Researchers at the Massachusetts Institute of Technology (MIT) recently published the results of their research into next-generation photovoltaic devices that could dramatically increase the efficiency of solar-energy generation. The devices use the principles of thermophotovoltaics—essentially turning sunlight into heat energy before reemitting it as light that is more suitable for the photovoltaic component of the cell. The scientists achieve this reemission by adding a layer of vertically aligned carbon nanotubes—extremely efficient absorbers of light across the entire visible spectrum—at the surface of the photovoltaic cell. This layer also contains an array of nanoscale photonic crystals that can reemit the heat generated by the nanotubes at the specific wavelengths that the photovoltaic cell can most efficiently transform to electricity.

The researchers—for the first time—were able to demonstrate a thermophotovoltaic cell that exhibits a higher efficiency than that of the underlying photovoltaic cell. At 6.8%, the efficiency of the prototype thermophotovoltaic device was relatively low—standard silicon photovoltaics can achieve efficiencies of between approximately 32% and 40%. However, the scientists claim that further research could result in additional gains.

Implications

Blocking sunlight from arriving at the surface of a photovoltaic cell in order to improve its efficiency may appear counterintuitive. However, thermophotovoltaics has the potential to reshape the solar-power industry and, by extension, the entire market for renewable energy. Any advance that significantly improves on existing technology could cause a shift in industry standards and prove to be highly lucrative for early adopters.

Perhaps one of the most significant implications of this technology arises from the ability to store and transport heat easily. Stored heat could find use in the generation of photovoltaic power at all times, completely removing the need for direct sunlight and simultaneously solving problems relating to energy storage and the intermittent nature of generating power by means of standard solar technology.

However, despite the promise, this technology is in the early phases of its developmental cycle. Significant levels of additional research—focusing primarily on issues related to scale and manufacturing—will be necessary before thermophotovoltaics is in a position to challenge existing methods of power generation.

Impacts/Disruptions

Several other methods of improving photovoltaic devices are currently in development. For example, stacking solar cells on top of one another in order to maximize the power output is one technique that can find use in overcoming the fundamental limitations of the technology. Alternative approaches within the field of thermophotovoltaics also exist. Researchers at the Australian National University and the University of California, Berkeley, are researching the use of nanoscale metamaterials that would function in a manner similar to that of the MIT research. Ultimately, efficiency and cost will determine which, if any, of these emerging technologies will become a commercial success.

If thermophotovoltaic technology does prove to be a commercial success, then it could potentially also find use in harvesting other sources of waste heat such as industrial processes or the heat generated by an automotive engine. The nanoscale nature of this technology also lends itself particularly well to the wide range of flexible photovoltaics applications that are set to proliferate in the coming five years.