Microwave Power Transmission for Space Solar Power Systems (SSPS)

Aayush Srivastava

Shubham Kumar Gupta

Introduction to SSPS

It is known that electromagnetic energy is associated with the propagation of the electromagnetic waves. We can use theoretically all electromagnetic waves for a Wireless Power Transmission (WPT). The difference between the WPT and communication systems is only efficiency because in communication systems transmitted energy is diffused to all directions. This is to say that all the technology for creating an SSPS is already existing, only difference is the scale of operation. Typical WPT is a point-to-point power transmission. It is proven that the power transmission efficiency can approach close to 100.The application of the WPT via microwave is a Space Solar Power Satellite (SSPS). The SSPS is a gigantic satellite designed as an electric power plant orbiting in the Geostationary Earth Orbit (GEO).

Components of an SSPS SystemSpace-based solar power essentially consists of three elements:

Fig. Schematic of a SSPS system

a means of collecting solar power in space, for example via solar concentrators, solar cells or a heat engine

a means of transmitting power to earth, for example via microwave or laser a means of receiving power on earth, for example via a microwave antenna (rectenna)

Solar Energy Collector

Solar energy collection can be done by following techniques-

Solar thermal electric energy generation concentrates the light from the sun to create heat, and that heat is used to run a heat engine, which turns a generator to make electricity. The working fluid that is heated by the concentrated sunlight can be a liquid or a gas. Different working fluids include water, oil, salts, air, nitrogen, helium, etc. Different engine types include steam engines, gas turbines, Stirling engines, etc. All of these engines can be quite efficient, often between 30% and 40%, and are capable of producing 10's to 100's of megawatts of power.

Photovoltaic, or PV energy conversion, on the other hand, directly converts the sun's light into electricity. Photovoltaic conversion uses semiconductor cells to directly convert photons into electrical power. The use of solar dynamic could reduce mass per watt. Most analyses of SSPS have focused on photovoltaic conversion (commonly known as “solar cells”).

Transmitters

Fig. Average Power vs Frequency for various transmitters

There are two types of microwave generators/amplifiers. One is a microwave tube and the other is a semiconductor amplifier. These have electric characteristics contrary to each other.The microwave tube, such as a cooker-type magnetron, can generate and amplify high power microwave (over kW) with a high voltage (over kV) imposed. Especially, magnetron is very economical. The semiconductor amplifier generate low power microwave (below 100W) with a low voltage (below fifteen volt) imposed. It is still expensive currently. Although there are some discussion concerning generation/amplifier efficiency, the microwave tube has higher efficiency (over 70%) and the semiconductor has lower efficiency (below 50%) in general. We have to choose tube/semiconductor case by case for the MPT system.

Fig. Typical parameters of the transmitting antenna of the SSPS

A. Microwave tube

1. Magnetron

Fig. Magnetron diagram

Magnetron is a crossed field tube in which E X B forces electrons emitted from the cathode to take cyclonical path to the anode. The magnetron is self-oscillatory device in which the anode contains a resonant radio frequency structure. The magnetron is suitable device for the MPT because of high efficiency and low cost and unsuitable device because of its unstable frequency and uncontrollable phase, however cooker-type magnetron is considered as noisy device.

2. Traveling Wave Tube Amplifier (TWTA）

The TWT is a linear beam tube with helix structure. The helix slow wave structure (SWS) slows the RF waves down to just below the velocity of the electron beam. In the TWT, the interaction between the RF waves and the electron beam is continuous along the length of the SWS. The TWT can be used for amplifier and we call it TWT amplifier (TWTA). The longer the tube, the higher gain. Applied frequency of the TWTA is very wide, from 1GHz-band to 60 GHz-band. Typical output power of the TWT is a few hundreds watts. The TWTA is widely used in television broadcasting satellites and communication satellites. The TWTA has a proven track record in space.

Fig. Travelling Wave Tube Amplifier

The Microwave Power Module combines the best aspects of TWT, semiconductor amplifiers, and state-of-the-art power supply technology into one package. This makes MPM into a good candidate for space application because it has high conversion efficiency, small size and low weight. In near future, we may consider the MPT system with TWTA.

3. Klystron

Fig. Klystron Transmitter

The klystron is also a linear beam tube with cavities. Electrons are emitted from the cathode and electron beam passes through the cavities. When RF inputs from input cavity, the electron beam is modulated and RF is amplified in last. The klystron is high power amplifier from tens of kilowatts to a few megawatts with high efficiency, over 70%. It requires a ponderous power supply and also a heavy magnet.

