11.1 Passive solar design

Passive solar design harnesses the sun's energy to heat and cool buildings naturally. This ancient technique, used by Romans and indigenous peoples, relies on building orientation, thermal mass, and natural ventilation to maintain comfortable indoor temperatures without mechanical systems.

Key principles include south-facing windows, thermal mass materials, and shading strategies. Trombe walls, direct gain systems, and indirect gain methods like roof ponds showcase the versatility of passive solar design in creating energy-efficient, comfortable spaces across various climates.

Origins of passive solar design

• Passive solar design harnesses the sun's energy for heating and cooling buildings without relying on mechanical systems
• Utilizes natural processes (heat transfer, convection, radiation) to maintain comfortable indoor temperatures
• Traces back to ancient civilizations (Romans, Anasazi) who oriented buildings to capture solar heat in winter and provide shade in summer

Key principles of passive solar design

Building orientation for solar gain

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• Orienting the building's longest axis along the east-west direction maximizes exposure to the sun's path
• South-facing windows (in the northern hemisphere) allow low-angle winter sun to penetrate deep into the space
  • Provides direct solar heating during colder months
  • Overhangs or shading devices block high-angle summer sun to prevent overheating
• North-facing windows are minimized to reduce heat loss

Thermal mass materials

• High-density materials (concrete, brick, stone, water) absorb and store solar heat during the day
• Release stored heat slowly at night to maintain stable indoor temperatures
  • Reduces temperature fluctuations and heating/cooling loads
• Thermal mass is typically placed in the direct path of sunlight (floors, walls) for maximum heat absorption

Natural ventilation techniques

• Passive solar buildings often incorporate strategies for natural ventilation to remove excess heat and provide fresh air
• Cross ventilation is achieved by placing openings on opposite sides of the building
  • Pressure differences between the windward and leeward sides draw air through the space
• Stack ventilation relies on the buoyancy of hot air rising to draw cooler air in from lower openings
  • Can be enhanced with a solar chimney or atrium

Shading strategies in passive solar

• Shading devices are crucial for preventing overheating and glare in passive solar buildings
• Fixed overhangs, awnings, or louvers are sized to block summer sun while allowing winter sun penetration
  • Calculated based on the building's latitude and sun angles throughout the year
• Deciduous trees or vines provide seasonal shading, allowing solar gain in winter when leaves have fallen
• Interior shades (blinds, curtains) give occupants control over solar gain and privacy

Trombe walls in passive solar

Components of Trombe walls

• A Trombe wall is a passive solar heating system named after French engineer Félix Trombe
• Consists of a thick, south-facing masonry wall (thermal mass) with a glass or plastic exterior layer
  • Air gap between the glazing and wall allows for heat transfer
• Vents at the top and bottom of the wall allow for controlled convective heat transfer to the interior

Heat transfer mechanisms in Trombe walls

• Solar radiation passes through the glazing and is absorbed by the dark-colored masonry wall
  • Thermal mass stores heat during the day
• Heat conducts slowly through the wall, radiating into the interior space hours later
  • Provides delayed heating to maintain comfort in the evening/night
• Convective heat transfer occurs as air in the gap is heated, rises, and enters the room through top vents
  • Cooler room air is drawn into the gap through bottom vents, creating a convective loop

Direct gain systems

Advantages vs disadvantages of direct gain

• Direct gain is the simplest passive solar heating strategy, allowing sunlight to directly enter and heat the living space
• Advantages:
  • Provides quick heating response and natural daylighting
  • Relatively low cost and easy to incorporate into building design
• Disadvantages:
  • Potential for overheating and glare if not properly controlled
  • Requires careful balancing of solar gain, thermal mass, and heat loss
  • May cause fading of interior finishes and furnishings over time

