Solar Energy

Solar Thermophotovoltaic

Solar radiation is one of Earth's most abundant renewable resources and the conversion of this solar energy into electricity can be accomplished by a number of methods. Among the most promising is solar thermal power generation, an approach that uses mirrors to concentrate sunlight on an absorbing surface, either a tower or a pipe, which in turn transfers heat to a fluid. The heat transfer fluid collects the thermal solar energy and is pumped to a heat exchanger that creates steam to power the turbine of an electric generator. While effective at generating electricity, a number of inefficiencies arise when running the equipment at extremely high temperatures and when operation is limited to periods of sufficient sunlight.

At the Device Research Laboratory, we are working to minimize these inefficiencies by studying micro-nano structures for enhanced thermal fluid properties. Micro/nano-particles can improve the fluid's ability to absorb solar radiation by decreasing emissive losses at high temperatures [1]. Further, thermal storage and heat transfer can be increased through the heat of fusion of phase-change micro/nano-particles [2], ultimately boosting the capacity factor of a concentrated solar power plant.

In addition, we research direct conversion of solar irradiation to electrical energy through the use of thermophotovoltaics (TPVs). We are investigating the overall performance and complex energy conversion mechanisms of solar TPVs through experiments [3-6] and a high-fidelity thermal electrical system-level model [7]. Since the efficiency of a TPV system is highly dependent on the optical properties of the thermal emitter and the PV cell, we work on incorporating advanced materials and designs to our prototype in order to demonstrate relatively high converter efficiencies.

The solar division of the Device Research Laboratory works in collaboration with the Solid State Solar Thermal Energy Conversion Center as well as the King Fahd University of Petroleum & Minerals.

Solar Aerogels for Solar-Thermal Applications

At DRL, we are developing materials with intrinsic spectral selectivity that can significantly improve the energy conversion efficiency of various solar-thermal systems. Aerogels are widely known for their thermal insulating properties due to its nanoporous structure. Monolithic silica aerogels that are both optically transparent and thermally insulating can significantly improve the efficiency of solar absorbers by acting as a spectral selective cover that allows solar radiation to transmit through but minimizes the absorber losses in the infrared spectrum. Heat losses due to solid and gas conduction can also be diminished due to the high porosity (> 90%) and pore sizes smaller than the mean free path of gas molecules which obviates the need for operation in vacuum. However, the transparency of silica aerogels in the solar spectrum is typically smaller than 85% which has prevented its adoption in solar-thermal applications. At our lab, we have developed optically transparent and thermally insulating (OTTI) silica aerogels [8-14] optimized for concentrated solar power (CSP) applications which can be even more transparent than glass (over 96% solar weighted transmittance). The OTTI aerogels could be useful for solar water heating, industrial process heat applications as well as energy efficient windows.

This work is supported by the ARPA-E FOCUS program in collaboration with Professor Gang Chen from the NanoEngineering Group at MIT.

  1. A. Lenert, E.N. Wang, "Optimization of nanofluid volumetric receivers for solar thermal energy conversion," Solar Energy, 86(1), p. 253-265, 2012.
  2. A. Lenert, Y. Nam, B.S. Yilbas, E.N. Wang, "Focusing of phase change microparticles for local heat transfer enhancement in laminar flows," International Journal of Heat and Mass Transfer, 56(1), p. 380-389, 2013.
  3. A. Lenert, W. Chan, Y. Nam, I. Celanovic, M. Soljacic, E.N. Wang, "Solar Thermophotovoltaic Energy Conversion with High-temperature Photonic Crystal Emitters" to be presented at ASME Summer Heat Transfer Conference, Minneapolis, MN, July 14-19, 2013.
  4. A. Lenert, D. M. Bierman, Y. Nam, W. R. Chan, I. Celanovic, M. Soljacic, and E. N. Wang, "A nanophotonic solar thermophotovoltaic device," Nat. Nanotechnol., vol. 9, no. 2, pp. 126-130, Jan. 2014
  5. D. M. Bierman, A. Lenert, W. R. Chan, B. Bhatia, I. Celanovic, M. Soljacic, and E. N. Wang, "Enhanced photovoltaic energy conversion using thermally based spectral shaping," Nat. Energy, vol. 1, no. 6, p. 16068, May 2016.
  6. Y. Nam, A. Lenert, Y. X. Yeng, P. Bermel, M. Soljacic, and E. N. Wang, "Solar thermophotovoltaic energy conversion systems with tantalum photonic crystal absorbers and emitters," in 2013 Transducers & Eurosensors XXVII: The 17th International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers & Eurosensors XXVII), 2013, pp. 1372-1375
  7. N. Miljkovic, E.N. Wang, "Modeling and Optimization of Hybrid Solar Thermoelectric Systems with Thermosyphons," Solar Energy, 85(11), p. 2845-2855, 2011.
  8. L. Zhao, B. Bhatia, S. Yang, E. Strobach, L.A. Weinstein, T.A. Cooper, G. Chen, E.N. Wang, "Harnessing Heat Beyond 200 C from Unconcentrated Sunlight with Nonevacuated Transparent Aerogels", ACS Nano. 13 (2019) 7508-7516.
  9. E. Strobach, B. Bhatia, S. Yang, L. Zhao, E.N. Wang, "High temperature stability of transparent silica aerogels for solar thermal applications", APL Mater. 7 (2019) 081104.
  10. L. Zhao, E. Strobach, B. Bhatia, S. Yang, A. Leroy, L. Zhang, E.N. Wang, "Theoretical and experimental investigation of haze in transparent aerogels", Opt. Express. 27 (2019) A39.
  11. E. Strobach, B. Bhatia, S. Yang, L. Zhao, E.N. Wang, "High Temperature Annealing for Structural Optimization of Silica Aerogels in Solar Thermal Applications", J. Non. Cryst. Solids. 462 (2017) 72-77.
  12. L.A. Weinstein, K. McEnaney, E. Strobach, S. Yang, B. Bhatia, L. Zhao, Y. Huang, J. Loomis, F. Cao, S. V. Boriskina, Z. Ren, E.N. Wang, G. Chen, "A Hybrid Electric and Thermal Solar Receiver", Joule. 2 (2018) 962-975.
  13. L. Zhao, S. Yang, B. Bhatia, E. Strobach, E.N. Wang, "Modeling silica aerogel optical performance by determining its radiative properties", AIP Adv. 6 (2016) 025123.
  14. L.A. Weinstein, J. Loomis, B. Bhatia, D.M. Bierman, E.N. Wang, G. Chen, "Concentrating Solar Power", Chem. Rev. 115 (2015) 12797-12838.
  15. L. Zhao, M. Blackman, L. Zhang, B. Bhatia, A. Leroy, E. Strobach, E.N. Wang, "Plasmonic absorption-induced haze suppression in random scattering media," Applied Physics Letters, 114(25), p. 251102, 2019.
  16. L. Zhao, B. Bhatia, S. Yang, E. Strobach, L.A. Weinstein, T.A. Cooper, G. Chen, E.N. Wang, "Harnessing Heat Beyond 200 C from Unconcentrated Sunlight with Nonevacuated Transparent Aerogels," ACS Nano, 2019, in press.