B. Semiconductor Amplifier

After 1980s, semiconductor device plays the lead in microwave world instead of the microwave tubes. Typical semiconductor device for microwave circuits are FET (Field Effect Transistor), HBT (Heterojunction Bipolar Transistor), and HEMT (High Electron Mobility Transistor). Present materials for the semiconductor device are Si for lower frequency below a few GHz and GaAs for higher frequency. We design microwave circuits with these semiconductor devices. It is easy to control a phase and amplitude through the microwave circuits with semiconductor devices, for example, amplifiers, phase shifters, modulators, and so on. For the microwave amplifiers, circuit design theoretically determines efficiency and gain. Semi-conductor amplifier is expected as high efficient amplifier for the MPT system. A weight of the microwave tube is lighter than that of the semiconductor amplifier when we compare the weight by power-weight ratio (kg/kW).

Transmitter Issues for Space Use

Largest MPT application is a SSPS in which over GW microwave will be transmitted from space to ground at distance of 36,000km. In the SSPS, we will use microwave transmitters in space. For space use, the microwave transmitter will be required lightness to reduce launch cost and higher efficiency to reduce heat problem.The microwave tube can generate/amplify higher power microwave than that by the semiconductor amplifier. Although it may seem that semiconductor amplifiers are light in weight, they have heavy power-weight ratio because output microwave power is very small.Heat reduction is most important problem in space. All lost power converts to heat. We need special heat reduction system in space. If we use high efficient microwave transmitters, we can reduce weight of heat reduction system. We should aim for over 80% efficiency for the microwave transmitter, which must include all loss in phase shifters, isolators, antennas, power circuits. Especially, the SSPS is a power station in space, therefore, heat reduction will be a serious problem.

Retro-directive Beam Control

Retro-directive arrays have the unique ability to retransmit signals back to its origination point without a priori knowledge. This is accomplished without complexity of digital signal processor and rely on purely analog circuitry offers system simplicity and high speed response.A microwave power transmission is suitable for a power transmission from/to moving transmitters/targets. Therefore, accurate target detection and high efficient beam forming are important. Retro-directive system is always used for SSPS.A corner reflector is most basic retro-directive system. The corner reflectors consist of perpendicular metal sheets, which meet at an apex. Incoming signals are reflected back in the direction of arrival through multiple reflections off the wall of the reflector.

Van Atta array is also a basic technique of the retro-directive system. This array is made up of pairs of antennas spaced equidistant from the center of the array, and connected with equal length transmission lines. The signal received by an antenna is re-radiated by its pair, thus the order of re-radiating elements are inverted with respect to the center of the array, achieving the proper phasing for retro directivity. Usual retro-directive system have phase conjugate circuits in each receiving/transmitting antenna, which play a same role as pairs of antennas spaced equidistant from the center of the array in Van Atta array. A signal transmitted from the target is received and re-radiated through the phase conjugate circuit to the direction of the target. The signal is called a pilot signal. We do not need any phase shifters for beam forming. The retro-directive system is usually used for satellite communication, wireless LAN, military, etc.

Fig. (a) two-sided corner reflector, (b) Van Atta Array, (c) retro-directive array

The retro-directive system unifies target detection with beam forming by the phase conjugate circuits. There are some methods for target detecting with pilot signal which is separated to beam forming. We call the method “software retro-directive”. Recto directive with the phase data from a pilot signal and for the beam forming with calculation of the optimum phase and amplitude distribution on the array.

Receivers

Point-to-point MPT system needs a large receiving area with a rectenna array because onerectenna element receives and creates only a few W. Especially for the SSPS, we need a huge rectenna site and a power network connected to the existing power networks on the ground.

A. Antennae

All antennas can be applied for both the MPT system and communication system, for example, Yagi-Uda antenna, horn antenna, parabolic antenna, micro-strip antenna, phased array antenna or any other type of antenna. To fixed target of the MPT system, we usually select a large parabolic antenna.

However, we have to use a phased array antenna for the MPT from/to moving transmitter/receiver which include the SPS because we have to control a microwave beam direction accurately and speedy. The phased array is a directive antenna which generate a beam form whose shape and direction by the relative phases and amplitudes of the waves at the individual antenna elements. It is possible to steer the direction of the microwave beam.

Fig. Phased Array Antennae

B. Rectenna

The word “rectenna” is composed of “rectifying circuit” and “antenna”. The rectenna can receive and rectify a micro wave power to DC. The rectenna is passive element with a rectifying diode, operated without any power source. The antenna of rectenna can be any of the antenna discussed above. The. The circuit, especially diode, mainly determines the RF-DC conversion efficiency. Silicon Schottky barrier diodes were usually used for the previous rectennas. New diode devices like SiC and GaN are expected to rectenna can also take any type of rectifying circuit such as single shunt full-wave rectifier, full-wave bridge rectifier, or other hybrid rectifiers increase the efficiency. The rectennas with FET or HEMT have appeared in recent years.

