Image Credit: Alex Wilson
More Energy Solutions
Last week I wrote about sunspaces and how they can be used to deliver passive solar heat to our homes. Another option for passive solar heating is the Trombe wall, or thermal storage wall.
While the sunspace is an “isolated-gain” solar system, a Trombe wall is an “indirect-gain” system. Here’s how it works: On the south side of a house you have a high-mass concrete or masonry wall whose exterior surface is painted a dark color. A layer of glass (or some other type of clear or translucent glazing) is held away from the wall surface by a few inches or more. Sunlight shines through the glazing and is absorbed by the dark wall, which heats up. The solar heat conducts into the wall where it is stored, and it gradually moves through the wall to the inner surface, where it radiates its warmth to the room.
A short history of Trombe walls
The Trombe wall is named after a French engineer Félix Trombe, who popularized this heating system in the early 1960s. The idea actually goes back a lot further. A thermal-mass wall was patented in 1881 by Edward Morse. In the U.S., interest in Trombe walls emerged in the 1970s, aided by researchers at Los Alamos National Laboratory in New Mexico.
I was fortunate enough to be working in Santa Fe in the late 70s, for the New Mexico Solar Energy Association, and I became particularly interested in Trombe walls. In fact, my first article in a national magazine was on Trombe wall retrofits — in Solar Age magazine in 1979. I also wrote the obscure Thermal Storage Wall Design Manual in 1979 (see photo), which gained some prominence among the small cadre of passive solar designers around that time.
Trombe walls are particularly well-suited to sunny climates that have high diurnal (day-night) temperature swings, such as the mountain-west. They don’t work as well in cloudy climates or where there isn’t a large diurnal temperature swing. In New Mexico, where homes have been built out of adobe (dried mud) bricks for hundreds of years, even an unglazed south wall will deliver some heat into the house — if you add a frame and layer of glazing on the outside of the wall the performance improves dramatically.
We are used to insulating walls, but with Trombe walls there is no insulation. The system works with a material that is both very heavy (high heat capacity) and fairly conductive (low R-value per inch). The trick is to choose the right material and size the wall thickness so that the solar heat makes it through to the inner surface by nighttime. If it’s too thick, it won’t be as effective at delivering solar heat, and if it’s too thin it will result in too much heat loss at night.
Tweaking a Trombe wall
An overhang is typically built that extends out over the Trombe wall above it. This will shade the wall from direct sun during the summer (when the sun is high overhead), but allow full solar exposure in the winter (when the sun is lower in the sky).
Top and bottom vents can be installed through the masonry wall to deliver more heat into the house during the daytime hours. Warm air in the space between the glazing and wall surface rises and enters the room, being replaced by air from the house entering through the lower vents in a convective loop. These vents should be closed at night so that the air circulation doesn’t reverse, with air next to the glazing cooling off and pulling in warm air from the room through the upper vents and delivering chilled air to the room through lower vents.
Vents through the glazing can also be installed and seasonally opened and closed. In the summer months — when you don’t want the Trombe wall delivering heat into the house — these vents are left open. Screens on the vents keep out insects and other unwanted visitors.
Like other passive solar heating systems, Trombe walls don’t require fans or pumps. Part of the house itself is turned into the solar heating system.
In addition to this Energy Solutions blog, Alex contributes to the weekly blog BuildingGreen’s Product of the Week, which profiles an interesting new green building product each week.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News, which is now in its 20th year. To keep up with his latest articles and musings, you can sign up for his Twitter feed.
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7 Comments
Direct Gain for New Construction
Different people may interpret reports differently, but didn't Doug Balcomb prove in the 80's that direct gain systems were more effective and less expensive than Trombe walls?
For retrofits on high mass walls like you describe, the Trombe wall design is great. I just hope that folks building a new home wouldn't think it's the best passive solar method.
Such climates are few and far between
Alex,
Félix Trombe was working in Europe and was looking for solutions that work with existing uninsulated buildings. It sounds like your experience in New Mexico in the 1970s reflected a set of similar conditions.
However, it's no longer the 1970s, and most of us don't live in New Mexico or Odeillo, France. In most of the country, there are months at a time when the weather gets cold and stays cold. It's very unwise to build an uninsulated wall in almost any climate in the U.S., so it's time to cross Trombe walls off our list of tricks -- unless, perhaps you are retrofitting adobe buildings in New Mexico.
