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Description
INTRODUCTION
Ventilation rates and air movements in buildings need to be quantified to answer the most basic indoor air quality (IAQ) question: "Is the proper amount of outdoor air reaching the occupants of the building to maintain acceptable indoor air quality?" This issue is of increasing concern as buildings become tighter and we rely more on mechanical ventilation systems to maintain acceptable IAQ (Posner et al. 2003). Nowadays, the largest part of electrical consumption in residential buildings is due to air-conditioning systems (Orme 2001). It is estimated that one-eighth of all energy used in Organization for Economic Cooperation and Development (OECD) countries is consumed in residential and service sector buildings, to meet the ventilation and air infiltration load (Liddament and Orme 1998). Carbon dioxide ([CO.sub.2]) emissions associated with high energy use should also be considered (Orme 2001).
Worldwide, there is a tendency toward naturally ventilated buildings (Simonson 2005). These buildings should satisfy required IAQ for the occupants and minimal (greenhouse gas) emissions to the environment (de Dear and Brager 2002; Snell et al. 2003). Especially for agricultural buildings, determination of ventilation rates in naturally ventilated buildings is crucial to meet with European environmental restrictions (NEC 2001).
Accurate ventilation rate measurements are essential for monitoring and controlling airflow through the building (ASHRAE 2005). A number of scientists worked on this issue in the last decades. Different solutions were proposed; however, none of them satisfies the required accuracy, reliability, and low cost needed for on-line control purposes of naturally ventilated buildings in practice. Table 1 shows a number of direct and indirect methods with their inaccuracies. A number of anemometers available on the market measure air speed at a local point very accurately. However, local measurements are not representative for whole sections, since the flow distribution along the section is not uniform and is time-varied. Additionally, using a number of sensors positioned at different places to calculate an average flow rate will be too complex and expensive for continuous on-line measurements. Although the free-running impeller is the most popular measuring technique in mechanically ventilated buildings (inaccuracy 5%), it fails most of the time in naturally ventilated buildings due to very low flow rates, the time-changing character of low flow rates, and noncircular inlet configurations (Berckmans et al. 1992). In addition, pressure distribution along the inlet brings another source of error (Van 't Ooster 1993). Indirect measuring techniques, including computer simulations such as computational fluid dynamics (CFD) calculations and gas balances, are quite popular for research purposes. Computer simulations require high expertness to decrease possible calculation and measurement errors. In addition, they can be suitable for design purposes, but not for on-line control of the ventilation rate. Use of metabolic [CO.sub.2] as a means to measure ventilation rate was reported in the literature (Stavova et al. 2006). However, imperfect mixing of [CO.sub.2] in space and measurements from unknown sources makes this technique inaccurate for further use. The tracer gas technique is one of the most commonly used methods for determination of ventilation rates in naturally ventilated buildings, but this technique assumes perfect mixing in space, which does not hold in reality.
Table 1. Overview of Available Ventilation Rate Measurement Techniques and Their Corresponding Accuracy on Total Ventilation Rate Measuring Ventilation Accuracy References Principal System Tracer gasses Natural 10%-30% Jung and Zeller (1994); Zhang et al. (1995); Demmers et al. (2001) Hot wire Natural 25% Krause and anemometer Janssen (1990); Watmuff (1995) [CO.sub.2] Natural 15%-40% Van 't Klooster balance and Heitlager (1994); Blanes and Pedersen (2005) CFD Natural 15%-40% Campen and Bot calculations (2003); Lee et al. (2005) Heat balance Natural 20%-40%... |
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