Energy Harvesting and Wireless Sensor Networks

Energy Harvesting Devices

Many kinds of harvesting devices become possible with Murata's technologies. Principles and characteristics of devices currently being developed are as below.

Converting a force into electricity with a piezoelectric material (Fig. 3)

When a force is applied to a piezoelectric material, electric energy is generated in proportion to the amount of distortion in the material. We may harvest this energy to operate equipment. Commonly, a thin layer of a piezoelectric material is pasted against a metal plate to allow for stress application. Such a device can be constructed relatively simply.

Fig. 3 Converting a force into electricity with a piezoelectric material

Fig. 3 Converting a force into electricity with a piezoelectric material

Converting vibration into electricity with a piezoelectric material

A resonator can be made in combination of a piezoelectric plate and a weight. Energy of a vibrating body transfers to a piezoelectric body when the oscillating frequency of a vibrating body and the resonating frequency of a piezoelectric body are synchronized. Combination of a piezoelectric oscillating plate and a weight may be designed to set this frequency to be anywhere in a wide frequency range between a few Hz to a few kHz.

Converting vibration into electricity with an electret material (Fig. 4)

An electret material is capable of storing a negative charge a long term. A positive charge is induced when bringing an electrode closer to the electret material, and the electrical charge escapes as the electrode moves away from the electret material. By alternating two motions, an alternating current may be generated. This device can be made low profile.

Fig. 4 Converting vibration into electricity with an electret material

Fig. 4 Converting vibration into electricity with an electret material

Converting temperature difference into electricity with a thermoelectric element (Fig. 5)

When temperature difference occurs between semiconductors, density difference in holes (or electrons) occurs from the Seebeck effect. Thus, electricity may be generated when a P-type semiconductor is connected to an N-type semiconductor.

However, since voltage generated from one pair of semiconductors is impractically low, a few dozen pairs are normally serially connected for generation. Murata developed an element, constructed similarly to a monolithic capacitor, having 50 serially connected P-N pairs.

Fig. 5 Converting temperature difference into electricity with a thermoelectric element

Fig. 5 Converting temperature difference into electricity with a thermoelectric element

Converting light into electricity with dye (Fig. 6)

With this technology, electric power is generated by the oxidation-reduction reaction of the dye. Specifically, when exposed to light, the dye adhering to a porous oxide semiconductor film enters an excited state emitting electrons. Electrons emitted flow towards a positive electrode, and return to the dye via an electrolyte.

Electric power generation results from this cycle. TiO2 normally used to form a porous semiconductor film, requires high-temperature sintering. Murata developed a photocell device with a thin, light and durable resin substrate, by replacing TiO2 with low-temperature sintered ZnO to form the porous film.

Fig. 6 Converting light into electricity with dye

Fig. 6 Converting light into electricity with dye

Sensor Network System

A node for a sensor network system consists of a sensor, a microprocessor and an RF module (Fig. 7) . Since energy gained from energy harvesting is small, energy utilization of the loading side must be very effective, in other words the system must support few features while being operable from minute energy.

Looking into sensors and microcontrollers operating with small power, we determined 100 µW to be the target value. Murata, then introduced an EnOcean® module to develop a wireless sensor node (Fig. 8) to utilize the energy harvested.

Fig.7 Sensor Network System

Fig.7 Sensor Network System

One obstacle previous wireless sensor network systems faced was with the exchanging of batteries. While utilization of energy harvesting may limit the available amount of energy thereby restricting the number of features supported, doing so may solve battery management problems.

Specification overview

Chipset EnOcean®E3000I (Dolphin)
Size: 13.0 x 8.0 x 2.1mm Max
Frequency: 315.0 or 868.3MHz
Modulation: ASK
Data rate: 125Kbps Max
Transmitting output: -2 to +6 dBm
Receiving sensitivity: -98dBm (315MHz)
-96dBm (868MHz)
Built-in CPU: 16MHz 8051 CPU (32KB Flash/2KB SRAM)
Compliant I/F: UART/SPI
Analog (10bit ADC)
Protocol: EnOcean specification
Fig. 8 Wireless sensor network systems

Fig. 8 Wireless sensor network systems

Final Remarks

We displayed energy harvesting demonstration units at CEATEC JAPAN 2011 (Fig. 9) . We are barely past the stage of matching an energy harvesting device with a sensor network, with several more hurdles left to clear. We will continue with our development efforts to complete a practical system as soon as possible.

Fig. 9 Energy harvesting demonstration units

Fig. 9 Energy harvesting demonstration units

Application Note