The Crestone Eagle, May 2006:

Designing thermal mass for passive solar heat storage
by Paul Shippee

Passive solar heating of homes is usually thought to be simple enough: just let the sun shine in through south-facing windows on winter days. Everyone knows that this technique works. Many people have enjoyed cost-free warmth and comfort from the sun in their homes on cold days.

However, during winter nights, which are much colder and also twice as long as winter days, the thermostat tells the furnace to turn on, and we return to our dependence on fossil fuels to keep ourselves warm until the sun returns.

In a conventional wood stick-frame house with sheetrock on the interior walls the daytime sunheating scenario described above might contribute 5-15 percent to the total 24 hour heating load of the house for sunny days. The other 85-95 percent of the required heat must be supplied by a fossil fuel source, such as propane or electric, or by a wood burning source.

Given the above conventional scenario it might be interesting to ask: What would it take to reverse those percentages in our southwestern high altitude climate? In other words, how can we get 85-95 percent of our total winter heating requirement from the sun and still retain the simplicity of passive solar heating?

You may remember that it was explained in the previous article in April that passive solar heating is defined as using the house itself as a solar collector. It is for this reason that I am using the word “simple” to describe passive solar home heating. It is not a mechanical system and does not depend on electricity for its operation. You might say it is a natural system, using natural means of heat flow to achieve comfort round the clock when designed properly.

It is common to oversimplify the application of passive solar. It seems that builders, designers and architects often tend to do just that. This is usually because they might not have taken the time and trouble to fully understand and appreciate the dynamic and critical role that thermal mass plays in achieving much higher passive solar heating percentages of up to 100 percent. The practical result of oversimplifying is that, because of inadequate thermal mass placement, the house overheats, then begins to get cold immediately at sundown.

What is this dynamic and critical role that thermal mass can play in a simple passive solar system? What is thermal mass? Thermal mass in the solar heating context refers to the capacity of dense materials to store significant quantities of heat, at a reasonable rate, as their temperature rises (and then release it as the temperature around them falls). The two main materials used for thermal mass in passive solar buildings are water (in containers) and earth materials in the form of adobe, concrete, earthen plaster, bricks, and stone. To get technical for a moment, a cubic foot of water can store nearly three times as much heat as a cubic foot of any earth material for each degree of temperature rise. You could say that water is more efficient, space-wise, than adobe or concrete for storing heat.

If a regular sheetrock house has a fair amount of south windows, it will overheat on sunny winter days and cool down rapidly after sundown. Such large interior temperature swings are unacceptable from a comfort point of view, and a portion of the collected solar heat is unusable and therefore wasted as well.

The remedy for this untenable situation is to add interior thermal mass materials so that interior temperature swings are kept to 10°F, say between 65°F and 75°F. The main functional idea behind deploying thermal mass materials is that they suck up excess solar heat during the day, thus preventing overheating of the house air. Then during the night this heat is slowly released into the rooms. The stored solar heat, you could say, is invited out of storage as the house air tries to cool down.

The volume, location, and surface area of heat storage materials can be a creative architectural element inside the house. The main design strategy is to balance the relationship between these four elements: desired solar heating fraction (percentage of heating by solar); amount of south glass area; quantity and location of thermal mass; and daily allowable temperature swings (comfort factor).

These quantities should all be strategically coordinated from an engineering point of view by using accurate numbers. Careful and deliberate sizing of these elements will produce the best results in terms of long-term fossil fuel savings, as well as having an attractive and comfortable home.

Lastly, the actual location of thermal mass materials in the home requires attention to solar architecture, as well as heat flow engineering details. The three most popular types of passive solar systems are direct gain, Trombe wall, and sunspace. A rule of thumb for designing a comfortable direct gain system, which lets sun shine directly into rooms, is to distribute the thermal mass materials over a large area of walls and floor, about nine or ten times the area of south glass and about 4-6 inches thick (for earth materials).

For a Trombe wall design, a 12-inch concrete wall is placed directly behind the south glass. For a sunspace design this same wall is moved back several feet from the glass to create a greenhouse area that is allowed to swing in temperature more than the house interiors. The solar rays heat one side of the wall and this heat begins to show up on the interior side at evening time.

From an economic point of view, the added building cost for deploying quantities of thermal mass materials can be compared to the future life cycle cost savings of the fossil fuels not used.

Free convection air flows, facilitated by various venting schemes, can augment the distribution of solar heat to the cooler interiors. The performance of any passive solar system can be greatly enhanced by employing moveable insulation to cover south windows during long winter nights. This aspect will be covered in detail in the next article to follow in June. For more information go to www.crestonesolarschool.com .

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