Bo Zhao, Kalsi Assistant Professor of mechanical engineering, and his doctoral student, Sina Jafari Ghalekohneh, have created recent architecture that improves the efficiency of solar energy harvesting to the thermodynamic limit. (University of Houston)

The nice inventor Thomas Edison once said, “As long as the sun shines, man will give you the chance to develop power in abundance.” His wasn’t the primary great mind to marvel on the notion of harnessing the ability of the sun; for hundreds of years inventors have been pondering and perfecting the solution to harvest solar energy.

They’ve done an incredible job with photovoltaic cells which convert sunlight directly into energy. And still, with all of the research, history and science behind it, there are limits to how much solar energy might be harvested and used — as its generation is restricted only to the daytime.

A University of Houston professor is constant the historic quest, reporting on a recent form of solar energy harvesting system that breaks the efficiency record of all existing technologies. And no less necessary, it clears the solution to use solar energy 24/7.

“With our architecture, the solar energy harvesting efficiency might be improved to the thermodynamic limit,” reports Bo Zhao, Kalsi Assistant Professor of mechanical engineering and his doctoral student Sina Jafari Ghalekohneh within the journal Physical Review Applied. The thermodynamic limit is absolutely the maximum theoretically possible conversion efficiency of sunlight into electricity.

Finding more efficient ways to harness solar energy is critical to transitioning to a carbon-free electric grid. In response to a recent study by the U.S. Department of Energy Solar Energy Technologies Office and the National Renewable Energy Laboratory, solar could account for as much as 40% of the nation’s electricity supply by 2035 and 45% by 2050, pending aggressive cost reductions, supportive policies and large-scale electrification.

How Does it Work?

Traditional solar thermophotovoltaics (STPV) depend on an intermediate layer to tailor sunlight for higher efficiency. The front side of the intermediate layer (the side facing the sun) is designed to soak up all photons coming from the sun. In this manner, solar energy is converted to thermal energy of the intermediate layer and elevates the temperature of the intermediate layer.

However the thermodynamic efficiency limit of STPVs, which has long been understood to be the blackbody limit (85.4%), continues to be far lower than the Landsberg limit (93.3%), the final word efficiency limit for solar energy harvesting.

“On this work, we show that the efficiency deficit is attributable to the inevitable back emission of the intermediate layer towards the sun resulting from the reciprocity of the system. We propose nonreciprocal STPV systems that utilize an intermediate layer with nonreciprocal radiative properties,” said Zhao. “Such a nonreciprocal intermediate layer can substantially suppress its back emission to the sun and funnel more photon flux towards the cell.

We show that, with such improvement, the nonreciprocal STPV system can reach the Landsberg limit, and practical STPV systems with single-junction photovoltaic cells also can experience a big efficiency boost.”

Besides improved efficiency, STPVs promise compactness and dispatchability (electricity that might be programmed on demand based on market needs).

In a single necessary application scenario, STPVs might be coupled with a cheap thermal energy storage unit to generate electricity 24/7.

“Our work highlights the good potential of nonreciprocal thermal photonic components in energy applications. The proposed system offers a recent pathway to enhance the performance of STPV systems significantly. It could pave the way in which for nonreciprocal systems to be implemented in practical STPV systems currently utilized in power plants,” said Zhao.

Publication Referenced within the Article:

Sina Jafari Ghalekohneh, Bo Zhao. Nonreciprocal Solar Thermophotovoltaics. Physical Review Applied, 2022; 18 (3) DOI: 10.1103/PhysRevApplied.18.034083

This text was written by the team on the University of Houston.


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