A thorough cost analysis helps answer the question
The concept of energy harvesting has been around for as long as windmills and waterwheels were used to harness energy. In the last decade, the possibilities of using low-power harvested energy have greatly expanded in the industrial market as devices became smaller, autonomous and required less power. Some of the more common ambient energy sources include:
Photovoltaic: Light is converted to usable energy. In this category, the sun is the obvious energy source, but energy can be scavenged from indoor sources of light, too. Light energy is the most common ambient energy source not only because the cells that harness light energy becoming less expensive, they’re also getting smaller and conversion efficiencies are increasing.
Vibration/piezoelectric: Much of our motor-driven machinery produces vibration. This energy is collected and transduced — often by magnets over a coil in a rotating device or by a generator activated by mechanical stress — then tuned to amplify and match the power needed for the application.
Thermo-electric: Dark spaces often have some form of temperature gradient, making these sources suitable where light and vibration-based energy harvesting isn’t possible. According to EnOcean, which specializes in self-powered wireless modules, wireless sensor applications, for instance, can power electronic circuits using temperature differentials as small as 2°C.1
Other ambient energy sources include wind and RF. There are many exciting applications where energy harvesting may be employed including consumer devices for the home and wearable medical devices. This article will focus on the industrial environment.
Is there ambient energy to be had?
We’re increasingly hearing the benefits of energy harvesting all the time. It is considered a desirable solution in equipment operating harsh, remote and dangerous environments where access is problematic, or in applications requiring retrofits, predictive maintenance or where batteries frequently wear out. Devices that rely on harvested energy do not require personnel to install and change batteries, saving time and money while presenting less safety risk. And, harvesting energy is environmentally friendly. Batteries contain heavy metals, and their safe disposal adds to the total cost of the device.
But while energy harvesting has gained attention over the last few years as an attractive option for industrial devices and networks, many of the considerations facing designers haven’t really changed during this time. The first and most obvious question is whether or not there is in fact energy available to be harvested from the intended environment. Sometimes environments give the impression that there’s ambient energy available but nothing can be done to harvest it.
Steve Grady, vice president of global marketing at Cymbet, notes that some of the heaviest and most rugged equipment can produce lots of impact motion and g-force, but the motions may lack the repetitive nature for tuning to create a desirable bandwidth for power output. “As our customers do a detailed analysis of their motion/vibration sources, they discover that, even though there seems like there’s a lot of energy, it’s a very, very complex energy harvesting situation.”
Another consideration is availability. Some of the best applications for energy harvesting present themselves when energy must be consumed right away. Scavenged sources of energy are not always reliably available. Without light, devices won’t always work when needed; and temperature and wind are highly variable and cannot easily be controlled. By choosing a primary battery to power the device, the engineer does not have to worry about “down times” in which harvested energy is inaccessible.
Analyze the total cost
While energy harvesting has enjoyed wider adoption these last few years, primary batteries, meanwhile, continue to improve. Some battery chemistries and constructions may last up to 20 years, far exceeding the expected lifetime of the intended end product, meaning they simply won’t need to be charged, replaced and disposed often enough to justify the costs of harvesting energy. In many cases, it all comes down to cost. For instance, some of the supporting components in an energy harvesting-based design can add up to be several-times more expensive than a design requiring just a coin-cell sized battery.
In the case of self-powered energy-harvesting-based modules, there’s more to the cost of a system than the up-front cost of energy harvesting. “Together with our customers, we usually do an amortization calculation,” explains Matthius Poppel, chief operating officer of EnOcean. The analysis typically “includes lead time or standby time of the batteries and the exchange costs of the batteries.”
The cost-analysis example in Figure 1 compares primary battery cells and an energy harvester for the same device, based on the harvester’s lifecycle capacity. The cost analysis should also factor the supporting components of the energy harvester along with design tradeoffs including battery change-out, product lifetime, power requirement, relation of battery size to overall product size, electrical requirements, assembly, transportation, safety and disposal.2
A third option: Hybrid
Deciding how to power the device does not necessarily have to be an “either-or” choice between energy harvesting and a primary battery-only strategy. Energy harvesting needs some compatibility between the output of the harvester and the size of the storage device. Otherwise a power and size mismatch occurs and the rechargeable battery can drain completely. Meanwhile, the primary-battery only strategy brings the risks of being the only source of power in the system, and will expire over time. However, the cost proposition and space allocation – particularly if enough space is available for a small primary battery anyway — may, in fact, allow for a hybrid configuration that includes the harvester, rechargeable device, and a primary battery. This hybrid approach includes a backup reservoir to help the system meet its reliability requirements should temperature gradients or vibrations, for instance, unexpectedly change.
There are many tradeoffs and considerations facing industrial device designers requiring low power. A cost analysis along with a thorough examination of the nearby ambient energy possibilities, the device’s life expectancy, and the disposal/transportation implications of battery power will go a long way to determine whether you should use harvested energy, a primary battery only, or a combination of both strategies.
1 “Thermoelectric Energy Harvesting,” EnOcean press release, July 1, 2011. (http://www.enocean.com/en/newsitem/thermoelectric-energy-harvesting/), accessed January 10, 2014.
2 Steve Grady, Cymbet Corporation (2014). Powering Wearable Technology and Internet of Everything Devices: What every product manager and designer needs to know. (unpublished, accessed January 10, 2014).