Design of a Low-Speed Cylindrical Pole Alternator Suitable for Bidirectional Flowing Water Body

Abstract

Hydropower energy from bidirectional water bodies such as river flow represents a largely underutilized renewable energy resource due to the challenges associated with low flow velocities, bidirectional motion, and variable operating conditions. This paper presents the design and evaluation of a low-speed alternator specifically developed for energy harvesting from such environments. The alternator design focuses on optimizing key electromagnetic parameters, including the number of poles and air-gap length, to enable effective power generation at very low rotational speeds without the use of mechanical gearing. A cylindrical alternator configuration incorporating 54 poles was implemented and experimentally evaluated. The prototype achieved an RMS output voltage of 13 V (open circuit condition) at a rotational speed of 40 rpm, which was identified as the minimum operational speed for useful energy extraction, with the output voltage increasing proportionally with rotational speed to approximately 26 V at 80 rpm. Experimental results further demonstrate a linear relationship between alternator speed and output frequency, highlighting an inherent limitation of low-speed alternators operating under variable flow conditions. The findings confirm the feasibility of direct-drive, low-speed alternator designs for bidirectional hydropower energy harvesting while underscoring the need for effective frequency stabilization. Ongoing work is directed toward the development of a frequency-conditioning mechanism to enable stable power delivery for practical applications.