Rectify this

Below is a cut and paste “review” of the idea behind light rectification as an alternative method of deriving electrical power from solar energy. To preempt comments, this is not a magic bullet solution, it is some way yet before it may be useful (if ever) and I am not suggesting that “all our energy problems are over hip hip hooray”. However, IF advances keep apace, the possibility of being able to mass print such a device as envisioned below may be a welcome development.

Rectification is part of the process of converting an AC signal into a DC signal. Simple half wave rectification uses a diode that allows the forward going current to pass and blocks the reverse going current. Additional components are then used to “smooth” the resulting signal. Full rectification uses a bridge rectifier (essentially a collection of diodes) arranged in such a way that both the input forward and reverse currents appear at the output of the circuit in the same direction. Again, when smoothed this becomes a DC current.

Rectification is not a new technology. The simple “cats whisker” rectifier (a piece of wire in contact with a semiconducting crystal) of simple crystal radio sets – perhaps a novelty for the Nintendo generation – was the heart of the detector.

Rectification of microwaves is achievable at high power and is one of the technologies behind schemes to transmit solar power from space to ground based receivers. It has also been demonstrated as a means of wireless transmission.

In theory, the same principle should be applicable to shorter wavelength radiation, if some fundamental limitations can be overcome.

Some background ;

New and emerging developments in solar energy
Solar Energy 76 (2004), 33 – 43

This paper describes some of the new and emerging developments, with special emphasis on: (1) nanoscale antennas for direct conversion of sunlight to electricity with potential conversion efficiencies approaching 80–90%;

Nanoscale antenna solar energy conversion: The current photovoltaic technologies rely on the quantum nature of light and semiconductors which are fundamentally limited by the band-gap energies. A revolutionary new approach suggested by Professor Robert Bailey in 1972 revolves around the wave nature of light. Professor Bailey suggested that broadband rectifying antennas could be used for solar to d.c. conversion. These rectennas would not have the fundamental limitation of semiconductor band-gap limiting their conversion efficiencies. Rectennas for solar conversion would have dimensions of the order of the wavelengths of solar radiation which falls mostly in the sub-micron range.
The challenges in actually achieving the objectives are many. This paper describes the challenges and approaches to their solution.

Direct solar energy conversion to electricity is conventionally done using photovoltaic cells, which makes use of the photovoltaic effect. The photovoltaic effect depends on interaction of photons, with energy equal to, or more than the band-gap of the photovoltaic materials. Over many years of research, the efficiency of photovoltaic cells has incrementally increased.

The antenna concept, on the other hand, relies on the fact that solar radiation is electromagnetic in nature. In other words, the waves are oscillating electric and magnetic fields propagating from the sun to the earth. The use of antennas for signal (and power) transmission and reception is a very well-developed science in the microwave frequencies. Brown (1984), of the Raytheon Company, a pioneer of microwave power transmission, coined the term ‘‘rectenna’’, for rectifying antenna. Reception and conversion of microwave radiation (at 2.45 GHz) at close to 90% efficiencies have been recorded in the literature.

The advantage of the antenna approach to solar energy conversion is that, primarily, conversion would not be band-gap limited as in photovoltaic cells. It is assumed while making this assertion that the process of rectifying these high frequency electromagnetic waves will not involve any such lossy quantum effects.

It is hypothesized that EM wave to d.c. conversion can be done at solar frequencies with potentially much higher conversion efficiencies than with present day photovoltaic technologies. The challenges in actually achieving this objective are many. They are summarized below:

• Scale: Antennas for solar conversion have to have dimensions ofthe order of wavelengths of solar radiation that falls in the sub-micron range. This size range brings up the associated challenges of fabrication, interconnection, quality control, mass-manufacturing and such.

• Material: What type of materials are best suited for working at solar frequencies? The antennas will probably have to be made of dielectric materials, the choice being decided by the effect on antenna losses.

• Rectification: The received waves will have to be rectified to d.c. to be useable. Such high frequency rectification would be quite a challenge, which will have to be overcome.

• Antenna properties: The antenna array will have to be optimized in order to achieve good conversion efficiencies. The antenna geometry and materials will have to be selected such that the array is broadband, of good beam-width, circularly polarized, and with a center frequency in the solar range.

• Impedance matching: Good impedance matching while working with broadband signals will be another challenge to be overcome.

