Building Geometry: Energy Use Effect (Energy Engineering)

Abstract

Energy service companies (ESCOs) use the energy use index (EUI) as a tool to evaluate a building’s potential for reduction in energy use. Select Energy Services, Inc. (SESI) has found that consideration of building geometry is useful in evaluating a building’s potential for energy use reduction. Building load and energy-use simulations using Trace® and PowerDOE®, respectively, were conducted to gain insight into how building geometry impacts heating, ventilation, and air-conditioning (HVAC) sizing and energy use. The ratio of gross wall area to gross floor area, Awan/Afloor, has been found to be a useful factor to consider when making EUI comparisons. Simulations suggest that buildings with higher Awan/Afloor ratios require higher central plant capacities and use more energy per unit area to satisfy the heating and cooling loads. Taking a building’s geometry (Awan/Afloor) into account while estimating savings potential may produce more accurate results.

INTRODUCTION

Select Energy Services, Inc. (SESI) has conducted a multitude of building evaluations in the course of its performance contracting and design work. Select Energy Services, Inc. has many energy engineers with real-world heating, ventilation, and air-conditioning (HVAC) design experience, which often provides insight into peculiarities. One such peculiarity is “Why do two buildings of the same usage and square footage exhibit energy use indexes (EUIs) significantly different from one another?” In an attempt to answer this question, SESI conducted a series of simulations which focused on building geometry and its contribution to heating and cooling loads and annual energy use.


The tools used in this analysis are Trace and PowerDOE®. Trace® is a software package published by C.D.S. Software that is used to determine equipment loads. PowerDOE is published by the Electric Power Research Institute, Inc. as a “front-end” for the U.S. Department of Energy’s DOE-2 building energy simulator. PowerDOE® is used to simulate annual building energy use. Both software packages allow easy and economical means to evaluate a building’s HVAC capacity requirements and resulting energy use.

ENERGY USE INDEX

Even before setting foot on site, an energy service company (ESCO) can get a preliminary estimate of the potential for energy cost reduction. This can be done by analyzing the fuel and electric rates, looking for credits or rate restructuring, and evaluating EUIs. Energy use index is defined as the ratio of total annual energy used, in kBtus, divided by the square footage of the building.

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The EUI is used as a barometer for estimating the potential for energy savings. However, it must be applied with discretion or an ESCO could pass on a great opportunity or overestimate the potential for energy cost savings.

How the Energy Use Index is Used

Once utility billing data, equipment data, and building square footage have been provided, an EUI evaluation can be conducted. The calculated EUI of the building is compared to an “ideal” EUI. The difference between the building EUI and the ideal EUI is the potential for energy savings (see Fig. 1). However, it is often cost prohibitive to attain the entire EUI differential, so ESCOs often prescribe a maximum, economically attainable, EUI improvement. Fifty percent is often used, but this depends on many factors.

From the above methodology, it is easy to see how comparing EUIs, based solely on square footage, can sometimes result in an inaccurate evaluation of the energy savings potential.

Energy use comparison.

Fig. 1 Energy use comparison.

BUILDING GEOMETRY

Building geometry is an important factor to consider from a design standpoint. It influences heat loss, heat gains, infiltration, and solar gains which influence the heating and cooling load. Typically, the more wall (including windows) area available, the higher the heating and cooling loads. The first 10-15 ft from the exterior wall is considered the perimeter zone. The perimeter zone heating/cooling load is constantly changing because it is under the influence of the weather via the building envelope (walls, windows, and roof) as well as internal loads (occupants, lights, equipment, etc.). Inside this area is the interior zone, which experiences much less heating/ cooling load variation (only internal loads). Therefore, if evaluating two buildings of equal floor area, use, occupancy, etc. the building with the larger interior zone will typically require less heating and cooling capacity and use less energy annually.

Building geometry is also an important factor to consider from an energy-use standpoint. If two buildings of same square footage, use, schedule, controls, occupancy, and construction exhibit significantly different EUIs, differences in building geometries may explain why. Because of its greater exposure to environmental conditions, the building with more wall area will likely have the higher EUI. Therefore, it is not uncommon to find that buildings with multiple floors or eccentric shapes use more energy than single-floor, rectangular buildings of the same square footage. For example, Figs. 2 and 3 illustrate two-story buildings, each with 32,400 ft2 of floor space. Building A has a layout found on many military installations and educational campuses, and its protruding wings resemble radiator fins, both in cross-section and in thermal effect.

Building A floor plan.

Fig. 2 Building A floor plan.

Building B floor plan.

Fig. 3 Building B floor plan.

Building B is another building of the same square footage, but in the shape of a rectangle, a more compact shape with less exposed surface area.

Given the same building load parameters, Trace® calculations indicate that Building A requires 15% additional cooling capacity and 25% more heating capacity than Building B. PowerDOE® energy use simulations indicate that Building A will use 15% more energy annually than Building B. This example shows that building geometry is indeed an important factor to consider when estimating the potential for energy use reduction based on EUI comparisons. With the realization that building geometry affects energy use, how can it be accounted for, during an EUI comparison?


