Nancy Friedrich
Microwaves & RF
As wireless-sensing components boost efficiency and other performance criteria, energy-harvesting solutions are moving beyond a niche and into everyday applications.
Energy is all around us – whether the sources are solar, electromagnetic, piezo-electric, or thermal. By “harvesting” even a fraction of this energy, engineering firms can deploy a growing number of sensing technologies for the greater good. Such sensing applications include wearable medical-monitoring devices, aircraft monitors, automotive monitors, and remote monitors for gas and energy sources. Given the varied sources of energy, harvesting also makes improved medical care and other benefits available in locations with little infrastructure, such as rural or impoverished areas. To help energy harvesting increase its global footprint, it is being applied to a growing number of solutions ranging from integrated circuits (ICs) to active and passive components.
Examples can be seen in the components of the “Energy Harvesting Solution To Go” kit from Energy Micro, Linear Technology, and Würth Elektronik. The two basic parts of this kit are an energy-harvesting board and the Giant Gecko starter kit. Both elements contain passive components from Würth Elektronik. Würth’s WE-EHPI power inductors derive their efficiency from the low ohmic resistance of each winding as well as a core that was especially developed for a rugged environment. Efficient electromagnetic-interference (EMI) suppression is realized by adding surface-mount-device (SMD) ferrite beads at each plug contacts.
The multisource energy-harvesting board has four voltage converters from Linear Technology, which are optimized for the different energy sources. For instance, the LTC3588 is offered for alternating-current (AC) sources to 20 V, such as piezo-electric and inductive energy generators. The Giant Gecko Starter Kit contains the energy-friendly microcontroller, EFM32. (The EFM32GG990F1024 consumes only 200 μA/MHz in active mode.) It also includes an ARM Cortex M3 microcontroller unit (MCU) with 48-MHz speed, 1024-kByte Flash memory; 128-kByte random-access memory (RAM); Universal Serial Bus (USB), liquid-crystal-display (LCD) control, low-energy sense, and more.
When one looks at the different aspects comprising an energy-harvesting solution, it is clear that this kit offers a solid starting point. In the case of short-range wireless applications, for example, energy-harvested wireless sensor nodes use low-cost ICs to perform sensing, signal-processing, communication, and data-collection functions, notes Mauricio Peres, Director of Business Development at Microsemi’s Ultra Low Power Group. They also include a low-power wireless-communications interface.
Peres details the various aspects of most energy-harvesting sensors: A sensor detects and quantifies any number of environmental parameters required in the application, while an energy-harvesting transducer converts some form of ambient energy to electricity. In addition, a power-management module is needed to channel the energy, regulate voltage supply, and implement the energy-storage management required by the sensor node. An MCU manages the signal from the sensor and communicates with the radio link. Finally, a radio link with or without an RF wakeup receiver function is required at the sensor node (see figure).
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| Figure 1. | Shown is a basic energy-harvesting wireless sensor. The output of the application sensor is typically connected to a MCU, which processes the signal created from measuring the parameter of interest (i.e., temperature, pressure, acceleration, etc.). |
The key to these systems is not just the various parts and how they are put together, however. Energy-harvesting solutions place unique and stringent demands on all of their components – for example, by demanding very high power efficiency. Peres explains, “The microcontroller (MCU) and radio must operate in low-power modes whenever possible in order to maximize the power-source lifetime. In the last year or so, both RF IC and MCU manufacturers have invested and launched lower-power-consumption solutions that can be used in short-range, energy-harvested wireless sensor nodes. As an example, radios and MCUs with power supply as low as 1.8 V and low supply current can be attained today for ultra-low-power wireless sensor design.”
There also is a need for microprocessors that can quickly move from deep sleep to idle and active modes, notes Mark Grazier, Low Power RF Product Marketing Manager, Wireless Connectivity Solutions at Texas Instruments. This capability reduces current consumption between transmit and receive transitions and, ultimately, the power usage. Grazier states, “The key to a more efficient radio architecture is to lower the energy required to transmit and receive packets of information. Energy-harvesting systems also require lower packet-error-rate radios, which eliminate having to retransmit packets of information, thereby reducing the total daily amount of energy provided by the energy harvester.”
Issues also arise due to the makeup of most wireless-based sensor networks. For example, the majority of those networks rely on duty-cycling to conserve power and restrict the usage of radio space. This generates peaks in the sensor’s current-consumption profile. Low peak current consumption in the radio transceiver reduces constraints on the wireless sensor’s power supply.
