Solar Power Satellites (SPS) using Wireless Power Transfer (WPT) to beam renewable energy to consumers on earth face three grand challenges: moving parts, heat dissipation, and radio interference. Solutions to each of these “show stoppers” are presented here. Further, a progressively more-complex pathway is described which starts where we are now and leads step-wise to implementation of large-scale Space Solar Power (SSP).

The first two grand challenges are addressed by a novel SPS design based on a thin-walled cylinder configuration of solar panels.  The remaining challenge is tackled through a newly-discovered antenna configuration which allows dramatic reduction in radio/telecom interference from so-called “sidelobes.”  The cost of this SPS (called the “tin can” for its resemblance to a soup tin with the “lid” antenna canted up at an angle) is made affordable through the use of raw materials already present in space.  The techniques known as In-Situ Resource Utilization (ISRU, or “living off the land”) provide for the refinement of minerals (powdered rock or “regolith”) from the moon or from asteroids into the pure metals and semiconductors needed to build the tin can SPS.  All these factors are brought together as the ultimate goal of a progression of value-added solutions leading to commercial feasibility of SSP.

As demand for energy increases worldwide, researchers are continually evaluating sources of renewable power.  One promising technology is space solar power (SSP).  There are multiple advantages to using SSP over conventional power sources such as fossil fuels, and even over conventional earth-based solar power.  Like other renewables such as wind, hydropower, and terrestrial solar, SSP doesn’t produce atmospheric carbon emissions like the majority of our current energy sources do.  The major drawbacks to terrestrial solar and wind is their unreliability. These intermittent power supplies have posed a significant drawback for the widespread adoption and the consumption of renewable energy.  SSP offers a way to collect energy using solar power satellites (SPS) with a continuous (“baseload” or “always-on”) reliable supply of electricity with no CO2 emissions. 

Space solar power is not a new idea; in fact, it has been discussed and researched since the 1970s.  Although decades old, we have not seen any significant deployment of this concept since it was first conceived.  The issue facing SSP is that it has a very high development cost, compared to more conventional sources of energy.  Building a SPS will require that a large amount of hardware be launched into orbit. These high upfront costs, along with the relatively untested technologies required for such a project, have prevented the manufacturing of a solar power satellite up to this point in time.  

There are several methods for reducing these costs and making this enabling technology a more economical energy source.  By producing on a sufficiently large scale, costs per unit of energy delivered will go down.  A 2015 study shows the potential for solar power satellites, whose infrastructure has been amortized over the first several units, to deliver wholesale electricity to a city at 36 USD/MWh2. This result depends on being able to harvest most of the materials needed for the manufacture of solar power satellites from the moon or from asteroids.  Known as ISRU (in-situ resource utilization), the pulverized soil of the moon can be processed to extract oxygen, silicon, aluminum, and iron.  By designing and producing advanced manufacturing stations in space and on the moon, powdered regolith can be used to produce pure oxygen plus silicon which can be processed into photovoltaic (PV) cells.3 The advantage of manufacturing on the moon – or other planetary surface - is that these locations require 24 times less energy to launch objects from the moon into orbit than from earth.

Prior SPS designs have run into some hurdles that are difficult to design around.  Our SPS design seeks to solve these “show stoppers” with its simplistic tin can design. A common problemfound in other SPS designs is the need for moving parts. If one portion of the satellite tracks the sun and another part has to face earth, these components need to rotate. This makes it very challenging to transfer power from the solar panels to the space antenna because complex mechanisms are needed to transfer huge amounts of power through rotating components.  Previous SPS systems have also required structural support across kilometer-scale lengths. The tin can SPS design uses centrifugal force to hold the structure in its desired configuration.  Furthermore, this innovative design can sway many degrees out of alignment, and the beam direction can be adjusted using diode-loaded transmission line infinitely-variable phase shifters to maintain transmission accurately to the ground-based rectenna (receiving antenna).

Thermal management is the second problem most frequently encountered by prior SPS designers. A total of 50 GW must be rejected radiatively. Approximately one third of the outer surface of the cylindrical cell is exposed to the sun. This heat transfers to the backside of the PV cells where it can radiate towards those solar cells presently in the dark. From there the heat is eventually radiated into space. Heat pipes and thermoelectric devices will also be utilized to aid heat flow from the sun-lit PV cells to the cold cells that are in the dark.

Lastly, beaming a multi-gigawatt power signal will tend to interfere with any other signals in the area. This interference is caused by side lobes that surround the main beam. A side lobe is like a echo of the primary power beam which until recently has been thought to be unavoidable. Our approach uses a newly-discovered configuration for broadside transmission of a phased array antenna that allows side lobe power levels to be arbitrarily reduced to insignificant levels. This new configuration will still produce high sidelobes when there is a significant fraction of failed or malfunctioning antenna elements. However, with proper design and maintenance a nearly-perfect “pencil beam” of power is delivered directly to the rectenna on earth, avoiding interference or “desense” of telecommunication and radio signals already in use around the globe.

