Improving the off-design efficiency of organic Rankine bottoming cycle by variable area nozzle turbine technology
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Organic Rankine cycles (ORCs) are expected to operate more at varied working conditions, especially when rapid growth in renewable energies put power generation technologies in variable load operation. A power cycle works with lower efficiency in part loads than expected in full load condition. Here, an organic Rankine bottoming cycle with a control strategy based on variable geometry turbine technology is proposed to boost part load efficiency of combined cycles offshore. The Variable Area Nozzle (VAN) turbine is selected to control cycle mass flow rate and pressure ratio independently. A design methodology is presented, and the performance of proposed design and developed control strategy is assessed by an in-house developed tool. With the suggested solution, part load ORC efficiency is kept close to design values while clearly outperforming the reference case with sliding pressure operation. The proposed solution assures efficient operation in a wide range of loads. The combined cycle efficiency showed a clear improvement compared to the reference case, resulting in thousands of tonnes of annual CO2 (carbon dioxide) emission reduction. Compactness, autonomous operation, and acceptable technology readiness level for VAN turbine solution facilitate application of presented design to offshore oil and gas installations.