Rectenna Element

The design of rectenna elements for WPT has usually been focused at 2.45GHz, but this frequency requires large physical dimensions for the individual components.  At frequencies higher than about 10GHz, components become expensive to produce and atmospheric absorption plays a significant role in reducing incident power, which reduces overall link efficiency (Figure 1).  Several factors must be considered when choosing a frequency for microwave power transmission, including:

  • Size of the aperture
  • Frequency dependence of overall system efficiency
  • Heat radiation in space associated with the inefficiency of components
  • Whether the transmission takes place only in space or also through Earth’s atmosphere
  • The existing state of the art of available components
  • The impact of using the selected frequency on other users of the electromagnetic spectrum

Given these considerations, 5.8GHz is selected as the frequency of operation due to its smaller component size, lower atmospheric losses (especially during rainfall), lower cost, pre-existing band licensing, and availability of several high-efficiency rectenna element designs.

Figure 1. Earth’s Atmospheric Attenuation Over the RF Range

Main Element Components

The rectenna is a key element of the power transmission system, serving to receive and convert microwave power into dc power.  Each rectenna element consists of: half-wave dipole, input low-pass filter, rectifying diode, output filter, and resistive load.  Once the microwave power is received by the individual antenna, a low-pass filter transforms the dipole impedance to the diode impedance and rejects higher order diode harmonics from radiating through the dipole.  The microwave power passes through the high frequency rectifying diodes.  A capacitor serves as a DC pass filter which is followed by a resistance across the output terminals to act as a load.  Figure 2 shows the function blocks of the rectenna.

Figure 2. Block Diagram of a Rectenna Element

Design Optimization

Optimization of the antenna design and the rectifying circuit is crucial to obtaining the highest conversion efficiency.  The diode must be matched to the microwave power level and to the DC load.  The physical distance between the capacitor and the diode is critical since the capacitor can be used to tune out the reactance of the diode, thereby improving conversion efficiency.  Inclusion of tuning stubs (Figure 3) can further match the impedances to push the efficiency even higher.

Figure 3. General Schematic of a Rectenna Element

From specified state of the art designs, a well-matched rectenna element can reach a peak efficiency of 82% when attention is given to considerations such as the shape of antenna, the type of diodes, the space between elements, and the load impedance, among other factors.

 

Rectenna Array

The rectenna elements will be arranged in a two-dimensional array in the shape of a circle.  The rectenna array will radiate in the broadside direction relative to array orientation.  The surface area of the overall structure is linked to the diameter of the transmitting antenna for a desired percentage of transmitted power, and both path distance and operating frequency are known.

When the element spacing is larger than ~0.8 λ, grating lobes (unwanted peaks in the radiation pattern in directions other than that of the main lobe) begin to appear.  Since the radiating element is a half-wave dipole, the spacing between elements must be larger than 0.5 λ to avoid overlapping.   In order to reduce coupling between adjacent elements, a practical element spacing of 0.7 λ will be used in the rectenna layout.

Since the side effects of using amplitude taper to control side lobe level include broadening of the main beam and a reduction in overall directivity, the transmitter will not use any windowing functions to modify the aperture distributions.  Placing the dipoles at 0.25 λ above a ground plane creates an image below the ground plane and makes the dipole directional.  An illustration of a small dipole array with ground plane is shown in Figure 4.

 

Figure 4. Illustration of Dipole Array

The shape of the full rectenna array is considered to be the aperture of the receiving antenna and can be approximated by the number of discrete elements.  Given that the diameter of the space transmitting dish is 1km, the required diameter of the rectenna is 5.3km.  Thus, the surface area of the rectenna array is 22km2, which in turn specifies the number of elements at approximately 16.8 billion.  The array designed is summarized in the following table.

Table 1. Design Parameters

Power Management

The large number of rectenna elements and large area of the rectenna array necessitate a distributed approach to power management.  Since the incident power will be on the order of several gigawatts, connecting all of the elements to a single output could result in a catastrophic loss of power if something in the system were to fail.  Large power plants in use today were designed with an individual capacity of about 1 GW.  Dividing the rectenna array into partitions of about 1 GW gives the design a level of robustness against equipment failure, and takes advantage of the fact that power management equipment already exists on this scale.  Alternatively, each array could be divided into more or fewer partitions depending on the specific needs of the geographical location and large customers in the area.  Figure 5 shows a breakdown of the equipment groupings.

