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Energy harvesting is a fundamental concept in engineering. Taking excess waste or available energy and recovering it for useful work makes sense, extending the useful life of power sources. For large-scale applications like transportation, implementing additional hardware to recover waste heat can add singles of a percent (< 10 percent) to the overall system efficiency. Here, the business case is the number of vehicles and the net-positive environmental benefit from converting the heat to work rather than releasing it into the atmosphere.
For electronics, similarly low-energy conversion efficiency (10 percent) has hindered the range of applicability of energy harvesters to low-power uses. The power contribution of ultra-low-power MCUs limits how large a battery that energy harvesting can replace, on the order of 1-1,000 µW/cm2. Still, the technical challenge to improve the energy conversion efficiency of electronic energy harvesting can enhance the device’s performance and extend or replace the battery life with significant efficiency improvements.
It is essential to differentiate maximum power from maximum efficiency. Though improving conversion efficiency increases power for the same input energy, this condition does not necessarily coincide with the highest power. Here are some ways to optimize typical ultra-low-power MCU energy harvesters to achieve maximum efficiency.
Applications suited for electronic renewable energy harvesting include wearable technology and off-grid wireless sensor networks. Though energy harvesting efficiency of these use cases is not very high, the critical metric is to compare the technology’s power contribution with a battery balanced against its cost. With self-powering devices in mind, the three solar energy harvesting areas are cell, space, and module efficiency.
Cell efficiency refers to a single cell that engineers size based on its expected solar load to produce the highest wattage over a given photovoltaic cell. Engineers optimize the power output to unit area ratio to increase efficiency. The third area is module efficiency, which considers the entire system in the efficiency calculation. The most effective way to improve solar energy harvesting efficiency is to target the cell level. Using advanced photovoltaic materials maximizes cell efficiency. And while increasing system efficiency sometimes conflicts with budgetary constraints, engineers can orient the more efficient cell material to achieve the higher power differential, yielding optimal efficiency for a given power condition.
The ultimate strength of a piezoelectric material limits how much acceleration it can handle. That level defines the material’s power density and determines the upper boundary for power output. The maximum power output occurs at the material’s resonance frequency.
Next, to optimize energy harvesting efficiency, it is essential to note that both efficiency and power output are strongly correlated to frequency. With the output power level dictated by the material, the fundamental frequency guides the piezoelectric element’s design. Tuning the piezoelectric energy harvesting solution to its resonance frequency minimizes destructive wave interference of the energy available for recapture, maximizing power output along with efficiency.
The Seebeck effect—converting heat directly into electrical energy—is the guiding principle of thermoelectric energy harvesting. Thermodynamics indicates that the efficiency of thermoelectric energy harvesting is highest in conditions with high-temperature differential. This gradient is the driving potential for energy transfer and enables a high-power density solution. However, especially in human-contact energy harvesters, it is not practical to realize a significant temperature differential (ΔT).
If ΔT cannot deliver high-efficiency energy harvesting, power conversion and thermal conductivity are other levers that can increase efficiency. Thicker materials transmit more energy (conduction), while low-loss conversion to energy reduces the inefficiency prevalent in converting one form of energy into another.
Energy harvesting is a critical step toward eliminating batteries in low-power applications. To ensure a commercially viable landscape, you should maximize the harvesting processes efficiency to extract as much useful work from the source energy as possible.
Current ultra-low-power (ULP) MCU energy harvesting solutions realize efficiencies of around 10 percent. This inefficient energy conversion makes the energy recapture viable for ULP use cases on the order of tens of microwatts per square centimeter. Your knowledge of how each traditional electronics energy harvesting method recovers energy helps you understand how likely the approach is to fully obsolete an application's battery.
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.
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