Optical Rectenna - High efficiency solar power generation
- 1 Background of technology, including the basic science foundation
- 2 Current state of the technology
- 3 Required inputs for energy generation
- 4 Organizations/researchers working with this technology
- 5 Reasons why the science and technology has not moved forward
- 6 Costs of facilities, production, now and projected future costs with improvements
- 7 Intellectual Property surrounding technology
- 8 Ability to be scaled
- 9 Environmental impact
- 10 Risks associated with a prize in this space
- 11 Positive energy tests to evaluate this technology
- 12 References
Background of technology, including the basic science foundation
[Note: There is also a page about Solar Technologies.]
In 1941, science fiction writer Isaac Asimov published the science fiction short story "Reason", in which a space station transmits energy collected from the Sun to various planets using microwave beams. The SBSP concept, originally known as satellite solar-power system (SSPS), was first described in November 1968.
Space-based solar power (SBSP) is the concept of collecting solar power in outer space and distributing it to Earth using a solar-power satellite or a solar-power system. It has been in research since the early 1970s.
In 1973 Peter Glaser was granted U.S. patent number 3781647 for his method of transmitting power over long distances (e.g. from an SPS to Earth's surface) using microwaves from a very large antenna (up to one square kilometer) on the satellite to a much larger one, now known as a rectenna, on the ground.
An antenna-coupled diode operating at optical frequencies, also called an optical rectenna, incorporates a submicron antenna and an ultra-high speed diode. The optical rectenna absorbs electromagnetic radiation and converts it to current. A diode rectifies the AC current, providing DC electrical power. Compared to conventional solar cells, which absorb photons and generate electron-hole pairs to provide electrical power, rectennas seem to rely on a classical electromagnetic wave view of light.
Optical rectennae could eventually generate electricity with twice  the efficiency as conventional solar cells, for a tenth of the cost. Unfortunately, as this technology is still in its infancy. However, it has been said that rectennae could become well-suited to mass-production once the method of construction has been perfected in practice.
The term 'rectenna' refers to a configuration in which an antenna receives incident light waves and the oscillating electric field generated by the light is converted into a direct current by a rectifying diode. This concept of coupling a solar light antenna to diodes to generate DC current was first proposed by Professor R.L. Bailey in 1972. The idea was patented in 1973, describing an array of conical metallic antennas coupled to diodes. The antenna spacing and position could be altered to optimise the output under different polarizations of light (circular, plane, random, etc.).
The genius of Bailey's idea is that it allows the generation of electrical energy from solar radiation without the use of semiconducting materials, as in conventional solar cells. Semiconductor atoms have electrons bound to them, but only require a small amount of energy (compared to insulators) to liberate them so they are free to conduct electricity. The amount of energy needed to liberate them is known as the 'band gap'. In solar cells, if the energy of the photon is less than the band gap, not enough energy is present to liberate an electron and the photon simply passes through the materials. But if the photon energy is greater than the band gap, the electron is liberated, but the excess energy is lost as heat. In either case, the energy is lost and solar cell efficiency is reduced. With the antenna technology, the efficiency of conversion could theoretically reach 86.8%, compared to the highest possible efficiency for conventional solar cells at 44%.
Current state of the technology
With all these hurdles (as well as other factors such as the coherence of light and the materials used), it was difficult to create a rectenna optimised for optical light. Even larger rectennae designed to receive long-wave radiation have usually failed to yield high efficiencies. And even when high efficiencies were seen, the polarization and coherence of the radiation source used were artificially optimised in a way that solar radiation is not.
In 2015, engineers at the Georgia Institute of Technology developed the first working optical rectenna by using carbon nanotubules. They had achieved good impedance matching by making the antenna part of the diode. In addition, the diode itself can achieve frequencies near 1PHz and has an ultra-low capacitance of a few attofarads (achieving a low time constant). The efficiency is still lower than 1% but can potentially be improved to 40-90% efficiency, if the combination of materials can be optimised and the waste heat harvested.
The rectennae can operate stably at temperatures between 5 and 77 degrees Celcius whereas conventional semiconductor solar cells require low temperatures for maximum efficiency.
While they are still difficult to build, the process itself is, in principle, simple and could potentially become well-suited to inexpensive mass production due to the relatively cheap materials.
Within the study of global weather modification programs, there lies the potential that satellites are already being used to collect energy from the sun for the purposes of retransmission to any other satellite in a global network (for coverage of the entire planet, including the dark side, at all times), with the ultimate purpose of directed energy discharge charge capability into any specific earth atmospheric location.
NASA has also conceptualized the idea of satellite solar energy collector and re-transmitter technology, using the term suntower, citing the early work of Peter Glaser in their technical paper.
Ultra-Low-Cost Solar Power
The billionaire backed Breakthrough Energy Coalition lists the greater field of Ultra-Low-Cost Solar (not necessariy Optical Rectenna) as a pathway among their landscape to a better energy future.
