Journal of Heat Island Institute International Vol. 12-2 (2017)

Academic Article

Effect of trees and greening of buildings on the indoor heating andcooling load – microscale numerical experiment
Robert von Tils** Institute of Meteorology and Climatology, Leibniz University Hannover
Corresponding author: Robert VON TILS, vontils@muk.uni-hannover.de
ABSTRACTA three-dimensional microscale model is used to study the effects of trees and greening of buildings on theindoor heating and cooling load during a winter and a summer scenario. Shading by evergreen trees increases theheating load but also reduces the cooling load. The green roof and walls decrease the surface temperature in thesummer scenario during daytime and increase it at nighttime. Overall is the cooling load decreased. An evergreen
facade also improves the building insulation and causes a lower heating load during winter.
Key Words : indoor climate, trees, green roof, green wall, heating load, microscale simulations

１. Introduction

condition is compared to measurements made by Eiband (13).

In developed countries, the average person spends most of their

２. Methods

time in an interior location. During summer people stay inGermany and north America 79 % to 90 % of their time indoors

The wind and temperature distribution is simulated with theand in winter even 88 % to 97 % (1)(2). Especially in private

micro-scale model ASMUS (Ausbreitungs- und Strömungs

households heating takes the largest amount of the total energy

Modell für Urbane Strukturen). It is based upon the Navier-Stokes

used there (3) and is most often provided by fossil fuels (4).

equations,

Whereas only 3 % of German households were using an air

thermodynamics and an equation for specific humidity. Buildings

conditioning in 2015 (5), this number is rising since 1990 (6).

and trees are represented by impermeable or semipermeable grid

Regarding this, generating of a comfortable indoor climate during

volumes. Their surface temperatures are calculated depending onsummer and winter without using more energy is very important

their specific orientation and physical properties by a surface

and may reduce carbon emissions.

energy budget (14). The model equations, boundary conditions forUrban greenery, such as green roofs, green facades and trees canground and building surfaces, including green roofs and trees arehave a positive effect on the local climate especially in summer

described by Gross (10).

by moderating the maximum temperatures (7)(8). The effects of

The calculation of the latent heat flux is modified after Bruse (15)those green elements on the outdoor climate can be studied using

to take the soil water content and solar radiation via stomata

a three- dimensional microscale meteorological models - forresistance into account (16).

thecontinuityequation,

thefirstlawof

example with ENVI-met (9) or ASMUS (10). Green walls and trees
have also an effect on the indoor climate and can reduce the2.1 Indoor Module

indoor temperature and cooling load during summer (11) (12).

The walls, floors and roof elements of a building are divided in to

The effects of those measures on the cooling and heating load

multiple layers of different depth, thermal conductivity andduring summer and winter conditions is investigated in thethermal diffusivity. Windows have multiple layers of equal widthpresent study. A numerical microscale model is used to simulateand physical properties that sum up to the U-value (overall heat

the outdoor and indoor climate of a single building and analyzetransfer coefficient) of real windows. A one-dimensional model isthe effect of greenery and other measures on the energy

used to calculate the temperature within all layers.

consumption for different orientated and situated rooms. The

Diffuse and direct shortwave radiation can partially transmit

simulation results for a building without green during a winterthrough windows in both directions. Longwave radiation is- 35 -absorbed by all surfaces. Each surface type can have a differentalbedo.

NThe temperature of the indoor surfaces is also calculated by anenergy budget, but the diffusion coefficient for heat K to calculate
the sensible heat flux by a flux-gradient relationship is empirically
fixed to 0.08 m²s-1 to match the heat transfer coefficient for indoor
surfaces in DIN EN 6946 (17)𝑄𝐻 = 𝑐𝑝 𝜌𝐾

S/RS

𝑇−𝑇𝑆𝑢𝑟𝑓(1)∆𝑧Figure 1: Floor plan of the simulated building. With window

cp is the specific heat, ρ the air density, Surf means Surface, T thepositions as small blue lines, heating grid points as red squares,

temperature at the next air grid point and ∆z is the distancecentered north room on 2nd floor N, centered south room on 2nd

between this grid point and the surface.

floor and reference to measurements S, centered south room on

A room is heated or cooled by a single grid box where the

3rd floor under the roof RS.

temperature is decreased or increased and the heating or coolingpower is calculated. An ideal air conditioning system is assumed

Table 1: Albedo and calculated heat transfer coefficients of walls,

for cooling that has an energy conversion efficiency of 1 like theceilings and floors according to material parameters (after (13)).

numerical and experimental heating system.

