By Frank Schmidt, CTO and Co-Founder, EnOcean
Harvesting energy from the environment is not new: Wind and water have served as energy sources for hundreds of years. Yet a wireless device capable of operating from ambient energy sources has only recently been achievable. Such a device is ideal for use in the sensor and control market. Principal energy sources are kinetic energy, light, and heat. Harvesting these sources is critical for achieving adequate energy to maintain a practical communications range for a wireless device.
Of course, power consumption may not be constant, and some deviceslike kinetic harvesting switchesdo not operate over long periods. Fast, reliable transmission protocols are therefore vital. Although energy may be freely available, extracting it from the environment is not trivial. Incredibly efficient micro-energy converters are needed to supply power to wireless sensors, switches, and controls. They are required to take advantage of energy harvested in the following ways:
Motion
Linear motion is the most obvious solution for any switching device. But finding suitable and sustainable methods to extract energy is not simple. Piezoelectric and inductive generators are the most common approaches. Piezoelectric devices are small, but also can be inefficient and mechanically unstable.
Inductive solutions are larger, but lower in cost and more efficient. With the latter, there is little mechanical movement and only a few Newtons in the switching action. As a result, sophisticated techniques are required to generate enough energy to power a wireless switch. It is possible to design a product to transmit three data packets or telegrams per button push. Such a product will allow on/off and dimming commands to be transmitted.
Light
Indoor solar cells are an ideal energy source for most areas. A small, eight-cell design can deliver 11 to 14 A at 3 to 4 V. Light sources can be variable in most buildings and absent at night. As a result, suitable energy management and storage schemes need to be employed to avoid shutting down devices in the absence of light sources.
Rechargeable nickel-cadmium or lithium batteries are low in cost. Yet their lifespan can be short, depending on energy usage. They also require complex charging circuits. If a maintenance-free system is required for decades, a better approach is to use a polyacenic-semiconductor (PAS) capacitor. A PAS is smaller than a double-layer capacitor. It has a high capacity per square millimeter with low self-discharge rates. The PAS also is environmentally friendly, as it contains no cadmium, mercury, or lead. A PAS coupled with an effective energy-management scheme makes battery-less PIRs, thermostats, and CO2 sensors simple to implement at minimum expense.
Temperature Differences
Differences in temperature can be used to power a number of remote applications. A thermoelectric device creates a voltage when there is a temperature variant on two junctions of two metalsa property discovered by Seebeck in 1821. Conversely, when a voltage is applied, it creates a temperature difference (known as the Peltier effect).
A number of low-cost Peltier elements are available that can be used "in reverse" as generators for small wireless monitors. The voltage generated from these elements is very low. It requires innovative direct-current-to-direct-current (DC-to-DC) conversion to transform the voltage to a level suitable for use by a typical wireless controller. For example, EnOcean's ECT310 low-voltage DC-to-DC converter uses a blocking oscillator design.
With a 2K temperature difference and a standard, low-cost Peltier element, the DC-to-DC converter starts operating at around 20 mV. Its output depends on the actual temperature difference of the Peltier element. An input voltage range of 20 to 50 mV corresponds to an output voltage range of 3 to 4 V. A typical thermo-driven sensor consists of a sensor element, a small Peltier element, a DC-to-DC converter, and a radio module.
System Implementation
Implementing a wireless system using harvested energy constrains a microcontroller design. A balanced approach is required to make sure that energy is available for sensing control. The start-up time of a microcontroller plays a strategic role. Such timing is usually influenced by oscillator delay. For example, crystals and ceramic resonators can take several milliseconds to stabilize.
RC oscillators, by contrast, provide fast startup. But they generally suffer from poor accuracy over temperature and supply voltage. To save time, it is advisable to use a microcontroller that can start with an RC oscillator and subsequently switch to a crystal oscillator.
In addition to saving energy, rapid switching of a sensor is particularly effective when measuring parameters that change slowly. Given suitable power strategies, it is possible to achieve an average current consumption just above the total current consumption of the continuously running processor blocks.
While several circuit blocks can be switched off, others must be operated permanently. For example, threshold switches activate the electronics and timers that trigger periodic activities, such as sensor readings. These circuit blocks rapidly dominate the entire energy requirements. As a result, they must be aggressively optimized. Because the timers of typical harvesting modules should only require approximately 20 nA, they are typically analog. These timers switch off all components during sleep periods. This approach can enable a power reserve via the PAS capacitor of up to one week with solar deviceseven in complete darkness.
