Free energy contributions (solar, internal)

Free heat gains refer to the heat and sometimes light that your building receives without consuming any energy: sunlight (through windows) and internal heat gains (occupants, lighting, equipment, cooking, IT systems, etc.). When managed properly, they reduce heating needs in winter; when poorly managed, they increase the temperature in summer and lead to overheating. The goal is not simply to "let in as much sunlight as possible," but to position, filter, store, and control these heat gains according to orientation, usage, and available thermal inertia.

Free contributions: operational definition

Solar heat gain depends on orientation (south is beneficial in winter, west is critical in summer), glazed area, solar factor (g), and light transmission (LT), as well as external shading devices (external venetian blinds, awnings, canopies) and building management system (BMS) controls (automation based on sunlight). Internal heat gains come from people (~70–100 W/person), lighting (LEDs vs. older fixtures), equipment (PCs, ovens, motors), and processes.

To maximize heat gain: capture heat at the right time (winter/daytime), buffer it with thermal inertia (heavy walls, slabs), redistribute it via ventilation, and switch off or relocate anything that generates unnecessary heat. To minimize heat loss: prioritize external protection, use energy-efficient LEDs and equipment, implement occupancy sensors, schedule programming, and, if necessary, night/free-cooling to dissipate heat at night. The STD/SED method helps quantify the winter benefit versus the summer risk.

Advantages, limitations and points to consider regarding free contributions

Interests

• Less heating in winter (solar + internal useful gains).
• Visual comfort (natural lighting) and a refined architectural image.
• OPEX reduced if the BMS intelligently controls blinds/times/lighting.
• Generator sizing sometimes reduced (heating power).

Boundaries

• Overheating possible in spring/summer (West-facing facades, glass roofs).
• More cold production is needed if internal inputs are high (IT, kitchens, retail).
• Glare and discomfort if solar management is insufficient.
• Gains vary depending on climate, use, inertia and operating discipline.

Points to consider

• Priority should be given to external protection (more effective than internal blinds).
• Glazing choice: higher g on the South side (winter), lower g on the West side (summer).
• Thermal inertia + night ventilation to store/purge calories.
• LEDs & controlled standby: reducing internal "parasitic" inputs.
• GTB/EMS: weather scenarios (closing of external blinds, load shifting), DH monitoring.
• Light/sun balance: aim for sufficient TL without overheating (glare).

Anecdote — “When Besançon heats up all by itself”

In Besançon, a brand-new media library benefited from a large south-facing façade. In winter, the sun streamed through the overhanging awning and naturally warmed the atrium; in spring, the adjustable sunshades closed automatically according to the amount of sunlight, while the concrete slab stored the day's heat. In summer, nighttime ventilation and exterior blinds removed the excess. The result: little heating from November to March, less air conditioning during the shoulder seasons, and delighted readers: "Here, even the novels warm up in the sun—without a bill.".