Renewable energy is at the forefront of global decarbonization efforts. Using green sources of energy that do not contain carbon, engineers have defined a path for infinite on-grid distributed power that does not harm the environment. The scale of the technical challenges and distance away from the current state means that the commercial viability of a fully renewable grid is still years away.
For on-grid electronic applications, semiconductor manufacturers have reduced the power draw substantially, which has made applications like ultra-low-power wireless sensors a reality. This innovation has improved grid power sustainability while engineers develop a renewable approach to grid power in parallel.
Shifting to renewable energy is an expensive wholesale change, so the industry employs intermediate techniques to aid the transition from fossil fuels to renewables. One of these approaches is energy harvesting, a form of waste energy recovery that captures available energy and returns it to a source device.
It is essential to distinguish between sustainable and renewable energy. Sustainable solutions extend existing fossil fuel-based approaches by supplementing them with auxiliary power. Renewable energy uses infinite sources such as wind and solar energy to generate power. Using these sources that do not contain hydrocarbons drastically reduces or removes them from the output energy. As a result, energy harvesting is a sustainable measure to extend grid-power or battery source energy.
Energy harvesting becomes renewable when it can replace finite source energy with infinite options. For off-grid applications requiring a battery, such as wearables and off-grid sensors, energy harvesting offers this kind of renewable option, using light, motion, or thermal energy to supply the battery’s entire power load.
Self-powering devices are the key to economic viability, and they significantly reduce replacement costs for the consumer while moving closer to a renewable landscape.
Green Energy Harvesting Strategies
Applications suited for electronic renewable energy harvesting include wearable technology and off-grid wireless sensor networks. Though energy harvesting efficiency is not very high, the critical metric is to compare the technology’s power contribution with a battery balanced against its cost. With the goal of self-powering devices in mind, it is essential to understand the fraction of power for three renewable sources of energy harvesting (light, motion, and thermal):
Light
Solar, or photovoltaic, energy, contributes 0.1mW/cm2 (indoor) to 100mW/cm2 (outdoor) at a harvesting efficiency around 10 percent (15-20 percent is possible). This level provides power of 10 (indoor) to 10,000µW/cm2 (outdoor) of additional power.
Motion
At nominal vibration/motion, piezoelectric energy harvesters recover 4µW/cm2 (human, measured at 1Hz and 50Hz) and 100µW/cm2 (machine, measured at 5Hz and 1,000Hz). Even high-performance piezoelectric materials operate at around 10 percent efficiency, similar to that of photovoltaic devices.
Thermal
Energy harvesting using thermoelectric media yields power contributions of 30µW/cm2 for human temperature differentials and up to 10,000µW/cm2 for industrial equipment. The efficiency is again around 10 percent for machines due to the high thermal conductivity of the metal. However, the small differential available from human body temperature yields a harvesting efficiency of only 0.15 percent from thermoelectric media, by far the lowest of the three sources. Because human body temperatures do not significantly vary, the ambient environment has a more profound effect on thermoelectric performance. These sources are more efficient on cold days due to higher temperature differential.
Economics of Energy Harvesting
The three principal renewable energy harvesting sources—light, motion, and thermal—provide auxiliary power on the order of 10-10,000µW/cm2. This contribution level means energy harvesting can replace ultra-low-power MCU batteries. Though the true economics depend on the geometry, design, and load conditions of a specific application, there is reason to believe that energy harvesting offers a definitive economic advantage over batteries.
Engineers have significantly improved the performance density of the batteries. Given that, energy harvesters may take up more space than batteries in-application. The packaging constraints of on-chip components challenge incorporating energy harvesters within the envelope.
However, batteries are finite and require humans to replace them. Because batteries consume limited natural resources, these approaches are non-renewable. They also use natural resources such as lithium and alkaline to produce a partially recyclable—at best—product. Also, humans must be engaged to replace them, and part designs must consider replacement, maintenance, and sensor downtime.
Replacement cost alone could translate to hundreds of hours per year, whereas renewable energy harvesting creates a passive, hands-free solution that does not adversely affect the environment.
Takeaway
The key to a self-powering design strategy that uses renewable energy harvesting is to start with ultra-low-power MCUs in applications that already have low power requirements, around 11,000W/cm2, such as wearables and remote wireless sensors. As renewable energy harvesting techniques improve, they will become more practical for larger battery-based devices. This approach can make IoT advancements less environmentally harmful as well as economically viable.
About the Author:
Adam Kimmel has nearly 20 years as a practicing engineer, R&D manager, and engineering content writer. He creates white papers, website copy, case studies, and blog posts in vertical markets including automotive, industrial/manufacturing, technology, and electronics. Adam has degrees in chemical and mechanical engineering and is the founder and principal at ASK Consulting Solutions, LLC, an engineering and technology content writing firm.