AlbedoU-value

Each room is ventilated through its outer walls to simulate typical

Facade0.3

0.44

leakage through window seams and doors and to take legal

Window-1.4

requirements for air exchange rates into account. The energy

Side wall

0.6

0.9(18)savingregulation

EnEV(Energieeinsparverordnung)Rear wall

0.60.65

requires a minimum exchange rate of 0.5 h-1 that is also used inCeiling

0.6

0.52

these simulations.

Roof

0.3

0.52

Floor

0.3

0.52

Green wall

0.3

0.43

Green roof

0.3

0.50

3 Setup of the numerical experiments
Eiband (13) has built a real size single 14 m² large and 2.5 m highroom that has one south orientated window with a size of 2 m²

3.1 Meteorological and HVAC conditions

and has a very effective insulation towards all “inner” walls, theA winter and a summer situation is simulated. The winter situation

floor and the roof. An artificial sun and a glass dome over the only

was created by Eiband (13) and represents a typical sunny winterexterior wall are used to simulate direct and diffuse shortwave

day in Munich, Germany at the 20th of February. The temperatureradiation through the window as well as different outdoor

outside varies between -5 °C and +5 °C with an averagetemperatures. The room is ventilated through the south facade

temperature of -0.53 °C and the sun is shining for 9 hours with awith an air exchange rate of 0.5 h-1 and is heated with a fan heater.

maximum shortwave irradiance of 480 W/m². The summer

For the simulations, this room is copied and mirrored to create asituation represents a clear hot summer day without clouds at thefull three-storey building with a flat roof and 6 rooms per floor.

20th of July. The outdoor temperature varies between 20°C in theHalf of them with a window to the south and the other half with aearly morning and 30°C in the early afternoon.

window to the north side (Fig. 1). The south orientated room in

A room is heated in winter when its average temperature is lower

the center of the 2nd floor corresponds with the real size room

than 20 °C. In summer a room is cooled when the roomfrom Eiband (13).

temperature is higher than 20 °C.

The rooms fulfill building insulation requirements in Germany
from 2004. The thermal properties of the different wall layers are

3.2 Initialisation and Spin-up

described in Table 1 (layer structure in detail in (13)).

It takes serveral days of simulation time for the temperatures

For these simulations, an equidistant Cartesian staggered grid

inside the walls to reach a steady state under periodic outdoor

with a mesh size of 0.5 m was used. Therefore, the windows areconditions and the three-dimensional simulation is compu-

12.5 % larger and have a size of 2.25 m².

tationally very intensive. Therefore, a one-dimensional model isused for each building wall, roof and floor element that is forcedon the outside with the diurnal cycle for the winter or summer
situation and on the inside with a constant room temperature.

- 36 -

3.3 Building scenarios

needed for heating and indoor temperature for unshaded south

For both meteorological scenarios four different building setups

orientated rooms caused by short-wave radiation from the sun

are simulated: the basic building (B), a building with three close

through the windows (Fig. 2).

large evergreen trees on the south side (14 m high, 10 m crown

Room S gains 5.7 kWh of energy per day by heating (Table 2) anddiameter) (T), a building with a green roof (0.2 m growing

about 2.0 kWh per day by incoming short-wave radiation throughmedium and 0.2 m thick vegetation on top) and green facades

the window. The same total is lost by heat transfer through the(during summer and winter) on all surfaces except for theouter façade (4.4 kWh per day) and due to the forced ventilation

windows (0.2 m thick vegetation) (G) and a building where all(3.3 kWh per day). In comparison to this room N gains almost nowindows are 66 % larger and have an area of 3.75 m² (W).

energy by short-wave radiation through the window but has aThe simulated green roof is covered with gras with an leaf area

about the same losses through the façade and ventilation andindex (LAI) of 6 and the green facades consist of ground-based

therefore has higher heating requirements. Room RS has alsoivy (LAI = 4). The water supply of the plants is optimal with aenergy losses through the roof and therefore also needs moresoil water content at field capacity. The evapotranspiration isenergy for heating than the rooms S and N.

mainly limited by the stomata resistance of the plants and its

For the basic scenario, the heating power in room S and the indoor

reaction to shortwave radiation.

temperature is similar to the measurements by Eiband. The
simulated reference room needs 8 % more energy than in the4 Results

laboratory experiment.

