Developments in semiconductor and transducer technology are making ambient energy harvesting a practical reality. An application of micro-power harvesting is wearable health monitoring systems such as blood pressure monitoring systems, EEG and ECG but the batteries needed to power such systems are either large ,providing a longer run time but making the system less user friendly, or small which make the system less autonomous. A source of energy harvesting is therefore required to power such systems.
Solar cells may not be efficient for these health monitors as most of the body sensing units are not directly exposed to sunlight. RF energy harvesting could provide a solution but the complexity reduces power efficiency. One of the most readily available sources of power for health monitoring systems is body heat as potentially over 100W of power in the form of waste heat is available to be harnessed from the human body. Thermoelectric sensors attached to the body are capable of generating electricity by sensing the difference between body and room temperature. An example is the wearable EEG system developed by IMEC.
This EEG monitor is a battery-free wireless 2-channel system powered by a hybrid power supply that uses body heat and ambient light. The hybrid power supply combines a thermoelectric generator that uses the heat released from a person's temples together with silicon photovoltaic cells. The entire system is wearable and integrated into a device resembling headphones. The system can provide more than 1mW on average indoors, which is more than adequate.
Thermoelectric generators using body heat typically show a drop in generated power when the ambient temperature is in range of the body temperature. When outside, the photovoltaic cells in the IMEC hybrid system counter this energy drop and ensure a continuous power generation. They also serve as part of the radiators for the thermoelectric generator, which are needed to obtain high efficiency. Each of the two radiators, on left and right sides of the head, has an external area of 4.8cm 2 that is made of high-efficiency Si photovoltaic cells. Thermally conductive comb-type structures (so-called thermal shunts) have been used to eliminate the thermal barrier between the skin and the thermopiles that is caused by the person's hair on the thermoelectric generator.
The benefits of energy harvesting for medical monitoring devices are a reduction in cost as self-powered wireless sensors do not require wires, conduits and are very easy to install. Maintenance costs are also reduced as energy harvesting allows for devices to function unattended and eliminates service visits to replace batteries. A great advantage is a reduction in size and weight of the equipment which would not have to house batteries.
Top Image: Wearable EEG monitor. Source: IMEC
For more attend: Energy Harvesting and Storage Europe 2009
