Surface Technology — 2 — 334 1. Introduction Energy harvesting technology is a technology for "harvesting" (harvesting) dilute energy that exists in various forms in the surrounding environment, such as light, vibration, heat, and radio waves, and converting it into electric power 1)-3 ). Also called energy harvesting technology.
Harvesting energy from the surrounding environment brings to mind large-scale power generation facilities such as mega solar, wind power, hydro power, geothermal power, and wave power. Not included in energy harvesting. The so-called energy harvesting is an energy conversion technology that can be used as a stand-alone power source for small electronic devices, with an output of μW to mW, at most several watts. In the past, such small-scale power generation technology had limited uses, but recent advances in low power consumption technology have expanded its applications.
In particular, growing interest in the Internet of Things (IoT), cyber physical systems (CPS), and trillion sensors has led to the emergence of key technologies for realizing independent power supply drive for wireless sensors. As such, there are growing expectations for energy harvesting. At the current level of energy harvesting technology, it is not a power supply technology that can be used universally everywhere, but worldwide research is becoming more active year by year, and technological progress is remarkable. This article outlines the latest trends in energy harvesting technology and describes its relationship with surface technology.
2. Overview of technology 2.1 Various energy harvesting technologies As mentioned above, energy harvesting is the conversion of light, vibration, heat, radio waves, and other sparse energies in the environment into electrical energy. . However, not all thermal energy is available, so it would be more accurate to say exergy (effective energy). He can convert not only temperature difference exergy but also pressure difference exergy, humidity difference exergy, concentration difference exergy, etc. into electrical energy. Because there are various forms of energy (exergy) in the environment, there are also various technologies for converting them into electrical energy. Therefore, the term energy harvesting includes various technologies. Specifically, the technologies listed in Table 1 are the main targets of various research and development. In the following, we will introduce recent research and development trends for practical use of individual energy harvesting technologies and related technologies. 2.2 Light energy utilization technology There is abundant visible light emitted from the sun and indoor lighting in the environment. Some light sources include near-ultraviolet or near-infrared rays.
So-called solar cells correspond to energy harvesting technology that uses environmental electromagnetic waves with visible light or near-visible wavelengths. There are various solar cell technologies, but among them, the technology that is attracting attention from the viewpoint of energy harvesting is the technology that can efficiently generate power from natural light such as in the shade or cloudy weather, or from indoor lighting. be. Amorphous silicon solar cells from TDK and Panasonic are used in practical products such as calculators and wristwatches. Dye-sensitized solar cells and perovskite solar cells are attracting attention as solar cell technologies that are more efficient than amorphous silicon solar cells in indoor environments. 0093 Emerging Trends in Energy Harvesting Technologies Keiji TAKEUCHI a NTT Data Institute of Management Consulting, Inc.(10th floor, JA Kyosai Building, 2-7-9, Hirakawa-cho, Chiyoda-ku,Tokyo 102- 0093) DM-DM-DM-DM-DM-DM RICKKEN Keywords : Energy Harvesting, Internet of Things ----------------------------------------------------------------―――――――――――――――――――――――――――――――― Small Special Feature: Energy Harvesting (Energy Harvesting) --------------------------------------------------------------------------- Energy source Major energy harvesting technologies Visible light Various solar cells (dye-sensitized solar cells, perovskite solar cells, organic thin-film solar cells, etc.) Mechanical energy Electromagnetic Induction, electrostatic induction (electret, electroactive polymer, triboelectrification, etc.), piezoelectric effect, inverse magnetostrictive effect Thermal energy Heat engines, etc. Radio wave energy Rectenna Others Biofuel cells, microbial fuel cells, etc. Table 1 Main energy harvesting technologies — 3 — Vol. For dye-sensitized solar cells, Fujikura has started selling samples of modules that have passed various durability tests. Ricoh has also developed an all-solid-state dye-sensitized solar cell. Perovskite solar cells are a technology that was born in 2009 at the Miyasaka Laboratory of Toin University of Yokohama, but currently research groups in Europe and South Korea are competing to improve conversion efficiency.
The energy conversion efficiency has already exceeded 22% and is said to have room for further improvement. Researchers of organic solar cells all over the world are participating in research on perovskite solar cells, and early commercialization is expected (Fig. 1)4). At present, the solar cells with the highest energy conversion efficiency for sunlight and indoor light are thin-film GaAs solar cells (excluding those using concentrators and multi-junction technology). Thin-film GaAs solar cells are produced and sold by Alta Devices, a US venture company. Due to the high price of the company's modules, it was difficult to develop markets other than military applications. there is 2.3 Mechanical energy utilization technology In order to convert mechanical energy into electric power, first, mechanical energy existing in the environment is taken into the device, and then the energy is converted into electrical energy.
