Design Methodology Development for High-Energy-Efficiency Buildings in Algerian Sahara Climatic Context

Abstract

In Algeria, the rapid increase in population and urbanization, evolving comfort needs, subsidized electricity prices, and climate change has significantly contributed to higher energy consumption for heating and cooling as well as greenhouse gas emissions, particularly in southern regions characterized by hot and arid climates. Most recent constructions in Algeria are highly energy-intensive, unlike traditional Saharan architecture, which is far more environmentally friendly. This paper presents eco-friendly and cost-effective design methods and solutions inspired by Saharan architecture to guide architects and project owners during the design phase of buildings in hot climate regions. A numerical simulation was performed using EnergyPlus 9.2 to compare the energy consumption of a semi-collective residential building in Béni Abbès with four design alternatives inspired by vernacular architecture, “O”, “L”, “U”, and rectangular configurations. The findings showed that the O-shape configuration achieved the highest cooling energy savings (38.55% on the ground floor, 27.68% on the first floor), followed by the L-shape (31% and 32%), U-shape (28% and 29%), and rectangular shape (26% and 25%), highlighting the effectiveness of form optimization in enhancing energy efficiency. The results obtained demonstrate the energy efficiency of the four variants compared with the initial cases, with a reduction in cooling needs while using the same materials. This reduction could reach up to 39% during the hot season. The pay-back period for the investment was estimated at approximately six years for the city of Béni Abbès and around five years for the city of Adrar. By incorporating full insulation into all four variants, a maximum reduction in air conditioning consumption of approximately 53% was observed for the “O” variant in Béni Abbès compared with the initial case without insulation. In Adrar, this reduction reached around 48% for the same variant. Passive design elements, such as shape optimization, compact urban fabric, patio integration, and window shading, offer moderate energy savings with a shorter payback period, whereas complete insulation achieves higher energy savings but requires a longer time to offset the investment costs.

1. Introduction

In recent years, Algeria has experienced an exponential increase in energy consumption, particularly in the tertiary sector, which includes residential (or domestic) housing and commercial facilities, and accounts for more than 47% of final consumption. This sector recorded a 7.6% rise in consumption, increasing from 23.4 million tons of oil equivalent (Mtoe) in 2021 to 25.2 Mtoe in 2022 [1]. This increase in demand is attributed to several factors including:

-

A transformation in lifestyles and growing comfort requirements;

-

The increasing use of energy-intensive appliances such as air conditioners, often with poor energy ratings;

-

Energy prices that are too low and heavily subsidized, encouraging wasteful consumption;

-

Inadequate architectural designs that fail to consider regional climatic conditions;

-

Rapid population growth and urbanization [2].

To address the housing crisis, the Algerian government has made significant investments by constructing millions of subsidized housing units under various programs. However, these efforts have not been sufficient to meet the growing demand [3,4]. Under social pressure and aiming to satisfy as many people as possible, the authorities prioritized cost and the speed of execution over quality and energy efficiency. This approach has resulted in housing that does not comply with thermal regulations, is poorly adapted to specific climatic conditions, and is both uncomfortable and highly energy-intensive, particularly in the southern regions of the country, which are characterized by an arid and challenging climate [5,6].

Traditional Saharan architecture, once environmentally friendly, has gradually been replaced by standardized constructions poorly suited to their surroundings. As a result, Saharan cities have lost their environmental advantages and now heavily rely on cooling appliances, which are highly energy-intensive, especially during heatwaves. During such periods, the massive use of these devices becomes vital, leading to record overconsumption, the overloading of electrical grids, and frequent power outages. This situation has contributed to an increase in deaths among the most vulnerable populations.

Currently, the population can afford this high energy consumption due to subsidized prices, with rates starting at 1.77 DZD/kWh (0.013 USD/kWh), which is significantly below the actual production cost [7]. However, as energy subsidies place a growing burden on public finances and the energy demand continues to rise, there is an increasing need for energy-efficient solutions, such as sustainable architecture, to reduce the dependence on conventional energy sources.

In light of these challenges, Algeria must now act to preserve its natural resources and the environment by constructing ecological, comfortable, and energy-efficient buildings [8,9,10,11,12]. To achieve this, studying traditional Saharan vernacular architecture—particularly its passive heating and cooling techniques [13,14,15,16,17,18]—has provided a valuable foundation for designing alternatives that are better suited to current needs [19,20].

Numerous studies have addressed the topic of energy efficiency. Khaoula et al. [21] and Bassoud et al. [22] examined the thermal comfort of vernacular dwellings in hot and arid climates. Their findings demonstrated that traditional Saharan housing could achieve acceptable thermal conditions during hot periods without relying on mechanical air conditioning. Additionally, other researchers have explored the energy efficiency and thermal comfort of underground buildings in southern Algeria, concluding that structures buried at a depth of 2.34 m could significantly reduce the energy needs during the summer season [23]. Several studies [24,25,26,27] have also shown that interior courtyards can serve as a passive and effective strategy to cope with hot and arid climatic conditions. Furthermore, numerous studies [28,29,30,31] have emphasized the importance of incorporating appropriate design aspects and making sound decisions from the early phases of building design. Among these aspects, the choice of building shape and compactness has a direct impact on energy efficiency [32,33,34], as does the selection of envelope materials [35,36,37,38,39,40]. Additionally, using an urban fabric that minimizes heat exchange between interior and exterior spaces could further enhance energy performance [41]. A thoughtful selection of these elements would enable the construction of buildings that combine thermal comfort with low energy consumption [42]. While this study employed deterministic EnergyPlus simulations, recent advances in stochastic machine learning models (Liu et al. [43,44,45,46,47,48]) suggest that integrating predictive algorithms such as random forest and gradient boosting could further refine energy efficiency assessments by accounting for material variability and environmental uncertainties.

