Solar thermal needs no moving parts or electrical knowledge and can be built entirely from salvaged materials; a flat plate collector made from an old radiator and a window pane will produce hot water every sunny day for decades. The sun delivers roughly 1,000 watts per square meter at peak, and the collector should be tilted at your latitude angle facing true south in the northern hemisphere or true north in the southern.
Solar thermal is the most accessible renewable energy technology for a post-collapse homestead. Unlike hydro or wind, it requires no moving parts, no electrical knowledge, and can be built entirely from salvaged materials. A flat plate collector made from an old radiator and a window pane will produce hot water every sunny day for decades.
Solar Energy Fundamentals
Insolation & Angle
The sun delivers roughly 1,000 watts per square meter at peak (clear sky, noon, direct angle). Your actual harvest depends on:
- Latitude: Determines sun angle. At 45°N latitude, a south-facing collector tilted at 45° captures the most annual energy
- Season: Summer delivers 2-3x more energy than winter at temperate latitudes
- Cloud cover: Overcast days still deliver 10-25% of clear-sky energy
- Collector tilt: Aim for an angle equal to your latitude for year-round use, latitude +15° for winter optimization, latitude -15° for summer
Rule of thumb: Tilt your collector at the same angle as your latitude, facing true south (northern hemisphere) or true north (southern hemisphere).
Absorption vs Reflection
- Matte black surfaces absorb 90-95% of solar radiation
- Shiny metal reflects 70-90% (useless as an absorber, great as a reflector)
- Glass transmits 85-90% of visible light but blocks infrared (heat) radiation — this is the greenhouse effect that makes collectors work
Paint your absorber surface matte black. Use salvaged flat black spray paint, wood stove paint, soot mixed with oil, or lamp black mixed with any binder.
The Greenhouse Effect in Collectors
Glass (or clear plastic) over the absorber creates a heat trap:
- Sunlight passes through the glass and hits the black absorber
- The absorber heats up and radiates infrared energy
- Glass is opaque to infrared — the heat cannot escape back through the glass
- Temperature inside the collector rises well above ambient (60-90°C for water heating, 150°C+ for cooking)
Flat Plate Collectors
The workhorse of solar water heating. A flat plate collector can heat water to 50-80°C, sufficient for bathing, dishwashing, laundry, and space heating.
Absorber Plate Construction
The absorber is a black metal surface with water flowing through or across it:
Option 1 — Salvaged car radiator:
- An automotive radiator is already a flat plate heat exchanger. Paint it matte black
- Connect garden hose to inlet and outlet
- Mount behind glass in an insulated box
- One of the simplest and most effective designs
Option 2 — Copper pipe on sheet metal:
- Solder or clamp 12-15 mm copper pipes in a serpentine pattern onto a flat sheet of copper or steel
- Paint everything matte black
- The metal sheet absorbs heat and conducts it to the pipes
- Space pipes 10-15 cm apart for good coverage
Option 3 — Corrugated metal roofing:
- Lay two sheets of corrugated metal face-to-face, creating channels
- Seal the edges with silicone, solder, or folded/crimped joints
- Water flows through the corrugation channels
- Paint exposed surface black
- Cheap and effective if you can get the seals watertight
Glazing
The transparent cover that creates the greenhouse effect:
- Window glass (best): Durable, high transmission, blocks infrared. Salvage from buildings
- Greenhouse polycarbonate: Nearly as good, lighter, shatter-resistant
- Clear plastic sheeting: Works for 1-2 seasons but degrades in UV. Emergency option
- Glass shower doors / sliding doors: Large pieces ideal for big collectors
Leave a 2-3 cm air gap between glazing and absorber. A second layer of glazing improves performance in cold weather but reduces transmission.
Insulation & Frame
- Build a shallow box from wood (5-10 cm deep)
- Line the bottom and sides with insulation that will take the stagnation temperature, not the working temperature. A sealed collector left in the sun with no flow reaches roughly 120-220 °C against the plate (derived in The Greenhouse Effect in Collectors), and that is the temperature the insulation actually has to survive, because stagnation is normal — it happens every time the pump stops, the tank is full, or nobody draws water for a day.
Rigid foam and packed straw do not belong in a solar collector
Expanded and extruded polystyrene soften below 100 °C and will slump, shrink away from the plate and melt. Polyurethane and polyisocyanurate char and give off smoke at collector stagnation temperatures. Packed straw against a plate at 150-200 °C will slowly pyrolyse, and pyrolysed straw smoulders — a slow, hidden, self-sustaining fire inside a sealed box you have screwed to your roof. Each of these has been recommended for collector boxes, including by an earlier version of this line, and each is wrong.
Use mineral wool or unfaced fibreglass against the plate: both are glass or rock and neither burns. If you have neither, use an air gap with a reflective foil or sheet-metal layer behind it and accept the lower performance — a collector that works less well is better than one that smoulders. Keep any organic insulation, and any paper or foil facing with an adhesive, outside the hot zone entirely.
Insulation & Frame works the material choice and the 5 cm thickness through.
- Place the absorber plate in the box
- Cover with glazing, sealed against rain but with small weep holes at the bottom for condensation drainage
- Overall size: 1-2 square meters per person for domestic hot water
Plumbing Connections
Water enters at the bottom of the collector (cold) and exits at the top (hot). Use salvaged copper, steel, or high-temperature plastic pipe. Garden hose works for temporary setups but degrades in UV and heat.
