Abstract:
Systems and methods for an improved solar/infrared conversion efficiency using multiple rectennas, one for each band of the spectrum. Each rectenna is optimally efficient for each spectrum band. An antenna receives at least one of a visible or infrared spectrum. Rectifying circuits coupled to the antenna generate a current based on a portion of the spectrum received by the at least one antenna. Each rectenna operates efficiently using a different operating voltage. The operating voltages are based on the selected load resistor and the current-voltage characteristics for the diode of the rectifying circuit at the associated spectrum portion.

Description:
PRIORITY CLAIM 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/758,578 filed Oct. 25, 2012. 
    
    
     FIELD 
     Embodiments relate generally to rectennas (i.e., antenna-coupled diodes), and, more particularly, to more efficient rectenna solar cells. 
     BACKGROUND 
     It has been assumed implicitly by researchers in the field of rectennas (i.e., antenna-coupled diodes) that rectenna solar cells can provide efficient power conversion for the full solar spectrum. This would appear to be the case because rectennas operating in the microwave band can efficiently convert a broad range of frequencies. However, in the visible and near-infrared spectrums this is not the case. 
     Ordinarily diodes (as used in rectennas) act as mixers, producing sum and difference frequencies in response to input frequencies. This mixing effect would be expected to homogenize the input frequencies so that a single operating voltage would suffice for power conversion. This mixing effect increases with increasing input power. However, the maximum solar power that can be obtained by each rectenna element is approximately 0.3 microwatts due to coherence limitations. This does not provide sufficient power to produce significant mixing in the diode. 
     At the frequencies of visible and infrared light, the operation of an optical rectenna is fundamentally quantum mechanical in nature. A monochromatic (single-frequency) light source can, in principle, be converted to electrical power by an optical rectenna with 100% efficiency. However, this high conversion efficiency is available only at one operating voltage, corresponding to the photon energy (in eV) divided by the electronic charge q. That means that the entire solar spectrum cannot be efficiently converted to electrical power using optical rectennas operating at any single voltage. 
     BRIEF SUMMARY 
     The present invention provides an improved solar/infrared conversion efficiency using multiple rectennas, one for each band of the spectrum. Each rectenna is optimally efficient for each spectral band. 
     According to one set of embodiments, a method is provided. The method includes receiving a visible or infrared spectrum at an antenna of a rectenna. A rectifying circuit (diode, filter, load) is coupled to each antenna. At the rectifying circuit coupled to the antenna, a current is generated based on a portion of the spectrum received by the antenna. Each of the rectennas operates efficiently using a different operating voltage. The selected operating voltages are based on the selected load resistor and the current-voltage characteristics for the diode of the rectenna at the previously identified spectral portion. The efficiency of rectifying multiple narrow spectral portions is high compared to the rectification of the entire spectrum. 
     According to another set of embodiments, a system is provided. The system includes at least one antenna that receives at least one of a visible or infrared spectrum and at least one rectifying circuit that is coupled to the at least one antenna. The rectifying circuits generate a current based on a portion of the spectrum received by the at least one antenna. Each rectifying circuit operates at a different operating voltage. 
     In one aspect of the invention, the rectennas include a diode and a load resistor coupled in parallel to the antenna. 
     In another aspect of the invention, the load resistor is a variable load resistor. 
     In still another aspect of the invention, the system includes a spectral splitting device that splits the received spectrum into at least two spectral bands. The rectennas are located on a single substrate configured to receive the split spectrum bands. 
     In yet another aspect of the invention, at least one of the rectennas is located on a first substrate that allows at least a portion of the spectrum to pass through. Another of the rectennas is located on a second substrate. The second substrate is located further from the source of the spectrum. 
     In a further aspect of the invention, the antenna comprises at least one dipole antenna or bowtie antenna or any frequency selective surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is described in conjunction with the appended figures: 
         FIG. 1  shows a schematic diagram of an exemplary portion of a spectral splitting rectenna system; 
         FIG. 2  shows a perspective view of a portion of a planar spectral splitting rectenna system; 
         FIG. 3  shows a top view of a portion of a planar spectral splitting rectenna system; 
         FIG. 4  shows a perspective view an exemplary multi-planar spectral splitting rectenna; 
         FIG. 5  shows an exemplary circuit diagram of a single rectenna; and 
         FIG. 6  shows a graph of current-voltage characteristics for an exemplary rectenna diode. 
     
    
    
