Energy harvesting is a key technology for enabling long-term, maintenance-free operation of low-power electronic devices, such as wireless sensors. By capturing waste energy from the environment, from such sources as lighting, temperature differentials, vibrations, and radio waves (RF energy), it can be reused to operate low-power electronic devices. Of these micro-power energy sources, energy from RF transmitters can provide unique benefits including predictable and consistent power over distance, and enable the energy harvester to be untethered from the power source.

Ambient RF energy is currently available from billions of radio transmitters around the world, including mobile telephones, handheld radios, mobile base stations, and television/ radio broadcast stations. The ability to capture some of this energy will enable the creation of new, batteryfree devices and allow battery-powered devices to be wirelessly tricklecharged. An alternative to ambient RF energy is to use dedicated transmitters to send power, which enables wireless power systems to be engineered for higher performance. This solution will be preferred in many applications, but it has the result of added cost. Government regulations limit the output power of radios using unlicensed frequency bands to 4 W effective isotropic radiated power (EIRP), as in the case of radio-frequency- identification (RFID) interrogators. As a comparison, earlier generations of mobile phones based on analog technology had maximum transmission power of 3.6 W, and Powercast's new TX91501 transmitter is 3 W.

An obvious appeal of ambient RF energy harvesting is that it is essentially "free" energy. Devices with this capability can potentially have mobility while charging, however, many ambient RF energy harvesting implementations will require a directional antenna pointed toward a known source, such as a mobile base station. A long-term vision for the mobile phone industry is to be able to harvest enough ambient RF energy to match the standby power of a mobile handset. If this were possible, then mobile phones could have continuous standby capability instead of only several days. While this particular application is not practical at present, several system-level factors are converging to enable ambient RF energy harvesting for other applications. These include the increased availability of low-power components, increased transmitter devices as sources of energy, improved RF sensitivity for passive RF harvesters, and increased availability of low-equivalent-series-resistance (ESR) double-layer capacitors, also known as supercapacitors.

Manufacturers of low-power electronic components, such as microcontrollers, are in a literal "race to the bottom" in terms of providing high performance at decreasing power consumption. Datasheets and other opmarketing efforts from these companies proudly claim sleep currents in the low nanoampere range, as well as on-chip DC/DC converters that can boost power from batteries at voltages of less than 1 V. Other components, such as sensing elements, are increasingly being designed as passive devices that can lower the overall system power consumption. This is especially important for battery-free devices. With sufficient real-time energy harvesting, battery-free devices can run continuously, but if the energy is too low it must be stored until there are enough joules for a cycle of operation. As component power levels decrease, systems powered by energyharvesting techniques can operate more frequently.

The number of radio transmitters, especially for mobile base stations and handsets, continues to increase. ABI Research and iSupply estimate the number of mobile phone subscriptions has recently surpassed 5 billion, and the ITU estimates there are over 1 billion subscriptions for mobile broadband. Also, consider the number of WiFi routers and wireless end devices such as laptops. In some urban environments, it is possible to literally detect hundreds of WiFi access points. At short range, such as within the same room, it is possible to harvest a tiny amount of energy from a typical WiFi router transmitting at a power level of 50 to 100 mW. For longer-range operation, larger antennas with higher gain are needed for practical harvesting of RF energy from mobile base stations and broadcast radio towers. In 2005, Powercast demonstrated ambient RF energy harvesting at 1.5 miles (~2.4 km) from a small, 5-kW AM radio station.

Passive RF receivers, or RF energyharvesting devices such as the P2110 Powerharvester receiver from Powercast, operate with RF input of -11 dBm or higher. Improving the RF sensitivity allows for RF-to-DC power conversion at greater distances from an RF energy source, but as the range increases the available power decreases and the charge time increases. Low-leakage energy storage is essential, especially at very low input power, to minimize the harvested energy that is lost and to make the energy harvesting process as efficient as possible.

An important performance aspect of an RF energy harvester is the ability to work over a wide range of operating conditions, including variations of input power and output load resistance. For example, the Powercast RF energy-harvesting components do not require additional energy-consuming circuitry for maximum power point tracking (MPPT) as is required with other energy-harvesting technologies such as the conversion of solar energy into electrical energy. Powercast's components maintain high RF-to-DC conversion efficiency over a wide operating range that enables scalability across applications and OEM devices. RF energy-harvesting circuits that can accommodate multi-band or wideband frequency ranges, and automatic frequency tuning, will further increase the power output, potentially expand mobility options, and simplify installation. Powercast's components are designed with a standard 50-O input impedance to reduce design time and accommodate the use of off-the-shelf antennas.

