Application of the Building Height Concept to Energy-Efficient Heating and Cooling for Saharan Buildings

The intensive use of energies from exhaustible natural resources has motivated some scientists to propose experimental environmental works on atmospheric emissions in urban areas [1]. In this context, the geometry of thermal structures is an essential factor in determining the reached comfort. The shape factor is a measure of the building’s compactness. According to a literature review, several contributions have revealed that the building design has a significant effect on both the thermal performance and energy needs. Martaa and Belinda [2] have proposed a simplified model to expect heating and cooling energy needs for a building subject to a Spanish climate. They reported that the compactness factor is one of the determining and preponderant factors. Li et al. [3] have provided some guidelines which enable to compute shape compactness based on the inertia moment. As expected in reference [4], it is proved that the stated concepts that have a direct link with the geometric properties can improve the energy efficiency in buildings. In 2012, on the basis of the research studies achieved by Parasonis et al. [5], it has been indicated that the relationship between the building shape and its energy performance was significant. On the one hand, the geometric efficiency depends both on its dimensions and proportions; on the other hand heat losses through the envelope elements constitute a large part of the total energy needs. However, in some Algerian sites, such as Algiers and Ghardaïa, optimal compactness is an additional measure to consider; increasing the compactness index has a contrasting effect, negative for the heating and positive for the cooling, meanwhile, the savings for cooling needs are larger than the disadvantage of increased heating needs [6]. Other research work, conducted by Ourghi et al. [7] allowed us to obtain an analysis tool to predict the effect of the geometric shape for an office on its annual cooling and total energy use. The same objective was addressed by AlAnzi et al. [8]. The studies take into account several building forms including rectangular, L-shape, U-shape, and H-shape. For this purpose, a compactness index was used to assess the impact of shape on the energy efficiency of office buildings. Furthermore, in previous work, Danielski et al. [9] have shown that designing buildings with lower shape factor will result in lower specific heat demand; the impact of this parameter factor varies significantly as function of different thermal envelope properties for different climate circumstances. For an appropriate occupation scenario, the change in specific heat demand varied from 12 to 52 kWh/m²/year. The shape factor has a sensible impact on this specific heat demand with lower thermal properties and for cold climates.

Additionally, a large number of contributions have used only roof area to calculate the energy saving of green roofs. The main objective pursued by Park [10] was to conclude the most effective building to install green roofs in Harrisburg. All finding results demonstrated that indoor temperature of buildings and energy demands are affected by building shapes. An experimental study was carried out to determine the relationship between building compactness and indoor temperature after the integration of green roofs during the summer season. The approach adopted was based on four physical models tested for 54 days. Indoor temperatures can be reduced by 8.1 °C for a less compact building compared to a more compact building (4.6 °C). These results are more apparent on warm days. Another paper [11] aims to set a new understanding for building compactness assessment which can contribute to originate building morphologies in terms of comfort and thermal performance. On the basis of the cost optimal level methodology, some authors [12] have announced that the choice of the best energy efficiency measures underlined the importance of the building typology. Another research work led by Kadraoui et al [13] confirmed that the building envelope is the main source of heat loss. The integration of passive architectural concepts (such as compactness) is required to improve the building's energy performance.

In the field study of thermal buildings, a change in the size, particularly the concept height, without variation of the ceiling and floor surfaces, systematically causes a change in compactness index. This article wants to emphasize the effect of the building compactness by addressing such issues as the height of buildings and the different contact modes with the external environment. The assessment criteria will be based on a well-defined lifestyle and occupancy scenario, the indoor comfort temperature and the required energy consumption expressed in Kwh/year/m². It should be noted that the few existing works in these severe conditions (Saharan climate) do not deal properly with the problem. In addition, the uniqueness and asset of this contribution consist in applying a specific method to label any building and estimate energy needs. The combination of different approaches provided a new performing model.

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Figure 2. Masonry composition and building material proprieties

The energy balance has to deal with the physical parameters, thermal properties, building design, climatic conditions…etc.

In the heating season and during inter-seasons, consumption and heating energy needs for buildings are given by the following formula [17-18]:

${{Q}_{\text{Needs}}}=\,\,\,\left| \begin{align}  & \, \\ & \,\,{{Q}_{Envelop}}\,\text{-}\left( \,{{\text{Q}}_{\text{Occup}}}+{{Q}_{Elc}}\, \right)\,\,\pm {{Q}_{\text{Solar}}}\, \\\end{align} \right|+{{\text{Q}}_{\text{DHW}}}+{{\text{Q}}_{\text{tot }\!\!\_\!\!\text{ elec }\!\!\_\!\!\text{ appl}}}$  (1)

In the cooling season, equation 2 has to be used [17-18]:

${{Q}_{\text{Needs}}}=\,{{Q}_{Envelop}}\,+\left( \,{{\text{Q}}_{\text{Occup}}}+{{Q}_{Elc}} \right)\,\,+{{\text{Q}}_{\text{DHW}}}+{{\text{Q}}_{\text{tot }\!\!\_\!\!\text{ elec }\!\!\_\!\!\text{ appl}}}+{{Q}_{\text{Solar}}}$   (2)

3.1 Energetic needs due to the building thermal envelope

Heating or/and cooling needs due to the building thermal envelope are defined by equation 3 [17-18]:

${{Q}_{Envelop}}=\,\,\,24\,\,D{{P}_{envelop}}\,\,Dj$   (3)

Detailed calculations are provided in reference [19-20].

