POLYMER-ENHANCED ENERGY HARVESTING FROM STREAMING
POTENTIAL
T. Nguyen, Y. Xie, L. J. de Vreede, A. van den Berg, J.C.T. Eijkel
BIOS lab on chip group, MESA+ Institution of Nanotechnology, University of Twente, the Netherlands.
ABSTRACT
In this contribution, we present the experimental results of energy conversion from the streaming potential when a
polymer, polyacrylic acid (PAA) with concentration from 200 ppm to 4000 ppm in background electrolyte KCl solution
was used as the working fluid. The results show that when PAA was added in KCl 0.01 mM solution, the energy
conversion efficiency of the system was enhanced a factor of 447 as compared to the case without polymer. An
enhancement factor of 257 was also observed when PAA was in the higher ionic strength background solution, KCl 1
mM. These are the first experimental demonstrations of this effect.
KEYWORDS
Energy conversion, efficiency, streaming potential, polymer.
INTRODUCTION
Energy harvesting from the streaming potential is based on the electrokinetic phenomena which are associated with
interfacial charges. In general, every surface obtains a surface electrical charge when brought into contact with a polar
medium. These interfacial charges, in turn, influence the ion distribution in the polar medium and lead to the formation of
the electrical double layer (EDL). The streaming potential is generated by pressure-driven transport of the net charged
liquid in the EDL. The main goal of researchers in the field is to increase the energy conversion efficiency (Eff) of the
systems.
The Eff equals the ratio of (electrical) output power (P
out
) and (hydrodynamic) input power (P
in
):
Eff =
P
out
(electrical)
P
in
(hydrodynamic)
(1)
Recently, Berli et al. [1] predicted theoretically that addition of polymer to the working fluid in a microfluidic channel
can enhance the Eff. However, this prediction has not yet been investigated experimentally.
When non-adsorption polymers are introduced into a microchannel, depletion layers near the channel walls are
formed due to the repulsive force between polymer chains and the walls, which is of entropic origin (Fig. 1a). The
thickness of these layers () will be approximately equal to the radius of gyration of the polymers (R
g
). This results in
two different viscosity zones in the channel, one of low viscosity (
s
) within and the other of high viscosity (
p
) outside
the polymer depletion layers. [2] Figure 1b shows the approximate predicted velocity profile of the fluid flow for
polymer solutions (red curve) and for normal viscosity electrolyte solutions (black curve). The decrease of the bulk
velocity on polymer addition will decrease the hydrodynamic input power P
in
. Because the thickness of the depletion
layers can be varied from a few tens to hundreds of nanometers depending on the polymers of choice, it can be made
larger than the thickness of the EDL, so that the transport of charge and hence P
out
remain unaffected. Thus, one can gain
Eff by reducing the volumetric flow rate in the bulk liquid without affecting the electrokinetic phenomenon which
happens only inside the EDL (Fig1. b).
In this paper, we present for the first time the results from experiments of energy conversion from streaming potential
when polymer solutions were used as working fluids, and show that the energy conversion efficiency can be strongly
enhanced.
Figure 1: (a) a scheme of depletion layers when polymer is added to the working fluid. (b) predicted velocity profile of
the fluid flow for polymer solutions, (red curve) and for normal electrolyte solutions, (black curve)
Depletion layer (free of polymers)
Depletion layer (free of polymers)
(a) (b)
16th International Conference on
Miniaturized Systems for Chemistry and Life Sciences
October 28 - November 1, 2012, Okinawa, Japan 978-0-9798064-5-2/μTAS 2012/$20©12CBMS-0001
1987