Performance Study Of Ship Waste Heat Recovery System Based On Heat Storage

With the rapid development of economic globalization,ships as the main means of transportation,ship diesel engines consume fossil fuels and produce a large amount of greenhouse gases,but only about half of the energy is converted into output work,and the remaining part is dissipated in the form of heat energy in the surrounding environment.In order to reduce carbon emissions,China has put forward the emission targets of"carbon neutral"and"carbon peaking"to the international community in 2020.In order to accomplish the"double carbon"target,the development of green ships and green navigation in the shipping industry has been the general trend.The high density of supercritical CO₂ is beneficial to the compactness of turbomachinery design.At the same time,CO₂ is easy to obtain,non-polluting,stable physicochemical properties,good heat transfer performance and other characteristics,which helps to promote the development of green ships.The ship waste heat recovery（WHR）system can efficiently recover heat from the main engine exhaust,improve energy utilization and reduce greenhouse gas emissions.Due to the influence of ship sailing conditions,the main engine exhaust heat source varies in phases,which leads to the reduction of the operation efficiency of the waste heat recovery system and even the inability to operate at certain times due to the high volatility of exhaust heat.Therefore,this study proposes a CO₂-based WHR system combined with cascaded latent heat storage（CLTES）,establishes thermodynamic models under design and non-design conditions,and determines the distribution strategy of the exhaust under the system design conditions through theoretical analysis to ensure the efficient and continuous operation of the WHR system throughout the voyage,and evaluates the thermo-economic performance of the proposed system.The main research of this thesis is as follows:（1）A mathematical model of Supercritical CO₂ Brayton Cycle（SCBC）and Transcritical CO₂ Rankine Cycle（TCRC）is established to compare the system performance of the two power cycles under different ship main engine exhaust conditions from the thermodynamic point of view.The results show that for four-stroke ship main engine exhaust waste heat recovery,the SCBC system has higher efficiency in terms of the barnacle;for two-stroke ship main engine exhaust waste heat recovery,the TCRC has better performance.In addition,in order to predict the system performance of WHR system under non-design conditions,non-design models of key system components（turbine,mass pump and heat exchanger）were established and verified the correctness of the models.（2）For ship exhaust waste heat storage,the heat storage performance of sensible heat storage and latent heat storage is compared.In addition,based on entropy optimization theory,cascaded latent heat storage is proposed for overcoming the problem that a single phase change material（PCM）can only store heat at a certain temperature band.Materials with suitable phase change temperatures are selected according to the theoretical melting temperatures of different cascade thermal storage systems,and the performance of secondary,tertiary,and quaternary-CLTES systems is compared.The results show that the sensible heat storage with thermal oil,due to the high heat storage temperature,increases the net output work and heat output by 13.0%and 12.5%,respectively,compared with the tertiary-CLTES system,however,the heat storage volume is 3.8 times higher than the latent heat storage.In the CLTES system,the available energy increases as the number of series PCMs increases,but the degree of enhancement gradually decreases,and the 3-CLTES heat storage system improves the efficiency by 4.3%compared to the 2-CLTES bilinear system,while the 4-CLTES bilinear efficiency improves by only 1%compared to the 3-CLTES.（3）A ship waste heat recovery scheme combining cross-critical CO₂ Rankine cycle and cascade latent heat storage is proposed,and the influence of the main design parameters on the system performance is analyzed.The effect of exhaust temperature,turbine inlet pressure,temperature,condensing temperature and component isentropic efficiency on the system performance was investigated by taking the waste heat source of the cruise ship"Bikal"as the research object.The system design parameters were determined through sensitivity analysis of key design parameters.Based on the actual sailing conditions,the thermal performance of the proposed system in each flight segment was investigated.The results show that there exists an optimal turbine inlet pressure（16MPa）,and the system has the lowest cost per unit of thermal energy at this pressure.Based on the study of the isentropic efficiency of the system components,it is shown that the turbine isentropic efficiency has a greater effect on the thermal and economic performance of the proposed system than the pump isentropic efficiency.From the whole voyage,the total net output work and thermal energy of the proposed system are 5356k Wh and 6958k Wh,respectively,which are 19.1%and 7.9%improved compared with the conventional WHR system and 17.9%reduction in the unit wired energy cost.（4）The NSGA-II optimization algorithm was used to further optimize the designed system after sensitivity analysis with the optimized targets of the efficiency and cost per unit of heat and power,and the results showed that the net output power of the optimized system decreased by 5.2%,but the thermal energy output increased by 20.6%,and the cost per unit of heat and power decreased by 10.9%.Based on the existing ship energy efficiency index（EEXI）analysis,the optimized waste heat recovery system showed that the EEXI was reduced to 14.82 g CO₂/ton·nm.