Fig. Schematic of rectenna circuit

The single shunt full-wave rectifier is always used for the rectenna. In an ideal situation, 100% of the received microwave power should be converted into DC power. The RF-DC conversion efficiency of the rectenna with a diode depends on the microwave power input intensity and the connected load. It has the optimum microwave power input intensity and the optimum load to achieve maximum efficiency. When the power or load is not matched the optimum, the efficiency becomes quite low. If the input voltage to the diode is lower than the junction voltage or is higher than the breakdown voltage, the diode does not show a rectifying characteristic. As a result, the RF-DC conversion efficiency drops with a lower or higher input than the optimum.The SPS requires a rectenna array whose diameter of over a km.

C. Cyclotron Wave Converter

If we would like to use a parabolic antenna as a MPT receiver, we have to use Cyclotron WaveConverter (CWC) instead of the rectenna. The CWC is a microwave tube to rectify high power microwave directly into DC.

Fig. Schematic Picture of Cyclotron Wave Converter

Efficiency of the SSPS

We classify the SSPS efficiency roughly into three stages; 1. DC-RF conversion efficiency which includes losses caused by beam forming, 2. Beam collection efficiency which means ratio of all radiated power to collected power on a

receiving antenna, and 3. RF-DC conversion efficiency.

1. DC-RF Conversion Efficiency

Fig. DC-RF Conversion Efficiency of TWTA

If we do not have to steer a microwave beam electrically in a MPT, we can use a microwave transmitter with high DC-RF conversion efficiency over 70-80 % like microwave tubes.However, if we need to steer a microwave beam electrically without any grating lobes, we have to use phase shifters with high loss. Especially in the SPS system, the optimum and economical size of the transmitting phased array and microwave power are calculated as around a few km and over a few GW, respectively.

Fig. Implementation of microwave transmission with a high power microwave oscillator andphase-shifters for high precision control of microwave beam direction to large angles

without grating lobes

2. Beam Collection Efficiency

The beam collection efficiency depends on the transmitter and receiver aperture areas, thewavelength, and the separation distance between the two antennas. Decline of the efficiency is caused by phase/frequency/amplitude error on a phased array.Phase/frequency/amplitude error on a phased array causes difference of beam direction and rise of sidelobes. If we have enough large number of elements, the difference of the beam direction is negligible. The rise of the sidelobe decreases antenna gain and beam collection efficiency.Therefore, we have to fundamentally suppress the grating lobes for a MPT system.

3. RF-DC Conversion Efficiency

The RF-DC conversion efficiency of the rectenna or the CWC is over 80 %. Decline of the efficiency is caused by array connection loss, change of optimum operation point of the rectenna array caused by change of connected load, trouble of the rectenna, and any losses on the systems, for example, DC/AC conversion, cables, etc. However, it is easier to keep high efficiency than that on the other two stages of the SSPS

Efficiency of 2.45GHz Rectenna Efficiency of 5.8GHz Rectenna

Fig. Efficiency of Rectenna Element

.Advantages of SSPSThe SSPS concept is attractive because space has several major advantages over the Earth's surface for the collection of solar power.

There is no air in space, so the collecting surfaces could receive much more intense sunlight, unobstructed by the filtering effects of atmospheric gasses, cloud cover, there is no night, dust to be cleaned, clouds and other weather events. Consequently, the intensity in orbit is approximately 144% of the maximum attainable intensity on Earth's surface.

A satellite could be illuminated over 99% of the time, and be in Earth's shadow a maximum of only 72 minutes per night at the spring and fall equinoxes at local midnight. Orbiting satellites can be exposed to a consistently high degree of solar radiation, generally for 24 hours per day, whereas the average earth surface solar panels currently collect power for an average of 29% per day.

Power could be relatively quickly redirected directly to areas that need it most. A collecting satellite could possibly direct power on demand to different surface locations based on geographical base-load or peak load power needs. Typical contracts would be for base-load, continuous power, since peaking power is ephemeral.

Elimination of plant and wildlife interference.

Disadvantages of SSPS

The large cost of launching a satellite into space Inaccessibility: Maintenance of an earth-based solar panel is relatively simple, but construction and

maintenance on a solar panel in space would typically be done tele-robotically. In addition to cost, astronauts working in GEO orbit are exposed to unacceptably high radiation dangers and risk and cost about one thousand times more than the same task done tele-robotically.

After being decommissioned, parts of it may stay in orbit and become space debris. This space debris can create trouble for other space satellites.

