Mitsubishi Heavy Industries, Ltd. (MHI) has conducted ground demonstration testing of “wireless power transmission,” a new technology presently under development to serve as the core technology of the space solar power systems (SSPS) that are expected to be the power generation systems of the future. With successful completion of the test at the company’s Kobe Shipyard & Machinery Works, MHI has now verified the viability of long-distance wireless power transmission.
In the ground demonstration test, 10 kilowatts (kW) of power was sent from a transmitting unit by microwave. The reception of power was confirmed at a receiver unit located at a distance of 500 meters (m) away by the illumination of LED lights, using part of power transmitted. The transmission distance and power load mark new milestones in Japan with respect to length and volume of wireless power transmission. The testing also confirmed the performance of the advanced control system technology used to regulate the direction of the microwave beam so that it does not veer from the targeted receiver unit.
Wireless power transmission technology aims to eliminate the cable connections conventionally necessary for transmitting electricity, and the newly successful test results lead the way to applying the technology in numerous terrestrial fields. The achievement of wireless power transmission over long distances will not only facilitate the transmission of power to locations where installation of power cables has been difficult or dangerous; it is also expected to contribute to transmission of power from offshore wind turbines and various other applications in the future. One readily conceivable application is wireless transmission of power to electric vehicles.
The wireless power transmission being developed for SSPS usage is referred to as a radio emission technology, and once the technology is achieved it will enable wireless transmission of power over unprecedented distances.
The SSPS is being developed as a system that will generate power on a geostationary satellite at 36,000 kilometers above the earth using solar cell panels; the generated power will be transmitted to earth by microwave/laser – i.e. without relying on cables – and the power received on the ground will be converted to electrical energy. As the power source is environmentally clean and inexhaustible, the SSPS is highly anticipated to become a mainstay energy source that will simultaneously solve both environmental and energy issues.
Going forward MHI aims to pursue expanded practical applications of this advanced aerospace technology in a quest to contribute to social progress, while simultaneously further advancing Japanese technology toward the realization of tomorrow’s SSPS’s.
Feature Green TechSolar
How Japan Plans to Build an Orbital Solar Farm
JAXA wants to make the sci-fi idea of space-based solar power a reality
By Susumu Sasaki
Posted 24 Apr 2014 | 14:00 GMT
Illustration: John MacNeill
Here Comes the Sun: Mirrors in orbit would reflect sunlight onto huge solar panels, and the resulting power would be beamed down to Earth.
Imagine looking out over Tokyo Bay from high above and seeing a man-made island in the harbor, 3 kilometers long. A massive net is stretched over the island and studded with 5 billion tiny rectifying antennas, which convert microwave energy into DC electricity. Also on the island is a substation that sends that electricity coursing through a submarine cable to Tokyo, to help keep the factories of the Keihin industrial zone humming and the neon lights of Shibuya shining bright.
But you can’t even see the most interesting part. Several giant solar collectors in geosynchronous orbit are beaming microwaves down to the island from 36 000 km above Earth.
It’s been the subject of many previous studies and the stuff of sci-fi for decades, but space-based solar power could at last become a reality—and within 25 years, according to a proposal from researchers at the Japan Aerospace Exploration Agency (JAXA). The agency, which leads the world in research on space-based solar power systems, now has a technology road map that suggests a series of ground and orbital demonstrations leading to the development in the 2030s of a 1-gigawatt commercial system—about the same output as a typical nuclear power plant.
It’s an ambitious plan, to be sure. But a combination of technical and social factors is giving it currency, especially in Japan. On the technical front, recent advances in wireless power transmission allow moving antennas to coordinate in order to send a precise beam across vast distances. At the same time, heightened public concerns about the climatic effects of greenhouse gases produced by the burning of fossil fuels are prompting a look at alternatives. Renewable energy technologies to harvest the sun and the wind are constantly improving, but large-scale solar and wind farms occupy huge swaths of land, and they provide only intermittent power. Space-based solar collectors in geosynchronous orbit, on the other hand, could generate power nearly 24 hours a day. Japan has a particular interest in finding a practical clean energy source: The accident at the Fukushima Daiichi nuclear power plant prompted an exhaustive and systematic search for alternatives, yet Japan lacks both fossil fuel resources and empty land suitable for renewable power installations.
