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Energy  > An Overview of Modeling Energy Performance in Concrete Buildings
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This section presents summary results of energy performance modeling conducted on prototypical residential and commercial buildings, comparing a range of wall types in six cities, representing five climate zones. Links to summaries of results are at the bottom of the page.
 
Overview
 
Field tests and analytical studies demonstrate that for most climates, buildings constructed with concrete use less energy for heating and cooling compared to buildings constructed with lighter weight materials.
 
The inherent energy efficiency of concrete construction derives from concrete’s thermal mass properties. Concrete acts like a heat “sponge,” which absorbs heat energy and thus moderates indoor temperatures and peak heating and cooling loads.
 
As a result, the peak heating and cooling demand and annual energy performance of high mass buildings are often reduced. In addition, the HVAC system capacity of an efficient, high mass building may be less than a lighter building of the same size.
 
While building mass reduces energy consumption in nearly all North American climate zones, it is most effective in areas and during seasons that see large daily temperature swings.
 
Energy Modeling
 
Energy modeling, or energy simulation, is a method for predicting the energy consumption of a building. 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 software, such as DOE-2, that can predict 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 concrete building envelope systems.
 
 
Modeling Methodology
 
There are three methods to simulate whole-building energy use, though only two are used in practice: the heat balance method and the weighting factor method. Both methods are dynamic in that they account for the beneficial effects of concrete’s thermal mass. And both methods require expert knowledge to use and interpret results.

The heat balance method is an application of the first law of thermodynamics. It is numerically intensive and requires significant computing power. Very simply, a heat balance equation is written for each surface in a space and for the space air itself. These equations are solved simultaneously for surface and air temperatures. The temperatures are then used to calculate heat flows. This method is used in energy simulation software such as EnergyPlus, IES, and Tas.

The weighting factor method requires much less computation, though in some respects it is just as complicated as the heat balance method. The weighting factors are first determined using heat balance methods. These are then used to determine response factors for the transient heat flow in walls and weighting factors for the thermal response of building spaces. And like before, the building systems are simulated to respond to these heat flows. When weighting factors are used, they should always be based on the actual properties of the room being modeled including wall construction, furniture type, furniture fraction, and furniture weight. This method is used in the energy simulation software DOE-2 which is the calculation engine in several energy simulation software tools. Work for the Portland Cement Association by CTLGroup (see linked reports below) has been done using DOE-2 and a menu-driven interface called VisualDOE. A detailed discussion of building energy simulation software is available in the Building Energy Software Tools Directory from the U.S. Department of Energy’s Office of Energy Efficiency & Renewable Energy.

There are two approaches to comparing whole-building energy use in two or more buildings. One approach is to compare buildings that are modeled as realistically as possible. For example, the thermodynamic effects of the building structure, the HVAC systems, lighting, and occupancy are all simulated. In order that the comparison be realistic, variables that depend on occupant behaviour (ventilation, lights, thermostat setpoints, etc.) and those that depend on the level of quality exercised by builders (infiltration, HVAC efficiency, etc.) should be kept constant. This is the approach taken in the work done for the Portland Cement Association.
 
Energy Use of Single-Family Houses With Various Exterior Walls (2001)
Gajda, John, R&D Serial No. 2518, Portland Cement Association, 50 pages.
 
Modeling Energy Performance of Concrete Buildings for LEED-NC v2.1 EA Credit 1 (2005), Marceau, Medgar L. and Martha G. VanGeem. Portland Cement Association. Item Code: SN2880
 
These studies were based on energy simulation software using the DOE 2.1E calculation engine. Modeling was performed so that the only differences for a given location were the exterior wall type and the capacity of the HVAC system. The HVAC system capacity was automatically sized to maintain the thermostat settings by the analysis software. The HVAC system was otherwise identical for all building types as were building orientation, shading, equipment power density, lighting requirements, operational parameters, air infiltration, fresh air requirements, and occupancy. Besides differences in construction materials, model parameters were varied to reflect differing code and, for commercial buildings, ASHRAE 90.1 thermal performance requirements for the various climates as well as the local energy costs. A summary of findings are presented here by climate zone and building type.
Findings

A summary of findings are presented here by climate zone and building type
 
Climate/Building Type
Residential
AAC, CMU, ICF sandwich panel, and cast-in-place vs code-compliant, wood frame,
and steel frame walls
 
Commercial

Five construction types,
light to heavy mass

Hot Humid

Miami, FL>>

Miami, FL>>

Mixed-dry

Phoenix, AZ>>

Phoenix, AZ>>

Mixed-Humid

Memphis, TN>>

Memphis, TN>>

Marine

Seattle/Tacoma, WA >>

Salem, OR>>

Cold

Chicago, IL>>

Chicago, IL>>

Very Cold

Boulder, CO>>

Denver, CO>>


The other approach is to model only the components of interest. For example, to study just the effects of thermal mass, one can model the space loads in a building without considering the interactions with the HVAC system. This is the approach taken in recent work done for the Cement Association of Canada and the Portland Cement Association. These studies isolated the passive thermal mass effects of concrete from the building’s HVAC equipment. One study was done for the for a prototype commercial building in Canadian climate zones, and another study evaluated a mixed-use building prototype (multi-family with ground floor retail) in US climate zones. Both studies are under final review and will be posted in the reference library when available.
 
Regardless which simulation method is used or which comparison approach is used, the results of simulation must be interpreted properly. Whole building energy simulation does not predict energy use, because building energy use is dependent on the local level of quality exercised by builders, how a building’s systems are operated and maintained, and occupant behavior. Whole building energy simulation is ideal for comparing competing alternatives because factors that are too variable to predict can be controlled in the simulated environment.