Scientists from the University of Colorado are developing a new type of “rectenna” to efficiently “harvest” thermal emissions (waste heat) radiated from devices (a rectenna converts electromagnetic radiation to DC current).

Currently rectennas work best at low frequencies, but most heat is at higher radiation frequencies — up to the 100 THz (100 trillion cycles per second) range. So Won Park and his colleagues found a way to enhance thermal emission of hot bodies at the lower end of the spectrum (around 1 THz): by manipulating the surface of the object.

A metamaterial for engineering thermal emission

Park’s team uses software to analyze how the nanoscale topology of a surface — its bumps, holes or grooves — changes the way that electromagnetic radiation interacts with the surface. In some instances the geometry supports the formation of a wave of rippling electronic charges, called a plasmon, that hugs the surface.

“We design the surface to support a surface wave, because the presence of the wave offers a new avenue for engineering thermal emission,” Park said. For the case of optimizing thermal energy harvesting, the researchers found they could “spectrally tune” a surface to emit more radiation at 1 THz frequency.

The researchers first optimized the design, which consists of a copper plate with a regular array of tiny holes, using simulations. They then built the design in the lab and confirmed that the plate did indeed produce the type of surface waves predicted by the simulations.

The researchers also used computer modeling to design a bowtie-shaped antenna that would effectively capture the enhanced thermal emission. Simulations predict that an antenna placed near the holey surface could capture 10,000 to 100,000 times more thermal energy than an antenna in open space.

The team is in the process of experimentally testing this prediction and hopes to have new results to report soon. The results will also help the team calculate how rectenna thermal energy harvesting might compare to other ways of harvesting waste heat, such as thermoelectric materials.

The researchers described the system at the AVS 62nd International Symposium and Exhibition in San Jose, Calif. today (Monday, Oct. 19). The research is funded in part by a grant from Redwave Energy Inc.

Abstract of Metamaterial Enhanced Rectenna for Efficient Energy Harvesting

Rectenna solar cell offers an important alternative to the conventional semiconductor solar cell technology. Direct rectification of electromagnetic radiation faces many challenges one of which is the high frequency of operation. Thermal emission from hot bodies peaks at 10 ~ 100 THz while solar radiation has its maximum at around 600 THz. One may circumvent this difficulty if sufficiently strong thermal radiation is available at lower frequencies. In general, thermal emission is described well by the theory of blackbody radiation while the property of the non-black surface is characterized by its emissivity. When the surface supports surface waves, however, the properties of thermal emission can deviate substantially from the blackbody radiation, offering a new avenue for engineering thermal emission. For example, spatially coherent and spectrally selective thermal emission may be achieved. The presence of surface waves also means enhanced local density of states near the surface, which consequently leads to strongly modified thermal emission intensity and spectrum in the near field. In this paper, we report a metamaterial design to achieve enhanced thermal emission at 1 THz.

Two types of metamaterial designs were investigated: a 1D array of parallel trenches and a 2D array of holes etched on copper. The metamaterial surface was designed to support surface waves resembling the surface plasmon on metal surface. Numerical simulations by the finite element method confirmed the presence of surface waves and strong electric field near the surface at 1 THz. The strongly enhanced electric field is the direct consequence of enhanced local density of states. To further confirm the surface modes can be excited by thermal emission, we also conducted finite-difference time-domain simulations in which thermal emission was calculated by using the fluctuation dissipation theorem. Once the enhanced thermal emission is confirmed, a bowtie antenna was placed close to the metamaterial surface to capture the enhanced thermal emission in the near field. The antenna was optimized to maximize the electromagnetic energy delivered to the antenna gap. Since the antenna should couple efficiently with the surface modes, the optimal antenna design became quite different from the free space bowtie antenna operating at the same frequency. The optimized metamaterial and antenna design resulted in an antenna voltage of 10 mV at 1 THz, three orders of magnitude larger than the free space antenna. Such a large enhancement makes the metamaterial approach a highly promising route to efficient energy harvesting with rectenna.

references:
Dawei Lu, Won Park, Pat Brady. Metamaterial Enhanced Rectenna for Efficient Energy Harvesting. Proceedings of the AVS 62nd International Symposium and Exhibition, Oct. 19, 2015; EM+AS+SS-MoM-6

http://www.kurzweilai.net/a-metamaterial-that-enhances-thermal-energy-harvesting