• Energy Research

Abstract Green roofs are complex systems governed by intricate transport phenomena, which are frequently solved using simplified and empirical models. This paper describes a numerical model capable of solving the conservation equations that govern the unsteady nonlinear coupled moisture and heat energy transport through a multi-layer green roof composed of a structural support, a water storage layer, growing medium and canopy. To get an accurate insight into the role of different variables that affect the hygrothermal behaviour of green roofs, the temperature on the outer surface, as well as the outflow and inflow heat fluxes, were computed for different roof models and environmental conditions. A sensitivity analysis was performed to understand the role of the different layers and the canopy’s geometrical composition (e.g. vegetation coverage, plant height and leaf area index) in the energy balance of the building’s roof. Finally, to foresee the behaviour of the full canopy system under real climate conditions, weather data with distinct climatic characteristics from Braganca (Portugal) and Seville (Spain) were used. The simulated green roofs use insulation cork boards (ICBs) to replace both the water storage and insulation layers. Due to the intrinsic thermal characteristics of ICB (an ICB layer of 0.2 m allowed us to reduce the heat flux by about 58% compared with an ICB layer of 0.05 m), these roofs are expected to improve interior comfort and save energy. Although the ICB and soil layers made the greatest contribution to the thermal insulation, the characteristics of the vegetation were found to be of substantial importance to the overall performance of the green roof. The leaf area index (LAI) was the most relevant vegetation variable (a change from LAI = 2 to LAI = 5 decreased the inflow heat flux by about 27%), while difference in plant height did not lead to any significant change in inflow heat flux.

A mathematical model that governs unsteady coupled moisture and heat energy transport through an exterior wall covered with vegetation is described. The unknown temperature and moisture content of the plants and canopy air are represented by a system of nonlinear ordinary differential equations (ODEs). The transport of moisture and heat through the support structure, which includes insulation and soil layers, is defined in a series of nonlinear partial differential equations (PDEs). After setting out the model, this article presents and discusses a set of numerical applications. First, a simplified system consisting of a brick wall covered with climbing vegetation is used to study the role of individual variables (e.g., wind speed, minimum stomatal internal leaf resistance, leaf area index, and short-wave extinction coefficient) on the hygrothermal behaviour of the green wall. Thereafter, more complex green wall systems comprising a bare concrete wall, mortar, cork-based insulation (ICB), soil and vegetation are used to evaluate the influence of the thermal insulation and substrate layers on the heat flux distribution over time at the interior surface of the wall, and on the evolution of the relative humidity, water content, and temperature throughout the cross section of the green wall. The numerical experiments proved that vegetation can effectively reduce exterior facade surface temperatures, heat flux through the building envelope and daily temperature fluctuations.

Abstract Green roofs are increasingly being used to improve the energy balance and stormwater management of buildings. This work examines the thermal behaviour, water drainage and vegetation growth of two green roof systems: a conventional system containing extruded polystyrene and polyethylene, and an alternative cork-based system. In the new system, an eco-friendly expanded cork layer is used to provide thermal resistance and water storage capacity in a more sustainable way, as cork is a natural material. This material can also be used as an uncoated finishing layer for architectural purposes. The main goal of this work was to see if the new system is reliable in real environmental conditions and to compare its behaviour to that of the conventional system for a full year. This work presents the results of a fully functioning prototype built in Portugal with a technical substrate 10 cm thick and planted with a variety of vegetation. The reference and proposed systems were installed side by side and monitored under spring, summer, autumn and winter conditions. The thermal insulation and the water drainage and storage capacity provided by the cork-based green roof were found to be similar to those of the reference solution, with the expanded cork layer showing a more marked temperature delay and a more effective management of rainfall events under dry conditions. Additionally, it was found that the growth and health of plants in the cork-based green roof were comparable to those of the reference solution, indicating the good performance of the cork system.

Green roofs are made up of several components, including those belonging to the waterproofing and drainage layers, substrate, and vegetation. Of these, the substrate is undoubtedly one of the most important layers of a green roof, contributing not only to the healthy growth of vegetation but also to the water retention capacity and thermal behaviour of the whole solution. Although green roofs are widely recognized as sustainable solutions, it is possible to further improve their environmental performance by developing more ecological substrates that contain industrial by-products. Bearing this objective in mind, sixteen newly developed substrates were characterized in terms of thermal conductivity, specific heat, emissivity, water vapour transmission, hygroscopic sorption, and water retention/drainage capacity. These properties are extremely relevant when solving heat and mass transfer problems as well as for water management prediction. Two reference substrates were also studied for comparison purposes. The results showed that the new ecological substrates have properties that make them comparable to conventional substrates already available on the market. Additionally, the results showed that temperature, moisture content, and density play an important role in the behaviour of substrates of this kind and have a significant influence on many of the studied properties.

Innovative toilets can save resources, but have higher embodied impacts associated with materials and electronic components. This article presents an environmental life-cycle assessment (LCA) of an innovative multifunctional toilet (WashOne) for two alternative configurations (with or without washlet), comparing its performance with those of conventional systems (toilet and bidet). Additionally, two scenario analyses were conducted: (i) user behavior (alternative washlet use patterns) and (ii) user location (Portugal, Germany, the Netherlands, Sweden and Saudi Arabia). The results show that the WashOne with washlet has a better global environmental performance than the conventional system, even for low use. It also reveals that the use phase has the highest contribution to impacts due to electricity consumption. User location analysis further shows that Sweden has the lowest environmental impact, while Germany and the Netherlands have the highest potential for impact reduction when changing from a conventional system to the WashOne. Based on the overall results, some recommendations are provided to enhance the environmental performance of innovative toilet systems, namely the optimization of the washlet use patterns. This article highlights the importance of performing a LCA at an early stage of the development of innovative toilets by identifying the critical issues and hotspots to improve their design and performance.