Glazing considerations for direct gain

• Glazing type, size, and location are critical factors in direct gain system performance
• South-facing windows should be sized to provide sufficient solar gain without excessive heat loss
  • Typically 7-12% of floor area in cold climates, 10-15% in temperate climates
• High-performance glazing (low-e, double or triple pane) reduces heat loss and improves thermal comfort
  • Selective coatings allow high solar transmittance while minimizing infrared heat loss
• Clerestory windows or skylights can distribute light and heat deeper into the space

Indirect gain systems

Thermal storage walls

• Thermal storage walls, also known as indirect gain systems, collect and store solar heat in a mass wall behind glazing
• The wall, typically made of concrete, masonry, or water containers, acts as a heat sink during the day
  • Absorbs solar radiation and slowly releases heat to the interior over time
• An air gap and insulation between the wall and glazing helps regulate heat transfer
• Provides more evenly distributed heat and reduces potential for overheating compared to direct gain

Roof ponds for passive cooling

• Roof ponds are a type of indirect gain system used for passive cooling in hot climates
• Shallow water containers or bags are placed on the roof, often with movable insulation panels
  • During summer days, the insulation covers the pond, shielding it from solar gain
  • At night, the insulation is removed, allowing the pond to radiate heat to the cool night sky
• The cooled water absorbs heat from the interior space below, providing a cooling effect
• Roof ponds can also be used for heating in winter by reversing the insulation operation

Isolated gain systems

• Isolated gain systems collect solar heat in a separate space, such as a sunspace or atrium, and distribute it to living areas as needed
• The solar collection space is thermally isolated from the main living areas, allowing for better control of heat transfer
  • Doors, vents, or fans can be used to regulate heat flow between the spaces
• Sunspaces can also serve as a buffer zone, reducing heat loss from the main building envelope
• Isolated gain systems offer flexibility in design and can provide additional living or growing space

Passive solar in vernacular architecture

Ancient Roman passive solar techniques

• The ancient Romans were early adopters of passive solar principles in their architecture
• They oriented buildings to the south to capture winter sun and used thick stone walls for thermal mass
  • The Baths of Caracalla utilized large south-facing windows to heat the interior spaces
• Courtyards and atriums provided natural light and ventilation, with shading from colonnades or porticoes
• The Romans also used glazed windows (albeit rare) and heated floors (hypocaust system) for thermal comfort

Passive solar in indigenous dwellings

• Many indigenous cultures around the world have incorporated passive solar strategies in their traditional dwellings
• The Anasazi people of the American Southwest built south-facing cliff dwellings that captured winter sun and shaded summer sun
  • Thick adobe walls provided thermal mass to regulate interior temperatures
• Navajo hogans and Pueblo adobe houses also utilized passive solar principles for heating and cooling
• Inuit igloos used the insulating properties of snow and the heat-trapping geometry of the dome to maintain warmth in extreme cold

Pioneering architects in passive solar

George Fred Keck's Solar Park development

• George Fred Keck was an American architect and early proponent of passive solar design in the 1930s and 40s
• He designed the "Solar House" for the 1933 Chicago World's Fair, showcasing passive solar principles
  • Large south-facing windows, overhanging eaves, and a concrete floor for thermal mass
• Keck's Solar Park development in Glenview, Illinois (1940) was a pioneering community of passive solar homes
  • Homes were oriented to the south with large windows and thermal mass for solar heating
  • Demonstrated the potential for energy savings and comfort in residential applications

Passive solar in Frank Lloyd Wright's designs

• Frank Lloyd Wright, a renowned American architect, incorporated passive solar principles in many of his designs
• His Prairie style homes featured low-pitched roofs with deep overhangs for shading and large windows for natural light
  • The Robie House (1910) and Jacobs House (1937) exemplify these principles
• Wright's Usonian houses in the 1930s and 40s further explored passive solar strategies
  • Clerestory windows, concrete slab floors, and radiant heating systems for thermal comfort
  • The Herbert Jacobs House (1944) was an early example of passive solar heating in a Usonian design