A stepwise process is required (Figure 2). The former method of capturing the mechanical energy in the environment into the device includes (1) a method in which the device is deformed by an external force, (2) a method in which the blades receive the flow of air or water, and (3) changes in the acceleration of the external environment. There are methods such as so-called vibration power generation that utilize the inertial force generated in the internal weight by (vibration or impact). The lighting switches sold by EnOcean in Germany and Pulse Switch in the United States are commercialized using method (1). Power is generated by pressing the switch with a finger, and the lighting ON/OFF control signal is transmitted wirelessly. It has the advantage of eliminating the need for wiring inside walls and facilitating layout changes, and is becoming popular mainly in Europe. Examples of the practical use of method (2) include automatic faucets for toilets sold by LIXIL and TOTO.
The flow of water is used to generate electricity to drive an infrared sensor and an electromagnetic valve. As a practical application of method (3), there is a vibration power generation device sold by Perpetuum in the United Kingdom. There are many types that resonate with the environmental frequency and amplify the amount of power generated. Products that are tuned to 50/60/100/120 Hz assuming that the vibration source is a motor that vibrates in synchronization with the frequency of the commercial power supply. are on sale.
Figure 1 Perovskite solar cells with various structures Mechanical energy uptake into external environmental power generation devices G G G Force x Displacement Density x Velocity 3 Acceleration 2 Conversion to energy Pushing, stepping, hitting, bending Running, swimming Walking, swinging limbs, etc. N S ＋＋＋＋ －－－－ Fig. 2 Power generation technology from mechanical energy Surface technology — 4 — 336 Review When frequency changes I can not cope. Also, it is not compatible with inverter-type motors whose frequency fluctuates. Therefore, in order to generate power efficiently even with more general environmental vibrations, extensive research has been conducted on vibration power generation devices that are compatible with broadband vibration sources or that can automatically tune to the prevailing frequency in the environment. .
As a method for broadening the bandwidth, the type that uses the nonlinearity of the spring, especially the type that uses a bistable structure (for example, a structure with two stable points such as a mechanical switch) has been well studied. However, due to reasons such as unstable operation depending on vibration conditions, it has not yet reached the level of practical use. On the other hand, Takenaka Corporation and Panasonic are prototyping two mass point devices. It is academically uninteresting, but it is believed that it will be put to practical use soon. On the other hand, the latter of the two-step process, the principle of converting the mechanical energy taken inside the device into electrical energy, is known to have four types: electromagnetic induction, piezoelectric effect, electrostatic induction, and inverse magnetostrictive effect (Fig. 3). ). Electromagnetic induction generates an induced current through the relative motion of a coil and a magnet, and the piezoelectric effect generates surface charges by distorting piezoelectric materials such as PZT.
There are several variations of electrostatic induction, but in electret power generation, which has been extensively researched in Japan, an electret that is electrified by injecting an electric charge is used as one electrode of a capacitor, and the counter electrode is moved. Generate power by changing capacity. E. Research on triboelectrification, which electrifies two materials by bringing them into contact without using a rectolet, is also gaining momentum, mainly in the United States. These electrostatic induction methods are particularly closely related to surface technology. To generate power using the inverse magnetostriction effect, the magnetostrictive material that deforms when a magnetic field is applied is distorted to change the surrounding magnetic field and generate an induced current in the coil. As a magnetostrictive material for power generation, Galfenol developed by the US military and his Terfenol-D are used. Among the four power generation principles, electromagnetic induction has the highest power generation efficiency when the device is large. However, when trying to fabricate small and thin devices for energy harvesting applications, depending on the conditions, other power generation principles may be more advantageous.
Especially for wearables and implants, devices that are small, thin, flexible, and biocompatible are desired, and device research using surface technology is actively carried out. 2.4 Thermal energy utilization technology Various power generation methods using heat (temperature difference exergy) are being researched (Fig. 4). Heat engines (Rankine cycle, etc.) are widely used in large-scale power generation facilities and have high power generation efficiency. However, in small power generation for energy harvesting applications, the efficiency of the heat engine decreases, so thermoelectric power generation devices, which do not lose efficiency even when miniaturized, are advantageous. Marlow Industries, Perpetua, and others in the United States sell a combination of a thermoelectric generation module, a heat sink, and a booster circuit so that it can be used immediately as a power source for wireless sensors.
