In Passive solar heating, the building envelope composed of walls, floor, roof and windows is designed to collect, store, reflect and distribute solar energy in form of heat in the winter and reject heat in summer. Energy from sun is being used, or controlled, through the physical makeup of the spaces.

Passive solar design depends on the location of the building, orientation, geometry, envelope and other factors to achieve the target.

Solar passive design technique puts forth light upon the orientation of the building as it affects the solar radiation, daylight, and wind. In a hot and humid climate, the orientation of the buildings should be along the long axes in the east-west direction. This will eventually place the longest façade in north and south direction along with a short wall facing the east and west direction ( Figure 1 ).

The form of the building can affect solar access, wind exposure, rate of heat loss or heat gain through the external envelope of the structure as well as airflow pattern around the structure, which will also affect the ventilation. The compactness of the structure can be measured with the help of the ratio of surface area to the volume (S/V). The lowest S/V ratio is believed to be of circular geometry. Thus, the circular form of the building becomes the most energy-efficient in a hot and humid climate.

The building envelope is what separates the interior of the building from the exterior; it consists of windows, outside walls, roofs, and floors.

Windows: to balance the amount of light admitted into the structure with the control of solar heat gain and conduction of energy. Up to 40% of a home’s heating energy can be lost and up to 87% gained through glazing. Window performance is a combination of several factors.

Thermal transmittance of the walls is the heat flow rate in a steady state divided by area and the temperature difference between the surroundings on each side of a system. Solar heat gain coefficient (SHGC), represents the fraction of solar heat that enters the window and becomes heat; it includes both directly transmitted and absorbed solar radiation. The lower the SHGC, the less solar heat the window transmits through the glazing from the exterior to the interior and the greater shading capability. Visible transmittance (VT) of the glass VT (0 to 1): percentage of visible spectrum that is transmitted through the glazing. LSG: The ratio SHGC/VT: Light-to-solar-gain (LSG) ratio is a gauge of the relative efficiency of different glass types in transmitting daylight while blocking heat gains.

It is important not only to make sure to specify high-performance windows but also to carefully consider how they are incorporated into the building design. Solar heat gain through appropriately placed windows can help offset the amount of heat a building needs during colder months. During the summer months, this needs to be counteracted with shading to prevent too much heat from the sun from getting into the building, causing overheating.

Walls: walls consist a major element in a building envelope. One of the characteristics is the thermal transmittance, which represents the heat flow rate in the steady state divided by area and the temperature difference between the surroundings on each side of a system. Thermal mass of the exterior surface that receives direct sunlight during the day and placement of insulation with respect to the building façade.

Roof: a roof of a high-performance building is especially important because it is a major area for heat transmission due to its generally large area and exposure to the sun:

Using surfaces with high albedo (a measure of the reflectivity of solar radiation) for roofing can reduce the ambient air temperature so that the entire area is cooler, which helps reduce the thermal load on the building as well as the surrounding neighborhood.

The Solar Reflectance Index (SRI): measures how hot materials are in the sun–used to easily describe the amount of solar energy reflected by roofing materials. A building with light-colored shingles and an SRI of 54 would reflect 54% of incident solar energy, & would be very cool relative to a building with conventional dark shingles.

Insulation and thermal bridges: Insulation is a critical element of the building envelope.

Insulation acts as a barrier to heat flow and is essential for keeping the home warm in winter and cool in summer. Some types of insulation can also help with weatherproofing and soundproofing. Passive House makes the most of the envelope by super-insulating the building in order to minimize heat loss. The result is a significant increase in the thermal performance expected from the building envelope. When a material bypasses the insulation, it is known as a thermal bridge and can significantly reduce the effectiveness of insulation, especially if that material is very conductive, like metal. Minimizing repeating thermal bridges and aiming for continuous insulation where possible, helps make the most of the insulation within the building envelope.

ECOTECT software is an environmental analysis tool that allows architects and designers to simulate building performance right in conceptual phase [18] . It allows to calculate building’s energy consumption by simulating its context within the environment, especially on dealing with solar heat, its nature for day-lighting, natural airflow for ventilation, and its energy consumption for man-made systems such as Air Conditioning and Lighting.

Lebanon, with a total area of 10.452 km², is located in the East Mediterranean and extends over some 210 km along the coast and 50 km inland. The climate in Lebanon is characterized in general by the existence of a cold winter season, a hot summer season and two mild mid-seasons: the summer season extends from July till September with August being the hottest month.

Lebanon is divided into four regions based on temperature, relative humidity and solar radiation as per Table 1 . These climatic parameters affect the heating and cooling requirements in buildings. Beirut, which is our case study, fall under Zone 1: Coastal, characterized by a warm and short winter, and a hot and humid summer.

