Energy harvesting is rapidly maturing to become an important sector in the electronics industry, having risen over the past five years to take its place as a competent technology enabling an expanding market. While the pioneering companies behind the technology have faced many challenges in that time, the market now faces one of its biggest obstacles; convincing system integrators that it is a viable alternative to battery packs.
According to some leading companies in the field, the technology has proved it delivers the levels of energy needed for its target applications, in a reliable, sustainable way. The number of installations using harvested energy grows daily, and many forms of energy harvesting now exist; from thermopiles to vibration, solar to RF.
As they all deliver relatively modest amounts of energy, they also share a common application area, which today is predominantly providing power for wireless sensor nodes, used for monitoring industrial/automation equipment and machinery.
Micropelt’s Thermoelectric Generator (TEG) uses MEMS technology to sandwich n-type and p-type substrates to harness Seebeck’s Law.
But even with significant demand now coming from end-users in this field, it seems system integrators are yet to be fully convinced of harvested energy’s credentials. This is due in part to the fragmentation that currently exists amongst competing suppliers as, while standards exist for batteries, the same isn’t yet true for harvested energy. This places another hurdle in the way of their being specified, but one that is comparatively easier to overcome than the laws of physics.
Micropelt’s TEG device shown on a pencil, can be directly mounted on a heatsink to maximize the temperature gradient across its faces.
Because they increasingly target the same class of applications as batteries; typically those that need limited power often sourced from replaceable, removable, rechargeable or renewable cells, this breakdown in the supply chain threatens to impede the future progress of energy harvesting.
However, recent efforts backed by the ISA (Instrumentation, Systems and
Automation Society) have created a working group that intends to develop standards for the interchangeability of power modules for wireless sensor nodes (WSNs), which will cover the electrical and mechanical characteristics of power modules whether they be batteries, fuel cells or energy harvesters.
With a focus on wireless sensors as used within an industrial setting, requiring average power of around 1mW, the ISA100.18 working group will also take into account the needs of wireless systems such as those covered by ISA100.11a, as well as commercially dominant protocols including WirelessHART and ZigBee. But high on the deliverables will be a standard for power module interchangeability that will ensure that any (compliant) application will be able to use batteries, fuel cells or energy harvesters indiscriminately. This, it is hoped, will promote the use of energy harvesting modules within new installations by promoting interchangeability. The first draft detailing a standard connector has
recently been published for comment.
A number of companies have now commercialized their energy harvesting
technologies and are already seeing success. Typically the underlying technology employs a well-known and well understood electromechanical or chemical effect. For example at least two companies are currently producing harvesters that exploit Seebeck’s Law to create a current flowing between two substrates; one n-type, one p-type. Current flow between the substrates is caused by a thermal differential. The effect itself has been known for many years and is synonymous with the Peltier Effect, which is the reverse effect used to create a heat pump.
All that is needed to generate energy is a relative temperature difference between the two sides of the substrate ‘sandwich’, a significant advantage of this effect is that the n- and p-type pairs can be ‘stacked’ side by side and wired in series to create greater potential differences.
The symbiotic nature of energy harvesting in wireless sensor nodes enables many use-cases, like this thermostatic radiator valve.
Micropelt, which started as a project by Infineon but is now VC funded, employs the effect to create both Peltier coolers and thermoelectric generators (TEGs). It uses MEMS technology to create micro-TEGs which are capable of producing power in the 10mW range, and is currently commissioning its first volume production line in Halle/Saale, Germany, which will eventually produce 10 million parts per year.
The Perpetuum free-standing harvester combines electromagnetic vibration energy harvesting technology with a selectable suite of energy charge, storage and management options.
According to Micropelt’s Vice President of Business Development, Burkhard Habbe, the biggest challenge the company now sees is supporting its target markets and customers through to volume deployment, but that market acceptance will require the energy harvesting industry to further prove its maturity by developing and adopting standards, while educating and supporting system integrators: “Cross-disciplinary consulting and design houses are very desirable and we work on establishing those in our efforts to speed market development.”
