Energy performance simulation in buildings composed of natural and organic materials within the Moroccan rural areas of the Atlas Mountains

The construction field uses up over one-third of the global energy consumption and contribute to 40% of CO2 emissions according to the International Energy Agency (IEA) and the 2020 annual reporter of United Nation, Goal 11 (Make cities inclusive, safe, resilient and sustainable) which discusses sustainable, safe and efficient buildings. Therefore, Morocco has a commitment to this program by publishing the law 47-09 of energy efficiency. This work aims to study the energy efficiency of two types of building, a conventional and a natural building. Conventional building is constructed using concrete, while the natural one uses sand clay and straws. As for the technique of making the natural building, it perpetually follows the same approach accustomed in rural zones of Atlas Mountains in Morocco. In this research we also simulate, temperature and humidity variation inside these buildings using TRNSYS software. Sketch Up software was employed to design these houses. The weather database is used for a typical meteorological year (TMY). In the case of natural building, many building configurations were simulated: roof insulation, floor insulation, different types of glazing and sun protection. What's more, the thermal comfort is revealed to be more conspicuous in the case of natural building.

the greenhouse gas CO2 (carbon dioxide) released into the atmosphere, in comparison to conventional houses. Traditional building materials are very efficient to maintain indoor thermal comfort and to trim down external emissions.
Many studies have taken a keen interest in energy efficiency of houses built by natural materials like adobe, clay, or rammed earth, occasionally mixed with natural fibers such as rice husk [5][6] and studied Direct Solar Floor (DSF) and Earth-Air Heat Exchanger (EAHE) of these types of housses [7].
Césaire and al. [8] improved the thermal comfort of houses in a hot-dry climate by using compressed earth blocks instead of hollow concrete blocks in the design of exterior walls. Moreover, the profiles of thermal discomfort vary depending on the wall designs and building spaces. Cassandra and al. [9] tested the thermal conductivity and the vapor transmission performance, as well as the combustibility of the material. It has been established that wood and hemp batt insulation products show thermal conductivity in the same range as expanded polystyrene, the vapor can diffuse and flow through the natural insulation up to three times more than with cellular synthetic insulation. Natural insulation materials are highly combustible, nontheless, they have short ignition times, making them safer than the expanded polystyrene.
Peñaloza and Jaramillo [10] evaluated the energy savings that can be obtained by installing an EAHE in a tropical climate in Colombia. The enhancement of energy effectiveness, in the construction sector, can be attained by the development of new materials, with improved properties, superinsulating accoutrements, and phase change accoutrements [11].
In our study, we simulate thermal caracteristics of natural and conventional houses by using three types of glazing (simple, double and triple), roof and low floor insulation also fixed sun protection in order to compare thermal comfort inside theses houses. Data of temperature and humidity outside the buildings applied for a typical meteorological year (TMY).

2.1-The numerical model
In the present study we used the numerical software TRNSYS 16 and type 56 [12] to simulate the variation of temperature and humidity during the year betwixt conventional and natural buildings to eventually draw up conclusions. In the case of natural buildings, the materials put to use are sand clay coupled with straw [13], as for the conventional buildings, the applied material is concrete. Numerous studies were based on this modelisation for the purpose of evaluating the thermal performance of each building and its energy needs [14]. The formulation of the material plays an important role in augmenting it's energy efficiency: for example, the use of fibre decreases the thermal conductivity of the material [15]. TRNSYS uses a multizone approache, where building zones are represented by nodes that can exchange many type of flux ( Figure 1).

Figure 1: Multizone modelisation approch
The convective heat flux to an air node "i" is calculated as follows: Q̇s urf,i : Convective heat flux from inside all surfaces (KJ/h) where: U w,i : Transmission coefficient of a wall in zone "i"(W/m 2 .K), A w,i : Surface of a wall in zone "i"(m 2 ), T wall,i : Wall surface temperature in zone "i"(K), T air : Air temperature (K). Q̇c plg,i : Convective gains due to air flow between zones (KJ/h).
Q̇c plg,i = V̇ρCp(T zone,i − T air ) Where: T zone,i : temperature of the zone adjacent to zone "i".

2.2-Radiative Heat Flows (only) to the Walls and Windows
Meanwhile, the radiative transfert on an air nodal in the TRNSYS type 56 distinguishes between short wavelength exchanges that are less than 2.5 µm and wavelength greater than 2.5 µm; the radiative exchange of a wall surface temperature node is expressed in equation ( The specific radiative heat flow at a given wall.

