Managing Harvested Energy
An important consideration in the development of an energy harvesting sensor node is to ensure that there is always enough energy available to power the system, as shown in Figure 2.
Figure 2. Typical Energy Harvesting System
This energy harvesting system uses a solar cell array to harvest energy. A solar cell unit, such as a Sanyo AM-1815, delivers approximately 40 µA when a 200 Lx light level is available. It is reasonable to expect this level of light in an office with a window but no direct sunlight on the cell. The 40 µA of current that the array generates is fed into a power management circuit and trickle-charged into a TFB. When selecting power management chips, it is necessary to pay attention to the leakage current characteristics, which are normally only a few microamperes. However, with only 40 µA coming into the TFB, this tiny amount of leakage must be understood and accounted for. A thin-film battery, such as the Infinite Power Solutions MEC101-7SES, provides a 0.7 mAh capacity, which is a reasonable amount of energy for a wireless sensor node system. At a 200 Lx level of light, this TFB charges up fully in around 17.5 hours.
This combination of solar cell, power management and storage technologies provides an adequate level of energy for a wireless sensor node. The next important decision in the design process is the selection of a low-power MCU and wireless transceiver combination that can operate effectively from a limited energy source. A wireless MCU with extremely low power consumption and high-performance radio characteristics is an ideal choice. The Silicon Labs Si1012 wireless MCU, for instance, uses a programmable sub-GHz ISM radio in a single-chip configuration with an ultra-low-power MCU. This device, which also includes an on-chip temperature sensor, is essentially a wireless sensor node on a chip.
With the hardware configured as shown in Figure 2, the control problem to be considered is how to operate the wireless sensor node at a duty ratio that does not deplete the TFB capacity that is itself being trickle-charged by the solar cell. Using the low-power design techniques discussed earlier, it is possible to reduce the average current of the wireless sensor node to around 51 µA (including power management leakage) while transmitting sensor data every second for three minutes. That is low enough to allow the system to operate and stay fully charged in minimal lighting conditions.
If the light input is reduced to 0 Lx, the wireless sensor node continues to operate and transmit for 64 hours before the TFB capacity is exhausted (assuming the three-minute transmit period is repeated every 20 minutes). A simple spreadsheet detailing expected input energy (i.e. how much light is available) versus output energy (how often the node is required to transmit) is the only tool that a designer needs to optimize the system. If more than adequate light is expected, this energy can be used to increase the range of the transmitter. This type of system allows a range up to 300 feet, depending on the exact conditions.
Many different types of energy harvesting sources can be used to power a wireless sensor node instead of using a solar cell (or even in combination with solar energy). If a wireless sensor node is placed in a location without ready access to a light source, the node can be powered by thermal, vibration (piezoelectric) or radio wave energy harvesting sources. The power management, storage and wireless sensor node circuits are essentially the same as those used in the solar cell example. Regardless of the harvested energy source, the system design principles are the same: a limited source of energy is captured and stored in a TFB and then used to power an ultra-low-power wireless sensor node.
The ability to power wireless sensor nodes from harvested energy sources allows embedded designers to offer systems with significantly reduced cost of ownership for the end-user as well as benefits to the environment. The cost of replacing batteries housed in out-of-the-way sensor node locations can be quite significant. These wireless sensor nodes, for example, can be embedded in structures, such as buildings or bridges, or even buried underground. The three key enabling technologies needed to create self-sustaining wireless sensor nodes are readily available today: cost-effective energy harvester devices, small and efficient energy storage devices and single-chip ultra-low-power wireless MCUs. Wireless sensor nodes powered by harvested energy sources will soon become commercially viable and commonplace technologies used in our homes, offices, factories and infrastructure.
About the Author
Ross Bannatyne serves as an MCU marketing manager for Silicon Laboratories’ Embedded Mixed-Signal division. Prior to joining Silicon Laboratories, Mr. Bannatyne spent 12 years at Motorola where he served in systems engineering and marketing positions. Most recently, he served as channel market segment manager for Motorola’s Semiconductor Products Sector (now Freescale Semiconductor) where he managed the Americas region 8- and 16-bit microcontroller business. Mr. Bannatyne is the author of numerous technical articles as well as two textbooks: Using Microprocessors and Microcomputers
(Prentice-Hall) and Electronic Control Systems
(SAE). Mr. Bannatyne holds a master’s degree in business administration from the University of Texas at Austin and a bachelor’s degree in electrical engineering with honors from the University of Edinburgh in Scotland.