Thermal Effects of Vertical Greening in Summer: An Investigation on Evapotranspiration and Shading of Façade Greening in Vienna

Abstract

Global urbanization is advancing, and with it, the densification of cities. Due to increased sealing of open spaces and the re-densification of existing urban settings, green spaces in the city are becoming scarcer. At the same time, greening within the urban fabric is known for its positive effects on the environment and decisively counteracts the urban heat effect. This study deals with the benefits of green façades for the environment as a cooling measure. Two façade greening systems, one trough and one cassette system, consisting of curtain wall elements with a basic metal structure, installed at a south-facing outdoor wall of a school building in Vienna, Austria, were taken under metrological examination. In order to evaluate the cooling effect caused by evapotranspiration, the amount of water evaporated was calculated using the difference of inflow and outflow. Furthermore, the surface temperatures of the greened and non-greened walls were measured to display the influence of the interaction of shading and evapotranspiration on the surrounding microclimate. The investigated vertical greening system with an area of 58 m² has an average evaporation capacity of 101.38 L per day in the summer. The maximum surface temperature difference was measured to be 11.6 °C.

1. Introduction

Due to the assumption that two thirds of the world population will live in urban areas by 2050 [1] the issue of urban greening is becoming increasingly important. The climate of cities is known to be very different compared to that of surrounding rural areas and a clear temperature difference is observable. This phenomenon is called the “Urban Heat Island” (UHI) effect. One of the main reasons for its occurrence is the sealing of permeable surfaces—as it happens, for example, when constructing buildings and infrastructure on previous open, green spaces. Furthermore, vertical building surfaces cause a reflection of the sun rays and thus heat up the environment. Vertical greening is a strategy to counteract this effect [2,3,4].

One of the measures in the “Urban Heat Island Strategy Plan” by the Vienna Environmental Protection Department is the protection and expansion of green and open spaces, as they provide a cooling effect through shading and evapotranspiration [4]. However, according to the report “Global warming of 1.5 °C” by the Intergovernmental Panel on Climate Change (global material resource) [5], a general increase in surface temperature has been observed worldwide and not only in cities. This makes the issue of cooling in summer of rising importance. Façade greening can contribute to this as an effective cooling measure.

The microclimate of a city is influenced by various urban characteristics, such as building density, the nature of its open spaces, the orientation of buildings in relation to solar radiation and wind movements, as well as the surface materials and colors of roofs, façades, courtyards and streets. The synergistic effects of planning decisions that have an impact on the microclimate can lead to a significant improvement in the residents’ sense of comfort [6].

Green façades have a cooling effect on the building and the surrounding climate [7,8]. Furthermore, the cities are becoming increasingly dense, which makes the space-saving characteristic of vertical greening an optimal solution. Façade greening represents an interface between buildings and urban planning. In addition to the previous, conventional tasks of buildings, a greened building takes over one more with a façade greening.

Façade greening also has numerous other advantages, including air quality improvement, sound reduction through adsorption, supply of oxygen and of course the aesthetic aspect [9,10].

There are various categories of façade greening. Among the ground-based are the climbing plants, which are divided into self-climbers and scaffold climbers. They take root in the soil in front of the building. The self-climbers, such as ivy and wild vine can hold themselves to the wall by means of roots and adhesive discs. Scaffold climbers need climbing aids such as ropes or trellises.

Facade-bound greeneries are structures that are attached directly to the building. These curtain wall elements are either plant troughs that are attached to rails or vertically suspended elements from which the plants grow out horizontally [11].

The microclimate in a street canyon is strongly influenced by the surrounding buildings and their structure, color, construction, insulation and surface temperatures [3].

The building envelope is heated up by sun rays. However, a green surface intercepts the rays and thus prevents their reflection, which leads to the reduction in temperature in the immediate surroundings. In consequence, the surface temperatures of a building are significantly reduced by vertical greening, which is also proven by a number of studies [7,12].

In a study examining the exterior surface temperature of a south façade with a vertical greening façade, it is shown that the mean reduction is 1.5 °C and ~9 °C maximum [13].

