Power Transmission by Laser

Electric power sharing among spacecraft is advantageous for many missions because it makes the spacecraft simpler, more compact and reduces mass, whilst enhancing the power requirements for the mission. Conventional electric cables cannot be used to transfer electricity from the ground to space due to the huge distances and spacecraft velocities involved. At present, the only way to transfer power over long distances in space is by electromagnetic radiation.  

In the case of electromagnetic radiation closer to the visible region of the spectrum (tens of micrometers to tens of nanometers), power can be transmitted by converting electricity into a laser beam that is then pointed at a photovoltaic cell. This mechanism is generally known as “power beaming” because the power is beamed at a receiver that can convert it to electrical energy. At the receiver, special photovoltaic laser power converters which are optimized for monochromatic light conversion are applied.

Lasers are the most powerful and efficient tools for this purpose. Despite recent advancements in laser power transmission technologies , space applications have not been mastered due to currently unresolved challenges. Electric power transmission through the vacuum of space by laser is, therefore, a key-technology limiting space exploration.

The main motivations for this technology are the following:

• Several smaller spacecraft can be powered by one large orbital power plant;
• Due to mass, size, economic or the other constraints the use of a large power plant is either not possible or prohibited on a spacecraft;
• Large specific mass or short lifetimes of current alternative spacecraft power systems;
• Spacecraft power supply during peak power periods.

The particular examples may be:  

• Power supply of spacecraft in low  orbits (for example in LEO) where atmospheric drag force acting on spacecraft becomes significant;
• Power supply of spacecraft in a planet’s shadow or in the deep space missions;
• Power supply of space rovers;

An example of such a kind of application is an interorbital tug docked to a payload container (PLC) charges its electric propulsion system that it uses to transfer the PLC to GEO. It will receive power from one or several energy stations (ES) that are located in such a way that at least one station should be in the field of view of the tug at any one time, whilst the distances between tug and ES should be minimal throughout the flight.

It should be noted that the transport of any payload, for example to GEO, starts by launching the PLC with the payload (and a starting stock of energy for the interorbital tug) into a low earth orbit (LEO).

The advantages of the technology:

• It is flexible for different mission scenarios;
• It can serve several spacecraft missions;
• It can also be used for communications;
• It does not interfere with the other radio-frequency band communication channels;
• At long distance the overall power transportation system by laser is more efficient than using physical connections such as cables.

The main disadvantages of the technology include:

• Requirement of a direct line of sight with the target;
• Atmospheric absorption, and absorption and scattering by clouds, fog, rain, etc., cause up to 100 % losses;
• Conversion between electricity and light is limited. Photovoltaic cells achieve 40 – 50 % efficiency. /1, 3-4/ (The conversion efficiency of laser light into electricity is much higher than that of sun light into electricity).
• Laser radiation could be hazardous.

The current status of the technology is described in the following sections of this report. The technology may also be mentioned as “energy beaming” and “wireless energy transmission”.