A batch water heater is a dark tank of water placed in the sun that stores and collects heat in the same vessel, with no pump and no separate collector. How hot it gets depends on how many litres sit behind each square metre of sun-facing area: about 17 K of rise at 100 litres per square metre, 29 K at 60, and only 4.3 K at 400. Use a shallow, glazed, insulated tank for warm-to-hot water and a bare drum only for pre-warming.
Part of Solar Thermal Collectors
A batch water heater combines collector and storage in one vessel, the simplest solar hot-water design, filled once and drawn off later.
The Rule Is Litres Per Square Metre
Every kilowatt-hour that lands on the tank face warms the water inside it. The temperature rise in a day is
ΔT = (daily energy absorbed per m² of sun-facing area) ÷ (water mass per m² × 4.18 kJ/kg·K)
so a given sun delivers a big rise to a small volume and a small rise to a large one. Inputs, assumed for the example: 5 kWh/m² per day arriving, 40% of it ends up in the water (a glazed box; a bare drum is lower), so 2.0 kWh/m² = 7,200 kJ/m².
- 40 L/m²: 7,200 ÷ (40 × 4.18) = 43 K.
- 60 L/m²: 7,200 ÷ (60 × 4.18) = 29 K.
- 100 L/m²: 7,200 ÷ (100 × 4.18) = 17 K.
- 400 L/m²: 7,200 ÷ (400 × 4.18) = 4.3 K.
Equivalently, ΔT ≈ 1,720 ÷ (litres per square metre) in kelvin, for that sun and efficiency. Change the 5 kWh or 40% and the numbers move proportionally; they are not universal (Insolation & Angle).
A large drum in the open gives the 400 L/m² end of the table. A shallow glazed box gives the 40-60 L/m² end.
Two Designs
- Black barrel, the crude version (Black Barrel Design).
- Bread box or ICS collector, a tank inside an insulated, glazed, reflective box ( ICS Collector).
Sizing against household need is in Sizing for Household Use.
What You Need
- A metal tank — drum, pressure vessel, old water-heater tank
- Matte black heat-resistant paint
- A vent at the top and a draw-off tap at the bottom
- Glazing and insulation for the box version
- A stand that holds the weight (1 L of water is 1 kg)
Method: Fill, Heat, Draw
Step 1: Place the tank where the sun reaches it from mid-morning to late afternoon, facing the equator.
Step 2: Fill it before the sun is strong, so the cold water collects the heat all day. Leave the vent open.
Step 3: Draw hot water from the top in the evening (hot water is lighter, so it floats at the top).
Step 4: Refill in the morning. Water left in the tank loses its heat overnight unless the tank is insulated.
Common Mistakes
- Judging a design by its tank volume. What counts is litres per square metre of sun-facing area.
- Sealing the vent. Water expands as it warms, about 2.7% between 20°C and 80°C (densities 998.2 and 971.8 kg/m³, handbook values recalled); in a full sealed tank that expansion can rupture the tank.
- Leaving it unshaded and empty. An empty black tank in sun overheats its paint and seals.
Safety
A tank in the sun scalds and can burst if sealed
Water above about 60°C scalds in seconds, so mix with cold water before it reaches skin, and keep children and the elderly away from the draw-off. A sealed tank warming in the sun can burst; keep the vent open and clear. A glazed batch heater with no water draw can stagnate above boiling in a long hot spell, so keep it vented and keep the discharge away from people (Plumbing Connections).
What’s Next
- Learn next: ICS Collector — the improved version
- Combine with: Tank Placement — supporting the weight