Cryogenic Exergy Utilization Research Articles

Synergies of fuel cell system thermal management and cryogenic hydrogen exergy utilization

Low-temperature polymer electrolyte fuel cell systems (FCSs) need to reject large amounts of low temperature heat. Often a mobile FCS’s cooling capacity limits the FCS power output. Cryogenic hydrogen is typically utilized as a direct heat sink using heat exchangers (HXs), even though HXs destroy most hydrogen exergy. This paper investigates synergies between FCS thermal management and cryogenic hydrogen exergy utilization in terms of their benchmark performance: the FCS coolant circuit supplies heat at coolant temperature level to a so named reversible cryogenic exergy utilization system (rCEUS) comprised of thermodynamically ideal heat engine processes. The rCEUS converts this heat partly to electrical energy (the value of which equals the hydrogen exergy) and rejects remaining heat to hydrogen to heat it to coolant temperature. The rCEUS output power is used to support the FCS, so the FCS rejects less heat and a significant fraction of this heat is utilized by the rCEUS. As a result, significantly less heat has to be transferred to ambient and the fuel demand decreases. In this paper, three hydrogen storage options are compared: liquid hydrogen, subcooled liquid hydrogen and cryo-compressed hydrogen. Different para- and orthohydrogen compositions are evaluated. For typical FCS operating points, rejected FCS heat to ambient is reducible by 40–67%. FCS power demand is reducible by 14–31%. FCS rejected heat to ambient reduction is 4.5–8 times larger than that of conventional HXs. Calculations are based on hydrogen’s lower heating value.

A novel combined system for LNG cold energy utilization to capture carbon dioxide in the flue gas from the magnesite processing industry

A novel system which include a new concept called cascade nested organic Rankine cycle (ORC) was proposed based on LNG (liquified natural gas) cryogenic exergy utilization. The flue gas from the magnesite processing industry and LNG was used as heat source and heat sink for ORC respectively. Meanwhile CO2 in the flue gas was captured by LNG. The mixed working fluid, regenerator and LNG regasification pressure play a key role in the system. Furthermore, the generating power capacity, the exergy efficiency, the CO2 capture quantity and the cold energy utilization efficiency were calculated as criteria to evaluate the system performance. The results indicate that the selection of suitable mixed working fluid and the installation of regenerator can improve the system performance effectively. Under the given condition, the exergy efficiency of the integrated system, the efficiency of LNG cold energy utilization, the output power of the integrated system and the amount of CO2 capture reaches to 56.9%, 20.81%, 22.59 kWh/tLNG and 0.29tCO2/tLNG respectively. Moreover, as the LNG gasification pressure increases, the exergy efficiency and cold energy utilization efficiency increases, while the CO2 capture quantity and power generation decreases.

Application of the Stirling engine driven with cryogenic exergy of LNG (liquefied natural gas) for the production of electricity

LNG (liquefied natural gas) delivered by means of sea-ships is pressurized and then regasified before its introduction to the system of pipelines. The utilization of cryogenic exergy of LNG for electricity production without combustion of any its portion is analyzed. For the conversion of LNG cryogenic exergy into electricity, the Stirling engine is proposed to be applied. The theoretical thermodynamic model of Stirling engine has been applied. This model is used to investigate the influence of pinch temperature in heat exchangers, engine compression ratio and dead volumes ratios on the thermodynamic parameters of the Stirling engine. The results of simulation represent the input data for investigations of thermodynamic performance of the proposed system.In order to evaluate the thermodynamic performance of the proposed process, an exergy analysis has been applied. The exergy efficiency and influence of design and operational parameters on exergy losses are determined for each of the proposed system configurations. The obtained results represent the background for advanced exergy-based analyses, including thermo-ecological cost.

Utilization of the cryogenic exergy of liquid natural gas (LNG) for the production of electricity

Liquid natural gas (LNG) delivered by means of sea-ships is compressed and then evaporated before its introduction to the system of pipelines. The possibilities of the utilization of cryogenic exergy of LNG for electricity production without any additional combustion of any its portion, have been analyzed. Three variants of the plant have been investigated. A cascade system with two working fluids has been analyzed in two first of them. The economic optimization proved that the optimum temperature difference in the LNG evaporation is higher than initially assumed. Therefore, a third variant of the plant has been analyzed, with ethane as a single working fluid. Only the third variant has been analyzed in detail. The economic optimization of the minimum heat transfer temperature difference (pinch value) in particular heat exchangers has been performed and then the optimum heat transfer area of heat exchangers has been calculated. The influence of the changing ambient temperature on the efficiency of the cold power plant has been analyzed. The own cost of the produced electricity, the annual profit and the pay-back of the plant have been evaluated. The attained reduction of CO2 emission in national economy has been calculated. The analysis of the sensitivity of economical profit on the deviation of economic data has been determined.