Peres notes that these constraints are even more important for wireless sensors that run from harvested energy sources. “Often, energy-harvester transducers have higher output impedance than batteries,” he explains. “The micro-power management layer between the energy transducer and the sensor converts the supply characteristics – including source impedance. Therefore, the low peak-current consumption in the radio transceiver reduces constraints on the power supply of the wireless sensor.”
No matter their approach, the call for more efficiency is echoed by energy-harvesting systems of all types so that they can transmit data more often. Yet the extent to which each type of approach is actually harvesting energy varies greatly at this point. Solar harvesters are currently the most prevalent source of energy harvesting, as they operate between 25% to 50% efficiency per cm2 (see table). As they are more widely deployed, their cost will decrease per cm2.
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The state of RF energy harvesting
For their part, RF energy-harvesting solutions are currently more a darling of the labs than a commonly used solution in their own right. A typical RF energy-harvesting system also looks quite different from its counterparts. John Bazinet, Product Line Manager, Power Products at Linear Technologies, provides a picture of a typical solution by dividing it into two parts: the receiver [tuned antenna, rectifier, storage element (capacitor), DC-DC converter] and transmitter [directed RF energy (PowerCast, for example) or ambient RF (WiFi, cellular, radio)]. Typically, he notes, RF energy systems have four components including a tuned antenna, an input storage element, power-management circuitry, and an output storage element.
Like their counterparts, RF energy-harvesting systems are in need of a number of performance enhancements. Bazinet explains that RF energy-harvesting solutions require improvements to the following (many of which are already in the works): directed RF energy source (not ambient), higher efficiency, ultra-low quiescent current, wider-input-range DC-DC, low-power MCUs, and RF transceivers.
The microwave and RF industry in particular could better serve these systems by providing lower-power RF transceivers. Even if these demands are satisfied, however, Bazinet notes that “directed RF energy systems are very niche.” If they are using ambient RF, they have much less available energy compared to photovoltaic or thermal energy harvesters. RF energy harvesting also will have to overcome typical RF-centric issues, such as limited radio range due to building materials, locations, and more.
As RF energy harvesting finds its way, other energy-harvesting solutions are already broadening their reach. TI’s Grazier summarizes, “Solar harvesters, over time, will continue to improve their efficiency so that they can expand their usage for both outdoor and indoor applications, where available lighting sources are available. Thermal harvesting solutions are also finding their way into building applications, where they can maximize the temperature differential between the outdoor and indoor temperature on a window. Thermal harvesting is also being used on body-worn devices. Overall, energy harvesting has a promising future as more and more products move from R&D development into main stream products.”
New solutions are starting to appear more steadily, reinforcing this point. The AS3953 NFiC (Near Field Communications interface Chip) from ams AG provides a high-data-rate interface between an NFC device, such as a smartphone, and any host microcontroller with a standard Serial Peripheral Interface (SPI). Because it operates on energy harvested from an NFC reader’s RF emissions, the interface chip) requires no external power source and a maximum of one external capacitor. The AS3953 features a configurable wake-up interrupt, enabling a zero-power system design while in shut down. The device can draw up to 5 mA of harvested energy from the external magnetic field. With an internal power-management circuit, it also can supply harvested energy to the application.
Another recent debut promises to solve the longstanding indoor-location-detection problem that has plagued emergency responders. ROHM, in collaboration with Ritsumeikan University and Information Services International - Dentsu, Ltd. (ISID), announced Guidepost Cell. Using the low-power IEEE 802.11-compliant wireless beacon protocol, this indoor location-detection infrastructure supplies precise positional data to smartphones and other mobile devices. The solution is powered by dye-sensitized solar cells (DSCs) that harvest energy from indoor lights, eliminating the need for an external power supply while reducing installation and electricity costs. The DSCs promise to generate 48 μW/cm2 under 1000 lux.
These are just two examples of myriad opportunities. At this point, the potential for these solutions knows no bounds – as long as they can achieve their efficiency goals and other performance metrics. Going forward, such solutions will increasingly be miniaturized to allow their use in more individual wellness and health applications. Thus, while ICs and active and passive components raise their performance while reining in efficiency, engineers also will be taming the old demons of range, interference, and size.
