Energy harvesting devices

Materials World magazine
,
3 Jul 2011
GoSense Wireless OEM ultra-low power sensor node. Image courtesy of Matthew Steinberg

Concrete cancer, kinetic watches and Antarctic instrumentation interspersed discussion at the Balancing the Instrumentation Budget for Field Deployment conference held in London, UK, on 8 June.

Ken Ball pointed to his left ear. He has been using a hearing aid for years. But, each time a new model is released, it uses more power, and the batteries last less time. Furthermore, he receives scant warning before these batteries run out. He smiled wryly at the rows of seated delegates. Could anybody offer a solution? Perhaps a baseball cap incorporating thermodynamics could be applied to remedy the situation. Anyone? Anything?

This light-hearted diversion did not form part of his talk on energy conversion from piezoelectric sources, yet Ball, of Microdul in Kingsbridge, UK, summed up the thrust behind the recent Balancing the Instrumentation Budget for Field Deployment event held in London, UK. While engineers are pressing ahead with novel sensors and instruments, powering them in cost-effective, sustainable ways is just as pertinent.

From innovation to application

Roger Hazelden, Technology Leader for Sensors and Optoelectronics at TRW Conekt, in Solihull, UK, outlined the advantages and challenges inherent in applying sensors in cars and aeroplanes. In the company’s tyre pressure monitoring system, a wheelmounted sensor model with a lithium button cell battery is attached.

To reduce cost, shut down, sleep and power modes are employed, and the device only transmits every few minutes. But, despite streamlining the monitoring system, incorporating energy harvesting devices (EHDs) is still not financially viable. ‘No one is going to use an EHD until it costs less than a battery,’ explains Hazelden, adding that button cell batteries cost less than a euro each.

Paul Mitcheson, of Imperial College London, who focuses on next generation EHDs, echoed Hazelden’s concerns. He noted, ‘A lot of universities are designing in isolation’, and emphasised the importance of designing with an awareness of the entire power chain. ‘By the time you apply power processing to a device, the efficiency goes down,’ he explained.

Mitcheson selected the Seiko kinetic watch – which converts kinetic movement in the arm into electrical energy – as an ‘outstanding’ example of an EHD. While this system illustrates what can be achieved, he cautioned that, ‘the human body is extremely difficult to generate power from’. He added that applying the same technology to a biomedical application would be a lot more difficult, as these devices are often a lot smaller and are required to incorporate several different applications.

Concrete solution

Nevertheless, the conference produced several examples of novel, cost-efficient sensors and EHDs.

Matthew Steinberg, of GoSense Wireless in Cambridge, UK, has created ultra low-power electronics for standalone data collection using integral sensors. Their restivity sensor node incorporates four stainless steel rings around an epoxy resin casing, which can be embedded into steel-reinforced concrete structures. The sensors are said to provide contactless data transfer at short range using a radio frequency identification reader (RFID). According to Steinberg, one advantage of the technology is that, ‘it can be fully passive, it doesn’t need a battery, and it can scavenge from the RFID field’.

The device can be used to monitor the ravages of concrete cancer in bridges. Other applications the company is developing include a wearable UV safety sensor with a data monitor to protect against melanoma from sun exposure, in addition to standard OEM sensor nodes (see image). Steinberg claims, ‘If you measure the concrete restivity [in a bridge] once a day, the sensor is only awake for 120ms. The device is asleep for 99.99% of the time’.

Max Pastena of SSBV, in Portsmouth, UK, has developed a thermoacoustic generator that can convert heat flux into acoustic power, which is then converted into electrical power. The system is composed of a linear tube that is divided into hot and cold parts using glass or carbon fibre stacks.

Pastena admits, ‘The efficiency is not very high (up to 50W, depending on tube size). It is not an alternative power generator, but it can be added to another generator’. The technology can be used to garner energy from solar panels, for heat recovery on the hot parts of exhaust pipes and in space satellites.

Polar power

Michael Rose, of the British Antarctic Survey, discussed the limitations of instruments in extreme climes, for example, the use of lithium ion cell batteries is limited in Antarctica. This is due to a paucity of data on how the batteries react to charging and discharging when the environment changes from very cold to warm. With transport also a major hindrance, his team has looked towards alternative methods of generating power.

One such device is the Seebeck effect generator – a thermoelectric energy harvester used to power sea-ice instrumentation during the polar winter due to the unavailability or unreliability of other devices. The device exploits the temperature differential between the sea beneath the ice and the colder air above the ice. It consists of a condenser, an evaporator and a connecting heat pipe. The pipe transports the acquired heat up the condenser section, where it is released into the ambient environment using thermoelectric generators and heat sinks to create electricity.

He explains that the team must generate, ‘a heat flow of 100W to get the electrical power of 1/3W.’ While this might not literally set their world alight, he points out that the system is ‘more environmentally friendly than dropping a lead acid battery into the sea’.

In some ways, this technology encapsulates the current state of EHD technology. Despite impressive innovation in thermal and acoustic energy harvesting, and the development of stoic sensors with modest appetites, power generating capacity needs to rise and cost needs to fall if these devices are to be widely embraced.