LNG cold energy utilization: Prospects and challenges

Exergy analysis of solar thermal collectors and processes

In the present paper, a review is carried out of the current state of thermodynamic cycles that exploit the exergy released during liquefied natural gas (LNG) regasification, with the objective of improving power plant efficiency. A study of the exergy available in the LNG is carried out and is divided in thermal and mechanical exergy. The mechanical aspect can only be recovered through the direct expansion of LNG. As for the thermal aspect, power plants based on Rankine, Brayton, and Kalina cycles, as well as combined cycles and cycles with CO2 capture are evaluated. The choice of proper cycle for a better exergetic use is linked with the quality of the heat source of the cycle. The Rankine cycle is most suitable when of low grade, while the Brayton is more appropriate when disposing of a medium or high grade heat source. When the temperature of the heat source is high, the combination of Brayton, Rankine and direct expansion is most favourable in order to obtain good efficiency. Also established is the selection criteria of the working fluids. Ethane and ethylene are put forth as the most suitable to operate with low temperature Rankine cycles and CO2 with high temperatures. In the case of Brayton cycles, helium and nitrogen are those recommended.

Review of thermal cycles exploiting the exergy of liquefied natural gas in the regasification process

Natural gas is one of the cleanest forms of fossil fuels and its usage is rising since the importance of environment-friendly conversion systems has been seen Liquefied natural gas (LNG) and pipeline technologies are two present methods for the natural gas transportation between the exporter and importer This review presents the LNG cold utilization systems which are discussed on the applications such as separation processes, cold food storage, cryogenic carbon dioxide (CO 2 ) capture, power generation, etc The minimum power consumption and flow rate are detected as 045 kWh/kg and 8 ton/hour, respectively for the air separation processes Exergetic and energetic efficiencies are increased by nearly 40% of those of the conventional separation processes In the desalination system, it is inferred that the LNG cold utilization has provided nearly 50% energy saving while 2 kg ice melt water is produced by 1 kg equivalent LNG cold energy in the freeze seawater desalination For the cold storage systems, payback periods are found less than 5 years by using the LNG cold energy Energetic and exergetic efficiency are increased significantly by using the LNG cryogenic exergy Moreover, it is used in the cryogenic CO 2 capture systems, and there is no dramatic decrement in the overall efficiency values while the other CO 2 methods decrease the overall efficiency The LNG cold utilized power generation systems are discussed in detail with different alternatives including the gas, steam, combined and Stirling systems The minimum thermal efficiencies are found nearly 40% and 20% for the gas and steam cycles The selection of working fluids in the cycles is also discussed Lastly, the importance of economic and environmental aspects of LNG cold utilization systems is discussed Basic modeling equations are presented for the designed LNG cold utilization cycle with respect to the thermoeconomic and exergoenvironmental methods

Wen-Ji Rao

Papers

A combined cycle utilizing LNG and low-temperature solar energy