Natural gas is an important natural source of energy. Approximately a third of natural gas is traded globally in the form of liquefied natural gas (LNG), which must be regasificated before it is used as a fuel for power generation. In the process, adequate cold energy is produced and cannot be used in industries, for example thtaking away by seawater. So far a limited number of energy conversion systems have been proposed to recover the liquefied natural gas cold energy, include the Bryton cycle, the combined cycle, and the Rankine cycle.
The traditional Stirling engine presents a good perspective for enhancing the energy efficiencies for power generators. Unfortunately, the shortcomings of the typical Stirling engines for cold energy applications are founded on the complex mechanical moving parts at non-ambient temperatures that generate more challenges for seals as well as lubrication, therefore causing instabilities for long-term operations.
Thermoacoustic Stirling engine is a unique form of the Stirling engine that has been focused by both the academic and industrial researchers. This engine implements acoustic tubes as opposed to mechanical pistons in order to maintain the proper working conditions for the Stirling cycle. This leads to more reliability, elementary structures, and reduced operational costs as opposed to the typical Stirling engines. Applying the thermoacoustic Stirling engine to generate electric energy by coupling acoustic-electric converters with it is a promising application.
Adequate cold energy can be released from the process of regasification of LNG, and gas turbine generators can use the natural gasas a fuel, and release the waste heat. Therefore, implementing thermoacoustic Stirling engines to recover liquefied natural gas cold energy and the waste heat from the exhausted gases could provide an effective pathway for improving the efficiency of the gas power generators.
Nanyang Technological University researchers in Singapore led by Professor Fei Duan proposed a thermoacoustic Stirling electric generator that can simultaneously recover liquefied natural gas cold energy and low temperature waste heat for the medium-sized liquefied natural gas power generators. The bulky resonator in the traditional generators was replaced with a pair of commercial linear alternators in the proposed system. Their research work is published in peer-reviewed journal, Energy.
The authors performed numerical simulation as guided by the thermoacoustic theory in a bid to characterize and optimize the system’s operations. The impact of the back volume of linear alternators, regenerator length, and feedback tube length on the performance were analyzed. The authors further studied the distribution of important parameters, such as volume flow rate, pressure, acoustic power, exergy flow, and phase difference.
The research team observed that the feedback tube length and the back volume were critical to the output performance of the system. When the back volume was increased from 13.78L to 80L, there was a benefit for both exergy and electric power efficiency. The feedback tube length had a bigger impact on the performance. When the regenerator length was 70mm and the feedback tube length 1.46m, the acoustic field indicated that zero phase difference between volume flow and pressure was realized in the regenerator. By analyzing the exergy flow, the authors indicated that the cryogenic exergy of the liquefied natural gas as well as the exergy from the low-grade heat were the principle driving sources for the thermoacoustic Stirling system.
The optimized system could reach an output power of 2.3kW with the highest exergy efficiency of 0.253 at 4MPa Helium gas within the predetermined low and high temperature. The study by Kai Wang, Swapnil Dubey, Fook Hoong Choo, and Fei Duan provide a constructive guideline for designing such thermoacoustic Stirling electric generators.
Kai Wang, Swapnil Dubey, Fook Hoong Choo, Fei Duan. Thermoacoustic Stirling power generation from LNG cold energy and low-temperature waste heat. Energy, volume 127 (2017), pages 280-290.
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