FOUNDATION HEAT TRANSFER MODULE FOR ENERGYPLUS PROGRAM

New calculation procedures for calculating conduction heat transfer for foundations are described. These procedures are first validated and then implemented within EnergyPlus source code (beta version 5.0). Selected results of the implementation are presented for a 3-zone low -rise building. The results indicated that the developed foundation heat transfer module accounts better than existing EnergyPlus module for the ground mass and its effects on reducing the hourly and seasonal fluctuations of slab surface temperatures. INTRODUCTION The energy performance of the above-grade portion of the buildings is generally well understood. Hourly prediction of heat transfer from walls exposed to ambient has helped improve the thermal efficiency of building envelopes. Unfortunately, the attention to foundation heat transfer has lagged behind other building components. Today, a quantitative understanding of foundation heat transfer is needed to accurately predict and thus improve the overall energy performance of a building. It is estimated that a basement kept uninsulated may contribute up to 60 percent of the heat loss in a tightly-sealed home that is well insulated above-grade (Labs et al., 1988). The ground-coupling heat transfer, in almost all the existing hourly building simulation programs, is treated in a primitive way by defining a simple steady-state U-value. This crude treatment stems from the lack of a straightforward and easy approach to calculate transfer functions and/or response factors for building foundations. Major advances in knowledge of earth-contact heat transfer have been acquired since the 1970's. Sterling and Meixel (1981), Claridge (1988), and more recently Krarti (2000) provide detailed review of the state-of-the-art ground heat transfer work. Most of the dynamic models developed for foundation heat transfer are based on numerical methods and thus require hours of computer time and are, therefore, inappropriate for use in a program like EnergyPlus, which is attended to perform annual simulation of the above-ground building in few minutes. An analytical technique called the Interzone Temperature Profile Estimation (ITPE) method developed by Krarti et al. (1988) has been applied to several ground-coupling heat transfer problems. In particular, ITPE solutions have been developed to calculate heat transfer from slab-on-grade floors, basements, and earth-sheltered buildings with commonly used insulation configurations (Krarti, 1990, 1993, and 1994). The ITPE method combines both analytical and numerical techniques to obtain two-dimensional and three-dimensional solutions of heat conduction equation. Because it is based on an analytical solution, the ITPE method handles any value of thermal insulation R-value, water table depth, and soil thermal properties. In a typical ITPE formalism, the ground (or any conductive medium) is first divided into several zones of regular shapes by “imaginary” surfaces. The geometry and the boundary conditions determine these imaginary surfaces that divide the ground medium. Then, the temperature distribution is determined in each zone by solving the heat conduction equation by an analytical technique. Along the imaginary surfaces, the temperature profiles are not known. However, these temperature profiles are determined using the heat flux continuity between the zones. For more details on the formalism and the applications of the ITPE method, refer to Krarti et al. (1990, 1993, and 1994). Several studies compared the results from the ITPE method with measured data and with predictions based on detailed numerical solutions (Krarti et al., 1985 and 1995, and Yuill and Wray, 1987). In general, good agreement was found. While the ITPE method calculates the foundation heat flow in less than one minute of computer time, the numerical solutions require several hours of computer run time. A study by Krarti et al. (1993) showed that the ITPE formalism can be applied to generate response factors for building foundation. These response factors can be generated in one or two minutes of computer time and are suitable for use with most hourly building simulation programs including EnergyPlus. The response factors are calculated only once by the pre-processor of the simulation program and then used to calculate ground heat fluxes at any time-step. More recently, Chuangchid and Krarti (1998) found that three-dimensional foundation heat transfer from either buildings or refrigerated structures (with low Seventh International IBPSA Conference Rio de Janeiro, Brazil August 13-15, 2001