This Green-in-Practice will explain how to use the physical properties of concrete to optimize the energy efficiency of your green building. You will learn about how energy is used in buildings and about concrete’s thermal mass properties, which you can use to minimize peak heating and cooling loads. Finally, we’ll discuss energy modeling, the computer tool used to analyze and optimize the building envelope and systems for peak energy efficiency. Energy modeling is required to achieve points under LEED’s Energy Optimization Credit, EA 1.
How Energy Is Used In Buildings
Buildings are designed to meet occupants’ needs for thermally comfortable, well-lit, well ventilated spaces. Energy in buildings provides for lighting, appliances and equipment, and service hot water. It is also used to condition interior spaces with ventilation and temperature control. Energy used for temperature control provides heating, cooling, and pumping. How energy consumption is distributed among these various uses depends on the local climate and on the type of building. Figure 1 illustrate a typical energy use breakdown in a cooling-dominated and a heating dominated climate.
Figure 1. Typical Energy Use Breakdown for Cooling-Dominated and Heating Dominated Climates [[This is Figure 4.2 from the CA Guide]]
In addition to climate, the building type and size affect energy usage and consumption. Buildings that are smaller than 50,000 square feet have energy demands that are "skin dominated." That is, heating and cooling energy demands are largely driven by climate and are heavily influenced by how well-suited the building skin, or envelope, is for that climate. Examples of typical skin-load dominated buildings include single-family homes and low-rise multi-family housing, small warehouses, and small retail facilities.
Buildings that are larger than 50,000 square feet typically have energy demands that are "load dominated." That is, heating and cooling energy demands are largely driven by the internal loads of the building (predominantly the heat produced by lighting, equipment and people), which are large enough to dwarf climate-related energy demands. Examples include schools, offices, and large retail complexes. Such buildings can require cooling year-round.
Thermal Mass in Buildings
A range of construction products and materials with different weights (masses) are available to designers. Depending on the choice, the resulting building can be considered as a light weight, medium weight, or heavy weight building. In some situations, you can take advantage of the mass of a medium or heavy weight building to save energy. In this context, the mass is referred to as “thermal” mass.
Consider this heavy weight residence in a sunny, hot climate (Figure 2). In the summer, the family spends the daytime in interior rooms on the first floor, which stay cool due to the thick massive walls and roof. After dark they move out onto the open roof, which quickly loses its heat to the clear night sky and offers a cool, well-ventilated area. In winter, the pattern is reversed. The family spends daytime hours on the roof in the sunshine. At night they retreat to rooms on the upper story, where the walls have retained some of the daytime sun’s heat energy.
Figure 2. Example of massive construction in a hot climate. [[figure 4.1 of CA Guide]]
In modern buildings in North America, this kind of peripatetic occupancy is usually impractical. Nonetheless, there are still many ways to use thermal mass as a design strategy to improve energy performance without compromising thermal comfort.
How Does Thermal Mass Affect Energy Performance?
The thermal function of thermally massive envelope components is complex. Consider that building heating and cooling systems must respond to changing conditions in outside air temperature, occupant and equipment activity, and incident solar energy over the course of a day. For a thermally massive building, the same external and internal loads exist but the building responds differently than a light-weight building because it has much greater capacity to absorb and retain heat. This produces three effects (See Figure 3):
1. Mass moderates indoor temperature fluctuations, reducing spikes in temperature.
2. Massive wall and roof slow the transfer of heat through the building envelope.
3. Mass can store energy thus shifting demand to off-peak time periods, potentially reducing peak loads and avoiding peak utility rate periods where time-of-use rate structures are used.
Figure 3. Time Lag and Temperature Damping [[Figure 4.4 of CA Guide]]
Thermal mass is more effective in reducing cooling loads than heating loads. The most energy is saved when reversals in heat flow occur within a wall during the day. So, thermal mass is most effective in places and in seasons with large daily temperature fluctuations above and below the balance point temperature (BPT) of the building. The balance point temperature is the outdoor temperature below which heating will be required in the building because internal heat gains are less than heat loss through envelope and ventilation. (For a typical four-story office building the balance point is about 50oF (10°C)). For these conditions, the mass can be cooled by natural ventilation during the night, and then be allowed to “float” during the day. When outdoor temperatures are at their peak, the inside of the building remains cool because the heat penetration through the mass is delayed. Although few climates are this ideal, thermal mass in building envelopes can still improve energy performance in most climates. Often, the benefits are greatest during summer and fall, when conditions most closely approximate the “ideal” climate described above. In heating-dominated climates, thermal mass can be used to effectively collect and store solar gains or store heat provided by the mechanical system to allow it to operate during off-peak hours. In the cooling season, when daytime peak electricity rates apply, a building with thermal mass can be “charged” or cooled in the early morning hours. The result will be less energy used during the peak hours when electricity rates are higher.
