Some thoughts on power management

The concept of energy harvesting has been around for more than 10 years, but in real-world environments, systems powered by ambient energy have always been bulky, complex, and expensive. However, some markets have successfully adopted energy harvesting methods, such as transportation infrastructure, wireless medical devices, tire pressure monitoring, and building automation markets. In particular, in building automation systems, such as occupancy sensors, thermostats, and even light-operated switches, the power or control wiring that was usually used in the previous installation is no longer required. Instead, they use local energy harvesting systems.

　　One major application of energy harvesting systems is wireless sensors in building automation systems. To illustrate, consider the distribution of energy use in the United States. Buildings are the number one user of energy production each year, accounting for approximately 38% of total energy consumption, followed by transportation and industry, each accounting for 28% of total energy consumption. Furthermore, buildings can be further divided into commercial buildings and residential buildings, which account for 17% and 21% of the 38% of energy consumption, respectively. The 21% figure for residential buildings can be further divided, with heating, ventilation and air conditioning (HVAC) accounting for approximately three-quarters of the total energy consumption for residential buildings. With energy use currently expected to double between 2003 and 2030, the use of building automation systems could save up to 30% of energy [Source: "World Energy, Technology and Climate policy outlook (WETO)", jointly written by several EU research institutions].

　　Similarly, a wireless network using energy harvesting can connect any number of sensors in a building to adjust the temperature or turn off lights in non-critical areas when the building or rooms are unoccupied, thereby reducing HVAC and electricity costs. In addition, the cost of energy harvesting electronics is often lower than the cost of running power lines or the routine maintenance costs required to replace batteries, so there is a clear economic benefit to using harvested energy for powering.

　　However, if each node requires its own external power source, then many wireless sensor networks lose their advantage. While power management technology does continue to advance, allowing electronic circuits to operate longer on a given power source, there is a limit to this, and powering with harvested energy provides a complementary approach. Therefore, energy harvesting is a method of powering or supplementing wireless sensor nodes by converting local ambient energy into usable electrical energy.

　　Since all wireless sensor nodes can now operate on a few hundred μW to tens of mW, it is feasible to power them with non-traditional power sources. This has led to the emergence of energy harvesting, which can be used to charge, supplement or replace batteries in systems where using batteries is inconvenient, impractical, expensive or dangerous. Obviously, if the battery replacement cycle can be extended from 2 years to 5 or 7 years, the maintenance cost savings achieved will be huge.

　　A typical energy harvesting configuration or wireless sensor node (WSN) consists of four blocks, as shown in Figure 1. These are: 1) an ambient energy source; 2) a transducer component and power conversion circuitry to power downstream electronics; 3) a sensing component that connects the node to the real world and a computing component (consisting of a microprocessor or microcontroller that processes the measurements and stores them in memory); and 4) a communication component consisting of a short-range radio that enables wireless communication with neighboring nodes and the outside world.

　　

　　Figure 1: Block diagram of the main components of a typical energy harvesting system or wireless sensor node

　　Examples of ambient energy sources include a thermoelectric generator (TEG) or thermopile connected to a heat source such as an HVAC duct, or a piezoelectric transducer connected to a mechanical vibration source such as a window pane. In the case of a heat source, a compact thermoelectric device (often called a transducer) can convert a small temperature difference into electrical energy. In the presence of mechanical vibration or strain, a piezoelectric device can be used to convert this mechanical energy into electrical energy.

　　Once generated, the energy can be converted and conditioned by energy harvesting circuits into a suitable form to power downstream electronics. Thus, a microprocessor can wake up a sensor to obtain a reading or measurement, which can then be processed by an analog-to-digital converter for transmission via an ultra-low power wireless transceiver (typical transmission current levels are 20-30 mA for 1-10 ms).

　　The most advanced and readily available energy harvesting technologies (e.g., vibration energy harvesting and indoor photovoltaics) can produce milliwatts of power under typical operating conditions. While such low powers may seem limited, several years of work with harvesting components have shown that such technologies are roughly comparable to long-life primary batteries, both in terms of energy supply and cost per unit of energy provided. In addition, systems using energy harvesting can generally be recharged after depletion, which is not possible with primary battery-powered systems.