Although several studies have explored the principles of Saharan vernacular architecture, few have focused on the application and adaptation of these concepts in contemporary architecture and their impact on energy consumption. The objective of this article was to propose solutions and recommendations aimed at improving the thermal comfort of buildings while reducing the reliance on cooling devices, all with limited investment costs. The goal is to provide guidelines and insights for designers to support decision-making during the design phases of residential projects in hot and arid climates. This paper addresses this gap by presenting eco-friendly and cost-effective design methods inspired by Saharan architecture to guide architects and project owners during the design phase of buildings in hot climate regions. A numerical simulation using EnergyPlus 9.2 was conducted to compare the energy consumption of a semi-collective residential building in Béni Abbès with four design alternatives inspired by vernacular architecture. The findings highlight the energy efficiency of the four variants compared with the initial cases, achieving up to a 39% reduction in cooling demand during the hot season while utilizing the same materials. The investment’s payback period was estimated at approximately six years in Béni Abbès and around five years in Adrar. With full insulation applied to all four variants, the “O” variant in Béni Abbès achieved a maximum reduction of approximately 53% in air conditioning consumption compared with the initial uninsulated case. In Adrar, the same variant recorded a reduction of around 48%.

2. Materials and Methods

This study was based on a methodology designed to analyze the impact of architectural decisions, inspired by Saharan vernacular architecture, on the energy efficiency of residential buildings in arid desert environments and can be divided into two parts:

The first part of this research involved the validation of the EnergyPlus model, followed by a comparison of the energy performance of an existing multi-zone residential building in southwestern Algeria with four alternative architectural designs. These alternatives were inspired by passive techniques from traditional Saharan architecture, adapted to meet the current comfort needs. To ensure a fair comparison, the various designs studied had the same habitable surface area and volume. Additionally, the envelope materials used in the initial case were retained, with variations in the shape factor (SF), which represents the ratio between the external envelope surface area and the habitable volume [49].

In this context, four geometric shapes for the buildings were proposed: rectangular, O-shaped, C-shaped, and L-shaped. These proposals also included variations in orientation and ventilation rates and were studied under two distinct Saharan climates, represented by the cities of Béni Abbès and Adrar.

The second part of the study focused on the impact of orientation choices, envelope materials, and the effect of solar protection on energy consumption as well as analyzing their economic impact and payback period.

Figure 1 provides a concise overview of the different steps followed in our study, each of which is detailed in the following sections.

2.1. Climatic Study

Algeria is characterized by a significant climatic diversity, with considerable variations across regions due to key meteorological parameters such as temperature, rainfall, relative humidity, and wind speed, among others. According to the Algerian Technical Regulatory Document D.T.R C 3.2 [50], the country’s climate is classified into five zones, as illustrated in Figure 2:

-

Zone A: Mediterranean climate, marked by hot and humid summers and mild winters.

-

Zone B: Semi-arid Mediterranean climate, characterized by hot, dry summers and cold winters.

-

Zone C: Arid to semi-arid climate, featuring very hot summers and cold, dry winters.

-

Zone D: Saharan climate, defined by extremely high summer temperatures and mild winters.

-

Zone D’: Extreme Saharan climate, where summers are exceptionally hot with record-breaking temperatures, while winters remain mild with almost no precipitation.

These climatic characteristics directly impact the built environment and are essential for analyzing the thermal performance of buildings. In our study, we focused on two specific zones:

-

Zone D: Representing the Saharan climate, exemplified by the city of Béni Abbès.

-

Zone D’: Representing the extreme Saharan climate, illustrated by the city of Adrar.

This analysis aimed to deepen the understanding of the implications of Saharan climatic conditions on the thermal performance of buildings in these two distinct contexts.

2.2. Case Study

2.2.1. Reference Case Study

For this study, a two-story semi-collective residential building with a cross (†) shape was chosen, located in the city of Béni Abbès at coordinates 30°4′59.175″ N and −2°50′4.219″ E, with an altitude of 471 m (See Figure 3a). This building is part of a 60/1500 housing project developed under the LPL (Public Rental Housing) program, initiated by the project owner OPGI (Office of Promotion and Real Estate Management). The program was specifically designed to provide housing for the most disadvantaged social groups living in precarious and/or unsanitary conditions in Algeria [51].

This building was selected because the semi-collective housing model is widely used in the southern regions of the country. Figure 3b shows the site plan of the project, oriented directly westward, along with the typical floor plan (Figure 3c). Each floor consists of a living room, hallway, two bedrooms, a kitchen, a bathroom, and sanitary facilities, with a total habitable area of 67.91 m² and a shape factor of 0.69 (

Table 1. The different studied shapes and their geometric characteristics.

Shape	Usable Floor Area m2	Height m	Perimeter m	Volume m3	Exterior Area m2	Shape Factor (SF) m2/m3
	67.91	6.12	44.7	415.61	249.45	0.69
	67.93	6.12	41.3	415.73	169.55	0.40
	67.66	6.12	47.5	414.08	179.41	0.43
	67.76	6.12	50.9	414.69	173.62	0.41
	67.72	6.12	40.2	414.44	163.63	0.39

).

The external walls are constructed as double-layered walls made of hollow bricks, each layer being 10 cm thick and separated by a 5 cm air gap. These walls are coated on both the interior and exterior sides with a cement render. The internal walls consist of single-layer hollow bricks, 10 cm thick, also finished on both sides with cement mortar.

The ceiling structure, from the exterior to the interior, is composed of the following elements:

-

A waterproofing system of type 36 S;

-

A sloped concrete layer with an average thickness of 7 cm;

-

Thermal insulation made of 4 cm thick polystyrene;

-

A hollow-core slab of (16 + 5) cm;

-

A mortar render finish.

The windows are made of wooden frames fitted with single glazing and exhibit poor airtightness against external air.

2.2.2. Building Configurations: “O”, “L”, “U”, and Rectangular Shapes

To compare the energy efficiency of the reference building, four architectural configurations were designed to adapt to the climatic constraints, optimize thermal efficiency, and integrate harmoniously into a Saharan environment. These configurations also meet the current comfort needs while respecting the specifications for social housing established by the OPGI (Office of Promotion and Real Estate Management). The studied shapes—“O”, “L”, “U”, and rectangular—were inspired by vernacular architecture principles, combining thermal comfort, functionality, and durability through the following strategies:

• Utilization of a Patio as a Thermal Regulator:

The “O”, “L”, and “U” configurations were organized around an interior courtyard (Figure 4), with respective dimensions of 3.1 × 2.4 m, 6.05 × 2.4 m, and 3.5 × 2.7 m. These patios, drawn from vernacular architecture, play a central role in thermal regulation. They create shaded areas and promote air circulation, thus contributing to the cooling of interior spaces during hot periods. Additionally, these features shield windows from direct solar exposure.

