Solar Passive Building Design Strategies

Solar Passive building strategies are ones which work without use of any mechanical and electrical devices that are used in active solar building strategies. The building sector uses about 50% of the world’s energy in construction, operation and maintenance.

Of this, 85% is operational energy i.e. mainly used for space heating and cooling and only 15% is the embodied energy. This points at the importance of careful planning of the thermal parameters during the design stage. This climate sensitive approach to design aims at providing a high standard of thermal comfort quality, energy savings and environmental benefits. Many modern computer modelling software are available for analysing and deciding on the design of the various passive strategies by simulating the data and calculating performance predictions till desired result is achieved.

Some metrics for Solar Passive Building

Thermal comfort is defined as the optimal thermal condition in which the least extra effort is required to maintain the human body’s thermal balance. The comfort temperature in terms of outside dry bulb temperature can be defined using Fanger’s equation of adaptive comfort : Tm=17.6+0.31 toa (mean) where Tm is thermal comfort in degree Celsius, and toa mean is the mean outside air temperature. A variation of ± 0.5C is considered to define the comfort zone. The various factors that affect human comfort are broadly divided into two types as below –

1. Environmental factors – Air temperature, Temperature of surrounding surfaces, Relative humidity, Air velocity
2. Physiological factors – Clothing, Activities, Age, Sex

Other factors affecting thermal comfort are the macro and micro climatic factors. The macro climatic factors are topography, water bodies, altitude, vegetation, urbanization, diurnal variations in temperature. The micro climatic factors are ground surface, landscape and built form.

The Sun-path for passive design – The seasonal variations in sun’s path in terms of the 47-degree difference in the altitude of the sun at solar noon between summer and winter forms the basis of passive solar design. Combining this information with the local climatic data, heating and cooling requirements helps determine the time of the year when solar gain will be beneficial for thermal comfort and when it should be blocked from entering the building.

Building physics –

Indoor climate depends largely on the outdoor climate especially in buildings that are passively heated or cooled. Heat flows from warmer areas to cooler areas and is transferred by three mechanisms: 1) conduction, 2) convection and 3) radiation.

Conduction refers to the direct heat flow through matter and results from actual physical contact of one part of the same body with another part, or of one body with another. Convection is the movement of air that occurs as heated air becomes less dense and then rises, cooler air is thus drawn in to fill the space left by the displaced warm air. Radiation is the process of heat transfer from warmer surfaces to cooler surfaces that face each other but may not be in contact.

Solar Passive Building Design –

It is important to analyse the site, location, climate, local sunpath, solar orientation of the building, the prevailing insolation (i.e. latitude/ sunshine/ clouds/ precipitation) and project requirements to know the strategies which will be suitable for a project.  In Passive solar building design, the windows, walls, floors are made to collect, store, reflect and distribute solar energy in the form of heat during winter and rejecting the solar heat when it is summer. Movable shutters, shades, screens, curtains etc can accommodate the day to day and hour-to-hour solar gain and insulation requirements.

Building form can affect solar access and wind exposures as well as the rate of heat loss or heat gain through the external envelope. Buildings exposure to external elements can be contained through compact building envelope, use of sheltering and buffering.

Compactness and zoning calls for modulating the built form in terms of its built mass proportions, density and size, surface to volume ratios, zoning the built form on site as per wind directions and solar orientation. Compactness of a building is measured using a ratio of surface area to volume i.e. compactness = S/V where S=surface area and V=volume.

Optimizing the Street orientation, street width to building height ratio on site for compact planning. Street orientation determines the azimuth upto which solar radiation can be cut off in buildings. Ratio of street width to building height determines the altitude upto which solar radiation can be cut off.

Perimeter to area (P/A) ratios indicate radiative gains or losses and efficient ventilation. Low P/A ratios are suitable for hot-dry climates.

Building Orientation is important with regards to the wind direction and surrounding built forms. Amount of solar radiation falling on surfaces varies depending on the view and exposure to the sun. in tropical climates, east and west receive maximum solar radiation during summer. Southward orientation has radiation during winters due to the low sun angle which can be used for heat gain during the cold periods. North has brief period of solar radiation in mornings and later afternoons in summer.

