Cool roof technology in London: An experimental and modelling study

Abstract

One of the primary reasons for the application of cool materials is their energy and associated environmental impact on the built environment. Cool materials are usually applied on the roof of buildings to reduce cooling energy demand. The relative benefits of this reduction depend on the construction of the building, external weather conditions and use of the building. This paper examines the impact from the application of a reflective paint on a flat roof in a naturally ventilated office building in the area of London, UK where the climate is moderate with high heating demand by buildings.The environmental conditions (internal/external air and surface temperatures) of the building were monitored before and after the application of the cool roof during the summer. It was found that internal temperatures were reduced after the application of the cool roof. The building was modelled using TRNSYS and the model was calibrated successfully using the measurements. A parametric analysis was carried out by varying the reflectivity and insulation of the roof and ventilation rate; the heating and cooling demand for a year was calculated using the Summer Design Year for London as the weather file. It was found that cooling demand is significantly reduced, heating demand is increased and the total energy savings vary between 1 and 8.5% relative to an albedo of 0.1 for the same conditions. In free floating (naturally ventilated) buildings summer comfort is improved but there is a penalty of increased heating energy during the winter. Thermal comfort can be improved by an average of 2.5 o C (operative temperature difference for a change of 0.5 in albedo) but heating demand could be increased by 10% for a ventilation rate of 2 air changes per hour. The results indicate that in the case of temperate climates the type, operation and thermal characteristics of the building should be considered carefully to determine potential benefits of the application of cool roof technology. For the examined case-study, it was found that a roof reflecticity of 0.6-0.7 is the 2 optimum value to achieve energy savings in a cooled office, improve summer internal thermal conditions in a non-cooled office (albeit with some heating energy penalty). It indicates that it is a suitable strategy for refurbishment of existing offices to improve energy efficiency or internal environmental conditions in the summer and should be considered in the design of new offices together with other passive energy efficient strategies.

Assessment of the Impact of Cool Roofs in Temperate Climates through a Comparative Experimental Campaign in Outdoor Test Cells

“…The majority of those researches calculates the increased heating demand through energy and microclimatic simulations [21,28,31,33,36,38]; some authors, such as Kolokotroni et al [33], Stavrakakis et al [38] and Pisello et al [37] have performed on-site monitoring of some variables mainly to validate the building simulation model. Finally, some researches analyse the energy performances of cool roofs though a comparative assessment with respect to green roofs [28], or balance cooling savings with the increasing of the heating consumptions in winter [28,35].…”

“…Many other authors have instead analysed and demonstrated the effectiveness and the benefits of adopting cool roofs to reduce the energy consumptions of buildings for summer cooling in different climates [9,20,23,[26][27][28][29][30][31][32], considering buildings with different end uses [29,31,33,34].…”

“…Most of the researches have demonstrated that cool roofs involve an annual energy penalty due to their performance in winter [35], considering both analysis of the whole urbanized areas through global climate models [21,31], and specific climatic contexts [28,33,[36][37][38]. In contrast, some other authors have demonstrated that winter heating penalties for cool roofs are negligible in cold climates-even without the effect of snow- [21,39], depending on: the ratio of cloudy to sunny days (which increases during winter), less total solar radiation available on the roofs, the effect of snow.…”

Skylight Application for Natural Lighting in a Building; the University of Tehran Branch of Melli Bank Case Study

“…This increased the reflection effect more than 88%. To take advantage of the positive effect of cooled roof technology and decrease the cooling energy consumption in summer [15,16], the coating was used to cover the entire roof. Since in OFFICE projects for these types of buildings in all climates the potential decrease in lighting energy use is limited [12], the angle of the reflectors was modified to produce the best possible effect.…”

Reducing cooling demands in a hot dry climate: A simulation study for non-insulated passive cool roof thermal performance in residential buildings

“…The reflectivity varies from about 0.1 for a very black colours to 0.8 for a very white colour. Generally, the rejection of solar gain is the main goal of passive cooling strategies [38] especially in hot climates. Direct sunlight onto a roof not only affects the indoor thermal comfort; thermal radiation coming from the roof materials affects the micro climate around a building [5].…”