Photovoltaics

Space platforms using lasers to beam solar energy back to Earth is no longer stuff of science fiction. Professor Stephen Sweeney, Head of Photonics at the University of Surrey, writes on the latest developments of space-based solar power.

By Dr Stephen Sweeney, University of Surrey

Science fiction becoming a reality

It's the stuff of science fiction - space platforms using lasers to beam solar energy back to Earth - but the reality of a solar power satellite system has become much closer.

In an Astrium-led project, trials have begun at the Advanced Technology Institute at the University of Surrey working closely with space technology giant Astrium. The technological approach has the potential to be scaled up to become a major and long-term source of energy - 'clean', inexhaustible and accessible for any area of the planet.

The idea was first discussed in the 1960s when Dr Peter Glaser proposed that a way of meeting the growth in energy needs from both the developed and developing worlds was to use orbiting satellites which convert solar energy into a radio frequency energy beam. At the time, the available technology didn't match the ambition. Since then, a number of nations have invested in research on the basis of the huge potential for the technology in dealing with energy supply problems.

The first attempt at conceptual design of a working system was led by NASA (US National Aeronautics and Space Administration) between 1977 and 1980, which was based on a fleet of 60 satellites generating five gigawatts. On paper the system proposed was considered to be too optimistic in terms of the photovoltaic cell (solar panel) performance and continued to be too great a technical challenge.

The price tag for the concept - a one-off start-up cost $1 billion for a system which would last 30 years - was considered to be prohibitive. Japan's Institute of Space and Astronautical Science (ISAS) developed two prototype Space Solar Power Satellites during the 1980s, along with a Lunar Solar Power option, with photovoltaic platforms set up on the Moon.

Photonics and PV cells

Most recently, projects have been developed based on the ongoing advances in laser technologies (photonics) and the quality and efficiency of photovoltaic cells. NASA developed "Sun Tower", a tethered solar satellite system with modules for power generation and transmission designed to function in a 'middle-earth' orbit (a distance commonly used by weather satellites and spy satellites) but which proved impractical; The National Space Development Agency of Japan (NASDA) invited companies to submit proposals for a functioning solar power satellite generating between 10 kilowatts and 1 megawatt, and research and laboratory projects continue.

Scientists at the University of Surrey in the UK working with Astrium now believe they are in a position to take a lead in developing a working system. With technologies available now, a single launched satellite would be capable of providing roughly 10 kilowatts to the ground end-user with a laser power transmission system.

Once proven, a first stage would be to set up a multi-satellite system capable of beaming power to people in isolated areas with no access to electricity power grids. These kind of ‘off-grid’ power sources would then be operational in advance of the development of the real goal, the very large scale Mega- and Giga-watt orbiting solar power stations which could provide significant supplies of power to grids, not necessarily as a single solution to energy needs, but as an important part of a suite of renewable energy sources.

The challenges

The most challenging aspect of the technology is the effective transmission of energy without losing power over the huge distances and the different atmospheric conditions involved. A laser system, for example, is sensitive to atmospheric conditions, with the potential for a sizeable reduction in power level when passing through clouds. Without the guarantee of constant power, this means that currently the solar ray could not be used as the single source of power for applications.

To circumvent this, work is currently underway based on using satellites to convert sunlight into an infrared laser beam, targeted to a receiving 'spot' on the Earth's surface where it is captured by an array of highly efficient photovoltaic cells. In this way the sun's full energy can be captured 24 hours a day, with no deterioration in power through the atmosphere due to a transmission “window” in the atmosphere in a particular part of the infrared spectrum.

The laser is designed to work at a long wavelength - as opposed to visible short wavelengths or harmful Ultraviolet rays - and is therefore safe to humans and animals. Despite crossing distances of around 20,000 kilometres, it is believed that the beam can be guided to an accuracy of 10-30 metres.

Initial ground based trials are in progress and a first prototype is expected to be launched into space in 2016 to beam low levels of energy, and the technology scaled up to the point where many megawatts of power will stream into energy grids.

Stephen Sweeney is Professor of Physics and Head of Photonics group at the University of Surrey. s.sweeney@surrey.ac.uk