• Low Shape Factor:

Each configuration features a low shape factor of approximately 0.4, with habitable areas close to 67 m² across the various variants and a floor height of 3.06 m per level (Table 1):

-

“O” shape: Habitable area of 67.93 m², shape factor of 0.40;

-

“L” shape: Habitable area of 67.66 m², shape factor of 0.43;

-

“U” shape: Habitable area of 67.76 m², shape factor of 0.41;

-

Rectangular shape: Habitable area of 67.72 m², shape factor of 0.39.

These low shape factors, which varied slightly among the four variants, minimize thermal exchanges between the interior and exterior, ensuring thermal comfort and energy efficiency.

• Semi-Buried Layout:

A semi-buried layout was adopted for all configurations, with a level difference of 1.02 m between the interior and exterior (Figure 5). This approach reduces the surface area of walls exposed to the external environment, thereby limiting thermal exchanges and heat losses. This technique, commonly used in vernacular architecture, also exploits the thermal inertia of the ground to provide natural cooling in summer and a comfortable ambient temperature in winter.

• Compact Urban Fabric:

The “O”, “L”, and “U” configurations were integrated into a compact urban fabric where three facades were shared with adjacent buildings (Figure 6). Inspired by the principles of Saharan urban density, this arrangement allows buildings to mutually protect one another, reducing exposed wall surfaces and thermal exchanges.

• Narrow Streets and Solar Protection Systems:

The main facades of the four configurations, along with the secondary facade of the rectangular building, face a narrow 6-m-wide street, as illustrated in Figure 7a. These streets are equipped with overhanging solar shades, reminiscent of the narrow, shaded alleys of traditional ksour (Figure 7b). These features protect against excessive heat and provide optimal shading. Compact buildings and narrow streets have lower heating and cooling demands compared with isolated buildings with wide streets [52].

• Window Protection from Solar Rays:

For the “O”, “L”, and “U” configurations, windows open onto the patio or a loggia, providing additional protection against direct solar rays. For the rectangular configuration, the second facade parallel to the main facade also faces a 6-m-wide narrow street equipped with 1.2-m solar shades on each side (Figure 8).

Table 2. Use of passive systems for the different variants studied.

Passive Systems	Initial Case	“O” Shape	“L” Shape	“U” Shape	Rectangular
Use of a patio	X	✔	✔	✔	X
Semi-buried implantation	X	✔	✔	✔	✔
Narrow street and solar protection system	X	✔	✔	✔	✔
Use of a compact urban fabric	X	✔	✔	✔	✔
Protection of windows against solar rays	X	✔	✔	✔	✔

lists the various passive systems used for the five case studies.

2.3. Numerical Simulation

To better assess and understand the thermal behavior of the different variants, a dynamic numerical simulation approach was adopted. The choice of simulation tool was EnergyPlus software 9.2, widely recognized and validated in the field of energy research applied to buildings. This choice is motivated by its powerful features, its ability to integrate across multiple platforms, and the rigorous validation of its simulation algorithm, making it an essential reference in this field [53].

For geometric modeling, SketchUp software, integrating the OpenStudio 2.9.1 plug-in (Figure 9), was used. This allows users to create precise geometry tailored to the needs of EnergyPlus [54].

2.3.1. Thermal Zoning

Each level was divided into five thermal zones (Figure 10), with set point temperatures of 20 °C for heating and 27 °C for cooling. The infiltration rate used in the simulation was 1 volume per hour, and the nighttime ventilation rate was 10 volumes per hour.

2.3.2. Internal Gain and Occupancy Schedule

Each apartment is occupied by a family of four. The occupancy schedule was based on the actual lifestyle of the residents. It was assumed that the homemaker remained in the apartment throughout the day, while the other family members were away from 8:00 AM to 5:00 PM on weekdays. On weekends, all occupants left the apartment between 5:00 PM and 7:00 PM (Figure 11).

Other internal gains came from appliances (television, laptop, hairdryer, iron, vacuum cleaner, and electric lighting), as seen in

Table 3. Average daily energy consumption for electrical appliances [55].

Device	Power (W)		Duration of the Use per Day
LCD TV with integrated demo	140		6 h
Refrigerator 250 L capacity	175		12 h
Lighting	Kitchen	40	2 h
	Living room	60	3 h
	Master bedroom	40	2 h
	Child’s room	40	2 h
Vacuum cleaner	720		12
Hair dryer	450		5 min
Iron	925		15 min
laptop	60		1 h

.

2.3.3. Material Characteristics

The characteristics of the construction materials used in the simulation were defined in accordance with the Algerian Technical Regulatory Document D.T.R C 3.2 [50].

Table 4. Properties of the materials making the envelope [50].

No.	Building Element	Layer	Materials	Thickness (m)	Thermal Conductivity (W.m−1.K−1)	Density (Kg/m3)	Specific Heat Capacity (J.Kg−1.K−1)
1	Exterior wall	01	Mortar	0.02	1.4	2200	1080
		02	Hollow brick	0.1	0.48	900	936
		03	Air cavity	0.05	0.024	1.22	1
		04	Hollow brick	0.1	0.48	900	936
		05	Mortar	0.02	1.4	2200	1080
2	Partition wall	01	Mortar	0.02	1.4	2200	1080
		02	Hollow brick	0.1	0.48	900	936
		03	Mortar	0.02	1.4	2200	1080
3	Ground floor	01	Concrete	0.1	1.75	2500	1080
		02	Concrete screed	0.07	1.75	2200	1080
		03	Tilling	0.02	2.1	2200	936
4	Internal floor	01	Tilling	0.02	2.1	2200	936
		02	Mortar	0.07	1.75	2200	1080
		03	Concrete slab with hollow bloc	0.21	1.45	1080	1450
		04	Mortar	0.02	1.4	2200	1080
5	Roof	01	Tightness	0.01	0.23	1000	1656
		02	Concrete screed	0.08	1.75	2200	1080
		03	Polystyrene	0.04	0.038	30	1404
		04	Concrete slab with hollow bloc	0.21	1.45	1080	1450
		05	Mortar	0.02	1.4	2200	1080

presents the properties of the various materials that make up the building envelope.