     In the appended figures, similar components and/or features can have the same reference label. Further, various components of the same type can be distinguished by following the reference label by a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label. 
     DETAILED DESCRIPTION 
     Ordinarily a diode rectifier in a rectenna would not require different operating voltages for different frequencies. However, quantum rectification (i.e., photon-assisted tunneling) that takes place for visible and near-infrared photons requires photon-energy-dependent (or similarly wavelength-dependent) operating voltages. 
     In order to take advantage of the photon-energy-dependent voltage, the present invention passively or actively splits the solar spectrum into different spectral bands, each of which is absorbed by rectennas having an operating voltage that optimizes the power conversion efficiency for that band. 
     As shown in  FIG. 1 , a radiation source (e.g., the sun, or any source of thermal radiation like a hot body) provides radiation  10  (visible/infrared) that is split by a spectral splitting device  12  into previously defined spectral frequency bands  20 . The previously defined spectral frequency bands  20  are directed by the spectral splitting device  12  to rectennas  30  having customized operating voltages. The operating voltage is determined by selecting the resistance of the load for a given input power. The given input power is determined by the received spectral band. Each rectenna  30  is designed to absorb radiation over a well defined spectral band corresponding to the operating voltage for that rectenna. 
     In one embodiment as shown in  FIG. 2 , splitting of the spectrum  10  is accomplished optically, using a wavelength-selective device  40 . A prism or a grating will suitably spatially split the spectrum into spectral components. The wavelength-selective device  40  sends spectral bands  21  to a group of frequency-insensitive rectennas  31  located on a substrate  34  (e.g., printed circuit board). The rectennas  31  are coplanar, with different spectral bands  21  being processed by spatially separated rectennas. In one embodiment, the antennas used in the rectennas  31  include broadband bowtie antennas. 
     As shown in  FIG. 3 , a substrate  50  includes frequency-sensitive rectennas  52  that absorb only a predefined band of wavelengths. Thus, the spectral splitting is done passively. The frequency-sensitive rectennas  52  include wavelength-selective dipole antennas  54 . The spectrum is incident on the surface of the substrate  50 . The frequency-sensitive rectennas  52  include a diode  51  and a load resistor  60 . The diodes  51  and the load resistor  60  are configured to operate at a particular operating voltage. An example diode  51  includes a metal insulator metal (MIM) diode. 
     Because the dipole antennas  54  can absorb radiation over an area that is larger than their physical size, the dipole antennas  54  that are sensitive to different spectral bands are arrayed over the substrate surface  50 . Different spectral portions of the total radiation (incident over the large area) are absorbed by the selective antennas occupying no more footprint than the area occupied by the incident radiation. As a result, spectral splitting is achieved by spatially separated rectennas on the same plane and does not require a grating. 
     As shown in  FIG. 4 , a rectenna system  70  includes stacks of semi-permeable substrates  72  that include one or more frequency-sensitive rectenna  74 . Each level of the stack of rectennas absorbs different frequency bands. Thus, non-absorbed frequency bands penetrate to the next substrate layer  72 . The material chosen depends on the incident spectrum (visible, NIR, long wavelength IR). Examples include quartz, fused silica, transparent polymers/plastics, calcium fluoride and potassium bromide IR transparent substrates. 
     In another embodiment, a single broadband antenna absorbs the entire spectrum of interest, or a substantial fraction of it. The electric current coming from the antenna is split into frequency bands, and each frequency band of current is directed to a separate diode whose operating voltage is tuned for that band. One way to accomplish the splitting is with the use of passive inductor-capacitor (LC) filters, as is conventionally used to filter ac signals. 
       FIG. 5  shows an exemplary rectenna circuit  90 . The rectenna circuit  90  includes an adjustable load resistor  92 , an antenna (frequency sensitive or insensitive)  98 , a diode  94  and a DC output filter  96 . The DC output filter  96  blocks AC power dissipation in the load resistor  92 . The adjustable load resistor  92  is adjusted to provide a desired voltage across the diode  94 . 
       FIG. 6  shows current-voltage [I(V)] characteristics for a rectenna diode (such as the diode  94 ). The solid curve  100  is the I(V) for an exemplary rectenna in the dark (i.e., not exposed to the spectral source), and the dashed curve  102  shows the I(V) under illumination from a specific spectral band. The voltage at which the dashed curve  102  crosses the Voltage axis is equal to the photon energy divided by the electron charge. The location where the curve  102  crosses the v-axis indicates the maximum voltage that the rectenna can extract from the incident spectrum. Beyond this voltage, the current is negative and the rectenna cannot generate any energy. For an ideal rectenna diode illuminated by a monochromatic source, the rectenna efficiency approaches 100% when the bias voltage is equal to the photon energy divided by the electron charge. Therefore, for a high-photon-energy slice (band) of the spectrum the dashed line  102  extends farther to the left on the Voltage axis than for a low-photon-energy slice. A load-line  106  intersects the rectenna illuminated I(V) (the dashed curve  102 ) at the operating voltage V o . Rectenna output power is the area under the dashed curve  102  at the load-line  106  intersection. The slope of the load-line  106  and hence the operating voltage can be chosen for high rectenna efficiency by tuning the load impedance. The illuminated I(V) curve (dashed curve in  FIG. 6 ) is a function of the power and the energies of the different photons in the incident spectral band. 
     All the different spectral bands (i.e., frequencies) can be absorbed using rectennas composed of the same materials. The spectral sensitivity is determined by the operating voltage, and can also be affected by the antenna design and size. This provides a great reduction in the cost of making a wide spectral band rectenna system. This is particularly advantageous over spectrum splitting in conventional solar cells, where different materials are required for each spectral region. 
     For optimum efficiency, the diode I(V) characteristic has to be such that the diode impedance presented to a slice of the spectrum is matched to the antenna impedance. This results in maximum power transfer from the antenna to the diode. Also, the diode must have a fast response time so that its RC time constant is less than the inverse of (2 times π times the maximum frequency in the incident band). 
     The present invention applies to solar radiation and to any broad optical or infrared spectrum (e.g., near, mid, and long-wave IR spectrum). For example, power conversion from a hot object or source that produces blackbody radiation in the infrared would also benefit from the spectrum-splitting approach that is taught in detail herein. 
     Other examples and implementations are within the scope and spirit of the disclosure and appended claims. For example, features implementing functions can also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. Also, as used herein, including in the claims, “or” as used in a list of items prefaced by “at least one of” indicates a disjunctive list such that, for example, a list of “at least one of A, B, or C” means A or B or C or AB or AC or BC or ABC (i.e., A and B and C). Further, the term “exemplary” does not mean that the described example is preferred or better than other examples. 
     Various changes, substitutions, and alterations to the techniques described herein can be made without departing from the technology of the teachings as defined by the appended claims. Moreover, the scope of the disclosure and claims is not limited to the particular aspects of the process, machine, manufacture, composition of matter, means, methods, and actions described above. Processes, machines, manufacture, compositions of matter, means, methods, or actions, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding aspects described herein can be utilized. Accordingly, the appended claims include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or actions.