This thesis shows that this efficiency is limited mainly by viscous friction in the jetting micropore, surface energy formation in jet and droplets and air friction. Furthermore a significant loss fraction is attributed to the unequally spread velocity of the droplets, causing non-optimal harvesting of the energy. We show (1) how the working principle of the system can be explained, (2) what loss factors are present and which ones are significant, (3) how measurements can be performed to evaluate the behaviour of the system, (4) how viscous friction in the pore behaves as function of pressure and pore radius, (5) the effects and losses attributed to surface energy formation (6) how air friction on a stream of microdroplets in a large volume of air behaves as function of flow rate, droplet size and electric field (7) how the electric field and loading resistance can be optimally tuned to harvest all energy (8) what limitations are caused by the breakdown characteristics of air, (9) how induction of extra charge can be described and used to lower the required electric field and (10) how the system efficiency is expected to behave when tuning the parameters pore-target distance, pore radius and applied pressure. Viscous friction in the micropore, surface creation, air friction and velocity dispersion are expected to cause losses of approximately 30%, 20%, 20% and 10% of the original input power for the current system operating at 1.4bar, 15mm distance and with 5µm pore radius. The thus predicted theoretical efficiency of 20% is slightly lower than measured values, most likely caused by inaccuracies in the modelling air friction in the initial millimetres of the air trajectory and of viscous friction. From the developed model we predict that much higher efficiencies can be obtained by increasing the radius of the pore, where a limiting factor is the charge density that can be induced. Mechanical efficiencies over 80% (excluding the effect of velocity spreading) are predicted for 15µm pore radii, if the device is not limited by the charge density.
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:53 electrotechnology
Programme:Electrical Engineering MSc (60353)
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