The meteorological file TMY (Typical Meteorological Year) used for the simulations was obtained from the OneBuilding.org website [56] for the two regions studied: Béni Abbès, located in a Saharan climate, and Adrar, characterized by an extreme Saharan climate.

2.3.4. Validation of the Model

The validation of the building’s energy model, developed using EnergyPlus software, are essential steps to ensure the reliability of the simulation results. In this context, an indoor ambient temperature measurement campaign was conducted in the living room on the first floor of the initial variant, located in Zone 1 (Figure 10). Temperature readings were recorded hourly using Arexx measuring devices (Zwolle, The Netherlands) (Figure 12), equipped with a powerful transmitter and an external temperature probe with an accuracy of ±0.5 °C.

The measurements were taken on a winter day, 12 March. To ensure representative readings of the indoor thermal conditions, the sensor was placed at the center of the room at a height of 1.4 m. Figure 13 illustrates the comparison between the measured and simulated temperatures for this day.

For the validation of the model, two indices defined by the ASHRAE 14 [57] standard were used: the mean bias error (MBE) and the coefficient of variation of the root-mean-square error (CV[RMSE]). The acceptable thresholds for these indices are set at ≤±10% for MBE and ≤30% for CV[RMSE], particularly for hourly data. These parameters were calculated using Equations (1) and (2):

$$\mathrm{M}\mathrm{B}\mathrm{E}\left(\mathrm{\%}\right)={\displaystyle \frac{{\sum }_{i=1}^{N_p}(M_i-S_i)}{N_p\times \mathrm{m}}}\times 100$$

(1)

$$\mathrm{C}\mathrm{V}[\mathrm{R}\mathrm{M}\mathrm{S}\mathrm{E}]\left(\mathrm{\%}\right)={\displaystyle \frac{1}{m}}\times \sqrt{{\displaystyle \frac{{\sum }_{i=1}^{N_p}{(M_i-S_i)}^2}{N_p}}}\times 100$$

(2)

where:

• ${\mathit{M}}_{\mathit{i}}$: Measured value at time step i;
• ${\mathit{S}}_{\mathit{i}}:$ Simulated value at time step i;
• ${\mathit{N}}_{\mathit{p}}:$ Total number of measurements;
• m: Average of measured values.

The mean bias error (MBE) was −2.74%, while the coefficient of variation of the root- mean-square error CV[RMSE] reached 6.12%. These values meet the criteria defined by ASHRAE 14 (MBE ≤ ±10% and CV[RMSE] ≤ 30%), indicating that the model was properly validated.

3. Results and Discussion

A total of 140 simulations were carried out in the context of the Saharan climate to assess and compare the energy efficiency of an initial cross-shaped configuration (“†”) with four alternative configurations inspired by passive heating and cooling techniques from traditional desert architecture: the “O”, “L”, “U”, and rectangular configurations.

3.1. Ambient Temperature

Figure 14 illustrates the variations in indoor ambient temperature in the living room based on the different building shapes throughout the day. The lowest average temperatures were recorded on 13 January for Béni Abbès and 7 January for Adrar. The highest temperatures were recorded on 5 July for Béni Abbès and 11 July for Adrar.

For the coldest average day (Figure 14), in Béni Abbès, there was a significant variation in outdoor temperatures, with a minimum of 3.23 °C at 1:00 AM, reaching a maximum of 16 °C at 2:00 PM. In Adrar, the minimum temperature was 1.35 °C at 7:00 AM, and the maximum reached 16.39 °C at 4:00 PM.

On the other hand, the indoor temperatures of the buildings remained relatively stable throughout the day, with less variation than the outdoor temperatures in both cities. The temperatures varied between approximately 14 °C and 17 °C in Béni Abbès, and between around 12 °C and 16 °C in Adrar, regardless of the building type. This was mainly due to internal heat gains, which help maintain thermal consistency inside the spaces.

The “L”-shaped building performed the best for both cities, with temperatures ranging from 14.97 °C to 17.71 °C in Béni Abbès, and between 13.66 °C and 17.1 °C in Adrar.

For the hottest day on average (Figure 15), the recorded outdoor temperatures in the city of Béni Abbès ranged from 34.52 °C at 01:00 to a peak of 46.17 °C at 14:00, before gradually decreasing to 36.91 °C at midnight. In contrast, in the city of Adrar, temperatures varied between 29.93 °C at 07:00 and 46.64 °C at 15:00. These high values reflect the difficult and extreme climatic conditions faced by both cities.

Regarding indoor temperatures, differences were observed depending on the building shapes, with values ranging from 35 °C to 39 °C. The initial form recorded the highest temperatures, with a minimum of 36.21 °C at 06:00 and a maximum of 40.08 °C at 22:00.

On the other hand, the other forms exhibited better thermal performance, with the “O” shape showing the most favorable results. The temperature ranged from 34.19 °C at 01:00 to 37.18 °C at 22:00.

3.2. Energy Consumption

The highest temperatures were recorded on 5 July for Béni Abbès and 11 July for Adrar. Figure 16 illustrates the energy consumption related to heating and air conditioning for the different building forms in the cities of Béni Abbès and Adrar. The results show that for heating, only slight variations in consumption were observed. For the city of Béni Abbès, a slight decrease in energy consumption was noted for the “L”, “U”, and rectangular configurations compared with the initial case, with a maximum reduction of approximately 2 kWh/m² for both levels. In contrast, the “O” shape recorded a slight increase in consumption, reaching 1.04 kWh/m² for the ground floor, rising from 29.30 kWh/m² in the initial case to 30.34 kWh/m², an increase of 3.42%. For the first floor, the consumption rose from 31.77 kWh/m² to 35.22 kWh/m², an increase of 10.86%. These results stem from the fact that for this form, the majority of openings are protected from direct solar radiation, reducing the internal gains compared with the initial case.

