Energy Harvesting and Wireless Sensor Networks
Although the term "energy harvesting" has been gaining popularity lately, the amount of energy harvestable from our immediate surroundings is not large. In fact, most energy harvesting experiments are dealing with less than 1mW of energy. While few applications benefit from such small amounts of energy, we are beginning to find out that energy harvesting can compensate for the weakness in wireless sensor networks (WSN) when used as their energy source.
This article will introduce Murata's energy harvesting-related efforts.
What is Energy Harvesting?
Energy harvesting is a series of processes including collecting energy around us, converting it to electricity and operating small equipment with it. We all know that we can generate a lot of electric power if we use fuel, but that is not called energy harvesting. Energy harvesting does not include electric energy generated from conscious human effort either. The key idea here is being able to generate electricity "unconsciously."
Careless harvesting may end up overburdening an energy source by trying to get as much energy as possible. With all honesty, it is impossible to generate a large amount of energy through energy harvesting anyway. Such attempts would result in creating devices unrealistically large or costly, defeating the purpose.
Therefore, energy harvesting only makes sense when it is done through the effective and thrifty utilization of small amounts of harvested energy.
Energy Around Us
Figure 2 lists kinds of energy we can find around us. The unit for energy is joule (J) and 1J is 1 watt second. This list reveals that the level of harvestable energy around us is very small. In comparison, the level of energy used by equipment around us is quite high. We often receive inquiries about the plausibility of charging mobile phones with energy harvesting.
It should be evident from the right column of Fig. 2 that it will be very difficult. We are also often asked if it is possible for us humans to generate enough energy to supply household energy. Judging for the calories we consume from food, it is easy to surmise that the energy we can possibly generate is also small. Furthermore, three quarters of the calories are used by our basal metabolism. The amount of energy a person may generate per day, without conscious effort, is less than 1mJ. The key mission of energy harvesting is the effective utilization of such small amounts of energy.
Fig. 2 Energy comparison
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
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
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
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
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
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.
||13.0 x 8.0 x 2.1mm Max
||315.0 or 868.3MHz
||-2 to +6 dBm
||16MHz 8051 CPU (32KB Flash/2KB SRAM)
Analog (10bit ADC)
Fig. 8 Wireless sensor network systems
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