Transient Thermodynamic, Sensitivity and Multi-Objective Design Analysis of a Multilayer PCM-Assisted Evacuated Tube Solar Collector

Transient thermodynamic analysis of a multilayer phase-change material (PCM)–assisted evacuated tube solar collector (PCM–ETSC) is presented. A compact enthalpy-based numerical model is formulated to capture coupled heat transfer among the absorber tube, a mixed-mean heat-transfer fluid (HTF) control volume, and multiple PCM layers with staggered melting temperatures under time-dependent irradiance. Performance is evaluated using solar-referenced thermal and exergy efficiencies and reported over a sunlit window defined by G (t) > 0.1 Gmax. For the baseline run, the simulated temperature ranges are Tf∈[298.000,306.489] and Tt∈[298.000,310.432] K with PCM, compared with Tf∈[298.000,306.774] and Tt∈[298.000,310.848] K without PCM, corresponding to reductions in peak HTF and absorber temperatures of 0.284 and 0.416 K, respectively. The sunlit-window means are η¯solar = 0.8788 (87.88%) and ψ¯solar = 0.0098 (0.98%), while the maximum instantaneous solar exergy efficiency reaches 0.0133. A time-step refinement from Δt=0.2to0.1s leaves η¯solar unchanged to six decimal places (0.878817 vs. 0.878816) and reduces the RMS global energy-balance residual from 0.086164 to 0.048758 J per step, supporting numerical consistency. Multilayer phase change is evidenced by layer-resolved liquid-fraction histories; using fl≥0.99 as a completion metric, none of the three PCM layers reach completion within the 6 h horizon for the reported baseline (peak temperatures remain below the threshold implied by Tm,k and the mushy band). One-at-a-time sensitivity studies quantify the influence of HTF mass flow rate, uniform PCM thickness, and a common melting-temperature shift. Multi-objective optimization using NSGA-II (MATLAB gamultiobj) maps the trade-off between η¯solar∈[0.780,0.908] and ψ¯solar∈[0.007,0.018], with representative thermal-oriented, exergy-oriented, and compromise designs reported explicitly. The results show that first-law and second-law metrics diverge under transient charging and that multilayer PCM integration can buffer peak temperatures while enabling design trade-offs between delivered heat quantity and quality.