2.3-Envelope scale transfer modeling
The Mitalas and Arsenault methode is used in TRNSYS to model the walls because it allows accurate calculation of near walls in transient regime.  The wall is considered as a black box and defined by four coefficients (as, bs, cs, ds) which represents wall thermal behavior, these time series coefficients are determined in the TRNBuild program which integrates the thermo-physical properties of each layer constituting the wall.

2.4-Thermal load of a zone
The temperature in a building zone (thermal node) is calculated by TRNSYS using equation (9): C i : The thermal capacity of zone i (J.kg -1 .K -1 ) , Q̇i : The net thermal gain (KJ/h), it's a function of T i (K) and temperatures of all the other zones adjacent to zone i.

2.5-Calculation of convective coefficients and thermal inertia
In TRNSYS the default calculation of convection heat transfer coefficients is only valid for internal building surfaces, these coefficients are calculated according to the equation (10). TRNSYS software consideres the thermal capacity of a thermal zone as equal to 1.2 multiplied by the zone surface value.

2.6-TRNSYS inputs and outputs
In all simulations using TRNSYS the following considerations are taken into account: initial temperature and humidity of the air inside all thermal zones are acquired at 20°C and 50% respectively, no internal heat gain (unoccupied buillding), a constant infiltration rate of 0.5 for all areas and soil temperature equal to 14°C. TRNSYS outputs are temporal variation of temperature and zones humidity.

2.7.1-Dimension and zoning of building
The studied building is a single floor that includes a single zone (Figure 3) of 60 m 2 of surface and 180 m 3 of volume, with two windows in the East and West and a main façade oriented to the north. Table 1 portrays windows characteristics of this building.

2.7.2-Constitutions and thermo-physical characteristics of the building walls
The walls components for the natural building are divulged in table 2 as for the ones that concern the conventional building they're illustrated in Table 3. Table 4 on the other hand, uncovers the thermo-physical characteristics of the building walls material used in the natural and conventional buildings.   In this study we used one type of insulation for natural building, which is wood fibres, and the used insulations thicknesses are 5cm, 10 cm, and 15cm.

Natural single-zone building
TRNSYS simulations of temperature and humidity for natural single-zone building along the TMY are showcased in Figure (4-a). As we can see, both inside and outside temperatures of this building are higher in the summer season. Yet, in the winter, inside temperatures are greater than outside temperatures and this behavior is reversed in the summer. Moreover, during the whole year, inside temperatures has trivial fluctuations compared to the outside ones. All this highlights why a natural building had an energy efficiency. Figure (4-b) compares the outside and inside humidity of the building.
Ostensibly, during the winter, the inside humidity is less than that of the outside and the contrary is observed during summer.  where we notice a dwindle in temperature reaching up to 2°C compared to the natural sigle-zone building with single glazing. Still, there was no notable impact on simulated humidity for the three type of glazing compared to single glazing (Figure 7(b)).

Comparison between a natural building and a conventional building
TRNSYS simulations of temperature for natural single-zone building are compared with that of a conventional building along the TMY in Figure 10(a). The figure demonstrates that a natural building is somewhat synonymous with an energy efficient building owing to the fact that the tempretaure is curtailed, especially during summer.
Howbeit, the conventional building provides better results when it comes to humidity, since humidity in natural building is higher than that found in conventional building ( Figure 10(b)).

Conclusions
The main aim of this paper was to study the energy efficiency of a conventional and a natural building. Concerning the conventional building; construction material was concrete, while in the natural building the construction material was sand clay and straw. Sketch Up and TRNSYS 16 software were used respectively to design these houses and to simulate temperature and humidity inside building. As for the weather database, it was used for a typical meteorological year (TMY). In the case of the natural building, simulations results exhibit that, during summer seasons, the building efficiency can be enhanced by using roof insulation, different types of glazing or sun windows protections. However, simulations demonstrate that insulating the floor of the natural building can increase inside temperature during summer, especially when increasing the thickness of the insulation. During winter seasons, simulations demonstrate that the building efficiency can be enhanced by using roof insulation, floor insulation or Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 23 December 2021 different types of glazing. So, the thermal comfort is more guaranteed in the case of natural building in comparison to a conventional building. As perspectives of this study, we plan to focus on natural buildings that closely mimic and imitate those found in rural zones of the Atlas Mountains in Morocco. So, future simulations may subdivide the natural building to three or four zones and incorporate home furniture and residents.
Also, simulations can be coupled with an optimization procedure to find optimal configuration including in the same natural building both floor insulation, roof insulation and glazing.