06-05-2014, 11:29 AM
SOLAR NANTENNA ELECTROMAGNETIC COLLECTORS
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ABSTRACT
This research explores a new efficient approach for producing
electricity from the abundant energy of the sun. A nantenna
electromagnetic collector (NEC) has been designed,
prototyped, and tested. Proof of concept has been validated.
The NEC devices target mid-infrared wavelengths, where
conventional photovoltaic (PV) solar cells are inefficient and
where there is an abundance of solar energy. The initial concept
of designing NEC was based on scaling of radio frequency
antenna theory. This approach has proven unsuccessful by
many due to not fully understanding and accounting for the
optical behavior of materials in the THz region. Also, until
recent years the nanofabrication methods were not available to
fabricate the optical antenna elements. We have addressed and
overcome both technology barriers.
INTRODUCTION
Full spectrum incident and reflective (readmitted)
electromagnetic (EM) radiation originating from the sun
provides a constant energy source to the earth. Approximately
30% of this energy is reflected back to space from the
atmosphere, 19% is absorbed by atmospheric gases and
reradiated to the earth’s surface in the mid-IR range (7-14 um),
and 51% is absorbed by the surface or organic life and
reradiated at around 10 um [1]. The energy reaching the earth in
both the visible and IR regions and the reradiated IR energy are
under-utilized by current technology.
Several approaches have been pursued to harvest energy from
the sun. Conversion of solar energy to electricity using
photovoltaic cells is the most common. An alternative to
photovoltaics is the rectenna, which is a combination of a
receiving antenna and a rectifier. The initial rectenna concept
was demonstrated for microwave power transmission by
Raytheon Company in 1964 [2]. This illustrated the ability to
capture electromagnetic energy and convert it to DC power at
efficiencies approaching 84% [3]. Since then much research
has been performed to extend the concept of rectennas to the
infrared and visible regime for solar power conversion.
Progress has been made in fabrication and characterization of
metal-insulator-metal diodes for use in an infrared rectenna [4-
5].
Economical Alternative to PV
We have developed an alternative energy harvesting approach
based on nantennas that absorb the incident solar radiation. In
contrast to PV, which are quantum devices and limited by
material bandgaps, antennas rely on natural resonance and
bandwidth of operation as a function of physical antenna
geometries.
The NECs can be configured as frequency selective surfaces to
efficiently absorb the entire solar spectrum. Rather than
generating single electron-hole pairs as in the PV, the incoming
electromagnetic field from the sun induces a time-changing
current in the antenna. Efficient collection of the incident
radiation is dependent upon proper design of antenna resonance
and impedance matching of the antenna. Recent advances in
nanotechnology have provided a pathway for large-scale
fabrication of nantennas.
THEORY OF OPERATION
We have designed nantenna elements that capture
electromagnetic energy from naturally occurring solar radiation
and thermal earth radiation. The size of the antenna is relative
to the wavelength of light we intend to harvest. The basic
theory of operation is as follows: The incident electromagnetic
radiation (flux) produces a standing-wave electrical current in
the finite antenna array structure. Absorption of the incoming
EM radiation energy occurs at the designed resonant frequency
of the antenna [7].
When an antenna is excited into a resonance mode it induces a
cyclic plasma movement of free electrons from the metal
antenna.
The electrons freely flow along the antenna
generating alternating current at the same frequency as the
resonance. Electromagnetic modeling illustrates the current
flow is toward the antenna feedpoint. In a balanced antenna,
the feedpoint is located at the point of lowest impedance.
Figure 1 was acquired from modeling the electromagnetic
properties of an infrared spiral antenna. The e-field is clearly
This provides a
concentrated at the center feedpoint.
convenience point to collect energy and transport it to other
circuitry for conversion.
Analytical Model – RLC Circuit
To model NEC structures it is first necessary to understand the
electrical equivalent circuit and basic theory of operation. The
primary antenna structure studied in the initial design of an
NEC is a periodic array of square-loop antennas. Its RLC
circuit analog is shown in Figure 4. The electrical behavior of
the structure is described as follows. The metal loops give
inductance to the NEC as thermally-excited radiation induces
current. The gaps between the metallic loops and the gap
within the loop compose capacitors with a dielectric fill. A
resistance is present because the antenna is composed of lossy
metallic elements on a dielectric substrate. The resulting RLC
circuit has a resonance “tuned” filter behavior [7].
PROOF OF CONCEPT THROUGH MODELING
Frequency Selective Surface (FSS) structures have been
successfully designed and implemented for use in radio (RF)
and microwave frequency applications. The classic laws of
physics apply for adapting microwave applications to the
infrared. Innovative INL research has further optimized FSS
designs for operation in the Terahertz (THz) and infrared (IR)
spectrums.
It is recognized that several numerical analysis techniques can
be employed for electromagnetic analysis. The basis for our
research and development is the adaptation of the ‘Method of
Moments’ technique, which is a numerical computational
method of solving linear partial differential equations
associated with electromagnetic fields. Ohio State University
has implemented a method of moments based algorithm in a
software product, termed Periodic Method of Moments (PMM),
which was developed for designing military RF frequency
selective surfaces. The details of this code are discussed in [9].
Ohio State’s PMM serves as the analysis engine for the INL
‘design and modeling’ methods.
CONCLUSIONS AND FUTURE WORK
Both modeling and experimental measurements demonstrate
that the individual nantennas can absorb close to 90 percent of
the available in-band energy. Optimization techniques, such as,
increasing the radial field size could potentially increase this
efficiency to even higher percentages. More extensive research
needs to be performed on energy conversion methods to derive
overall system electricity generation efficiency. The circuits can
be made from any of a number of different conducting metals.
The nantennas can be formed on thin, flexible materials like
polyethylene.
Further laboratory evaluations of the flexible substrate NEC
prototypes are planned. Manufacturing methods will continue
to be refined to support roll-to-roll manufacturing of the
nanostructures. Future work will focus on designing the
nantenna structure for operation in other wavelengths. By
further shaping the spectral emission of the NEC it may be
possible to concurrently collect energy in the visible, near-
infrared and mid-infrared regions.