Plans for recovering LNG cold energy have become an important focus in the LNG industry and show great potential for increasing energy efficiency and reducing carbon emission. Standard LNG terminals regasify LNG from seawater or air, wasting a huge amount of cold energy and also posing a threat to marine ecology.
A research team at Nanyang Technological University (NTU) has proposed and demonstrated an effective approach for LNG power generation through a combined power generation system. “Our studies show that small-scale LNG cold utilization systems can be a sustainable solution in the LNG trade,” says Prof. Fei Duan, the team leader and principal investigator for the project, “especially for the inland and stranded markets where the pipeline natural gas method or the large-scale LNG regasification plants are not economically feasible.”
The group’s idea is to simultaneously recover the untapped cold energy released by the LNG regasification and the exhaust heat from a gas turbine to generate extra electricity through a Stirling engine . “We hope to provide an efficient and reliable solution for LNG power generation through the design, development and integration of cutting-edge energy technologies, including a micro gas turbine, a novel Stirling engine, an efficient LNG regasification unit and phase change material (PCM) thermal energy storage,” says Duan. In the combined cycle, LNG cold energy and waste heat from the exhaust gas of a microgas turbine serve as the cold sink and heat source for the Stirling engine, respectively.
The Stirling engine is the key subsystem for recovering the cold energy from LNG and the waste heat from the exhaust gas of the gas turbine. The researchers assessed the feasibility of different types of Stirling engines for this special application and concluded that conventional Stirling engines were not a good option due to the high requirement of the reciprocating well-sealed pistons at both cryogenic and high temperatures—a challenge for manufacturing precision, assembly accuracy and oil lubrication.
“We proposed and developed a special Stirling power generator  for the project,” says Dr. Kai Wang, the group’s main researcher. “Its thermoacoustic Stirling engine employs an acoustic wave instead of pistons to complete the fluid flow and heat transfer processes for energy conversion. It does not have any high-precision moving parts at either cryogenic or high temperature regions, so requirements for manufacturing precision and assembly accuracy are much lower, and oil lubrication is totally unnecessary.”
In conventional thermoacoustic Stirling engines, a long and bulky resonator is typically necessary to provide the resonance for acoustic oscillation. All of the previous thermoacoustic Stirling power generators were designed for utilizing thermal energy above room temperature, most of which requires a driving temperature of 400 to 700ºC.
The NTU thermoacoustic Stirling power generator, however, is capable of recovering both cold energy and waste heat simultaneously with a much more compact structure. The design uses a pair of linear alternators, directly coupled with the thermoacoustic energy conversion loop, to replace the bulky gas resonator.The team has completed the fabrication, integration and tests of the thermo-acoustic Stirling power generator with a 30-kW microgas turbine and related energy recovery units at NTU’s Energy Systems Laboratory. Exhaust heat from the microgas turbine is collected through a waste heat recovery unit and delivered to the hot end of the thermoacoustic Stirling power generator through thermal oil. The selected molten salt—as PCM—is integrated in the thermal oil loop for thermal energy storage and stabilization. Liquid nitrogen is used as the cold source for the thermoacoustic Stirling power generator while compressed natural gas is supplied to the microgas turbine as fuel in the prototype combined system, as the retail supply of LNG is not available in Singapore at this stage.
The prototype thermoacoustic Stirling power generator oscillated successfully at the designed frequency of 55 Hz with helium gas at 30 bar. The engine stably generated a maximum output mechanical power of 3 kW and electric power of 1 kW at 295ºC. Instant mechanical and electric powers reached up to 3.4 kW and 1.2 kW, respectively.
“We were excited to achieve this result with the proof-of-concept engine, since it is the first prototype ever developed in the world with this unique design for LNG power generation,” says Wang. “It proves that our thermoacoustic concept is technically feasible for recovering cold energy and waste heat simultaneously. And we believe that further optimizations, reached by solving the detailed problems we met in the prototype, will lead to significant improvements in an updated version.”
Based on the test results, a thermodynamic assessment of the combined system showed a promising performance improvement over conventional LNG power generation system. Extra electric power, at a range up to 10 percent, was generated from the waste energy when the microgas turbine was operated at different power levels. It demonstrated the technical feasibility and possibility of increasing the energy efficiency for LNG power generation applications by using the proposed approach. The thermoeconomic analysis indicated that the combined system is more profitable than the single power generation system.
“This innovative idea could enable on-site power generation at the LNG terminal and off-site power generation where LNG or the other cryogenic fluid is available,” Duan says, “as the readily available cold energy and waste heat allow the optimum conversion of heat energy to mechanical energy necessary for power generation. This encourages the efficient use of energy and reduces the amount of energy drawn from the grid and operating costs for the end users.”
Singapore’s Energy Market Authority administered and funded the project through its National Research Foundation Energy Innovation Research Program. Additional researchers included Dr. Swapnil Dubey and Prof. Fook Hoong Choo, who were co-PIs on the project; Prof. Seth Sanders, who was the collaborator; Dr. Lu Qiu, who worked on the gas turbine and efficient heat exchanger study; Dr. Chenzhen Ji and Zhen Qin, who contributed to PCM thermal energy storage; and Kanbur Baris Burak, who conducted the thermoeconomic assessments.
 Fei Duan, Swapnil Dubey, Fook Hoong Choo, Lu Qiu, Kai Wang, Power Generation System and Method. PCT Application No.: PCT/SG2016/050446, US Application No.: 15/759,826.
 Kai Wang, Swapnil Dubey, Fook Hoong Choo, Fei Duan, “Thermoacoustic Stirling power generation from LNG cold energy and low-temperature waste heat,” Energy, Vol.127, 2017. ■