Finite-volume thermal analysis of building roofs under two-dimensional periodic conditions

A finite-volume implicit procedure is presented for the determination of the thermal performance of two-dimensional composite building roof elements. The elements are subjected to steady periodic change in ambient temperature, solar radiation and non-linear radiation exchange on the outside surface and to a constant indoor-air-temperature on the inside surface. The solar radiation is calculated by the ASHRAE clear-sky model. TEACH-C computer program is utilized to solve the finite-volume equations. The validity and accuracy of the numerical model are checked by comparing results with exact analytical solutions for simpler problems. The model versatility is demonstrated by an application to a "Hordi-type" (rib-slab) structure used in roofs in Saudi Arabia. The climatic data of Riyadh for a representative day for July are employed. The Hordi units used are made of an insulation material to further increase the overall thermal resistance (R-value) of the roof. Sample results show that the ribs of the reinforced concrete slab act as thermal bridges which degrade the roof R-value appreciably.King Saud Universit

Thermal performance of building roof elements

The study concerns the evaluation and comparison of the thermal performance of building roof elements subject to periodic changes in ambient temperature, solar radiation and nonlinear radiation exchange. A numerical model, based on the finite-volume method and using the implicit formulation, is developed and applied for six variants of a typical roof structure used in the construction of buildings in Saudi Arabia. The climatic conditions of the city of Riyadh are employed for representative days for July and January. The study gives the detailed temperature and heat flux variations with time and the relative importance of the various heat-transfer components as well as the daily averaged roof heat-transfer load, dynamic R-values and the radiative heat-transfer coefficient. The results show that the inclusion of a 5-cm thick molded polystyrene layer reduces the roof heat-transfer load to one-third of its value in an identical roof section without insulation. Using a polyurethane layer instead, reduces the load to less than one-quarter. A slightly better thermal performance is achieved by locating the insulation layer closer to the inside surface of the roof structure but this exposes the water proofing membrane layer to larger temperature fluctuations.King Saud Universit

Heat transfer characteristics and optimum insulation thickness for Hordi roofs using a pseudo one-dimensional dynamic model

Rib-slab (Hordi) roofs are commonly used in the construction of buildings in Saudi Arabia. Compared to the traditional solid-slab roofs, Hordi roofs are lighter and have higher thermal and acoustic resistances. Despite their widespread use in recent years, only little information is available regarding their thermal characteristics in general, and none under optimised insulation conditions. The geometrical configuration of the Hordi units is strictly two-dimensional, however, the heat flow across the roof is predominantly one-dimensional. By modeling the layers in a Hordi unit by thermal resistances connected in parallel, an "equivalent" one-dimensional model is obtained for the roof. A finite-volume time-dependent implicit procedure, which has been previously validated, is used in conjunction with this model to compute the yearly heat transmission loads for different insulation layer thickness while accounting for solar radiation and long wave radiation exchange with the sky. These loads, calculated under steady periodic conditions using the climatic data of Riyadh, are used in an economic model in order to determine the optimum insulation thickness (Lopt). Three roof configurations are investigated all with a polystyrene insulation layer. Roof R1 is a traditional solid-slab roof, roof R2 is a rib-slab roof with Hordi units made of polystyrene, and roof R3 is a rib-slab roof with Hordi units made of concrete. Under the conditions of the present study, values of L opt obtained are: 4.8, 3.7 and 4.2 cm, for R1, R2 and R3, respectively. Under optimum conditions, R2 gives the lowest total cost of insulation and energy consumption over the lifetime of the building, while R3 gives the lowest decrement factor (0.0047) and the longest time lag (13.2 h); R1 is found to be the least favoured roof in these respects. The average R-value for the three roofs is about 2.2 m2 K/W.King Saud Universit

Optimum insulation thickness for building walls in a hot-dry climate

The optimum thickness of an insulation layer in a typical building wall is determined under steady periodic conditions using the climatic data of Riyadh. A finite-volume implicit procedure, which has been previously validated, is used to compute the yearly heat transmission loads for various insulation thicknesses. These loads are input to an economic model, based on the present worth method, in order to minimise the total cost of insulation and energy consumption over the lifetime of the building. Cooling and heating loads are integrated separately over the year and treated with different costs in the economic analysis. The wall yearly transmission loads, yearly-averaged dynamic R-value, time lag and decrement factor are presented versus insulation thickness and compared for different wall orientations. A parametric study is performed to establish the sensitivity of the results to changes in economic parameters. The optimum insulation thickness is found to increase with the cost of electricity, building lifetime and inflation rate; and decrease with increasing cost of insulation material, coefficient of performance of air-conditioning equipment and discount rate. The results also show that the wall orientation has a significant effect on the thermal behaviour but a relatively smaller effect on the total cost and consequently the optimum insulation thickness. The south-facing wall is the most favourite orientation since it gives about 12% lower yearly transmission load and 5% lower total cost compared to the least favourite orientation which is the west-facing wall. Among the insulation materials investigated, molded polystyrene is found to be the most economical type with an optimum thickness of 9.3 cm.King Saud Universit

Optimized monthly-fixed thermostat-setting scheme for maximum energy-savings and thermal comfort in air-conditioned spaces

The present study deals with energy conservation in buildings via reduction of cooling-and-heating transmission loads through walls by optimizing the indoor air-temperature settings. Maximum energy-saving and thermal-comfort are obtained for both yearly- and monthly-fixed thermostat settings. The transmission loads are calculated under the climatic conditions of Riyadh by using a dynamic heat-transfer model based on the finite-volume implicit procedure, which has been validated previously. The study utilizes a basic thermal-comfort chart where indoor air temperatures are selected inside the summer and winter comfort-zones, as a function of relative humidity, in a manner to provide the highest comfort-level while maximizing energy savings. The yearly-fixed thermostat settings range between 21.6 °C and 24.1 °C (70.9 °F and 75.4 °F), and those for the optimized monthly-fixed settings range between 20.1 °C and 26.2 °C (68.2 °F and 79.1 °F). For the yearly-fixed thermostat settings, the results show that about a 10% reduction in yearly cooling transmission load can be achieved per 1 °C increase in thermostat setting. Despite a corresponding increase of about 14% in yearly heating transmission load, a net saving in the yearly total energy cost of about 4% can still be affected per 1 °C increase in thermostat setting within the comfort zone. However, much bigger savings are achieved by utilizing an optimized monthly-fixed thermostat setting scheme developed in this study. Savings in energy cost between 26.8% and 33.6% compared with the yearly-fixed settings are obtained depending on the value of yearly-fixed setting. The corresponding reductions in peak loads compared with the yearly-fixed settings range between 13.5% and 25.6% in summer, and between 15.1% and 31.9% in winter depending on the yearly-fixed setting. These percentage savings in energy cost and reductions in peak loads are conservative since the yearly-fixed settings are themselves selected for high annual energy-savings while maintaining a high level of thermal comfort throughout the year. © 2007 Elsevier Ltd. All rights reserved.King Saud Universit

Numerical modelling of two-dimensional shallow-water flows

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