By Randy Frank, Contributing Editor
With the latest energy harvesting techniques, wireless sensing systems can avoid batteries or at least the need to change or recharge batteries. To make these systems a reality, many companies are developing the required pieces of the energy harvesting puzzle. It certainly takes more than an energy conversion technology.
A shown in Figure 3, a complete energy harvesting system has several aspects. The potential energy sources include piezoelectric (kinetic), thermoelectric, photovoltaic, and electromagnetic energy (RFID). Since the energy converted from any of these sources is rather limited, it must be consumed very judiciously by all of the other components of the wireless sensing node.
Thermoelectric technology is just one of the areas that have provided recent advances. One company, Micropelt, a spin-off from Infineon Technologies, takes advantage of semiconductor processing technology.
Micropelt uses a thin-film microelectronic process to develop a silicon-based thermogenerator. The working structure consists of leg pairs of n- and p-type material separately produced and optimized on two different wafers to generate a voltage. Figure 1 shows the two-wafer structure.
Fig. 1: Micropelt’s thermogenerator consists of two wafers, one with n-type material and the other with p-type material. Source: Micropelt.
The generated voltage is directly proportional to the number of leg pairs and the temperature difference between the top and bottom sides times the Seebeck coefficient. More than 100 leg pairs are possible on a square millimeter and a temperature difference (ΔT) as small as 25° C can generate an open circuit voltage in excess of 2.5 V. With a heat load of 1 W applied to thermogenerator with 450 leg pairs, over 2.5 mW can be generated with a ΔT of only 25° C, as shown in Figure 2. The products target applications around room temperature.
The choice of the energy harvesting technique depends on the energy source or sources that the wireless sensor would normally experience. In some instances, more than one energy harvesting technique is used to ensure a sufficient amount of energy. In any case, an energy storage element, typically either a supercapacitor or battery, stores the harvested energy.
A power management block, such as an integrated circuit, handles the conversion of the voltage level to appropriate levels for other portions of the system including charging the battery or supercapacitor. In some cases, this requires boosting the voltage from a level of a few volts to several volts required for the sensor and transceiver. For example, Linear Technology offers power management products for piezo, solar and thermo energy harvesting applications.
In addition to the wireless communication protocol, the transceivfiger must also have extremely low power in both active and wait or even sleep modes. Finally, a very low power consumption microcontroller (MCU) takes care of the proper operation of the complete wireless sensor node. Companies including Texas Instruments, Microchip, and others have developed MCUs with extremely low power consumption and specifically applied them to energy harvesting applications.
Taking into account the delicate balancing of available energy versus power consumption at the right time is the major challenge of the energy harvesting system designer. To simplify entry into energy harvesting, many companies have partnered to provide most of the critical pieces of the energy harvesting system in development and evaluation kits for just about any technology, including RF, piezoelectric, thermoelectric, and solar. If the right combination of energy source, wireless protocol and sensor are in the kit, it certainly seems like an obvious place to start.