Differences in Reliability and Predictability of Harvested Energy from Battery-less Intermittently Powered Systems

(1) North Carolina School of Science and Mathematics, Durham, North Carolina, (2) Department of Computer Science at the University of North Carolina at Chapel Hill, Chapel Hill, North Carolina

Solar and radio frequency harvesters serve as a viable alternative energy source to batteries in many cases where the battery cannot be easily replaced. However, energy harvesters do not consistently produce enough energy to sustain an energy consumer; thus, both the energy availability and execution of the energy-consuming process are intermittent. By simulating intermittent systems with large-scale energy demands using specifically-designed circuit models, the harvested voltage and other parameters such as the voltages across the capacitor and the load were determined. We plotted these data, for both harvested solar and harvested radio frequency energy, to make probability plots depicting the likelihood that energy will be available now given that N number of energy events have occurred. Additionally, we designated a metric as the η-factor, which was calculated from these probability plots for the solar and radio frequency data to quantify the reliability of the power source. The η-factor for harvested solar energy was statistically significantly higher than the η-factor for harvested radio frequency energy, meaning harvested solar energy was more consistently available than harvested radio frequency energy. Finally, we collected data to determine the effects on the output voltage of various obstacles between the radio frequency transmitter and receiver. We found that obstacles like metal and people caused a more pronounced drop in the amount of energy harvested when compared to other obstacles like foam or wood. Quantifying the reliability of different harvested sources would help in identifying the most practical and efficient forms of renewable energy; determining which obstacles cause the most obstruction to a signal can aid in the strategic placement of harvesters for maximum energy efficiency.

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