Figure 1 shows the performance of the Powercast P2110 Powerharvester receiver at several frequencies, including the Industrial-Scientific-Medical (ISM) frequency band with center frequency of 915 MHz.

There are several options to store the energy captured from energy harvesting, including traditional rechargeable batteries, emerging thinfilm batteries, and capacitors. Many advances have been made in lithiumion (Li-ion), nickel-metal-hydride (NiMH), and thin-film batteries in the last 20 years. Energy densities have increased, package sizes have become smaller, and these products have been successfully shown to sustain micropower sensor applications for long periods of time. A downside is that, like disposable batteries, rechargeable batteries have limited life cycles and will eventually have to be replaced. This is why energy harvesting and alternative energy storage options, such as supercapacitors, are being considered and investigated across many industries.

Traditional supercapacitors or electrochemical double-layer capacitors (EDLCs) as they are known, feature ESR values to hundreds of Ohms at either 2.5 or 5 V, and have been used for energy-backup applications for more than 30 years. Applications include backup power for clocks in consumer applications like videocassette recorders (VCRs), radios, and other electronic systems. These clocks use less than 10 μA current at low voltages or are used in real-time-clock (RTC) applications in many electronic circuits. These low-power applications found EDLC devices to be an excellent compromise between batteries, which had to be constantly replaced, and electrostatic/electrolytic capacitors, which did not provide enough charge storage in practical sized packages like "button" cells.

Low-ESR EDLCs have been developed in response to customer requirements over the last 10 years for capacitors capable of providing currents to several amperes at voltages approaching 5 V for high-pulse-power applications. The result is low-profile, low- ESR (20 to 50 mO), high-capacitance (6.8 mF to 1 F) EDLCs with voltage ratings of 2.5 to 20.0 V. These components provide high current pulses of several amperes required in, for example, wireless barcode scanners, smart metering systems, and for many types of GSM/GPRS cellular applications. These low-ESR devices are now being designed in new applications like micropower energyharvesting systems because they offer two unique characteristics: low leakage current and low ESR. They are now preferred over other capacitors or over other small batteries that have been tested for these and other similar applications. BestCap components from AVX Corp., for example, deliver the low ESRs, low leakage currents, and high current pulses suited for ambient energy harvesting. They feature low ESR values along with low leakage currents of less than a few microamperes.

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Figure 2 is a schematic diagram showing a cross-sectional view of an EDLC. It shows two active nanoparticle carbon layers surrounded by an electrolyte with a "separator" layer between. These carbon layers are in contact with current collectors that carry the current to the outside world. The two carbon layers consist of two capacitors in series, hence the name Double Layer Capacitor or DLC. Since the charge carriers within the capacitor are ionic in nature, the term electrochemical DLC (or EDLC) is used. This diagram shows a simple schematic where the primary concentration of charges is at the current collector-carbon interface. The capacitance (C) is directly proportional to the active area (A) and inversely proportional to the separation distance (d) between these charges (or C a A/d). The separation between opposing charges for a double-layer capacitor is in the nanometer range, and this is why the capacitance in EDLCs is so large (because this separation is several orders of magnitude smaller compared to a separation between charges in an electrostatic capacitor).

BestCap devices, which are based on an aqueous electrolyte, utilize protons, which are the smallest ionic species, as charge carriers. This capacitor design approach results in significantly lower ESR per unit of active area compared to other supercapacitor technologies where larger ionic species may be used. The Best- Cap architecture also features lower leakage current due to its design, and these devices have enhanced reliability. This also offers the potential to build a variety of capacitors within the same package, and the result is the flexibility to have a variety of voltage ratings for capacitors in one package size. No external balancing is required within this package

Ambient radio waves are present in large numbers over an ever-increasing range of frequencies and power levels, especially in highly populated urban areas. These radio waves represent a unique and widely available source of micropower if this free-flowing RF energy can be effectively and efficiently harvested. The growing number of wireless transmitters is naturally resulting in increased RF power density and availability. Dedicated power transmitters further enable engineered and predictable wireless power solutions. With continued decreases in the power consumption of electronic components, increased sensitivity of passive RF receivers, and improved performance of low ESR double-layer supercapacitors, the practical applications for wire-free charging by means of RF energy harvesting will continue to grow.