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.

A fast and robust computational model of a spiral-wound vacuum-enhanced air gap membrane distillation (V-AGMD) module at the pilot-scale is proposed and implemented. In contrast with data-driven models available in the literature, a physics-based approach is adopted for more reliable generalization beyond the validation dataset. A total of 86 experimental results, of which 41 are described in this work and 45 come from independent sources available in the literature, are used in the validation effort with quite favorable results. With the confidence on the robustness of the methodology due to the wide range of operational parameters and the use of spiral-wound modules of four different sizes included in the validation comparisons, a physical analysis is conducted varying the air gap pressure, the number of feedwater channels, the feedwater flow rate, and the membrane area. Improvements of up to 60% in both water productivity and energy efficiency can be achieved by intensifying the vacuum in the air gap or decreasing the number of feedwater channels. These parameters achieve performance gains due to less resistance in the air gap for vapor to migrate through it, in the former case, and a reduced temperature polarization effect, in the latter case. Smaller flow rates favor energy efficiency at the expense of water productivity by simultaneously decreasing transport-phenomena-related irreversibility and the partial pressure difference across the membrane and the air gap. In addition, this tradeoff between energy efficiency and driving force is shown to lead to an optimum value for the membrane area beyond which the permeate flow rate through the membrane starts to fall due to the small driving force. An illustrative case is predicted to achieve energy efficiency metrics, such as a gain-output ratio of 12.7, competitive with multi-effect distillation.

This study focused on evaluating the energy performance and economic viability of a direct expansion photovoltaic-Thermal Heat Pump (PV-T/HP) system for residential heating applications in various regions of Brazil. A detailed mathematical model was implemented to compare the PV-T/HP system with other solar heating technologies, as photovoltaic with solar evacuated tubes collector (PV+TSC) and photovoltaic with electrical heat water (PV+EHW), analyzing key metrics such as electrical efficiency, thermal efficiency, and levelized cost of heat (LCOH). The study revealed variations ranging from 7.5 to 2.5 in the Coefficient of Performance (COP) of the heat pump under different solar irradiation and ambient temperature conditions, emphasizing the necessity for future optimization to enhance the system performance. Results indicated that cities with the lower ambient temperature such as Belo Horizonte and Porto Alegre exhibited favorable conditions for the PV-T/HP system, leading to increased energy generation and positive net electricity production. The study highlighted the need for tailored strategies to control system operations, limit compressor consumption, and improve overall efficiency. These findings contribute to the understanding of PV-T/HP systems in residential heating applications, showcasing their potential for sustainable energy solutions in Brazil.

An experimental investigation was carried out to study the convection heat transfer and pressure drop of oil-based multi-walled carbon nanotube nanofluids. The necessary data were acquired for laminar flow in the thermal entrance region. Nanofluids were produced at different mass concentrations (0.005

Single-walled carbon nanotube–water nanofluids were tested in a 1–5 TEMA E shell and coil heat exchanger. Cold nanofluid, flowing inside the coil, was heated by hot water flowing in the shell side. Volumetric fraction of nanoparticles, inlet temperature of nanofluid, and mass flow rate of nanofluids ranged from 0 to 0.21%, 2.3 to 23.4 °C, and 40 to 90 g/s, respectively. For a given Reynolds number, at the coil side, pure base fluid (φ = 0%) performed better than low-concentration nanofluid samples (φ = 0.035% and 0.053%) and was nearly equivalent to the nanofluid of highest concentration, φ = 0.21%. The thermal conductivity enhancement factor of the nanofluid ranged from 0 to 0.2 and to 0.45, at inlet temperatures of 30 °C and 50 °C, respectively. It is believed to work in favor of a better performance of the nanofluid samples. On the other hand, the unusual (literature-wise) low temperature of the nanofluid further amplified the enhancement of the nanofluid viscosity, with a reduction effect on the Reynolds number. Besides, other thermal resistances of the heat exchanger work toward an attenuation of the enhancement effect that nanoparticles may have in the heat exchanger performance.

This work is concerned with a comparison of measured and theoretical thermophysical properties of different nanofluids and using the properties to evaluate their suitability as a secondary working fluid in refrigeration systems, aiming at the external thermodynamic losses. Single-Walled Carbon Nanotubes (SWCNT) and silver nanoparticles were dispersed in distilled water to produce the nanofluid samples. After that and by using the regression, a neural network (NN), an adaptive neuro-fuzzy inference system (ANFIS) and through obtained experimental data, new correlations were extracted to determine the thermophysical properties of nanofluids. Then, a numerical simulation was performed taking into account a mathematical model for thermodynamic optimization of a double-pipe heat exchanger. A comparison between the results obtained by the measured data or those which is obtained by soft-computing techniques and those evaluated theoretically by previous available models were carried out, showing a notable difference among the results. The results obtained with the experimental or soft-computing data show that in the case that the heat exchanger is optimized to minimize the external entropy generation, silver nanofluids (at any volumetric concentration) and the SWCNT nanofluids (at lower concentrations) presented positive results, whereas using the former existing theoretical data indicated that silver nanofluids showed negative results and SWCNT nanofluids presented positive results in all volumetric concentrations. (C) 2019 Elsevier Ltd. All rights reserved.

The development of nanofluids is extremely important for improving the thermal conductivity of the base fluids. One of the ways to improve the safety of nuclear power plants is related to improving their heat transfer capacity. The study of new fluids that improve the rate of removal of heat is fundamental to obtain greater efficiency of energy systems. Among the several factors that compromise the efficiency of energy systems, it is possible to highlight the thermophysical limitations of conventional fluids. These limitations inhibit, quite significantly, some industrial applications. This paper presents a work proposal to be carried out in the Thermo-Hydraulic Laboratory of the CDTN, whose objective is the study of the improvement of the heat transfer characteristics of the refrigerant, used in the primary circuit of nuclear reactors, through the addition of nanoparticles. Carbon nanotubes will be used mainly in light water, which is the most commonly used cooling fluid for nuclear reactors. As support to the work, an experimental workbench will be designed, assembled and calibrated in order to study the thermo-hydraulic behavior of these components.

SWCNT-water (single walled carbon nanotube) nanofluid was tested as a secondary fluid for a 4-9 kW indirect vapor compression refrigeration system. The evaporator, with boiling refrigerant HCFC-22 extracting heat from the nanofluid, was of the brazed plate counter-flow type. A semi-hermetic compressor, an electronic expansion valve (EEV) and an air-cooled condenser were the other main components of the refrigeration cycle. Tests were carried out with the experimental apparatus operating over a range of different volumetric fractions of nanoparticles (0-0.21%) as well as nanofluid inlet temperatures (30-40 degrees C) and mass flow rates (40-80 g/s). Overall, the performance of the system working with nanofluid as a secondary fluid was superior to that where just the base fluid (i.e., pure water) circulated in the secondary fluid loop, at the same mass flow rate and inlet temperature. The enhanced thermal conductivity of the nanofluid is believed to be the main reason why the refrigeration system with the nanofluid loop, if compared to that with pure water, presented a higher refrigerating capacity. (C) 2016 Published by Elsevier Ltd.