The amount of solar energy that hits Earth every day is large enough to power the world many times over with carbon-free electricity. Continuous technology innovation and increasing scale of the solar power technologies we have today has resulted in dramatic cost reductions and a clear path for solar power to achieve electricity costs of 5c/kWh or less that are directly cost competitive with traditional forms of electricity generation in many parts of the world. However, cost reductions significantly below these levels will be required to 1.) allow large penetrations of solar power onto the grid to be competitive from a system-wide perspective – to account for the added costs of energy storage and other grid upgrades that may be required to accommodate growing amounts of solar – and 2.) allow solar power to cost effectively decarbonize other sectors through the production of low carbon transportation fuels and industrial materials. A new generation of transformational solar power technologies, beyond what we have today, will need to be developed to achieve these dramatic continued reductions in the cost of solar power.
Required inputs for energy generation
The inputs for any energy generation process can be represented as shown in the System Representation.  The efficiency of the system is represented by the output energy divided by the input energy. In this case the input energy is an external "free" source of energy (the Sun). The higher the efficiency of energy conversion the smaller the area required to produce a given amount of power. As noted above though, the polarisation of the input energy (light) can be relevant to the efficiency of the device.
As with solar cells, optical rectennae do no require any energy inputs (other than solar radiation) to operate.
Organizations/researchers working with this technology
- The Georgia Institute of Technology
- H. Y. He - University of Science and Technology of China; University of California
- S. T. Pi - University of California
- Z. Q. Bai - University of California
- M. Banik - University of California
- V. A. Apkarian - University of California
- R. Q. Wu - University of California
- Somayeh M. A. Mirzaee - Queen's University, Kingston, ON, Canada
- Jean-Michel Nunzi - Queen's University, Kingston, ON, Canada
Other Organizations in Advanced Solar
- The Center for Revolutionary Solar Photoconversion (CRSP) a research center of the Colorado Energy Research Collaboratory, is to conduct basic and applied research that will result in scientific and technological revolutions in solar energy conversion. The scientific advances generated in CRSP will underpin future renewable energy technologies that exhibit highly efficient and low-cost production of both fuels and electricity via direct solar-conversion processes.
Reasons why the science and technology has not moved forward
Bailey's design requires that the spacing between adjacent antennas and other similar dimensions are close to the wavelength of the light the rectenna is designed to receive. This equates to structures smaller than a micrometre, which were difficult to construct prior to recent developments in nanotechnology. In addition, rectennae this small present problems for quality control, interconnectedness and mass-production.
The rectifying diodes themselves need to be able to turn on and off at frequencies matching the frequency of the light, so ultra-high frequency diodes needed to be developed before rectennae became viable. This required the time constant of the diode to be low, which required that the diodes be very small - leading us back to the nanotechnology problem above. Also, if each antenna is coupled to an individual diode (Bailey's design), the diodes need to be able to work at low voltages because each antenna would only produce a small voltage. Furthermore, impedance matching between antenna and diode must be good, so as to improve the efficiency of energy transfer.
We will specifically discuss the following important issues:-
- The coherence of sunlight, which is not an issue for conventional solar cells, but is of crucial concern for rectenna solar cells.
- A quantum theory of rectification at optical frequencies.
- Diode challenges and potential solutions, including MIM structures and new concepts.
- Antenna constraints.
- Ultimate and practical power conversion efficiency limits, including heat harvesting.
Costs of facilities, production, now and projected future costs with improvements
Researchers at GeorgiaTech have estimated that costs for rectennae could ultimately drop to one-tenth the cost of solar. Nevertheless, like with solar, it is important to consider not only panel costs but also balance-of-system costs in evaluating the overall cost of electricity generated from this form of energy.
Intellectual Property surrounding technology
- US3760257 A - Bailey's original design
- US8115683 B1 - A nano-scale rectenna which converts solar energy to heat energy, then heat energy to electric.
- US9255840 B2 - A system in which the light is split into various frequencies which are then redirected to rectennae optimised for those frequencies
- US7799988 B2 - A rectenna made of metallic-carbon nanotubules, acting as both antenna and diode.
- US20110100440 A1 - An optical rectenna coated in a 'nongaseous conductive medium'
- US9346563 B1 - A solar-powered space weapon, using optical rectennae as a power source
- EP2909865 A1 - The patent for the first working optical rectenna and the unique method of constructing it
- US 3781647 A - Method and apparatus for converting solar radiation to electrical power
Ability to be scaled
Scalability of this technology would be the same order of magnitude as solar. With innovation and careful development rectenna solar cells have the potential to provide an exciting new photovoltaics technology.
The energy generation process can be represented as shown in the System Representation. For this technology to be scaled up consideration needs to be given to the logistics of scaling up of the deployment of solar panels (and their manufacture), and to the environmental impact arising from waste (end of life decommissioning) and land use. The promised higher solar conversion efficiency helps reduce both of these potential impacts.
