Sarah Alghamdi, Hally ElKony

Saudi Arabia participation in causing global warming is continuously increasing; it became one of the world’s ten highest emitting countries due to its large consumption of energy. The residential sector is the greatest consumer of local energy, consuming more than half the country’s annual production of electric power. Accordingly, and based on the country’s strategic plan for sustainable development, this study investigates the shading impact of the regulated street’s vegetation on the energy demand of a typical Saudi house in a typical neighborhood in Riyadh City. The investigation proceeded through simulation by using Energyplus software engine. The modeling is based on initial data collected from literature. Parametric study of different parameters and multiple variables is conducted; In hand to find out what impact the municipality guidelines of street’s vegetation, as enforced and applied, can have on energy consumption? Thirty models (cases) were designed and simulated; where results found that the municipal guidelines for street vegetation were found beneficial in the matter of reducing energy. There was reduction in the electrical consumption of the tested housing unit, as it ranged from minor amount of 0.1% to a significant one of 7.2%. The CO₂ equivalent produced by consumption of electricity was reduced almost 5% in some cases. This study is a modest attempt to shed light on the importance of vegetation within a residential neighborhood; the findings will help identify new horizons to improve the existing regulations regarding the energy consumption. Furthermore, Findings is an attempt to encourage vegetating residential neighborhoods not only for its influence on the buildings’ energy performance but also to have breathable sustainable city, through reducing the greenhouse gasses (GHG) emissions and supporting the country’s sustainable development plan.

Keywords: Vegetation – Street Trees – Shading – Typical Saudi House – Regulation – Energy Demand – GHG.

1. Introduction:

Saudi Arabia is one of the highest greenhouse gas (GHG) emitting countries in the world, due to its large consumption of energy (Enerdata, 2013). The kingdom’s energy consumption is aggravated by the annual increase in population, which in turn makes the residential sector the largest consumer of local energy. In 2007, the electricity consumed by the Saudi residential sector was more than the sum of all other sectors. It accounted for about 53% of total energy consumption, Figure 1; and this high proportion still applies now (Ministry of Water and Electricity, 2007). This can be reduced if houses were built per certain specification to be thermally efficient (Ahmed, 2004). While constructing thermally efficient houses is important to maintain its energy consumption, yet a simple shading mechanisms can greatly reduce building surface temperature thus its energy demand. Vegetation's shade can provide significant energy savings specially if strategically located near buildings as demonstrated by several modelling and empirical studies (Berry, Livesley, & Aye, 2013).

Figure 1: Distribution of Saudi electricity consumption in 2007, by consuming sector

Urban vegetation characterizations have a crucial effect not only on the energy demand but also on urban microclimate and indoor comfort levels, where each tree has its specific impact. That should be considered for assessment in the site examined and in building simulations (Fahmy, Sharples, & Eltrapolsi, 2009). The net radiation received by building walls and roof surfaces is a primary driver of building microclimates. Tree canopies absorb and reflect large proportions of both solar and terrestrial radiation received by a building, which in summer can reduce the difference between internal and external building temperatures and directly reduce energy use for internal space cooling (Berry, Livesley, & Aye, 2013).

The street tree is controlled type of urban vegetation, which is “at the side of a street or in a public place to enhance the view” (Oxford Dictionaries, 2017), according to certain characteristics related to certain factors. It has many great benefits environmentally, socially and economically; mullaney et al. discussed and emphasized its importance where it “play an integral role in supporting healthy urban communities and they have a significant social impact by improving human health, reducing crime, increasing community interaction and boosting property values” (Mullaney, Lucke, & Trueman, 2015).

Whereas, the trees root behavior can cause some damage to pavement and infrastructure that might need high cost maintenance and tree replacement. Moreover, in some cases the growth of trees can obstruct the pedestrian movement. However, these issues and many others can be preventable by making appropriate choices prior to planting (Mullaney, Lucke, & Trueman, 2015). The strategic decisions on the early stages of the building design process not only can prevent the possible damage one tree can cause, but also, it have a notable impact on buildings’ overall energy performances. During initial phases, numerical simulations can be a key tool to improve buildings' energy efficiency (Calceranoa & Martinelli, 2016)

Many studies explored the benefits of street trees and its relation to the energy performance of buildings. In his study, Simpson identified several possible permutations that control the tree shading effect; such as building type, tree type, tree size, canopy density, tree location on each building surface and its orientation, solar angle, season and microclimate (Simpson, 2002). On the other hand, Hwang et al. limited the provision of single tree shade onto a building to the “function of two factors: tree form and tree placement. Tree form describes the physical attributes of a tree, whereas tree placement describes its location relative to a building targeted for shading” (Hwang, Wiseman, & Thomas, 2015)

Calceranoa et al. did numerical optimization through dynamic simulation to measure the influence of urban microclimate on the energy balance of buildings. To reduce the energy consumption of the residential sector, and to identify the optimal position of trees around a buildings. As a function of the maximum reduction in energy consumption for cooling during summer season; considering only the shading effect of trees. The results confirm the significant influence of vegetation’s shading effect for energy savings: the reduction of energy consumption in some optimized configurations can reach 48.5%. The relative incidence of trees on energy consumption reduction decreases progressively as their number around the building increases; therefore, even a limited number of trees can have a paramount effect. The interaction between trees is a factor of utmost importance, and the tree effect must always be considered as a combined effect, avoiding shade overlapping that decreases energy savings with the increasing density of the tree crown (Calceranoa & Martinelli, 2016).