Passive solar design tools

Sun path diagrams

• Sun path diagrams are graphical tools used to visualize the sun's position and path across the sky at a given location
• They represent the sun's altitude and azimuth angles for different times of day and year
  • Altitude is the angle of the sun above the horizon, ranging from 0° at sunrise/sunset to 90° at zenith
  • Azimuth is the angle of the sun along the horizon, with 0° being true south (in the northern hemisphere)
• Sun path diagrams are used to design shading devices, optimize building orientation, and assess solar access
• They can be created using physical sun path models, computer software, or mobile apps

Calculating solar heat gain

• Calculating solar heat gain is essential for sizing passive solar systems and assessing their performance
• Solar heat gain is the amount of solar radiation that enters a building through windows, skylights, or other openings
  • Measured in Btu/h or W/m²
• Factors affecting solar heat gain include:
  • Window size, orientation, and shading
  • Glazing type and solar heat gain coefficient (SHGC)
  • Local climate and latitude
• Solar heat gain can be calculated using the following formula:
  • $Q = A \times SHGC \times I$
  • Where $Q$ is the solar heat gain (Btu/h), $A$ is the window area (ft²), $SHGC$ is the solar heat gain coefficient, and $I$ is the incident solar radiation (Btu/h·ft²)
• Passive solar design aims to balance solar heat gain with heat loss to maintain comfortable indoor temperatures

Limitations of passive solar design

Climate-specific challenges

• Passive solar design is most effective in climates with abundant solar radiation and moderate temperature swings
  • Cold climates with prolonged cloudy periods may require supplemental heating
  • Hot, humid climates may face challenges with overheating and moisture control
• Passive cooling strategies, such as natural ventilation and shading, may be insufficient in extreme heat
  • Active cooling systems or dehumidification may be necessary to maintain comfort
• Microclimates, such as urban heat islands or shaded sites, can impact the effectiveness of passive solar strategies
  • Site-specific analysis is crucial for optimizing passive solar design

Balancing solar gain vs heat loss

• A key challenge in passive solar design is balancing solar heat gain with heat loss through the building envelope
• Excessive glazing or insufficient insulation can lead to overheating in summer and excessive heat loss in winter
  • Careful sizing and placement of windows, along with high-performance insulation, is essential
• Thermal mass must be properly sized and located to effectively store and release heat without causing temperature swings
  • Insufficient thermal mass can result in overheating, while excessive mass can delay heat transfer
• Passive solar buildings must also consider the impact of occupant behavior, such as opening windows or adjusting shading devices
  • Occupant education and user-friendly controls are important for optimal performance

Passive solar in contemporary architecture

Integration with active solar systems

• Contemporary passive solar buildings often integrate active solar systems for enhanced performance and flexibility
• Photovoltaic (PV) panels can be used to generate electricity for lighting, appliances, and mechanical systems
  • PV can also power active ventilation or cooling systems to supplement passive strategies
• Solar thermal collectors can provide hot water for domestic use or space heating
  • Integrating solar thermal with radiant floor heating or thermal mass can improve overall system efficiency
• Active solar systems can help offset energy demands during periods of low solar gain or extreme weather conditions
• Careful design integration is necessary to optimize the performance of both passive and active solar components

Passive solar in net-zero energy buildings

• Passive solar design is a key strategy in achieving net-zero energy buildings (NZEBs)
  • NZEBs produce as much energy as they consume on an annual basis
• Passive solar principles help reduce heating, cooling, and lighting loads, minimizing the need for active systems
  • High-performance envelopes, daylighting, and natural ventilation are critical components
• Passive solar design is often combined with energy-efficient HVAC systems, appliances, and lighting to further reduce energy consumption
• On-site renewable energy generation, such as PV or wind turbines, is used to offset remaining energy demands
• Passive solar NZEBs demonstrate the potential for sustainable, energy-independent buildings that prioritize occupant comfort and well-being