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Enhanced efficiency of solar-driven thermoelectric generator with femtosecond lasertextured metals Taek Yong Hwang, A. Y. Vorobyev, and Chunlei Guo* 1

The Institute of Optics, University of Rochester, Rochester, New York 14627, USA * [email protected]

Abstract: Through femtosecond laser irradiation, we produce in this work a unique type of surface nanostructure on Al that have enhanced absorption at UV and visible but a relatively small emissivity in infrared. By integrating this laser-treated Al to a solar-driven thermoelectric generator, we show that the thermoelectric generator integrated with the femtosecond laser-treated Al foil generates a significantly higher power than the ones without. Our study shows that our technique can dramatically enhance the efficiency of solar-driven thermoelectric devices that may lead to a leap forward in solar energy harnessing. ©2011 Optical Society of America OCIS codes: (320.2250) Femtosecond phenomena; (350.3390) Laser materials processing; (350.6050) Solar energy.

References and links 1. 2. 3. 4. 5. 6. 7.

M. Xie and D. M. Gruen, “Potential Impact of ZT = 4 Thermoelectric Materials on Solar Thermal Energy Conversion Technologies,” J. Phys. Chem. B 114(45), 14339–14342 (2010). A. I. Hochbaum, R. Chen, R. D. Delgado, W. Liang, E. C. Garnett, M. Najarian, A. Majumdar, and P. Yang, “Enhanced thermoelectric performance of rough silicon nanowires,” Nature 451(7175), 163–167 (2008). T. C. Kandpal, A. K. Singhal, and S. S. Mathur, “Optimum power from a solar thermal power plant using solar concentrators,” Energy Convers. Manage. 23(2), 103–106 (1983). S. A. Omer and D. G. Infield, “Design optimization of thermoelectric devices for solar power generation,” Sol. Energy Mater. Sol. Cells 53(1-2), 67–82 (1998). F. J. DiSalvo, “Thermoelectric cooling and power generation,” Science 285(5428), 703–706 (1999). C. G. Granqvist, “Solar Energy Materials,” Adv. Mater. (Deerfield Beach Fla.) 15(21), 1789–1803 (2003). A. Reja and R. J. Ram, “Solar Thermoelectric Generator for micro-Power Applications,” in Optics and Photonics for Advanced Energy Technology, OSA Technical Digest (CD) (Optical Society of America, 2009), ThC11. http://www.opticsinfobase.org/abstract.cfm?URI=Energy-2009-ThC11

A. I. Boukai, Y. Bunimovich, J. Tahir-Kheli, J.-K. Yu, W. A. Goddard 3rd, and J. R. Heath, “Silicon nanowires as efficient thermoelectric materials,” Nature 451(7175), 168–171 (2008). 9. R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O’Quinn, “Thin-film thermoelectric devices with high room-temperature figures of merit,” Nature 413(6856), 597–602 (2001). 10. J. Chen, “Thermodynamic analysis of a solar-driven thermoelectric generator,” J. Appl. Phys. 79(5), 2717–2721 (1996). 11. G. S. Nolas, J. L. Cohn, G. A. Slack, and S. B. Schujman, “Semiconducting Ge clathrates: Promising candidates for thermoelectric applications,” Appl. Phys. Lett. 73(2), 178–180 (1998). 12. A. Y. Vorobyev and C. Guo, “Solar Absorber Surfaces Treated by Femtosecond Laser,” in 2010 International Conference on Biosciences (BIOSCIENCESWORLD), 2010), 135–138. 13. A. Y. Vorobyev, V. S. Makin, and C. Guo, “Brighter light sources from black metal: significant increase in emission efficiency of incandescent light sources,” Phys. Rev. Lett. 102(23), 234301 (2009). 14. A. Y. Vorobyev and C. Guo, “Colorizing metals with femtosecond laser pulses,” Appl. Phys. Lett. 92(4), 041914–041913 (2008). 15. J. J. Brophy, Basic electronics for scientist, 5th ed. (McGraw-Hill Publishing Company, Singapore, 1990). 16. M. Cobble, “Calculations of Generator Performance,” in CRC Handbook of Thermoelectrics (CRC Press, 1995). 17. F. Merola, L. Miccio, S. Coppola, V. Vespini, M. Paturzo, S. Grilli, and P. Ferraro, “Exploring the capabilities of Digital Holography as tool for testing optical microstructures,” 3D Research. 2, 1–8 (2011). 18. F. Joud, N. Warnasooriya, P. Bun, F. Verpillat, S. Suck, G. Tessier, M. Atlan, P. Desbiolles, M. Coppey-Moisan, M. Abboud, and M. Gross, “3D exploration of light scattering from live cells in the presence of gold nanomarkers using holographic microscopy,” 3D Res. 2, 1–8 (2011). 19. T. Y. Hwang, A. Y. Vorobyev, and C. Guo, “Ultrafast dynamics of femtosecond laser-induced nanostructure formation on metals,” Appl. Phys. Lett. 95(12), 123111 (2009). 8.