Solar Power Satellite Design

Multiple considerations must be factored in and trade spaces explored when designing a solar power satellite.  Figure 1 shows the “tin can” design first proposed by IUPUI Professor Peter J. Schubert in 201412.  The basic design of this satellite is a hollow cylindrical shell composed of photovoltaic cells on the exterior surface.  The shell is tethered to a central copper core or spire by using flexible polymer cables.  At one end of the spire is a hexagonal transmitting antenna (“spacetenna”), where power electronics convert DC current from the solar panels into radio-frequency power signals delivered through a phased array of individual antenna elements, and thence to earth.

The entire assembly rotates slowly, completing one revolution every 24 hours.  This permits the spacetenna to remain consistently pointing – at least substantially - at its dedicated rectenna. The angle at which the spacetenna is canted relative to the axis of the spire (which points to solar north) mirrors the latitude of the earth rectenna.

The simple cylindrical design solves many issues that have faced other designs.  The most important improvement is that this approach doesn’t require a high-power slip ring from the PV panels to the spacetenna because it is not necessary to separately rotate the sun-facing and earth facing sides of the satellite.2 The cylinder design, slowly rotating at the same rate as the earth, allows each and every panel to receive power for about one third of the time.  This way the panels can be expected to last up to three times as long. 

The number of solar panels needed is of course greater than those required on a flat or planar solar farm, but the significant benefit is that the hot solar cells facing the sun can reject their heat through the hollow center of the shell to the cool cells on the dark side.  Because the shell rotates with the spacetenna, the need for a slip ring is eliminated.  This will reduce energy losses caused by imperfect electrical connections and sharply reduce the risk of catastrophic electric failure from an unfortunately-located micrometeorite strike. 

The spacetenna is the transmitting antenna attached to the copper spire at the center of the satellite shell.  Large utility-scale inverters and transformers are needed for the electric power to be converted into a 2.45 GHz power signal and distributed as needed across the spacetenna. These power conversion devices are heavy and must be supplied from earth for the foreseeable future.  Waste heat from conversion can be radiatively rejected up through the central axis of the tin can shell towards cold space in the direction of solar north (see Figure 3).  Heat pipes can be used to spread the heat to additional radiators to maintain proper operating temperatures. 

The 1978 Reference Design5 assumes a spacetenna of almost 1 km diameter with a tapered Gaussian distribution of power across its face.  Through the Friis equation, this size of spacetenna can deliver power to a rectenna of approximately 10 km diameter.  The beamed microwave power is rectified to DC and then fed into a public utility grid4. 

Figure 3: Interior of satellite showing the spacetenna and copper spire to which the outer shell is tethered.

The first segment is the Power Transmission and Management of the solar array. The function is to convert, regulate and transmit the electric power of solar array modules. A majority of the power is transmitted to the rotary joints. The remainder of the power is distributed to the service devices on the sub-arrays, including to the energy storage equipment needed to supply power during eclipse. The voltage of the output power of solar array modules is 500 V. The power is converted to 5000 V when transmitted to the rotary joints. So, for a solar sub-array, there are two output power buses and each bus is 5000 V, 4800 A.

The second segment is Power Transmission and Management on the main structure. The function is to convert, regulate and transmit the electric power of solar sub-arrays. A majority of the power is supplied to the electric power interfaces of the antenna. A part of the available power is distributed to service devices (including electric thrusters) and the remainder of the power is used for energy storage to supply power for service devices during eclipse. The voltage of output power of solar sub-arrays is 5000 V. The power is converted to 20 kV and is transmitted to the antenna. So, for the antenna, there are two input power buses and each bus is 20 kV, 50 kA.

The third segment is Power Transmission and Management on the antenna. The function is to convert, regulate and transmit the electric power of the antenna. The voltage of input power of the antenna is 20 kV. A majority of the power is converted to 5 kV and is transmitted to the microwave generators to generate microwave. Some of this power is distributed to service devices (including electric thrusters) and the remainder of the power is used for energy storage to supply power for service devices during eclipse.

Figure 5 shows the profile of such a beam’s far-field region taken through the horizontal axis of Fig. 4.  Compared to prior designs limited to a -40 dB side lobe (0.01 percent of primary beam), Fig. 5 shows an amazing -240 dB reduction in side lobe power (10-24).  At this level, side lobes will not cause appreciable interference with cell phones, emergency responder radios, aircraft send/receive, or amateur radio equipment.

Figure 4: Triangular phased array configuration for spacetenna elements.

Figure 5: 240 dB Reduction > 0.2 degrees in 4000x4000 element.

The spacetenna is located at the end of the central spire of the tin can SPS. With the mass centroid so aligned wobbling of the entire structure is minimized. Gravity gradient considerations will likely require a counter-balancing mass at the opposite end of the spire to prevent torque on the structure by earth’s gravity. All told, with this approach, we achieve excellent side lobe reduction and have "knocked-down" one of the shop-stoppers: radio interference.