Batch Water Heaters
The simplest solar hot water system: a tank of water sitting in the sun.
Black Barrel Design
- Paint a 200-liter / 55-gallon metal drum matte black
- Fill with water via a hose or bucket
- Place in direct sun, ideally behind a glass or plastic windscreen
- By afternoon, water is a few degrees warmer, not hot — see the arithmetic below
- Drain from a spigot at the bottom
A bare drum cannot reach bath temperature, however well you build it
This is a limit of the collecting area, not of the workmanship. A 55-gallon drum presents about 0.5 m² to the sun. At a generous 5 kWh/m² a day that is 2.5 kWh, or 9 MJ, and 9 MJ into 200 kg of water is 9,000,000 ÷ (200 × 4,180) = a rise of 10.8 K with perfect absorption and no losses at all. In practice expect 4-8 K. So a drum starting at 15 °C finishes the day in the low twenties — pleasant for rinsing, and nowhere near the 40-60 °C an earlier version of this line promised, or the “hot bath for a family every evening” it concluded with.
The fix is not a better drum, it is less water per unit of collecting area. Put 40-80 litres behind a square metre of glazing in an insulated box and the same day’s sun is working on a quarter of the mass through twice the aperture, which is why the Bread Box design below genuinely reaches bathing temperature and this one does not. Use the drum as pre-heated feed for something else, or as a summer rinse, and size a real collector if you want hot water.
Bread Box / ICS Collector
An insulated, glazed box containing one or two tanks:
- Build an insulated box large enough for a 40-80 liter tank (old water heater tank, pressure vessel, or welded drum)
- Paint the tank black
- Mount the box tilted toward the sun, glazed side facing south
- Insulate the back and sides heavily
- Cold water enters at the bottom, hot water exits at the top
- The insulated box retains heat for hours after sunset
This design can deliver water at 50-70°C reliably, enough for comfortable bathing and dishwashing.
Thermosiphon Systems
How Thermosiphon Works
Thermosiphon is passive circulation — no pump needed:
- Hot water is less dense than cold water
- When the collector heats water, the hot water rises naturally
- Cold water from the bottom of the storage tank flows down to replace it
- This creates a continuous circulation loop powered entirely by heat
Critical requirement: The bottom of the storage tank must be above the top of the collector. If the tank is lower than the collector, thermosiphon will not work — you would need a pump.
Tank Placement
- Mount the storage tank on a platform, shelf, or attic space at least 30 cm above the top of the collector
- A typical household system uses a 100-200 liter insulated tank
- Insulate the tank heavily — wrapped in blankets, foam, or straw in a box
- Pipe runs between collector and tank should be as short as possible and well-insulated
Freeze Protection
In climates where temperatures drop below 0°C:
- Drain-back system: Tilt the collector so water drains back to the tank when circulation stops (requires careful plumbing slope)
- Antifreeze loop: Fill the collector loop with a water-antifreeze mix (salvaged car coolant) and use a heat exchanger coil in the tank to transfer heat to clean water
- Drain-down: Manually drain the collector before freezing nights. Simple but requires attention
Solar Cooking & Pasteurization
Panel Cooker
The simplest solar cooker — reflective panels focus sunlight on a black pot:
- Cut a large piece of cardboard into a funnel or petal shape (the CooKit design)
- Cover the reflective side with aluminum foil, shiny side out
- Place a black pot in a clear plastic bag (creates greenhouse effect)
- Set the pot in the focal area of the reflector
- Orient toward the sun
- Reaches 100-120°C — enough to cook rice, beans, stews, and bake bread
Cooking time: roughly 2x longer than conventional cooking. Start the meal 2-3 hours before you want to eat.
Box Cooker
More effective and stable than a panel cooker:
- Two nested boxes (outer 60x60 cm, inner 40x40 cm) with insulation between
- Line the inner box with reflective material
- Paint the bottom of the inner box black (or place a black metal tray)
- Cover the top with glass or clear plastic
- Add a reflective lid that props open to bounce extra light into the box
- Place black pots inside, close the glass lid, aim at the sun
- Reaches 150-180°C — can bake, roast, and sterilize
Water Pasteurization
Solar heat can make water safe to drink without fuel:
- SODIS (Solar Disinfection): Fill clear PET plastic bottles with water, lay in direct sun for 6 hours (or 2 days if cloudy). UV radiation kills pathogens. Simple, proven, WHO-endorsed
- Solar pasteurization: Heat water to 65°C for 30 minutes using any solar collector or cooker. This kills all common waterborne pathogens
- WAPI (Water Pasteurization Indicator): A small tube with wax that melts at 65°C, confirming pasteurization temperature was reached. You can make one from a glass tube and soy wax
System Sizing
For a single household:
- Hot water: 1-2 m² of flat plate collector + 100-200 liter storage tank = 40-80 liters of hot water per sunny day
- Cooking: One box cooker handles 1 meal per day for 4 people in sunny conditions
- Water pasteurization: 6-10 PET bottles per day via SODIS = 10-15 liters of safe water
Next Steps
- heat-storage-systems — Store your solar-heated water for overnight use
- concentrated-solar-power — Phase 3: focus sunlight for metalworking temperatures
- district-heating — Phase 4: community-scale hot water distribution