Soon after we humans invented silicon-based photovoltaic cells to convert sunlight directly into electricity, more than 60 years ago, we realized that space would be the best place to perform that conversion. The concept was first proposed formally in 1968 by the American aerospace engineer Peter Glaser. In a seminal paper, he acknowledged the challenges of constructing, launching, and operating these satellites but argued that improved photovoltaics and easier access to space would soon make them achievable. In the 1970s, NASA and the U.S. Department of Energy carried out serious studies on space-based solar power, and over the decades since, various types of solar power satellites (SPSs) have been proposed. No such satellites have been orbited yet because of concerns regarding costs and technical feasibility. The relevant technologies have made great strides in recent years, however. It’s time to take another look at space-based solar power.
A commercial SPS capable of producing 1 GW would be a magnificent structure weighing more than 10 000 metric tons and measuring several kilometers across. To complete and operate an electricity system based on such satellites, we would have to demonstrate mastery of six different disciplines: wireless power transmission, space transportation, construction of large structures in orbit, satellite attitude and orbit control, power generation, and power management. Of those six challenges, it’s the wireless power transmission that remains the most daunting. So that’s where JAXA has focused its research.
img how to beam clean
Illustration: John MacNeill
The Japan Aerospace Exploration Agency is working on several models for solar-collecting satellites, which would fly in geosynchronous orbit 36 000 kilometers above their receiving stations. With the basic model [top left-hand side], the photovoltaic-topped panel’s efficiency would decrease as the world turned away from the sun. The advanced model [top right-hand side] would feature two mirrors to reflect sunlight onto two photovoltaic panels. This model would be more difficult to build, but it could generate electricity continuously.In either model, the photovoltaic panels would generate DC current, which would be converted to microwaves aboard the satellite. The satellite’s many microwave-transmitting antenna panels would receive a pilot signal from the ground, allowing each transmitting panel to separately aim its piece of the microwave beam at the receiving station far below.Once the microwave beam hits the receiving station, rectifying antennas would change the microwaves back to DC current. An on-site converter would change that current to AC power, which could be fed into the grid.
Wireless power transmission has been the subject of investigation since Nikola Tesla’s experiments at the end of the 19th century. Tesla famously began building a 57-meter tower on New York’s Long Island in 1901, hoping to use it to beam power to such targets as moving airships, but his funding was canceled before he could realize his dream.
To send power over distances measured in millimeters or centimeters—for example, to charge an electric toothbrush from its base or an electric vehicle from a roadway—electromagnetic induction works fine. But transmitting power over longer distances can be accomplished efficiently only by converting electricity into either a laser or a microwave beam.
The laser method’s main advantages and disadvantages both relate to its short wavelength, which would be around 1 micrometer for this application. Such wavelengths can be transmitted and received by relatively small components: The transmitting optics in space would measure about 1 meter for a 1-GW installation, and the receiving station on the ground would be several hundred meters long. However, the short-wavelength laser would often be blocked by the atmosphere; water molecules in clouds would absorb or scatter the laser beam, as they do sunlight. No one wants a space-based solar power system that works only when the sky is clear.
But microwaves—for example, ones with wavelengths between 5 and 10 centimeters—would have no such problems in transmission. Microwaves also have an efficiency advantage for a space-based solar power system, where power must be converted twice: first from DC power to microwaves aboard the satellite, then from microwaves to DC power on the ground. In lab conditions, researchers have achieved about 80 percent efficiency in that power conversion on both ends. Electronics companies are now striving to achieve such rates in commercially available components, such as in power amplifiers based on gallium nitride semiconductors, which could be used in the microwave transmitters.
In their pursuit of an optimal design for the satellite, JAXA researchers are working on two different concepts. In the more basic one, a huge square panel (measuring 2 km per side) would be covered with photovoltaic elements on its top surface and transmission antennas on its bottom. This panel would be suspended by 10-km-long tether wires from a small bus, which would house the satellite’s controls and communication systems.
Using a technique called gravity gradient stabilization, the bus would act as a counterweight to the huge panel. The panel, which would be closer to Earth, would experience more gravitational pull down toward the planet and less centrifugal force away from it, while the bus would be tugged upward by the opposite effects. This balance of forces would keep the satellite in a stable orbit, so it wouldn’t need any active attitude-control system, saving millions of dollars in fuel costs.
Japan’s Space based solar power roadmap
Basic research 2014–2020
2014:Demonstration on the ground
2017:1 kilowatt satellite experiment
Development phase 2021–2030
2021:100 kilowatt satellite experiment
2024:2 Megawatt satellite experiment
2028: 200 Megawatt demonstration power station
Commercial Phase 2031–2040
2031:1 Gigawatt full-scale power station
2037:Commercial space-based solar power industry (one launch per year).
SOURCES – Mitsubishi, JAXA (Japan Space Agency)