Development of a hybrid mixed-mode solar dryer for product drying

**Background:** Agriculture-based countries face 20–30% post-harvest losses due to poor handling and lack of cold chain infrastructure. Drying is the most economical preservation method, but conventional sun drying causes nutrient loss and contamination, while mechanical dryers are too energy-intensive for remote areas. Existing solar dryers are often too expensive for small farmers in developing countries. This study aimed to develop a low-cost hybrid mixed-mode solar dryer (HMSD) with uniform drying rate and evaluate it on three commonly grown agricultural products.

**Methods:** The study was conducted at Pir Mehr Ali Shah, Arid Agriculture University, Rawalpindi, Pakistan. The HMSD was fabricated from mild steel sheets and included a 1.5 m heating section, a 3 m drying section with four stainless steel perforated trays, three DC fans (1000–1200 RPM each), a 50-W PV panel, and a battery backup. The collection section was tilted at 33° to maximize solar intensity. A 3 mm glass cover with 95–98% transmittance was used. The total width was 1 m with a cross-section area of 0.112 m² (36% heating, 64% drying). Fresh peach (Freestone), apple (Golden), and chili (Anaheim) were purchased locally. Ten kg of each fruit were washed, blanched (dipped in boiled water for 2–5 min then plunged into ice water), and sliced (3 mm thick for apple; peach halved; chili left whole). Three replicates were prepared. Drying performance was evaluated under one-fan, two-fan, and three-fan operating conditions. Solar intensity (W/m²), humidity (%), temperature (°C), and air speed (m/s) were measured using a pyranometer (LP02-05, ±0.18×10⁻⁶ W/m²), humidity/temperature sensors (Mi BT, 0–99°C, 0–99%), and an anemometer (UT363S, 0.4–30 m/s). Thermal efficiency was calculated using standard equations. Quality analysis included proximate analysis (AOAC methods), total plate count (TPC) test (incubated at 37°C for 48 h), and color testing using a colorimeter (L*, a*, b* scale). Regression analysis with exponential decay fitting was performed at a 5% significance level.

**Key Results:** In zero-load testing, the maximum collector temperature was 71°C at 2:00 p.m. (ambient 37°C), with an average collector temperature of 56°C and average tray temperature of 53°C. For apple slices with one fan: maximum solar irradiance 435 W/m², air velocity 0.38 m/s, drying rate 0.86 kg/h at 1:30 p.m. With two fans: maximum solar irradiance 430 W/m², drying rate 1.157 kg/h, air velocity 0.38 m/s at 1:30 p.m. With three fans: maximum solar irradiance 535 W/m², air velocity 0.48 m/s, drying rate 1.251 kg/h at 1:30 p.m. Average initial moisture content was 89% for peach, 87% for apple, and 75% for chili. Average final moisture content was 16% for peach, 15% for apple, and 11% for chili. Drying times were 17 h for peach, 16 h for apple, and 14 h for chili. Thermal energy used: 22.2 kWh (peach), 20.3 kWh (apple), 18.5 kWh (chili). The average system thermal efficiency was 33%. Regression analysis showed exponential decay curves with R² values of 0.99 (peach), 0.98 (apple), and 0.99 (chili). Proximate analysis of dried products: peach (moisture 16.5%, ash 3%, crude protein 2.5%, fat 18%, fiber 0%, carbohydrates 60%); apple (moisture 15.4%, ash 2.2%, crude protein 0.04%, fat 2.87%, fiber 0%, carbohydrates 79.4%); chili (moisture 11%, ash 1.1%, crude protein 2%, fat 1.5%, fiber 8.5%, carbohydrates 75.9%). TPC results: peach 1.9×10⁴ cfu/g, apple 1.2×10⁴ cfu/g, chili 1.7×10⁴ cfu/g — all within safe limits. Color analysis: peach (L*=53, a=11, b=32, light golden); apple (L*=59, a=14, b=44, light red and yellow); chili (L*=28, a=25, b=17, deep red).

**Clinical Implications:** This study does not involve clinical or patient outcomes. However, the findings have public health and food safety implications. The HMSD produced dried agricultural products with moisture content below 20%, preventing fungal growth (which requires >80% moisture). Bacterial counts were within safe limits, and nutritional composition (low ash, protein, fat; high carbohydrates) was within standard food ranges suitable for powder processing and sale. The system's low cost ($500 total investment) and short payback period (0.44 years, or 160 days) compared favorably to other systems ($670–$4600; payback 0.65–1.89 years). With 185–290 sunny days per year in Pakistan and neighboring countries, the HMSD offers a viable, low-operating-cost solution for small farmers to reduce post-harvest losses and improve food preservation in resource-limited settings.