Green walls and building energy consumption

  • J.A. Oosterlee

Student thesis: Master

Abstract

A green wall, where foliage covers the external wall of a building, is considered to be a sustainable solution as it has many benefits for the direct environment. However, the advantages for the consumer (owner or building occupant) remain limited. One of these advantages is the potential energy savings that come from a reduction in the building's cooling load. The covering foliage has three properties that will negatively influence the amount of heat transfer taking place between the indoor and the outdoor climate. The first property of the foliage cover is the reduction of wind speed, or convective heat transfer, along the wall. This influence mainly causes the heat resistance of the external air film to rise from 0.04 to a maximum of 0.17 m2 K W-1, dependent on the density of the foliage. The second property of the foliage is the reduction of solar radiation dissipating in the external wall. The leaf area index determines the size of this influence. The reduced temperature of the external wall causes less heat transfer to the indoor climate. Finally, the third property of the foliage is the evaporation of water inside the leafs. This process converts sensible heat into latent heat, which is beneficial for the Urban Heat Island effect. The energy needed for this process is mainly taken from incoming solar radiation, but on rare occasions with strong wind and hot and dry air, the sensible heat can be taken from the air, causing a small reduction in air temperature. This property is however not quantified here. Living wall systems could also contribute to energy savings through their insulating capacity. The Rc value of three systems has been computed using THERM 6.3. The Rc value of the Greenwave system based on soil varies between the 0.1 and 0.2 m2 K W-1 dependent on the wetness of the growing medium. For the Wonderwall system based on fabric this ranges between the 0.2 and 0.3 m2 K W-1. The insulating capacity of the LivePanel system (rock wool) was found to be more significant: 0.9 - 1.5 m2 K W-1. The results of the quantification of the first two foliage properties and the Rc value of the LivePanel are subjected in a HAMBASE energy consumption simulation. In this study a small Dutch office building is subjected to quantify the potential energy savings aspect of a foliage covering the building's external walls (40% glass). Other investigated variables are the thermal mass of the building, the amount of thermal insulation and the application of a green roof. The results show that applying a foliage cover led to a decrease in annual cooling load, but this was balanced by an increase in annual heating load. In every examined situation, the annual energy consumption decreased by a mere 1%. Changing the model's structure from heavyweight to lightweight, while maintaining the Rl at 3.5 m2 K W-1, induced to a 41% increase in energy consumption. The increase was lower (24%) when the Rl of the heavyweight model was changed to 2.5 m2 K W-1. Applying a green roof on the heavyweight model led to a 2% decrease. Attaching the LivePanel system to the poorly insulated heavyweight model (Rl ? 2.5 m2 K W-1) caused the annual energy consumption to drop from 18,350 kWh to 14,650 kWh. Despite the annual savings of about € 815.-, the application's annual maintenance costs (estimated between € 2,520.- and € 8,400.-) are not relieved.
Date of Award28 Feb 2013
Original languageEnglish
SupervisorC.P.W. Geurts (Supervisor 1), H.J.H. (Jos) Brouwers (Supervisor 2), Jos J.N. Lichtenberg (Supervisor 2) & Henk L. Schellen (Supervisor 2)

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