Frederick C. Winkelmann

EnergyPlus: creating a new-generation building energy simulation program

Abstract Many of the popular building energy simulation programs around the world are reaching maturity — some use simulation methods (and even code) that originated in the 1960s. For more than two decades, the US government supported development of two hourly building energy simulation programs, BLAST and DOE-2. Designed in the days of mainframe computers, expanding their capabilities further has become difficult, time-consuming, and expensive. At the same time, the 30 years have seen significant advances in analysis and computational methods and power — providing an opportunity for significant improvement in these tools. In 1996, a US federal agency began developing a new building energy simulation tool, EnergyPlus, building on development experience with two existing programs: DOE-2 and BLAST. EnergyPlus includes a number of innovative simulation features — such as variable time steps, user-configurable modular systems that are integrated with a heat and mass balance-based zone simulation — and input and output data structures tailored to facilitate third party module and interface development. Other planned simulation capabilities include multizone airflow, and electric power and solar thermal and photovoltaic simulation. Beta testing of EnergyPlus began in late 1999 and the first release is scheduled for early 2001.

Moisture effects on conduction loads

Abstract The effects of moisture on sensible and latent conduction loads are shown by using a heat and mass transfer model with variable material properties, under varying boundary conditions. This model was then simplified to reduce calculation time and used to predict conduction peak load (CPL) and yearly integrated wall conduction heat flux (YHF) in three different cities: Singapore (hot/humid), Seattle (cold/humid) and Phoenix (hot/dry). The room air temperature and relative humidity were calculated with the building energy simulation program DOE-2.1E. The materials studied were aerated cellular concrete (ACC), brick (BRK), lime mortar (LMT) and wood. It is shown that the effects of moisture can be very significant and that simplified mathematical models can reduce the calculation time with varying effects on accuracy.

Two- and three-dimensional natural and mixed convection simulation using modular zonal models in buildings

We demonstrate the use of the zonal model approach, which is a simplified method for calculating natural and mixed convection in rooms. Zonal models use a coarse grid and use balance equations, state equations, hydrostatic pressure drop equations and power law equations of the form m=CΔPn. The advantages of the zonal approach and its modular implementation are discussed. The zonal model resolution of nonlinear equation systems is demonstrated for three cases: a 2-D room, a 3-D room and a pair of 3-D rooms separated by a partition with an opening. A sensitivity analysis with respect to physical parameters and grid coarseness is presented. Results are compared to computational fluid dynamics (CPD) calculations and experimental data.

An empirical correlation for the outside convective air-film coefficient for horizontal roofs

Abstract From measurements of surface heat transfer on the roofs of two commercial buildings in Northern California we have developed a correlation that expresses the outside convective air-film coefficient for flat, horizontal roofs as a function of surface-to-air temperature difference, wind speed, wind direction, roof size, and surface roughness. When used in detailed building energy analysis programs, this correlation is expected to give more accurate calculation of roof loads, which are sensitive to outside surface convection. In our analysis, about 90% of the variance of the data was explained by a model that combined standard flat-plate equations for natural and forced convection and that took surface roughness into account. We give expressions for the convective air-film coefficient: (1) at an arbitrary point on a convex-shaped roof, for a given wind direction; (2) averaged over a strip along the wind direction; and (3) averaged over a rectangular roof for a given wind direction.

Comparison of DOE-2 with temperature measurements in the Pala test houses

Abstract The predictions of version 2.1E of the DOE-2 program for building energy analysis have been compared with measurements in the Pala test houses near San Diego, CA. This work was part of the California Institute for Energy Efficiency ‘Alternatives to Compressor Cooling in California Transition Zones’ project in which DOE-2 was used for parametric analysis of cooling strategies that reduce peak electrical demand. To establish the validity of DOE-2 for this kind of analysis the program was compared with room air temperature measurements in a ‘low-mass’ house with conventional insulated stud wall construction and a ‘high-mass’ house with insulated concrete walls. To test different aspects of the DOE-2 calculation, four different unoccupied, unconditioned thermal configurations of these houses were considered: unshaded windows, shaded windows, white exterior surfaces, and forced night ventilation. In all cases DOE-2 agreed well with the air temperature measurements, with a mean deviation between simulation and measurement ranging from 0.2 to 1.0 K depending on configuration and type of house. Comparisons with inside surface temperature measurements also showed good agreement. Agreement between predictions and measurements improved when a more accurate calculation of foundation heat transfer was used, the ground surface temperature was calculated, and the normal 7-day ‘warm-up’ period in DOE-2 was extended to 11 days for the high-mass house.

Symbolic Modeling in Building Energy Simulation

Abstract We show how symbolic modeling is used in the Simulation Problem Analysis and Research Kernel (SPARK) for solving complex problems in building energy simulation. After a brief overview of SPARK, we describe its symbolic interface, which reads equations that are entered in symbolic form and automatically generates a program that solves the equations. The application of this method to solving the partial differential equations for two-dimensional heat flow is illustrated.

Two- and three-dimensional natural and mixed convection simulation using modular zonal models

We demonstrate the use of the zonal model approach, which is a simplified method for calculating natural and mixed convection in rooms. Zonal models use a coarse grid and use balance equations, state equations, hydrostatic pressure drop equations and power law equations of the form {ital m} = {ital C}{Delta}{sup {ital n}}. The advantage of the zonal approach and its modular implementation are discussed. The zonal model resolution of nonlinear equation systems is demonstrated for three cases: a 2-D room, a 3-D room and a pair of 3-D rooms separated by a partition with an opening. A sensitivity analysis with respect to physical parameters and grid coarseness is presented. Results are compared to computational fluid dynamics (CFD) calculations and experimental data.

The electrical analog: RC networks for heat transfer calculations

A brief introduction to the analogy between electricity flow and heat flow is given and some simple examples of RC networks applied to heat conduction through building walls are described. (AIP)