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Solar and Thermal Energy Harvesting Textile Composites for Aerospace Applications

Energy harvesting devices in the form of fibers could be woven into lightweight, strong textiles for integration with structural composites.

The proposed research focuses on developing novel energy harvesting devices that can be integrated with loadbearing structures in an air vehicle (e.g. a UAV). Several ambient energy sources are available on a UAV: light, heat, and vibration. The amount of energy available from light and heat exceeds that in vibration, so this work focuses on the first two modes of harvesting.

In the Photovoltaic Fiber Structure, very thin active organic layers and metallic electrodes are deposited concentrically around a fiber core, and light is coupled in through the outer electrode.
The approach is to create energy harvesting devices in the form of long fibers that eventually could be woven into lightweight, high-strength, multifunctional textiles for seamless integration with aerospace structural composites. The fiber form factor is a powerful paradigm for these energy conversion devices, since it can lead to improved light trapping in the organic photovoltaic (PV) cells, and allow for a high density of thermocouple junctions without the use of costly patterning techniques, significantly enhancing the cost-benefit performance.

The initial focus was on modeling and experimentally demonstrating prototype devices consisting of single fibers capable of the thermoelectric (TE) and PV modes of energy conversion. The results obtained were highly encouraging, and have opened up several exciting new research directions. In a solar cell geometry, the active organic layers and metallic electrodes are formed concentrically around a fiber core, and light is coupled in through the outer electrode. This structure is quite different from the conventional planar PV cells, and requires special considerations in its design and for predicting its optoelectronic performance.

Fresh advances in modeling OPV devices on fibers include the application of multilayer dielectric coatings to fiber bundles. This architecture maximizes light in-coupling in individual fibers, and takes advantage of photon recycling in multi-fiber arrays. The modeling combines ray-tracing and transfer-matrix simulations at multiple length scales. Each component of the model has been independently validated by experiments.

Improved power conversion efficiency of planar OPV cells was demonstrated using a metal-organic-metal layer structure. Importantly, these devices now match the efficiency of conventional ITO-based cells, which were improved. The ITO-free device exhibits a slightly lower short circuit current density (JSC), but compensates with a higher open circuit voltage (VOC). Further analysis of how JSC varies with anode thickness reveals that the device performs unexpectedly better than the far-field transmittance of the anode would suggest. The enhanced performance is due to the microcavity effects dominating the thin-film OPV cell, in which the far-field optical transmission of the electrode is less important than its ability to place the antinode of the optical field close to the donor-acceptor junction in the organic layers. Detailed optical modeling enables mapping of the performance of a wide range of electrode materials, and predicts that silver is not far from the conventionally employed ITO with respect to the JSC values it can allow.

Conversion of heat to electricity (thermoelectric generation) can be accomplished by connecting two dissimilar materials (metals or semiconductors) in a series of junctions, and sandwiching the junctions between a hot source and a cold sink. The voltage produced by the junction is proportional to the temperature gradient between the hot and cold sides. The conventional series-connected junction geometry can be reproduced in the form of thin-film segments deposited along fibers. Weaving these fibers can position the junctions as required for power generation. The TE generator is optimized by maximizing the temperature gradient, minimizing the thermal conductivity, and maximizing the Seebeck coefficient and electrical conductivity.

Woven thermoelectric generators have been demonstrated utilizing several TE fibers at once. Several fiber diameters have been explored, varying also the TE segment length and weave density, and spanning square inches. For smaller fibers, increased weave density, and greater temperature gradients, the power density increases dramatically. The thinness and flexibility of these mats suggests that multilayer TE fabrics can be used to efficiently span temperature gradients using individual layers tuned to work at their maximum ZT point.

This work was done by Max Shtein and Kevin Pipe of the University of Michigan, and Peter Peumans of Stanford University for the Air Force Office of Scientific Research. AFOSR-0004

Topics:
Aircraft Coatings, colorants and finishes Coatings, colorants, and finishes Composite materials Conductivity Fabrics Fibers Materials Materials properties Metals Semiconductors Simulation and modeling Textiles Unmanned aerial vehicles Vehicles
This Brief includes a Technical Support Package (TSP).
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Solar and Thermal Energy Harvesting Textile Composites for Aerospace Applications

(reference AFOSR-0004) is currently available for download from the TSP library.

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    Defense Tech Briefs Magazine

    This article first appeared in the December, 2012 issue of Defense Tech Briefs Magazine (Vol. 6 No. 6).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document outlines a research project focused on developing fiber-based energy harvesting devices that convert light and heat into electricity, specifically for integration into aerospace structures, such as unmanned aerial vehicles (UAVs). Conducted by a team from the University of Michigan, the project spans from January 2007 to September 2009 and aims to enhance the efficiency and functionality of photovoltaic (PV) and thermoelectric (TE) devices.

    In the first year, the research team designed and demonstrated prototype PV and TE devices on polymer fibers. The initial focus was on modeling and experimental validation of these energy conversion technologies. The results from the first year laid the groundwork for further advancements in the subsequent years.

    By the second year, significant improvements were achieved in the performance of the energy harvesting devices. The team demonstrated individual solar cells made from molecular organic compounds, which were deposited onto long fibers. These PV fibers showed comparable performance to traditional planar solar cells, with the added advantage of enhanced light trapping capabilities. This feature is particularly beneficial for UAV applications, where the angle of sunlight varies during flight. The research also explored methods to improve the efficiency of optical in-coupling in thin-film organic PV cells.

    In the third year, the team achieved a notable milestone by developing ITO-free, vacuum-deposited, planar, small-molecular organic solar cells that exhibited double the power conversion efficiency of previous devices, reaching approximately 2%. This advancement represents a significant step forward in the quest for more efficient energy harvesting technologies.

    The project also emphasized the potential for integrating these energy harvesting fibers into lightweight, high-strength multifunctional textiles, which could be woven into structural composites for aerospace applications. The fiber form factor allows for improved light trapping and a higher density of thermocouple junctions, enhancing the overall cost-benefit performance of the devices.

    Overall, the research demonstrates promising advancements in the field of energy harvesting, with the potential to revolutionize how energy is captured and utilized in aerospace applications. The findings have opened new research directions and have led to plans for further development, including the possibility of creating a start-up company focused on multifunctional energy conversion devices.

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