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Passive Solar

Passive House, or passivhaus, is sometimes confused with passive solar, and although the latter is an important component in Passive House design, the terms are not interchangeable.

Passive solar refers to the strategy of using the building itself – the windows, walls, floors –  without added equipment, to collect, store, and distribute solar energy as heat. A part of passive solar design is also the control of unwanted solar energy in the summer, through the use of overhangs etc. The idea of passive solar contrasts with active solar, which uses equipment (e.g. photo-voltaic panels, or solar hot water collectors) to do the same.

Passive solar requires thoughtful consideration of the local climate, solar access, building siting and orientation, landscaping etc.

There are several types of Passive Solar. The first, and most basic, is Direct Gain, where the interior space is heated directly through south-facing windows (of course this assumes the building is located in the northern hemisphere).

 

 

 

 

 

 

 

In Indirect Gain, a thermal mass, for example a “trombe wall”, is located between the south-facing windows and the space to be heated. The advantage in this method is that the transfer of heat to the interior is delayed, so a thermal mass heated during the day may release its heat to the interior at night.

 

 

 

 

 

 

 

The third type is Isolated Gain, using a separate Sunspace, or Greenhouse, to borrow heat from as needed.

Some Passive Solar Fundamentals:

  • Orientation – if possible, orient the long axis of the building in the east-west direction, to maximize southern exposure. Ideally there will be unobstructed access to the sun during most of the day, and the principle use spaces of the building will be located on the south side, with service spaces (e.g. example bathrooms, mechanical, storage) on the north side.
  • Windows (free solar heat generators) – in general, optimize the amount of windows on the south side of the building, and minimize the amount of windows on the other three sides.
  • Control – use the architecture itself (eaves, awnings, exterior shades, sliding screens etc.), to block summer sun, but allow winter sun to penetrate interior. The latitude determines the ratio of depth of overhang to height of glazing. You can also use the landscaping for control. Deciduous trees on south side can block unwanted summer sun, but allow the winter sun to pass through. Evergreen trees on the east and west sides can block unwanted solar gain.
  • Thermal mass – Thermal mass refers to a material that can absorb the solar heat that enters a building – it can be an exposed concrete floor, ceramic tile, even gypsum wallboard.
  • Distribution – Thermal mass distributes the heat by radiation; In indirect or isolated passive solar, distribution can be by radiation, convection, or assisted by mechanical means.

Some Passive Solar Challenges:

  • Passive solar design guidelines often assume a large, flat, unobstructed site with no trees. In urban areas, lots oriented east-west typically have (sometimes tall) neighbors tight to the south, while lots oriented north-south will have a short face on the south side, neither of which is ideal. Sites on north facing slopes are not ideal – sometimes the site itself can block the sun (esp. when the sun is low in winter, when you need the solar gain the most). Conversely, sites on south facing slopes are preferred.
  • Seattle homes are sometimes designed as “View Machines”, and often that view is to the west – maximizing windows for view can be at odds with passive solar ideals.
  • Shading or screening of south-facing windows, to minimize summer heat-gain, can make rooms darker in our already gray winter months.
  • Remodels – passive solar design guides often assume you’re building a new house from the ground up, and so have more opportunity for optimal siting, orientation etc. A remodel or addition project has more constraints, e.g. existing architecture to relate to, structural issues that may make large areas of glazing difficult, etc.

That being said, an existing house can be remodeled to incorporate passive solar strategies, e.g. adding more windows on the south side, adding awnings over south facing windows, or adding thermal mass on the interior.

Without going into detail, I’ll list a few innovative ideas relating to passive solar design:

  • Annualized geo-solar – this refers to capturing warm season solar heat and storing it for several months, until it’s needed in the cold season. A variation on the Thermal Flywheel idea;
  • Phase change materials – usually eutectic salts, materials that store solar energy as latent heat. The sun heats and melts the material during the day – at night the material reverts to a solid state, and the stored heat is released. Phase change materials can be incredibly efficient in storing heat – as much as 80 times as effective as water;
  • Living Walls, depending on the plant type, can allow winter sun through, but will block the sun when it’s filled out in the warmer months;
  • Planning for future active solar – I like to think of this as another passive solar fundamental. Configure the roof to maximize solar orientation and access for potential future PV and solar hot water systems. In projects not installing a solar system, pre-pipe for future installation.

The heat-gain benefits of passive solar design should always be complemented by strategies to minimize heat-loss, such as adding insulation (beyond code), using high-performance windows, making the building super air-tight, using an HRV, using high-efficiency lighting, plumbing fixtures, appliances and systems, etc. This meshes with the goals of Passive House (you knew I was going to circle back to that, didn’t you?) – to equalize, as much as possible, the heat loss through the envelope of the building, with the heat gains, both external (solar) and internal (peoples bodies, appliances, lighting, etc.).

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