Energy harvesting (EH) solves a central issue of wireless sensor nodes: regular replacement of the primary cells is no longer necessary and this in turn can help reduce the total cost of ownership. While EH is not possible for all sensors in every circumstance, it is a viable energy supply option for a wide range of devices. Furthermore, fully autonomous products can help understand and control industrial processes better and therefore make them more profitable.
The authors: Tilo Merlin Global Product Manager Temperauture Products, ABB Products Automation Horst Schwanzer Technology Manager Temperature Products, ABB Products Automation
Everyday life in the twenty-first century would be unthinkable without wireless communication. Yet even the most sophisticated tablet computer or smartphone has to be plugged in frequently. Wireless technology based on batteries or accumulators is not really feasible in an industrial environment. ABB proves not only that wireless communication is a feasible technology but also that autonomous instruments are within reach. Initial practical experience has meanwhile been gained and the first products will appear on the market in the summer.
Energy harvesting
Replacing batteries on a regular basis is not always an option since – depending on the plant setup – this could offset the savings from using wireless devices. Instead, energy harvesting (EH) is seen as one possible solution that overcomes this issue by creating truly autonomous devices. EH converts the energy available in the process into usable electrical energy, which in turn serves to power wireless devices. Typical energy sources include hot and cold processes, solar radiation, vibration and kinetic energy from flowing media or moving parts. The most widespread mechanisms are solar radiation as well as thermoelectric and kinetic converters.
Common transducer technologies
Although photovoltaics is nowadays a robust and established technology, its application indoors is rather limited. While the outdoor intensity can reach approximately 1000 W/m2, indoor values are typically in the region of 1 W/m2. In other words, the amount of energy that can be harvested is restricted.
Thermoelectric generators (TEG) harvest electrical energy from thermal energy (i.e. from the temperature gradients between hot or cold processes and ambient) using the Seebeck effect. While the efficiency of TEGs is rather low – typically less than 1 % – the technology is extremely robust and stable. Large temperature reservoirs are often present, especially in the process industry. Hence, a lot of heat is available and the power that can be delivered by commercially available TEGs is sufficient to maintain a large number of wireless sensor nodes in a variety of scenarios.
Mechanical movement such as vibrations can be directly converted into electrical energy with the help of different transducer mechanisms:
- Electromagnetic mechanisms use a flexibly mounted coil, which moves inside the static magnetic field of a small permanent magnet. This induces a voltage as described by Faraday’s law.
- Piezoelectric transducers are based on piezoelectric materials. Kinetic motion displaces a seismic mass, which induces a mechanical stress on the piezoelectric material.
- Electrostatic transducers function with a charged variable capacitor. When me-chanical forces are applied, work is performed against the attraction of the oppositely charged capacitor plates. As a result, a change in capacity induces a current flow in a closed circuit.
In short, all kinetic converter principles are based on a mechanical resonator, and the systems can only deliver a reasonable power output if the resonance frequency of the harvesting device matches the external excitation frequency. The use of variable-frequency drives in the process actually limits the options for vibration harvesting systems.
System components and architecture
Energy harvesting can be a discontinuous process: for example, in the case of outdoor photovoltaic applications, day-night cycles will lead to unstable power sources. Plant downtimes can result in different process temperatures, which may influence the energy delivered by TEGs, while variable-frequency drives can cause power yields of vibration harvesters to vary. On the other hand, there could be times when the energy harvesting system supplies more energy than is actually needed.
The power consumption profile of typical wireless sensor nodes is also discontinuous: depending on the duty cycle and update rate of the sensor, peak loads may have to be buffered because the EH systems are not able to support these high short-term currents.
Every EH system needs a buffer to cope with periods when the harvesting device is unable to supply enough energy for the sensor node. Popular buffers include:
- Special super or hybrid-layer capacitors, which tolerate high peak currents
- Rechargeable secondary cells
- Conventional primary cells – these cannot store an excessive amount of energy from the EH system but they can be used to provide power at times when the system cannot
- Typical industrial primary cells with a very long shelf life and low self-discharge rates
One drawback of conventional, lithium-ion based secondary cells is the limited number of discharge/charge cycles. Harvesting devices and buffers need an appropriate power management (PM) system to achieve a truly autonomous power supply. The PM’s most important functions are to adjust the output voltage and current characteristics of the EH system to the input requirements of the electrical consumer and to switch smoothly between energy buffers and the various EH sources.
Autonomous temperature transmitter
ABB’s R&D department has developed a completely autonomous temperature transmitter with a fully integrated EH system. Thermoelectric generators have been integrated into the transmitter in such a way that its handling, stability and form factor are unchanged while its lifetime and functionality are considerably enhanced. The device also includes a smart energy buffer solution for occasions when the process temperature is insufficient to generate enough power.
The overall size of the temperature transmitter selected prevented the integration of conventional TEGs, which normally have macroscopic dimensions of around 10 to 20 cm2. Instead, novel micro-thermoelectric generators (micro-TEGs), manufactured in a wafer-based process, were used. The principal challenge of integrating these two devices was to ensure that the stability and robustness of the transmitter were maintained.
In most cases the process is warmer than the ambient air temperature, which means the hot side of the TEGs needs to be coupled to the process with optimal thermal conductivity. Extensive numerical simulations were carried out to maximise the heat flow through the TEGs. The other (cold) side must be cooled and is therefore coupled to the ambient air by means of a heat sink. This needs to be positioned at a sufficient distance to allow applications where the process pipe is covered with a thick insulation layer.
With a minimum difference of about 30 K between the process and ambient temperatures, the system is able to generate enough energy to supply both the measurement components and the wireless communication electronics. With temperature gradients greater than 30 K, more energy is generated than is needed and this could be used to permit faster refresh rates, for example.
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