- By Mike Tillou
- December 1st, 2011
As energy prices continue to increase, K-12 schools look for ways to conserve energy and minimize their energy expenditures. A growing number of school districts have turned to engineers who use software-based energy modeling as a predictive and analytical tool to improve the performance of their buildings. What is energy modeling and how can it help facility managers operate their school buildings more efficiently?
Establishing an Energy Target
Prior to creating the energy model, the facility manager and engineer must establish energy goals. Is the target to meet the minimum required energy codes? Some schools may adopt a more proactive energy approach and target energy usage at 20 to 30 percent or more below the code requirement. Will LEED certification be applied for and, if so, at what level (Silver, Gold, Platinum)? In the most ambitious cases, the building may be designed as a so-called net-zero energy building — a building that creates or saves as much energy as it uses. The energy target selected by the facility manager and school district will in turn impact how the energy model is ultimately used.
The Energy Model
Energy modeling is a software-based mathematical model of a building. After energy targets are established, thermal data about the building — its size, massing, wall types, roof, windows — are entered into the model along with solar and climate information for the building’s location. Information on the building’s heating, ventilation and cooling system along with heating and cooling loads are then layered into the model. The result is an accurate model that can be used by the design team to test alternative building layouts, wall and roof construction and HVAC equipment to optimize energy usage. Some energy modeling software allows renewable energy systems like electric photovoltaics to be directly modeled, so the benefits on energy performance can be immediately seen.
When a new school facility is being designed, energy modeling should be employed from the earliest conceptual stages through design development. The initial energy model should be created once the architect has identified the basic massing of the building along with preliminary information about wall and roof construction and fenestration. At this stage, alternatives in massing and building orientation can be explored for their effect on energy consumption. As the building’s shape and size are firmed up, the effect of the building envelope (walls, roofs and windows) can be examined, and opportunities for fixed shading can be explored. Placement and number of windows, type of glazing systems and their insulating qualities can all be adjusted for energy considerations. As the building design is refined, the energy model is applied to predict energy usage. This iterative process is repeated several times during design until a final design is achieved that meets the energy goals that were set initially.
For a recently designed elementary school in Illinois, energy modeling was started early in conceptual design and used continuously throughout the design process. The project was able to target a 52-percent reduction in energy consumption compared to the energy code baseline. The previous elementary school design for the same school district only achieved 30-percent energy savings, in part, because energy modeling was not used as rigorously.
Energy modeling can help in existing facilities, as well as new construction. At a 40-year-old high school in Massachusetts, school board members suspected that air infiltration problems and an aging school infrastructure were symptomatic of a very energy inefficient school. A simple energy model was developed and then calibrated against actual energy usage, as documented in the school’s utility bills. When the energy usage was compared to similar buildings in similar climates, it was found that the high school actually used less energy than the similar buildings. Further investigation found that ventilation louvers had been blocked off and air infiltration actually replaced conditioned supply ventilation. The study also revealed that facility personnel were operating and maintaining the antiquated building pneumatic control system extremely well, thus contributing to the building’s unexpected level of energy efficiency.
Calibrating the energy model until it closely matches the existing energy use patterns is the key to improving energy consumption in existing buildings. Where documentation in the form of utility bills exists, that calibration, while sometimes challenging, helps reveal and zero in on the right corrective actions required for a building.
In a recently completed classroom building in Maryland, energy usage was considerably higher than had been predicted during design. The energy model was reviewed, but no inaccuracies were detected. An inspection of the building’s HVAC system revealed that a supply air sensor was incorrectly placed within an air-handling duct, causing the chillers to cool air far more than was necessary. After the sensor location was corrected, a drop in energy usage was immediately evident.
In an educational laboratory building in Missouri, the owner opted to construct the building using a design/build approach where the architect completed the design
up to an agreed upon point, then the contractor took over and finalized the design and construction. After the facility was completed, but only partially occupied
(less than 30 percent), cooling use was observed to be 80 percent of the amount anticipated in a fully occupied building. Facility managers questioned how this could be and blamed the faulty design of the building. By calibrating an energy model to the actual observed energy use, an energy engineer was able to demonstrate that the cooling load was independent of the internal space loads because of the high air change rates required in the laboratory spaces. The model confirmed that if conditioned makeup air was supplied to unoccupied spaces at design rates, the cooling load would occur at the same rate as had been observed. Using an energy model as an analytical tool, the temperature and volume of makeup air was adjusted in unoccupied spaces, thus reducing overall building energy use.
Level of Resolution
Energy modeling is as much an art as it is a science, and the experience of the engineer creating the model is crucial. The level of resolution within the energy model is key; higher resolution yields more detailed information, even room-by-room energy usage, while a lower resolution may only predict energy use of the building as a whole or zones within a building. Of course the greater the resolution, the more time consuming and costly the energy model becomes. Finding a balance between resolution and adequacy of the prediction is the art of energy modeling. As more engineers claim to be energy modelers, it is important to seek out credentialed professionals like those who have earned ASHRAE’s Building Energy Modeling Professional designation, and who can help determine the proper level of effort for a given project.
Predicting and Tracking
In the most advanced use of energy modeling, real-time predictions can be created. Using actual weather data, monthly energy use can be projected and then compared against actual energy use. By plugging in anticipated data, energy use can be predicted in advance, allowing troubleshooting of problems before they occur. In the most sophisticated scenario, energy measurement and modeling can be brought to the desktop of the facility manager.
Energy modeling is becoming an accepted discipline within the engineering world. K-12 schools who consult with an engineer experienced in modeling can reap benefits in both newly designed facilities and existing buildings. As energy costs continue to climb, the ability to predict, analyze and then correct building energy performance can provide significant cost savings, an important consideration for every school district.
Mike Tillou can be reached at