Thermo/Piezo-Electrics (T-PEGs)

 

A major thrust in our group is the search for cost effective, efficient ways of utilizing Thermoelectric and Thermo/PV systems. The approach we have taken is to examine thermal energy scavenging opportunities in organic matrix nanocomposite materials. Though this approach is bound to be less efficient than say BiTe in thermoelectrics or the novel thermal fluids used in T/PV, it has the cost advantage of manufacturability in large sheets, and simple direct applique form factors.


The "Power Fabrics" project has several partners:

    International Thermodyne Charlotte NC

    NanotechLabs Inc. Yadkinville NC

    Sineurop Inc. Stuttgart Germany


Ultimately the TEP materials developed will be integrated into TEP devices for structural components of buildings and transportation as well as in PV systems.


Thermoelectric Power:


TEP is the generation of a potential difference across a material in which there is a temperature gradient. It is described by the Seebeck Coefficient which is the ratio of the voltage across the material to the temperature difference:

 

S = - dV/dT


+S represents majority hole carriers, -S majority electron. As carriers are heated due to T(hot) they diffuse to the opposite end of the material while their immobile opposite carriers are left behind. This in turn creates an E field. Equilibrium is then reached between hot carriers traveling to T(cold) and cold carriers returning due to the effect of E. The voltage generated can then be measured with a voltmeter.

The figure of merit FOM, typically used to compare thermoelectric devices/materials is given by:





Where s is the electric conductivity, S the Seebeck coefficient, and k the thermal conductivity. Usually this is expressed as ZT where Z is multiplied by the average temperature of operation T. ZT = 1 is good, while ZT ~ 2-3 is the best reported. We would like to get to ZT > 4.


We measure S and ZT using a home built system that allows temperature control down to 20K and up to 400K shown right.


Thermopower and Heterogeneity:


The systems we study are CNT meshes embedded into electroactive polymers.  Here is some typical data taken from a SWNT mat:























 

CarrollResearch © 2016

courtesy NYTimes:Ken Bennett Photographer

The focus of our power “scavenging” development is a meta-material platform with the form factor of a fabric and the ability to convert both kinetic and thermal energy into electricity.  Moreover the intrinsic coupling of the kinetic energy scavenging with the thermal energy scavenging yields a synergistic effect which we have termed T-PEG (Thermo - Piezo electric power generation).

Carbon Nanotubes

Matrix composites of Carbon Nanotubes are one approach to damping thermal conductivity while maintaining electrical conductivity.  In heterogenous systems such as this, interactions with the host matrix are rarely considered, but we have found they can make a big difference.