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Virginia Tech

Virginia Tech has one of the largest energy harvesting programs in the world, encompassing inorganic and organic photovoltaics, piezoelectrics and thermoelectric. For example, in an Energy Harvesting Workshop there in 2009, Professor Scott Huxtable of Virginia Tech pointed out that traditional thermoelectric theory predicts that miniaturisation can be achieved with thermogenerators without loss of performance (figure of merit X temperature difference). However, experimental results with miniaturised thermoelectrics do not comply with this theory. He therefore defines an effective figure of merit that takes account of interfaces and parasitic conductances and this gives a more appropriate scaleability. Entrance and exit conductances are important. Optimum miniaturised device performance can be achieved with an element length 10-200 micrometers. Experimentation is being carried out on devices made with nanoparticles, nanotubes and pulverise, press, sinter procedures. Thermoreflectance is used for the tricky thermal conductivity measurement. Romny Scientific is involved in this ongoing research.
 
Mohammad Amin Karami of Virginia Tech CIMSS noted that miniaturisation to scale of piezoelectric cantilevers for energy harvesting is also misguided but MEMS versions can be achieved and optimised. He observed that, although capacitive and electrodynamic harvesting of vibration exists, piezoelectric harvesting receives by far the most attention these days. We note that this is despite one if the most widely used vibration harvesters being from Perpetuum in the UK which uses electrodynamics having found piezoelectric vibration harvesting something of a dead end. In his theoretical study, Karami found good correlation with experimental results. Several design recommendations resulted including use of segmented electrodes with appropriate circuitry and the observation that optimum electrical load is extremely sensitive to both the short circuit and open circuit resonance frequency. Piezoelectric constants of bulk materials are not a sufficient guide to their suitability for use in a given miniature piezoelectric harvesting device.
 
Gurpreet Singh of Virginia Tech reported on nanotechnology solutions to sensors and power generation. He employs one dimensional nanostructures.
 
Virginia Tech is partnered with the University of Texas, Clemson University and the University of California San Diego on some of its energy harvesting work.