In a previous article, I explained the concept of harvesting solar energy from space using an SSPS (Space Solar Power System). One of the major challenges associated with this technology is the ability to transport collected energy to Earth.
In this article, we will go over the two major viable methods of space-to-Earth power transmission: lasers and microwaves. We will also briefly discuss a hybrid approach that combines laser and microwave transmission.
Safety of Wireless Power Transmission
In the proposed designs, the laser beams will operate at skin- and eye-safe wavelengths, with intensity comparable to normal sun exposure, and the intensity of the microwave radiation will be about one-sixth of that of noon sunlight. Operating at these levels ensures that either transmission mode will be safe for humans, animals, and plants.
Space Solar Power Transmission
The laser beam and microwave power transmission systems are currently the most promising technologies for wirelessly transmitting power over the long distance from a satellite in orbit to the surface of the Earth. The two methods differ in size, mode of operation, efficiency, and cost.
Laser Beam Wireless Power Transmission
In the laser beam wireless power transmission technique, a laser beam sends concentrated light to a photovoltaic cell receiver through the vacuum of space and the atmosphere. The receiver converts the energy back into electricity through these steps:
- The DC power harvested in space is used to generate a single wavelength (monochromatic) light beam.
- A set of optics shapes the laser light according to the required beam size.
- A control system ensures that the laser is pointed at the intended receiver site on Earth.
The mode of operation of the photovoltaic receiver is similar to that of solar power harvesting in which the sunlight falling on solar cells produces electricity. However, this method uses high-intensity laser beams on specialized photovoltaic cells and allows for higher efficiency than what is currently possible with solar cells. Mirrors and telescopes can be used to aim the laser beam at any receiver directly below the satellite with an unobstructed-line-of-sight transmission path.
Advantages of Laser Beam Transmission
- Does not interfere with TV, radio, Wi-Fi, cell phone and other communication signals
- Requires smaller transmission and receiving equipment compared to microwave transmission. (For example, a 1GW installation would require about a one-meter diameter transmitting optics and a ground receiver of several hundred meters across.)
Disadvantages of Laser Beam Power Transmission
- Suffers from atmospheric losses due to environmental factors such as rain and clouds and hence cannot provide continuous power
- Has a low conversion efficiency
- May require huge battery storage systems on the ground
- Carries the risk of causing skin and eye damage if not well managed
Microwave Wireless Power Transmission
A microwave power transmission system consists of the source of the RF energy, a transmit antenna, a transmission medium or channel, and a rectifying antenna usually referred to as the rectenna. The transmission process involves:
- Conversion of the DC power from solar cells to microwave (RF) energy
- Generating and concentrating a microwave beam that can be aimed at fixed locations corresponding to the receivers on the Earth’s surface
- Collection of the RF energy and conversion into electricity at the receiver station
Depiction of a possible space solar power system. Image courtesy of the National Space Society blog.
The solar arrays attached to a typical satellite generating 1.6GW in space and an average of 1GW on Earth would measure about 5 to 6 square kilometers and use a transmitting antenna array with a diameter of about 1 km. The large transmitter array ensures that the transmitted beam will have low divergence, and lower beam divergence means that the RF energy will be more spatially concentrated when it reaches the surface of the Earth.
The rectenna is made up of an array of dipole antennas, with fast-switching diodes across the dipole elements. The microwave energy induces alternating current in the antennas. This is then rectified by the diodes to produce DC voltage, which can power DC devices or be converted to AC using an inverter. Schottky diodes are preferred because of their low forward voltage drop, which reduces power dissipation, and fast switching speeds.
One of the most efficient frequencies for the microwave beam is 2.45GHz. This frequency is located in an ISM band, allows for low-cost power components, and does not experience significant attenuation from gases or moisture in the atmosphere.
The table below lists various details for four different space-solar-power systems:
Important characteristics of four space-solar-power systems.
Advantages of Microwave Wireless Power Transmission
- Benefits from highly-developed microwave technology, capable of achieving efficiencies of up to 85%
- Achieves lower atmospheric attenuation
Disadvantages of Microwave Wireless Power Transmission
- Requires management of the energy lost during conversion of DC to microwaves
- May cause RF interference
- Requires large transmission and receiving equipment
Laser–Microwave Hybrid Wireless Power Transmission System
Each of the two wireless power transmission methods, microwave- and laser-based, has advantages and disadvantages. In an effort to devise an optimal system, some researchers have considered a hybrid approach.
In such a system, a laser would transmit power from a solar array to an in-orbit base station (a photovoltaic array platform). The base station would convert energy from the laser into electricity and then into microwave radiation, which is transmitted to the receiver station on Earth. Thus, the laser beam is used where it does not experience significant attenuation from the atmosphere, then transmission changes to microwave radiation, which is much less subject to atmospheric attenuation.
The Japan Aerospace Exploration Agency hopes to have a commercial space-solar-power system operational within 25 years. Only time will tell if this is an achievable goal. The technological and economic challenges facing space solar power are far from trivial, and all three proposed methods will require much research and testing before they become feasible solutions for large-scale power generation. But history shows that human beings can accomplish amazing things when sufficient motivation is present.