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Mass Absorption |
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Mass Absorption refers to the process of storing energy in building materials for use at a later time. Materials used in this application are commonly referred to as thermal mass. These materials absorb heat from the sun during the day and slowly release it as temperatures drop later in the day or at night. The slow release of stored heat in the mass prevents rapid temperature fluctuations in the building space. In addition to absorbing solar energy, mass walls also provide the benefit of dampening heat swings in buildings by absorbing internal heat. The materials commonly used for mass absorption applications are masonry, concrete, wallboard, and even water, and are an integral part of many passive solar designs in residential and commercial buildings. In using mass absorption, some elements of the building, or the entire building itself, make use of the natural energy characteristics in materials and air created by exposure to the sun. Implementing mass absorption techniques into a whole building design strategy can reduce or even eliminate the need for mechanical cooling and heating. Additionally, the use of thermal mass is inexpensive compared to the cost of mechanical systems, is usually simple in design, has few if any moving parts, and requires minimal to no maintenance.
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Contents
1.Definition
2. Use/Application
a. Established Techniques
b. Emerging Trends
3. Use an Integrated Approach
4. Resources
5. Associated Strategies
6. Case Studies |
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Definition
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Mass Absorption refers to the process of storing energy in building materials for use at a later time. Materials used in this application are commonly referred to as thermal mass. These materials absorb heat from the sun during the day and slowly release it as temperatures drop later in the day or at night. The slow release of stored heat in the mass prevents rapid temperature fluctuations in the building space. In addition to absorbing solar energy, mass walls also provide the benefit of dampening heat swings in buildings by absorbing internal heat. The materials commonly used for mass absorption applications are masonry, concrete, wallboard, and even water, and are an integral part of many passive solar designs in residential and commercial buildings. In using mass absorption, some elements of the building, or the entire building itself, make use of the natural energy characteristics in materials and air created by exposure to the sun. Implementing mass absorption techniques into a whole building design strategy can reduce or even eliminate the need for mechanical cooling and heating. Additionally, the use of thermal mass is inexpensive compared to the cost of mechanical systems, is usually simple in design, has few if any moving parts, and requires minimal to no maintenance.
Incorporating thermal mass into buildings is utilized throughout the world in both heating and cooling climates and has been shown to result in buildings that have low energy costs, reduced maintenance, and superior comfort. Currently, much of the literature pertaining to the use of mass absorption addresses heating concerns. However, installing a mass storage medium in a building in a hot climate with cool nighttime temperatures can also be an appropriate application. Because of the lag time associated with thermal mass, heat that is absorbed by the mass doesn’t start to heat the space until later in the day, when outside temperatures are beginning to drop. Cooler nighttime air is then used to cool the mass which will be ready to absorb heat again the next day. This is a proven method of reducing cooling loads in hot, dry climates.
Proper use of mass absorption includes: appropriate solar orientation of the building to take advantage of how the sun moves across the sky, proper ventilation, window placement, optimum mass-to-glass ratios, and knowledge of the local climate.
A specific strategy is the use of a Trombe wall. The wall typically consists of an 8–16 inch-thick masonry wall on the south side of a building, with glazing mounted about 1 inch or less in front of the wall's surface. Solar heat is absorbed by the wall's dark-colored outside surface and stored in the wall's mass, where it radiates into the interior space. The Trombe wall distributes or releases heat over a period of several hours. Solar heat migrates through the wall, reaching its rear surface in the late afternoon or early evening. When the indoor temperature falls below that of the wall's surface, heat begins to radiate and transfer into the interior space.
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Use / Application
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In applying mass absorption to either a residential or commercial building, the designer will need to pay particular attention to the sun to minimize heating and cooling needs. Given the proper building site, just about any type of architecture style can integrate mass absorption into the design. Although proper design for thermal mass in a building need not be complex, it does involve knowledge of solar geometry, window technology, the local climate, and familiarity with the sites’ solar and wind patterns, terrain, and vegetation.
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The use of mass absorption can also aid energy conservation efforts because the overall building design directly relates to its overall energy use. Buildings that utilize thermal storage can use the sun to offset heating, cooling, and lighting loads. This will reduce the need for the building to consume energy from other sources while still providing a comfortable environment inside. When the sun's rays enter a building they are partly reflected and partly absorbed by the surfaces. The absorbed radiation heats these surfaces, which in turn, raises the temperature of the surrounding air to various degrees depending on the density of the material.
For instance, a floor material such as ceramic tile on a bed of grout would tend to heat up fairly slowly. Because of this energy absorption process, the surface temperature of the ceramic tile would remain relatively low and would not significantly raise the temperature of the surrounding air. This type of high density surface has excellent thermal mass properties. Therefore, when the sun stops shining on this floor surface, it would give off its heat to the room fairly slowly as the absorbed heat deep in the grout worked its way back to the surface of the ceramic tile. However, a lighter density surface would have poor thermal mass properties (such as a vinyl floor on plywood) and would tend to heat up relatively quickly when exposed to the sun. Since the surface temperature of the vinyl would be high, it would raise the temperature of the surrounding air. Once the sun stops shining on this floor, the mass would tend to give off its heat to the room quite quickly and cool back to room temperature.
