Heat transfer through the building envelope is due to the temperature difference and may occur through one, two or all of the three heat transfer modes – conduction, convection and radiation. While conduction is based on the thermal property (thermal conductivity) of the envelope material and the temperature difference between the outside and the inside surfaces, convection occurs at the junction where the fluid (in motion) is in contact with the envelope surface. On the other hand, radiation arises when envelope bodies emit photons or radiant energy.
In a whole building energy model, the building envelope characteristics such as the U-factor are input in the software to estimate the building energy consumption. Although it may seem as simple as keying the U-factor of the building envelope in the software, care should be exercised when using software tools for Net Zero Energy buildings. This is due to complex building envelope systems and the methodologies used for heat transfer analysis to compute U-factor. Obviously, the downside of incorrect input of U-factor is over- or under-estimation of building energy consumption.
Hourly building energy programs used for energy consumption calculation cannot accurately represent the transient and multi-dimensional effects of envelope heat transfer. In addition, each of the hourly building energy modeling software uses a different method to calculate heat flow transfer for envelope assemblies. It is a matter of concern that EnergyPlus™ software promoted by the US Department of Energy has not resolved this fundamental, essential problem of integrating a two- or three-dimensional heat flow algorithm; instead, it developed (and is developing) algorithms for new emerging technologies. However, EnergyPlus™ has improved ground heat transfer modeling through links to three-dimensional finite difference ground models. Until such programs with built-in multi-dimensional heat flow analysis exists, it is crucial to develop U-factors using other auxiliary programs, and to input the relevant and more accurate data in existing hourly building energy programs in order to determine whole building energy consumption. Considering that almost all of building energy consumption studies are computed using such hourly energy programs, it is evident that the building energy consumption data is either over- or under-estimated and is in direct opposition to the objective of Net Zero Energy building design.
It is possible to generate a series of response factors or transfer functions for the envelope, however complex it may be, and to modify the existing hourly energy programs’ source codes for accurate results. The “equivalent wall” concept, a simple one-dimensional multi-layer structure that replicates the thermal properties of an actual wall, including the dynamic thermal behavior, provides a step forward in envelope heat transfer modeling (Kosny and Kossecka, 2002). Currently, the Oak Ridge National Laboratory has developed an Interactive Internet-based Envelope Material Database for whole-Building Energy Simulation Program. The hotbox test is used to calibrate the R-value situated in the envelope material database. In addition, the database provides a direct link to hotbox testing results, advanced three-dimensional heat transfer simulations, and whole-building energy analysis (ORNL, 2009). However, the database possesses only a few number of envelope configurations for use by designers.
Therefore, each input parameter, issues that relate to accuracy (or measurement of inaccuracies thereof), scientific basis for any alternatives provided in lieu of current inputs and secondary / auxiliary computation methods, etc., need utmost attention until the current building energy programs raise to our expectations.
Write to him (Ravi) at [email protected].
Ravi Srinivasan is teaching three courses on Building Envelope Design for Net Zero Energy buildings:
NZE 521: Modeling Building Envelope & Internal Loads- June 16, 8:30am-5:30pm.
NZE 621: Building Envelope & Internal Loads Optimization- July 19-20, 8:30am-5:30pm.
