A methodology towards delivery of net zero carbon building in hot arid climate with reference to low residential buildings — the western desert in Egypt

Net zero carbon building (NZCB) is considered an important approach for reducing carbon emissions (CE), which may be due to the exponential rise of energy use and greenhouse gas emissions in buildings industry. Delivery of NZCB on its life cycle is considered a challenge due to its complexity and research deficiency for examining CE life cycle assessment (LCA) to reach NZCB in early design phase, especially in hot arid climates. The present proposal aims to develop an experimental methodology for NZCB in hot arid climate, with reference to the western desert region in Egypt as an experimental location due to its hot climatic characteristic, which includes the most common climate in Africa. The study was held on three models for a single floor residential unit with fixed area 110 m2, using DesignBuilder software for annual simulation and One-click LCA software for 50 years LC simulation. The effect of conventional construction materials replacement, as a passive technique, and application of solar panels, as an active technique, was examined. Simulation results indicated that there was a reduction in carbon emissions through LCA reached approximately 85% when applying both passive and active techniques on the experimental models, as well as a reduction of approximately 101% in energy consumption. Implementation and integration between passive and active systems in early design phase are evident for achieving net zero CE target in hot arid climate.

are few case studies for NZCBs; however, there is a growing number of NZCBs in relatively hot and humid subtropical climates [13] but, unfortunately, very limited attempts to deliver NZCB in arid regions like Egypt. As per study for evaluation of existing NZCBs, integration of three main strategies to achieve net zero is as follows: (1) reducing energy demand through the use of low-energy passive design measures; (2) increasing efficiency through using energy-efficient building systems and technologies; and (3) using RE sources like photovoltaic (PV) panels to supply the remaining energy demand and mitigate CE target [13,14]. Integration of systems and technologies, which are based on principles of sustainable site planning, energy use reduction, low-carbon construction and material selection, water use minimization, building flexibility and adaptability, had been integrated into NZCB [15]. There is limitation in previous research for addressing NZCB outlines and challenges that face delivering NZCB which fracture our NZCBs understanding, therefore, more researches on NZCB outlines and challenges are needed.
NZCB delivery system is based on eight boundaries, which include a detailed description and role for each NZCB boundary (e.g., geographic, climatic, density, building life cycle, etc.) [16]. These boundaries are considered as an innovative approach for examining NZCBs in terms of system integration for future research and practice. Noticing that the early design phase parameters connect and integrate all NZCB boundaries, the criteria for each outline can be analyzed in the early design phase with respect for project's circumstances.
The challenges that face NZCB delivery can be summarized in lack of understanding NZCB principles, unclear NZCB policies with conflict in NZCB management and priorities, and with underlying lack of theoretical grounds knowledge as well as NZCB boundaries. These main challenges are (1) designed target vs. operating target, (2) testing and commissioning, and (3) occupant behavior, comfort, and satisfaction. The designed target vs. operating target can exceed 3.5-5 times initial design estimations in examined case studies which make early design phase simulation for each design parameter a necessary process [13]. Reliability of RE generation is an essential factor for achieving net zero carbon objective and to be effective for a specific context. Finding the best combination between passive and active RE systems design strategies is also essential for achieving best CE target [14,15].
By reflecting previous literature on the case study location, the western desert in Egypt is rich with climate-responsive vernacular architecture. Settlements in the Dakhlah Oasis are collected structures with a strong defensive character; constructions are bound to each other to cope with the hot arid environment. These settlements are characterized by traditional mud-brick Islamic architecture with wind catchers for natural ventilation enhancement (Fig. 2) [17]. On the other hand, the Egyptian government was building housing projects in Al-Wadi Al-Jadid governorate with the same conventional construction materials (concrete and fired bricks) with no consideration to climatic conditions (Fig. 3).
Summarizing the research's knowledge gaps for this research, limitation in conducted studies for standardizing an approach of residential structures CE estimation over their life cycle stages in hot arid climates, with no distinctive definition for NZCB in such climatic region. In addition, there is deficiency in embodied CE data for conventional and alternative construction materials in the case study region. Additionally, the absence of understanding among designers and developers results in climatic responsive design application limitations in Egypt. Addressing LCA as a tool for CE reduction in buildings in early design phase can be a valuable tool in CE mitigation [6]. Moreover, the raise of awareness to nonprofessional users and designers to such problem has a great role in due course. The next part of this research investigated the building's life cycle assessment based on carbon emissions (LCCO 2 ), embodied CE for applying conventional and alternative construction materials. It was felt necessary to define the methodology towards delivery of NZCB in the western desert in Egypt through design strategies and techniques  integration, which may be applicable for a governmental typology of low-rise residential buildings in a hot arid climate. The experimental yearly and LCA simulations were conducted by using DesignBuilder and One-click LCA software.