The house is a single floor one, with a rectangular shape of 12.5 m x 8 m, and a height of 3 m, the long façades face the east and west. The envelope has an infiltration rate of 0.5 air change per hour (ACH) with a U-value of 4.021 w/m². C. The external walls are composed of 1.5 cm plaster, 15 cm hollow block, and 1.5 cm plaster. Slabs are made from reinforced concrete without insulation, same for the roof. Windows are made from 6mm single glass panes.

Table 2 summarizes the U values of the base case model, while Figure 2 shows the building floor plan.

To note that the envelope thermal transmittance (U) is higher than the maximum value set by the local building code for the same region of 2.5 W/(m² K) [19] .

The house is divided into two zones. The first one is composed of bedrooms, and it is assumed that is occupied only at night during sleeping hours, while the second one is the kitchen with salon and living room, and it is assumed to be occupied during the day and in the evening. The zoning is used to design the operation such as number of people which is equal to 3, the type of active system which is mixed that is used to provide for both heating and cooling for the house, and operating hours depending on each zone schedule.

The weather file used to simulate Lebanon weather data for Beirut area is the one of Rafiq Hariri International Airport [20] . For this case study, we will adopt the following parameters: internal loads, including occupants, lighting, and appliances, are approximated to be 5 w/m² of sensible load, and 3 w/m² of latent load. The thermal comfort band is set to be between 18℃ and 26℃.

Individual household energy consumption in Lebanon estimates vary from 0.45 to 2.2 MWh per household [21] . Although no studies were found identifying per capita residential energy consumption, several values were obtained for total per capita energy consumption varying from 2.0 to 2.6 MWh capita [22] - [24] .

In 2007, final power consumption remains concentrated at 65% in the residential sector and the tertiary sector, then at 30% in the industrial sector, and finally at 5% in the other sectors (agricultural and administrative).

In Lebanon, 67% of housing consists of flats in several-floor buildings. 52% of the housing units are 4 to 5 rooms. The average housing area in Lebanon is 129.3 m². 21.8% of the housing units are power heated, 25% use fuel oil, 27.3% gas (LPG), 17.6% wood or coal [25] ( Figure 3 ).

The direct normal irradiation and the annual sun path analysis for the base case model are shown in Figure 4 and Figure 5 respectively.

The simulation of the base case model showed that the annual cooling load is 27.25 kw/m² with a peak of 3.95 kw on the 9th of July, while the annual heating load is 1.68 kw/m² with a peak of 0.99 kw on the 6th of February.

Table 3 summarize the monthly cooling and heating loaded in wh, while Figure 6 represent the combined heating and cooling loads during the year.

To know how to interact with the model and what improvement to do, we will check the passive gains breakdown graph in Figure 7 . In order to evaluate the passive design strategies that will be studied, different analysis functions were used, mainly the ones related to passive gains. The first function used is Conduction loads through the fabric; these loads refer only to the gains due to differentials in air temperature between inside and outside the space. The second function used is Indirect solar loads through opaque objects, which refers to additional gains due to the effects of incident solar radiation on the external surface of exposed opaque objects.

The solar radiation acts to raise the external surface temperature which in turn increases the conducted heat flow. Also, Direct solar gains through transparent objects is a function used to analyze the effectiveness of passive strategies; these loads refer to solar radiation entering the space through a window.

Many fundamental parameters for the passive design concept will be studied to evaluate their function and role on different passive design strategies.

The orientation of the house influences cooling and ventilation load. The natural ventilation flow rate that is needed for passive cooling during hot season which constitutes 5 months of the year. A range of orientations has been modelled and tested using Ecotect weather tools to determine the best orientation of the house in terms of heating and cooling loads.

Simulating the different orientations showed that the best orientation is to have the longest facade with more window/wall ratio facing north (192.5˚ angle) as in Figure 8 , in order to minimize the solar gains during hot season. Results are presented in Table 4 .

A study done in residential estate planning in 2013, in a climate similar to our base case conditions, Nanjing in China, Ma’anshan belongs to hot-summer and cold-winter region [18] .

Different elements of the envelope were studied, such as walls, slabs, roofs and glazing. Each having many alternatives, using Ecotect software, we were able to identify the best option resulting in the lowest cooling load.

In our case, we will choose option W1 from Table 5 since material are available in market, in order to achieve better load optimization.

The slab doesn’t have much impact on the heating and cooling loads, so we will pick an alternative consisting of 10cm concrete floor on top of 1.5 m soil + parquet 5 cm, with a total load of 28.73 KWh/m².

A small improvement in roof will lead to a high saving in cooling load since it is the most Exposed to solar radiation and receives huge solar heat. We will adopt R2 from Table 6 , since material are available in the market considering better improvement in terms of cooling loads.