For any highly dynamic processes that need to be analyzed, it also is worth doing the following: pre-processing the data in the sensor, reducing the data to be transmitted, and only transmitting minimal measurement data as needed. Simple, well-architected software reduces execution timesaving more energy in the process. Another element to consider when waking a central processing unit (CPU) is oscillator startup and power-down times (see figure). Finally, minimizing writing to memory will save power.
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Connectivity To Other Systems
Normally, energy-harvesting devices need to communicate with a powered device, such as a light or HVAC controller. As these typically require permanent power anyway, a harvesting network can make use of them for communication and control. In a standard system, an energy-harvesting switch can transmit to a receiver that has a triac or relay. Powered devices can act as repeaters to span greater distances. In addition, they can assist two-way communications. As an energy-harvesting device can only listen for short periods, the powered device can be a postmaster/mailbox for non-powered devices, which can pick up commands after they wake up again.
Taking this one stage further, the simple receiver can be replaced by a gateway or controller that can communicate to the main system infrastructure over TCP/IP. Many building systems are already in place using BACnet, LonWorks, or KNX. It is easy to add a bridge or gateway so the EnOcean wireless standard and incumbent protocols can simply interact with each other.
Examples abound in the sector that recognized the advantages of energy harvesting earliest: building automation. Energy-autonomous building-automation solutions range from room thermostats with set-point temperature settings to maintenance-free motion detectors and window handles. In this last example, a radiator is automatically shut down when a window is opened to reduce energy consumption. Installing occupancy sensors that turn off lights in vacant rooms can save up to 40% on energy and operating costs.
Another key area for energy-harvesting technology is industrial automation, as wireless technology presents an ideal alternative for process optimization, monitoring, and control. In the automotive industry, for example, energy-harvesting technology can be used for wireless cable harness testing. Instead of conventional cabled switches, energy-harvesting wireless switches are used with an energy generator. Just pressing a button unit produces enough energy to determine whether individual components are correctly attached to a cable harness. A further advantage is that the classic cabling on the backplane or underside of a board is not applicable.
Additional forms of energy harvesting are close to moving from the research labs into production. For example, large agricultural sites, areas of natural beauty, or sports fields (e.g., golf courses) could be fitted with maintenance-free sensors to ensure that crops or plants receive an optimal supply of water and nutrients. And forests or bush areas could use sensors to provide early warning of fires.
In logistics, energy-harvesting technology could be used to control air freight containers and communicate their parametersfor instance, position or temperature. To date, technologies like GSM or GPS are not used for logistic purposes in air freight because of their active transmitting components. International regulations prohibit radio components that transmit permanently in flight. An extra challenge is the independence from batteries as a power source, as their regular replacement costs time and money. This is an opportunity for energy-harvesting wireless technology, which can send extremely short data telegrams and execute functions very quickly. In addition, those units that are not momentarily needed are always cut out.
Such technology is suitable for a variety of logistics systems as well. For example, take sea freight containers or supermarket cold chains. These can be governed with time offsets to reduce the cost of peak power needs. Suitable temperature sensors are fitted at critical points in a freezer to ensure that foodstuffs do not thaw. When a specified temperature is reached, these sensors send a wireless signal to a controller to the effect that power to a freezer must be restored.
In the future, energy-harvesting-powered sensors will play a major role in providing data to care and medical staff about the whereabouts, health, and comfort levels of patients in healthcare facilities. For example, a sensor placed inside the mattress will send signals stating whether the bed is occupied. Door or carpet sensors will provide data of where people are (e.g., fallen out of bed or in the bathroom).
In automobiles, it will be possible to remove the batteries from the tires in the tire-pressure monitoring system via energy-harvesting-based sensors. Such sensors will provide huge savings in the hundreds of millions of batteries that need to be disposed of annually. In aviation, thermal-driven sensors on the outside of the aircraft will save expensive and heavy cabling. Even simple passenger lighting, call button, or window dimmers could be replaced with battery-less wireless solutions, again reducing the amount of copper cable required.
Most likely, mechanical (linear motion), solar, and thermal sources will remain the major forms of energy for such sensors in the near future. RF energy harvesting, which is currently being tested by a number of groups, may be another. It uses the energy from accessible radio signals. Another form could be rotational motionfor example, harvesting energy from the flow of water or gas in a meter. No matter its source, energy harvesting is part of a future that could span every industry.
Frank Schmidt
CTO and Co-Founder
EnOcean GmbH
Kolpingring 18a, D-82041
Oberhaching, Germany
+49.89.67 34 689 0
FAX: +49.89.67 34 689 50