Methodology
Implementing inventory from literature review to be an input data for experimental/ simulation part, Fig. 4 illustrates three experimental models with fixed area (110 m 2 ) with reference to the Egyptian governmental prototype in the western desert. Base model is the typical prototype that was constructed by the Ministry of Housing. Alternative model is the same design of base model but with alternative construction materials. NZCB model had the same area with the implementation of passive and active techniques. Simulation process consisted of two main parts: modeling and simulation on DesignBuilder software for annual readings and then implementation of data on One-click LCA for LCA readings. Optimization process in simulation of NZCB model was applied for achieving zero LCA CE target. In order to achieve precise experimental results, system boundaries were investigated as follows:

Methodology of system boundaries
The functional unit for this research is kg CO2e/m 2 /50 years for a residential building, and the target is to reach net zero carbon target on its LCA. There were assumptions for simulation process, which were constants for the three models, such as the service life of construction materials that were estimated with the same life span of the building (50 years). Traveling distances for the site transportation were assumed with 60 km per truck which is adequate for average transportation distance in conventional construction site in Egypt. The end of life scenarios was neglected in simulations due to its minor impact on the whole LCA [6]; at last, the main aim to use passive techniques was for overcoming mechanical HVAC and reducing the operational CE without testing indoor thermal comfort in detail, as it has several considerations that need to be carefully studied in further research.

Geographic and climatic outlines
The experimental study was located in Al-Kharj, Al-Wadi Al-Jadid, the western desert in Egypt, with climatic classification of hyper-arid climate. Table 1 showed values for climatic characteristics with reference to Bahariya Oasis weather station in the western desert, as presented for warmest and coldest months and their temperature values, relative humidity ranges, global radiation, and average annual rainfall. Almost 73% of the year is outside the human comfort range, with average cooling hours needed were 2795 out of 8760 yearly hours [19]. These climatic data were implemented in simulation software as a part of CE simulation.

Model parameters and architectural style
The Egyptian government attempted to follow vernacular architecture typology to harmonize with the geographic nature in new housing projects in the western desert (Figs. 5 and 6). This residential prototype had an area of 110 m 2 on footprint, 5% of total land plot [20][21][22]. The architectural style is characterized with domes and vaults on the roof formation; however, it was built with the conventional construction materials used in construction industry in Egypt (masonry blocks and reinforced concrete), without taking into consideration applying any vernacular passive techniques for natural ventilation like wind catchers. The prototype floor plan had living spaces for a family of four individuals as illustrated in Fig. 7.

Conventional and alternate construction material properties
Experimental models followed literature data gathering for their construction materials, base model was built with conventional construction materials (concrete and masonry blocks), while alternative model and NZCB were built with alternative construction materials for better environmental and carbon emissions performance (CEB and low-E concrete). Table 2 described the general information for the experimental three models. Also Tables 3 and 4 illustrated the conventional and the alternative construction materials specification which were used as an input data for annual simulation study.

Ventilation methods and vernacular passive solutions application
The base model followed the governmental typology with partially mechanical conditioned with split units without taking into consideration any passive techniques application for natural ventilation enhancement (Fig. 8). However, the western desert accommodates buildings with vernacular architectural style, which is responsive to hot arid climate in terms of indoor thermal environment and human comfort, using passive techniques in construction materials selection and natural ventilation enhancement (Fig. 9). The alternative model and NZCB model used natural ventilation techniques as presented in Fig. 10 without adding new masses for wind catchers for maintaining the same construction materials volume.

Simulation process
Three models were modeled on DesignBuilder using building block technique, and then comparable simulation for annual performance and carbon emissions took place. The first model is the base model (BM), which mimics the governmental typology for lowrise houses in western desert with its conventional construction materials and with partially air-conditioned split system. The second model is the alternative model (AM) with application of passive techniques, which are replacing conventional construction materials with alternative materials with lower embodied carbon, and natural ventilation enhancement for overcoming the AC usage. The third model is testing the selective active systems on the alternative model which is the implementation of onsite PV panels to reach net zero carbon building (NZCB).