Glazing and shading are critical in passive design strategies since they have a high-impact on both passive heating and passive cooling of the building. Double-glazing window with different thicknesses of panels and air void has been studied and the output was very remarkable.

Moreover, different shading element proportions have been modelled and tested to control direct solar gains during hot season to avoid increasing thermal storage and undermining the effect of this setting is related to the wind speed and direction by a parameter defined as wind sensitivity used in the thermal simulation of natural ventilation that is used to passively cool the house.

Option G1 will be adopted from Table 7 since the difference from other types is considerable, and will help achieve our target related to load demand optimization.

Adding shading element as overhang for west façade windows which will receive the most solar direct + diffusion radiation, as shown in Figure 9 , will help reduce the cooling needed from base case model 27.25 Wh/m² to 23.73 Wh/m² while heating load will almost remain the same at 1.61 Wh/m².

Based on the alternatives simulated previously, we will combine the best options of each case to get the best model in term of passively reducing energy consumption.

Combination of best alternatives without shading devices will give a space load of 17.63 KWh/m² ( Table 8 ), representing a reduction of 39% from the base case model, which is remarkable.

Adding the impact of shading devices will improve the reduction to 46.4%.

Simu_lation results of best-case model are shown in Figure 10 , while the passive gain breakdown of the optimized model is presented in Figure 11 .

Comparing the passive gains breakdown will show a reduction in the red color related to conduction and yellow color related to direct solar gains, which is logical due to the improvement applied for the envelope including insulation, glazing with shading and best orientation.

As a percentage of total energy saving, as stated above, on average 40% of all energy consumed by buildings worldwide is used for space heating and cooling, we get 0.464 × 0.4 = 0.185 meaning that about 18.5% of the total energy bill of a house.

To calculate financial savings, assuming the cost of 1 KWh on average is 0.22$ [27] , and average annual electricity consumption per capita in Lebanon is 3,495.68 KWh [28] , and we have 3 habitants in the apartment, we get: 3495.68 × 3 × 0.22 × 0.185 = 426 $ of total annual saving, which is remarkable.

After discussing different passive design strategies in order to achieve the maximum energy reduction to our base case model, we will focus on using renewable energy system to cover the remaining energy needed, notably the solar photovoltaic panels system.

Solar photovoltaic systems generally consist of six individual components: the solar PV array, a charge controller, a battery bank, an inverter, a utility meter, and an electric grid. The correct installation of all of these components determines how efficient the solar panels are. However, a charge controller and battery bank are optional.

In order to determine the total electric power needed on annual basis, we will consider that the building is using efficient lighting appliance and electric equipment. The equipment and systems with their corresponding electric power consumption that are taken into consideration are shown in Table 9 .

To design the PV system in a way to cover all the building’s annual energy needed, we will use an average per day of: 5,516/365 = 15.1 KWh/day.

The average solar insolation of the building under study 5.2 KWh/m²/day is shown in Figure 12 .

To estimate the electricity generated by a photovoltaic system, we will use the following formula:

$E=A\times r\times H\times PR$ [30] (1)

Where: E = energy (KWh);

A= total area of solar panels (m²);

r = solar panel yield (%);

H = annual average solar radiation on tilted panels;

PR = performance ration, coefficient for losses.

In our case, using 8 PV module Longi LR5-72HPH of 545 w, we have:

A = 2.2 × 1.1 = 2.42m²; r = 21.3%; PR = 0.8; Average daily solar radiation = 5.2 KWh/m² We get E = 8 × 2.42 × 0.213 × 5.2 × 0.8 = 17.15 KWh/day > 15.1 KWh/day.

So, we will use 8 panels of Longi LR5-72HPH.

For the battery bank calculation, we will assume that we need backup system for 24 hours, and we need the system to produce 4A, so the total power needed will be: 4 × 24 × 220 = 21,120 Wh.

Using losses factors of 80% for DOD and 60% for total discharge [30] and using a system of 48 V, size needed will be a follow:

$\frac{Power}{LossxDODpowerxSystem\left(V\right)}$ (2)

Applying the formula, we will get: $\frac{21120}{0.8\times 0.6\times 48}=916.6A$; so, we will use 5 batteries of 200 A each.

For the inverter size, since the system is 48 V and we have a system of 4360 w (8 × 545), we will choose an inverter of 5 Kva and 48 V. We are using a system of 8 × 545/20 = 19.1 A and 48 V, so the charge controller will be of 20 A and 48 V.

System cost: We will use price from Lebanon market to approximate the cost of the system.

8 panels of 545 W are at 8 × 100 = 800 $.

5 batteries of 200 A are at 5 * 180 = 900 $.

Inverter 5 Kva and 48 V is at 350 $.

Combiner box with charge controller, wires, utility meter, chassis for panels and workmanship will cost about 800 $.

So, the total cost of the system will be 2850 $.