Published on April 28th, 2018 | by James Ayre0
Solar Walls, Trombe Walls, & Passive Solar Heating 101
April 28th, 2018 by James Ayre
Solar walls, glazed solar collectors, and so-called Trombe walls are all different types of passive solar heating technologies based around the use of materials meant to absorb solar radiation (generally, dark-colored materials since dark colors absorb the heat better) and thermal mass. The end goal is to provide space heating, and often ventilation as well.
The means by which this space heating occurs varies by design. Glazed solar collectors work similarly to solar thermal water heaters — as external structures that absorb solar radiation as heat and then redistribute it. A Trombe wall, on the other hand, is located in the building itself, usually right up near a glass window and outfitted with or without a venting system.
Unglazed solar collector heaters are another variant — one whereby sheets of dark perforated metal are used as external on building walls where solar gain is good — but they are usually just used to precondition air before it is drawn into a ventilation system (thereby improving system performance and cutting costs).
As you’ll note, such approaches to passive solar heating are broadly similar to those we discussed in our earlier article about passive solar house design. The basic idea is simply to absorb solar radiation coming through windows in dark materials that can function well as thermal mass, with this heat then re-emitted from the thermal mass at a later time (and over a long time period) as infrared radiation.
The potential value in using a passive solar wall heater of some kind largely lies with the fact that they are relatively easy modifications/additions to existing structures. In other words, you don’t have to remodel your home, as you would if you were to design an in-space passive solar heating home.
Despite the relative ease of building or installing a Trombe wall or a glazed solar collector, the systems can function pretty effectively as heaters in regions where sunlight levels are relatively high during the cold months.
Below, I’m going to provide a bit more in the way of details on the different varieties of solar wall heaters. Enjoy.
Glazed Solar Collectors — External Systems That Function Like Solar Thermal Water Heaters
Glazed solar collectors, commonly known as solar walls, usually look quite a lot like solar thermal water heaters do, with the primary differences being that they are designed with the heating of air in mind rather than water heating and that they are mounted on a sun-facing wall rather than a roof.
That said, systems can be designed to circulate water rather than air — with the water heated by this process then being used inside a building as thermal mass (which will modulate internal temperatures quite a bit through the night if there’s a fair amount of it).
The basic idea is that the air from a building be drawn into the external solar wall heater through convection, heated up in the dark glazed labyrinth of solar tubing, and then recirculated back into the building. As the process is based around convection, two holes are required to be made into the wall in question — one for cooler air to be drawn into via convection, and one for warmed air, which is released into the room.
As should be obvious here, such heaters only work when the air inside the solar wall is warmer than the air inside the source room. Hence the reality that vacuum-insulated solar collectors are considerably more effective than other designs in this application.
As should also be obvious here, placement and orientation are highly important when installing or designing such a system — to the degree that bad placement will make the system ineffective, and a particularly good placement will allow for very effective heating at the exact desired portions of the day.
Trombe Wall — Internal Passive Solar Wall Heater
What’s a Trombe wall? It’s a type of passive solar wall heating system that utilizes a “wall” made of material that’s effective at absorbing solar radiation, in combination with thermal mass, and is located just behind a window or wall of glass.
Imagine a large dark “wall” pressed up perpendicularly almost right next to a window, with vents located both near the bottom and near the top. The sunlight coming through this window is absorbed by the dark-colored “wall,” and then re-radiated out as infrared that greatly warms the air in the limited space between the wall and the window.
This air can then be widely circulated throughout the adjoining room in question simply by placing and utilizing vents both at the bottom and top of the wall — with both vents open, convection will occur, and thus cool room air will be drawn in at the bottom and released at the top after being warmed.
For such a system to function effectively, it needs to be sited so as to maximize solar gain during the colder months of the year (when heating is desired) — as is true of effectively all passive solar heating technologies. The quantity and placement of thermal mass, venting systems, the orientation, and the possible use of window quilts at night (to prevent heat loss when it’s colder outside), will all modify effectiveness substantially as well. The type of window utilized — whether double-glazed or triple-glazed — will of course limit the amount of solar radiation making it inside somewhat. They will also minimize heat losses through the window at night.
Variations on the design include the use of a venting system leading outside — which effectively turns the design into a solar chimney; that is, a means of increasing ventilation and/or cooling rates during the hot months.
While the idea/design has likely been around for much longer, what we know of as the “Trombe wall” came to modern attention in 1881 when Edward S. Morse patented a design. Widespread application didn’t follow until the 1960s or so when Félix Trombe, a French engineer, began experimenting with passive solar wall designs — hence the common name.
As a final note here, despite the use of the word “wall” in the name, there’s no real need to make a Trombe wall completely block a window — doing so will of course increase performance, but spacing can be left at the top to allow for normal window use if so desired.
Other Passive Solar Wall Heater Ideas
Among the many other passive solar wall heater design possibilities out there, some of the most interesting are those relating to greenhouses or solariums. As noted in our article on passive solar home design basics, attached greenhouses or solariums can make sense from the perspective of increasing solar gain and daylighting when they are properly located and built.
An effective means of increasing this utility is to include large amounts of thermal mass and also dark materials that can easily absorb solar radiation and then re-emit it later. While dark stonework is itself a possibility, a common solution that has been seen is to combine said dark stonework with a body of water — water itself being quite effective both at absorbing solar radiation and also functioning as thermal mass; with the effects being amplified when paired with dark stone or concrete. Alternately, glazed solar collectors of various types can be used to increase heat gain in such spaces during the day, through various means.
To further tie this in with our earlier discussion of solar walls, one of the many advantages to the use of attached greenhouses in a passive solar home is that it can be easily opened or closed to the rest of the house depending upon the time of day. In other words, the primary weak point of solar wall design (heat loss at night through the window or collector in question) can be largely curtailed simply by closing the door to the greenhouse at night.
Alternately, though, one can simply mitigate such potential problems through the use of a window quilt or something similar. Greenhouses and solariums of course have their other advantages as well.
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