Cool and green roofs to reduce cooling energy demand in storage buildings: simulation study for distinct climates

Abstract

Goods storage buildings surround urban areas and they usually have large areas of roof. In many climates such buildings require protection from external conditions for the goods and materials and for relative comfort of workers. These buildings may provide an excellent opportunity to use cool and green roofing construction techniques because they cover a large area exposed to solar radiation compared to residential dwellings and other commercial buildings and they are usually single-storey with the roof encompassing the entire internal volume. It is for this reason that any modifications to the U-values or solar reflectance properties of the building envelope are very influential towards the energy demand from the HVAC system of the building. This paper presents a comprehensive computational analysis on the use of green and cool roofing techniques applied to a model that represents a typical steel goods storage building considering the local thermal building practice. The investigation is carried out using Energy Plus to compare the energy efficiency and related environmental impact in four distinct climates and six cities in Brazil with different climatic conditions; these are: • hot and dry: Abu Dhabi UAE; Petrolina, Brazil, Brasília, Brazil • hot and humid: Manaus, Brazil; Fortaleza, Brazil • cold winter – hot summer: Wuhan, China; Santa Maria, Brazil • mild winter and humid mild summer: London, UK; São Paulo, Brazil • cold winter and mild summer: Stockholm, Sweden For each location a parametric analysis is carried out through thermal simulations to calculate energy demand to provide heating and cooling to the structure with an analysis of the CO2 equivalent (CO2e) emissions due to these consumptions. The green roof simulation results are shown to have a consistently positive impact on energy efficiency. Throughout all the simulation climates reductions of varying magnitude of both heating and cooling energy demand are recognised, which results in subsequent reductions in energy operational costs and CO2e emissions. The cool roof simulations provide both positive and negative results for the parameters that are explored in this paper. Case-studies in hot and dry climates (Abu Dhabi, Petrolina and Brasília) as well as hot and humid (Manaus, Fortaleza) experience a rejection of solar heat gain leading to a reduced cooling load and the resultant benefits. Conversely, cool roofs are shown to have a detrimental to energy efficiency and emissions in climates that predominantly require heating energy and well insulated external envelope which is confirmed within the cases of London and Stockholm. Cool roofs are shown to provide significant reductions in combined heating and cooling energy demands in cold winter and hot summer climates (Wuhan, Santa Maria). Despite this effect, higher emission rates might occur in these locations due to the increased heating energy consumption exceeding the corresponding reduction in cooling benefits when the structure is well insulated. A positive effect is observed for less well insulated cases indicating that in this case a cool roof may be a good cost effective strategy to reduce energy demand.