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Received 21 Apr 2011; revised 20 May 2011; accepted 20 May 2011; published 9 Jun 2011

4 July 2011 / Vol. 19, No. S4 / OPTICS EXPRESS A824

20. A. Y. Vorobyev and C. Guo, “Femtosecond laser nanostructuring of metals,” Opt. Express 14(6), 2164–2169 (2006). 21. J. Agassi, “The kirchhoff-planck radiation law,” Science 156(3771), 30–37 (1967). 22. A. D. Raki, “Algorithm for the determination of intrinsic optical constants of metal films: application to aluminum,” Appl. Opt. 34(22), 4755–4767 (1995).

1. Introduction It takes only one hour for the Earth to capture enough energy from the sun to power the entire world for a whole year of 2008 [1]. If we efficiently utilize this virtually unlimited energy from the Sun, we can significantly reduce environmental impacts from harmful air pollutant and greenhouse gas emissions by reducing the electric power generated from fossil fuels that is currently responsible for powering approximately 90% of the world consumption [2]. In the past decade, solar energy industry has grown rapidly and therefore, it is foreseeable that solar energy can gradually replace fossil fuels with increasing conversion efficiency of the solar energy. One of the most attractive technologies that converts solar energy to electrical energy is the solar-thermal technology [1, 3–6]. When solar radiation turns into heat, the heat can directly produce electricity using thermoelectric (TE) devices or convert to electricity through the Stirling or Rankine engine with electromagnetic induction method. Due to higher efficiency of the Stirling engine (~30% of Carnot efficiency) than TE devices (~10% of Carnot efficiency) [5], the solar-thermal method with the Stirling engine has been more broadly used by institutions such as Southern California Edison, Sandia National Laboratory, and Infinia Stirling Engine Technology [1]. However, the Stirling engine requires a regular maintenance due to its moving parts and pressurized gases or fluids and thus, the engine faces a higher rate of failure. Furthermore, a failure of the Stirling engine may generate environmental problems due to the leaking gases or fluids [5]. On the other hand, the TE devices are solid state devices that have no moving parts and a long lifetime of 15-20 years [7]. Therefore, the TE devices have much lower failure rate, and thus significantly reducing the maintenance cost. The major obstacle to use the TE devices for power generation is a lower solar energy conversion efficiency. However, the lower maintenance cost of the TE devices may compensate for a lower conversion efficiency because solar power plants are usually built in rural and remote areas [1]. Besides, a recent theoretical work has showed that the TE device efficiency may reach over 30% and high-performance materials for constructing more efficient TE devices have been continuously developed [2, 8–11]. Therefore, there is potential to develop a high-efficiency TE device that can approach the Stirling engine efficiency in the future. Recently, we have developed a number of techniques that allow us to produce unique surface structures that can transfer a shiny piece of metals pitch black, creating the so-called black metal, using femtosecond (fs) laser beam irradiation without any chemical etchants [12]. The fs laser-induced nanostructures on metals showed a significant enhanced absorption of metal surfaces in the wavelength range of 0.25 – 2.5 μm [12–14], where most of solar radiation energy is distributed at the sea level [6]. Moreover, the fs laser-induced nanostructures can be more selectively produced in areas as small as a tightly focused laser spot or as large as needed. These nanostructured metal surfaces can also be used at an elevated temperature without sacrificing the thermal stability because the nanostructures are directly produced on metals [13]. Through fs laser irradiation, we produce in this work a unique type of surface nanostructure on Al that have enhanced absorption at UV and visible but a relatively small emissivity in infrared. By integrating this laser-treated Al to a solar-driven thermoelectric generator (TEG), we show that the TEG integrated with the fs-laser treated Al foil generates a significantly higher power than the ones without. Our study shows that our technique can dramatically enhance the efficiency of solar-driven thermoelectric devices that may lead to a leap forward in solar energy harnessing.