Another study, by means of in situ measurements, showed that a surface temperature reduction by vertical greening systems is even possible to the extent of 34 °C [14].

The extent to which surface temperatures of façades with greening can be decreased compared to a non-greened wall depends very much on the solar radiation and the orientation of the façade [8].

As part of a study, a simulation program, “Energy Plus” was used to show how this shading effect can significantly reduce the energy consumption of a building. Two cities with warm climates, Hong Kong and Wuhan, China were examined. For a well-insulated residential building with summer nighttime ventilation through windows, the annual reduction in cooling measures was found to be of up to 3% of the energy consumption [7].

Green façades have an impact not only on their surroundings, but also on the interior of a building. A variant study was carried out to determine the effects of different methods on hydrothermal indoor climate during a heatwave. Measurements were taken from different buildings, which served as a basis for the simulation. Exterior shading, airing during the night, change of the color of the façade greening and loggia greening were investigated. The results showed that the most effective option is exterior shading [2].

Thermal comfort perception for pedestrians can be influenced by greened façades in their vicinity, depending on their orientation [8].

A study in Madrid, Spain showed the reductions in the air temperature due to façade greening. The highest air temperature reduction as measured in situ ranges between 2.5 °C and 2.9 °C [15].

There are many studies about façade greening and its cooling effects, but most of them deal with the cooling effect due to shading or its combination with transpiration. There are only a few that study evapotranspiration in particular.

There are various ways to determine transpiration rates. Hoelscher et al. [12] used the sap flow measurements, which are based on the heat balance method. For each greening several main stems were equipped with a sensor system, wrapped with insulating foam and protected against solar radiation and precipitation. The investigation on a west-facing wall on a clear summer day showed that the all-day cooling is 39.9% dependent on transpiration and 60.1% on shading. This agrees with the ratios of the results of another study by Convertino et al. [16]. They further found that the contribution of transpiration is higher on cloudy days, increasing up to 73.0%.

Convertino et al. [16] measured evapotranspiration with a load cell and also evaluated it via the energy balance of the green layer. The contribution of the latent heat exchange to the cooling effect by the greenery was evaluated.

A study in Australia investigating cooling by green façades to quantify the relative effects of evapotranspiration and shading considers sunny and dry weather conditions in 2021. The research showed that the temperatures behind the green façade are up to 7 °C cooler than those behind the shaded façade. To distinguish the evapotranspiration effect from the shading effect, the greened façade was compared with the shaded façade and set in relation to the ambient temperature. This resulted in an average contribution of 25–30% to the total cooling effect for both orientations [17].

Within the context of a research project in Germany, the evapotranspiration performance of two green roofs and a green façade, with plants in troughs, is being investigated with long-term measurements and accompanying simulations. Two methods are used. The actual evapotranspiration (ETP) can be measured with lysimeters. Several units were integrated into two green roofs to measure the water content of the substrate. A second, more simple method to determine the actual ETP is to measure the difference between precipitation and runoff, which was also applied in our research and is described in this paper. The measured data will be used to create long-term simulations and forecasts, but the project is still in development. For the green façade system, irrigation was monitored continuously to infer evapotranspiration. The ETP rate of the south-facing green façade in July and August was equivalent to an average cooling value of 157 kWh per day [18].

A study in Italy [19] proposed a simplified relationship to quantify latent heat due to evapotranspiration. They determined that net radiation is the most important influencing parameter for latent heat transfer. The formula based on net radiation as an input parameter proved to be the most suitable. Such correlations are especially important for the development of Simulation programs.

The microclimate in a city has countless influencing factors as stated above, which is why it is essential to use various methods to determine the contribution of a façade greening.

In this article, shading and evapotranspiration were examined in more detail since these two effects make a major contribution to the cooling of the microclimate by greening, especially in urban areas. The goal of this study is to look at the evapotranspiration of two different façade greening systems separately and evaluate them and their relevance of outdoor thermal comfort. Furthermore, the surface temperatures underlying different greening systems were considered in comparison to a non-greened wall.