Regarding energy consumption for air conditioning, significant reductions were observed for the four alternative configurations compared with the initial case, with higher overall consumption for the city of Adrar compared with Béni Abbès. The “O” configuration proved to be the most efficient for both levels. On the ground floor, the annual consumption decreased from 111.34 kWh/m² in the initial case to 68.42 kWh/m², a saving of 42.92 kWh/m², corresponding to a 38.55% reduction. For the first floor, the consumption decreased from 190.74 kWh/m² to 137.93 kWh/m², a saving of 52.81 kWh/m², corresponding to a 27.68% reduction. These performances can be explained by an improvement in the form factor, which dropped from 0.69 in the initial case to 0.4 for the “O” form as well as the application of various techniques and principles of vernacular architecture mentioned in the previous section.

Economical Study

Table 5 presents a comparative analysis of different architectural forms in terms of the annual energy savings, financial costs, and payback period. The total annual energy savings (AES) were calculated as the sum of the annual savings related to cooling and heating using the following formula:

$$\mathbf{A}\mathbf{E}\mathbf{S}=\mathbf{H}\mathbf{e}\mathbf{a}\mathbf{t}\mathbf{i}\mathbf{n}\mathbf{g} \mathbf{E}\mathbf{n}\mathbf{e}\mathbf{r}\mathbf{g}\mathbf{y} \mathbf{S}\mathbf{a}\mathbf{v}\mathbf{i}\mathbf{n}\mathbf{g}\mathbf{s}+\mathbf{C}\mathbf{o}\mathbf{o}\mathbf{l}\mathbf{i}\mathbf{n}\mathbf{g} \mathbf{E}\mathbf{n}\mathbf{e}\mathbf{r}\mathbf{g}\mathbf{y} \mathbf{S}\mathbf{a}\mathbf{v}\mathbf{i}\mathbf{n}\mathbf{g}\mathbf{s}$$

(3)

According to the data in the table, the total energy savings were significantly higher for the city of Adrar compared with Béni Abbès. Specifically, in Béni Abbès, the total energy savings were approximately 5000 kWh/year for all four configurations. Conversely, in Adrar, the maximum savings reached around 7715 kWh/year for the “L” form, representing a difference of approximately 2715 kWh/year compared with Béni Abbès. The minimum savings in Adrar were about 6418 kWh/year for the “U” form, which was still about 1418 kWh/year higher than in Béni Abbès. This demonstrates that all four alternatives studied provide good thermal performance, suitable for desert and extreme desert climates.

The financial savings results from the annual energy savings were calculated by multiplying the total energy savings by the unit energy cost. According to the Ministry of Energy, the average electricity tariff for the population is 4.01 DZD/kWh (approximately 0.029 USD/kWh), while the actual production cost of 1 kWh is approximately 5.4 DZD (around 0.04 USD/kWh) [58]. These rates are extremely low compared with those in most other countries due to government subsidies. In this analysis, energy prices were based on the actual production costs rather than the subsidized rates.

As per the table, the highest financial savings were recorded for the “L” form in Adrar’s climate, with savings reaching 318.07 USD/year.

For semi-collective social housing, the average construction cost subsidized by the government is approximately 45,000 DZD/m² (331.23 USD/m²). This relatively low cost limits the use of high-performance construction materials, which are expensive.

The additional investments include works not accounted for in the baseline case:

-

Mass excavation to a depth of 1.02 m and a reinforced concrete retaining wall adjacent to the external roadway to enable semi-underground construction;

-

Solar protection in the form of cantilevered reinforced concrete slabs along the main façade for “O”, “L”, and “U” forms, and along both the main and rear façades for rectangular buildings.

The construction costs were determined by contacting local contractors to gather price information, allowing for the calculation of additional investment amounts for each variant (Table 5). The “O” form required the lowest additional investment, amounting to USD 1165.95, representing a 5.18% increase in the initial project cost. Conversely, the rectangular form entailed the highest additional investment, amounting to USD 1527.80, representing a 6.79% increase in the initial project cost.

The payback period is defined as the time required to recover the additional investment through the savings achieved and is calculated using the following formula:

$$\mathit{T}\mathit{e}=\mathit{C}\mathit{o}/\left(\right(\mathit{C}\mathit{a}\mathit{p}-\mathit{C}\mathit{a}\mathit{v})\times \mathit{P})$$

(4)

where:

• Te: is the payback period (in years);
• Co: is the cost of additional investment (in USD);
• Cav: is the energy consumption before the investment (in kWh/year);
• Cap: is the energy consumption after the investment (in kWh/year);
• P: is the unit price of energy (in USD per kWh).

The payback period varies depending on the building’s form and location. In this regard, it was observed that the payback period was faster in Adrar compared with Béni Abbès. The “O” form proved to be the most advantageous, offering a quick return on investment with low additional costs. Specifically, the payback period was 5.89 years for Béni Abbès and 4.26 years for Adrar.

Conversely, the rectangular form had the longest payback period, reaching 7.88 years in Béni Abbès and 5.82 years in Adrar.

Table 5. The investment cost and payback compared with the initial case for the cities of Béni Abbès and Adrar.

	Shape	Annual Energy Gain Heating (kWh/year)	Annual Energy Gain Cooling (kWh/year)	Total Annual Energy Gain (kWh/year)	Total Annual Financial Gain (USD)	Project Cost (USD)	Additional Investment Amount (USD)	Impact on Project Cost (%)	Payback Period (Years)
Beni Abbes	Initial Case	/	/	/	/	22,491.4	/	/	/
	“O” Shape	−235.30	5215.51	4980.20	197.93	23,663.98	1165.95	5.18%	5.89
	“L” Shape	128.71	4911.73	5040.45	200.32	23,828.66	1420.05	6.31%	7.09
	“U” Shape	77.02	4922.22	4999.24	198.69	23,795.50	1353.78	6.02%	6.81
	Rectangular	9.163	4867.10	4876.26	193.80	23,956.28	1527.80	6.79%	7.88
Adrar	Initial Case	/	/	/	/	22,491.4	/	/	/
	“O” Shape	−11.59	6892.42	6880.82	273.47	23,663.98	1165.95	5.18%	4.26
	“L” Shape	287.45	7715.64	8003.09	318.07	23,828.66	1420.05	6.31%	4.46
	“U” Shape	233.51	6418.75	6418.75	255.10	23,795.50	1353.78	6.02%	5.31
	Rectangular	156.49	6448.25	6604.74	262.49	23,956.28	1527.80	6.79%	5.82

Table 5. The investment cost and payback compared with the initial case for the cities of Béni Abbès and Adrar.