Figure 5. Earth Station Equipment Groupings

Ultimately the array must be divided in some way because of the distributed nature of the harvesting implementation and the need to protect the overall system from equipment failure.  The array will be subdivided into many tiles, each of which will have dimensions of approximately 3 m x 2.25 m.  Every tile will be interconnected in an arrangement similar to Figure 6, with bypass diodes on each string to protect against underperforming elements.  Like PV arrays, operating the tiles at power levels lower than peak power would cause a reduction in output across the entire rectenna array.  Since the incident power levels will not be uniform across the array, tracking the maximum power point at each tile and using a DC/DC converter to transform the voltage at the load can ensure maximum power transfer and minimize the associated losses.  This peak power tracking will regulate the output voltage of the receiver tile and increase the stability of the overall system.

Figure 6. Sample Element Interconnections

The manufacturing of the tiles will be expensive at first, but given that each rectenna array will require over 3.2 million tiles, the overall price per tile will benefit greatly from economies of scale and iterative process improvements.  The tiles are sized such that they can be prefabricated and then transported via truck to the installation location.

 

Power Transmission

Each tile will output the same DC voltage level at different currents.  An analysis for each location (depending on the intended use of the power) must be performed to determine whether to integrate an inverter directly onto each tile for power transmission, or whether to connect a number of tiles in series/parallel (DC line) before inverting for transmission.  Figure 6 below illustrates a sample DC line configuration of two tiles.   Each tile has many elements connected in series, and the output of the two tiles are connected in parallel and fed into an inverter.  Also present are necessary circuit protection components and breakers.

Figure 7. Sample DC Line Configuration

The inverters would be synchronized and connected to one of several substations built underground directly beneath the rectenna array for transformation to HVAC and transmission to an offsite load.  The substation and inverter designs must be performed with additional knowledge of the geographic location and existing power grid operational parameters in order for the rectenna array to smoothly integrate into existing infrastructure.

 

Safety

The use of microwave power is perhaps the most controversial aspect of a space solar power system. An assessment by the US National Reserch Council has recommended more focus on environmental, health and safety issues. Recently, the electromagnetic pollution on the earth has been acknowledged by both industry and researchers. In the SERT Program standard, the center of beam power densities is limited to the range of 100-200W/m2 to assure environmental health and safety. [5]

In the system of this project, with a target power capacity of 5GW, the power density above the rectenna area is up to 56.65 W/m2, and it drops off to about 1 W/cm2 at a distance of 5.3km from the rectenna center where is beyond the perimeter of the rectenna. The sidelobe peaks are less than 0.5 W/cm in this case. Since intensity levels rapidly decrease outside the focus area, nearby people and wildlife would remain completely unaffected. More information about safety issues considerations, please refer to [5].

Exposure to dangerous radiation stemming from an SSP system can be mitigated in several ways. To minimize exposure on the Earth’s surface, access to dangerous areas can be controlled by fencing and security personnel. Minimizing aircraft exposure can be achieved through the implementation of no-fly zones or controlled flight areas. Aircraft flying near the exposure sites can be shielded with protective materials.

The exposure of birds and other wildlife should also be considered. Current experiments show minimal or no effects on birds, monkeys, cows, and other animals who have been exposed to SSP level radiation [6].

 

 

References

http://www.rfcafe.com/references/electrical/ew-radar-handbook/rf-atmospheric-absorption-ducting.htm

J. O. McSpadden, Lu Fan, and Kai Chang, “A high conversion efficiency 5.8 GHz rectenna,” Proceedings of the IEEE MTT-S International Microwave Symposium Digest, 1997, pp. 547-550.

http://www.tpub.com/content/neets/14183/css/14183_159.htm

http://www.activefrance.com/Antennas/Introduction%20to%20Phased%20Array%20Design.pdf

http://hante.home.xs4all.nl/zonnepaneel.html

http://kc7ekk-solar.blogspot.com/2010/12/to-grid-tie-or-not-to-grid-tie.html

http://permanent.com/p-sps-bm.htm