Because of the interrelation between climate, internal loads and its absorptive nature, thermal mass:
• Works best in commercial building applications
• Works well in residential applications
• Works best when mass is exposed on the inside surface
• Works well regardless of the placement of mass
Comfort & Control Considerations
Reducing energy consumption can reduce the operating costs of a building. However, if energy-saving strategies compromise occupant comfort, the productivity of workers in a building will be reduced. Since salary costs per workspace are about 100 times higher larger than energy costs, even a 1% change in productivity represents as much as the building’s entire energy bill. For this reason, the importance of occupant comfort is recognized by building designers with the use of standards like ASHRAE 55-1992 (Thermal Environmental Conditions for Human Occupancy) and ASHRAE 62-1999 (Ventilation for Acceptable Indoor Air Quality).
A person’s perception of thermal comfort depends in part on the temperature of surrounding air and objects. Although normal human temperature is 98.6oF (37oC), a comfortable ambient temperature for humans is about 68oF (20oC) (although this varies with the individual and clothing). Humans rely on this temperature difference because our bodies are constantly generating heat and need to reject this heat to the cooler air and objects.
Designing for thermal mass strategies requires taking into consideration the temperatures of thermal mass elements over the diurnal cycle, as well as their line-of-sight relationship with building occupants. The line-of-sight relationship is important because the massive elements of the building will radiate warmth or cold to any humans within sight line. ASHRAE 55-1992 offers guidance on acceptable parameters in situations where occupants are exposed to warm or cold walls and ceilings. In general, radiant walls are more acceptable than ceilings. In either case, clothing choices by occupants can mitigate the radiant effect.
Energy Modeling and Thermal Mass
Capturing the benefits of thermal mass in the design of your building requires an accurate prediction of the building’s energy usage. The analysis considers the building’s numerous thermal characteristics including the materials of the walls and rest of the building envelope, the size and orientation of the building, how the building is occupied and operated, and the local climate.
Accurate analysis of high mass buildings requires complex energy modeling software, such as DOE 2, that can predict annual energy use on an hourly basis. Hourly analysis is required because the steady-state R-value traditionally used to measure energy performance does not accurately reflect the complex, dynamic thermal behavior of massive building envelope systems.
For LEED version 2.2 projects pursuing oneor more of the 10 points available under EA Credit EA 1, Optimize Energy Efficiency, Option 1, the energy model must demonstrate a percentage improvement in the proposed building performance rating compared to the baseline building performance rating per ASHRAE/IESNA Standard 90.1-2004 (without amendments) using the Building Performance Rating Method in Appendix G of the Standard. This credit applies to all energy systems, includeing HVAC, service hot water, interior lighting, plug loads, exterior lighting, and elevators. The baseline buildings is a lightweight building. The minimum energy cost savings percentage for each point threshold is as follows:
Studies have shown that using concrete as floors, walls, or both will save enough energy to help gain LEED points in most climates. Accurately modeling thermal mass takes time. DOE-2 accounts for the thermal mass effect in a space using one of two methods: custom weighting factors or precalculated weighting factors. In general, the custom weighting factor method requires the most amount of user input but produces the most accurate results. The DOE-2 reference manuals suggest using custom weighting factors for masonry buildings and heavy construction. Precalculated weighting factors are not recommended. Custom weighting factors are based on the actual properties of the room being modeled including wall construction, furniture type, furniture fraction, and furniture weight.
Peak Load Savings
Incorporating thermal mass into a building’s design will help reduce peak and non-peak heating and cooling demands on the mechanical systems, resulting in increased LEED points for energy efficiency.
Reduced peak demand may also allow downsizing mechanical systems such as chillers and air handling equipment. Smaller equipment and shaving of peak loads saves on one-time capital costs of equipment and electrical connections and yields savings in ongoing energy usage (kWh) and energy demand (kW) costs. Potentially, operator and maintenance costs may be reduced as well.
Mass works well in commercial applications by delaying the peak summer load, which generally occurs around 3:00 pm until offices begin to close. As a case in point, the blackout in the Northeast in August 2003 occurred at 3:05 pm. A shift in peak load would have helped alleviate the demand and possibly alleviated this peak power problem.
Thermal Properties of Concrete
Thermal resistance (R-values) and thermal transmittance (U-factors) do not take into account the effects of thermal mass, and by themselves, are inadequate in describing the heat transfer properties of construction assemblies with significant amounts of thermal mass. Only computer programs such as DOE-2 and EnergyPlus that take into account hourly heat transfer on an annual basis are adequate in determining energy loss in buildings with mass walls and roofs. The heat flow through the wall is dependent on the materials’ unit weight (density), thermal conductivity, and specific heat.
Specific heat is defined as the amount of heat energy (in Btu) required to raise the temperature of one pound of a material by one degree Fahrenheit. Specific heat describes a material's ability to store heat energy. The specific heat of concrete and masonry can generally be assumed to be 0.2 Btu/lb·°F. (ASHRAE Handbook of Fundamentals, 2005)
Heat Capacity (HC) is the amount of heat energy required to raise the temperature of a mass one degree Fahrenheit. Heat capacity is per square foot of wall area (Btu/ft2·°F) and includes all layers in a wall. For a single layer wall, HC is calculated by multiplying the density of the material times its thickness (in ft) times the specific heat of the material. HC for a multilayered wall is the sum of the heat capacities for each layer.
Values of heat capacity, thermal resistance, and thermal transmittance for concrete and masonry are presented in Appendix A of ASHRAE Standard 90.1-2004. Thermal conductivities are presented in the ASHRAE Handbook of Fundamentals.
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