	Shape	Annual Energy Gain Heating (kWh/year)	Annual Energy Gain Cooling (kWh/year)	Total Annual Energy Gain (kWh/year)	Total Annual Financial Gain (USD)	Project Cost (USD)	Additional Investment Amount (USD)	Impact on Project Cost (%)	Payback Period (Years)
Beni Abbes	Initial Case	/	/	/	/	22,491.4	/	/	/
	“O” Shape	−235.30	5215.51	4980.20	197.93	23,663.98	1165.95	5.18%	5.89
	“L” Shape	128.71	4911.73	5040.45	200.32	23,828.66	1420.05	6.31%	7.09
	“U” Shape	77.02	4922.22	4999.24	198.69	23,795.50	1353.78	6.02%	6.81
	Rectangular	9.163	4867.10	4876.26	193.80	23,956.28	1527.80	6.79%	7.88
Adrar	Initial Case	/	/	/	/	22,491.4	/	/	/
	“O” Shape	−11.59	6892.42	6880.82	273.47	23,663.98	1165.95	5.18%	4.26
	“L” Shape	287.45	7715.64	8003.09	318.07	23,828.66	1420.05	6.31%	4.46
	“U” Shape	233.51	6418.75	6418.75	255.10	23,795.50	1353.78	6.02%	5.31
	Rectangular	156.49	6448.25	6604.74	262.49	23,956.28	1527.80	6.79%	5.82

3.3. The Effect of Orientation on Energy Consumption

Figure 17 and Figure 18 show the differences in the energy consumption of buildings based on their architectural form and orientation (west, north, east, south) for the cities of Béni Abbès (Figure 17) and Adrar (Figure 18). The results revealed a slight variation in heating consumption depending on the orientations and building forms. However, the most significant variation was observed for the initial case with a north orientation, where the heating consumption on the first floor decreased from 31.77 kWh/m² for the west orientation to 24.58 kWh/m² for Béni Abbès and from 26.53 kWh/m² to 20.54 kWh/m² for Adrar.

This decrease can be explained by the presence of a large portion of the openings on the rear facade, which faces south. This orientation promotes solar gains, thereby increasing thermal gains and reducing the energy consumption for heating.

For air conditioning, there was a noticeable decrease in energy consumption across the four architectural forms compared with the initial configuration for different orientations in both Béni Abbès and Adrar. A pattern was also evident between the results for the north and south orientations as well as between the east and west orientations. However, a notable exception arose with the “L” shape, which has openings on three distinct orientations, affecting its energy consumption. The north–south orientation proved to be the most favorable for all five architectural forms, offering better energy efficiency. In contrast, the east–west orientation was less advantageous and should be avoided due to the increased energy consumption it causes.

3.4. The Effect of Thermal Insulation of the External Envelope on Energy Consumption

To study the effect of the exterior envelope on the energy consumption of buildings, four alternatives were proposed:

Alternative 01 (Figure 19): Wall Insulation

This alternative aims to improve the insulation of the exterior walls by replacing the 0.10-m-thick exterior brick with a 0.15-m brick. A layer of 0.10-m-thick polystyrene insulation is added, followed by the installation of plasterboard panels.

Alternative 02: Roof Insulation

Roof insulation is optimized by replacing the existing concrete slope form and the 0.04-m-thick polystyrene insulation with:

-

A slope form made of insulating mineral foam produced with Airium technology, with an average thickness of 0.10 m.

-

A layer of 0.17-m-thick fiberglass insulation.

-

A suspended plasterboard ceiling with a thickness of 0.013 m.

Alternative 03: Window Replacement

In this variant, single-glazed windows are replaced by double-glazed windows containing argon gas, with a thickness of 0.013 m.

Alternative 04: Combined Approach

This alternative combines the previous three approaches by integrating wall insulation, roof insulation, and the use of double-glazed windows.

Table 6. Properties of the materials used for the four alternatives [50].

No.	Building Element	Layer	Composition	Thickness (m)	Thermal Conductivity (W.m−1.K−1)	Density (Kg/m3)	Specific Heat Capacity (J.Kg−1.K−1)
1	Exterior wall	01	Mortar	0.02	1.4	2200	1080
		02	Hollow brick	0.1	0.48	900	936
		03	Air cavity	0.05	0.024	1.22	1008
		04	Hollow brick	0.15	0.48	900	936
		05	Polystyrene	0.1	1.4	2200	1080
		06	Plasterboard	0.013	0.35	900	1460
2	Roof	01	Tightness	0.01	0.23	1000	1656
		02	Airium slope shape	0.15	0.09	400	1001
		03	Concrete slab with hollow block	0.21	1.45	1080	1450
		04	Glass wool	0.17	0.037	25	612
		05	Plasterboard	0.0013	0.35	900	1460

presents the properties of the materials comprising the building envelope for the four alternatives.

Figure 20 and Figure 21 illustrate the effect of exterior envelope insulation on the energy consumption of the five architectural forms for the cities of Béni Abbès and Adrar.

The results showed a significant reduction in energy consumption with improved insulation. Full envelope insulation represented the most significant results, followed by wall insulation and window insulation. On the ground floor, the heating consumption without insulation for the different architectural forms was approximately 27–30 kWh/m² for Béni Abbès and about 20–23 kWh/m² for Adrar. After full insulation, these values decreased to 16–21 kWh/m² for Béni Abbès and 11–15 kWh/m² for Adrar, representing a maximum reduction of about 45% for Béni Abbès and around 50% for Adrar.

On the first floor, the heating consumption without insulation for the different architectural forms was around 31–35 kWh/m² for Béni Abbès and about 22–27 kWh/m² for Adrar. After full insulation, these values decreased to 14–22 kWh/m² for Béni Abbès and 9–16 kWh/m² for Adrar, representing a maximum reduction of about 54% for Béni Abbès and about 63% for Adrar.

