What is Wireless Power Transmission?
Wireless Power Transmission (WPT) is the efficient transmission of electrical power from one point to another through a vacuum or an atmosphere without the use of wire or any other substance. WPT can be used for applications where either an instantaneous amount or a continuous delivery of energy is needed, but where conventional wires are unaffordable, inconvenient, expensive, hazardous, unwanted or impossible. The power can be transmitted using microwaves, millimeter waves or lasers. The microwave spectrum is defined as electromagnetic energy ranging from approximately 1 GHz to 1000 GHz in frequency. The most common applications are within the 1 to 40 GHz range.
The history of Wireless Power Transmission
Maxwell’s theory of electromagnetism, published in 1865, mentions electromagnetic waves moving at the speed of light, and concluded that light itself was just such a wave. In 1886 Hertz performed a successful experiment with pulsed wireless energy transfer. He produced an apparatus that produced and detected microwaves in the UHF region.
Tesla also performed experiments in the field of pulsed wireless energy transfer in 1899. His Magnifying Transmitter, an early type of Tesla Coil that measured 16 meters in diameter, could transmit tens of thousands of watts without wires. Tesla supposedly managed to light 200 lamps, without wires, from 40 kilometers away, although no documentation from Tesla’s own records were ever published. In 1897 he filed his first patents for the Wardenclyffe tower, which was meant to be a pilot plant for his “World Wireless System” to broadcast energy around the globe. The facility was never fully operational and was not completed due to economic problems.
What equipment is needed?
The components for WPT include a microwave source, a transmitting antenna and a receiving antenna.
The microwave source consists of electron tubes or solid-state devices with electronics to control power output. The slotted waveguide antenna, parabolic dish and microstrip patch are the most popular types. Due to high efficiency (>95%) and high power handling capacity, the slotted waveguide antenna seems to be the best option for power transmission.
The combination of receiving and converting unit is called a rectenna (rectifying antenna) that is used to directly convert microwave energy into DC electricity. It includes a mesh of dipoles and diodes for absorbing microwave energy from a transmitter and converting it into electric power. Its elements are usually arranged in a multi-element phased array with a mesh pattern reflector element to make it directional.
WTP for space solar
The largest potential application for microwave power transmission is space solar power satellites (SPS). In this application, solar power is captured in space and converted into electricity. The electricity is converted into microwaves and transmitted to the earth. The microwave power will be captured with antennae and converted into electricity. NASA is still investigating the possibilities of SPS. One of the problems is the high investment cost due to space transport.
Power transfer for bridging applications
A powerful, focused beam in the microwave or laser range can travel long distances. There are two methods of wireless power transmission for bridging applications. One is the direct method, from by array to rectenna. A line of sight is needed and is therefore limited to short (< 40 km) distances. Above 40 kilometers, huge structures are needed to compensate for the curvature of the earth. Another method is via a relay reflector between the transmitter and rectenna. This reflector needs to be at an altitude that is visible for both transmitter and rectenna.
A pilot project was conducted to supply power to rural areas in Alaska. The system used consisted of a 2.45 GHz phased array design. The distances to be bridged were between 1 and 22 kilometers. The pilot project was not converted into a working project.
Grand Bassin, La Reunion
This WPT project was to supply electricity to a remote mountain village by beaming electrical power to the tourist village of Grand Bassin on the island of La Reunion. The plan was to build a microwave link operating at 2.45 GHz over a distance of 700 meters and delivering 10 kW with an overall efficiency of 57%. A prototype was built and presented at the Wireless Power Transmission conference in 2001, but the estimated cost rendered the project uneconomic.
In 2008 a long-range WPT demonstration was realized on one of the islands of Hawaii. It involved the wireless transmission of energy over a distance of 148 kilometers. Although the amount of power transmitted, 20 Watts, was barely enough to power a small compact fluorescent light bulb, the system was limited by the budget not the physics. If they had been able to afford more solar panels, more phased array transmitters and better receivers, better results could have been achieved.
A general public perception that microwaves are harmful has been a major obstacle for the acceptance of WPT. A major concern is that long-term exposure to low levels of microwaves might be unsafe and could even cause cancer. Scientific research indicates that heating of humans exposed to the radiation is the only known effect, although there are also claims of low-level non-thermal effects.
A clearly relevant bio-effect is the effect of microwave radiation on birds, the so-called “fried bird effect”. Research has been carried out at 2.45 GHz. The outcome showed slight thermal effects that probably are welcome in the winter and to be avoided in the summer. Larger birds tend to experience more heat stress then small birds.
The overall conclusion of bio-effects research is that microwave exposures are generally harmless except for the case of penetrating exposure to intense fields far above existing exposure limits.
Compatibility with other radio services and applications
It is assumed that WPT systems working with microwaves use frequency bands around 2.45 GHz or 5.8 GHz. These bands are already allocated in the ITU-R radio regulation to a number of radio services. They are also designated for industrial science and medical applications. The 2.45 GHz band is also used for radio LANs and microwave ovens, and the 5.8 GHz is used heavily for various applications like radiolocation service and Dedicated Short-Range Communications.
Microwave wireless power transmission could supply power to places that are difficult to reach. Especially small communities in rural areas could be supplied with power using WPT. However, it is clear that cost is a major issue to the implementation of WPT systems, with estimated costs ranging from $10k to $1 million per MW per km.
More successful demonstration projects could help the development and utilization of this technology, although further investigations concerning compatibility and safety are needed to clarify these issues.
Space Solar Power has recently gained public interest because of global warming and the energy independency goals of the US and the EU.
Application note: Wireless Power Transmission http://www.leonardo-energy.org/white-paper/wireless-power-transmission
Leonardo ENERGY's "Wireless Bible": http://www.leonardo-energy.org/good-practice-guide/wireless-energy-transmission