Energy Harvesting Wireless Lighting Control Systems

Frank Schmidt, EnOcean GmbH, Oberhaching, Germany

Energy can be found everywhere — in the movement of doors and windows or machine components, the vibration of motors, changing temperature or variances in luminance level. These energy sources, which usually remain unused, can be tapped by means of energy harvesting to power electronic devices and transmit wireless signals. This principle is the basis of energy harvesting wireless technology. In the course of ten years, the technology has opened up three different sources of energy to power wireless modules: motion, light and temperature differences.

But it’s not only the energy harvesting that makes the wireless technology work. The two additional keys are ultra-low power electronics and reliable wireless communications. Depending on the energy requirements, this complete package enables several wireless applications, which work without cables and batteries.

Mechanical Energy Converter

A core element of energy harvesting wireless is a mechanical energy converter called ECO 200 that converts mechanical energy into electrical energy, which it makes immediately available.

A magnetic flux is passed through two magnetically conductive laminations by a small but very strong magnet, enclosed in a U-shaped core. An induction coil is wrapped around this core. The magnetic parts are held in position by a plastic frame and a spring-loaded clamp. The U-shaped core leading through the coil is movable — it can take up two positions, each of which touches the opposite magnetic poles — and in each end position the magnetic flux is reversed in the U-core. This design ensures maximum magnetic flux alteration through the coil with minimal movement of the core — and therefore high efficiency — and wastes the smallest amount of energy possible. Moreover, the energy output does not depend on the speed of actuation. A mechanical energy store in the form of a leaf spring takes care of this. It forms the interface to the actuation of the energy converter. As the leaf spring is bent more, it stores mechanical energy until the magnetic forces can no longer hold the U-core in its position. If the spring forces exceed the holding power of approximately 3.5 newtons (N), the core flips quickly into its second position, accelerated by the spring. This generates a voltage pulse in the induction coil.

With an energy output of 120 μWs, a stabilized voltage of 2V and a wireless batteryless module, it is possible to transmit three radio telegrams per operation. This energy conversion can be used for functions such as door and blind control or light control and dimming.

The maximum allowed contact travel of 0.04 inches enables a typical completion of more than 300,000 switching cycles. With shorter contact travel (the converter switches after 0.03 inches of spring deviation, at the latest) significantly more than a million switching cycles are possible. This means a switch can be operated 100 times a day for more than 25 years.

Mini Solar Cell

Light is one of the most popular sources of renewable energy. Due to miniaturized solar modules no larger than 13mm × 35mm and covering eight cells, indoor light can also be used to supply electricity for ultra-low power wireless radio modules. An example for this is the solar-powered STM 330 sensor module, which can measure temperature in a room, for example. At 200 lux the solar cells generate an operating voltage of 3V. With 3.6 hours of charging in daytime, this provides the STM 330 with enough power to transmit a measured value every 15 minutes in an uninterrupted operation mode.

An additional charge capacitor can ensure an adequate power reserve to bridge intervals when little or no light energy can be harvested. In complete darkness the fully charged energy storage ensures reliable operation for four days in complete darkness. In addition, the radio modules save energy by executing all operations of sensors and actuators very rapidly, and promptly turning off when they are not needed. For this purpose the sensor modules incorporate special timers that only draw about 100 nanoampere of current, fully deactivating all other components during sleep phases and waking them again when they are required to operate.

The module provides a user configurable cyclic wake up. After wake up the external sensors are supplied with energy and after a configurable delay (default 2 ms) the internal microcontroller reads the status of the connected sensors. A radio telegram will be transmitted in case of a change of any digital input value compared to the last sending or in case of a significant change of measured analogue values. In the case of no relevant input change, a redundant retransmission signal is sent after a user configurable number of wake-ups to announce all current values. With default settings the module wakes up every 100 seconds and transmits a life signal every 15 minutes. In addition to the cyclic wake up, a wake up can be triggered externally using a wake input or the internal learn (LRN) button.

Thermo Energy Generation

Temperature differences contain a lot of energy and therefore are ideally suited as a third source. Just the cooling of a drop of water by 1 degree Celsius releases energy for about 20,000 EnOcean wireless telegrams. That is enough to operate not just a wireless sensor but even a number of wireless actuators. The energy is delivered by thermo generators.

