Experimental study of a miniature organic Rankine cycle unit using ocean thermal energy

Abstract

Miniature ocean thermal energy conversion (OTEC) unit is an attractive candidate for sustainable energy supply of autonomous ocean observation platforms. Due to the variability of power load, the OTEC remains susceptible to operation under off-design conditions. A miniature ORC-based OTEC unit was developed and constructed. The performance of the unit under design and off-design was experimentally examined. The effects of operating parameters (i.e., pump frequency and turbine frequency) on power generation and thermal efficiency were evaluate using response surface methodology. The results indicate that the unit can produce 0.917 kW net power with a thermal efficiency of 1.46% at 27 ℃ heat-source temperature and 6 ℃ cold-source temperature. Off-design steady-state assessment allows a wide operability range of working-fluid flow rate between 0.18 kg/s and 0.28 kg/s for R134a, corresponding to net power varying between −92 W and 166 W, as well as net thermal efficiency varying between −0.25% and 0.31%. Pump frequency is the primary operating variable affecting the unit performance, followed by turbine frequency. The optimal turbine frequency maximizing thermal efficiency increases with pump frequency, varying between 105 and 160 Hz. When the pump frequency is 32 Hz, the optimal turbine frequency is approximately 160 Hz, which corresponds to the maximum net output of 166 W. The current study offers experimental and theoretical supports for the design and operation of ORC-based OTEC unit.