# Passive Solar Design ## Table of Contents - [[#Overview]] - [[#Solar Geometry Fundamentals]] - [[#Direct Gain Systems]] - [[#Indirect Gain Systems]] - [[#Isolated Gain Systems]] - [[#Thermal Storage Design]] - [[#Sizing South-Facing Glazing]] - [[#Solar Heat Gain Coefficient]] - [[#Overheating Prevention]] - [[#Integration with Building Design]] - [[#Performance Estimation Methods]] - [[#Key References and Standards]] --- ## Overview Passive solar design exploits solar radiation as a free heating source by collecting, storing, and distributing solar energy through the building fabric without mechanical systems. The approach is most relevant in heating-dominated and temperate climates, though solar gain management (admission and exclusion) is a concern in all climate zones. The three canonical system types -- direct gain, indirect gain, and isolated gain -- each offer distinct advantages depending on programme, climate severity, and architectural intent. This article connects to [[Solar Shading Design]] for summer solar exclusion, [[Thermal Mass and Energy Storage]] for thermal storage principles, and [[Daylighting Fundamentals]] for the visual component of solar admission. --- ## Solar Geometry Fundamentals Understanding solar angles is prerequisite to any passive solar design. ### Key Angles - **Solar altitude (alpha)** -- angle of the sun above the horizon - **Solar azimuth (phi)** -- angle of the sun measured from south (northern hemisphere) or north (southern hemisphere) - **Declination (delta)** -- seasonal tilt of earth's axis, ranging from +23.45 deg (summer solstice) to -23.45 deg (winter solstice) ### Critical Dates for Design | Date | Significance | |------|-------------| | 21 December (N. hemisphere) | Winter solstice -- lowest sun altitude, maximum heating demand, design for maximum solar admission | | 21 June (N. hemisphere) | Summer solstice -- highest sun altitude, design for complete shading | | 21 March / 21 September | Equinoxes -- equal day/night, transitional shading design | ### Solar Altitude at Noon Solar altitude at solar noon can be calculated as: **alpha = 90 - latitude + declination** Example: At latitude 40 deg N, winter solstice: alpha = 90 - 40 + (-23.45) = 26.55 deg This low angle means winter sun penetrates deep into south-facing rooms, which is the fundamental principle enabling passive solar heating. --- ## Direct Gain Systems The simplest and most common passive solar approach. Solar radiation enters directly through glazing into the occupied space, where it is absorbed by internal thermal mass. ### Components 1. **South-facing glazing** (north-facing in southern hemisphere) with high solar transmittance 2. **Internal thermal mass** -- floor slab, internal walls, or dedicated mass elements to absorb and re-radiate heat 3. **Insulated envelope** -- to retain collected heat 4. **Operable shading** -- to prevent summer overheating ### Design Rules - Distribute mass across floor and walls rather than concentrating in one element - Mass surface area should be **6-9 times** the south glazing area for effective absorption - Mass thickness: first 100 mm is most effective; beyond 150 mm, diminishing returns - Mass surfaces should be dark-coloured (solar absorptance > 0.7) where directly sunlit - Avoid carpeting or covering mass floors in sunlit zones - Maximum room depth for effective solar penetration: approximately **1.5 times** the window head height ### Advantages - Lowest cost of all passive solar systems - Provides daylighting simultaneously - No intermediate elements between sun and space ### Limitations - Glare potential from large south-facing glazing - Temperature swings if mass is insufficient - Privacy concerns with large glazed areas - Furniture and occupants may block mass surfaces --- ## Indirect Gain Systems An intermediate thermal mass element is placed between the sun and the occupied space. The mass absorbs radiation and transfers heat inward by conduction, convection, and radiation with a time delay. ### Trombe Wall Named after Felix Trombe and Jacques Michel (1960s, Odeillo, France). A south-facing mass wall (200-400 mm thick) behind a layer of glazing, with a small air gap (50-150 