Getting Ready for Harvest Season

06.22.2010 // Posted by: Murray Slovick // Posted in: Articles, New Technology

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Energy harvesting, that is, which is opening the door for applications not feasible with traditional battery powered systems

Capacitors are so prevalent in circuit design that any new application runs the immediate risk of a “been here, done that,” reaction from engineers.

Lately, however, the folks on the “green” side of technology development have been spending a lot of time selecting caps (and supercapacitors) for an innovative app called energy harvesting that is proving to be an interesting project for even the most time-tested component specifiers.

The objective of an energy-harvesting system –consisting of an energy generator, energy storage and power management electronics-- is to scavenge electrical energy from the environment around us, providing anywhere from tens of microwatts up to some milliwatts of energy, which is sufficient to meet the energy needs of wireless sensor systems for everything from medical applications to industrial monitoring networks.

Because of variations in its power and voltage over time the output of an energy harvester is not yet directly suited as a power supply for conventional ICs (even though the threshold voltages of chips has increasingly scaled down), so a power management circuit is required that can handle very low feeding power and be able to adapt the electrical energy obtained to the requirements of the load.

This usually means matching voltage levels, regulating the supply voltage and minimizing the power consumption of the application device (all without the power management IC itself being a significant source of power consumption).

Energy Harvesting Sources

An energy harvesting circuit can collect intermittent or continuous energy input from a variety of sources:

• Mechanical energy resulting from vibration, stress and strain. When a piezoelectric transducer is stressed mechanically by a force, its electrodes receive a charge that may be collected, stored and delivered to power circuits or processors. MEMs vibrational energy harvesters operating in the frequency domain between 150 and 1000Hz are particularly well suited to convert vibrations from machines, engines and other industrial appliances into electricity. In the lab IMEC (Leuven, Belgium) has developed piezoelectric energy harvesters capable of generating up to 85μW.
• 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. In cities and very populated areas there is a large number of potential RF sources including broadcast radio and television, mobile telephony, wireless networks, etc. At 100 m from the typical 100W cellular base station, for example, there is about 800μW /m2 of energy available. Your GSM handset is good for about 0.1 μW/cm2
• Solar energy from light (using photovoltaic devices) once the exclusive domain of space science, has now come down in price to where solar powered watches and phones are available. Approximate available power: indoor light 10 μW or less, outdoors 0.10-15mW/cm2
• Temperature differentials (via thermoelectric generators or thermopiles attached to a heat generating source, such as an HVAC duct, furnaces, combustion engines etc.) Heat from industrial equipment has potential electrical power on the order of 1–10 mW/cm2; the human body can generate 15-30 μW/cm2

Energy Storage

The intermittent nature of environment-based energy sources means that: 1) there are likely to be extended time intervals before sufficient energy has been captured; and 2) as a result energy must be stored/buffered via an on-board battery, a capacitor bank, a supercapacitor or a hybrid system such as a MEMS capacitor connected directly to a rechargeable battery driving charge and energy into the battery when needed.

Examining storage options, the supercapacitor’s high power and fast recharge characteristics make it a good choice to buffer a high-power load from a low-power, energy-harvesting source.

It helps that while batteries store large amounts of energy they cannot provide high power or fast recharge. When large power bursts are required, e. g., upon application start-up, if there is an insufficient energy buffer the result can be excessively long start-up times.

Furthermore, since rechargeable batteries use an electro-chemical process, the number of times they can be recharged is limited while supercapacitors can be recharged in seconds and offer hundreds of thousands of charge/discharge cycles before needing to be replaced.

Two Typical Solutions

Energy harvesting solutions are advancing at a fast pace. Two commercial RF energy harvesting platforms are on the market from Powercast: the P1000 series for battery charging, and the P2000 series for capacitor charging. The P2000 series has a 3.3 V nominal output and can directly power a number of board-level components.

The company’s Powercast P1110 and P2110 receivers are capable of converting radio waves in the range of 850-950 MHz into DC power. At the recent Embedded Systems Conference Chicago (co-located this year with Sensors Expo) Powercast noted that at its facility outside Pittsburgh ambient RF energy is consistently in the – 25 to -20 dBm range (-20 dBm is 0.01 mW) thanks to a cell tower located 900 feet away.

Powercast demonstrated the P2110 at the exhibition storing received energy into a capacitor, and then performing a voltage boost to supply wireless module components with a regulated voltage. The P2110 extracted -11 dBm (approximately 0.1 mW) of power and provided up to 5.25 V (regulated).

Turning to vibrational energy harvesters Linear Technology has introduced two chips, the LTC3108 step-up converter/power manager and the LTC3588-1 energy harvesting power IC. The latter part is designed to interface directly to a piezoelectric power source, rectify a voltage waveform, store harvested energy on an external capacitor then bleed off any excess power via an internal shunt regulator, and maintain a regulated output voltage by means of a synchronous buck regulator.

After the output voltage is brought into regulation any excess energy is stored on the input capacitor; when a load exists at the output the buck can efficiently transfer energy stored at a high voltage to the regulated output.

Harvested energy can be stored on the input capacitor or the output capacitor. The input capacitor should be sized to store enough energy to provide output power for the length of time required. This may involve using a large capacitor. A standard surface mount ceramic capacitor can be used for COUT, though some applications may be better suited to a low leakage aluminum electrolytic capacitor or a supercapacitor.

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