February 23, 2009 The concept of Space-Based Solar Power (SBSP) has been doing the rounds for decades with fantastic claims of 24 hour a day solar power beamed from space via microwave to any point on earth. A start up company called Space Energy, Inc says it plans to develop SBSP satellites to generate and transmit electricity to receivers on the Earth's surface. To do this, the company plans to create and launch a prototype satellite into low earth orbit (LEO). The hitch: this concept is based on as yet unproven technology.

SBSP was theorized over 40 years ago by renowned scientist Dr. Peter Glaser. Since then, in response to periodic energy crises, the idea has been re-evaluated from time to time by the U.S. Department of Energy, NASA, major aerospace companies and countries such as Japan and India.

Solar power satellites are large arrays of photovoltaic panels assembled in orbit, which use microwave radio waves to transmit solar power to large receiving antennas on Earth. The resulting power can either supplement, or be a substitute for, conventional electricity sources.

The advantage of placing solar collectors in geosynchronous Earth orbit (GEO), about 36,000 kilometres (22,500 miles) above Earth, is that it uses the constant and unobstructed output of the Sun, unaffected by the Earth's day/night cycle.

By contrast, ground-based solar power provides a vital and valuable addition to the Earth's energy needs, but is limited by these factors:

  • Weather
  • Variable seasons
  • Atmospheric blocking of sunlight
  • Poor direct sunlight at higher and lower latitudes
Because none of these factors applies in outer-space, an orbiting SBSP station can supposedly provide an estimated 6-8 times more power than a comparable solar cell on the Earth's surface.

Here’s where the entire concept falls flat. Space Energy, Inc claims that a successful long-range wireless power transmission test was conducted in mid-2008, that supposedly transmitted a microwave beam (similar to the kind that would be used to transmit energy from space to Earth) between two Hawaiian Islands across 148 kilometres - more than the distance from the surface of the Earth to the boundary of space. They claim this test demonstrated the technical feasibility of transmitting SBSP to Earth.

Less than 1/1000th of 1% received

Unfortunately for Space Energy, Inc and the entire concept of space based solar power, the actual test results conducted for a Discovery channel documentary proved a total failure. The former NASA executive and physicist who organized the experiment, John Mankins, admitted in a press conference that the $1 Million budget spent of the experiment resulted in less than 1/1000th of 1% of the power transmitted being received on the other island.

The most successful test of wireless power transmission over any distance at high efficiency was conducted by Bill Brown in 1975. Using a NASA deep space tracking dish they transmitted 30kw over 1.6 km (1 mile) at 82.5% efficiency at the Goldstone Deep Space Communication Complex. A Since Geostationary orbit is 36,000 km (22,500 miles) away from earth the space based power station needs to efficiently transmit power over twenty thousand times further than has ever been achieved to date.

Researchers at the Japan Aerospace Exploration Agency (JAXA) have begun to develop the hardware for a SBSP satellite they hope to launch by 2030. They will begin testing this month of a microwave power transmission system designed to beam the power from the satellites to Earth. In a series of experiments to be conducted at the Taiki Multi-Purpose Aerospace Park in Hokkaido, the researchers will use a 2.4-meter-diameter transmission antenna to send a microwave beam over 50 meters to a rectenna (rectifying antenna) that converts the microwave energy into electricity and powers a household heater. The researchers expect these initial tests to provide valuable engineering data that will pave the way for JAXA to build larger, more powerful systems.

This hardware will then have to be scaled up from 50 meters to a distance of 36,000 km and from a 2.4 meter antenna to a ground receiving stations that measure 3 kilometres across and that can receive 1 gigawatt (1 million kilowatts) of electricity — enough to power approximately 500,000 homes.

Japan developing Laser transmission

The Japanese scientists have also experimented with direct conversion of sunlight into a high powered laser to transmit power wirelessly back to earth using light. Using solar plates made from chromium, a ceramic material that absorbs the sunlight, and neodymium, which converts it into laser light, these solar panels demonstrated a 42% solar-to-laser energy conversion efficiency – an impressive figure that outperforms previous technology by a factor of four. Unfortunately cloud cover has an adverse effect of laser transmissions, perhaps why they are also testing a microwave system.

Comparing the energy provided by Space Based power with a conventional ground based solar PV or thermal system of the same 3 km size as the receiving antenna shows that earth based systems are more than competitive. Using a value of 1.4kW per square meter as the radiant solar intensity at the Earth's surface with a 20% power conversion efficiency a ground based solar plant could generate 2.5 gigawatt (only during daylight of course) compared to the Space satellites 1 gigawatt. Many solar thermal power generation plants now incorporate thermal energy storage therefore providing the 24 hour a day base load at a small fraction of the cost of developing and launching solar power station that would be larger than the International Space Station into geostationary orbit.

Paul Evans