Regarding air conditioning on the ground floor, consumption without insulation ranged from 69–111 kWh/m² for Béni Abbès and from 120–177 kWh/m² for Adrar. With full insulation, it decreased to 47–71 kWh/m² for Béni Abbès and 83–116 kWh/m² for Adrar, representing a maximum reduction of about 35% for both cities. For the first floor, consumption without insulation ranged from 139–190 kWh/m² for Béni Abbès and from 216–281 kWh/m² for Adrar. With full insulation, it decreased to 87–123 kWh/m² for Béni Abbès and 144–188 kWh/m² for Adrar, representing a maximum reduction of about 34% for both cities.

Economic Impact Study

Table 7. Investment cost and amortization of insulation for the cities of Béni Abbès and Adrar.

Shape	Type of Insulation	Total Annual Gain (kWh/year)		Total Gain (USD)		Additional Investment Amount (USD)	Impact on Project Cost (%)	Payback Period (Years)
		Béni Abbès	Adrar	Béni Abbès	Adrar	Béni Abbès and Adrar	Béni Abbès and Adrar	Béni Abbès	Adrar
Initial Case	Wall insulation	4947.85	6609.22	196.66	262.70	3206.95	14.25	16.3	12.20
	Double glazing	838.20	1242.74	33.31	49.39	423.97	1.88	12.72	8.58
	Roof insulation	1143.76	1293.29	45.46	51.40	1599.55	7.11	35.18	31.11
	Total insulation	7455.05	9902.66	296.32	393.60	5230.48	23.25	17.65	13.28
“O” Shape	Wall insulation	2640.54	3635.13	104.95	144.48	1073.50	4.77	10.22	7.42
	Double glazing	595.62	910.28	23.67	36.18	461.07	2.045	19.47	12.74
	Roof insulation	1371.71	1631.66	54.52	64.85	1600.02	7.11	29.34	24.67
	Total insulation	5158.87	6905.41	205.05	274.47	3134.6	13.93	15.28	11.42
“L” Shape	Wall insulation	2200.78	1626.14	87.475	64.63	1204.82	5.35	13.77	18.64
	Double glazing	1594.55	894.850	63.38	35.56	461.07	2.05	7.27	12.96
	Roof insulation	2078.40	1235.27	82.61	49.1	1593.67	7.08	19.29	32.45
	Total insulation	4136.69	4152.14	164.42	165.03	3259.56	14.49	19.82	19.75
“U” Shape	Wall insulation	2777.76	3848.83	110.41	152.98	1509.42	6.71	13.67	9.86
	Double glazing	582.30	887.82	23.145	35.28	461.07	2.05	19.92	13.06
	Roof insulation	1526.11	1792.29	60.66	71.24	1596.02	7.096	26.31	22.40
	Total insulation	5321.05	7190.58	211.5	285.80	3566.52	15.85	16.86	12.47
Rectangular	Wall insulation	3278.71	4368.87	130.32	173.65	1072.94	4.77	8.23	6.17
	Double glazing	528.86	822.91	21.02	32.71	461.07	2.05	21.93	14.09
	Roof insulation	1066.16	1213.91	42.37	48.25	1595.08	7.09	37.64	33.05
	Total insulation	5343.4	7036.2	212.38	279.67	3129.10	13.91	14.73	11.18

presents a comparative analysis in terms of the annual energy gain, financial cost, and payback period for different types of insulation (walls, openings, roof, and total insulation) across the five architectural forms studied for the cities of Béni Abbès and Adrar. The results showed that thermal insulation provided energy and economic gains while having an impact on project cost and payback time. The payback time was shorter in Béni Abbès than in Adrar.

It was noted that the initial case offered the best results in terms of the annual energy gains for total insulation, reaching 7455.05 kWh/m² in Béni Abbès and 9902.66 kWh/m² in Adrar. This can be explained by a high form factor of 0.69, which implies the insulation of a larger surface area of walls in contact with the exterior. This influences the investment cost, which is relatively high compared with the other forms, reaching about USD 5230.48 with an impact of 23.25% on the project cost. However, the payback period remained moderate, with 17.65 years for Béni Abbès and 13.28 years for Adrar. In contrast, the payback period for double glazing was the shortest, at 12.72 years for Béni Abbès and 8.58 years for Adrar. This result can be explained by the fact that the openings in the initial case are not protected from direct solar radiation, unlike the other four configurations, where they are protected by courtyards, overhangs, balconies, or other structures. The longest payback period was associated with roof insulation, with 35.18 years for Béni Abbès and 31.11 years for Adrar.

For total insulation of the buildings, the rectangular form had the least impact on the project cost, with an increase of 13.91% as well as the shortest payback period of 14.73 years for Béni Abbès and 11.18 years for Adrar. In contrast, the “L” shape presented the longest payback period, reaching 19.82 years for Béni Abbès and 19.75 years for Adrar. For wall insulation, the rectangular form proved to be the most efficient, with a payback period of only 8.23 years for Béni Abbès and 6.17 years for Adrar, while having a low impact on project cost with a 4.76% increase. On the other hand, the initial case showed the longest payback period for Béni Abbès, reaching 16.3 years with an additional investment rate of 14.24%, while for Adrar, the “L” shape had the longest payback period of 18.64 years with an additional investment rate of 5.35%. For roof insulation, the payback period remained relatively high. In Béni Abbès, the “L” shape had the shortest payback period at 19.29 years, with an investment cost increase of 7.08%. In Adrar, the “U” shape stood out with the shortest payback period, estimated at 22.4 years, with an investment cost increase of 7.09%. In contrast, the longest payback period was observed for the rectangular form, with a duration of 37.64 years for Béni Abbès and 33.05 years for Adrar.

3.5. Impact of the Width of Horizontal Solar Protections on Heating and Cooling Energy Consumption Based on Orientation

To study the impact of horizontal solar protection width on heating and cooling energy consumption in the cities of Béni Abbès and Adrar, the master bedroom on the first floor of the rectangular variant (Zone 3) was selected for analysis, as illustrated in Figure 10e. This choice was justified by the fact that the openings in this zone directly face the exterior, allowing for a more precise comparison.

Figure 22 and Figure 23 illustrate the effect of horizontal solar protection width on heating and cooling energy consumption based on orientation for the master bedroom on the first floor of the rectangular-shaped building in Béni Abbès (Figure 22) and Adrar (Figure 23).

The results showed a reduction in cooling energy consumption and a slight increase in heating energy consumption as the width of the solar protection increased for the south, east, and west orientations. However, for the north orientation, the energy consumption varied only slightly.

