Energy harvesting by exploiting vortex-induced vibration from a modified cruciform structure

Abstract

From off-grid charging of electronic devices to energising independent wireless sensor networks, the demand for stand-alone, low-power generators from renewable energy sources is becoming more prevalent. A cruciform energy harvester has been shown to output consistent power in the order of 1 mW when the reduced velocity U*, exceeds 15. However, this output is insufficient and its onset too late for realworld applications. Thus, this study seeks to remedy these two shortcomings by investigating cruciforms oscillators at various cruciform angles. To fulfill these goals, the Reynolds-Averaged Navier-Stokes simulation were performed, and the results for the 90° cruciform were compared against experimental data for validation. The experiment uses a similar 90° cruciform in an open flow channel. Assessments were made on the vibration amplitude, frequency, lift amplitude and lift frequency at cruciform angles 90°, 67.5°, 45°, 22.5° and 0°. The Reynolds number range was 1.1x103 Re ≤ 14.6x103 and Scruton number 9.94, which was consistent with similar studies. Hilbert-Huang analysis of the 90° cruciform indicated that a lot of energy from the free stream was wasted in the production of non-performing Karman vortices. A larger lift was possible if streamwise vortices were produced instead. When 45 ≤ a(°) ≤ 67.5, asymmetries in the vortical structures prevented high-amplitude vibrations from taking place. However, when 0 ≤ (°) ≤ 22.5, a high-degree of symmetry among the vortical structures led to an early onset of high-amplitude vibration. Power generated by the cruciform was in the order of 1 mW for a 90° cruciform, below 1m Wwhen 45 ≤ a(°) ≤ 67.5, and in the order of 10 mW when 0 ≤ a(°) ≤ 22.5. Unification of the power generation and energy harvesting efficiency results produced a map that describes the power and efficiency of the harvester in the a(°)-U* parameter space. This uncovers three distinct regions of power generation: pure cruciform region as cruciform angle tends to 90°, steep-angle region between 45 ≤ a(°) ≤ 67.5, and shallow-angle region between 0 ≤ a(°) ≤ 22.5. Maximum efficiency occurs close to 0.8 m/s when cruciform angle is 90°, close to 0.2 m/s at 67.5°, and close to 0.4 m/s at 0°. This power and efficiency map makes it possible for future engineers to tailor the design of their cruciform energy harvester to their specific power and efficiency needs.