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Simulation evaluation of the ventilation effect on the performance of a VRV system in cooling mode--part II, simulation evaluation.

Article Excerpt
INTRODUCTION

Residential and commercial buildings require air conditioning and ventilation due to modern societies' demand for thermal comfort and healthy environments. The multisplit variable refrigerant volume (VRV) system is finding its way into residential and commercial buildings because of its precise capacity control and individualized thermal comfort capabilities. Since the first multisplit VRV system was installed more than 20 years ago in Asia (Goetzler 2007), this technology has been widely studied, both experimentally (Xia et al. 2002; Masuda et al. 1991; Hai et al. 2006; Hu and Yang 2005; Choi and Kim 2003; Zhou et al. 2007a, 2007b; Watanabe et al. 2004) and numerically (Wu et al. 2005; Xia et al. 2003, 2004; Shi et al. 2003, 2008; Park and Min 2001; Zhou et al. 2007c; Shah et al. 2004; Shao et al. 2008). Despite the large number of previous studies, there is little research on the energy consumption characteristics of the multisplit-type system from the dynamic building energy simulation perspective due to the lack of both test data sets and experimentally validated simulation design tools (Zhou et al. 2008). Since there is no well-known energy simulation software available that can be used for the energy analysis of multisplit-type systems, a new model, which is applicable to the EnergyPlus (DOE 2005) building energy simulation software, was developed (Zhou et al. 2007b). The developed model was experimentally validated (Zhou et al. 2007a, 2008). The average monitored and predicted data for the total provided cooling energy and power consumption were found to be within 25% and 28%, respectively.

Since multisplit VRV systems cannot provide any ventilation, which is required by ASHRAE's regulations for office environments (Goetzler 2007), additional ventilation systems must be installed. That's why the integration of the multisplit VRV system and the ventilation system gains importance in actual applications. In this study, the effect of ventilation on indoor temperature control, thermal comfort, outdoor unit energy consumption, and the efficiency of the multisplit VRV system and energy saving options are investigated numerically with the multisplit VRV module obtained from the literature (Zhou et al. 2007b).

SIMULATION MODELS AND EXPERIMENTAL VALIDATION

Multisplit VRV System Module

A mathematical model of the multisplit VRV system was developed based on the existing air-cooled direct expansion (DX) coil in EnergyPlus, which determines the part-load performance of the coil by utilizing numerical expressions of the total cooling capacity and the energy input ratio as a function of the part-load fraction through regression analysis. The single DX coil is used as an indoor unit of the multisplit VRV system. EnergyPlus uses five performance curves to describe the change in total cooling capacity and efficiency at the part-load conditions of the DX coil:

* total cooling capacity modifier curve as a function of temperature

* total cooling capacity modifier curve as a function of flow fraction

* energy input ratio modifier curve as a function of temperature

* energy input ratio modifier curve as a function of flow fraction

* part-load fraction correlation as a function of part-load ratio

The detailed formulation of the multisplit VRV system for the EnergyPlus module can be found in Zhou et al. (2008). The data required for inputs to the EnergyPlus multisplit VRV module, including the standard cooling capacities of the outdoor and indoor units; power input to the outdoor unit under varying outdoor and indoor conditions; and the airflow rates of each indoor unit, were obtained from manufacturer cataloges.

Heat Recovery Ventilation System

Since heat recovery ventilation (HRV) system units are well known and have been extensively studied and used for ventilation purposes in various applications (Dieckmann et al. 2003; Asiedu et al. 2005; Juodis 2006; Lazzarin and Gasparella 1998), EnergyPlus already has a stand-alone simulation module for that kind of system, known as an energy recovery ventilator (ERV). This module was used for the simulation part of the HRV units. Stand-alone ERVs consist of supply and exhaust air fans and an air-to-air heat exchanger. This module...

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