Dynamic Thermal Characteristics

DYNAMIC THERMAL CHARACTERISTICS

Jan Kosny
Oak Ridge National Laboratory

The steady-state R-value traditionally used to measure energy performance of a building envelope does not accurately reflect the dynamic thermal behavior of all complex building envelope systems. Dynamic thermal characteristics for complex building envelope components can be generated during dynamic hotbox tests or by using analytical calculation methods. One of the first dynamic test methods for determining transfer function coefficients for complex wall specimens using calibrated hotbox was developed by NIST (Burch et.al. 1990). ASHRAE Research Project 515-RP (NRC Canada, 1991), measured the dynamic heat transmission characteristics of seven generic walls using a guarded hot box procedure. Combined experimental and theoretical dynamic thermal characteristic analysis has been performed by ORNL for complex massive wall configurations (Kosny et. al. 1998). Theoretical methods are very useful in whole building energy modeling. Whole-building energy simulation programs like DOE-2, Blast, Energy-10, or just coming, Energy Plus, use one-dimensional calculation engines, which are inaccurate in simulations of complex building envelope components. To enable more accurate whole-building energy simulations, simplified one-dimensional descriptions of walls or roofs have to be developed.

Currently, the standard modeling practice is to replace complex material configurations by one-dimensional multi-layer structures with similar R-values and similar material arrangements. Unfortunately, such simplifications cannot accurately represent the complicated two- and three-dimensional dynamic heat transfer that takes place in many currently used wall and roof assemblies (Kosny & Kossecka 2000, Enermodal 2001). To increase the accuracy of the whole building energy simulations, simplified models can be developed based on classical heat transfer theory and resistance- capacitance analogues. One of the first useful approaches for the development of such models is described in (Jóhannesson, 1981), which represents component materials in an assembly as an equivalent network of thermal resistance and thermal capacitance. The Solar Energy Laboratory at the University of Wisconsin-Madison has developed a theoretical procedure for analyzing wall assemblies with three-dimensional thermal bridges (Seem et al., 1989). Formal relationships, which describe in a quantitative way the effect of structure on dynamic thermal behavior of walls and follow from the integral formulae for the heat flow across the surfaces of a wall in a finite time interval, were introduced in (Kossecka - 1992). A definition of thermal structure factors and analysis of relationships between the structure factors, response factors and z-transfer function coefficients have been derived by Kossecka and Kosny (1997). Thermal structure factors appear in expressions for the asymptotic heat flow across the surface of a wall, for boundary conditions independent of time. Another flexible methodology is described in (Mao and Jóhannesson 1997), although this approach is more suited to thermal bridges with metal framing and metal-skin walls. One of the first practical attempts to improve accuracy of whole building energy simulations using two-dimensional input data was performed by LBNL (Bazjanac et al. 1996 and Huang et al. 1996). A more general three-dimensional approach is the equivalent wall method described in (Kossecka & Kosny 1997, Enermodal 2001).

References

1.	Bazjanac, V. H. Feustel, and Y.J. Huang, 1996. "The modeling of two-dimensional heat flow in DOE-2 simulation", Consultant Report under Contract No. 400-93-023, Energy Efficiency Division, California Energy Commission, Sacramento CA.
2.	Burch D.M., Zarr R.R., Licitra B.A., 1990 “A Dynamic Test Method for Determining Transfer Function Coefficients for a Wall Specimen Using Calibrated Hot Box” ASTM - Insulation Materials, Testing and Application, ASTM 1030, pp. 345-361.
3.	Enermodal - 2001 (J. Kosny, E. Kossecka, S. Carpenter, T. Forrest) “ Modeling Two-and-Three Dimensional Heat Transfer Through Composite Wall and Roof Assemblies in Transient Energy Simulation Programs’ ASHRAE research project - 1145-TRP March 2001.
4.	Huang, Y.J., V. Bazjanac, H. Feustel, and J. Trowbridge 1996. "Two-dimensional wall response factors", DOE-2 User News, Vol. 17, No. 3, pp. 6-12, Lawrence Berkeley National Laboratory, Berkeley CA.
5.	Jóhannesson, G, and O. Cberg. 1981. “Thermal Bridges in Sheet Steel Construction”, Report TVBH-3006, Lund, Sweden.
6.	Kosny J., J. E. Christian, A. O. Desjarlais, E. Kossecka, L. Berrenberg, 1998- “Performance Check Between Whole Building Thermal Performance Criteria and Exterior Wall Measured Clear Wall R-value, Thermal Bridging, and Airtightness” - ASHRAE Transactions 1998, v.104 pt 2.
7.	Kosny J., E. Kossecka, A. O. Desjarlais, J.E. Christian -1998a “Dynamic Thermal Performance of Concrete and Masonry Walls ” - DOE, ASHRAE, ORNL Conference -Thermal Envelopes VII, Clear Water, Florida - Dec. 1998.
8.	Kosny J., E. Kossecka 2000“ Computer Modeling of Complex Wall Assemblies - Some Accuracy Problems” presented at International Building Physics Conference, Eindhoven, The Netherlands '2000
9.	Kossecka E. 1992. Heat transfer through building wall elements of complex structure, Arch. Civ. Engn., 38(1-2), 117-126.
10.	Kossecka E., J. Kosny, 1997 “Equivalent Wall as a Dynamic Model of a Complex Thermal Structure” - Journal of Thermal Insulation and Building Envelopes, vol. 20, January 1997.
11.	Mao G., Johanensson G. 1997 “ Dynamic Calculation of Thermal Bridges” Report Thermal Bridges; Efficient Models for Energy Analysis in Buildings’ Bulletin 173, ISSN 07346-5918, Stockholm.
12.	NAHB Research Center 1999 “Insulating Concrete Forms: Comparative Thermal Performance” prepared for U.S. Dept. of Housing and Urban Development. August 1999.
13.	NRC Canada 1991 “Dynamic Heat Transmission Characteristics of Seven Generic Wall Specimen” ASHRAE Research Project 515-RP, IRC Contract CR5880.
14.	Seem J.E., S.A. Klein W.A. Beckman, J.W. Mitchell. 1989. Transfer Functions for Efficient Calculation of Multidimensional Heat Transfer. J. of Heat Transfer, Vol. 111, 5-12.