An innovation that helps to improve deployment would be useful, so that large scale solar farms can be deployed quickly and cheaply.
Energy technologies can be broadly categorised using the Sustainability Scale. This takes into account all aspects of the technologies life-cycle, including its dependencies.
Based on sunlight, this solution requires large surface areas to scale. If carbon nanotubes are the key ingredient, material resource use seems less of a limiting factor.
Emissions / waste
No emissions. No information yet on end-of-lifecycle waste.
Risks associated with a prize in this space
Risks are associated with all radical innovations, and that can be due to several factors. A good technology might not succeed in the marketplace due to poor marketing and promotion. The perceived safety and environmental impact of a technology is also important to successful adoption.    Fortunately, solar power seems to be relatively well accepted, except for some concerns over land use    and where it displaces agriculture.
Poor implementation of a technology can also prevent successful adoption of a good technology. These are risks that come into effect after the awarding of an energy technology prize, but perhaps the associated challenge can provide post award support to ensure that these risks are reduced. In this case the following practical guidance for deploying solar farms may be useful. 
In addition, of course, there can be risks associated with the technical efficacy of the technology itself, and the logistics surrounding its development, operation and decommissioning.
Conversely, if we think outside of the box then solar power [in general] can be scaled up vastly by: having huge solar farms in the deserts; and by the creation of large scale solar structures in Earth orbit (see Collecting solar power in space).
Positive energy tests to evaluate this technology
This is especially crucial for technologies that are as yet untested, or have not yet generated large amounts of verifiable performance data. What conditions would need to be met for this technology to be considered unequivocally “verified” or “validated”?
An experimental procedure is discussed for a carbon nanotube rectenna. 
The inputs and outputs for any energy generation process can be represented as shown in the System Representation. The efficiency of the system is represented by the output energy divided by the input energy. There are many laboratories that should be able to determine the net efficiency of this solar technology - see Laboratories.
- Overview of optical rectennas for solar energy harvesting Zixu Zhu, Saumil Joshi, Bradley Pelz and Garret Moddel University of Colorado, Boulder (United States)
- Solar energy collection by antennas
- New and emerging developments in solar energy
- Optical Rectenna Could Double Solar Cell Efficiency
- Solar Cells Will be Made Obsolete by 3D rectennas aiming at 40-to-90% efficiency
- Photovoltaic Technologies Beyond the Horizon: Optical Rectenna Solar Cell
- A Proposed New Concept for a Solar-Energy Converter
- Electromagnetic wave energy converter
- P/N Junctions
- Overview of optical rectennas for solar energy harvesting
- The rectenna device: From theory to practice
- First Optical Rectenna – Combined Rectifier and Antenna – Converts Light to DC Current
- A carbon nanotube optical rectenna
- Solar Cells Made Obsolete
- Bostock A. (2017). System Representation, Energy Wiki
- Stark Effect and Nonlinear Impedance of the Asymmetric Ag-CO-Ag Junction: An Optical Rectenna
- Evidence of optical rectification in Ag nanoparticles and its application in rectenna device
- Electromagnetic wave energy converter US 3760257 A
- New and emerging developments in solar energy
- Rectenna solar energy harvester
- Spectrum splitting using optical rectennas
- Apparatus and system for a single element solar cell
- Optical Rectification Device and Method of Making Same 
- Solar powered space weapon
- Multilayer coatings formed on aligned arrays of carbon nanotubes
- Will Rectenna Solar Cells Be Practical - Garret Moddel
- Sustainability Scale
- Slovic and Weber (2013). Perception of Risk Posed by Extreme Events, Regulation of Toxic Substances and Hazardous Waste (2nd edition) (Applegate, Gabba, Laitos, and Sachs, Editors), Foundation Press, Forthcoming
- Michael Siegrist, Heinz Gutscher & Timothy C. Earle (2006). Perception of risk: the influence of general trust, and general confidence, Journal of Risk Research: Volume 8, 2005 - Issue 2
- Linda Steg and Inge Sievers (2016). Cultural Theory and Individual Perceptions of Environmental Risks, Environment and Behavior: Vol 32, Issue 2, pp. 250 - 269, First published date: July-26-2016
- Alona Armstrong (2014). Solar is booming but solar parks could have unintended climate consequences, Guardian sustainable business
- Solar Energy Development Environmental Considerations, the web site for Programmatic Environmental Impact Statement for Solar Energy Development in Six Southwestern States (Solar PEIS)
- Mid Devon Gazette (2012). News story: Go ahead for solar farms plan despite 'eyesore' objection
- CLA (2015). Guidance Note for CLA members: OVERVIEW of SOLAR FARMS, CLA: membership organisation for owners of land, property and business in rural England and Wales
- The falling costs of US solar power, in 7 charts
- How cheap does solar power need to get before it takes over the world?