2. Materials and Methods

Within the framework of a research project different façade greening systems and interior greening were installed on a school building in Vienna, Austria. The aim was, among others, to investigate their building physics’ behavior and influence of the systems on the microclimate [20]. The school building was built during the Wilhelminian period. The exterior walls are made of solid brick with interior and exterior plaster and have a thickness of 60 cm.

2.1. Studied Greening Systems

In this present study, two vertical greening systems, one based on aluminum troughs and one cassette system, were investigated. These two systems will be presented more in detail in the following subsection.

The south-facing façade is located in the schoolyard of the respective school building in the city center of Vienna, Austria and thus situated in a dense urban area.

(a)

System A: Façade greening based on aluminum troughs:

Figure 1 shows on the left side the linear trough system consisting of aluminum troughs. The troughs are arranged horizontally and at a distance of 25 cm above each other and function as a cascade system. They are filled with a substrate (plant granules) and are provided with different plant species, such as herbs and perennials. The troughs are lined with fleece, which acts as a reservoir and filter. The irrigation system is connected directly to the water pipe and is distributed to the individual rows by using soaker hoses. At the very bottom, there is a collection trough for the excess water, which is directed to the sewer system. The system is installed around three windows and has a total area of 58 m².

In the aluminum trough system, the water runs from the top to the bottom through the overflow openings. The system has therefore higher humidity at the bottom than at the top, which was compensated for by choosing plant species that have different requirements (Sedum in the upper rows, Bergenia in the lower ones). The exact plant species are listed in the Appendix A.

(b)

System B: Façade greening system based on cassette system:

The greening on the right side in Figure 1 shows the cassette system, consisting of individual modules. These metal cassettes have a grid structure at the front with openings for the plants. A continuous fleece is integrated in the façade basket and distributes the water supplied by soaker hoses evenly. The system consists of two parts that together have an area of 14 m².

Due to frequent irrigation failures, the plant spectrum changed considerably during the project period. Thus, the most suitable plants for this system were those with high stress tolerance, which are listed in Appendix A.

The view in Figure 2 shows positions of the greening systems on the south façade. Both systems have a distance of 6 cm from the wall. System A is blinded on the top and bottom sides and therefore has only a small back ventilation while System B is not blinded. Figure 3 shows the structure of both systems.

2.2. Measurement Methodology

Two measurement methods were selected to illustrate the cooling effect of green façades. In order to draw conclusions about the evapotranspiration performance of a vertical greening system, the first method includes the measure of the inlet and outlet and the subsequent calculation of the evaporation energy.

As a second method, the surface temperatures of the exterior walls were measured. This method describes the cooling by shading and includes the comparison of the greened and non-greened façade.

2.2.1. Evaporation Balance

Energy is required for water to evaporate, as well as for any change in state of aggregation. This energy is extracted from the environment in form of heat, thus creating the cooling effect. In greening systems this happens via surface of the plants and the substrate resulting in evapotranspiration cooling the environment.

In order to investigate the cooling effect of building greenery by evapotranspiration, the aluminum trough system showed in Figure 1 on the left side was studied.

The enthalpy of vaporization is the amount of energy required to vaporize a liquid at a given temperature [14]. To infer the energy of evaporation, the difference between inlet and outlet was calculated. The total inflow is composed of the irrigation and the rainfall.

As previously described, the system is irrigated by means of soaker drip hoses as shown in Figure 4. Using water meters, the water supply of the entire system was measured over the period studied.

In order to measure the discharge, a measuring system (shown in Figure 5) with a balance beam was installed, which measured the weight of the collection tray (bottle). When it is full of water, the magnet valve opens and closes again at empty weight.

A data logger was used to record the weight of the collection bottle every 30 s.

