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Harvest & Conversion
Power Harvest to DC
The power harvest collection farm consists of mirrors and photovoltaic (PV) arrays. There are two large mirror structures, each on opposite sides of the satellite. The mirrors are used to focus the sunlight onto the PV arrays. See Figure 1 for an artists rendition of this design.
Mirrors
A highly reflective flexible film called polyimide was selected because of it's 94% reflective index [2]. It will be mounted to an aluminum mesh structure. The metal mesh structure will have a carbon nanotube supporting structure that will not only support the aluminum, but will also connect to the main satellite.
The large mirror structure is 5km by 5km and is made up of 2,500 smaller mirrors that are 100m by 100m in size. Each of these smaller mirrors are independently tilted slightly so that they focus the sunlight onto the PV array.
Figure 1. Power harvest prototype [1]
The approximate cost of the mirror structure is $1.2B and is broken down on the budget sheet.
Stretched Lens Arrays
A photovoltaic (PV) structure is needed to convert the sunlight to DC energy. The Stretched Lens Array (SLA) design, as shown in Figure 2, was selected as this medium because they are eight times more efficient than standard photovoltaic solar panels for the same weight and size [2]. These arrays focus the sunlight onto a narrow strip of PV cells that rest on a radiator. The radiator will then output DC energy and will lead into a gyrotron for conversion to RF.
The development of the SLA has progressed in recent years and the company that manufactures them, Entech, has predicted that within the next 5-10 years the SLA will have the power capability of 1MW for every 500W/kg specific power and 120 kW/m^3 stowed power with an efficiency of 60% [6]. The irradiance of each of the SLAs is 10.9 kW/m^2. These parameters require us to have a total PV array size of 1.5km^2.
Power Conversion from DC to RF
The power conversion for the satellite consist of gyrotrons and waveguides. The decision to use gyrotrons for converting DC to RF energy was selected over several other technologies. Some of these factors included conversion efficiency, operating voltage, cost per watt, and operating frequency. The one factor that narrowed our selection down the most was operating frequency. Most of the currently available technologies only work efficiently below 6 GHz. With our operating frequency at 24 GHz, the technologies available are outlined in the table below [4].
Figure 2. Stretched Lens Array [3]
The power requirements for each satellite are 10 GW. A comparison of each of these technologies along with the cost of toting cargo into space at $1100/kg gives the following results
The obvious choice is to use the permanent-magnet gyrotron. A downside of the gyrotrons power converters is that they require, at present time, very large magnetic fields, a high thermal loading requiring active cooling, and high operating voltages [2]. Some consideration will need to be given to reduce the thermal loading and active cooling requirements and we will work closely with the manufacturer to overcome these obstacles.
The gyrotrons will mount to the back radiator of the SLA. The output of the SLA will directly couple into the gyrotron. The RF output of the gyrotron will work over K-band waveguide and will feed into a waveguide combiner network to feed into our dish antenna. A flow chart of this process is shown in Figure 3.
The waveguide combiner network will consist of 3,000 asymmetric waveguide couplers to combine the RF power output from the gyrotrons and will feed into the dish antenna for power transfer. A asymmetric waveguide coupler, as show in Figure 4, is an innovative way to combine the K-Band 24GHz signal.
Future Power Conversion
Entech, the company that makes the Stretched Lens Arrays, has also made some SLAs that have converted sunlight into lasers using a GaAs PV cell. As this company appears to be versatile in their capabilities, we will commission Entech to investigate converting sunlight directly to 24 GHz to bypass the gyrotrons altogether for a second generation design.
Atmospheric Losses
The losses through the atmosphere at 24GHz without any rain attenuation is about 0.5dB. Earth stations were selected in more arid regions of the earth so that rain attenuation at this frequency will not be an issue.
Power Link to DC
The antenna farm on the ground is modeled in Figure 5. The energy collected by these dish antennas will pass through a rectifying circuit to convert the 24GHz power to DC. This rectifying circuit will consist of a many high power output rectifiers. The output of this rectifying circuit will pass through a low pass filter into a power management circuit which will then pass into the main power grid. The reason for the low pass filter is to prevent any RF energy into the main power grid and allow it to be rectified to DC. Before the power is sent to the grid, it will pass through a switch which will store the energy for future use [2].
The power management circuit will have the following capabilities:
Just In Time - Power that is not throttled and is sent directly to the grid
Storage - Keep the power in a storage medium such as batteries for usage at a later point
Use at a Different Location - Ability to throttle which parts of the community get power at certain times
REFERENCES
[1] Space-Based Solar Power As an Opportunity for Strategic Security, Phase 0 Architecture Feasibility Study, National Security Space Office, Release 0.1, 10 October 2007.
[2] Space-based Solar Power: Possible Defense Applications and Opportunities for NRL Contributions, NRL/FR/7650-09-10,179 23 October 2009
Figure 3. DC/RF Flow Chart (click to enlarge)
Figure 4. Waveguide coupler (click to enlarge)
[3] http://www.stretchedlensarray.com
[4] Advances in microwave and radio frequency processing: report from the 8th International Conference on Microwave and High-Frequency Heating held in Bayreuth, Germany, September 3-7, 2001, ISBN: 3540432523
[5] M. O’Neill, J. Howell, J. Fikes, L. Lollar, C. Carrington, N. Suzuki, M. Piszczor, D. Hoppe, M. Eskenazi, D. Aiken, M. Fulton, H. Brandhorst, M. Schuller, and A.J. McDanal, “Stretched Lens Array SquareRigger: A New Space Array for High-Power Missions,” 4th World Conference on Photovoltaic Energy Conversion, Waikoiala, Hawaii, May 7-12, 2006.
[6] M. O’Neill, 1,000 W/kg Solar Concentrator Arrays for Far-Term Space Missions, American Institue of Physics
Figure 5. Scaled model of the antennas on the ground.
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