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Fig. 1. (color online) (a) Picture of three TEG modules used in our experiment. From left to right are the bare module, one covered by a laser-treated Al foil, and one covered with untreated Al foil. (b) A schematic of the TEG module and the measurement circuit. A and V represent ammeter and voltmeter, respectively.

2. Experimental setup We first prepare two Al foils with a thickness of 1 mm. The initial root-mean-squared (RMS) surface roughness of Al foils is 0.978 μm, measured by a UV laser-scanning confocal microscope. We then produce extensive nanostructures on one of the Al foils and the other Al foil is not treated for comparison. To produce the nanostructures, we employ an amplified Ti: sapphire femtosecond laser system that generates 65-fs pulses with a pulse energy of 0.9 mJ at a 1 kHz repetition rate with the central wavelength of 790 nm. A linearly polarized laser beam is slightly focused normally onto the Al surface at a 1/e intensity spot size of 200 µm in diameter. Next, the two Al foils, one treated by fs laser pulses and one without, are each attached to a TEG module. Each TEG module has an aluminum heat sink for dissipating heat from the TEG modules, as shown in Fig. 1 (a). Thermal compound (Sliver) is uniformly applied to all interfaces to increase their thermal conductivity. To effectively extract power generated from the TEG modules, we attach a load resistor (Rload) of ~1.8 Ω, which has nearly the same resistance as the TEG module (1.7 - 1.8 Ω including the whole resistance of the TEG system except Rload), as shown in Fig. 1 (b). All measurements are performed in clear days when the solar irradiance is in a range of 960 - 1080 W/m2. All the sample surfaces face the sun normally during the data collection. Surface structures on Al are studied with a scanning electron microscope (SEM) and a UV laser-scanning confocal microscope (UV-LSCM) following fs laser pulse irradiation. The total reflectance of the samples is measured over a wavelength range of 0.25 – 16 μm using a Perkin-Elmer Lambda 900 and a Brucker FTIR spectrometers with an integrating sphere. 3. Results and discussion We set up three TEG modules as shown in Fig. 1 (a) to convert solar radiation to electricity under direct sun exposure. Based on Thevenin’s theorem, each TEG module can be simply replaced by a single DC voltage source (VDC) and a single series resistor (Rinternal), as shown in

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Fig. 1 (b) [15]. Therefore, the output power (P) can be extracted by attaching a load resistor to the TEG module through the equation P = Rload I2system, where Isystem is the electrical current that equals to VDC / (Rinternal + Rcircuit + Rload) and Rcircuit is the resistance from the rest of the electrical connections [16]. Figure 2 shows the SEM and UV-LSCM images of fs laser-treated Al surfaces. Both images are obtained after the laser treatment. Recent studies showed that 3-dimensional surface morphological profiles can be monitored in real time using digital holography, which may be useful to further understand the fs laser surface structuring [17, 18]. To produce nanostructures, fs pulses are irradiated on Al surface at a fluence of 2.86 J/cm2. As shown in Fig. 2, large groove structures are formed on Al with the mean peak-to-valley height of about 70 μm due to the raster scanning of the sample [Fig. 2 (a) and (d)]. The groove structures are densely covered by extensive nanostructures and microstructures [Fig. 2 (b) and (c)]. These nano- and micro-structures are produced on Al by nonuniform energy deposition following fs laser pulse irradiation [19, 20].

