Advances in High Efficiency Solar-Powered Lasers

FONTS

Solar-Powered Lasers for Magnesium Production

Solar-powered Lasers for Magnesium Production

Secondary Concentrator and Pumping Cavity: Liang Group

Dielectric totally internally reflecting concentrator (DTIRC)

• Ray A is focused onto the end face of the rod by direct focusing from the curved face of the DTIRC
• Ray B is focused by indirect total internal reflections from the side walls
• For side-pumping, ray C is focused into the conical cavity, with the zigzag passage of the rays within the cavity ensuring a multi-pass side-pumping to the rod

Laser Gain Medium

The solar laser head used by Liang (2013)

Introduction

Magnesium production application

Challenges of solar-powered lasing

Critical design areas

• Laser gain medium
• Primary concentrator and solar tracker
• Secondary concentrator and pumping cavity

Conclusions

Future prospects for magnesium application

Nd:YAG single-crystal properties

• excellent thermal conductivity
• high quantum efficiency
• superior mechanical strength

Movement of rays through the Liang (2011) system, using DTIRC

• Silica aspheric lens
• Compresses the concentrated solar radiation from the focal zone onto the laser rod
• Compound parabolic concentrator (2D-CPC)
• Ensures that irradiance is larger at the output aperture than at the entrance aperture
• V-shaped cavity

Laser Gain Medium

Chromium-codoped Nd:YAG ceramic media

• Conversion efficiency greatly improved
• broad absorption band
• high thermal conductivity
• Yabe group, 2007 - 2009

Secondary Concentrator and Pumping Cavity: Yabe Group

Results

Importance of Secondary Concentrator: Yabe Group

Conclusions

Magnesium Production

Fluorescence yield at 1064 nm for pumping sources of varying wavelengths for both Cr-codoped Nd:YAG and Nd:YAG media; Cr:ND:YAG 1.8 times greater [Yabe 2007]

Yabe 2007

Cylindrical cavity with conical mirror

Yabe 2009

Additional cylindrical mirror

Yabe 2007

Primary concentrator

• Collects sunlight initially into a small spot
• Fresnel lens systems allowed for an increase in collection efficiency from less than 1 W*m-2 to 30 W*m-2

• Yabe group also found greater efficiency gains using Nd:YAG crystal instead of Cr:Nd:YAG ceramics
• Yabe et al. (2012) attribute this unexpected result due to the difference in scattering coefficients of the two media despite similar saturation gains
• Cr:Nd:YAG ceramics have a scattering coefficient of 0.004 cm, twice that of the Nd:YAG crystal.
• Optimized Cr:Nd:YAG ceramics with low scattering loss and high concentration of chromium ions could still offer efficiency gains due to the material’s broad band absorption and high productivity.

Nd:YAG single-crystal rod

• Liang and Almeida 2011-2013
• Smaller and less expensive
• Similar results at cw operation
• 19.3 W [Liang 2011] vs 20 W [Yabe 2009]
• Average output power of Cr-codoped rods are higher than that of nondoped rods at low repetition rates, but the ratio gradually decreases with increasing repetition rates.
• Thermal effect of Cr-codoped media is higher than nondoped media at higher repetition rates

Incoming sunlight is focused on the left side of the cavity. L is the length of the tapered part of the inner mirror. [Yabe 2007]

• Ohkubo et al. (2012) succeeded in producing magnesium from MgO by direct solar-pumped laser
• Same configuration as the Yabe (2009) system
• 53 W laser achieved deposition efficiency of 2.3 mg/kJ

Yabe 2012

Hybrid pumping scheme

New secondary concentrator: liquid light-guide lens (LLGL)

Secondary concentrator and optimal cavity design

• Focuses the sunlight on the focal zone onto the lens
• Required for maximum efficiency
• LLGL doubled max output power [Yabe 2012]

Yabe 2009

There is a difference between the refraction index of air and the coolant. The coolant is in a glass cooling blanket attached to the laser rod, and can thus serve as a lens to further concentrate pumping power onto the laser rod. [Yabe 2012]

Addition of the LLGL doubled output power compared to an identical configuration without the secondary concentrator

Laser medium

• Nd:YAG rod smaller, less expensive
• Nd:YAG improved efficiency over Cr:Nd:YAG
• Optimized Cr:Nd:YAG with low scattering loss and high ion concentration could be preferable

Application: Magnesium Production

Introduction

Liang 2011

Laser Gain Medium

• Sun provides more energy to the Earth in one hour than is needed for the energy requirements of the entire population in a year
• Harvest and storage of solar energy difficult
• Solar-pumped lasers could play a role in shifting power consumption to carbon-neutral sources
• Traditional anticipated applications:
• Off-grid locations such as spacecraft
• Space-based solar power
• New application:
• Magnesium production in a renewable energy cycle

• Reaction of magnesium with water yields high amounts of heat and hydrogen
• Can be used for turbines, reciprocal engines, and fuel cells
• Cost-effective, fossil-fuel-free energy cycle
• Magnesium is abundant, but resulting MgO must then be deoxidized in order to repeat the cycle
• Current methods energy-intensive
• Laser as alternate energy source
• Dissociation of MgO in equilibrium requires a temperature of at least 4000 K [Yabe 2006]
• Requires laser intensity of 10^5 W / cm^2 [Yabe 2012]
• For cycle to be practical source of renewable energy, must use solar-powered lasers

