A key application of energy harvesting systems is radio sensors in building automation systems. In the United States, buildings are the number one user of energy production on an annual basis, closely followed by the transportation and industrial segments.
A wireless network utilizing an energy harvesting technique can link any number of sensors together in a building to reduce HVAC and electricity costs by adjusting the temperature or turning off lights to nonessential areas when the building or rooms within are unoccupied. Furthermore, the cost of energy harvesting electronics is often lower than running supply wires, or the routine maintenance required to replace batteries, so there is clearly an economic gain to be had by adopting a harvested power technique.
Nevertheless, many of the advantages of a wireless sensor network disappear if each node requires its own external power source. Even though ongoing power management developments have enabled electronic circuits to operate longer for a given power supply, this has its limitations, and power energy harvesting provides a complementary approach. Thus, energy harvesting is a means of powering wireless sensor nodes by converting local ambient energy into useable electrical energy. Ambient energy sources include light, heat differentials, mechanical vibration, transmitted RF signals, or any source that can produce an electrical charge through a transducer. These energy sources are all around us and they can be converted into an electrical energy by using a suitable transducer, such as a thermoelectric generator (TEG) for temperature differential, a piezoelectric element for vibration, a photovoltaic cell for sunlight (or indoor lighting), and even galvanic energy from moisture. These so-called “free” energy sources can be used to autonomously power electronic components and systems.
With entirely wireless sensor nodes now capable of operating at microwatt average power levels, it is feasible to power them from nontraditional sources. This has led to energy harvesting, which provides the power to charge, supplement, or replace batteries in systems where battery use is inconvenient, impractical, expensive, or dangerous. It can also eliminate the need for wires to carry power or to transmit data.
A typical energy harvesting configuration or wireless sensor node (WSN) is comprised of four blocks, as illustrated in Figure 1. These are:
- Ambient energy sources
- A transducer element and a power conversion circuit to power downstream electronics
- A sensing component that links the node to the physical world and a computing component consisting of a microprocessor or a microcontroller that processes measurement data and stores them in memory
- A communication component consisting of a short-range radio for wireless communication with neighboring nodes and the outside world.
Examples of ambient energy sources include TEGs (or thermopiles) attached to a heat-generating source such as HVAC ducts, or a piezoelectric transducer attached to a vibrating mechanical source such as a windowpane. In the case of a heat source, a compact thermoelectric device can convert small temperature differences into electrical energy. In the case where there are mechanical vibrations or strain, a piezoelectric device can be used to convert these into electrical energy.
Once the electrical energy has been produced, it can then be converted by an energy harvesting circuit and modified into a suitable form to power the downstream electronics. Thus, a microprocessor can wake up a sensor to take a reading or measurement, which can then be manipulated by an analog-to-digital converter (ADC) for transmission via an ultra low power wireless transceiver.