Another company favoring the thermoelectric generator solution is Perpetua, which produces a flexible thermoelectric film. Jerry Wiant, Vice President of Marketing at Perpetua, believes that education is the company’s biggest short-term challenge, but feels energy harvesting is destined to be accepted on a wide scale: “The widely published reports of the pending explosive growth in wireless sensor network deployments is already intensifying the demand for energy harvesting solutions,” explained Wiant. “It is becoming clear that battery-changing labour costs and environmental concerns of disposing of batteries are going to be the primary drivers for widespread harvesting adoption.”
With technology already shipping in production quantities, Perpetuum, a spin-off from the UK’s University of Southampton but now also a VC-backed company, has developed its solution around a different principal; electromagnetic induction. Using the same basic techniques seen in countless electric motors - albeit in reverse - the company’s technology also targets wireless sensing in industrial automation.
Predominantly the technology harnesses vibration created by the machines being monitored, making the wireless sensing node symbiotic with its power source. For this reason there is little need for storing excess power, but Perpetuum’s President, Roy Freeland, who is also the co-chair of the ISA100.18 Power Sources Working Group, does believe that better power management and its storage will be critical to the future success of energy harvesting.
One of the objectives of the Working Group is to address power management, which could include defining the average power output that any particular method of energy harvesting must produce.
This is perhaps more relevant for energy harvested using vibration because, as Freeland explained, the energy produced is dependent on the efficiency of the converter which is, in turn, dependent on the method used to harness the energy. This requires a ‘tuned’ solution which is optimal at the resonant frequency of the vibration source. For machinery running AC induction motors, for example, this may be 100Hz, but for an energy harvester attached to a locomotive, the frequency may be significantly different and prone to change. Perpetuum’s technology differentiator is the way it dynamical adjusts to these changes, to
maximize the energy harvested.
Thinergy’s solid-state flexible and rechargeable thin-film cells are only 0.17mm thick.
While low-power, autonomous wireless sensing is clearly the main application for harvested energy today, it is commonly felt that the amount of energy available from existing solutions isn’t impeding its use in others: “We’re beyond the stage where power is that important, we’ve shown it can be done and the fundamental breakthroughs were made years ago,” explained Freeland.
Micropelt’s Habbe agreed: “This is not a true bottleneck, even though there is demand for higher power levels. Higher power levels are usually just a matter of cost, as they can be achieved by combining multiple basic harvester elements, but this premium will go down as volumes grow.”
Here, again, the need for better energy storage was raised, by Habbe: “Here is indeed a gap to fill. A ‘pure’ wireless monitoring sensor simply runs off a capacitor or a perfectly rechargeable small thin film battery, because there is no need for monitoring during the non-operational times of its host. However, if continuous access to, or heartbeat from the sensor is critical, there needs to be a bulk energy storage with considerable capacity, able to sustain operation over
multiple weeks without the supply of harvested energy.”
It seems that if wireless sensing is to become ubiquitous, creating and transmitting an unprecedented amount of data continuously, the issue of energy storage is the next challenge faced. Several companies already target harvesting applications with energy storage solutions, such as Cymbet and Infinite Power Solutions, both of which produce energy storage solutions for applications powered by harvested energy.
Roy Freeland pointed out that wireless sensing isn’t always about networking, although it has become synonymous with, say, ZigBee, where mesh networking is the default topology. Wireless sensing nodes will be less dependent on mesh networks, but better energy storage solutions could enable a greater level of networking, which inherently demands more power.
Freeland also sees applications that are less dependent on wireless connectivity; closed control loops that simply need harvested energy to power a microcontroller that must wake occasionally for maintenance, but agrees that energy harvesting will drive ubiquitous computing.
To reach the level of ubiquitous sensing that many feel is on the roadmap, energy harvesting coupled with improved energy storage will be fundamental, as Micropelt’s Burkhard Habbe, explained: “Energy harvesting will become pervasive as more autonomous systems need power independent of batteries. In fact, to many of those, energy harvesting will be the key enabler, because it is simply impossible to constantly maintain billions of ‘smart dust’ devices in structures, buildings, machines, production assets, cattle, pets and, lastly, humans.”
- Courtesy of EE Times Europe.