But such low-cost Peltier elements have a pronounced drawback, namely that they only produce very small voltages of about 10 mV per degree Kelvin. Electronic circuitry connected to this, a sensor module for example, needs a typical supply voltage of 3V. The ECT 310 DC/DC converter closes this gap. This optimized oscillator already starts to resonate upwards of 10mV input voltage. On 20 mV or more (i.e. about 2°C) a useful output voltage of more than 3V is generated. To enable this exceptionally high converter efficiency of 30%, the output voltage is only roughly regulated to less than 5V over the entire input voltage range up to 500 mV. This is similar to the unregulated supply voltage from solar cells.

A central component of the ECT 310 is the coil, a transformer with high gain of 1:100. The dimensions of the ECT module in surface-mount technology are 16mm × 16mm × 5 mm.

Even when using a heatsink, approximately 100 μW of energy is already produced for a temperature difference of only 7°K. A typical EnOcean wireless module that wakes every two minutes to send a telegram needs about 5 μW. The remaining 95 μW is enough to power a number of actuators, to drive heating valve actuators, air flaps or other mechanical devices.

EnOcean Radio

Besides efficient energy converters and ultra-low power wireless modules, the radio technology plays a special role. The principles of EnOcean’s radio is enshrined in the new standard, ISO/IEC 14543-3-10, which provides a “Wireless Short-Packet (WSP) protocol optimized for energy harvesting - Architecture and lower layer protocols.” It is the only standard specifically designed to keep the energy consumption of sensors and switches extremely low, including energy harvesting wireless solutions. The standard offers a physical and data link layer as well as the network layer. The EnOcean Alliance creates the application profiles that sit on top of ISO/IEC 14543-3-10 and are defined in order to achieve interoperability between products from different vendors. These application level protocols are referred to as EEPs (EnOcean Equipment Profiles). The standard can be downloaded from www.iso.org .

Telegrams are just 0.7 milliseconds in duration and are transmitted at a data rate of 125 kilobits per second. Although transmitted power is up to 10 milliwatts, the wireless transmission used here only has an energy requirement of 50 microwatt seconds for a single telegram. That is about the same as the power needed to lift a weight of 1 gram by 5 millimeters. The short telegram is randomly repeated twice in the space of about 40 milliseconds to prevent transmission errors. Transmitting data packets in random intervals makes the probability of collision extremely small.

Installation and parallel operation of a large number of wireless switches and sensors in restricted space consequently poses no problem. Each module comes with a unique 32-bit identification number to exclude any possibility of overlap with other wireless sensors. For additional security requirements, such as for access control systems and other critical applications, data encryption for secure wireless communication can be included as well. The telegram’s range covers a distance of up to 900 feet in open spaces or 90 feet inside buildings.

To be suitable for worldwide use, the EnOcean modules communicate in the frequency bands 868 MHz (Europe), 315 MHz (North America and international) and 902 MHz (North America). The sub 1 GHz frequency is less crowded than higher bands, such as 2.4 GHz for example. Therefore it provides a safeguard against other wireless transmitters, while offering fast system response and elimination of data collisions. This makes the sub 1 GHZ bands much more suitable for reliable automation. In addition, the radio waves have twice the range of, e.g. 2.4 GHz signals, and double the penetration through materials such as walls and furniture.

System Intelligence

Products and systems enabled by the energy-autonomous wireless technology can be integrated into all common building automation systems, regardless of whether they communicate over Ethernet/IP, KNX, BACnet or LON. So, energy harvesting wireless systems are found in all kinds of buildings, in industrial plant and other sectors such as smart home systems, ambient assisted living or M2M applications.

Because the technology eliminates the need to pull wiring and to change batteries, the maintenance requirements of the devices are extremely low. Due to its special features, energy harvesting wireless technology has the ideal characteristics to connect the final communication level in automation applications, bringing the intelligence to each single device in a system. In the future, this will pave the way to the “Internet of Things (IoT)”, where each end node, that is, each device, should have one (at least virtual) IP address in the network.

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