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In this study, a solar dryer incorporating a flat plate heat exchanger to recover heat from exhaust air was investigated to explore the usability of different types of collectors in the system. Their usability was further evaluated through economic and environmental analyses. Moreover, the thermodynamic performance of the solar dryers was evaluated under two weather conditions in Nepal. Apple drying experiments were conducted for 8 h from 09:00 to 17:00 from February to April 2023 on solar dryers and with open sun drying (OSD) in Dhulikhel, Nepal. The results showed that the low emissivity (low-e) coated aluminum collector was more efficient in terms of collector efficiency and drying rate than collectors with flat or v-corrugated GI absorbers. The average collector efficiency and drying rate were found to be 89 % and 107 g/(h × m2) using the low-e coated aluminum collectors, while the values were 50 % and 84 g/(h × m2) for the GI sheet collector. The results indicated a slight improvement in the performance of v-corrugated collectors, with values of 53 % and 89 g/(h × m2). The drying rate for OSD was found to be 78 g/(h × m2), which was lower than for the dryers with all collectors used in this study. In terms of economic analysis, the dryer equipped with the low-e coated collector was found to be superior, with a payback time of 1.61 years compared to the dryers with flat or v-corrugated GI collectors. Reducing the thermal losses due to radiation by using the low-e coated absorber was shown to be more important than increasing the absorber area using v-corrugated GI sheets.

In this study, a novel adaptive solar dryer with a coating of polymer-dispersed liquid crystals (PDLC) was designed, built, and characterized to evaluate its optical properties and thermal performance by modulating the transmission of solar radiation toward the drying chamber. PDLC films can electrically switch between opaque and transparent states, making them ideal for controlling solar radiation. Unlike conventional direct dryers, the proposed solar drying system takes advantage of direct solar radiation, known for its rapid drying process, while minimizing the impact on product properties. The results show that the PDLC coating exhibited a significant difference in transmittance between the “on” and “off” states under ambient conditions. At 400 nm, the transmittance varied by 20 %, increasing to 30 % at 450 nm. The solar dryer with dynamic control of solar irradiance constantly maintained a temperature range of 52.19 °C–57 °C. Lower relative humidity values were achieved compared to the uncontrolled solar dryer, resulting in a uniform irradiance distribution within the drying cabinet. The solar dryer with dynamic control of solar irradiance (acrylic and PDLC film) presents a total thermal resistance (0.02817 mK/W) exhibiting better heat permanence inside the drying cabinet compared to polycarbonate (one of the materials most used for drying systems). The system's thermal efficiency ensures reliable operation regardless of weather conditions, efficiently capturing and converting solar energy into heat.

The present work presents the effective application of waste heat rejected from window air conditioning (AC) in a waste heat dryer (WHD) for drying plantain banana slices. The comparison between drying with the waste heat dryer coupled with an AC unit (ACWHD) and a natural convective indirect solar dryer (SD) reveals significant differences in drying kinetics, supply air conditions, and thermodynamic performance. The drying kinetics of banana slices obtained with ACWHD and SD were validated by revisiting the concepts of dryer performance index (DPI) and generalized drying curve. The temperature of the hot air supplied to the drying chamber in ACWHD was 48 °C while the relative humidity of the supply air was below 30 %. The thermodynamic performance assessment revealed that the exergetic efficiency was about 45 % and 16 % for ACWHD and SD respectively. The results of exergy based sustainable index and environment impact factor indicated the potential application of the waste heat of residual air from AC units for drying applications. Thus, the results of the present study imply that this novel idea has appreciable potential for commercialization with large-scale central air conditioning units in commercial complexes and industries where the magnitude of heat rejection will be multi-fold.

Drying assumes a pivotal role in the preservation of high-moisture products over extended durations. Despite the widespread utilization of solar dryers owing to their energy-efficient nature, they are often plagued by prolonged drying durations, subsequently impacting the quantity of dried products. This investigation endeavors to introduce an innovative technique aimed at augmenting the efficacy of solar drying processes. Specifically, the study integrates forced convection solar drying with electrohydrodynamic (EHD) drying. The experimentation entails the drying of pineapple slices conducted between December 2023 and January 2024 in Nakorn Pathom, Thailand. The pineapple slices, featuring a ring-shaped geometry, are subjected to varying corona voltages ranging from 7 to 10 kV. The findings reveal a direct correlation between the corona voltage and the diffusion coefficient of the pineapple slices, wherein an escalation in corona voltage leads to an elevation in the diffusion coefficient, consequently resulting in a reduction of moisture ratio during the drying process. Consequently, the novel drying approach exhibits superior efficiency compared to traditional solar drying techniques, particularly as corona voltages increase. Moreover, the study employs Buckingham-Pi terms to predict the fluctuations in the diffusion coefficient, yielding a diffusion coefficient function with an R2 value of 0.996. Subsequently, a novel kinetics model for the drying process is proposed, boasting an average R2 value of 0.98. This model is envisioned to serve as a vital tool for enhancing and advancing forced convection solar-EHD dryers in forthcoming industrial applications.

