Lighting heat gain parameters: experimental method

HVAC & R Research, March, 2007 by Chanvit Chantrasrisalai, Daniel E. Fisher

The energy dissipated by lights is a significant contributor to the space heat gain and the space cooling load in many commercial buildings. To account for the heat gain due to lights, both of ASHRAE's new cooling load calculation procedures require the conditioned space/ceiling plenum split and the radiative/convective split as input data. This paper addresses the need to experimentally determine the lighting heat gain parameters for a range of common luminaires under realistic operating conditions. The paper presents both the measurement procedures and the computational procedures required to obtain derived results. The paper also discusses the uncertainty analysis and the accuracy of experimental results and compares different techniques that can be used to obtain the lighting heat gain parameters. Estimated uncertainties in the conditioned space, the ceiling plenum, and the convective fractions are relatively high. These uncertainties vary between [ or -]0.06 and [ or -]0.19. Estimated uncertainties in the shortwave and the longwave radiative fractions are relatively low, varying between [ or -]0.01 and [ or -]0.08 but mostly less than [ or -]0.03. A companion paper presents experimental results along with their estimated uncertainties, discusses the effects of various parameters on the measured results, and provides guidelines for the application of the experimental results.

INTRODUCTION

The heat gain due to lights constitutes a significant contribution to the hourly cooling load in many commercial buildings. Although all of the electrical power input to the lighting system is eventually converted to heat, the transport of lighting energy is very complex, particularly for a recessed luminaire, involving all three heat transfer mechanisms--radiation, convection, and conduction--to two spaces, the conditioned space and the ceiling plenum. Figure 1 illustrates the transport of lighting energy for various luminaires typically used in commercial buildings. The distribution of lighting energy is dependent on numerous variables, including the type of luminaire and lamp, the building construction, the room airflow configuration, and the setpoints of the conditioned space.

In order to account for the heat gain due to lights, both of ASHRAE's new cooling load calculation procedures (Pedersen et al. 1998) use a simple lighting heat gain model. The model requires two lighting heat gain parameters--the conditioned space/ceiling plenum split and the radiative/convective split (Spitler et al. 1997). The conditioned space/ceiling plenum split is the fraction of the lighting power converted to the lighting heat gain of the conditioned space and the fraction of the lighting power converted to the ceiling plenum's lighting heat gain. These fractions are only required for in-ceiling (or recessed) luminaires since it can be assumed that the heat generated by all other luminaires is entirely dissipated in the conditioned space. On the other hand, the radiative/convective split is the fraction of the lighting heat gain of the conditioned space that is transferred as radiation and the fraction that is transferred as convection. These fractions are required for both in-ceiling and non-in-ceiling luminaires. In the heat balance (HB) method, the radiative component of the conditioned space lighting heat gain participates in the inside surface heat balance with some prescribed radiative distribution, while the convective component is assumed to go immediately to the air heat balance (i.e., instantaneously becomes cooling load). The HB method treats shortwave and longwave radiation due to lights separately, meaning that shortwave radiation due to lights can be lost from the space through transparent surfaces. The radiant time series (RTS) method, on the other hand, does not distinguish shortwave and longwave radiation. The RTS method uses the so-called nonsolar radiant time factors to convert the radiative component of the conditioned space lighting heat gain into cooling load. Like the HB method, the convective component is assumed to become cooling load immediately.

[FIGURE 1 OMITTED]

Published lighting heat gain parameters in the 2005 ASHRAE Handbook--Fundamentals (ASHRAE 2005) do not present the conditioned space/ceiling plenum "heat gain" split. In addition, the 2005 Handbook data may be obsolete due to recent advancements in lighting technologies. The current research addresses the need to provide relevant lighting heat gain parameters for a range of common luminaires. The lighting heat gain parameters can be economically determined by detailed lighting models (Chung and Loveday 1998a, 1998b; Sowell and O'Brien 1973; Sowell 1990, 1993; Walton 1993). However, these lighting models have only been validated for one type of luminaire considered in the current study, namely, the recessed luminaire with acrylic lens. In addition, the lighting models require appropriate correlations of convection coefficients in order to predict accurate results (Chung and Loveday 1998a, 1998b; Sowell and O'Brien 1973; Sowell 1990, 1993; Walton 1993). Unfortunately, such correlations are not currently available for most luminaires. Therefore, experimental methods are preferable to numerical methods for the current study.