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D.6.2 Designing Using Computer Simulations
Computer simulation tools are an essential component of the whole-building design process. Designers depend on these tools to understand factors affecting the performance of the particular building they are designing, assist with decisions relating to building massing and adjacencies, determine the optimal combination of design solutions, predict the actual building performance, and identify performance problems after construction and commissioning are completed. To achieve these benefits, designers should begin depending on computer simulations of the proposed building early in the design process.
The thermal performance of any building entails complex interactions between the exterior environment and the internal loads that must be mediated by the building envelope and mechanical systems. The difficulty is that these various external and internal load conditions and associated utility loads are constantly changing from hour to hour and season to season. Also, the number of potential interacting design alternatives and possible trade-offs is extremely large. Computer simulations are the only practical way to predict the dynamic energy and energy cost performance for a large number of design solutions.
Building thermal simulation software tools that provide hour-by-hour analysis have been in use for more than 30 years. These software tools rely on an annual weather file for the site that provides the external hourly climate data. While several weather data formats are available, they Typical Meteorological Year (TMY2) format is commonly used.
To simulate a building using these tools, describe building parameters, such as assembly construction, volumes, and number of floors, along with their respective orientations. Account for the hourly variation in internal loads using hourly schedules. Also enter HVAC equipment and operation schedules and lighting schedules.
The simulation analyzes the conditions at the beginning hour and then the results are passed along to the next hour and so on throughout the simulation period. This process allows for the inclusion of thermal capacitance (mass) effects and solar impacts over time. The output of the simulation typically includes annual energy and cost data as well as hourly performance reports of the various building components.
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SIMPLIFIED PEAK LOAD CALCULATIONS VERSUS HOURLY LOAD SIMULATIONS |
Accurate energy code-compliant base-case computer models give the design team typical energy and energy cost profiles for a building of similar type, size, and location to the one they are about to design. The design team uses this information to develop a design concept to minimize these energy loads and energy costs from the very outset. At this stage, the design team can manipulate the building massing, zoning, siting, orientation, internal organization, and appearance of the facades without adding significantly to the cost of design.
As the design progresses, the design team compares the simulations of the proposed design alternatives to each other and to the base-case building simulation to understand the energy use, energy cost, and peak load implications of alternatives. Begin initiating computer simulations early in the design process for maximum effectiveness.
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Pre-design – Simulation helps identify and prioritize potential envelope-based energy efficiency strategies.
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Schematic design phase – Add the building massing, fenestration, and envelope constructions to the model to determine if energy targets are still being met.
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Design development – Test the performance of the full building together with the HVAC systems.
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Construction – Evaluate how design changes proposed during construction will affect the building performance before implementing the change.
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Commissioning – Run a simulation of the asbuilt construction to provide a baseline building performance that can be used for actual performance comparisons.
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Post-Occupancy – Periodically update the simulation after the building is occupied to reflect variations in operations, use patterns, and unique climate conditions. These conditions may dramatically affect the actual performance of the building.
The computer energy simulation provides a method to test the integration of various design solutions to verify that they are meeting design goals. Decisions about building form, materials, and systems can be tested and adjusted to improve performance.
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