Indirect Gain Passive Solar Homes

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How passive solar thermal trombe walls workThe single most widely applicable passive solar design option is indirect gain passive solar. The option works in every climate and every location, from the hottest and sunniest to the coldest and darkest.

"Indirect gain passive solar" translates into thermal storage walls (or Trombe walls). These walls are made from dense or earthen materials such as rammed earth, poured concrete, or concrete blocks. A pane of clear glass is fixed to the wall, about 3-6 inches away from the surface, which allows the sun's light to penetrate and heat the wall. The energy generated is either stored in the wall or escapes into the air between the wall and the glass, heating the air. The warm air either vents out to the sides or can be captured and directed.

Trombe walls are designed for night heating. They absorb the sun's warmth and are built so that the stored heat being taken in on the outside reaches the interior surface at sunset, or soon afterwards. As the inside of the home cools, the wall starts to release its heat and extends the comfortable warm period into the night. The amount of time it takes for the heat to get from the outside to the inside depends on the wall thickness, density and design, and ranges from a few hours to an entire day.

Calculating solar thermal wall thickness by materialMost Trombe walls are between 8 and 18 inches thick, though some are as thin as 6 inches and some as thick as 24 inches. Thicker walls take longer to transfer the heat from outside but they also maintain a more stable interior surface temperature. Trombe walls can hold windows, the same as any other wall

The guts of a thermal wall are usually made of dense material, as we mentioned earlier. However, some designers use water, because it can store more heat (though it releases that heat quicker than masonry). Plastic tubes filled with water run inside the wall and are usually visible on the interior wall because they're more efficient that way. Covering them reduces efficiency but can make them more attractive!

As with any kind of solar glazing, Trombe walls must be designed to prevent overheating. This is usually achieved with overhangs, though some people choose to install movable, insulated panels that cover the glass. The rigid foam panels option is more flexible, as it can also be used to prevent nighttime heat losses, especially in cold climates, though it requires a bit of physical effort from the homeowner.

One final note on thermal walls: many people are reluctant to install them because they think the walls will look ugly. The truth is that, from a distance, they look just like any other solar glazing. It's only when you get up close that you notice there's a wall behind the glass. They can contain windows, so it's not like living in a prison cell, and the interior can be covered with attractive plaster or stucco without significantly reducing heat transfer.