A novel Brayton cycle with the integration of liquid hydrogen cryogenic exergy utilization

Stored or transported liquid hydrogen for use in power generation needs to be vaporized before combustion. Much energy was invested in the H 2 liquefaction process, and recovery of as much of this energy as possible in the re-evaporation process will contribute to both the overall energy budget of the hydrogen use process, and to environmental impact reduction. A new gas turbine cycle is proposed with liquefied hydrogen (LH 2) cryogenic exergy utilization. It is a semi-closed recuperative gas turbine cycle with nitrogen as the working fluid. By integration with the liquid H 2 evaporation process, the inlet temperature of the compressor is kept very low, and thus the required compression work could be reduced significantly. Internally fired combustion is employed to allow a very high turbine inlet temperature, and a higher average heat input temperature is achieved also by internal heat recuperation. As a result, the cycle has very attractive thermal performance with a predicted energy efficiency over 73%. The choice of nitrogen as the working fluid is to allow the use of air as the oxidant in the combustor. The oxygen in the air combines with the fuel H 2 to form water, which is easily separated from the N 2 by condensation, leaving the N 2 as the working fluid. The quantity of this working fluid in the system is maintained constant by continuously evacuating from the system the same amount that is introduced with the air. The cycle is environmentally friendly because no CO 2 and other pollutant are emitted. An exergy analysis is conducted to identify the exergy changes in the components and the potential for further system improvement. The biggest exergy loss is found occurring in the LH 2 evaporator due to the relatively high heat transfer temperature difference, dictated by the fixed temperatures of the LH 2 and of the ambient combustion air, which are far apart. The exergy efficiency is 45%. The system has a back-work ratio only 1 4 of that in a Brayton cycle with ambient as the heat sink, and thus can produce 72.7% more work, with the LH 2 cryogenic exergy utilization efficiency of 50%.

A novel near-zero CO 2 emission thermal cycle with LNG cryogenic exergy utilization

A novel liquefied natural gas (LNG) fueled power plant is proposed, which has virtually zero CO 2 and other emissions and a high efficiency. Natural gas is fired in highly enriched oxygen and recycled CO 2 flue gas. The plant operates in a quasi-combined cycle mode with a supercritical CO 2 Rankine-like cycle and a CO 2 Brayton cycle, interconnected by the heat transfer process in the recuperation system. By coupling with the LNG evaporation system as the cycle cold sink, the cycle condensation process can be achieved at a temperature much lower than ambient and high-pressure liquid CO 2 ready for disposal can be withdrawn from the cycle without consuming additional power. The net energy and exergy efficiencies are found to be over 65 and 50%, respectively. In the case computed (but not optimized), the required total heat exchanger area is estimated to be about 460 m 2/MW electricity produced. Besides electricity and condensed CO 2, the byproducts of the plant are H 2O, liquid N 2 and A r.

Proposal and Analysis of a Novel Zero CO2 Emission Cycle With Liquid Natural Gas Cryogenic Exergy Utilization

A novel liquefied natural gas (LNG) fueled power plant is proposed, which has virtually zero CO2 and other emissions and a high efficiency. Natural gas is fired in highly enriched oxygen and recycled CO2 flue gas. The plant operates in a quasi-combined cycle mode with a supercritical CO2 Rankine type cycle and a CO2 Brayton cycle, interconnected by the heat transfer process in the recuperation system. By coupling with the LNG evaporation system as the cycle cold sink, the cycle condensation process can be achieved at a temperature much lower than ambient, and high-pressure liquid CO2 ready for disposal can be withdrawn from the cycle without consuming additional power. Good use of the coldness exergy and internal exergy recovery produced a net energy and exergy efficiencies of a base-case cycle over 65% and 50%, respectively, which can be increased up to 68% and 54% when reheat is used. Cycle variants incorporating reheat, intercooling, and reheat+intercooling, as well as no use of LNG coldness, are also defined and analyzed for comparison. The approximate heat transfer area needed for the different cycle variants is also computed. Besides electricity and condensed CO2, the byproducts of the plant are H2O, liquid N2 and Ar.

Novel cogeneration power system with liquefied natural gas (LNG) cryogenic exergy utilization

We propose a new cogeneration power system with two energy sources of fuel chemical energy and liquefied natural gas (LNG) cryogenic energy, and two outputs of electrical power and cooling power. Due to the advanced integration of system and cascade utilization of LNG cryogenic energy, the system has excellent energy saving: chemical energy of fuel and LNG cryogenic energy are saved by 7.5–12.2% and 13.2–14.3%, respectively. As CO 2 is selected as working fluid and oxygen as fuel oxidizer, CO 2 is easily recovered as a liquid with LNG vaporization. In this paper, the typical recuperative Rankine cycle and the corresponding cogeneration system are described and a detailed thermodynamic analysis is carried out to reveal the principle of the cycle and system. Furthermore, the influence of key parameters on performance is discussed. Considering the engineering application, the technical advantages and concerns are pointed out.

Performance analysis of liquid hydrogen fueled supersonic aircraft engines. On the utilization of cryogenic exergy of liquid hydrogen.

In supersonic aircraft engines with higher flight Mach number, the temperature of suction air is raised considerably by aerodynamic heating, which causes the degradation of engine performances. In this report, a precool engine cycle is proposed in which a precooler is installed in the upstream side of the compressor. A new evaluation method and the optimum structure of the precooler were proposed. Then the relationship among the effect of suction air cooling, flight Mach number and altitude was made clear. With the turbojet engine equipped with a precooler, suction air can be cooled by liquid fuel hydrogen of very low temperature. Hydrogen is, at the same time, evaporated and heated. An ideal precooling cycle was devised, and the effect of suction air cooling on the therm000efficiency was evaluated. The cycle with suction air cooling was applied to an actual engine and its effect was evaluated. As the result, it became clear that the addition of a precooler decreased fuel consumption, and the precooling was less effective with the increase in the altitude. (12 figs, 1 tab, 9 refs)