Hwang et al. highlighted the importance of improving the tree planting guidelines. Their study examined the Impact of vegetation shading on the energy conservation by simulating the shading cast by single tree onto a prototypical residential building. In hand to characterize daily and seasonal patterns of tree shade, and find whether the guideline of regional tree planting provides appropriate recommendations for maximizing the benefit of tree shade. They expressed in their study that “the phrase (plant the right tree in the right place) has become a widespread philosophy for maximizing tree benefits and minimizing costs in urban areas.” However, That shouldn’t be the case as their findings showed the importance of strategic tree selection and placement for optimizing shade, which has positive benefits for energy reduction and human comfort (Hwang, Wiseman, & Thomas, 2015).

Donovan et al. Estimated the shading effect of urban trees on summertime electricity use and found it significantly effective, but that the magnitude of the effect depended upon three factors: the crown area, its distance from a house, and the aspect relative to a house if it is north, east, etc. Also, their results demonstrated that shade trees reduce carbon emissions from electricity generation. However, they pointed out a valid point regarding the constraints many homeowners face as they are sometimes either unable to plant trees or are at least unable to do so in the optimal locations. It is recommended that setting aside space for shade trees should be considered when planning residential development, not simply something that is done as an afterthought (Donovan & Butry, 2009).

Accordingly, this research shed light on the problem of energy consumption of the Saudi residential sector; from the perspective of sustainability. By adapting the street vegetation as partial solution, and to explore its potential in this regard. Moreover, to encourage planting the city of Riyadh toward satisfying the country’s sustainable strategy.

2. Research objective and method

This study's importance emanates from its area of investigation, as the residential sector is the highest consumer of local energy. It aims to measure the shading impact of street trees on the electricity consumption of typical Saudi house in typical neighborhood in Riyadh city. The investigation proceeded through simulation by using Energyplus software engine. The modeling is based on initial data collected from literature, such as characteristics of the building, operating conditions (occupancy, lighting, air-conditioning and equipment) and air conditioning system, specifications of construction materials, vegetation regulations, weather data, etc. Parametric study of different parameters and multiple variables is conducted; In hand to find out what impact the municipality guidelines of street’s vegetation, as enforced and applied, can have on energy consumption of houses?

3. Parametric investigation:

The investigation of the vegetation’s shading impact on building’s energy performance depends on multiple parameters. This study was limited to explore the shading impact of street trees, as the latter has been regulated by the municipality of Riyadh city. Based on that and what have been reviewed in literature, some parameters were defined, which will be discussed later in section 3.5 of this study. EnergyPlus package is adopted as the simulation software; which is one of the most commonly used software for building thermal dynamic analysis and suited for optimization studies (Calceranoa & Martinelli, 2016). It offers considerable flexibility, a vast amount of detail and extensive documentation that enhances its accessibility. An additional feature of EnergyPlus that makes it especially suitable for this research is the availability of the Google SketchUp plug-in (OpenStudio Legacy), which simplifies the generation of geometric data of complex shapes. However, to avoid complex input data in the simulation design, energy consumption calculations were performed by modelling cooling loads with ideal HVAC system for two thermal zones for a typical Saudi house located in typical nieghbourhood in Riyadh city. The simulation was designed to evaluate the building’s energy performance under the shading impact of street trees through multiple cases. Some collected data were used as inputs entered in the EnergyPlus engine, then simulated for a whole year through 4 timesteps per hour. At first, the base-case model was created, then its energy performance was validated. Ensuring the study credibility was essential because all the other models, yet to come, were designed depending on its results and compared to it. Two outcome amounts were of interest and had been validated: electrical consumption and GHG. Moreover, the solar incident received by walls subjected to tree shading was measured and used as a guide to identify the shading pattern occurred during the day.

As stated earlier, Modelling the study cases needed some essential data to be used as inputs into the EnergyPlus engine. Data about the location, the weather, building characteristics, site layout and tree type and its characteristics was collected from different resources as appropriate.