Design builder models input data
Design Settings & Activity The experimental models were built on DesignBuilder interface, with orientation of western entrance façade as presented in Fig. 11. Unit floor plan, as presented in Fig. 7, is a typical one floor home with entrance lobby, reception, kitchen, three bedrooms, and bathroom. Implementation of activity template of residential dwelling occupied with four persons and standard metabolic rate (0.5) and activity of seated quiet template; natural ventilation technique was introduced in the activity settings. Occupancy of home appliances was introduced as well with power intensity of 13 W/m 2 . Construction materials input Implementation of construction materials specifications for three models, as concluded from the literature review, into DesignBuilder interface took place. The conventional materials were applied as presented in Table 3 for base model (BM); walling material is fired bricks of 120 mm thickness with outer/inner cement plastering with thickness 20 mm without insulation, and the roofing system is reinforced concrete 200 mm thick with outer/inner 20 mm plastering without insulation. The openings of base model were introduced as uninsulated with single glazed of 6-mm clear glass with R-value 0.15 m 2 K/W [25].
As for the alternative and NZCB models, alternative materials were applied as presented in Table 4 with their heat specification and embodied CE data as a passive technique. Walling and roofing materials are CEB 380 mm thick with inner/outer cement mortar for plastering with wall insulation for better thermal performance; openings were introduced as insulated 6mm double-glazed glass complied with Egyptian energy-efficient building code with R-value 0.58 m 2 K/W [14]; both models had the same wall/window ratio (opening dimensions 900 mm × 900 mm in each space). Figures 12 and 13 illustrated the construction materials in wall sections for BM, AM, and NZCB with their thermal specifications and embodied CE, which were implemented into DesignBuilder interface.

One-click LCA platform input data for LCA simulation
One-click LCA platform is an automated whole life cycle assessment online database software that helps in calculation and reduction of the environmental impacts of certain building or product as well as its embodied CE calculation. For early carbon optimization, it amends a large generic and environmental product declarations (EPD) database for global manufacturers which is considered among the largest of its kind, and it is also designed to find easily the information for material EPDs; it also complies with EN/ ISO standards and 40+ certifications [26,27]. The software provides also the embodied   carbon benchmark for construction materials, which gives information for the LCA first stage for any construction project, that can be useful for building industry professionals in analysis for whole LCA [28]. Importing gbXML models and simulated annual data from DesignBuilder to Oneclick LCA platform of the three models for LCA simulation with reference to LCA tool EN-15978. Importing data includes the scope of construction process, project's type, and structure type. Mapping construction materials for each model took place by linking each construction material (conventional and alternative) with local suppliers' database on One-click LCA platform; this process is important for calculating embodied, LCA, CE, and energy consumption.
Implementing building floor(s) area and annual energy consumption (Table 5) for each model as concluded from DesignBuilder simulation output. Afterwards, importing constant data for the three models of construction site operations phase with reference to construction methods followed in Egypt, like transportation distances, was estimated with 60 km per truck. Finally, importing LCA calculation period with 50 years.

Estimation of site PV needed to achieve net zero target and its LCA CE
RETScreen Expert PV simulation software is a reliable tool for estimation of PV capacity, cost, and CE [14,29]. Using it as a tool for estimation, the solar power needed to reach net zero carbon target with reference to geo-location for Al-Wadi Al-Jadid governorate that was implemented into the software to calculate the exact power needed. In order to reach zero carbon target in the NZCB model, selection and implementation The PV system also emits 0.83 tons CO 2 e per year with reference to emission factor 45 gCO 2 -e/kWh [30]. This data was imported again to One-click LCA software to offset CE and achieve zero CE target.

Results and discussion
Three models were simulated on DesignBuilder and One-click LCA software. The base model (BM) was following the governmental prototype in Al-Kharj Oasis -the western desert with conventional construction materials as discussed in literature review; the alternative model (AM) was a transitional model between BM and the NZCB model, with the application of low CE alternative construction materials as one of many passive techniques can be applied; and the last model is the NZCB model which followed the alternative model with implementation of 10 KW solar panels array as an active technique to offset CE; results for simulation on both software were presented in Table 6 as follows:

Construction materials embodied CE and their benchmark
Based on carbon emissions simulation data on DesignBuilder for BM, AM, and NZCB construction materials, a drop in embodied carbon emissions is in favor of alternative construction materials with 56% than conventional materials; this emphasizes the importance and priority of alternative materials implementation and replacement in construction industry. Figure 14 shows the reduction in embodied carbon emissions. Figure 15 shows One-click LCA results for carbon benchmarking between BM and AM.

