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« Efficient Artificial and Site Lighting |
Table of Contents |
Energy Source Ramifications » |
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Energy Modeling |
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Energy modeling, or simulation, is the practice of using computer-based programs to model the energy performance of an entire building or the systems within a building. This whole-building modeling provides valuable information about the building and system energy use as well as operating costs.
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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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THERM anaylsis of a curtain wall detail
| Energy modeling, or simulation, is the practice of using computer-based programs to model the energy performance of an entire building or the systems within a building. This whole-building modeling provides valuable information about the building and system energy use as well as operating costs.
Whole-building simulation is typically performed for an entire year using typical meteorological year weather data. An important aspect of whole-building modeling is that it accounts for the interaction between different elements of the building, such as the impact of lighting on space conditioning loads or the impact of daylighting on electrical lighting loads. The impact of different building uses and occupancy patterns is also accounted for.
The most common whole-building simulation tool for larger commercial buildings is DOE-2, which was originally developed by the U.S. Department of Energy and is now the basis for many other tools. For small commercial buildings, the National Renewable Energy Laboratory (NREL) developed Energy-10. Commonly used residential building simulation tools are REMRate, EnergyGauge, TREAT, and Micropas.
In addition to whole-building modeling, the energy perfor-mance of individual systems may be modeled for a short-term or design condition. Software programs such as THERM can perform heat transfer analyses using the finite element method to evaluate thermal bridging in a wall detail and a whole-wall R-value. Daylighting modeling tools such as Radiance can simulate the performance and impact of daylighting strategies for a specific space. The specific findings from these programs can then be integrated into the whole-building simulation. |
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Use / Application |
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Energy modeling can and should be used throughout the design process to optimize the building design for energy performance and reduced carbon emissions. Energy modeling has been applied to building design for decades, but with the advancement and proliferation of computers and the increased emphasis on building operating costs, the use of modeling is more common and often required. It is a valuable tool to assist architects in assessing the impact of various design decisions.
The size, complexity, and potential types of mechanical sys-tems will affect which of the many modeling tools is most appropriate for the project, and the tool may change from one design phase to the next. Thus, it is beneficial to have someone in-house or as a consultant who is familiar with the alternative simulation tools and knows their capabilities in terms of such variables as the number of floors, the number of zones, the ability to define schedules, and the mechanical equipment options.
Using a tool that can handle large complex buildings for a simple building can result in a waste of time and expense. Tools have been developed, such as Energy-10, specifically to model smaller buildings. Alternatively, the time and expense to develop a model for a building only to learn late in the process that it cannot model the type of mechanical system that is being proposed by the mechanical / electrical / plumbing firm is also a wasted effort.
There is a cost associated with energy modeling, and it varies with the complexity of the building. A speculative developer may find it difficult to bear costs that will accrue savings to the future building owner in the form of reduced utility bills. However, this should be presented as a feature of the building and worthy of a price premium. The building will also be more comfortable and perform more reliably. In many instances, the cost of model-ing can be recovered through the elimination of unnecessary factors of safety, allowing credit for specification trade-offs, and/or qualifying for available incentives.
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Established Techniques
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Energy cost by end use
| Energy modeling can be applied to various degrees in each phase of the design process.
Conceptual Design
During conceptual design, a very simplified model that uses basic assumptions for many of the inputs can be used to examine large-scale impacts such as building configuration and orientation.
Schematic Design
At schematic design, a more detailed energy model is useful for identifying the primary energy uses. Understanding which loads (space heating, cooling, lighting, water heating, etc.) are dominant provides guidance on what aspects of the building design should be targeted for energy savings. The greater the energy use, the greater the opportunity for cost-effective energy savings. For instance, the designers could test alternative percentages of façade glass area on the principal orientations for energy and aesthetic implications.
Modeled building geometry |
Design Development
During design development, a detailed energy model should be used to conduct parametric analyses to evaluate alternative specifications and more fully understand the trade-offs between initial cost and life-cycle cost. The accuracy of the model is important at this stage; therefore, providing as much information as possible on the expected use and occupancy schedules of the building is important. The phrase “garbage in, garbage out” is particularly relevant to modeling at this stage. It is important to keep the modeler informed of all design and specification changes, and all model inputs and assumptions should be carefully reviewed. For example, using a default 8-to-5 Monday–Friday operating schedule for an office facility that operates 24/7 would significantly underestimate the building’s electricity use for lighting and equipment. Using the wrong glazing specifications could significantly affect the heating and cooling load predictions.
Construction Documents
At this phase, the energy modeling will be necessary to document compliance with codes such as the Energy Cost Budget method in the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard 90.1 or the Total Building Performance section of the IECC. In many instances, energy modeling is also necessary to demonstrate qualification for specific certification programs, such as Energy Star and the U.S. Green Building Council’s Leadership in Energy and Environmental Design or incentives such as the federal tax credits.
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Emerging Trends |
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With the widespread use of computers, the use of energy modeling is common and expected. New tools and applications are being introduced all of the time.
EnergyPlus is one of the newest whole-building simulation tools. The Department of Energy has supported its development for several years as a replacement for DOE-2.1E. It was officially released in April 2001, but its lack of user-friendliness has limited its use to date.
Developing the input file for geometrically complex buildings can be time-consuming. Green Building Studio (GBS) is a relatively new tool that seamlessly links architectural 3-D CAD building designs with energy analysis. The GBS Web service automatically generates geometrically accurate, detailed input files for major energy simulation programs.
With the emergence of building information modeling (BIM), there is interest in integrating BIM and energy modeling. Most energy modeling is currently done independent of BIM because there are still significant limitations in BIM energy modeling capabilities. However, software developers are working to make it easier to derive the energy model from BIM. |
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Use an Integrated Approach |
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A new way of thinking must be adopted in order 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 proper use of energy modeling throughout the building design process will inform the decision-making process on the energy use implications of various alternative design choices. This will result in a building that has been optimized to reduce energy loads and thus energy requirements.
- Specify energy-efficient equipment and technologies.
- Energy modeling provides the opportunity to evaluate the annual impact on energy use and operating costs of different types of equipment and systems with different efficiencies.
- Use renewable strategies and purchase green power.
- Many energy-modeling tools can simulate the performance of renewable energy strategies, such as waste heat recovery, solar thermal systems, and solar electric systems.
- Using modeling to optimize the energy performance of the building will reduce the expense of purchasing green power, which is typically more per kilowatt-hour than the standard electricity price.
- Using modeling can help reduce peak time energy demand and thus reduce peak power demand pressure on utilities and resulting carbon emissions.
- Educate building owners, operators, and occupants.
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Last modified at 2/27/2009 11:33 PM by jamie nace
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