Carolina Palma Naveira Cotta

High-Concentration Photovoltaic (HCPV) systems operate under high heat fluxes, on the order of MW/m², requiring effective thermal management. Experimental studies of minichannel cooling for HCPV seldom extend from the laboratory to complete modules operating outdoors. This study experimentally evaluates seven water-cooled minichannel heat exchangers at three scales: a laboratory setup with electrical cartridge heating, outdoor single-cell monomodules, and complete 25-cell modules. Six were micromachined in aluminum, and one was manufactured in a cobalt–chromium–tungsten (CoCrW) alloy by Direct Metal Laser Sintering (DMLS). The total thermal resistance ranged from 0.24 to 0.84 °C/W, while pressure drops remained below 2 kPa at 60 mL/min. An empirical Nusselt number correlation for the tested straight-channel heat exchangers (22 ≤ Re ≤ 107) predicted 96% of the experimental data within ±30%. Under outdoor operation, the two modules, one equipped with the aluminum heat exchangers and the other with the CoCrW heat exchangers, achieved electrical efficiencies of 23.02% and 22.01% and combined efficiencies of 43.62% and 42.33%, respectively, with cell temperatures between 40 and 50 °C. The exergy content of the recovered thermal energy was 2–3%, so the module exergy efficiency of approximately 25% was determined mainly by the electrical output. Overall, the results demonstrate the potential of minichannel heat exchangers for simultaneous thermal and electrical energy recovery in HCPV modules.

The physics and modelling of cooling and freezing of droplets in contact with a colder substrate are of interest in various engineering applications. This work provides experimental results of this process employing infrared thermography for temperature measurements at the droplet’s surface. Also, a high-speed camera is employed to observe the recalescence period and measure the freezing front movement and the droplet shape change. Three substrates are prepared with distinct wettability ranges, i.e. one hydrophilic and two hydrophobic surfaces. From the experimental observation of a solidification front parallel to the substrate plane, a mixed lumped-differential model of the heat transfer process based on the Coupled Integral Equations Approach is proposed, reformulating the two-dimensional partial differential formulation in cylindrical coordinates into a one-dimensional transient energy equation for the droplet external surface temperature. Direct comparisons of the experimental and theoretical results for the supercooling period show excellent agreement for the droplet surface temperatures at different heights and for different values of the substrate–droplet contact angle. It is also shown that the classical partial lumped system analysis does not provide adequate predictions in the present problem. Finally, the dynamics of the recalescence and freezing stages are experimentally evaluated and physically interpreted. This article is part of the theme issue ‘Heat and mass transfer in frost and ice’.

Membrane distillation (MD) is an evolving thermal separation technique most frequently aimed at water desalination, compatible with low-grade heat sources such as waste heat from thermal engines, solar collectors, and high-concentration photovoltaic panels. This study presents a comprehensive theoretical–experimental evaluation of three commercial membranes of different materials (PE, PVDF, and PTFE), tested for two distinct MD modules—a Direct Contact Membrane Distillation (DCMD) module and an Air Gap Membrane Distillation (AGMD) module—analyzing the impact of key operational parameters on the performance of the individual membranes in each configuration. The results showed that increasing the feed saline concentration from 7 g/L to 70 g/L led to distillate flux reductions of 12.2% in the DCMD module and 42.9% in the AGMD one, averaged over the whole set of experiments. The increase in feed temperature from 65 °C to 85 °C resulted in distillate fluxes up to 2.36 times higher in the DCMD module and 2.70 times higher in the AGMD one. The PE-made membrane demonstrated the highest distillate fluxes, while the PVDF and PTFE membranes exhibited superior performance under high-salinity conditions in the AGMD module. Membranes with high contact angles, such as PTFE with 143.4°, performed better under high salinity conditions. Variations in operational parameters, such as flow rate and temperature, markedly affect the temperature and concentration polarization effects. The analyses underscored the necessity of a careful selection of membrane type for each distillation configuration by the specific characteristics of the process and its operational conditions. In addition to experimental findings, the proposed heat and mass transfer-reduced model showed good agreement with experimental data, with deviations within ±15%, effectively capturing the influence of operational parameters. Theoretical predictions showed good agreement with experimental data, confirming the model’s validity, which can be applied to optimization methodologies to improve the membrane distillation process.

Membrane distillation (MD) has emerged as a promising thermal separation technology aimed mainly at water desalination, compatible with low-grade heat sources such as waste heat from thermal engines, solar collectors, and high concentration photovoltaic panels. This study presents a comprehensive theoretical-experimental evaluation of direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD), focusing on the influence of different membrane materials (PVDF, PTFE, and PE) and operational parameters on performance. Ex-periments were conducted in a flat-sheet MD module under varying NaCl concentrations (7 to 70 g/L), feed temperatures (65–85°C), and flow rates (0.2–0.8 L/min). The DCMD configuration achieved significantly higher permeate flux, with PTFE membranes achieving up to 33.59 kg/m²h at 85°C, attributed to the high hydrophobicity (contact angle: 143.4°) and porosity (85%). Con-versely, AGMD exhibited lower distillate fluxes (maximum 24.77 kg/m²h with PE) but outper-formed DCMD in specific energy consumption (SEC), achieving values as low as 1390.14 kWh/m³ with PTFE at 85°C, in a setup without any heat recovery from either the feed or distillate streams. The thermal efficiency of AGMD reached 0.54, 17% higher than DCMD with the same membrane. Salinity impacted performance differently: PTFE exhibited consistent flux across NaCl concen-trations, while PE showed a reduction of 31.3% at 70 g/L. The proposed heat and mass transfer reduced model showed good agreement with experimental data, with deviations within ±15%, effectively capturing the influence of operational parameters. These findings underscore the trade-offs between DCMD's higher fluxes and AGMD's higher energy efficiencies, offering quantitative insights to guide the design and optimization of MD systems for desalination and wastewater treatment.