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 e�ciency 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 Du�n 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 micro uidics, but for which can be shown that the mechanism is radically di�erent. Measurements show conversion e�ciencies from pressure mechnical energy to electrical en- ergy of over 30%. This thesis shows that this e�ciency is limited mainly by viscous friction in the jetting micropore, surface energy formation in jet and droplets and air friction. Further- more a signi�cant 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 signi�cant, (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 e�ects 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 ow rate, droplet size and electric �eld (7) how the electric �eld 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 �eld and (10) how the system e�ciency 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 e�ciency 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 e�ciencies can be obtained by increasing the radius of the pore, where a limiting factor is the charge density that can be induced. Mechanical e�ciencies over 80% (excluding the e�ect 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)
Link to this item:http://purl.utwente.nl/essays/69500
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