Beatrice Lang
The concept of transmitting electricity through the air has been around for over a century, with Nikola Tesla first envisioning a future where power stations would directly beam electricity into homes and businesses. Tesla's research laid the foundation for modern methods of wireless power transfer, including magnetic induction and far-field techniques. While Tesla's ideas were ahead of his time, modern advancements in technology have brought the concept of wireless power transmission back into the spotlight. Recent experiments by MIT and companies like eCoupled have demonstrated the potential for wireless power transfer over short distances, and Japan's space agency is even developing a solar-satellite that would beam power back to Earth with microwaves.
| Characteristics | Values |
|---|---|
| Technology | Wireless power transfer |
| Methods | Inductive coupling, radiative techniques, magnetic induction |
| Applications | Charging handheld devices, RFID tags, induction cooking, wirelessly charging implantable medical devices, powering drone aircraft |
| History | Nikola Tesla first envisioned wireless power transfer in the early 1900s; the first long-distance wireless power transmission was achieved in the 1960s by William C. Brown |
| Challenges | Limited distance range, aiming the transmitter at the receiver, developing a standard frequency for gadgets |
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What You'll Learn
Nikola Tesla's wireless power distribution system
Nikola Tesla, a Serbian-American engineer, is known for his pioneering work in the field of wireless power transmission. He envisioned a "World Wireless System", which could transmit electricity and information without the need for physical power lines. This idea led to the construction of the Wardenclyffe Tower on Long Island, New York, in 1901. The tower, standing at 57 metres (187 feet) tall, was designed to be a transmitter for wireless power and communication.
Tesla's system was based on 19th-century ideas of electrical conduction and telegraphy, with an electrical charge being conducted through the ground and returned through the air. He theorised that if he injected electric current into the Earth at the right frequency, he could harness the planet's electrical charge and cause it to resonate at a frequency that could be tapped anywhere to run devices or carry a signal. This concept, known as "telegeodynamics", unfortunately never progressed beyond the prototype stage as the device was not powerful enough to transmit energy effectively through the Earth.
In addition to his work on the Wardenclyffe Tower, Tesla also conducted experiments at high altitudes in Colorado Springs in 1899. He proposed a system of balloons to suspend transmitting and receiving electrodes above 30,000 feet (9,100 metres) in altitude, where he believed the pressure would allow him to send high voltages over long distances. However, he incorrectly concluded that he could use the entire globe of the Earth to conduct electrical energy.
While Tesla's dream of a fully operational wireless power system never came to fruition, his ideas were visionary and ahead of their time. Many of his concepts have influenced modern wireless technologies, including Wi-Fi, wireless charging, and long-distance communication. Today, the Tesla Science Center at Wardenclyffe continues to preserve his legacy and advance research inspired by his pioneering work.
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Wireless power transfer using inductive coupling
Wireless power transfer (WPT) is the transmission of electrical energy without wires as a physical link. In a wireless power transmission system, an electrically powered transmitter device generates a time-varying electromagnetic field that transmits power across space to a receiver device. The receiver device then extracts power from the field and supplies it to an electrical load.
Wireless power transfer by employing inductive coupling has been well-refined in recent years. It works on the principle of electromagnetic induction, where a time-varying voltage in a coil generates a fluctuating electromagnetic field. If another coil is placed in this field, the voltage in the first coil is induced across the terminals of the secondary coil. Power can be transferred via inductive coupling in a simple, durable, safe, and efficient manner.
Ordinary inductive coupling can only achieve high efficiency when the coils are very close together, usually adjacent. In most modern inductive systems, resonant inductive coupling is used, in which the efficiency is increased by using resonant circuits. This can achieve high efficiencies at greater distances than non-resonant inductive coupling. Resonant inductive coupling is a form of inductive coupling in which power is transferred by magnetic fields between two resonant circuits, one in the transmitter and one in the receiver. Each resonant circuit consists of a coil of wire connected to a capacitor, or a self-resonant coil or other resonator with internal capacitance. The two are tuned to resonate at the same resonant frequency.
A drawback of resonant coupling theory is that at close ranges when the two resonant circuits are tightly coupled, the resonant frequency of the system is no longer constant but "splits" into two resonant peaks, so the maximum power transfer no longer occurs at the original resonant frequency and the oscillator frequency must be tuned to the new resonance peak. However, the possibilities of using resonant coupling to increase transmission range have only recently been explored.
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Power beaming with electromagnetic radiation
The basic principle behind power beaming with electromagnetic radiation involves converting electricity into a laser beam or high-frequency microwaves, which can then be focused into narrow beams and directed towards a receiver. At the receiver, the electromagnetic radiation is captured and converted back into electricity. This process is often referred to as "power beaming" because the power is literally beamed at a receiver, much like a flashlight beam.
One of the key advantages of using electromagnetic radiation for power beaming is its ability to transmit energy over long distances. This makes it suitable for applications such as solar power satellites, wireless-powered drone aircraft, and even powering spacecraft leaving Earth's orbit. For example, a proposed short-term demonstration project by a New Zealand firm involves beaming solar energy from low Earth orbit to supply electricity to developing nations near the equator.
