Micro-Energy Harvesting is possible through two identified sources of energy: 1) the energy provided by a living being like an animal or a human; 2) the energy supplied by the environment. Human Energy Harvesting technologies enable the power independance of portable handheld electronic gadgets, and thus reduce the dependancy of the battery and mains power. Based the power requirement of portable devices, Human Energy Harvesting techniques can either increase the battery backup time of devices, or can fully replace batteries in micro-energy devices.

Human Power Harvesting Portable Electronics

Examples of battery-powered portable devices, where Human Energy Harvesting techniques can help either battery backup time or can fully replace batteries with micro-energy harvesting systems.

Advances in semiconductor and microfabrication techniques has stimulated an increasing level of interest in microenergy harvesting. Industrial applications of energy harvesters include amongst others distributed wireless sensor nodes for structural health monitoring, embedded and implanted sensor nodes for medical applications, machine monitoring, power supplies for autonomous vehicles, automobile tyre pressure sensor monitoring systems and the wireless switching of appliances in building services. Micro-energy harvesting is indispensable for any application where conventional power sources cannot be used or are not economically viable.

A large number of today’s wearable or portable devices are mainly powered by batteries which are fundamentally energy reservoirs and are characterised by their energy density. There is a huge amount of battery-powered portable devices that we use in our daily lives: cell phones, camcorders, laptops, remote controls, PDAs (Personal Digital Assistant), I-pod media players, to name just a few, are the most frequently used.

Micro-energy harvesting aims at finding clean and economic alternatives to bulky batteries as the power supply of low-power micro-sensors and actuators. In short, micro-energy harvesting is a way to make unused energy useful.

Human Energy essentially appears in the form of mechanical, thermal, and chemical energy. Human energy can be either active or passive. Human active energy is the energy produced intentionally by a human to perform a task. It is an old concept which aims at increasing the ratio of time of use with respect to time of charge. Radio receivers, phone battery chargers and electric torches are examples of devices found in the market arena which are powered by human active energy. The energy domain exploited in such devices is mechanical energy which is provided by winding a hand crank, or shaking the device. Human-active-energy powered devices offer a good ratio between charging time and use time.

By human passive energy, the energy is unconsciously produced by the human. It is the most attractive since it eliminates the power maintenance in wearable and portable devices. Human passive energy appears in the form of thermal and kinetic energy. Among commercial products, the first devices were wristwatches due to its very low power consumption (less than 50 nW in modern watch circuits).

Both kinetic and thermal energy powered watches have been commercialised. With kinetic powered watches, the output power is 5 µW in normal conditions and up to 1 mW when the watch is forcibly shaken. With thermal powered watches, around 1.5 µW is generated when the temperature difference is 1 – 3° C. Miniature thermoelectric generators are also developed that convert body heat flow into electricity. It is claimed that it can generate 40 µW at 3 V with 5 degree difference in temaperature. Electronic wristwatches, attachable medical devices, self powered heat sensors and mobile electronics are potential application of such generators.

Human chemical energy can be exploited for producing biobatteries within the human body or in diagnostic tests, using the body’s fluids. Researchers has already developed biobatteries that harvest energy from the biological ion gradient. A few are developing biobatteries that can be integrated into cheap, disposable credit-card sized biochips healthcare kits) for desease detection. These biobatteries supply electricity on contact with biofluids such as urine or blood.