Modeling and experimental validation of an indirect active hybrid solar dryer for mint leaves: Coupled thermal and moisture-transfer analysis

Design and Experimental Study of an Indirect Forced Convection PV/T Solar Dryer in a Humid Tropical Zone

Ginger is highly perishable because of its high moisture content, and conventional sun drying is often constrained by variable weather conditions, contamination, and prolonged drying periods. Indirect forced-convection photovoltaic/thermal solar dryers offer a promising approach by simultaneously supplying thermal and electrical energy for controlled drying in humid tropical environments. This study presents the design, construction, and experimental evaluation of an indirect forced-convection photovoltaic/thermal (PV/T) solar dryer for ginger under humid tropical conditions. The system integrates a 150 W PV module, an air-heating section, a drying chamber with three trays, and a fan powered by the photovoltaic component. Experiments were conducted using a 2 kg batch of ginger while monitoring solar irradiance, ambient and collector temperatures, relative humidity, air velocity, sample mass, and drying time. The influence of airflow was assessed at volumetric flow rates of 7, 5, and 3 m³/h, corresponding to air velocities of 2.8, 2.0, and 1.2 m/s, respectively. Lower airflow increased the temperature rise across the PV/T collector and improved thermal performance. At 1.2 m/s, the air-temperature increase between the collector inlet and outlet reached approximately 10°C, the maximum collector-outlet temperature was 54.2°C, the maximum PV-module surface temperature was 62.44°C, and thermal efficiency reached 18.28%. Ginger on the tray nearest the hot-air inlet reached the target moisture content after 380 minutes, whereas the upper tray required 620 minutes. The results show that airflow rate and tray position influenced heat transfer, drying rate, and drying time. The developed PV/T dryer provided simultaneous electrical and thermal energy for forced-convection drying, while the observed differences among trays indicate a need for improved airflow distribution.

Influence of climatic conditions on moisture storage in bio-based multilayer walls: a coupled heat and moisture transfer study

Investigation of the Heating Performance of a Solar-Assisted Inter-Cooling Air Source Heat Pump System

This paper proposes a novel solar-assisted inter-cooling air source heat pump (SCAS–HP) system that integrates a solar collector/evaporator into the intermediate injection branch to utilize solar energy for increasing the refrigerant flow rate at intermediate pressure. A numerical model of the system was developed and validated against the literature data, and then employed to investigate performance under varying operating conditions. Three configurations were examined—two-stage throttling (T–SCAS–HP), single-stage throttling (S–SCAS–HP), and parallel evaporators (P–SCAS–HP)—and their performance was compared with a conventional vapor injection air source heat pump (VI–ASHP) across solar radiation intensities of 10–1000 W/m2, outdoor air temperatures of −20 °C to 10 °C, and outlet water temperatures of 35 °C and 55 °C. Hourly performance, economics, and CO2 reduction were further assessed for Lhasa, Beijing, and Harbin. The results show that the COPh improvement of SCAS–HP over VI–ASHP rises with increasing solar radiation intensity. Furthermore, the solar radiation threshold at which the COPh of SCAS–HP begins to exceed that of VI–ASHP lies within the range of 100–200 W/m2. Moreover, the COPh increase rate of SCAS–HP rises with larger solar collector area and higher outdoor air temperature, but declines as the outlet water temperature rises. Among the three configurations, T–SCAS–HP outperformed S–SCAS–HP and P–SCAS–HP by 0.37 and 0.42 in COPh at outlet water temperatures of 35 °C and 55 °C, respectively. In typical-day simulations, the COPh showed a relative increase of 50.9% compared to that of the VI-ASHP system, corresponding to an absolute rise from 3.73 to 5.62 with a 130 m2 collector at 35 °C outlet water temperature. The energy saving rate ranked highest in Lhasa, followed by Beijing and Harbin. The shortest payback period (5.6 years) was achieved in Harbin with a 130 m2 collector area at the outlet water temperature of 55 °C. The proposed system is particularly suitable for large heating demand and long heating seasons in cold climates with abundant solar resources.

