Minimum Energy Performance is a prerequisite of the Energy and Atmospheric section of the LEED requirements.  This is accomplished by developing A Computer Building Model.  Many people are not familiar with a Computer Building Models, but Computer Building Models have been used in the HVAC industry since the early 70’s.  The first Building Modeling program was developed by Southwest Research in San Antonio , Texas for the US Post Office.  The US Post Office facilities had wisely varying energy use, and HVAC equipment types.  The Post Office wanted a method to estimate the energy consumption of a facility, and quantify the equipment types and sizes to be installed in new facilities.  This program was titled The Post Office Program, and a commercial version titled Merriweather Program was marketed by the principle author of the program. 

Several Building Modeling programs were developed during the 80’s.  The new modeling programs fell into two categories depending on the results.  Some programs are Building Load programs, and the objective of the program is to determine the proper size of the HVAC equipment required to meet the needs of the facility.  The others are Building Simulation, or Building Energy Programs.  These programs simulate the operation of the facility on an hourly basis, and provide annual energy use, as well as equipment sizing information.

Why use one program over the other?  The building load programs are quicker and easier to setup and run.  Initially there was a significant difference in the amount of computer time required to obtain results.  The building load programs generally calculate data for one weekday, one Saturday, and one Sunday per month, and a few holidays.  On the other hand, the building simulation programs calculate data for each hour of the year.  During the period of the 8086 computers, a load program would require 15 minutes to run, while a simulation program would be started at 5:00 pm, and the results would be available the next morning.  The improvement in computer technology has eliminated this difference, and now a building simulation for a 200,000 sq. ft. building requires less then 2 minutes.

Why does LEED require a model?

ASHRAE standard 90.1 establishes the Minimum Energy Performance for buildings by defining the minimum standards for building components.  Walls, windows, lighting, water heating, and HVAC equipment are addressed in the standard.  The initial model is termed the “Basis”, or “Baseline” model.  The baseline model incorporates the building configuration, occupancies, and operating schedules, and the minimum requirements (insulation values, and efficiencies) of ASHRAE standard 90.1.  The results establish the Energy Cost Budget.  This is the maximum amount of energy that the design can use annually. 

The second and succeeding models use the same building configuration, occupancies, and operating schedules as the Baseline.  Other parameters such as lighting levels, improved equipment efficiencies, and high performance glazing are changed to reflect the values used in the actual design.  The results are termed the Design Energy Cost.  Hopefully the Design Energy Cost is less then the Energy Cost Budget.  If the Design Energy Cost is greater then the Energy Cost Budget, adjustments will have to be made in the building design to achieve a Design Energy Cost less then or equal to the Energy Cost Budget.

The data from the simulations is used to substantiate points claimed for EA credit 1, Optimized Energy Performance.  The building models can be used during the design process to test the effect of different energy conservation measures.  The alternate designs can be defined, and the results reviewed relative to the cost of the alternative.  If the results are not cost effective then the alternative can be discarded with a minimum of effort.  Energy conservation measures that are effective in one climate or building type may not be effective in another.  Modeling allows the design team to quickly evaluate alternatives, and only include the cost effective alternatives in the project design.

For example, an air to air heat exchanger with an enthalpy section is placed between an outside air intake air stream, and an exhaust air stream.  This is an excellent system component in the DFW area.  The number of operating hours when the outside air is above the discharge temperature of the HVAC equipment is significant and excellent performance during the cooling season is achieved.  On the surface it would appear that such a system would work well in Albuquerque .  Modeling indicated that the system does not create a favorable situation during the cooling season in Albuquerque , because the number of hours of operation in the cooling mode is low.  But, reviewing the data from the heating season indicated substantial saving could be accrued by installing the system.  In this case the system was beneficial to the project, and the modeling was required to demonstrate the effectiveness in the heating mode.

The building simulation should remain a part of the design documents so that there is a record of how the building was designed, and the intended operation.  Over the life of a building many things occur.  The use of spaces is changed, new equipment is installed, or new operating strategies are common changes over the life of a building.  Incorporating these changes in the building simulation developed during the building design phase will provide the building operating personnel and future designers information about how the changes will effect the overall operation of the facility.

The Building Simulation is a valuable tool in the design and operation of facilities.  The ability to quickly evaluate design and operation alternatives over a year of operation provides design professionals, and operating personnel the information they need to make well informed decisions about the facility.  For more information about Building Simulations, and available tools, visit the following WEB sites.

• http://www.doe2.com/

• http://gundog.lbl.gov/

Dale Ball is a Board member of the USGBC North Texas Chapter, and a consulting engineer with TAC America's. With 30 years of experience in the design, construction, and operation of facilities, Dale has worked on major projects at various locations throughout the United States. For additional information regarding this article, please contact Dale at dale.ball@tac.com.  All comments are the sole responsibility of the author. 

This article was originally posted 2/6/07.