A solar drying shed warms air in a south-facing glazed collector with a dark surface and passes it up through slatted racks to a high vent, reaching 45-65 °C on sunny days and drying fruit, vegetables, herbs and thin-sliced meat in 1-3 days. As a sizing check, drying 100 kg of fresh fruit from 85% to 15% moisture in 3 days takes about 198 MJ of evaporation heat, roughly 16 m² of collector at assumed values, and about 0.13 m³/s of airflow through vents of about 0.17 m² each.
Part of Drying Sheds & Ice Houses
A solar drying shed is a screened shed with a glazed solar collector that warms air and draws it over food racks to remove moisture faster and more cleanly than open-air drying.
The Shed Is A Heat Engine For Evaporation, So It Can Be Sized With A Heat Balance
A drying shed is built with screened sides that keep insects out and let air through, a dark or black-painted roof that absorbs heat and warms the interior air, an angled south-facing glazed panel on one side (like a cold frame lid) that acts as a solar collector, slatted drying racks that let air circulate around every side of the food, and openings at the bottom and top. Air heated by the collector rises, pulling fresh air in at the bottom and through the racks. This is much faster and more hygienic than open-air drying. Drying for Storage and Drying Methods cover the food side.
Two numbers decide whether a shed works: how much heat the collector delivers, and how much air moves through. The sum below turns the 1-3 day claim into the area and airflow it needs. It uses physical constants and assumed values for sunshine and collector efficiency, all labelled, because no measured tolerance for this particular design is in the sources. The result is a sizing check, not a performance guarantee.
What You Need
- A collector: a glazed panel facing the sun with a dark absorbing surface behind it
- Racks: slatted, so air passes on every side of the food
- Vents: an inlet at the bottom and an outlet at the top, screened against insects
- A thermometer in the drying chamber, and a way to check the weight of a sample tray
- Thin, even slices of food
Method: Size The Shed
Step 1: Find the water to remove. Water removed = fresh mass − final mass, where final mass = dry solids / (1 − target moisture).
Step 2: Convert to evaporation heat. Heat = water removed × latent heat of vaporization. Use about 2.4 MJ/kg at drying temperatures (steam-table value for water near 40 °C, recalled).
Step 3: Find the collector area. Area = heat needed per day / (daily sunshine × collector efficiency).
Step 4: Find the airflow. Each kilogram of dry air can carry away only so much water, set by how much heat it gives up as it cools through the racks.
Step 5: Size the vents. Use the buoyancy flow formula between a low inlet and a high outlet.
Step 6: Build, then measure. Weigh a sample tray morning and evening. The weight trend, not the sum, tells you whether the shed is working.
Worked Example
Inputs (assumed or recalled): 100 kg of fresh fruit at 85% moisture on a wet basis; target 15% moisture; dry time 3 days; daily sunshine of 5 kWh/m² on a horizontal surface; collector efficiency 0.3; ambient air 30 °C and 50% relative humidity; air heated to 50 °C in the collector and leaving the racks at 35 °C; 8 sun hours a day; specific heat of air 1,005 J/(kg·K); air density 1.2 kg/m³.
Water to remove. Dry solids = 100 × (1 − 0.85) = 15 kg. Final mass = 15 / (1 − 0.15) = 17.65 kg. Water removed = 100 − 17.65 = 82.35 kg.
Evaporation heat. 82.35 × 2.4 = 197.6 MJ, about 198 MJ.
Collector area, evaporation heat only. Sunshine = 5 × 3.6 = 18 MJ/m² a day. Delivered = 18 × 0.3 = 5.4 MJ/m² a day. For one day: 197.6 / 5.4 = 36.6 m². For 3 days: 197.6 / (5.4 × 3) = 12.2 m².
Airflow. Water per day = 82.35 / 3 = 27.45 kg, or 27.45 / 8 = 3.43 kg per sun hour. Inlet humidity ratio at 30 °C and 50% (saturation vapour pressure about 42.3 hPa by the Magnus formula, recalled): vapour pressure = 21.2 hPa, ratio = 0.622 × 21.2 / (1,013 − 21.2) = 0.0133 kg/kg. Air cooling from 50 °C to 35 °C releases 1.005 × 15 = 15.1 kJ per kg of dry air, which evaporates 15.1 / 2,400 = 0.00628 kg of water (if all that heat goes to evaporation, an upper bound). Dry air needed = 3.43 / 0.00628 = 546 kg/h, or 546 / 1.2 = 455 m³/h = 0.126 m³/s, about 0.13 m³/s. The outlet air then carries 0.0133 + 0.00628 = 0.0196 kg/kg, which at 35 °C is about 55% relative humidity, drying air that can still take more water.
Collector area including the air heating. Heat to warm the airflow from 30 °C to 50 °C = (546 / 3,600) × 1,005 × 20 = 3,050 W, about 3.05 kW. Over 8 sun hours = 3.05 × 8 × 3,600 = 87.8 MJ a day. Collector area = 87.8 / 5.4 = 16.3 m², about 16 m². This is larger than the 12.2 m² from evaporation heat alone because the air leaves the racks still warm, carrying away sensible heat.
Vent area (buoyancy). Height between inlet and outlet H = 1.5 m, temperature difference between chamber and ambient assumed 15 K, T = 300 K, discharge coefficient 0.6 (recalled). Velocity = √(2 × 9.81 × 1.5 × 15 / 300) = √1.47 = 1.21 m/s. Flow per area = 0.6 × 1.21 = 0.73 m/s. Area = 0.126 / 0.73 = 0.17 m² for each of inlet and outlet, about 42 cm × 42 cm.
Per batch. 100 kg fresh needs about 16 m², so a 10 kg tray of fresh food needs about 1.6 m² of collector: 16.3 / 10 = 1.63 m² per 10 kg.
What The Sum Leaves Out
- Assumed values. Sunshine, efficiency, temperatures, humidity and Cd are assumptions or recalled textbook values for a sunny day, not measurements. Cloudy or humid weather lengthens the time; air already near saturation carries no water.
- Bound water. Food holds water more tightly as it dries, so the last few percent take longer and need more energy than the free-water latent heat used here.
- Thin slices. Drying time depends strongly on slice thickness; the sum assumes thin, even pieces.
- No tolerance claimed. The 1-3 day figure and the 45-65 °C range come from the parent page’s description of the shed. The sum shows what area and airflow they imply; it is not an independent test.
Common Mistakes
- Collector too small — the food dries in days longer than planned and spoils first.
- Too little airflow — the air saturates and drying stalls; vents are as important as the collector.
- Inlet vent above the outlet — no draw.
- Thick pieces — the outside dries and hardens while the inside stays wet.
Safety
Food safety and hot surfaces
Thin-sliced meat that dries too slowly spoils before it is dry, because it sits for days in the temperature range where bacteria grow. Use thin slices, dry quickly, keep flies off, and do not rely on the shed’s temperature alone to judge safety; see Sun Drying, which warns that drying fails in humid weather. Collector glazing and the dark absorber get hot enough to burn skin. If glass is used for the collector, mind broken-glass hazards; see Salvaged Glass. Do not add a fire or stove inside the shed, which brings the carbon monoxide hazard of Carbon Monoxide Risk.
What’s Next
- Learn next: Herb & Medicine Drying — the gentler end of drying
- Combine with: Ventilation & Overheating Prevention — the same buoyancy sum