Even there, I imagine that a good case could be made for retrofitting wall insulation instead of installing a Trombe wall -- and the maintenance issues associated with the insulated wall would be likely to be far less of a headache than maintaining a Trombe wall.
Applicability of Trombe walls
Martin,
I agree that Trombe walls are not a good solution for most climates and, at least with new construction, sacrificing insulation for this passive solar heating option would rarely make sense. I'm not sure I'd go as far as you in saying they never make sense, though. It would be interesting to carry out some detailed energy modeling of different options to answer that question.
Next week I'll be addressing an option, direct-gain passive solar, that — as Kevin Dickson points out — generally makes a lot more sense.
Trombe Walls
Alex et al:
While trombe walls are not for every climate, when we can use them we should. They are perhaps the most effective passive heating strategy available, when handled properly. This is the purpose of my remarks: suggesting to the public that trombe can be vented is poor practice. I also built many trombe walls in New Mexico in the 70's and it took only one project to learn that venting is death to any trombe wall. First of all, if you want direct gain, just design in a window. It's ludicrous to spend money creating a mass wall, than glazing over it, only to ruin its performance with vents.
Trombe walls rely on precise wall thickness and continuous heating, thus vents eliminate the heat impinging on the wall. Vents also allow dust and contaminants to degrade the glazing system making it impossible to clean interior surfaces. I recommend walls using either solid concrete or grout filled-CMU at 1" thickness per hour of insolation. Thus a 12" wall is usually ideal as it allows heating to begin at approximately 8 AM on a winter morning, giving heat back from 8 PM-* AM the following morning. An air-tight aluminum glazing system sealed to the mass wall is highly desirable as wood windows are prone to air leakage. Using selective surfaces on the mass wall can improve performance, but they're ugly, expensive and overall not worth having. Pick a dark color such as a deep red, and if you use CMU, don't pick a lightweight block. You want high mass. Lastly, provide a calculated roof overhang to eliminate solar gain during the cooling season. In northern latitudes we are blessed with a dramatic solar altitude change by season. If Native cultures could figure this out over two thousand years ago, we should be able to accommodate this device.
Anyone wishing additional information can view Ecostructure, September 2007 article "Circle of Life" regarding our projects at Navajo Prep, or see our website at http://www.tp-studio.com.
Response to Stephen
Stephen,
Your wrote, "Trombe walls ... are perhaps the most effective passive heating strategy available, when handled properly."
Almost every solar expert in the world would disagree with this statement, with the possible exception of the late Felix Trombe.
Trombe walls
Thank you for your comments Martin. I will, however, stand by my statement, based on my experience. Direct gain strategies are effective, so long as you need heat right away and there's adequate insolation. The problem is that daytime hours are frequently the time when heating may not be required. With trombe walls the mass materials provide the thermal storage that will allow heat transmission during nighttime when it is needed most. It also provides for those periods when sun light is not available by creating a heat sink. That these walls, when properly shaded, provide a cool sink in the summer is the clincher. Few passive strategies can come close to providing this year round benefit.
One system, as yet unmentioned, is the thermosiphon. I had the privilege of working with Mark Jones in the 70's in Santa Fe. Mark was the first individual to incorporate passive design into development housing. This can be viewed at Altimira on the road to the ski basin. Here, direct gain, trombe wall and thermosiphons are in evidence. Back to the thermosiphon: Mark tested a thermosiphon design on Lot 45 near the area now known as Monte Sereno. The Los Alamos National Laboratory Energy Group, under the direction of Robert Jones, tested the house for over a year and found it to be 98% efficient for heating and cooling! The only shortcoming to this performance milestone was the cost involved. The carefully designed, stone-filled arterial system under the floor slab is probably prohibitive to most home buyers. Thus I'm back to trombe walls: they are perhaps the most efficient passive strategy available to home owners anywhere on the Lower Colorado Plateau.
Thermal Storage Wall Design Manual
Alex's Booklet is still available for Download:
http://nmsea.org/lib/ThermalStorageWallDesignManual.pdf
I love the hand-drawn graphs!
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