The considered time period was from the beginning of August to the end of September 2017, since the cooling effect is particularly desirable in the summer months. Equation (1) shows how the evaporation energy is calculated. The amount of water that evaporated was calculated with the difference of inflow and outflow. To obtain the evaporation energy, the value was divided by the mass of one mole of water (18.02 g/mol) and multiplied with the standard evaporation enthalpy (43.99 kJ/mol), which is given at 25 °C.

$$\begin{gathered} \frac{\begin{array}{c}\left(Irrigation+Rainfall-Outflow\left[\mathbf{g}\right]\right)\end{array}}{ 1 mol water mass \left[\frac{\mathbf{g}}{\mathbf{m}\mathbf{o}\mathbf{l}}\right]}\times Evaporation enthapie\left[\frac{kJ}{mol}\right]\times \mathbf{0.000278} \\ =enthalpy of vaporization \left[\mathbf{k}\mathbf{W}\mathbf{h}\right] \end{gathered}$$

(1)

2.2.2. Comparison of Surface Temperatures of Greened and Non-Greened Wall

In order to evaluate the effect of the façade greening systems on the surface temperatures, sensors were installed at different positions. For this purpose, façade greening systems based on both the aluminum troughs and cassettes were considered, as shown in Figure 1.

On the façade, the surface temperatures were measured using PT 1000 sensors behind the greening systems and as a reference, near the non-greened façade. The sensors have a measuring accuracy of ± 0.35 °C. The measuring points are shown schematically in Figure 6.

3. Results

3.1. Evaporation Balance

The daily mean value of the evaporation energy is 64.70 kWh. It is strongly dependent on irrigation and precipitation, respectively. The maximum daily value is 121.80 kWh and has been reached on 11 August.

The diagrams in Figure 7 and Figure 8 show the daily mean values of the evaporation energy for the months of August and September, respectively. The diagram shows the amount of evaporative energy released by evapotranspiration on a daily basis. In order to adjust the water demand of the plants, irrigation was no longer applied daily starting from mid-August, and even more reduced in September. Both can be seen in the evaporative energy balance, as the values are very close to zero. It can be assumed that with higher evapotranspiration and thus higher evaporative energy, the building and the environment are cooled even more.

Table 1. MIN, MAX and MEAN values of evaporation amount and evaporation energy in September and August 2017.

	MIN	MAX	MEAN	TOTAL
September 2017
Evaporation quantity (L)	−5.28	170.26	80.35	2410
Evaporation energy (kWh)	−3.31	106.83	76.39
August 2017
Evaporation quantity (L)	−0.49	194.12	121.4	3774
Evaporation energy (kWh)	−0.30	121.80	51.85
September and August 2017
Evaporation quantity (L)	−5.28	194.12	101.38	6184
Evaporation energy (kWh)	−3.31	121.80	64.70

below summaries the results and shows the minimum, maximum and average values for the two months according to the diagrams in Figure 7 and Figure 8. Through

This evaporation process extracts heat energy from the environment, so higher values are better for the cooling effect of the environment.

3.2. Comparison of Surface Temperatures of Greened and Non-Greened Walls

Surface temperatures were measured at different measurement positions on two heat days and compared with each other, as described in Section 2.2.2 and shown schematically in Figure 6.

The following diagrams, Figure 9 and Figure 10, show the surface temperatures outside of a greened and a non-greened wall, with respect to the façade greenings based on aluminum troughs and cassettes. In each case, two heat days from July 2017 are shown.

The surface temperature of the non-greened wall is significantly higher than the wall shaded with the greening system. The value of the maximum differences of both surface temperatures at the same time is 11.6 °C for the cassette system and 9.3 °C for the trough system. The comparison of the two diagrams makes it obvious that the system based on the cassettes shows significantly higher values, which can be explained on the one hand by the different position on the façade and on the other hand by the significantly smaller area of the cassette system. The larger system is 58 m² and the smaller is 14.2 m². The global radiation has a direct, time-delayed influence on the surface temperature. This influence is higher for the cassette system which can be seen in Figure 10. Due to the lack of view of the night sky, the greened façade cools down less than the non-greened façade. This factor must be taken into account when planning a green façade, as the night cooling of the façade is important to counteract the summer overheating in the interior. This depends on several factors, such as the air circulation behind the greening system, but also the orientation and nature of the wall.

The lower fluctuations of the external surface temperatures and the UV protection increase the life expectancy of the façade.

There is a clear reduction of the surface temperature by the greening systems.