To achieve efficient power transmission, various factors need to be considered, such as the distance between the transmitter and receiver, the wavelength of the electromagnetic radiation, and the size of the transmitting and receiving antennas. For instance, in the case of microwave transmission, shorter wavelengths result in more directional power beaming, allowing for longer-distance power transmission. Additionally, the use of high-directivity antennas or well-collimated laser light can further enhance the range and efficiency of power beaming.
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Using solar cells to convert sunlight into microwaves
Nikola Tesla first attempted to transmit electricity wirelessly in 1901. He built a 57-metre tower as part of an experiment to transmit information and electricity wirelessly over long distances. However, he failed to get the electric power itself to travel very far.
Since then, several methods have been proposed for transmitting electricity through the air. One such method involves using solar cells to convert sunlight into microwaves.
Space-based solar power (SBSP) is a concept that involves collecting solar power in outer space using solar power satellites (SPS) and distributing it to Earth. The main idea is to convert sunlight into some other form of energy, such as microwaves, which can then be transmitted through the atmosphere to receivers on the Earth's surface.
There are two basic methods of conversion that have been studied: photovoltaic (PV) and solar dynamic (SD). Most analyses of SBSP have focused on photovoltaic conversion, which uses solar cells to directly convert sunlight into electricity. Solar dynamic, on the other hand, uses mirrors to concentrate light on a boiler, which can reduce mass per watt.
Wireless power transmission has been proposed as a means to transfer energy from collection to the Earth's surface using microwave or laser radiation at a variety of frequencies. This type of power transmission is also known as power beaming, where power is transferred by beams of electromagnetic radiation, such as microwaves or laser beams. These techniques can transport energy over long distances but require the transmitter to be aimed at the receiver.
Several organizations have demonstrated the feasibility of this concept. For example, on March 12, 2015, JAXA wirelessly beamed 1.8 kilowatts of power to a small receiver 50 meters away by converting electricity to microwaves and then back to electricity. Mitsubishi Heavy Industries also demonstrated the transmission of 10 kilowatts of power to a receiver unit located 500 meters away on the same day.
The SBSP concept is attractive because space has several advantages over the Earth's surface for collecting solar power. For instance, it is always solar noon in space, and collecting surfaces can receive much more intense sunlight due to the lack of reflection and absorption by the atmosphere.
However, there are also concerns associated with SBSP. The potential exposure of humans and animals on the ground to high-power microwave beams is a significant concern. Studies have shown that at certain intensities, humans experience significant deficits in spatial learning and memory. As of 2014, none of the SBSP proposals are economically viable due to the high space launch costs.
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Resonant inductive coupling to increase efficiency
Resonant inductive coupling is a wireless transmission of electrical energy between two magnetically coupled coils, which are part of a resonant circuit tuned to resonate at the same frequency as the driving frequency. This method of power transfer is also known as electrodynamic coupling or strongly coupled magnetic resonance.
In resonant inductive coupling, the efficiency is increased by using resonant circuits. This method can achieve high efficiency at greater distances than non-resonant inductive coupling. The primary coil is driven at the resonant frequency of the secondary side, allowing significant power transmission between the coils over a range of a few times the coil diameters at reasonable efficiency.
The secondary coil is capacitively loaded to form an LC circuit. When the primary coil is driven at the resonant frequency of the secondary coil, the phases of the magnetic fields of the two coils are synchronized. This results in the maximum voltage being generated on the secondary coil, while copper loss and heat generation on the primary coil are reduced, improving efficiency.
The energy transfer occurs through the oscillation of the magnetic field in the inductor and the electric field across the capacitor at the resonant frequency. This oscillation gradually decreases due to resistive and radiative losses, but most of the energy can still be transferred if the secondary coil absorbs more energy than is lost in each cycle.
Resonant inductive coupling has been applied in various applications, including implantable medical devices and road electrification, achieving over 75% transfer efficiency at an operating distance of less than 10 cm between coils. It has also been used in RFID tags, wireless power transmission, and non-contact power supply systems.
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Frequently asked questions
There are a few methods to transmit electricity wirelessly, including:
- Magnetic induction: Using a magnetic field to generate an electric current.
- Power beaming: Using beams of electromagnetic radiation, such as microwaves or laser beams, to transfer power.
- Wireless power distribution systems: Using balloons to suspend transmitting and receiving electrodes in the air at high altitudes to transmit high voltages over long distances.
Wireless power transmission has a variety of applications, including:
- Charging handheld devices such as phones and electric toothbrushes.
- Wireless power transfer in implantable medical devices like artificial cardiac pacemakers.
- Powering electric vehicles.
- Solar power satellites.
- Wireless-powered drone aircraft.
Some challenges include:
- Efficiency: Wireless power transmission is most efficient when the coils are very close together, usually adjacent. While resonant inductive coupling can increase efficiency at greater distances, creating a standard frequency for all devices remains an issue.
- Safety: Wireless power transmission using electromagnetic radiation requires the transmitter to be aimed at the receiver, posing potential safety concerns.
- Infrastructure: Implementing wireless power transmission on a large scale would require significant changes to existing infrastructure and may face opposition from stakeholders, as seen with Tesla's Wardenclyffe project.
