Making wireless instruments self-sufficient

10 July 2012

Suzanne Gill reports on a recent development in energy harvesting technology, which looks set to pave the way for the first generation of truly autonomous wireless instrumentation.

Today’s process industries are becoming more highly automated and instrumented, helping with process control and for monitoring the health of equipment in a bid to minimise process downtime.

Because wiring and installation can account for almost 90% of the total cost of a typical industrial device, it makes sound financial sense to explore the option of using wireless technology.

Traditionally, wireless devices were largely confined to specialist applications in remote installations, such as water distribution or oil and gas. However, as more wireless-enabled products come to market wireless instruments are able to communicate increasing amounts of useful information throughout plant networks.

Although the meshed architecture of WirelessHART networks, for example, provides a robust solution that is able to re-route information to bypass any malfunctioning nodes and to ensure a a secure service, it does require all instruments to act as relays, passing information around the network, which can require them to power up more frequently. This can have a negative impact on battery life of these instruments.

Forcing maintenance engineers to spend their time replacing batteries will offset the upfront savings of using wireless devices. Although there are a variety of strategies available for minimising power consumption and maximising battery life – such as configuring instruments to revert to low-power mode between measurement intervals – there is also an alternative that can virtually eliminate the routine replacement of batteries to create truly autonomous devices.

That alternative is energy harvesting (EH). It relies on harnessing the spare energy that is available in most industrial installations. Energy sources include hot and cold processes, solar radiation, vibration and kinetic energy from moving parts, which can all be converted into usable electrical power.

The most promising energy harvesting technologies convert solar radiation, thermoelectric and kinetic energy into electricity using photovoltaics, thermoelectric generators (TEGs) and kinetic converters respectively.

Photovoltaic technology is robust and well-established. However, while the intensity of the energy harvested in outdoor installations can reach 1,000W/m2, typical indoor installations often only manage 1W/m2. This can limit the practical situations in which photovoltaics can be used.

TEGs rely on the Seebeck effect to harvest electrical energy from thermal gradients between hot or cold processes and the ambient surroundings. They are not very efficient (usually less than 1%) but the technology is robust and stable enough to deliver the reliability demanded by the process industry. Most process industries are also populated by a ready supply of hot and cold processes, so a surplus of heat is generally available to power multiple wireless sensor nodes.

Mechanical movement, such as vibrations, can be converted into electrical energy using a variety of transducers. Electromagnetic mechanisms use a flexible mounted coil that moves inside a static magnetic field to induce a voltage according to Faraday’s law. Alternatively, piezoelectric transducers contain materials that generate electricity when put under mechanical stress. Electrostatic transducers are based on a charged variable capacitor. When mechanical forces are applied, work is done against the attraction of the oppositely charged capacitor plates, resulting in a change in capacity that induces a current.

All these mechanisms are based on a mechanical resonator that can only deliver a reasonable power output if the resonance frequency of the harvesting device matches the external excitation frequency. So, the use of energy-efficient variable-speed drives to run pumps and other process equipment, for example, limits the capacity vibration-harvesting systems.

Making it work
EH is often discontinuous. Photovoltaic systems, for example, only gather energy in daylight and a TEG powers down as the process cools down during plant shutdowns. At other times, these systems may be harvesting more energy than needed.

Similarly, the power consumption of a wireless sensor may also be discontinuous, depending on the duty cycle and the update rate of the instrument. Peak loads can occur that draw more power than the system can deliver. In many cases a buffer will be needed to help match the energy supply and demand.

ABB is currently trialling the first generation of a completely autonomous wireless temperature transmitter that relies on TEG technology. The TEG has been integrated into the device so that the instrument behaves essentially in the same way as a conventional temperature transmitter, with superior functionality and lifetime. The system also includes a buffer for times when the temperature gradient drops below the 30K needed to drive the TEG.

At around 10 to 20cm2, conventional TEGs would have been too big for this application, so ABB has used novel micro-TEGs, which are a wafer-based technology.

In most cases the process will be warmer than the ambient air, so the hot side of the TEG is coupled to the process to optimise the heat flow through the device. The cold side is coupled to the ambient air via a heat sink, which must be positioned far enough away from the process to allow for applications where the pipe or vessel is covered in a layer of insulation.

Autonomous future
The initial success of the WirelessHART-enabled temperature probe from ABB points the way towards increasing the use of truly autonomous, cost-effective instruments in the future. By eliminating the wiring costs of installing more sensor nodes and solving the problem of restricted access for regular battery replacement, EH instrumentation look set to increase production efficiency and improve asset management across a range of industrial sites.
Speaking to CEE about the development of these first instrument-integrated energy harvesting devices, Gareth Johnson, global wireless product manager at ABB, said: “We have installed around 100 prototype energy harvesting systems to help us improve system design and to clarify application suitability.”

The first of these prototype sites was at Robinson Brothers, and another is the recently unveiled carbon capture pilot plant at Imperial College in London. "We expect to have a saleable product available within 12 months,” said Johnston.

Using the energy harvesting technology, Robinson Brothers has been able to make remote temperature measurements without the need for a power supply to the transmitter. Combined with WirelessHART communications technology, this has eliminated the need for any cabling to the instrument.

Robinson Brothers is trialling the transmitter on the steam main which supplies its chemical manufacturing plant. “The transmitter has operating for about three months now and it’s ticking all the boxes without drawing any power from its back-up battery,” said Tom Rutter E&I manager at the plant. “It looks like it could go on forever, provided there is steam flowing through the line.”

The transmitter is powered by an on-board micro-thermoelectric generator, which is driven by the temperature difference between the steam pipe and the ambient surroundings.
The system at Robinson Brothers needs a minimum temperature difference of around 30°C, which is easily achieved in this application, where the steam flows at around 106°C and the ambient air is typically 26°C. The transmitter also has a built-in back-up battery which is not used during normal plant operation.

The transmitter was set up to send data wirelessly to a remote wireless gateway, which feeds the signal into the site’s existing Ethernet network and then to a data recorder.

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