Annual energy consumption
Based on the input data for the three models to DesignBuilder, BM was partially air-conditioned with un-insulated construction materials consumed amount of 32,285 kWh/yr., both AM and NZCB models are naturally ventilated with better construction materials insulation consumed 14,941 kWh/yr., and there is a massive drop with the amount of 53% in annual energy consumption between the first model and both second and third models, taking into consideration users environmental control culture, their behavior for natural ventilation, and application of high mass construction materials with proper insulation. Figure 16 illustrates the simulation results with application of solar panels as an active technique for NZCB which offsets the energy consumption and exports 3396 kWh/year to the official grid to overcome the embodied CE and PV system CE to reach zero CE target.

Passive and active techniques applied
Using passive techniques, that were popular in the western desert vernacular architecture, to be the experimental methods applied to differentiate between BM, AM, and NZCB simulations. These passive techniques were changing the conventional construction materials to low-carbon alternative materials in addition to the natural ventilation enhancement, which dropped annual energy consumption as stated in previous point. Regarding the active technique applied for NZCB model only, implementation of PV with 10.175 KW power which generates 18,337 kWh/year, and produces extra 41 Ton CO 2 e over its life cycle. The selection and calculation of PV sizing and its CE took into consideration offsetting the overall LCA CE and energy consumption for NZCB model. Figure 19 and Table 7 illustrated the three models of LCA as per elements; operational phase electricity use has the highest contribution in carbon emission elements with the amount of 1282 ton CO 2 e for (BM) LCA simulation, (AM) optimization reduced to amount 604 ton CO 2 e with passive techniques implementation, while (NZCB) scored −9.8 ton CO 2 e with almost 103% reduction lower than BM.   The consumed electricity was relatively low due to the application of climatic responsive design, which integrated various passive techniques such as high thermal mass envelope and wind catchers, in addition to the user's awareness with power consumption optimization.

Discussion
In comparison with NZCB model simulation results, the present study concluded that overall LCCO 2 reduction was 614 ton CO 2 e in 50 years (12.28 ton CO 2 e/year and 111 kg CO 2 e/m 2 /year) with the implementation of PV farm that produces 18,337 kWh. Contradictorily, the building may consume 14,941 kWh (135 kWh/m 2 ). Integration of various passive and active techniques has a great potential for a massive reduction in operational energy consumption, which can reduce the PV farm size and cost accordingly, in order to achieve overall LC net zero CE.

Conclusions
The research shows the possibility of achieving building with net zero carbon emissions on overall life cycle stages (from cradle to grave) with reference to time (e.g., 50 years) taking into consideration NZCB strategies, outlines, and its challenges" and its unit of measurement for carbon emissions performance that can be simulated/measured with reference to over life cycle (kg CO 2 e/m 2 /50 years = zero.) After examining NZCB outlines, we can add eight outline which is early design phase optimization regarding passive and active techniques implementation and integration. Putting scenarios for design stages with proper criteria will reduce dramatically the gap between early design CE and actual operational CE, which will ease the process for decision-makers regarding achieving net zero carbon target.
For reaching the best practice for NZCB in the western desert, integration of passive and active techniques is considered a viable solution for mitigation of carbon emissions and reaching net zero carbon buildings target over buildings life cycle. PV as an active technique used in this research can be increased in power for reaching net zero target to 10.175 KW system.

Limitations
In order to reach more detailed LCA CE simulation, software used in this research like DesignBuilder, One-click LCA, and RETScreen needs to accommodate more inventory data for suppliers and embodied CE data for various sustainable materials. Moreover, there is a considerable lack in CE studies in hot arid regions like Egypt, which makes comparison with previous researches with the same climatic settings difficult. This research was conducted to test the methodology for achieving NZCB in hot arid climate with reference to literature data and the case study, which can be taken as a guideline in design phase decision-making process. It did not address uncertainty analysis as some of the concluded or estimated key variables are subjected to change