DP_envelop: envelope and ventilation heat losses (W/K).

Dj: numbers of degree-days.

3.2 Domestic hot water "DHW" Requirements

To compute the DHW needs, the calculation should be based on the equation below. In any event, it is considered that the required volume of the hot water is 50 litters of hot water at 50 °C per day per person. Energy needs for the DHW production is given by the following equation [17-18]:

${{\text{Q}}_{\text{DHW}}}\text{ }=\text{ }\,\text{1}\text{.1628}\,\,\,{{\text{V}}_{\text{DHW}}}\,\,\,\text{N}{{\text{b}}_{\text{occup}}}\,\,\,\,\left( \,{{\text{T}}_{\text{DHW}}}\text{ - }{{\text{T}}_{\text{CW}}} \right)$   (4)

Q_DHW: energy needs required to produce DHW for one day, in Wh

V_DHW: required volume of the hot water (litters)

Nb_occp: number of persons occupying the building

T_DHW: temperature of the hot water at the filling point (°C).

T_CW: average monthly temperature of the cold water entering the storage tank or the DHW production coil (instant production).

3.3 Internal heat gains

The human being diffuses radiations in sensible (by the body at 37°C) and latent (by the production of water vapor via respiration and perspiration) heat form. Different values are given in the literature [17-18], the heat diffusion (W) from the occupants' activities are given by Table 1. The general equation that gives the values of internal gains is given by the following expression:

${{\text{Q}}_{\text{Occup}}}\text{ }=Cp\,\,\text{N}{{\text{b}}_{\text{occup}}}\,\,{{\text{D}}_{\text{pres/day}}}\,\,\text{N}{{\text{b}}_{\text{heated }\!\!\_\!\!\text{ days}}}$   (5)

Cp: the amount of heat given off by occupant (W/occupant).

D_pres/day: the period of presence during the day (h/day).

Nb_{heated_days}: Number of heated days (days/year).

The total amount of heat released by both equipment and lighting is determined according to the use and ignition mode of these electrical appliances. In this context, average values (default values) were adopted to define the internal loads in a building (Table 2).

Table 1. Cp & radiated heat per person [17-18]

Table 2. Cp & radiated heat per person [17-18]

3.4 Internal heat gains

Three input data must be taken into account according 1200 to Table 3.

P_{elec_5731.3appl}: the power of electrical appliances (W).

Nb_hours: the number of hours when the device is in an operational state during the day.

Nb_days: the number of days when the device is in an operational state during the year. Calculation, in kilowatt-hours, shall be as follows:

${{\text{Q}}_{\text{tot }\!\!\_\!\!\text{ elec }\!\!\_\!\!\text{ appl}}}\text{ }=\text{ N}{{\text{b}}_{\text{hours}}}\,\,\,\text{N}{{\text{b}}_{\text{days}}}\frac{{{\text{P}}_{\text{elec }\!\!\_\!\!\text{ appl}}}}{\text{1000}}$   (6)

Table 3. Average energy consumption per day for electrical appliances [17-18]

3.5 Passive solar gains

The solar gains depend on the incident solar radiation, the orientation of the receiving surfaces and some characteristics such as: shading, transmission and absorption coefficients. This energy gain will be calculated according to the following equation [17, 19]:

${{\text{Q}}_{\text{Solar}}}\text{ }=\text{ }\sum\limits_{\text{j}}{\,\,{{\text{I}}_{\text{Sj}}}}\sum\limits_{\text{n}}{\,\,{{\left( \,\text{A}\,\,{{\text{F}}_{\text{Shad}}}\,\,{{F}_{\operatorname{Re}d}}\,\,g\, \right)}_{\,nj}}}$  (7)

The first sum is made on all orientations j; the second is applicable on all surfaces n in different orientations "j"

I_S: solar irradiation per area unit (Wh/m²)

F_Red: reduction factor for window frames, equal to the ratio of the transparent surface of the window to its total area; its value is set at 0.8

F_Shad: shading factor; its value is set at 0.7

g: solar factor of the bay window; its value is set at 0.8 for single-glazed windows.