How to tap the sun's energy through heat as well as light

Jan 19, 2014 by David Chandler
A nanophotonic solar thermophotovoltaic device as viewed from the perspective of the incoming sunlight. Reflective mirrors boost the intensity of the light reaching the carbon nanotube absorber array (center), enabling the device to reach high temperatures and record-setting efficiencies. Credit: FELICE FRANKEL

A new approach to harvesting solar energy, developed by MIT researchers, could improve efficiency by using sunlight to heat a high-temperature material whose infrared radiation would then be collected by a conventional photovoltaic cell. This technique could also make it easier to store the energy for later use, the researchers say.

In this case, adding the extra step improves performance, because it makes it possible to take advantage of wavelengths of light that ordinarily go to waste. The process is described in a paper published this week in the journal Nature Nanotechnology, written by graduate student Andrej Lenert, associate professor of mechanical engineering Evelyn Wang, physics professor Marin Soljačić, principal research scientist Ivan Celanović, and three others.

A conventional silicon-based solar cell "doesn't take advantage of all the photons," Wang explains. That's because converting the energy of a photon into electricity requires that the photon's energy level match that of a characteristic of the photovoltaic (PV) material called a bandgap. Silicon's bandgap responds to many , but misses many others.

To address that limitation, the team inserted a two-layer absorber-emitter device—made of novel materials including carbon nanotubes and photonic crystals—between the and the PV cell. This intermediate material collects energy from a broad spectrum of sunlight, heating up in the process. When it heats up, as with a piece of iron that glows red hot, it emits light of a particular wavelength, which in this case is tuned to match the bandgap of the PV cell mounted nearby.

This basic concept has been explored for several years, since in theory such solar thermophotovoltaic (STPV) systems could provide a way to circumvent a theoretical limit on the energy-conversion efficiency of semiconductor-based photovoltaic devices. That limit, called the Shockley-Queisser limit, imposes a cap of 33.7 percent on such efficiency, but Wang says that with TPV systems, "the efficiency would be significantly higher—it could ideally be over 80 percent."

There have been many practical obstacles to realizing that potential; previous experiments have been unable to produce a STPV device with efficiency of greater than 1 percent. But Lenert, Wang, and their team have already produced an initial test device with a measured efficiency of 3.2 percent, and they say with further work they expect to be able to reach 20 percent efficiency—enough, they say, for a commercially viable product.

Optical image of the vacuum-enclosed device illustrating the energy conversion processes in a nanophotonic solar thermophotovoltaic device: Sunlight is converted to useful thermal emission, and ultimately electrical power, via a hot absorber-emitter (at center, glowing orange). Credit: MIT

The design of the two-layer absorber-emitter material is key to this improvement. Its outer layer, facing the sunlight, is an array of multiwalled carbon nanotubes, which very efficiently absorbs the light's energy and turns it to heat. This layer is bonded tightly to a layer of a photonic crystal, which is precisely engineered so that when it is heated by the attached layer of nanotubes, it "glows" with light whose peak intensity is mostly above the bandgap of the adjacent PV, ensuring that most of the energy collected by the absorber is then turned into electricity.

In their experiments, the researchers used simulated sunlight, and found that its peak efficiency came when its intensity was equivalent to a focusing system that concentrates sunlight by a factor of 750. This light heated the absorber-emitter to a temperature of 962 degrees Celsius.

This level of concentration is already much lower than in previous attempts at STPV systems, which concentrated sunlight by a factor of several thousand. But the MIT researchers say that after further optimization, it should be possible to get the same kind of enhancement at even lower sunlight concentrations, making the systems easier to operate.

Such a system, the team says, combines the advantages of , which turn sunlight directly into electricity, and solar thermal systems, which can have an advantage for delayed use because heat can be more easily stored than . The new solar thermophotovoltaic systems, they say, could provide efficiency because of their broadband absorption of sunlight; scalability and compactness, because they are based on existing chip-manufacturing technology; and ease of energy storage, because of their reliance on heat.

Some of the ways to further improve the system are quite straightforward. Since the intermediate stage of the system, the absorber-emitter, relies on high temperatures, its size is crucial: The larger an object, the less surface area it has in relation to its volume, so heat losses decline rapidly with increasing size. The initial tests were done on a 1-centimeter chip, but follow-up tests will be done with a 10-centimeter chip, they say.

Explore further: Scientists develop a more efficient and economical solar cell based on graphene and perovskite

More information: A nanophotonic solar thermophotovoltaic device, DOI: 10.1038/nnano.2013.286

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not rated yet Jan 20, 2014
80% efficient (even 50% efficient) would be awesome. Using concentrators that type of cell would be viable in most any weather.

This layer is bonded tightly to a layer of a photonic crystal, which is precisely engineered so that when it is heated by the attached layer of nanotubes, it "glows" with light whose peak intensity

Now if one were to couple this with a discrete emitter instead of a continuous one (e.g. a quantum dot) that is geared towards the bandgap of the cell...that might be something to ponder.
not rated yet Jan 20, 2014
It seems quite far from working surely on long time, and simple solar concentration by 10 to 50 with basic parabolic cylinders giving 150°C up to 300°C for basic electricity gernerators has an efficiency better over 20%, with the possibility to store underground simply this heat from summer to winter, to give electricity continuously 365 days even in the night continuously, using a simple geothermal store, already working.
not rated yet Jan 20, 2014
Beware that a gang of MIT "researchers" has already stole in Nature journals and, with further support of the Harvard's ones, in ASC Nano Lett both the ideas and money of taxpayers. There are numerous swindlers from David H. Koch Inst. for Integrative Cancer Research and Dept of Chemical Engineering, also with Dept of Chemistry and Chem. Biology and School of Eng and Applied Science of Harvard University at & .
Their plagiaristic compilation titled Macroporous nanowire nanoelectronic scaffolds for synthetic tissues (DOI: 10.1038/NMAT3404) and Outside Looking In: Nanotube Transistor Intracellular Sensors ( was funded by NIH Director's Pioneer Award (1DP1OD003900) and a McKnight Foundation Technol Innovations in Neurosc Award, also a Biotechnology Research Endowment from the Dept. of Anesthesiology at Children's Hospital Boston and NIH grants GM073626, DE01-3023/6516.

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