Article by: Asst.Prof. Yod Sukamongkol
Faculty of Engineering, Ramkhamheang University
In the power transmission system, the cable wire or grid line is generally needed to deliver the electrical power from power plants to users even used in the household appliances. Sometimes, it results in the electrical problems such as voltage drop or short circuit.
Therefore, the wireless power transmission system has been studied, researched and developed to solve those problems. Moreover, it can reduce cost, increase the transmission range and apply for varieties power plant locations.
This concept idea seems to be the future innovation but, in fact, it was already proposed and discovered since the early of 19th century by Nicolas Tesla who is best known for his contributions to the design of the modern electrical engineering. His wireless power transmission experiments were established in the Colorado Springs, USA. Due to the limitation of material science and communication technology, although the unsatisfied results were exposed, this made pronouncements on the possibility of wireless communication with his devices. However, these ideas are presently used as the fundamental of the practical wireless charging in the modern electrical appliances such as electric toothbrush, mobile phone, etc.
Rectenna
Normally, antenna is a transducer designed to transmit or receive electromagnetic waves (e.g., TV, radio or Wifi) through the electrical devices that transform the signal into light and sound. The rectenna, which stands for rectifying antenna, is used for converting electromagnetic energy into electrical energy actually direct current (DC) electricity.
The researcher from MIT and the Technical University of Madrid experimentally uses an antenna to capture electromagnetic waves (Wi-Fi) as alternating current (AC) waveforms which are sent to a two-dimensional semiconductor that converts them into DC. The results show that this device can produce about 40 microwatts which is enough to serve an LED or to drive a very small IC silicon chip. Thus, the rectenna not only just harvest energy from the abandon electromagnetic waves, but also can convert it into electrical energy that could be used as a power supply for electrical devices without wire and battery support.
Previously, rectennas had been made from silicon or gallium arsenide, which is rigid and expensive. Then, the research team used molybdenum disulfide (MoS2) to craft the flexible rectennas. The structure is also known as a Schottky diode resulting in higher speed operation. This means it can capture higher frequencies (gigahertz frequencies level) and fast enough to cover most of the radio-frequency bands used by our daily electronics, including Wi-Fi, Bluetooth, cellular LTE, and many others. The experimental result shows that the maximum conversion efficiency is 31% when the frequency of electromagnetic wave is 2.4 GHz.
According to the alternative flexible rectenna, it can be deployed over large areas such as wall, roof, road, bridge even through on portable devices such as flexible smartphones. It is relatively low cost at larger scales, so it could be used for much bigger applications. Then, the electrical energy can be generated everywhere that has the electromagnetic wave. In addition, as this rectenna can generate electricity by harvesting the electromagnetic wave with no batteries needed, this device could even be used in medical implants and swallowable sensors which can decrease the risk that the patient dies from the leak of toxic metal from batteries.
Optical Rectenna
Generally, solar energy can be converted to electrical energy by using solar cell, which absorbs photons and generate electron-hole pairs within P-N junction to provide electrical power. The optical rectenna, the first announce in 2015, relies on a classical electromagnetic wave view of sunlight. This device consists of a combination of a carbon nanotube antenna and diode rectifier. Optical rectennas operate by capturing the sunlight and electromagnetic wave to its antenna whose ends have been opened. The electromagnetic field creates an oscillation in the antenna, producing an alternating flow of electrons. When the electron flow reaches a peak at one end of the antenna, the diode closes, trapping the electrons, then re-opens to capture the next oscillation, creating a current flow, then the electrical energy is generated. This switching must occur at terahertz frequencies to synchronize those of the sunlight. To increase the conversion efficiency, the junction between the antenna and diode must provide minimal resistance to electrons flowing through it while open, yet prevent leakage while closed. Nowadays, the conversion efficiency of optical rectenna in the laboratory is below 1% while those of commercial solar cell is about 15%.
Far from the first objective to develop the wireless power transmission, in the present, the alternative antenna technologies are discovered to generate the electrical energy by harvesting energy from electromagnetic waves which is free, abundant and all around us. However, these technologies need more study and develop to improve its conversion efficiency to achieve commercial market and become one of the potential green energy resources in the future.