Development of a cascaded latent heat storage system for parabolic trough solar thermal power generation

摘要

Concentrated solar power (CSP) has the potential of fulfilling the world’selectricity needs. Parabolic-trough system using synthetic oil as the HTF withoperating temperature between 300 and 400o C, is the most matured CSPtechnology. A thermal storage system is required for the stable and costeffective operation of CSP plants. The current storage technology is the indirecttwo-tank system which is expensive and has high energy consumption due tothe need to prevent the storage material from freezing. Latent heat storage(LHS) systems offer higher storage density translating into smaller storage sizeand higher performance but suitable phase change materials (PCMs) have lowthermal conductivity, thus hindering the realization of their potential. The lowthermal conductivity can be solved by heat transfer enhancement in the PCM.There is also lack of suitable commercially-available PCMs to cover theoperating temperature range. In this study, a hybrid cascaded storage system(HCSS) consisting of a cascaded finned LHS and a high temperature sensibleor concrete tube register (CTR) stages was proposed and analysed viamodelling and simulation. Fluent CFD code and the Dymola simulationenvironment were employed.A validated CFD phase change model was used in determining the heattransfer characteristics during charging and discharging of a finned and unfinnedLHS shell-and-tube storage element. The effects of various finconfigurations were investigated and heat transfer coefficients that can be usedfor predicting the performance of the system were obtained. A model of theHCSS was then developed in the Dymola simulation environment. Simulationswere conducted considering the required boundary conditions of the system todevelop the best design of a system having a capacity of 875 MWhth, equivalentto 6 hours of full load operation of a 50 MWe power plant.The cascaded finned LHS section provided ~46% of the entire HCSS capacity.The HCSS and cascaded finned LHS section have volumetric specificcapacities 9.3% and 54% greater than that of the two-tank system, respectively.It has been estimated that the capital cost of the system is ~12% greater thanthat of the two-tank system. Considering that the passive HCSS has loweroperational and maintenance costs it will be more cost effective than the twotanksystem considering the life cycle of the system. There is no requirement ofkeeping the storage material above its melting temperature always. The HCSShas also the potential of even lower capital cost at higher capacities (>6 hoursof full load operation).