A new approach to solar thermophotovoltaics by the Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts (US), achieves a 3-fold conversion efficiency increase by using an absorber-emitter layer made of carbon nanotubes and photonic crystals to absorb solar energy in form of heat first. The solar thermophotovoltaic (STPV) system also holds promises for easier energy storage.

Ordinarily, those wavelengths of sunlight would not be harvested in a conventional silicon solar cell, because the semiconductor material’s band gap does not match photons in the infrared range. So the MIT team put a two-layer absorber-emitter device between the sunlight and the photovoltaic cell. This in-between layer is made of a sun-facing array of multi-walled carbon nanotubes that harvests solar radiation at infrared wavelengths, tightly bonded to a layer of photonic crystals engineered to emit light that matches the photovoltaic semiconductor’s band gap to generate extra electricity. MIT demonstrated this concept for a low band gap (0.55 eV) InGaAsSb cell, but it could be extended to other band gap cells in the future, says Evelyn Wang. The associate professor in MIT’s Department of Mechanical Engineering coauthored the paper describing the process — “A nanophotonic solar thermophotovoltaic device” — published in Nature Nanotechnology.

The basic concept behind such a STPV system is not new. The theory has raised hope in the research community in recent years as a way to overcome the Shockley-Queisser limit — energy conversion efficiencies of over 80% could be thinkable, according to MIT. In practice, however, previous prototype STPV devices designed by other labs barely made it to 1% efficiency. The conversion efficiency of MIT’s test device at hand stands at 3.2%. Wang hopes to reach a commercially viable 20% soon with further optimisations, which will require scaling up the components. “Right now, we have demonstrated this concept with a 1 sq cm device. Our experimentally validated models show that if we can get to a 10 cm by 10 cm device with existing components, the 20% efficiencies can be achieved.”

The novelty in MIT’s device is the design of the two-layer absorber-emitter, “which allows the tuning of the energy balance to achieve a 3-fold improvement in STPV efficiency compared to previous work,” Wang says. “Our work suggests the potential of solar thermophotovoltaics as being a viable energy conversion technology, that is highly efficient and dispatchable.”

The test device reached its peak efficiency when the intensity of the artificial light source reached the equivalent of 750 suns, heating the absorber-emitter to 963 degrees Celsius. In contrast to previous STPV trials, which simulated sunlight concentrated by level of several thousand, the light intensity here is already much lower, and the team expects to be able to decrease the concentration even further with improvements, making the system easier to operate.

What is more, combining the advantages of a solar photovoltaic system with those of a solar thermal system, MIT’s promising broadband technique could make it easier to store solar energy for later use: “Because we collect the solar energy in the form of heat, it can be more easily stored via thermal and chemical means,” Wang confirms, adding that, hopefully this “can be one approach to take advantage of one of earth's most abundant resources, the sun, to generate power and ultimately help solve the energy crisis.”

Written by Sandra Henderson, Research Editor Solar Novus Today