Microwaves are already commonly used on the surface of the Earth by telecommunications firms, and have an established history in space where they are used for satellite relays, and deep space radio communications with interplanetary missions, such as the Curiosity Rover on Mars.
Microwave WPT systems are also considered to be safer for wildlife and humans than laser-based WPT, which the 2011 IAA report on SBSP noted has the potential to be weaponised. The IAA report estimates that energy levels at the very center of a microwave beam receiving antenna on earth would be between 200-250 W/m2 – or around 20-25% the intensity of mid-day sun at the equator.
The most ambitious experiment to date occurred in May 2008 when Mankins – along with Japanese researchers - demonstrated a microwave beam, similar to one that could be used to transmit energy to Earth, between two Hawaiian Islands 150km (100miles) apart. The distance was chosen because it is equivalent to the thickness of the atmosphere that a microwave beam from space would have to penetrate.
But the experiment brought mixed results. “It was a good end-to-end test of key technologies at short range and that portion was quite successful. It was also the first use of solar power to drive such an experiment, so all the major aspects of the SPS technologies were tested together,” explains Mankins.
“However, the power levels were much too low to actually transmit power over the full distance. Also, given the short time, the beam control system didn't work as we wanted.”
Scientists like Tom Murphy, associate professor of physics at the University of California, San Diego, believe that anyone who wants to spend the $1m it cost to fund the experiment, would be better off saving their money.
“The problem with wireless power transmission is the diffraction of energy through the atmosphere. That's not something you're going to cheat on. Physics makes transmission cumbersome and that's always going to be the case with any SBSP system,” he says.
Even a cloud free sky contains water vapour. Each droplet takes its toll, scattering the radiation in the microwave beam causing significant energy loss between the orbiting solar farm and the terrestrial surface, says Murphy, who estimates that between generating microwave energy in space, beaming it to the ground, and converting back into electricity again, the entire process could be expected to operate with around 50% efficiency.
Murphy argues that even with new technologies and cheaper launch systems, SBSP will not become viable.
“The time scale, the expense, and the degree of complexity of doing something from space makes it so daunting a task that if you can at all achieve the same scientific result from the ground it's far and away a superior path,” he says.
It is a view that is backed up by the International Energy Agency (IEA) which is positive about the prospects for terrestrial solar power generation, but explicitly rules out SBSP in the long-term because the expense, “mostly due to the costs of putting the necessary elements into orbit, would be several orders of magnitude greater than the costs of generating electricity on Earth".
Another factor weighing against a bright future for SBSP is the resurgence of oil and particularly gas production in the United States, which could see the country become 95% energy independent by 2035, according to the IEA.
But, despite the critics – and seemingly against the odds - projects are still pushing ahead. At least on paper.
In September 2012 there was a flurry of announcements to coincide with the Space Power Symposium held in Naples, Italy. A division of the Russian Federal Space Agency (Roscosmos) revealed that it has a working prototype of a 100kW SBSP system in development; although no launch date was announced. And the China Association for Science and Technology (Cast) revealed more details of a 100kW SBSP demonstration, which it plans to put in low earth orbit is expected by 2025, followed by a fully-operational SBSP system in geostationary orbit by 2050.