MODEL AND ITS PREDICTING ON COOLING POTENTIAL OF NATURAL VENTILATION (NVCP) FOR LARGE SPACE BUILDINGS

. More and more large space buildings have occurred in China due to its multiple functional zones and architectural aesthetics, but heavy HVAC energy consumptions have been often accompanied. Effective use of natural ventilation may largely reduce the use of chillers during cooling periods. Field measurements were carried out on the large space in Nanjing, a city of south China. It was found out that linear thermal (cid:2)(cid:3)(cid:4)(cid:5)(cid:3)(cid:6)(cid:7)(cid:6)(cid:8)(cid:5)(cid:3)(cid:6)(cid:9)(cid:10)(cid:2)(cid:11)(cid:12)(cid:13)(cid:4)(cid:13)(cid:11)(cid:13)(cid:14)(cid:6)(cid:15)(cid:13)(cid:10)(cid:3)(cid:11)(cid:12)(cid:6)(cid:3)(cid:16)(cid:11)(cid:3)(cid:16)(cid:13)(cid:11)(cid:17)(cid:4)(cid:5)(cid:15)(cid:6)(cid:13)(cid:10)(cid:3)(cid:2)(cid:11)(cid:18)(cid:11)(cid:19)(cid:13)(cid:6)(cid:10)(cid:17)(cid:11)(cid:6)(cid:10)(cid:11)(cid:4)(cid:5)(cid:10)(cid:17)(cid:13)(cid:11)(cid:9)(cid:7)(cid:11)(cid:20)(cid:21)(cid:22)(cid:23)(cid:20)(cid:21)(cid:24)(cid:11)(cid:25)(cid:26)(cid:27)(cid:28)(cid:11)(cid:29)(cid:4)(cid:5)(cid:15)(cid:6)(cid:3)(cid:6)(cid:9)(cid:10)(cid:5)(cid:30)(cid:11)(cid:3)(cid:16)(cid:13)(cid:4)(cid:27)(cid:5)(cid:30)(cid:11)(cid:31)(cid:4)(cid:13)(cid:2)(cid:2)


Introduction
Based on reports by the International Energy Agency [1], the building and construction sector should be the main target of GHG (greenhouse gas) reduction efforts, DFFRXQWLQJ IRU RI ILQDO HQHUJ\ XVH LQ 9. Such single spaces include factory building, theatre and fire laboratory building, which has the characteristics of laUJH VSDWLDO VFDOH DQG FRPSOH[ structure. The energy consumption of heating, ventilation and air conditioning (HVAC) systems of large space buildings has been much higher due to internal heat sources and long operating hours [2]. However, stack effect ventilation is widely evident in the large space buildings with large height difference between lower and upper openings, so it can be utilized in some cases to reduce chiller energy consumption [3]. The internal thermal environment is non-uniform, leading to frequently vertical heat migration. The development of a modification model of conventional stack effect model based on thermal stratification and predicting influence on cooling potential have increasingly attracted researchers' attention.
Vertical thermal stratification is widely occurred in the large space buildings, and in which values vary with the envelope, indoor heat sources and ventilation conditions.  Fig. 2 The fluctuation amplitude of indoor temperature were less evident due to lower solar radiation entering through the glass. From 8 a.m. to 5 p.m., the indoor temperatures were lower than that outdoor for the Fire Laboratory Building. The indoor temperature is delayed by one hour to reach the peaking point than the outdoor temperature, which reflects the high thermal mass of the building wall. Fig. 2(b) compares the measured indoor air temperatures with height and tendencies for Group A of the atrium at DP SP DQG SP SP RQ 6HSWHPEHU It is apparent that the higher the height, the higher the temperature. The linear gradients were ILWWHG IRU HYHU\ KRXU EHLQJ LQ UDQJH RI -ႏP $OO correODWLRQ FRHIILFLHQWV ZHUH DSSUR[LPDWHO\ Finally, for large spaces with natural ventilation, the correlation between temperature and height can be written as follows: ‫ݐ‬ = ‫ݐ‬ + ߙ݄ (1) Where ‫ݐ‬ is the indoor air temperature at height of h, ႏ ‫ݐ‬ is the occupied air temperature at ground level, ႏ ߙ is the linear gradient in vertical direction, ႏP Equation (1) will be adopted in the following stack effect model.

Energy model
A physical model of the actual studied fire laboratory building was established in EnergyPlus 9.1 (Fig. 1). Meteorological information is based on the standard CSWD weather data for Nanjing [13]. The parameters of the envelope are provided in Table 1. There are no HVAC systems in the laboratory. Infiltration ventilation is considered. there are constant heat sources, such as lamps, occupants. Given the thermal stratification, vertical zoning setting is adopted through zone airflow module as shown in Fig. 1.

Mathematical model
A mathematical model of stack effect introducing vertically thermal stratification is developed in the matlab as shown in Fig. 3. At the neutral plane, pressure difference between indoor and outdoor is zero, and the air pressure is generally treated to be the local atmospheric pressure Patm (Pa). The lower the height of  The ideal gas state equation is that: is the driving force of natural airflow through opening a, so is pressure difference οܲ (Pa) through opening b, and they can be written as:

Simulated cases
Sensitivity analyze are carried out to understand some influences on the temperature tn, the height Hn and the flow rate l which include two values of H: 12m, 16P ten values of Rab that the lower opening is assumed to be constant: 1, 2, ……., 1 WKUHH YDOXHV RI T :P 2, :P 2 :P 2 monthly mean outdoor temperature for Nanjing, Chongqing and Guangzhou in Summer..

Fitted vertical temperature gradient
)LJ give the monthly mean outdoor temperature and the monthly fitted vertical temperature gradients for three cities and different heights. It is found that the gradients vary in range of -a ႏP The gradients varies greatly in different climatic provinces, such as Nanjing and Guangzhou. Due to warmer outdoor temperature in Guangzhou, the gradients is higher than the other two cities. Fig. 4. The vertical temperature gradient ߙ and outdoor mean temperature following with month.

Dimensionless neutral plane height
Dimensionless neutral plane height Hn+ LV GHILQHG DV the height ratio of neutral plane to the building. Fig. 5 shows close to the ground and the larger potential of thermal pressure ventilation.

Fig. 5.
The dimensionless neutral plane height Hn+ IROORZLQJ with month for a monthly temperature gradient ߙ and Rab = 5. Fig. 6 shows the mass flow rates G varying with Rab for different internal heat sources and building heights in August. As shown in the figure, the flow rate is proportional to Rab, and the higher height building and greater heat source, the larger flow rate per unit area of upper opening. However, the change of flow rate is not obvious for the different regions.

Conclusion
For a Fire Laboratory Building with concrete roof in campus of Nanjing Tech University of China, field measurements were conducted on it to find out its characteristics of non-uniform thermal environments during daytimes. It was found out that the indoor temperatures increased linearly with the increase of KHLJKW EHLQJ LQ UDQJH RI -ႏP A modified model of stack effect ventilation based on thermal stratification was developed. Temperature gradients of case-study output from Energyplus was input into matlab and the dimensionless neutral height and ventilation mass flow rate were simulated. For scenarios of two building heights, ten values of Rab and three values of q and Temperature gradient with cases, sensitivity analyses show the following: (1) the dimensionless neutral heights close to the ground and the higher potential of thermal pressure ventilation.
(2) The ventilation mass flow rate is positively affected by heat source, building height and Rab.