Fig. 2. (color online) (a) SEM images of the laser-treated Al surface. (b) A detailed view of the grooves at its peak. (c) A detailed view of the grooves at its valley. (d) The morphological profile of the grooves measured by UV-LSCM.

These structures cause a significant decrease in the reflectance of Al surface down to 47% in the UV and visible region and to 7- 40% in the infrared region. A lower absorptance with a wavelength at IR means it has a lower emissivity as well, thus reducing the radiative heat loss at the long wavelength region [21], as shown in Fig. 3. Due to the initial surface roughness, the untreated Al surface also shows some decrease in the reflectance comparing to the table value of a smooth Al surface. However, as seen clearly from Fig. 3, the reflectance of the untreated Al surface is much higher than the treated surface. The bare TEG module itself also shows a higher reflectance than the treated Al surface. The generated power of all three TEG modules increases with solar irradiance, as shown in Fig. 4. Within our irradiance measurement range of 960-1080W/m2, however, the TEG module covered with the laser-treated Al foil harvests between 4 to 9 times more power than the other two TEG modules without laser treatments. The solar-driven TEG efficiency (η) can be defined as the ratio of the generated electrical power output to the input heat from the Sun. If we assume that the Seebeck coefficient (α) is independent of temperature, i.e., the Thompson effect is negligible, the solar-driven TEG efficiency can be expressed by the following equation [10],

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

qh qh  qc P    (T )2 , EA qh EA

(1)

where E is the solar irradiance on a TEG, A is the absorber area, ΔT is the temperature difference between the hot and cold junctions of TEG, qh and qc are the net rates of the heat input at the hot junction (transfer from Al foil to TEG) and the heat loss at the cold junction (transfer from TEG to the heat sink), respectively. The TEG energy balance equation is (qh qc) = αΔT Isystem – (Rinternal + Rcircuit) I2system = Rload I2system = P is used to derive Eq. (1). This energy balance equation can be rewritten as (qh - qc) 0.25 α2 (ΔT)2/Rload with Rload Rinternal + Rcircuit. Accordingly, under identical solar radiant flux (EA), solar-driven TEG efficiency is linearly proportional to the output power and quadradically depends on the temperature difference between the hot and cold junctions.

Fig. 3. (color online) Total reflectance of a bare TEG, the laser-treated Al surface, and untreated Al surface in the wavelength range of 0.25 - 16 µm. Calculated reflectance of a smooth Al based on the Fresnel theory is also plotted.[ 22].

In our experiment, an enhanced absorptance in the UV and the visible from the lasertreated Al surface leads to a higher hot junction temperature than the untreated Al surface or bare TEG surface, while the heat sink maintains the cold junction nearly the same temperature among these three cases without solar irradiation. Therefore, the greater temperature

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difference between the hot and cold junctions leads to the 4 to 9 times higher efficiency for the solar-driven TEG module with laser-treated Al surface.

Fig. 4. (color online) Dependence of the generated electrical power on solar irradiation from the three TEG modules under direct sunlight.

3. Conclusions In conclusion, we significantly enhance the efficiency of a solar-driven thermoelectric generator by integrating a highly absorptive metal produced by femtosecond laser pulses. Under identical experimental conditions, we show that the solar-driven TEG integrated with the laser treated Al foil can generate a significant higher power than the ones without. Our study shows that our technique can dramatically enhance the efficiency of solar-driven thermoelectric devices that may lead to a leap forward in solar energy harnessing. Acknowledgments We would like to thank A. Heins, E. Christensen, and X. Lou for solar irradiance measurements and discussions. This work was supported by the US Air Force Office of Scientific Research and the National Science Foundation.

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