Building energy simulation

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Building energy simulation, also called building energy modeling (BEM) (or energy modeling in context), is a design tool to characterize energy flows connected to a building and predict their impact on comfort parameters and energy demands through mathematical models.

Video Building energy simulation

Background

A typical building energy model has inputs for local weather; building geometry; building envelope characteristics; internal heat gains from lighting, people and plug loads; heating, ventilation, and cooling (HVAC) system specifications; operation schedules and control strategies. A building energy simulation then uses mathematical models to represent building systems and their interactions in order to calculate thermal loads, system responses to those loads, and the resulting energy use, along with related metrics such as occupant thermal comfort, energy use and carbon emissions.

Maps Building energy simulation

Origins

The development of building energy simulation represents a combined effort between academia, the government, industry, and professional organizations. The advancement of computing technology in the 1960's replaced manual procedures to calculate transient heat transfer for determining building HVAC loads. In the United States, the 1970's energy crisis intensified these efforts, as reducing the energy consumption of buildings became an urgent domestic policy interest. The energy crisis also initiated development of U.S. building energy standards, beginning with ASHRAE 90-75. Federal and state agencies such as the Energy Research and Development Administration (ERDA), which is the now the U.S. Department of Energy, the U.S. Department of Defense, the National Science Foundation, and the California Energy Commission collaborated with national laboratories and universities to develop building energy simulation calculation engines such as DOE-2 Building Loads Analysis and System Thermodynamics (BLAST), and TRansient SYstems Simulations (TRNSYS). Since 2001, U.S. federal development for building energy modeling tools consolidated into a new calculation engine called EnergyPlus, which combined features from BLAST and DOE-2. A similar pattern of development of building energy simulation tools is reflected in the United Kingdom, where the UK's Engineering and Physical Science Research Council and the R&D Framework Programmes of the European Commission supported the development of the ESP-r. Codes and standards continue to the drive market demand, as simulation software can be used to demonstrate performance-based compliance. As a result, there is a growing list of software tools for building energy simulations.

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Applications

Building energy models may be developed for both new or existing buildings. Major use categories of building energy simulation include:

• Architectural Design: quantitatively compare design or retrofit options in order to inform a more energy-efficient building design
• HVAC Design: calculate thermal loads for sizing of mechanical equipment and help design and test system control strategies
• Building Performance Rating: demonstrate performance-based compliance with energy codes, green certification, and financial incentives
• Building Stock Analysis: support development of energy codes and standards and plan large scale energy efficiency programs

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Accuracy

In the context of building energy models, error refers to the discrepancy between simulation results and the actual measured performance of the building. This error can be due to simplifying assumptions by the calculation engine or approximations in model inputs.

The building energy model calculation engine is solving a system of linear differential equations. Given the complexity of building energy flows, it is generally not possible to find an analytical solution, so the simulation software employs other techniques, such as response function methods, or numerical methods in finite differences or finite volume, as an approximation. ASHRAE Standard 140-2011 Standard Method of Test for the Evaluation of Building Energy Analysis Computer Programs provides a method to validate the technical capability and range of applicability of computer programs to calculate thermal performance.

There are normally occurring uncertainties in building design and building energy assessment, which generally stem from approximations in model inputs, such as occupancy behavior. Calibration refers to the process of "tuning" or adjusting assumed energy model inputs to match observed data on observed energy use from the utilities or Building Energy Management System (BEMS). ASHRAE Guideline 14-2002 and 14-2014 provides performance indices criteria for building energy model calibration. The performance indices used are normalized mean bias error (NMBE), coefficient of variation (CV) of the root mean square error (RMSE), and R² (coefficient of determination). ASHARE recommends a R² greater than 0.75 for calibrated models. The criteria for NMBE and CV RMSE depends on if measured data is available at a monthly or hourly timescale.

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Software tools

There hundreds of software tools available for simulating the energy performance of buildings and building subsystems, which range in capability from whole-building energy simulations to model input calibration to building energy auditing.

Among whole-building energy simulation software tools, it is important to draw a distinction between the calculation engine, which dynamically solves equations rooted in thermodynamics and building science, and the interface, which provides a more user-friendly platform for entering inputs and viewing outputs. For some software packages, the calculation engine and the interface may be the same product. The table below summarizes some commonly used whole-building energy simulation tools.

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Performance-based compliance

In a performance-based approach, compliance with building energy codes or standards is based on the predicted energy use from a building energy simulation, rather than a prescriptive approach, which requires adherence to stipulated technologies or design features. Performance-based compliance provides greater flexibility in the building design as it allows designers to miss some prescriptive requirements if the impact on energy performance can be offset by exceeding other prescriptive requirements. The certifying agency provides details on energy model inputs, software specifications, and performance requirements.

The following is a list of U.S. based energy codes and standards that reference building energy simulations to demonstrate compliance:

• ASHRAE 90.1
• International Energy Conservation Code (IECC)
• Leadership in Energy and Environmental Design (LEED)
• California Title 24
• EnergyStar Multifamily High rise Program
• Passive House Institute US (PHIUS)
• Living Building Challenge

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Professional associations and certifications

Professional associations

• International Building Performance Simulation Association (IBPSA)
• American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE)

Certifications

• BEMP - Building Energy Modeling Professional, administered by ASHRAE
• BESA - Certified Building Energy Simulation Analyst, administered by AEE

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See also

• Energy modeling

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References

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External links

• Bldg-sim mailing list http://lists.onebuilding.org/listinfo.cgi/bldg-sim-onebuilding.org for building modeling professionals.
• Energy Modeling Instruction and Discussion http://energy-models.com/forum

Source of article : Wikipedia