| Optimizing energy structure and developing new energy are significant methods tosolve the problems of energy shortage and environmental pollution China facednowadays. The way to optimize energy structure is increasing the proportion of cleanenergy (natural gas). China will import a large number of liquefied natural gas (LNG)over the next20years. LNG needs to be re-gasified to natural gas before using, inwhich will release amount of cold exergy. Solar energy is the most original and cleanestenergy among all the new energy. It is obvious that research of utilizing cold enexergyof LNG and low-temperature solar energy is of great significance, which not onlyaccords with the national policy of developing to energy-saving society, but also hasprofound social and strategic meaning.Since thermal efficiency is low for the conventional low-temperature solar organicRankine cycle using water or air as heat sink, moreover, waste of vast cold energy andmarine ecological damage will appear when using sea water to gasify LNG. A combinedcycle utilizing low-temperature solar energy and cold energy of LNG is creativelyproposed in this paper. Then, selecting of working fluids, cycle compare, cycleoptimization and exergoeconomic analysis have been done in this paper. The followingconclusions are obtained:(1) Five objective functions which are cycle system thermal efficiency, exergyefficiency, area of solar collector, volume flow rate of turbine and mass flow rate ofworking fluid are taken into account when selecting the optimum working fluids for thecombined cycle. The results show that R143a, propane and propene which have bettercycle performance than other13fluids are the most suitable working fluids for thecombined cycle. Improvement of cycle performance is obtained after using regenerator,while the increment is different with different working fluids. R116has the mostincrement of cycle performance when using regenerator followed by ethane, ethane andR218, whereas, R143a, propane and propene are also the optimum working fluids forthe combined cycle.(2) Compare between the novel combined cycle and the conventional solar ORCusing water as heat sink and LNG direct expansion cycle is done based on a same network output. The results indicate that the novel combined cycle has better cycleperformance since higher cycle sistem thermal efficiency and exergy efficiency and lower mass flow rate and area of heat exchanger can be obtained.(3) Cycle optimization for the novel combined cycle is done based on geneticalgorithm. Sistem thermal efficiency and exergy efficiency are selected as objectivefunctions. The optimize parameters are evaporative pressure(Pe), condensingtemperature(Tc), vapor pressure(Pn3) and supply pressure(Pn5) of LNG. The results showthat the optimizing evaporative pressure, condensing temperature, vapor pressure andsupply pressureof LNG exist. The optimizing value will change as the change ofworking conditions,whereas, the variation is always among a certain range. The rangeof the optimizing evaporative pressure, condensing temperature, vapor and supplypressure of LNG is about1.4MPa~2.8MPa,193K~208K,3MPa~4.2MPa and0.3MParespectively. Moreover, the working condition points of maximum sistem thermalefficiency(point A) and exergy efficiency(point B) are different. Point A and B areoptimum operating condition for single objective optimization, both of which cannot betaken as the optimum condition for the combined cycle. Through careful analysis, theworking condition (point O) of multi-objective optimization is obtained, thecorresponding valueis: Pe=2.201MPa，Pn3=3.398MPa，Tc=-64.9℃，Pn5=0.305MPa.(4) Since the model and inclined angle of solar collector have great influence to thecombined cycle using solar energy as heat source, analysis for solar collector is alsoconducted. The results show that cycle performance is different with various solarcollectors. Collectorswith larger cut length of efficiency should be selected. Theoptimizinginclined angle of conventional non-concentrating collector change with time,date and address, therefore, solar automatic tracking system is necessary when usingthis type of solar collector. Whereas, CPC(compound parabolic collector) focusingdevice does not need to real-time track solar but adjusts the CPC inclined angleseasonally. Monthly, quarterly, semi-annual and annual best inclined angle and annualincrement of solar radiation are simulated in this paper. The results indicate thatadjusting inclined angle monthly has the largest increment of radiation while quarterly,semi-annual and annual adjustment have a similar increment of radiation.(5) To find the weakest link in the combined cycle, detail exergy analysis is done.The results show that the largest exergy loss appears in the heat exchanger, whichaccounts for75%of total exergy loss, moreover, turbine accounts for25%and theexergy loss of pumps can be ignored. Solar collector is the weakest componentfollowing by condenser and heat exchanger HX. The cause of exergy loss in thecondenser and heat exchanger HX is a considerable difference of energy taste between hot and cold fluid. Reducing heat transfer temperature difference in the condenser andutilizing the surplus cold energy in heat exchanger HX can be used to lessen exergyloss.(6) It is found through economical analyses that cost per unit power generation forthe combined cycle is about0.0504$/kWh as work output of turbine1is100kW. Profitof0.0784$/kWh can be obtained when operating the combined cycle. Moreover, thepayback time is about5.38years. Exergoeconomic analysis is also adopted here. Theresults indicate that the total economic factor is6.21%, which means more than90%ofsystem cost is related to exergy loss. Inducing cycle exergy loss or increasing the capitalcostis a good idea to increase the exergoeconomic performance of the combined cycle.(7) Since thecold energy of LNG still has a certain capability of being used andstability is not high with soalr energy as heat source for the novel combined cycle, thecorrespondingmeasures are proposed to enchance cycle performance in the paper. Theway to enhance cold energy utilizingefficiency is to combine this cycle with air separatesystem and refrigerator, while to avoid the problem caused by the instability of solarenergy, thermal storage device or refueling will be added. |
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