Solar Energy

Solar energy remains a vastly unexploited renewable energy source on Earth and beyond. In just one hour, our Earth receives enough energy in the form of sunlight to meet all of humanity’s energy needs for a whole year. While photovoltaic (PV), i.e., solar cells, has received interest, it can be expensive when considering the need for energy storage to enable dispatchability. At the DRL, we are working on three alternative solar-related projects to meet our future global energy needs at low cost: 1) Solar thermophotovoltaics (STPV) for high-efficiency baseload power generation; 2) Solar transparent silica aerogels for solar-thermal applications; and 3) Radiative cooling for passive building cooling and refrigeration.

Solar thermophotovoltaics

Concentrating solar power

Cavity based solar
absorbers and emitters

Transparent silica aerogels

Solar thermal collectors

Thermally insulating windows

Passive radiative cooling

Directional radiative cooling

Polyethylene aerogels for deep sub-ambient cooling

Passive cooling of buildings

Recent Progress

Silica Aerogels for Solar-Thermal Applications

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 the DRL, we have developed optically transparent and thermally insulating (OTTI) silica aerogels 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.
Passive radiative cooling

Radiative cooling relies on passive rejection of thermal radiation to the cold outer space through the high transparency spectral window of earth’s atmosphere to provide passive cooling on earth. Due to its passive approach, radiative cooling has recently been proposed as an attractive solution for passive cooling of residential and commercial buildings in hot climates. While the passive approach is attractive, ensuring high cooling power and reaching cold sub-ambient temperature at the emitter has proven difficult due to parasitic solar absorption of common materials as well as a lack of infrared transparent thermal insulation. At the DRL, we explore ways to decrease parasitic solar absorption and heat gains using novel system designs and materials. Our first approach relies on the fact that sunlight is highly directional in the sky, where most of the solar radiation is confined in a small solar disk. By blocking the direct path between the emitter and the small solar disk, we can decouple solar reflection and mid-infrared emission, enabling passive cooling 6 °C below ambient temperature using simple materials like aluminum foil, thermal insulation, and polyethylene sheet. Our second approach relies on a new solar reflecting and infrared transparent thermal insulation called polyethylene aerogel. This novel spectrally selective thermal insulation can be placed on any infrared emitting surface to reflect over 92% of the sunlight while still allowing radiative cooling and providing thermal insulation from the warmer ambient air. Using polyethylene aerogels, we have demonstrated sub-ambient cooling up to 13 °C below ambient temperature under direct sunlight. With our radiative cooling solutions, we envision applications in passive cooling of buildings as well as safe storage of food and medicines in rural areas with limited access to electricity.

Solar thermophotovoltaics
Although sunlight is abundant on earth and can easily supply all our energy needs, converting it to electricity presents several challenges. The efficiency of single junction photovoltaic (PV) cells is limited to around 33.7% by the Shockley–Queisser limit due to the spectral distribution of sunlight (UV, visible and near-infrared light) and the spectrally dependent radiative recombination in the PV cell. Furthermore, traditional PV cell can only operate during the daytime to generate power, creating a mismatch between the peak power generation (solar noon) and the peak demand (morning/evening). At the DRL, we tackle these challenges by researching direct conversion of solar irradiation to electrical energy using thermophotovoltaics (TPVs). Our approach uses an intermediate thermal absorber/emitter between the sun and the PV cell that absorbs all wavelengths of the sunlight (absorber side) while only reemitting light (emitter side) to the PV cells at wavelengths that can efficiently be converted to electricity, increasing the maximum theoretical efficiency to 86.8%. We are investigating the overall performance and complex energy conversion mechanisms of solar TPVs through experiments and a high-fidelity thermal electrical system-level model. 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 to demonstrate relatively high converter efficiencies. Future work will also look at incorporating thermal storage in the absorber emitter to extend power generation past the sunset and provide closer to a baseload power generation.
Related Publications
  • 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, 7508, 2019.. 
  • E. Strobach, B. Bhatia, S. Yang, L. Zhao, E.N. Wang, “High temperature stability of transparent silica aerogels for solar thermal applications,” APL Materials, 7, 081104, 2019. 
  • A. Leroy, B. Bhatia, C. C. Kelsall, A. Castillejo-Cuberos, L. Zhao, L. Zhang, A. M. Guzman, E. N. Wang. “High-performance subambient radiative cooling enabled by optically selective and thermally insulating polyethylene aerogel,” Science Advances, 5, eaat9480, 2019. 
  • B. Bhatia, A. Leroy, Y. Shen, L. Zhao, M. Gianello, D. Li, T. Gu, J. Hu, M. Soljačić, E. N. Wang. “Passive directional sub-ambient daytime radiative cooling,” Nature Communications, 9, 5001, 2018. 
  • D. M. Bierman, A. Lenert, W. R. Chan, B. Bhatia, I. Celanovic, M. Soljacic, E. N. Wang, “Enhanced photovoltaic energy conversion using thermally based spectral shaping,” Nature Energy, 1, 16068, 2016.
  • A. Lenert, D. M. Bierman, Y. Nam, W. R. Chan, I. Celanovic, M. Soljačić, and E. N. Wang, “A nanophotonic solar thermophotovoltaic device,” Nature Nanotechnology, 9, 126, 2014,” Nature Communications, 9, 1191 2018.