In Béni Abbès, for an east-facing orientation, the cooling energy consumption was 135.66 kWh/m² in the absence of solar protection, while the heating energy consumption was 31.78 kWh/m². The addition of solar protection reduced the cooling energy consumption, with a maximum reduction achieved at a width of 1.5 m, lowering it to 118.56 kWh/m², representing a savings of 17.1 kWh/m². However, this protection led to an increase in heating consumption, reaching 36.92 kWh/m², an increase of 5.14 kWh/m². This variation occurred because solar protection reduces solar radiation penetration, leading to lower indoor temperatures. As a result, cooling demand decreases during summer, while heating demand rises in winter.

For a south-facing orientation in Béni Abbès, the cooling and heating energy consumption changed from 126.07 kWh/m² and 11.9 kWh/m², respectively, without solar protection, to 108.05 kWh/m² and 20.71 kWh/m² with solar protection. This resulted in a cooling energy savings of 18.02 kWh/m² but an increase of 8.8 kWh/m² in heating consumption.

In Adrar, for an east-facing orientation, the cooling energy consumption reached 212 kWh/m², while the heating energy consumption was 25.95 kWh/m² in the absence of solar protection. The installation of 1.5-m-wide solar protection resulted in a maximum cooling energy reduction, bringing consumption down to 193.59 kWh/m², a savings of 18.39 kWh/m². However, this protection increased the heating consumption to 29.25 kWh/m², an increase of 3.3 kWh/m².

For a south-facing orientation in Adrar, the cooling and heating energy consumption shifted from 202.3 kWh/m² and 11.42 kWh/m² without solar protection, respectively, to 183.24 kWh/m² and 18.08 kWh/m² with solar protection. This represents a cooling energy savings of 19.05 kWh/m² but a rise of 6.66 kWh/m² in heating consumption.

4. Conclusions

The main goal of this study was to propose a method for designing energy-efficient buildings, helping designers make informed decisions in the Saharan climate. To achieve this, we compared the energy efficiency of an existing reference building in Béni Abbès with four architectural variants inspired by traditional Saharan architecture, following the specifications provided by the Algerian State.

The results highlight the strong potential of these variants to reduce energy consumption compared with the reference building. They also demonstrate the possibility of constructing energy-efficient buildings without excessive cost increases and with a relatively short payback time. For example, in Béni Abbès, the “O”-shaped variant showed a 5.18% increase in construction costs while using the same materials as the reference building. This configuration reduced the air conditioning consumption by about 38% on the ground floor and 27% on the first floor, with a payback time of 5.89 years. Similarly, in Adrar, the “L”-shaped variant increased the construction costs by 6.31%, offering a 31% reduction in air conditioning consumption on the ground floor and 32% on the first floor, with a payback time of 4.46 years. These results confirm the suitability of these solutions for the Saharan climate, ensuring a balance between energy performance, initial cost, and long-term investment profitability.

Improving the exterior envelope insulation is also key to optimizing the overall energy performance by enhancing passive measures and reducing heat transfer. For example, complete insulation of the “U”-shaped configuration increased the investment costs by 13.93%. However, it resulted in remarkable energy efficiency, reducing air conditioning consumption by 57% on the ground floor and 53% on the first floor in Béni Abbès, with a payback time of 15.28 years. In Adrar, the same configuration reduced the air conditioning consumption by 53% on the ground floor and 48% on the first floor, with a payback time of 11.42 years.

Solar protections also helped reduce the energy consumption for air conditioning, particularly for the south, east, and west orientations. In Zone 3 of the rectangular variant, for the city of Béni Abbès, the maximum energy saving recorded was 18.02 kWh/m² for a south-facing orientation with a solar protection width of 1.5 m. However, an increase in heating consumption of 8.8 kWh/m² was observed.

For the city of Adrar, the maximum savings reached 19.05 kWh/m² for a south-facing orientation with a 1.5-m-wide solar protection, accompanied by a 6.66 kWh/m² increase in heating consumption.

National policy should encourage the integration of passive techniques inspired by local vernacular architecture, especially in arid and desert climates. This approach could be implemented by mandating specific guidelines for designers in the early phases of projects. These parameters were proven to be effective in this study, in other previous research, and over the centuries in the vernacular architecture of desert regions.

The passive design strategies examined in this study, including compact urban fabric, thermal inertia, solar protection, and natural ventilation, can be scaled to larger buildings such as offices and public spaces. While the core principles remain effective, adaptations are needed to account for occupancy patterns and internal heat loads.

To ensure effective implementation, specific directives could be mandated in project specifications, guiding designers from the early design stages. These principles demonstrated their effectiveness in this study, in previous research, and through centuries of vernacular architecture in desert regions. Key parameters for optimizing energy efficiency include:

• Adhering to a shape coefficient (CF) that must not exceed a certain value defined according to the specific region;
• Using solar protection for openings and installing double-glazed windows;
• Selecting construction materials suited to the climate, prioritizing those with good insulating properties while ensuring a low investment cost and a quick payback period;
• Favoring north and south orientations;
• Adopting a compact urban fabric to reduce energy losses and optimize thermal exchanges;
• Integrating semi-underground buildings to benefit from the thermal inertia of the ground.

Though this approach may increase the initial project costs, investments will quickly pay off in desert climates due to energy savings. Authorities should prioritize funding thermal and energy improvements in buildings instead of subsidizing energy prices. Such a policy would:

• Significantly reduce the household energy consumption;
• Minimize environmental impact by lowering the greenhouse gas emissions related to energy production and consumption;
• Enhance the country’s energy export capacity, boosting the national economy;
• Preserve non-renewable energy resources for future generations;
• Prevent future costly and complex thermal rehabilitation by incorporating efficient and sustainable solutions from the start.

5. Limitations

This research had several limitations. First, it focused exclusively on the Saharan climate without considering other Algerian climates such as the Mediterranean, arid, and semi-arid climates. It would be relevant to expand the study to cover all climatic zones of the country in order to provide recommendations tailored to designers working in these specific regions. Furthermore, the study did not explore the profitability of renewable energy sources, which could enable the transformation of these buildings into fully energy-autonomous structures. These limitations can be handled in future works.