Secondary Concentrator and Pumping Cavity

Source: Yabe, Applied Physics Letters, 2007

Magnesium Production

Critical Design Areas

• High intensity required for magnesium reduction requires that not only laser output power but also beam quality be maximized
• While holding beam quality factor M2 < 1.1, Liang et al. (2013) still obtain a maximum output power of 8.1 W
• Yabe (2012) M^2 = 137.
• However, output power is 120 W, 15 times that of Liang (2013)
• Future research should determine if there is an optimum combination of beam quality factor and output power maximization that will increase the deposition efficiency

Uzbekistan's 1MW solar-powered Nd:YAG laser research facility

Laser Magnesium Production: Economic Feasibility

Deposition efficiency

• Reduced Mg mass/ input laser energy
• Can be improved by higher input laser intensity

Recent advances in efficiency and output power

• Fresnel lenses
• Nd:YAG medium (neodymium-doped yttrium aluminum garnet)

Primary groups

High intensity requires maximizing

• laser output power
• beam quality

Yabe 2007, 2009, 2012

• Tokyo Institute of Technology

Liang 2011, 2013

• Universidade Nova de Lisboa

Primary metrics

Primary Concentrator

Collection efficiency

• laser output / primary concentrator area

Output power

Beam quality factor

References Continued

References

Prior results

• 1 W using parabolic mirror [Young 1966]
• 500 W from a 660 m^2 collecting mirror system
• collection efficiency of only 0.76 W/m2 [Landoa 2003]

Heliostat

Primary Concentrator and Solar Tracker

Ohkubo, Tomomasa Yabe, Takashi, Dinh, Thanh. (2012). "Demonstration of Solar‐Pumped La-ser‐Induced Magnesium Production from Magnesium Oxide." Magnesium Technology 2012: Proceedings of a Symposium Sponsored by the Magnesium Committee of the Light Metals Division of the Minerals, Metals & Materials Society (TMS), Held during TMS 2012 Annual Meeting & Exhibition, Orlando Florida, USA, March11-15, 2012. By Suveen N. Mathaudhu, Wim H. Sillekens, Neal R. Neelameggham, and Norbert Hort. Warrendale, PA: Minerals, Metals and Materials Society. 55-58.

Yabe, T., Bagheri, B., Ohkubo, T., Uchida, S., Yoshida, K., Funatsu, T., et al. (2008). 100 W-class solar pumped laser for sustainable magnesium-hydrogen energy cycle. Journal of Applied Physics, 104(8) doi:http://dx.doi.org/10.1063/1.2998981

Yabe, T., Ohkubo, T., Uchida, S., Yoshida, K., Nakatsuka, M., Funatsu, T., et al. (2007). High-efficiency and economical solar-energy-pumped laser with fresnel lens and chromium codoped laser medium. Applied Physics Letters, 90(26) doi:http://dx.doi.org/10.1063/1.2753119

Yabe, T., Uchida, S., Ikuta, K., Yoshida, K., Baasandash, C., Mohamed, M. S., et al. (2006). Demonstrated fossil-fuel-free energy cycle using magnesium and laser. Applied Physics Letters, 89(26) doi:http://dx.doi.org/10.1063/1.2423320

Young, C. G. (1966). Applied Optics 5, 993.

Barlev, D., Vidu, R., & Stroeve, P. (2011). Innovation in concentrated solar power. Solar Energy Materials and Solar Cells, 95(10), 2703-2725.

Dinh, T. H., Ohkubo, T., Yabe, T., & Kuboyama, H. (2012). 120 watt continuous wave solar-pumped laser with a liquid light-guide lens and an Nd:YAG rod. Opt.Lett., 37(13), 2670-2672.

Graham-Rowe, Duncan (2010). Nature Photonics 4, 64 – 65. doi:10.1038/nphoton.2009.272

Landoa, M., et al. (2003). Optical Communications 222(371).

Liang, D., & Almeida, J. (2011). Highly efficient solar-pumped Nd:YAG laser. Opt.Express, 19(27), 26399-26405.

Liang, D., & Almeida, J. (2013). Solar-pumped TEM00 mode nd:YAG laser. Opt.Express, 21(21), 25107-25112.

Ohkubo, T., Yabe, T., Yoshida, K., Uchida, S., Funatsu, T., Bagheri, B., et al. (2009). Solar-pumped 80 W laser irradiated by a fresnel lens. Opt.Lett., 34(2), 175-177.

Fresnel lenses

• Replaced large mirror systems
• Compact design
• Increased collection efficiency
• Used by both Yabe and Liang groups in all years

Drawback

• Chromatic dispersion
• spreads the focal spots of different wavelength along the focal region
• Yabe (2009) calculated neglible effect (1.4 mm)
• Liang group (2011) accounts for chromatic dispersion with modified dielectric totally internally reflecting concentrator (DTIRC)

Non-refractive part of conventional lens removed in Fresnel lens

[Source: http://spie.org/x8645.xml]

The Yabe (2007) system, consisting of two pieces of Fresnel lens with dimensions 1.4 x 1.05 m^2.; the system moves together as one unit

A .9 m diameter Fresnel lens is mounted on a two-axis solar tracker in the Liang (2011) system

445 W for a sunlight intensity of 890 W/m2

675 W for a sunlight intensity of 779 W/m2 power at the focal point averaged over 2 min

66.4 percent of incident sunlight on the lens focused at the focal point

2009, 2012 designs using 2 x 2 m lens

78.6 percent of incident solar radiation focused to the focal zone

2013 design found 590 W for 890 W/m2 in 2013 design wtih 1.0 m diameter