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Performance comparison of conventional and chilled ceiling/displacement ventilation systems in Kuwait.

Article Excerpt
INTRODUCTION

Buildings in Kuwait account for more than 50% of all energy use in the country [Mesheshwari et al. (2005)]. In the summer, the HVAC systems represent 70% of peak load. The combined chilled ceiling and displacement ventilation (CC/DV) system is a system that has potential for energy savings with improved air quality in spaces where the cooling load does not exceed 100 W/[m.sup.2] [Novoselac and Srebric (2002)]. The CC/DV system has not been introduced in Kuwait HVAC market. The current paper is concerned with assessing the perspective of this system performance, applicability, and energy saving potential in Kuwait climate and building code.

In displacement ventilation, the cooler air entering the room at the floor level displaces the warmer room air that rises due to its natural buoyancy effect. Consequently, the bottom occupied zone contains the fresh cool air with no recirculation flow while the heat and contaminants produced by the room activities rise to the ceiling level where they are exhausted [Jiang et al. (1995) and Yuan et al. (2001)]. The chilled ceiling carries a portion of the sensible cooling load and the DV system carries the rest of the sensible load in addition to the latent cooling loads. Design and operation of integrated chilled ceiling (CC) and displacement ventilation (DV) system is not as straight forward as the case when one system is used in meeting the cooling load. The CC/DV modeling is more involved due to nonlinearity in heat transport processes and stratification present in the room. The system operation required setting of several parameters that need to be simultaneously specified to insure meeting thermal comfort, IAQ inside the space while condensation on the ceiling is prevented. The operational variables include: the supply air conditions (flow rate, temperature and humidity) of the DV system and the chilled ceiling temperature. Ghaddar et al. (2008) proposed design charts that are based on the ratio R of the CC cooling load to the total load, the thermal comfort represented by the temperature gradient (dT/dZ), and the amount of displaced air parameter (P = (Q/m)). Their design chart parameters included the temperature ranges of the supply air temperature and the chilled ceiling for any R in the feasible design regions where dT/dZ is less than 2.5 K/m and with the facility to read off the stratification height H and insure that it is above 1.2 m. Stratification height is the elevation at which the density gradients disappear in the rising air and the plume spreads horizontally at a minimum level for acceptable indoor air quality in the occupied zone. The design chart of Ghaddar et al. (2008) provided information to select an accepted design, but it does not give information about expected performance under transient conditions initiated by external solar and environmental load and internal load schedule. The charts are based on a wall-plume-multi-layer model of Ayoub et al. (2006) that is insensitive to position of the heat sources in the room. Ghali et al. (2007) extended the plume-multilayer to transient response to take into consideration variable load and heat storage in walls and predict system energy consumption. Their model is well-suited for the current application because it accounts for plumes associated with non-uniformly heated walls; and it accurately predicts the stratification height and the vertical air and wall temperature gradients as a function of supply conditions, and the chilled ceiling temperature. This model will be instrumental in the assessment of the CC/DV system performance in Kuwait, but needs to be validated with experiments for cases when transient external load is present which could be a substantial load for Kuwait weather. The climate conditions of Kuwait and building construction practices necessitate careful design of the combined system to achieve energy savings. The climate is characterized by a high solar load and well-insulated building envelops with conductance U less than 0.72 W/[m.sup.2]K.

The first objective of the work is to experimentally test the transient wall-plume-multilayer model when transient external load is present in the conditions of Kuwait to check the attainment of thermal comfort and indoor air quality with an accept able stratification height. The second objective is to investigate the potential for energy savings and economic feasibility of the use of the CC/DV system compared to current air conditioning practice of using conventional mixed convection systems in Kuwait for a case study of an office space. A design procedure based on Ghaddar et al. (2008) design charts will be followed to size each of the two subsystems (chilled ceiling and displacement ventilation) and determine the chilled ceiling load removal ratio R to the total...

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