To evaluate the effect of geometry on energy use, several hypothetical models were defined, constructed, and analyzed using Trace® and PowerDOE®. The model definitions include such parameters as use (school, office, warehouse), geographical location, schedule, overall heat transfer coefficients (wall, roof, windows), and others. This methodology provides the ability to evaluate the effect of geometry in buildings with distinctly different uses and geometries. Models of the buildings were constructed in Tracew and PowerDOEw using the same building parameters. These programs allow parameters BUILDING MODELS such as orientation, location, and geometry to be changed with a keystroke.

RESULTS

After conducting the building load and energy use simulations, a factor has emerged that explains why a building uses more or less energy per square foot than another. This factor takes into account differences in building geometry when evaluating energy use reduction potential. This geometric ratio (GR) is defined as the ratio of gross perimeter wall area (Awall) to gross floor area C4 floor).

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Comparisons of Tracew load calculations indicate that buildings with higher wall to floor area ratios require larger heating and cooling plants. The effect of building geometry has been found to be more pronounced as outdoor air requirements decrease. For example, Tracew load calculations (heating only) for a warehouse indicate that with each percentage point increase in the GR, the peak heating requirement increases approximately 1%. Geometry, in this example, has such a significant effect because envelope load is a larger percentage of the total heating plant requirement. Annual energy use predicted by PowerDOE®, on the other hand, is relatively flat for warehouse structures while showing significant geometric effects for school and office-type occupancies. As the outdoor air requirements increase, the contribution of envelope loads to the total heating and cooling load decreases.

Simulations have also shown that it is important to consider the percentage window area per unit wall area. The amount of window in a wall has a significant impact in the overall heat loss/gain of that exposure. Buildings with higher window to wall area ratios typically require larger heating and cooling capacities and use more energy annually per unit floor area than buildings with lower window to wall area ratios.

Trace® and PowerDOE® simulations also indicate that the orientation of a building is also an important factor to consider. If a building has a high (2 or higher) aspect (length/width) ratio and is oriented so that the long sides of the buildings are facing east-west, this building typically requires larger heating and cooling plant capacities and will consume more energy annually. Had this same building been oriented such that a long side was facing north-south, the heating and cooling plant capacity and energy use could have been reduced. Orientation of the long side of the building in a north-south direction also could have permitted more effective use of natural light to reduce lighting energy requirements which can further reduce the cooling load. Therefore, when considering the potential for energy reduction in this particular building, it is advisable to take the building’s orientation into account.

Differences in building energy use can also be explained by considering geometry as it relates to the original intended use of the building. On many military installations, especially those associated with airfields, there are numerous single level, high ceiling, marginally insulated buildings whose original intended use and design are not consistent with their present utilization. For example, storage buildings and aircraft hangars are often converted to office space (without upgrading the walls, windows, or roof insulation). As such, they have undergone numerous HVAC retrofits through the years as the hangar/storage space is further converted to office space. Buildings of this type typically use more energy than buildings whose original intended use was that of an office.

The age of a building, in conjunction with geometry, also helps explain differences in EUI. Older buildings have experienced much more wear and tear and typically have higher infiltration rates. Additionally, older buildings are typically constructed with lower R-value materials than contemporary construction. Heating, ventilation, and air-conditioning equipment is typically at, near, or far past its useful life and requires frequent maintenance. In addition, older buildings typically are not insulated as well. Due to space restrictions, many older buildings have rooms or wings that have been added to the original building. This addition can significantly increase the perimeter wall area with only a small increase in square footage. Finally, older buildings do not benefit from recent quality control and construction standards. As a result, it is not uncommon for older buildings to exhibit higher EUIs than similar buildings of recent construction. PowerDOEw modeling has shown that building geometry, as indicated by the GR, has more influence on energy use in older buildings.

How the Geometric Ratio can be Used

When evaluating the potential for energy use reduction, it may be to the ESCO’s advantage to take into account the GR of the buildings under consideration. The GR represents the influence of building geometry on energy use and can be used to gauge the effectiveness of certain energy conservation measures (ECMs). For example, ECMs associated with walls and windows such as window film, window replacements, and wall insulation upgrades may have a larger EUI impact in buildings with higher GRs. This is because wall and window conduction and solar gains are a higher percentage of the total HVAC load. Alternately, ECMs such as air-side economizers, lighting retrofits, and roof insulation upgrades may have a larger EUI impact in building with lower GRs. This is because the percentage of contribution of outdoor air, lighting, and roof conduction to the total building load is typically higher for buildings with lower GRs.

CONCLUSIONS

At the conclusion of the calculations and simulations using Trace® and PowerDOE®, SESI believes that it has, at least in small part, contributed to a better understanding of building geometry and its impact on heating and cooling requirements and energy use. This understanding can be used to better estimate the effectiveness of certain ECMs, which can help to avoid underestimation as well as overestimation of potential EUI improvement. This exercise has shown that there is value in considering building geometry while estimating the potential for energy use reduction.

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