The fundamentals of energy harvesting design

The proliferation of power-miser sensors, microcontrollers and RF transceivers is ushering in wireless sensor networks powered exclusively or supplemented by energy harvesting techniques. Ultra-low-power wireless protocols are ever more widely used and standards are in active development. Sensor networks unshackled from the ac mains and batteries open the possibility for greater reliability, less maintenance, and better safety. Applications unthinkable only a few years ago are now possible with energy harvesting techniques.

New power management products can squeeze usable power out of intermittent and often miniscule outputs from such energy harvesting transducers as TEGs (thermo-electric generators), photovoltaics, piezoelectric materials, and electromagnetics. But designing for these minute amounts of power requires a new way of specifying and analyzing power management devices.

The typical wireless sensor node includes an energy source, processor, and an RF link. The missing link in this system to date has been a means of power management. This function is necessary because transducers available to harvest power are often inconvenient to work with. They generally provide either an extremely low-voltage, low-impedance output, or a high-voltage, high-impedance output. The various elements in the typical system can be further divided into power producers/regulators (transducer and power management) and power users (everything else). Simply put, if the average energy the system can harvest exceeds the average power the remote sensor electronics requires, you have the possibility for an autonomous system.

Before initiating any design, it is worthwhile to run a quick feasibility analysis. This will determine whether energy harvesting techniques are even practical. The first step is to decide how often measurements must take place and be transmitted. We will call this the measurement frequency, F. Next, we can determine the amount of processing power necessary for handling the sensor, signal conditioning, data conversion and processing, plus the amount of power needed for transmitting data via the RF transceiver.

The accompanying table shows the typical power requirements for a widely used microcontroller and RF link system. These power requirements can vary from one manufacturer to another for a particular application. There are numerous choices, and they can be optimized for specific end applications. The table provides enough information to calculate the system duty cycle and average power. The duty cycle, D, of the system is defined as:

D = T_m + T_p + T_t × F

where T_m = measurement time, T_p = processing time, T_t = transmit time, F = measurement frequency. The average power, P_a, is simply

P_a = P × D + P_s

where P = total power and P_s = standby power, which is generally small enough to be ignored.

For example, assume the task is to design an autonomous indoor temperature sensor. This sensor will be deployed throughout a large office building and coupled with proximity sensors that can detect when a room is occupied and adjust the temperature accordingly. Deploying this type of sensor within a large building can reduce heating and cooling costs significantly.

The sensors require 500 µA at 3.3 V for 2 msec to measure temperature and detect an occupant. A low-power microcontroller must operate on this data for another 5 msec. The processor consumes 3 mA at 3.3 V when processing the data. Finally, the RF link requires 30 mA at 3.3 V for 30 msec to transmit the data. The desired measurement frequency is 0.2 Hz (one measurement every five seconds). Then

D = T_m + T_p + T_t × F

= (2 msec + 5 msec + 30 msec) × 0.2 Hz = 0.0074

P = (3.3 V × .500 µA) + (3.3 V × 0.003) + (3.3 V × 0.03)

= 110.6 mW

P_a= P × D + Ps

= 0.0074 × 0.1106 = 818 µW