…

1.2. Rectification
The biggest stumbling block in achieving the ASEC concept is the ability to rectify electromagnetic waves at the high frequency range of visible and IR radiation. Conventional rectification devices are available for frequencies that are several orders of magnitude lower. As stated earlier, recent developments in nanotechnologies and molecular electronics could prove to be very useful.

The article below from Energy Matters reports on one aspect of this, at the risk of being seen as over optimistic science by press release.

New Solar Panel Claimed To Achieve 95% Efficiency
Energy Matters, 17th May.

A chemical engineer (Patrick Pinhero) in the USA claims he has developed a new flexible solar panel which can capture up to 95 percent of available light energy by harnessing infrared light and areas of the solar spectrum standard photovoltaic (PV) cells ignore.

His team have developed a thin, moldable solar sheet that incorporates small antennas that are conventionally used to convert heat to energy in industrial processes. These antenna - or nantenna, as they are known, work in concert with the solar PV process, collecting solar irradiation in the near infrared and optical regions of the solar spectrum and converting it to electricity.

An open access document part authored by Patrick Pinhero gives some more detail.

SOLAR NANTENNA ELECTROMAGNETIC COLLECTORS
Presented at Energy Sustainability 2008, August 10-14.

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.
Several factors were critical in successful implementation of NEC including:

1) frequency-dependent modeling of antenna elements;

2) selection of materials with proper THz properties; and

3) novel manufacturing methods that enable economical large-scale manufacturing.

The work represents an important step toward the ultimate realization of a low-cost device that will collect, as well as convert this radiation into electricity, which will lead to a wide spectrum, high conversion efficiency, and low-cost solution to complement conventional PVs.

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.

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].

The major technical challenges continue to be in developing economical manufacturing methods for large-scale fabrication of antenna-based solar collectors. Further research is required to improve the efficiency of rectification of antenna induced terahertz currents to a usable DC signal. The material properties and behavior of antennas/circuits in the THz solar regions need to be further characterized.

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.

…

The nantenna radiation pattern displays angular reception characteristics, resulting in a wider angle of incidence exposure to thermal radiation than typical PV. Any flux from the sun that falls within the radial beam pattern of the antenna is collected. This property is a critical antenna characteristic that optimizes energy collection from the sun as it moves throughout the horizon. Thus, it may be possible to reduce the need for mechanical solar tracking mechanisms. It also provides designers another mechanism to increase the efficiency of antenna arrays through the expansion of the radial field. Antennas by themselves do not provide a means of converting the collected energy. This will need to be accomplished by associated circuitry such as rectifiers.

This research has demonstrated that infrared rays create an alternating current in the nantenna at THz frequencies. Commercial grade electronic components cannot operate at that switching rate without significant loss. Further research is planned to explore ways to perform high frequency rectification. This requires embedding a rectifier diode element into the antenna structure. One possible embodiment is metal insulator-insulator-metal (MIIM) tunneling-diodes.

The paper then goes on to discuss some proof of concept developments and manufacturing processes for the mass production of the nantenna (see original). But at the stage described in the paper above rectification problems remain. But this area of research has also seen some recent advances, perhaps as a result of the recent developments in Terahertz (THz) scanners being introduced in airports.

From Gizmag,

Revolutionary diode design cracks 50 year-old electronics speed barrier
Grant Banks November 8, 2010

Metal-insulator-metal (MIM) diodes might just be the technology that allows electronics achieve the next big leap in processing speed. Research into diode design conducted at the Oregon State University (OSU) has revealed this week cheaper and easier to manufacture MIM diodes that will also eliminate speed restrictions of electronic circuits that have baffled materials researchers since the 1960's.

MIM diodes allow almost instantaneous electron transfer through the insulator surface – a major step in eliminating the 50 year-old speed barrier found in transistor based electronics.

The full paper is available.

Advancing MIM Electronics: Amorphous Metal Electrodes
Advanced Materials Volume 23, Issue 1, pages 74–78, January 4, 2011

Effectively controlling quantum mechanical tunneling through an ultrathin dielectric represents a fundamental materials challenge in the quest for high-performance metal-insulator-metal (MIM) diodes. Such diodes are the basis for alternative approaches to conventional thin-film transistor technologies for large-area information displays,various types of hot electron transistors,2–6 ultrahigh speed discrete or antenna-coupled detectors, and optical rectennas.

At the very least it may soon be possible to intercept infra red radiation directly from the sun without the need of using some thermo chemical or thermo mechanical converter. I suspect there may be some  defense investment behind this, as one obvious application of such a system might be direct powering of small devices using a soldiers body heat.