Optimized Green Walls

Abstract

The construction, operation and maintenance of buildings consume more than 40% of primary energy in most countries. Out of this 40%, a large portion is related to the operational phase involving heat losses through a buildings envelope. To reduce this loss, several materials have been developed to reduce the thermal transmittance through a façade, even though they carry a heavy environmental burden. Furthermore, the unregulated and rapid expansion of urban environments led to a considerable amount of problems. Among them, the urban heat island effect demands special attention as it has been responsible for an increase of the energy consumption to higher mortality rates. In part, the use of materials with high thermal admittance is responsible for these effects. Vertical green systems have shown to be a potential solution to improve, among others, the thermal demands of buildings and to mitigate the urban heat island effect. However, due to the uncertainty associated with their design process and operation, the implementation of vegetation as a construction material in an urban setting is often overlooked. Therefore, an in-depth study of their performance under different configurations and climate conditions is needed.

The optimization study was based on the parametrization of a green façade and a living wall system which aimed to identify their response under variable initial conditions. A analysis of the essential parameters in the vegetation model was performed. Consequently, the leaf area index showed the highest effect, followed by the substrate thickness, leaf angle distribution, leaf surface albedo and finally by the moisture content of the substrate layer. Furthermore, a state-of-the-art computational work flow was developed through the integration of ENVI_met, Rhino/Grasshopper and modeFRONTIER, in combination with Python 3 scripting, to evaluate the performance of vertical greenery systems. The evaluation focused on the heat transmission through the façade of a single building, in comparison to a reference model. The work flow allowed the study of the impact of each parameter in the behavior of the system and led to the development of several design guidelines. The optimized result was tested in an urban setting to evaluate its potential as a mitigation strategy for the urban heat island effect.

The largest reduction in thermal transmission took place in equatorial, fully humid climate due to the low vapor pressure deficit; while the lowest in a temperate climate during winter conditions, suggesting the lower efficiency of the systems under cold weather. Furthermore, living wall systems have a significantly higher performance in comparison to green façades. On the other hand, the latent heat release associated with the evapotranspiration process has a strong correlation with the leaf area index, given the simultaneous action of the aerodynamic and bulk surface resistance. The optimized configuration of the vegetation was then derived based partly on this correlation. Moreover, the leaf angle distribution displayed a high correlation with the solar zenith angle and the leaf surface albedo with the intensity of the solar radiation and the ambient temperature. Additionally, among the substrate properties, the substrate thickness indicated a large potential in reducing heat transmission.

The effects of vertical green systems in an urban setting suggested an improvement of the environmental conditions. While the leaf area index is directly related to the decrease of wind speed and evaporative cooling, the leaf surface albedo influenced the amount of reflected shortwave radiation in a façade. Furthermore, the highest cooling potential was observed in desert climates as a result of the high vapor pressure deficit with temperature drops of up to 0.25 C.

The outcome of this research indicates that an optimized vertical greenery system is a suitable replacement for artificial insulating materials as a passive alternative to reduce energy demands in buildings. Indicating a decrease in heat transmission from 8% to 50% of the original heat flux. Furthermore, the findings of the urban study suggests that a decrease in the ambient temperature and an overall reduction of the negative impacts related to the urban heat island effect is possible.