The space environment is hostile; panels suffer about 8 times the degradation they would on Earth

Space debris is a major hazard to large objects in space, and all large structures such as SSPS systems have been mentioned as potential sources of orbital debris.

The broadcast frequency of the microwave downlink (if used) would require isolating the SSPS systems away from other satellites.

The large size and corresponding cost of the receiving station on the ground.

Safety ConcernsAt the Earth's surface, a suggested microwave beam would have a maximum intensity at its center, of 23 mW/cm2 (less than 1/4 the solar irradiation constant), and an intensity of less than 1 mW/cm2 outside the rectenna fence-line (the receiver's perimeter). These compare with current United States Occupational Safety and Health Act (OSHA) workplace exposure limits for microwaves, which are 10 mW/cm2,  - the limit itself being expressed in voluntary terms and ruled unenforceable for Federal OSHA enforcement purposes. A beam of this intensity is therefore at its center, of a similar magnitude to current safe workplace levels, even for long term or indefinite exposure. Outside the receiver, it is far less than the OSHA long-term levels. 

Over 95% of the beam energy will fall on the rectenna. The remaining microwave energy will be absorbed and dispersed well within standards currently imposed upon microwave emissions around the world. It is important for system efficiency that as much of the microwave radiation as possible be focused on the rectenna. Outside the rectenna, microwave intensities rapidly decrease, so nearby towns or other human activity should be completely unaffected.

Exposure to the beam is able to be minimized in other ways. On the ground, physical access is controllable (e.g., via fencing), and typical aircraft flying through the beam provide passengers with a protective metal shell (i.e., a Faraday Cage), which will intercept the microwaves. Other aircraft (balloons, ultralight, etc.) can avoid exposure by observing airflight control spaces, as is currently done for military and other controlled airspace.

The microwave beam intensity at ground level in the center of the beam would be designed and physically built into the system; simply, the transmitter would be too far away and too small to be able to increase the intensity to unsafe levels, even in principle.

In addition, a design constraint is that the microwave beam must not be so intense as to injure wildlife, particularly birds. Experiments with deliberate microwave irradiation at reasonable levels have failed to show negative effects even over multiple generations.

Some have suggested locating rectennas offshore, but this presents serious problems, including corrosion, mechanical stresses, and biological contamination.

A commonly proposed approach to ensuring fail-safe beam targeting is to use a retrodirective phased array antenna/rectenna. A "pilot" microwave beam emitted from the center of the rectenna on the ground establishes a phase front at the transmitting antenna. There, circuits in each of the antenna's subarrays compare the pilot beam's phase front with an internal clock phase to control the phase of the outgoing signal. This forces the transmitted beam to be centered precisely on the rectenna and to have a high degree of phase uniformity; if the pilot beam is lost for any reason (if the transmitting antenna is turned away from the rectenna, for example) the phase control value fails and the microwave power beam is automatically defocused. Such a system would be physically incapable of focusing its power beam anywhere that did not have a pilot beam transmitter.

Interaction with AtmosphereIn general, effect of atmosphere to microwave is quite small. There are absorption and scatter by air, rain, and irregularity of air refraction ratio. In 2.45 GHz and 5.8 GHz, the absorption by water vapor and oxygen dominate the effect in the air. Especially, it is enough to consider only absorption by the oxygen in the microwave frequency.

Fig. Microwave attenuation by rain Fig. Attenuation of microwaves   in dry air

When microwave from the SPS propagates through ionospheric plasmas, some interaction between the microwave and the ionospheric plasmas occurs. It is nonlinear interaction between intense microwave and the space plasmas that we have to investigate before the commercial SSPS. It is theoretically predicted there is a possibility of Ohmic heating of the plasmas, plasma hall effect by Ponderomotive force, thermal self-focusing effect of the microwave beam, and three-wave interactions and excitation of electrostatic waves in MHz bands

Future Development

Reusable launch systems are predicted to provide lower launch costs to low Earth orbit. Moon is the optimum location for solar power stations, and promotes lunar solar power. The main

advantage is construction largely from locally available lunar materials, using in-situ resource utilization, with a tele-operated mobile factory and crane to assemble the microwave reflectors, and rovers to assemble and pave solar cells which would significantly reduce launch costs compared to SSPS designs.

JAXA researchers hope to conduct the first microwave power transmission experiment in space, sending several kilowatts from low Earth orbit to the ground. This step, proposed for 2018, should test out the hardware.

JAXA’s technology road map calls for work to begin on a 100-kW SSPS demonstration around 2020.

A global organization could begin the construction of a 1-GW commercial SSPS in the 2030s.

Fig. JAXA’s Future development timeline for SSPS