Solar thermal (domestic hot water, backup)

Solar thermal energy captures the sun's heat to produce domestic hot water (DHW) and, in some cases, supplemental heating. Unlike photovoltaics, this system doesn't generate electricity: a heat transfer fluid is heated in solar collectors, and this heat is then transferred to a storage tank via a heat exchanger. The advantage is that it covers a significant portion of DHW needs (often 40–70% depending on climate and usage) and reduces the load on the boiler/heat pump during the shoulder seasons, with a robust technology if properly sized and managed.

Solar thermal energy: definition and operation

A typical system includes collectors (flat plate or vacuum tube), a primary circuit filled with glycol (or water/gravity drainback), a solar station (pump, valves, flow meter), a differential regulator (it compares collector/tank temperature), a heat exchanger and a storage tank.

The domestic hot water (DHW) system takes priority: the collectors charge the storage tank; if the target temperature is not reached, the backup system (boiler, heat pump, electric element) takes over. For supplemental heating, low-temperature emitters (underfloor heating, low-profile radiators) and a buffer tank are recommended. Key design considerations include: orientation/tilt, shading, collector surface area versus draw-off profile, circulation speed (ΔT), anti-legionella measures (high-temperature cycles), anti-scalding mixing valve at the DHW outlet, and managing stagnation (summer/absences).

Advantages, limitations and points of attention of solar thermal energy

Interests

• Sustainable reduction of supplementary kWh for domestic hot water (and mid-season heating).
• High yield per m² of collector (direct thermal kWh).
• Simple to use once properly adjusted (little electronics).
• Visible decarbonization and positive image (CAPEX often helps).

Boundaries

• Marked seasonality: strong in summer, weak in winter (supplementation essential).
• Risk of stagnation/overheating in summer if low drawdowns (holidays).
• Glycol circuit maintenance (pH, boiling point, leaks).
• Surface area and integration into the roof/facade need careful attention (loads, waterproofing).

Points to consider

• Sizing according to actual DHW needs (hotels, accommodations, collective kitchens).
• Sensor selection: flat plate (robust, economical) vs evacuated tubes (better at high temperatures/cold winds).
• Hydraulics: valves, expansion tank, anti-thermosiphon valve.
• Control: differential regulation, DHW priority, T°/kWh monitoring.
• Sanitary quality: anti-legionella, thermostatic mixer tap, clear instructions.

Anecdote — “Sunshine in Occitanie, peaceful showers in Béziers”

In a student residence in Béziers, shower times skyrocketed between 7 and 9 a.m. Rather than increasing the gas supply, the building manager added 28 m² of flat-plate solar collectors to the roof terrace and a large-capacity solar water heater, with the gas backup only needed in the late morning. The result: a reliable hot water supply during peak periods, lower gas bills, and stress-free maintenance thanks to a drainback loop (zero glycol, no overheating in summer). The building manager summed it up: "Here, the sun heats the water to the perfect temperature... and keeps the staff in good spirits."