During the winter scenario, there is a strong change in power

Figure 2: Outdoor temperature at 2 m height and short-wave radiation during the winter scenario, indoor temperatures and power needed
for heating in room S for different building scenarios (Basic, big Windows, Trees, Green roof and facades)- 37 -

Table 2: Energy for heating /cooling in different scenarios and rooms (see Figure 1)
EnergyEibandBasic[kWh/d]SSNRSSNRSSNRSSNRSWinter

5.3

5.7

8.7

13.9

8.6

9.1

17.3

4.2

7.1

11.9

5.6

9.6

13.5

Summer-

4.4

3.2

5.6

2.9

3.1

5.4

4.0

2.7

5.5

5.0

3.5

6.1

Trees

GreenWindows

Figure 3: Outdoor temperature at 2 m height and short-wave radiation during the summer scenario, outside wall surface temperaturesand power needed for heating in room S for different building scenarios (Basic, big Windows, Trees, Green roof and facades)
Bigger windows reduce the time the heating is needed, increase

During the summer scenario in the unshaded south rooms S andthe indoor temperature while the sun is shining but need more

RS is most of the cooling power used while the sun is shining

power at night due to the higher heat transfer coefficient of thethrough the windows (Figure 3). The rooms gain less energy perwindow surfaces compared to the wall facade. Therefore room S

day by ventilation (1 kWh) due to the lower differences between

and RS need about the same energy for heating than in the basic

indoor and outdoor temperature and also through the facade.

szenario.

Incoming short-wave radiation is heating the southern rooms

Almost zero short-wave radiation enters the rooms S and RS inmuch more than in the winter szenario (3,3 kWh per day).

the scenario with trees and causes that more heating is needed

The trees reduce the cooling load very strong by blocking thethere. The results and causes are very similar to the northshort wave radiation from entering the south rooms. The surface

orientated room N in the basic scenario.

temperature of the southern facades is also lower.

The green scenario has a lower heat transfer coefficients caused

Bigger windows are increasing the cooling load by the sameby the additional green roof and facade layer. This results in lower

properties that affected the winter situation.

heating requirements for all rooms.

The green roof and walls cause lower outside surface- 38 -

temperatures during daytime by evapotranspiration and higher

hitzewelle-noch-verzichtet-die-mehrheit-der-deutschen-surface temperatures during nighttime because of the stronger

auf-klimaanlagen/ (23.01.2017)

heat flux from the underlying vegetation layer. Therefore, the(6)overall effect on the cooling load is relatively small (Table 2).

C. Pout, E. R. Hitchin, Future environmental impacts ofroom air-conditioners in Europe, Build. Res. Inf. 37(2009), 358-368(7)
5 Summary and Discussion

S. E. Gill, J. F. Handley, A. R. Ennos, S. Pauleit, Adapting
cities for climate change: The role of green infrastructure,
Building Environ. 33 (2007), 115-133

The different results of the laboratory experiment and the(8)W. Kuttler, Climate change in urban areas, Part 1, Effects,simulated reference room are probably caused by differences inEnvironmental Science Europe (ESEU) – Springer open

the size of the window and the room itself due to the used grid(2011) 1-12resolution.

(9)

M. Bruse, H. Fleer, Simulating surface-plant-air

The room location and solar radiation cause the strongest effects

interactions inside urban environment with a tree

in the simulations for the heating and cooling load. The

dimensional numerical model, Environ. Modell. Softw.

ventilation of the room can have a strong impact on the heating

13 (1998), 374-384

and cooling load if there is a large differenze between indoor and(10)G. Gross, Effects of different vegetation on temperature

outdoor temperature. In other meteorological szenarios ventlationin an urban building environment. Micro-scale numerical

could be used to lower the energy demand of the rooms. Overall

experiments, Meteorologische Zeitschift, 21 (2012) 399-

the energy demand can be more than two times larger between

412

different rooms.

(11)

P. Hoeppe, Improving Indoor Thermal Comfort byThe rooms in the green building need up to 16 % less energy inChanging Outdoor Conditions, Energy and Building 15-

the summer and up to 26 % less energy in the winter scenario.

16 (1991), 743-747

This effect depends on the available soil

water for(12)F. Calcerano, L. Martinelli, Numerical optimization

evapotranspiration, the meteorological conditions and whether

through dynamic simulation of the position of trees

the green walls will lose their leaves during winter.

around a stand-alone building to reduce cooling energy

The simulated trees decrease the cooling load in the summer

consumption, Energy and Buildings 112 (2016), 234-243

scenario by up to 34 % and increase the heating load during winter(13)

W. Eiband, Modellraumprüfstand mit dynamischer

by up to 51 %. The negative effects during winter could be

Simulation

avoided by using deciduous trees.

Untersuchung von Raumheizsystemen, PhD Thesis,

The overall effects of the tree, green and window measures should

Technische Universität München (2004)

be evaluated on a long-term yearly basis and effects of the micro-(14)scale urban climate should be taken into account.

klimatischerEinflussfaktoren

zurR. B. Stull, An introduction to boundary layer
meteorology (1998), Kluwer Academic, London(15)
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