Solar collectors suit a heating network best as a summer hot-water source and a return-water preheater, not as a winter heat source: covering the hot water of 30 houses (157 kWh a day) at the corpus figure of 5 kWh per square metre per day and an assumed 40 to 50% efficiency takes 63 to 78 m² of collector, and 63 m² at 50% peaks near 31.5 kW at noon, about a quarter of the 120 kW winter peak. Give every collector loop a relief path for stagnation.
Part of District Heating
Where solar thermal collectors fit in a village hot-water network and how much area they take.
What Solar Is Good For Here
Winter is when heating is needed and when the sun is weakest, so solar gives little where the demand is highest. Summer is the reverse: the sun is strong and the only demand is hot water. The best use is therefore:
- Summer domestic hot water, so the boiler and the network can be shut down for the season (the reason is in Domestic Hot Water Takeoff).
- Shoulder-season preheat of the return water or the heat store.
This page gives no winter yield, because the corpus gives none and an assumed one would be a guess.
Worked Example: Collector Area For Summer Hot Water
Inputs: hot water demand 100 L per house per day heated from 10 to 55 °C (assumed): 100 × 4.18 × 45 = 18,810 kJ = 5.2 kWh per house per day; × 30 houses = 157 kWh/day. Sun: Solar Thermal Collectors uses a generous 5 kWh/m² of sun a day, and about 1,000 W/m² at the peak (Solar Energy Fundamentals); collector efficiency of 40 to 50% is an assumption, not a corpus figure.
- Area = 157 ÷ (5 × 0.5) = 63 m², or 157 ÷ (5 × 0.4) = 78 m².
- Per house: 2.1 to 2.6 m².
- Peak output at 1,000 W/m² and 50%: 63 × 1,000 × 0.5 = 31.5 kW, which is 31.5 ÷ 120 = 26% of the 120 kW winter peak of the reference village.
Flat plate collectors reach 50 to 80 °C (Flat Plate Collectors), enough for water at 55 °C only when the sun is strong. A 30-house collector field is a substantial building project.
Integration Options
- Direct to the households. Batch or thermosiphon heaters on each roof (Batch Water Heaters, Thermosiphon Systems). No network, no loss. This is what the network should hand over to in summer.
- A collector field at the plant, charging the heat store through a pump. Feeds the network whenever it runs.
- Return preheat. Collector water raises the temperature of the network return before the boiler. It only helps when the collector outlet is hotter than the return.
- Concentrating collectors (Concentrated Solar Power) reach 300 to 1,500 °C, but this page does not size them and the carrier’s idea of a trough heating the supply directly needs sourced yields.
Stagnation
A collector with no flow in full sun heats until it loses as much as it gains, which for a flat plate can exceed the boiling point of water. Fit a relief path so that a stopped pump cannot build pressure, as the corpus collector page requires. Keep the store and the pipes rated for the stagnation temperature, and use a controller that stops the pump when the store is full and a hot-water dump on loss of load.
Common Mistakes
- Treating the 1,000 W/m² peak as the daily average. The corpus warns against it; use kWh per square metre per day.
- Sizing for winter space heating. A winter solar yield cannot be quoted here.
- No relief path. A closed, full collector with a stopped pump is a pressure vessel in the sun.
Safety
A stopped collector is a boiler
A collector in full sun with no flow can reach well over 100 °C and rupture, scald, or throw boiling water from a vent. Always provide a relief path to a safe place, and mount collectors where a vent discharge cannot reach people or roof space.
What’s Next
- Learn next: Distribution Network — the network the solar heat joins
- Combine with: Seasonal Storage — banking summer heat for winter