On the analysis of cool roofs for cooling system efficiency

Cool roof is a well-documented passive cooling strategy for buildings in several climate conditions. The mechanism consists of the reduction of the heat load entering the roof, which is characterized by high solar reflectance and high thermal emittance. The purpose of this paper is to study the coupled effect produced by such a technology. First, the passive cooling contribution is quantified, then, the “active” contribution is investigated. This latter effect consists of the cool roof capability to decrease the suction air temperature of heat pump external units, when these units are located over the same roof. This “cooling” benefit produce an extra-increase of the energy performance of the heat pump in cooling mode, given that it produces the decrease of the temperature lift between the source and the output. In order to study this twofold effect, an industrial building with an office area located in Rome, Italy, was continuously monitored in summer 2012. The thermal behavior of the roof, of the indoor environment, and the energy requirement for cooling were evaluated. The main results showed that the cool roof allows to decrease the roof overheating up to 20°C. The office indoor air temperature was lowered, even if the same set-point temperature was kept constant during the whole campaign. The energy requirement for cooling decreased by about 34% during the working time of the office. In order to investigate the “active” contribution, suction air temperature was monitored and a new simple analytical model is proposed in order to estimate the cool roof effect in reducing the air overheating over the roof and, therefore, the temperature lift to be smothered with the cooling system.