Building thermal performance calculations are made for two primary reasons. They are made to size and select mechanical equipment or to predict the annual energy consumption of a structure. While these two tasks are not mutually exclusive and some programs can handle both tasks, they do tend to be conducted in isolation from each other. Richard Paradis, P.E., Director, Whole Building Design Guide at National Institute of Building Sciences, Washington D.C., USA briefs.
To decide what computer-based, energy analysis tool is best for your project, it is important to have a basic understanding of how these tools operate. In the past, few buildings benefited from energy analysis. Today energy analysis tools are becoming more common and are being applied earlier in the design process.
Energy programs are primarily designed to predict the annual energy consumed by a structure in terms of BTUs or pollution avoidance. Whereas, Sizing programs are primarily designed to calculate peak hourly loads during the heating and cooling seasons. Almost all buildings of any complexity have a sizing analysis of some kind run by an architect, engineer, or mechanical contractor.
Some software programs are designed to excel at one or two steps in this process while others tackle the whole problem comprehensively. Other tools use simplified methods to expedite input requirements or minimize run time, while still others are more detailed and precise.
Boundaries between energy analysis tools are beginning to blur as developers in different industries are converging to produce software that is more graphic, easier to use, and capable of greater accuracy. Nevertheless, important distinctions between software programs still remain and will do so for the foreseeable future.
Not only do energy analysis software programs have varying levels of accuracy; they are also intended to be used at different phases of the design process; and require very different levels of effort and cost. Most energy analysis tools can be classified as being one of four generic types. The software examples listed are meant to be indicative, not exhaustive.
The limits and benefits of energy analysis tools
Users of energy analysis tools should be aware that energy calculations, regardless of their sophistication, cannot precisely predict actual energy consumption. Factors such as construction quality, occupancy schedules, and maintenance procedures may vary markedly from assumptions contained in the analysis and skew results. However, this does not mean that energy analyses are not important tools.
It is also important for users of energy analysis tools to understand the interrelationships among all aspects of building design. Conducting an energy study of a new building or a major retrofit project is an excellent means by which to evaluate the relative energy performance of alternative designs.
In a correlation-based program, daily, monthly, or seasonal, the building performance is computed by comparing or correlating, the performance of the building in question against predetermined equations (or curves) that predict the performance of the building based on key thermal characteristics and climate information.
Correlation programs generally run quickly because they demand a minimum of computation, but this speed is at the expense of some accuracy. Also because of their relative simplicity, correlation programs are not able to evaluate the important trade-offs between certain interactive energy strategies such as daylighting and heating or thermal mass and cooling.
Screening tools are primarily used during budgeting and programming of retrofits. They are designed to evaluate project viability during the earliest stages of programming and often include some economic analysis capability. They also tend to be correlations rather than full hourly simulations.