With the rise of the Internet of Things and wearable devices, how to get rid of the battery, find an inexhaustible source of energy, and constantly supply power for such small, low-power electronic devices is becoming a hot research topic. This topic is called "energy harvesting technology."
The demand for energy harvesting is increasing, and a variety of research work is underway. One of them is to use the human body to provide "always" uninterrupted power for electronic devices. Energy harvesting technology requires an energy harvesting sensor that matches this type of energy source to convert the available energy from the surrounding environment into electrical energy . Typical energy sources include electromagnetic waves, wind, vibration, and thermal energy.
Batteries have always been the main source of power for small portable devices, but their use has been quite inconvenient. The batteries in these devices are very bulky and can only last for a limited time and must be constantly charged and replaced, which becomes expensive and time consuming. Moreover, the battery often causes inconvenience. For a mobile phone, the call is often interrupted because it suddenly loses power in half.
Energy harvesting techniques from the human body can be a method of replacing battery powered devices. For general small electronic devices, the use of body energy as an energy source will bring many benefits. Usually we don't have much awareness of our body. In fact, the human body produces energy through daily activities and intense physical activity, which is an excellent source of energy.
Extracting energy from the body is much more complicated than extracting energy from the machine because the size of the harvester is quite small and human motion occurs at lower frequencies . In the past, there were products such as watches, flashlights and radios that used manual power sources to work.
Currently, research is being conducted on energy extracted from the human body for low-power biomedical applications. For example, a medical device implanted in the body is powered to extend the life of the device. Otherwise, in order to replace the batteries in these devices, continuous operation is required. In the future, the application of human body power will gradually expand to use in wearable devices and even mobile phones.
We can use the energy generated by people's daily activities, such as from people's breathing, body heat, arm movements, walking, running and pedaling. These sensors, plus signal processing circuitry, convert these energy into electricity. This collector can be placed on a human body wearing such as clothes, watches, shoes or glasses to charge the electronic device without any adverse effect on the user.
Energy harvesting for three different sensors
To extract energy from the human body, three types of sensors are generally used , including pyroelectric sensors , electromagnetic sensors, and piezoelectric sensors . Thermoelectric sensing uses body heat, while electromagnetic sensing and piezoelectric sensing extract energy from body motion.
Heat is one of many sources of energy from the body. For example, in a newly developed wearable wireless EEG system, it uses body heat to work. The EEG system uses a pyroelectric sensor that produces more than 2mW of power at room temperature, and the system itself requires only 0.66mW of power, which is sufficient for the needs.
By attaching thermoelectric sensors to the body, these sensors are able to sense the temperature difference between the body and the room to establish an electrical potential. Studies have shown that although there is a large amount (about 100 W) of thermal energy in the body, only electric power in the milliwatt range can be obtained from body heat. The exposed parts of the body, such as the forehead, wrists, and head, are areas where the maximum amount of power can be extracted, while the parts of the body that are blocked by the clothes release less heat.
Studies have shown that when the thermoelectric sensor is placed in the correct position of the human body (generally placed on the torso of the body, where the body tissue temperature is the highest), electrical energy of approximately 10-30 μW/cm 2 can be generated. In addition, the thicker or smaller the sensor, the higher the amount of power generated.
The human body is also a large source of energy, which is produced by movements such as walking, running, jogging and related body parts. Some research on the use of electromagnetic energy is also underway, and electromagnetic sensors have been found to be useful for providing energy to wearable devices or other portable electronic devices. The electromagnetic sensor is similar in function to the pyroelectric sensor, extracting energy from body motion and converting it into a voltage, a current, and the like.
Researchers have developed an electromagnetic sensor mounted on the hip that can be used to capture the kinetic energy generated by hip movement during body movements such as walking or running. According to their research , kinetic energy extracted from human motion can be converted to electrical energy , which can then be used to power equipment equipped with people . From this study, it was found that when walking or running at a speed of 1.5 meters per second, a maximum open circuit voltage of 1 V (voltage when no external load is connected) and a power of 0.3-2.46 mW can be generated.
In order to make the kinetic energy collection more efficient, some people have thought of putting the sensor on the belt on the waist and then using a connecting rod in the form of a swing. One end of the connecting rod is connected to the sensor and the other end is connected to the surface of the thigh. The swing link located inside the thigh moves with the thigh, and the person swings around the hip when walking. Thus, the voltage generated by the sensor increases with walking speed. This shows that since power is a function of voltage, the amount of power produced by the sensor also increases with walking speed.
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