The use of solar energy for drying is considered one of the most environmentally friendly ways to reduce dependence on fossil fuels. The purpose of this experimental study is to determine how absorption plate design affects the evaporation rate of a solar food dryer in Baghdad, Iraq. Tests were performed on four different absorbent panel models. This dryer design consists of a section for heating solar air and one for preserving food. There is insulation at the back of the dining cabinet, which is made of wood. As food samples, orange slices were selected. Except for the absorb plate design, all four models have identical measurements. Matte black aluminum absorbent plate is featured on the Model-1. Both Models 2 and 3 have perforated cylindrical fins arranged uniformly and vertically in Model 2 and horizontally in Model 3. Model 4 uses horizontal fins and an absorber plate coated with 15 % silicon content. A fruit cabinet holds sliced fruit in five movable mesh trays. According to experimental results, the shape of the absorbent plate directly affects the temperature of the outlet air, which in turn affects the rate at which water is evaporated from fruit slices and the overall drying process. Model-1 solar air heaters could be improved by approximately 17 % by coating them with 15 % silicon. The efficiency of Models 2, 3, and 4 is increased by 20 %, 40 %, and 65 % by adding fins and tint over Model 1. Models 1, 2, 3, and 4 had moisture contents of 28 %, 25 %, 20 %, and 18 %, respectively, at 15 p.m. A solar dryer has been demonstrated to be suitable for successfully drying fruits and vegetables in Iraq's atmosphere through practical experiments.

With ever expanding population and accelerating energy challenges, the ultimate need to adopt green-energy solutions for the preservation and storage techniques for fruits and vegetables has become vital. Sun drying is an age-old method to preserve food and it has revived the popularity within the industrial sector to overcome the high energy consuming operations. This work presents the experimental investigations of a solar dryer with integrated thermal storage chamber. The thermal storage chamber employed paraffin wax as a phase change material (PCM). Incorporating PCM enhances the efficiency and efficacy of the solar dryer. PCM absorbs heat, causing it to melt during peak sunshine hours. Then, during periods of reduced sunlight, the PCM releases the stored heat, enabling continuous product drying. Particularly, the solar dryer successfully dried onions, apricots, and peas, with moisture removal percentages of 69.6 %, 65 %, and 75 % respectively from the total mass.

A solar air heater that operates in the mixed-mode double pass and an absorber plate has aluminum cans at the top and bottom of the absorber for red pepper drying experiments was conducted in Bahir Dar, Ethiopia climatic conditions. A total dying period of red pepper in a mixed-mode solar greenhouse dryer takes less time than an open solar drying system. From the experiment, the average daily dryer chamber temperature ranges between 39.93 0C and 47.02 0C and the average daily ambient temperature ranges between 24.890C and 27.530C. Solar insolation reached 973 W/m2 when the sun was at mid-day and minimum solar insolation was 220 W/m2 and 0.0383 kg per second of air are expelled. A newly designed mixed-mode double pass, integrated with an Aluminum can dryer and the traditional open sun dryer, achieves an efficiency of 46% and 28%, respectively. In the open sun dryer system and the mixed-mode solar dryer, the drying rates are 0.0003395 kg/s and 0.0000365 kg/s respectively. On eleven mathematical models, Microsoft Excel was used to perform the statistical analysis. Logarithmic models were found to be the best explanation of red pepper behavior in both mixed-mode and open-sun solar dryer systems. These models were tested by comparing the maximum value of the coefficient of determination R2 and the minimum value of reduced chi-square X2 and root mean square error RMSE between experimental and predicted moisture ratios.

This recent study evaluated a hybrid solar-assisted heat pump dryer (HS-AHPD) for drying paddy and its product quality. R22 is used as a working fluid in the heat pump system during the drying experiment. The HS-AHPD decreased the paddy's weight from 420 kg (31.67% db) to 370.60 kg (16.18% db) in 5.5 h with an average air temperature of 62.9 °C and an average relative humidity of 16.1% at a mass flow rate of 0.195 kg/s. The average drying rate, specific moisture extraction rate (SMER), and specific energy consumption (SEC) were 8.34 kg/s, 0.44 kg/kWh, and 4.69 kWh/kg, respectively. The HS-AHPD's average thermal efficiency and the exergy efficiency of the drying section were found up to 29.1% and 18.4%, respectively. The furnace and the solar collector contributed 19.7% and 12.9% of energy to the HS-AHPD, respectively. The improvement potential (IP) was in the range of 628.8–824.4W. The drying resulted in higher-quality products due to higher head rice yield (93.10 ± 1.044%), lower broken rice (4.41 ± 0.737%), and lower rice grouts (1.11 ± 0.271%).

In this study, the performance of direct forced convection household solar dryer for drying banana is presented. A solar dryer with a polycarbonate plates cover on a flat plate collector was constructed and it was designed in the parabolic shape. A polycarbonate cover is used to reduce heat losses while allowing the incident solar radiation to transmit into the dryer. Five batches of banana were dried in this solar dryer during January–July 2019. For each batch, 10 kg of bananas were dried. The results shown that the temperature of the drying air in the dryer varied between 35 °C to 60 °C from 8:00 a.m. to 6:00 p.m. The moisture content of the banana in the dryer was reduced from an initial value 72% (wet basis, wb) to a final value of 28% (wb) within 4 days, whereas the moisture content of the sun dried samples was reduced to 40% (wb) in the same period. There was a considerable reduction in drying time in the solar dryer as compared to natural sun drying with 48% saving drying time. High quality of solar dried product in terms of flavor, colour and texture were obtained. The payback period of the dryer is approximately 1.1 years.