As already pointed out in the introduction, a reduced surface temperature of the outer façade can have a positive effect on both the pedestrians and the people inside the building’s sense of comfort.

4. Discussion

It is difficult to represent and value the total cooling effect of a green area caused by evapotranspiration. Therefore, in the following a comparison of the cooling capacity with a beech tree is made and a method for the monetary evaluation of the cooling effect is presented.

A beech tree has an approximate average transpiration capacity of 30 L per day [21].

The studied vertical greening system with an area of 58 m² has an average evaporation capacity of 101.38 L per day in summer.

Thus, it can be said that the façade greening is equivalent to about 3.4 beeches in terms of evaporative capacity. Nevertheless, it should be noted that the façade greening must be artificially irrigated. The water consumption of this trough system without tank in the outdoor area is 0.35 m³/m²/year. [12] For the entire system, this would be 18,550 L per year. However, at places with insufficient space for trees, especially in the context of dense urban areas, vertical greening is a good alternative. A façade greening is an elaborate concept compared to a self-growing tree, but it has a time advantage if it has to be done quickly, as a tree needs decades to become fully grown.

Tudiwer et al. [22] determined the cooling costs for the economic evaluation of a façade greening. The presented method was used to qualitatively assess the economic effects of greening buildings. In particular, it deals with the evaluation of the evaporation energy in urban areas. The production costs are influenced by evaporation capacity, the total costs for the greening system, and the number of summer days. These cost models are usually used in the energy industry to compare different electricity prices. It is based on the ratio of the total cost of electricity generation to the electricity generated. The costs are incurred at different times and using the net present value method, they are to be discounted and calculated at time t = 0 to have a direct comparison.

In [22], the formula for the electricity production costs is adapted and the cooling production costs of the façade greening, located in Vienna, are calculated.

Considering several influencing factors, such as the number of summer days (desired cooling effect), the amount of heat extracted and the total costs, the cooling production costs were calculated to be 0.80 €/kWh. This indicates that with an investment of 0.80 € in this façade greening 1 kWh of thermal energy can be extracted from the environment. It should be noted that the calculations were made in 2019 and the current costs may differ.

The reduction in the surface temperatures of the exterior façade due to greening is clear. The south façade of a 5-storey administrative building in a university campus in Shanghai showed a maximum temperature difference of ~9 °C between the non-greened and greened façade, and up to 4.2 °C for the north façade.

A study conducted in the vicinity of the city center of Ljubljana, Slovenia showed a temperature reduction in a south façade of up to 34 °C.

In our study, the maximum value was 11.6 °C.

It can be concluded that this reduction in surface temperature of green façades depends on various factors, such as its location, orientation and the type of greening system.

5. Conclusions

The research presented in this article shows that façade greening is suitable as an effective measure for cooling dense inner cities in particular. It is shown that a façade greening with an area of 53 m² can reach the transpiration capacity of 3.4 beeches and accordingly cool the environment. Furthermore, the surface temperatures of the façade were also lower by more than 11 °C, due to the impact of the greening system. This can have a positive influence on the energetic behavior of the building, depending on its construction. This parameter also has an influence on the UHI effect, as less heated surfaces also emit less heat to their surroundings.

The measurements and calculations carried out within the framework of this research are based on the actual irrigation performance and the evapotranspiration achieved accordingly. A possible optimization could be carried out with regard to a maximum cooling capacity and a corresponding adjustment of the irrigation quantity, as well as the irrigation system, e.g., in the form of sprinklers, of course considering the needs of the plants.

Data evaluations of this study support the findings of other studies and prove the numerous advantages of green façade systems. The added value of green façades for the urban climate should be paid more attention to.

It is clear that more greening in the city promotes well-being and health and is be-coming a necessity at an increased rate. However, it is still not possible to assign a direct value to all the benefits, because health and well-being are difficult to value and do not always meet with the necessary appreciation in our predominantly business-oriented society.

The significant impact of greening on summer overheating in cities should definitely be given more attention. It is obvious that this topic is of great and ever-increasing significance considering the steady growth and densification of urban areas.