mm) between. **Operating modes:** | Mode | Configuration | Effect | |------|--------------|--------| | Daytime heating (convective) | Top and bottom vents open | Thermosyphon loop circulates warm air into room | | Time-delayed heating (conductive) | Vents closed | Heat conducts through wall, reaching interior 6-10 hours later | | Summer ventilation | Top vent to outside open, bottom room vent open | Draws room air out, inducing ventilation | | Summer exclusion | External shading deployed | Prevents solar gain entirely | **Wall specifications:** - Concrete or masonry, 200-400 mm thick - Exterior surface: dark colour (absorptance > 0.9) or selective surface coating - Glazing: single or double, with air gap of 50-100 mm - Vent area: approximately 1% of wall area at top and bottom each - Selective surface coatings (absorptance > 0.9, emittance < 0.2) improve performance by 15-30% ### Water Wall Replaces masonry with water-filled containers. Water has approximately twice the volumetric heat capacity of concrete, allowing thinner walls. Less common due to maintenance concerns, leakage risk, and aesthetic limitations. --- ## Isolated Gain Systems Solar collection occurs in a thermally separate space, with controlled heat transfer to the occupied building. ### Sunspace (Conservatory / Solar Greenhouse) - Glazed space on the south facade, thermally separated from the building by a mass wall or insulated partition with operable openings - Acts as a thermal buffer, reducing heat loss from the adjacent wall - Can provide usable amenity space in mild weather - **Not a habitable room** -- temperature swings of 15-30 degC are typical and acceptable **Design guidelines:** | Parameter | Recommendation | |-----------|---------------| | Glazing tilt | Vertical or 60-75 deg from horizontal (vertical is simpler, nearly as effective, and avoids summer overheating) | | Depth | 1.5-3.0 m | | Separating wall | Mass wall with operable vents, or insulated wall with openable doors/windows | | Floor | Thermal mass (dark tile on concrete slab) | | Summer ventilation | Operable roof vents and low-level inlets essential | ### Thermosyphon Air Panel A glazed, dark-surfaced collector panel mounted on the south wall, with top and bottom connections to the interior. Air heats in the panel and rises by natural convection into the room. Backdraft dampers prevent reverse flow at night. Simple, low-cost, and effective for supplementary heating. --- ## Thermal Storage Design Effective thermal storage is essential to all passive solar systems. ### Material Properties | Material | Density (kg/m3) | Specific Heat (J/kgK) | Volumetric Heat Capacity (kJ/m3K) | |----------|-----------------|----------------------|-----------------------------------| | Concrete | 2300 | 880 | 2024 | | Brick | 1700 | 800 | 1360 | | Water | 1000 | 4186 | 4186 | | Stone (granite) | 2600 | 900 | 2340 | | Adobe | 1550 | 900 | 1395 | | Phase change material (PCM) | 800-1600 | Latent: 150-250 kJ/kg | Variable | ### Sizing Guidelines - **Floor mass:** 100-150 mm concrete or stone slab in direct sunlit zone - **Wall mass:** 100-200 mm thickness for room-side exposure - **Mass-to-glass ratio:** 6:1 to 9:1 (mass surface area to south glass area) for direct gain - **Distribution:** spread mass across multiple surfaces to maximise surface area for absorption and re-radiation ### Phase Change Materials (PCMs) PCMs store and release heat at a specific temperature through solid-liquid phase transition. Encapsulated PCMs (typically melting at 21-26 degC) can be integrated into plasterboard, ceiling tiles, or floor systems, providing high thermal storage capacity in lightweight construction. --- ## Sizing South-Facing Glazing ### Rule of Thumb For heating-dominated climates (northern hemisphere), south-facing glazing area as a percentage of floor area: | Climate Severity | South Glazing / Floor Area | |-----------------|---------------------------| | Cold (< 3000 HDD base 18 degC) | 12-18% | | Moderate (3000-5500 HDD) | 7-12% | | Mild (> 5500 HDD) | 5-7% | These figures assume well-insulated envelopes (U-values < 0.3 W/m2K for walls, < 0.2 W/m2K for roofs) and adequate thermal mass. ### Glazing Performance