Much has been written about the benefits of wireless sensors and the potential of energy harvesting to provide power for the life of these devices. Disposable, long-life batteries will continue to be used in wireless sensor applications but, as the technologies mature, energy harvesting will create some shift in battery usage from primary to rechargeable batteries for applications that need higher power over the life of the device. The greatest potential, however, lies in a new class of devices that will be battery-free and thus enable applications that would have been prohibitively expensive due to the maintenance cost of eventual and repeated battery replacement.
The energy harvesting industry is developing technologies to take advantage of varied sources of micropower (power measured in milliwatts) including solar, vibration, thermal, and RF energy. For any specific installation, there will likely be a clear choice of the optimal energy harvesting technology to be used, but—depending on the application—all are capable of providing the micropower needed for wireless sensor applications.
Energy from Radio Waves
RF energy harvesting converts radio waves into DC power. This is accomplished by receiving radio waves with an antenna, converting the signal, and conditioning the output power, as shown in Figure 1.
 Figure 1. Overview of an RF energy harvesting system |
There are multiple approaches to converting an RF signal to DC power, (e.g., single-stage vs. multistage) depending on the desired operating parameter, such as power, efficiency, or voltage. The amount of power available for the end device depends on several factors including the source power, distance from the source, antenna gain, and conversion efficiency. For Powercast's components, the conversion efficiency of the received RF power to DC is typically between 50%–75%, over a 100X range of input power or load resistance, and even greater for specialized applications. The harvesting activation power is currently ~100 µW and the output power is up to 250 mW.
The sources for RF energy harvesting can be grouped into three general categories: intentional sources, anticipated ambient sources, and unknown ambient sources.
Intentional sources (e.g., dedicated power transmitters) provide the most control because the availability and amount of power can be controlled and engineered for the application. While specific applications may require higher power transmitters, intentional sources will generally be 4 W or less and comparable to the power of widely deployed RFID readers. Intentional sources can be deployed in a network similar to WiFi routers or mobile base stations where multiple transmitters provide coverage over a wide area. They can be operated as required for the application, such as keeping energy storage devices fully charged or providing power for device activation, and can provide power continuously, on a scheduled basis, or on demand.
Anticipated ambient sources are sources where, although there is no control, they can be relied on to act as sources of power on a regular or intermittent basis. An example of this is the concentration of mobile phones (i.e. people) expected at a given location such as bus stops or crowded sidewalks. There are estimated to be 3.5 billion GSM subscriptions globally and predicted to grow to 4 billion by 2012. Depending on transmit power, multiple phones in close proximity can provide several milliwatts of power. Additional examples of these sources include known radio, television, and mobile base station transmitters.
Unknown ambient sources are sources of RF energy of which there is no control and no knowledge (e.g., microwave radio links and mobile radios such as those used by police forces), but which still provide a continual or intermittent source of power.
Usable power from RF energy harvesting will typically be in the milliwatt and microwatt range based on the power limits from commercially available transmitters or the distance from sources such as radio and TV transmitters. The usable power or range can be greater for specialized military or industrial applications that use higher levels of transmission power. Comparisons are often made regarding the power density (i.e., W/cm3) of various energy harvesting technologies. While power density is a valid metric of comparison, it is also incomplete because each type of energy harvesting presents unique benefits. In the case of RF energy harvesting, for example, these are controllable and ambient power over distance, one-to-many wireless power distribution, mobility, embedded harvesting technology, and independence of weather conditions or time of day.
Harvester Requirements
RF energy harvesters (such as those in the sidebar, "RF Energy Harvesting Modules") can be simple or complex, depending on the performance and functionality required. A simple harvester, for example, may provide basic signal rectification and require external power management circuitry. A more complex harvester may combine the power management and other functionality within a single component. For maximum performance, design flexibility, and application flexibility, there are several important characteristics that a commercial RF energy harvester should provide. The harvester should have high sensitivity to enable it to harvest from ultralow levels of RF energy. It should have high efficiency to convert as much of that energy as possible into usable power. The efficiency range should be sufficiently broad to support a wide range of operating conditions such as input power, load resistance, and output voltage. The harvester should have intelligent power management capabilities that can be controlled or used by a microcontroller to optimize system-level power. And lastly, it should be easy to implement, such as having an input impedance of 50 ohms to be compatible with a wide selection of commercially available antennas, and packaged to participate in standard PCB manufacturing processes.
Implementation Options
Like other forms of energy harvesting, there are multiple ways to use RF energy harvesting in implementing a power system, including:
- Direct power (no energy storage)
- Battery-free energy storage (supercapacitor)
- Battery-recharging
- Remote power with battery backup
- Passive wireless switch (battery activation)
These implementation options provide significant flexibility in designing power systems for wireless sensors. RF energy harvesting can provide a device with the ability to receive power or replace energy when needed, or to trigger the activation of remote sensors that are completely dormant.
Applications
RF energy harvesting can be used for a number of wireless sensor applications, deployed both indoors and outdoors. Applications include ground-level agricultural sensors, HVAC and energy management, automated gauge and meter reading, structural health monitoring, location tracking, distributed pollution sensors, and rotational equipment sensors.
RF energy harvesting has great potential to power systems designed for indoor usage such as temperature, motion, and light sensors. Building interiors can often have low-light conditions that make solar energy harvesting methods unreliable or have no-light areas when sensors are located inside walls or above ceilings. Suitable thermal gradients are not likely to be available indoors, and vibrations are hopefully at a minimum. Thousands of watts of energy for HVAC and lighting can be controlled by using a few watts to operate one or more power transmitters, making the potential energy ROI quite significant. By eliminating the labor and cost of battery replacement, the financial ROI for this particular application is also very attractive.
Future Vision
Today, the most practical implementations of RF energy harvesting will require intentional sources to provide the energy. Wirelessly networking the transmitting sources to control how they operate can maximize the overall performance of a wireless power distribution system. Turning the power sources into access points completes the functionality to create a complete infrastructure for wireless power and data while also eliminating the need for battery replacement.
Ambient RF power levels will increase as more transmitting devices are put into use. A more significant factor in enabling pure ambient RF energy harvesting will be the introduction of devices that operate at lower and lower power levels. As device power consumption decreases, ambient RF energy harvesting will become more practical and available in more areas. The development of efficient multiband or wideband RF energy harvesters will also play an important role in the realization of widespread ambient harvesting over the next several years.
RF energy harvesting is a unique technology that can enable controllable, wireless power over distance, and scale to provide power to thousands of wireless sensors. Devices built with this wireless power technology can be sealed, embedded within structures, or made mobile, and battery replacement can be eliminated. With commercial RF energy harvesting components currently available, engineers can integrate this technology to provide embedded wireless power for their low-power wireless devices.
RF Energy Harvesting Modules
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