Experimental Investigation and CFD Modeling of Heat and Mass Transfer During Drying of Alfalfa Leaf Fraction in a Rotary Drum Dryer

The convective drying of agricultural materials is an energy-intensive process, and optimizing dryer design is critical for improving efficiency and product quality. This study presents a comprehensive heat and mass transfer model for the convective drying of alfalfa leaves in a rotary drum dryer. Freshly harvested leaves with an initial moisture content of approximately 70% (w.b.) were used as the test material. The proposed system features a simplified drum design aimed at enhancing process efficiency while reducing equipment complexity. The primary objective was to reduce the moisture content of alfalfa leaves to below 50% to ensure their quality during subsequent storage and transportation. To determine the optimal operating conditions, the kinematics of leaf motion inside the rotating drum and the associated heat and mass transfer phenomena were investigated through analytical modeling, numerical simulation, and experimental studies on a laboratory-scale physical model. An analytical model was developed to establish relationships between transverse kinematic characteristics (detachment condition, Froude number, drum inclination angle), average longitudinal velocity, and residence time. Numerical simulations based on the Navier–Stokes equations (continuity, momentum, and energy) provided detailed moisture content distributions within individual leaves under varying airflow orientations and drying durations. The novelty of this work lies in the integrated determination of optimized operating parameters through combined analytical, numerical, and experimental approaches. A regression model relating final moisture content to key process variables (air velocity, temperature of 60 °C, drum rotation frequency and mass of loaded material) was developed from experimental data, yielding practical recommendations for the design and operation of rotary drum dryers for alfalfa and similar agricultural materials.

Modeling the performance of a solar pond under Tripoli's climatic conditions, with an advanced physical model.

Solar ponds offer an effective solution for long-term solar energy harvesting and thermal storage, particularly in arid and semi-arid regions. This paper presents a numerical simulation framework for a solar pond with a salinity gradient, subjected to controlled thermal energy extraction rates of 10, 20, and 30 W/m² from the lower convection zone. The study aims to evaluate the operational feasibility of sustainable heat removal while maintaining the thermal gradient, salinity stability, and overall hydrodynamic integrity of the pond. In addition to this conventional thermal extraction, an advanced physical model is presented as a visualization, not incorporated into the current numerical simulation process. This model, intended to serve as a basis for future studies, integrates the solar pond with a dedicated flash evaporator chamber for low-pressure steam generation. The thermal energy extracted from the lower convection zone is transferred to the evaporator chamber, where saline or brackish water undergoes controlled flash evaporation to produce steam suitable for thermal applications or for generating electricity and desalinated water at low temperatures. The concentrated brine produced from the evaporation process is managed to minimize freshwater loss and control salt concentration gradients. A key innovation of the proposed system is a semi-closed salt management cycle, where salt depleted from the non-thermal zone (NCZ) due to diffusion and operational disturbances is continuously replenished through salt recovery and recycling from the evaporator unit. This approach significantly reduces external salt demand while maintaining the critical salinity gradient necessary for long-term pond stability. The numerical model incorporates interrelated constraints for heat transfer, mass diffusion, and buoyancy-driven flow, providing a scalable platform for detailed calculations and future optimization studies. Simulation results demonstrate the feasibility of maintaining heat extraction of up to 30 W/m² without destabilizing the NCZ, provided that adaptive salt recycling strategies and controlled salt withdrawal are implemented. The proposed configuration provides a consistent and scalable physical basis for modeling the integration of solar ponds with evaporative steam production, offering a promising pathway toward sustainable, low-cost thermal energy systems with improved resource efficiency.

Performance Evaluation of a Heat Pump System with Dual Condensers for Drying Herbs

A dual condenser heat pump dryer (HPD) was utilised for herb drying, demonstrating its capability to provide a cost-effective drying solution via economical energy consumption, expedited drying processes, and improved system coefficient of performance (COP). The drying properties, exergy energy, and techno-economic feasibility were comprehensively evaluated. The HPD employed refrigerant R-32, operating within a temperature range of 6 °C for the evaporator and 47 °C for the condenser to dry kesum and pandan leaves. The experimental results demonstrate that the dryer required a modest amount of electricity, 0.012 and 0.143 kW/kg of fresh kesum and pandan, respectively. The dryer effectively removed moisture during its passage through the dual condensers and evaporator, yielding 16.85 kg and 3.39 kg of condensate water from the initial mass of 50 kg and 5.5 kg, respectively. The specific moisture extraction rate (SMER) for the HPD mode was calculated at 11.23 and 2.26 kg/ kWh, and the average COP was 5.7 and 5.34 for kesum and pandan, respectively. The dual condenser improved the COP by approximately 11%. A maximum dryer productivity of about 0.75 kg/m²h is obtained at the air with a temperature of 47°C, velocity of 2.32 m/s and dryer surface load of 3.39 kg/m². The exergy efficiency of the thorough drying period varied from 56.85% to 96.33%. Moreover, the economic analysis determined a payback period of 3.24 months and 6.09 months for drying 24,000 kg and 2,640 kg of kesum and pandan leaves annually, with calculated annual incomes of 147,822.48 and 27,046.80 USD, respectively.