Publication Details
Abstract
Objective: The aim of this study is to make detailed comparative analysis of five different roof surface finishings – Standard Dark, White Reflective, Aluminum Metallic, Thermochromic, and Retro-Reflective – in terms of their capacity to decrease the heat conducted through the roof surface to the conditioned building space in hot–arid climate. Method: Transient heat–balance model for flat commercial roof is programmed in Python and modeled using Typical Meteorological Year (TMY) data for Baghdad International Airport, Iraq. The model calculates exterior surface temperatures hourly with a nonlinear energy balance equation, which considers solar radiation absorbed by the exterior surface, convection with the wind, longwave exchange with the sky, and conduction into the conditioned interior. The advanced coating behaviours are represented by a thermochromic reflectance function, dependent on temperature, and a high-reflectance approximation for the retro-reflective surface, with the angular dependence of the latter being not modelled in the current framework (constant ρ = 0.80). Timestep-sensitivity check shows that the hourly discretization causes less error of < 1.1% compared to a 10-minute reference grid, which is adequate for the annual energy accounting at the selected timestep. Using a two-parameter sensitivity analysis, it is shown that both the solar reflectance and thermal emittance have strong and coupled effects on the cooling demand, with the lowest cooling demand being for high solar reflectance and high thermal emittance. Results: The simulated annual roof-related cooling demand for a 100 m² roof area on a 1 story commercial building, with a Standard Dark roof at 24°C is 6,268.2 kWh. These values are only for the heat conducted through the roof assembly and should not be used as a whole building energy value. The total cooling energy saved was 53.9% of the Standard Dark when the dark surface was replaced with a White Reflective coating, resulting in an annual roof cooling energy of 2,888.7 kWh. This level of savings is attributed to the very absorptive baseline and the highly cooling-dominated climate, and is in the upper end of ranges reported in multi-study syntheses, which usually fall between 10-45% for whole-building analyses. Retro-Reflective and Thermochromic coatings result in annual savings of 3,075.7 and 3,560.5 kWh, respectively, and an Aluminum Metallic finish achieves 4,562.8 kWh (27.2% savings). An economic assessment indicates that White Reflective coatings have the most favorable 1.77-year payback period and the highest net savings of USD 1389.7 over a 10-year period of time in current Iraqi electricity tariffs, followed by Aluminum Metallic, Retro-Reflective and Thermochromic finishes. Novelty: The book offers an open and transparent computational framework, which can be customized for other climates and building types.