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Microclimate Simulations experiments in Prague, Czechia

Project objectives and characteristics

The objective of this study is to test the different features of the software Solene-Microclimat in a district of Prague, Czech Republic, where ECOTEN is located. Diverses variants are studied:

  • A variation in surface’s albedo (V1 and V2)
  • A variation in the quantity of vegetation in the central block studied (V3 and V4)
  • A test of the thermal model of a building with or without the rows of trees (V5 and V6) 

The functionalities tested in this case study are:

  • The thermo-radiative model coupled with a fluid flow model
  • The assessment of trees and vegetated soil 
  • The energy consumption computation 
  • The comfort index computation

The chosen scene is a housing block surrounded by eight other blocks. It is located in the Vinohrady district, close to Prague city centre. To the north there is a park and the rest of the stage is surrounded by the same type of buildings that make up a dense urban environment. The buildings are of average height, between 20 and 30 meters, consisting of 5 to 6 storeys. The walls are pastel coloured and the roofs are sloping. The floors are covered with bitumen, or white and black paving stones. Some of the streets are lined with trees. 

Methodology and Assumptions 

1. Modeling the variation in albedo values in a given area: V1 and V2 

To simulate the impact of different albedo on the microclimate, we need to identify each surface composing the ground or the buildings beforehand as well as the albedo level for each of these surfaces. Then, the software will compute.

For the first two variants, the albedo of facades, roofs and sidewalks are modified. In Prague most of the facades are coloured, often light coloured, but sometimes they are darker or have not been resurfaced for some time and are dark.  

Facades from 0,3 to 0,7 albedo index (V1, V2).
2. Modeling the variation in vegetation density in a given area: V3 and V4 

For V3 and V4, the albedo values are the same as V1 and V2 respectively. In addition, we simulate along the streets bordering the central block of the rows of trees. They are located 5 meters above the ground, the volume representing their foliage is 10 m high and 6 m wide.

The characteristic of the model to simulate tree volumes is the Leaf Area Density (LAD). It represents the total area of leaves per unit volume of canopy and is expressed in m2/m3. Leaf area density varies according to the species considered, the architecture and the season. 

Facades from 0,3 to 0,7 albedo index (V1, V2).
Vertical profile of average leaf density for deciduous trees (A) and conifers (B)
3. Thermal study of a building: V5 and V6 

The purpose of these two variants V5 and V6 is different from the first four ones, which are designed to test the building thermal module of Solene-Microclimat.

We modeled the different floors as well as the glazing surface and its types. Then, using these assumptions, we solved the equations of moisture and energy transport. The convection coefficients are computed using air velocities, and we run a thermo-radiative assessment to obtain the surface temperatures.

V5 represents the thermal assessment with the assumption of V1 (light coloured surface and no vegetation).

V6 reprensents the thermal assessment with the assumption of V3 (light coloured surface and rows of vegetation planted).

Simplified model with floors and glass surfaces represented for the building thermal assessment.

There are 3 options for setting assumptions on the temperature control for the Building Thermal Tool. Either the temperature is allowed to change freely, or it cools down above a set value, or it cools down all the time. We selected the second option, with a setpoint temperature of 26 °C.

Results and findings 

1. Outdoor comfort V1 – V4 

The aim of studying the first 4 variants was to simulate the urban microclimate in the scene in order to assess the outdoor comfort. In order to analyse the results, specific points had to be selected for study. We take as a referal the Point East (Est) and the Point South (Sud) and track their air and surface temperatures.

 Evolution of surface temperature (left) and air temperature (right) for the East point.
 Evolution of surface temperature (left) and air temperature (right) for the South point.

We gathered the average and max temperatures for both points. The first comparison (V2 – V1) is for a variation of albedo in the scene without vegetation, the second (V4 – V3) compares a variation in albedo in the scene with rows of trees and the third (V2 – V4) compares temperatures between scenes with and without trees and with the same albedo.

Firstly, we notice that the difference in air temperature is greater for the third comparison than for the other two (up to 8 times), this is particulary true for the western point than at the southern point. The maximum value of 7.25 °C for the West street corresponds to the temperature difference of a dark-coloured street with a trees VS treeless configuration.

This is a very important finding that underlines the gain in comfort provided by the shade of rows of trees. 

We then notice the smaller differences between the two streets for the cases of albedo variation. This can be explained by the fact that the heat comes from a rise in temperature that is much more evenly distributed between all surfaces, not only those sheltered or not from the sun, by means of inter-reflections and GLO radiation.

2. Thermal assessment of the Building V5 – V6 

We note the rapid increase in temperature of the top floor, due to the composition of the roof with very little insulation (20 cm of brick) and the albedo of 0.3 (tiles). The temperatures of the other floors increase according to their proximity to the top floor and rather slowly. There are three reasons for this: the heaviness of the configured interior walls (floors and interior walls made of 20 cm of brick), the characteristics of the windows (excellent insulation and low transmittance) and the low ventilation rate.

Evolution of the air temperature per floor, for V 5.

When comparing the 2 variants, it is logical to note greater differences for the first two levels, as they are located under the row of trees bordering the street front of the building. The top of the third storey is at the same level as the top surface of the trees, and part of the outer surface of the storey is shaded. On the other hand, there is very little difference between the two variants on the highest floor. Trees do not shade roofs so they have no influence on this configuration.

Internal temperature difference per level between variants V5 (without trees) and V6 (with trees).
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