South-facing glazing should have: - High **Solar Heat Gain Coefficient (SHGC)** for winter solar admission (> 0.5, ideally > 0.6) - Low **U-value** to minimise heat loss (< 1.4 W/m2K, ideally < 1.0 W/m2K) - **Low-e coating on surface 3** (inner pane, room side) to retain long-wave radiation while admitting solar This conflicts with east/west glazing requirements where low SHGC is preferred to reduce summer gains. --- ## Solar Heat Gain Coefficient The SHGC (also called g-value or total solar energy transmittance in European standards) represents the fraction of incident solar radiation that enters through the glazing: **SHGC = Direct transmittance + Absorbed fraction re-radiated inward** | Glazing Type | Typical SHGC | Typical U-value (W/m2K) | |-------------|-------------|------------------------| | Single clear | 0.86 | 5.8 | | Double clear | 0.76 | 2.8 | | Double, low-e (passive solar) | 0.58-0.65 | 1.2-1.6 | | Double, low-e (solar control) | 0.25-0.40 | 1.2-1.6 | | Triple, low-e, argon | 0.45-0.55 | 0.7-1.0 | **Design note:** Specify different glazing types by orientation. South glazing should maximise SHGC; east, west, and roof glazing should minimise SHGC while maintaining visible transmittance for daylighting. --- ## Overheating Prevention Passive solar buildings are inherently at risk of summer overheating. Prevention must be designed in from the outset. ### Hierarchy of Measures 1. **External shading** -- fixed overhangs, operable louvres, deciduous planting (see [[Solar Shading Design]]) 2. **Operable ventilation** -- night purge cooling, cross-ventilation 3. **Thermal mass** -- absorbs peak gains, reduces temperature swings 4. **Internal shading** -- blinds, curtains (less effective than external, as heat is already inside the envelope) 5. **Glazing specification** -- reduce SHGC on non-south orientations 6. **Mechanical cooling** -- last resort backup ### Fixed Overhang Sizing For south-facing glazing, a fixed horizontal overhang can be sized to admit winter sun and exclude summer sun: **Overhang depth = Window height / tan(summer noon altitude)** Refined approach: design for the date when shading should begin (typically 6-8 weeks before summer solstice to account for thermal lag). --- ## Integration with Building Design ### Architectural Considerations - South glazing need not be a continuous ribbon -- well-proportioned punched openings can be effective - Trombe walls can be combined with direct gain windows for variety and functional zoning - Sunspaces can serve as entrance lobbies, circulation, or seasonal dining areas - Thermal mass can be expressed architecturally (exposed concrete, stone, brick feature walls) - Avoid north-facing glazing in heating-dominated climates unless required for daylighting (specify low-U, low-SHGC) ### Common Mistakes - Over-glazing the south facade without adequate mass (causes overheating and temperature swings) - Using solar control glazing on south facades (defeats the purpose of passive solar gain) - Placing mass where it is not sunlit (mass behind furniture or in corners is less effective) - Inadequate summer ventilation strategy - Ignoring east/west glazing control while focusing on south --- ## Performance Estimation Methods | Method | Complexity | Application | |--------|-----------|-------------| | Solar Savings Fraction (SSF) | Low | Quick estimate of heating energy saved | | Load Collector Ratio (LCR) | Medium | Sizing and comparing passive solar systems | | Balcomb's correlations | Medium | Monthly heating performance estimation | | BRE Admittance Method | Medium | Peak temperature estimation with solar gains | | Dynamic simulation (EnergyPlus) | High | Detailed hourly performance, overheating analysis | --- ## Key References and Standards - Balcomb, J.D. (1992). *Passive Solar Buildings* - Mazria, E. (1979). *The Passive Solar Energy Book* - Lechner, N. (2014). *Heating, Cooling, Lighting* - CIBSE Guide A -- Environmental Design (solar gain calculations) - ASHRAE Handbook -- Fundamentals (solar data, fenestration) - Approved Document L (UK) -- limiting solar gains and U-values - Passivhaus Institute -- passive solar within ultra-low-energy envelope --- #environment #passive #solar