A generalized methodology for determining the total heat gain through building envelopes

Abstract

SUMMARYWe present a generalized methodology for determining the annual total heat gain through external walls and roofs of large air-conditioned buildings. The methodology is based on the concept of the overall thermal transfer value (O'ITV). Respective OTTV equations for building envelopes and roofs are developed through parametric simulations using the DOE-2 computer code. The equations are valid for buildings having different aspect ratios and wall masses. Appropriate coefficients for heat conduction through fenestrations and opaque walls and solar correction factors for wall facades of different orientations are computed from local weather data. The equations allow building designers to make accurate estimates of the total heat gain for the purpose of evaluating energy-efficient building envelope components and air-conditioning systems and plant options. The methodology is validated using DOE-2 computed heat gain results and can be applied to different classes of buildings, construction types and locations.KEY WORDS: overall thermal transfer value (OTTV); building envelope heat gain 1. INTRODUCTION The overall thermal transfer (OTTV) is a measure of the average heat gain into a building through its envelope. In tropical climates, the OTTV has been widely used as a criterion for achieving adequately designed building envelopes to reduce external heat gains and hence the cooling load of air-conditioned buildings. Although the latest ASHRAE Standard 90.1 (1989) recommends the use of an overall thermal transmittance value in its prescriptive and system performance criteria, there appears to be room for simplifying the calculation and yet obtaining an accurate estimate of the total heat gain into a building. This is especially desirable for buildings in tropical climates where cooling is required all year round and where the average indoor-outdoor temperature difference is smaller than 10 "C. For such applications, it is advantageous that the envelope thermal performance standards be closely correlated with the cooling load and hence the cooling energy use of the buildings. In this respect, Chou and Chang (1993) have shown how the OTTV can be used as a key parameter in estimating the cooling energy consumption of large air-conditioned buildings.The O W formulation takes into account the following three basic contributions to heat gain through the external wall of a building, namely: (i) heat conduction through opaque walls; (ii) heat conduction through fenestration; and (iii) solar radiation through fenestration. From ASH= Standard 90 (19751, the OTTV of the ith external wall of a building is expressed as (1) This equation is to be applied to every external wall of a given air-conditioned building so that the OTI'V of the entire building envelope can be obtained by taking the weighted average of the values of all the walls. For a square building with four identical walls, the OTTV of the building envelope may be expressed as

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