Solar Passive Building components

Opaque surfaces-

Conductivity (k) having unit of W/mK is defined as the rate of heat flow through a unit area of unit thickness of the material, by a unit temperature difference between two sides. Resistance (R ) is defined as thickness/ K-value. Transmittance (U-value) is defined as 1/R with unit of W/m²K. The lower the U-value for the material, the better the resistance of the material to transfer heat.

Thermal insulation helps reducing the U-value for the wall section. Insulation is always placed on the hotter side of the surface ie. Externally for summer cooling and internally for winter heating. Insulation is used in combination with heat storing material such as walls and floors and it should be placed inside in passively cooled buildings. Thermal storage and time lag play a significant role when one wants to take advantage of daytime surplus heat energy in evening. Time lag is the time delay between the peak outside temperature and the peak inside temperature.

Infiltration is the phenomenon of heat loss or heat gain through joints, cracks, opening of doors, faulty construction, and gaps in doors and windows. It is driven by two factors – wind pressure and thermal buoyancy. Control of infiltration can be implemented in the design process with proper detailing, orientation, window type selection, good quality construction materials.

Solar heat gain factor = transmitted solar energy / incident solar energy x 100. It is an important criterion for selecting materials with intense solar radiation. This value should not exceed 4% in warm-humid climates and 3% in hot-dry climatic zones.

Transparent surfaces – heat loss or gain through glazing is done by convection and radiation at its boundary surfaces and by conduction across the material itself. With single glazing the U-value depends on the surface’s resistance. With multi-layered units e.g. double glazing units, the reduction in U-value is due to the additional thermal resistance provided by the airspace between the layers, further reduction can be achieved by applying special coating to the glazing e.g. low-emissivity metallic coating.

In a Solar Passive Building, efficient window design needs to take into consideration the frame materials, glazing area, glazing materials, number of glazing layers, orientation, slope, external reflectors, shading devices, latitude and radiation of the place. The design of shading devices depends on the latitude of the place, azimuth angle of the sun and angle of the altitude. External shading devices can be fixed or movable, vertical or horizontal. Horizontal ones are effective for south and north direction wheareas vertical ones protect the windows from south west and northwest solar radiation. Internal shading devices include curtains, blinds, roller devices.

Advanced solar passive techniques

1. Passive solar heating – heating cost of the building can be reduced by improved insulation and reduced infiltration. Use of solar passive elements for heat gain and storage are provided.
   1. Direct gain – large vertical south facing window along with floor, internal wall, insulated external wall to be used as storage element.
   2. Indirect gain – the tromba wall, mass wall, water wall, roof pond are all indirect gain systems that combine collection, storage, and distribution functions within some part of the building envelope, which encloses the space.
   3. Insulated gain – solar collection is thermally isolated from the living spaces of the building and the energy transfer process from the collector to the space takes place through a thermos phonic loop. Eg. Sunspaces with under floor rock beds, isolated mass wall.
2. Passive cooling in a Solar Passive Building – For passive cooling first we need to control the amount of heat gained from solar radiation and the hot air that reaches the building. Then minimize the effect of unwanted solar heat within the building skin. Next reduce the internal heat gains from appliances and occupants and finally to use environmental heat sinks to absorb any remaining unwanted heat by applying the following –
   1. Evaporative cooling – direct evaporative cooling systems, indirect evaporative cooling systems
   2. Radiative cooling – nocturnal cooling, roof pond with movable insulation
   3. Ground cooling – earth berms and geothermal cooling
   4. Ventilation –
      1. natural ventilation (windows, single sided, cross ventilation, stack ventilation using chimneys or atrium, louvers, turrets, window towers),
      2. mechanical ventilation – mechanical extract, mechanical supply, balanced supply and extract

• hybrid ventilation systems in Solar Passive Building – these systems are combination of natural and mechanical systems. Types include seasonal hybrid, spatial hybrid.

3. Daylighting – daylighting provides illuminance, visual comfort, psychological comfort to the occupants. Factors affecting daylight distribution are room depth, height of the window head, shading devices and glazing type. Innovative daylighting systems include
   1. light shelves – internal or external light shelves
   2. mirrored louvers – fixed and movable
   3. light wells,
   4. atrium,
   5. hybrid daylighting systems i.e. using daylight as main strategy and artificial lighting as secondary one by using automatic switching controls.