The slow release of energy from massive building components contributes to load dampening, which is a result of the reduction in the extreme temperatures which cause spikes in cooling and heating loads. In other words, in the above examples, the massive building will not see internal temperatures as cold or as hot as the same building built with lighter materials. This means that peak cooling loads in cooling climates and peak heating loads in heating climates are significantly reduced. Drastic spikes in temperature are reduced increasing occupant comfort and energy savings. |
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Established Techniques
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Material Selection
Mass absorption works best with heavyweight materials such as concrete which provide a high level of thermal mass. The level of thermal mass of a material is often measured in terms of admittance which has units of Watts per square meter Kelvin (W/m2 K). An example of high thermal mass construction is a traditional brick and block wall with a plaster finish which has an admittance of around 6 W/m2 K. An example of a low thermal mass construction is a timber frame wall, which has an admittance value of approximately 0.85 W/m2 K. As can be seen from these examples, material selection is important for proper mass absorption as a heavyweight wall can have as much as seven times more thermal mass than a lightweight wall. Typical thermal mass building elements are poured concrete floors, ceramic tile floors on a bed of grout, hardwood flooring, brick or stone feature walls, multiple layers or thicker drywall, and water.
Residential Applications
Opportunities abound in the residential sector for use of mass absorption techniques. There are 2 basic types of passive solar systems that employ the use of thermal mass - Direct Gain and Indirect Gain systems.
In a direct gain system, thermal mass is incorporated into the living space in the floors or walls. When the ratio of south facing glazing to floor area begins to exceed approximately 8%, thermal mass should be installed to store excess energy and prevent overheating of the particular room. Adding mass will allow for the use of more glazing for increased solar contribution without overheating. For optimum and effective use of the thermal mass, it is important to charge the mass during sunlight hours and discharge it during the night. When adding mass, its use and location must be considered. For example the decision to add carpet over a floor surface, which acts as insulation, can reduce potential heat storage by as much as 70%. The benefits of proper application of mass absorption in a residential building will be comfort (warm in the winter and cool in the summer), economical (by decreasing heating and cooling loads), and durable through use of long-lasting, low maintenance materials.
In an indirect gain system, components such as sunspaces, thermal mass walls or trombe walls are used. These structures are designed to collect a large amount of heat during the day and release it at night. Sunspaces are similar to greenhouses, but they are generally not heated or cooled by the home’s mechanical equipment, and their primary function is to supply the house with heat. Thermal storage walls can be used in heating climates or cooling climates that have large variations in daily temperature. In heating climates, they generally consist of a floor to ceiling mass wall located in direct sunlight and are separated from the exterior by a glazing unit and an air space. The sun strikes the mass through the glass, heats up and radiates into the room. A trombe wall is essentially a thermal storage wall with vents evenly distributed along the bottom and top of the wall. Heated air flows through the top vents into the living space by natural convection, and cool air is drawn in from the living space through the bottom vents. In heating climates, it is recommended that dampers be installed to prevent the backflow of conditioned air from the living space into the trombe wall space when the trombe wall is too cold.
Commercial Applications
In commercial buildings, such as offices, mass absorption techniques can delay peak internal temperature up to five or six hours, which typically occur in the late afternoon or evening after the occupants have left. At this time, heat gains from the sun, occupants, lighting, and office equipment are greatly diminished and the building mass stops absorbing heat. As the evening progresses and the external air temperature drops, the use of an automatic ventilation system is an effective way of removing accumulated heat from the building and lowering its temperature for the following day. To make this heating and cooling cycle most effective, it is essential that heavyweight mass elements, such as floor slabs, are thermally exposed so heat can move freely between the internal environment and the concrete.
The ability of thermal mass to absorb heat results in a lower cooling load in buildings, which helps reduce associated energy use and CO2 emissions. In many cases, air conditioning can be avoided altogether through the use of thermal mass and effective ventilation, a combination that has been used very successfully in many commercial and public sector buildings.
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Emerging Trends |
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The increasing understanding of the link between fossil fuel use and the global environment, along with the ever increasing worldwide demand for energy, continues to push approaches that maximize energy and reduce overall emissions in the commercial and residential building sectors.
Currently, the biggest trend in mass absorption techniques is their use in whole building design strategies. Mass absorption techniques are most effective when used with complementary systems and approaches. The U.S. Department of Energy (DOE) outlines the variables that need to be considered when deciding to effectively implement mass absorption into a building. These are: building orientation including glazing and room layout; climate variables such as sun, wind, air temperature, and humidity; building use types to include occupancy schedules and use profiles; lighting and daylighting to include electric and natural light sources; the building envelope such as its geometry, insulation, fenestration, air leakage, ventilation, shading, thermal mass, and color; internal heat gains from people, lighting, office equipment, and machinery; HVAC systems, and controls; and energy costs to include the fuel sources, demand charges, and conversion efficiencies. |
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Use an Integrated Approach |
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A new way of thinking must be adopted to meet the goal of reducing carbon emissions associated with buildings. Your solutions can begin by integrating four possible methods. None works alone, and they are not all relevant in considering every strategy. However, considering the following tactics is necessary:
- Reduce the overall energy use in your building
- The implementation of mass absorption techniques can reduce the overall energy use of a building by decreasing the size of heating and cooling systems and thus cost.
- Specify energy efficient equipment and technologies
- Use an integrated system approach to spec out the most cost effective energy efficient equipment in a building that also includes thermal mass will reduce overall energy use and emissions.
- Use renewable strategies and purchase green power
- Use of mass absorption by utilizing present solar income to decrease energy loads and offset emissions of conventional building energy systems.
- Educate building owners, operators, and occupants
- On function and operations of installed technology
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Last modified at 2/27/2009 10:21 PM by jamie nace
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