3.1Typical Saudi house

The typical housing unit studied was described in an interim report on a KACST Project (Ahmad, 2004). It has a rectangular layout (W/L = 1.1) with its long axis oriented east–west; it comprises two storeys, each with an area of 262.5 m² and height of 3.5m. The characteristics of the building and its operating conditions of occupancy, lighting, air conditioning and equipment are listed in Table 1 (Ahmed, 2004), along with the U value for each construction component (i.e. items made of materials most commonly used in the country today) (Taleb & Sharples, 2011). This typical Saudi house was identified here as the base-case located in Riyadh city (Lat. 24° 72′ N, Long. 46° 47′ E, elevation 612 m).

3.2 Typical neighborhood

The separated villa site layout was adopted in the current study, as it represents the most common type. The site simulation model was designed in accordance to municipality minimum prerequisites, where the built area of the plot should not exceed 60% for the ground floor and 65% for the first floor. Also, it is regulated that all setbacks toward neighbors should be a minimum of 2 meters, while the setbacks toward surrounding streets should be at least one fifth of the street’s width. Furthermore, the site should be overlooking a street of width not less than 12 meters, as illustrated in Figure 2 (Ministry of Municipal and Rural Affairs, 2016).

3.3 Weather data

Located between latitudes 24° and 28° N and longitudes 44° and 48° E, the arid desert climate of Riyadh city is marked by extreme temperatures: very hot in summer and cold in winter, and great diurnal variation between night and day. In summer, the lowest average temperature ranges from 22–27°C and the highest from 40–43°C; while in winter, the lowest temperature ranges from 8–14°C and the highest ranges from 20–28°C (High Commission for the Development of Arriyadh, 2016). The detailed weather data for Riyadh was obtained from the EnergyPlus website (U.S. DOE, 2016), and then used in this study as a main component of the simulated models using the EnergyPlus simulation engine.

Figure 2: Typical neighbourhood site layout in Riyadh

Tree type

In the early 1980s, with the fast spread of Riyadh’s neighborhoods, the city municipality committed to increasing the presence of trees in the city streets (BaHammam, 2002). However, lately the works on spreading greenery decreased due to the lack of funds and the high expense of maintenance cost. BaHammam in his field study detected several planting patterns existed in the city streets; they were strongly related to street sizes, shapes, and functions. In the residential neighborhoods streets, through the many stages and years of planting the city, the Propsi tree where found the most used tree. Yet, in the last two decades, the Conocarpus Erectus tree has replaced many Prosopis trees. This tree species proves to be suitable for street planting due to its upright oval head, extremely fast-growth and the requirement for little maintenance and cleaning (BaHammam, 2002). Table 2 summarize some of the characteristics of the Propsi and Conocarpus Erectus trees; both are suitable for the hot-arid climate of Riyadh and requires reasonable amount of water for irrigation. The Conocarpus Erectus has an advantage of growth rate and size, where it can reach 20m in height. However, it should be noted that as much as a tree gets larger its roots also will grow to meet its needs; and causing possible damage to infrastructure, buildings, and pavement (High Commsion for the development of Arriyadh, 2014).

3.4. Regulations limitation

Municipality of Riyadh city has some regulation regarding the street vegetation in residential neighborhoods (Ministry of Municipal and Rural Affairs, 2016). Where the tree pit should be located near the property wall, far as possible from the street to leave enough space for the pedestrians to walk on the sidewalk. Its sides can be built of solid blocks with depth of 40 cm and width of 10 cm at the least. Moreover, trees should not be planted near doors or windows so it doesn’t block the sun beams or the view. As for tree type, the flowering plants should be avoided due to the large maintenance it needs. Also, any plants that needs special care or that have large roots or toxic in any way. Deciduous trees can be planted to have protection from sun heat at summer and benefit from that heat in winter, however it is better to plant evergreen trees.

Most side-walks, in neighborhoods streets, vary from 1.20 m to 2.00 m in width, where tree pits are aliened in every 6 meters apart, and their size are usually 0.6 X 0.6 m² (BaHammam, 2002). A detailed graph below illustrates the regulations considered in this study, see Figure 3.

Figure 3: the street vegetation regulations considered in this study

3.4 Parameters and designing variables

As the study is limited to investigate the shading impact of street trees on the energy performance of residential building, only electricity consumption amount was measured together with the amount of GHG produced based on that consumption. Other indirect effect is neglected such as solar access, evapotranspiration, wind deflection, etc. Accordingly, the investigation is designed depending on the collected data mentioned earlier in this section. Multiple parameters are defined and used to develop the study scenarios. As listed in Table 3, There are fixed and varied parameters. The correlations between them produced three main scenarios and nine subcases for each scenario, see Table 4. A reference case is designed for each scenario, in hand to have a comparable base to measure the impact of the change, Figure 4. In total, thirty models were developed, three reference cases and twenty-seven varied subcases. The three main scenarios differ in street orientation, where three orientation are specified: East, South, and west. It should be noted that the north orientation was neglected due to its insignificant solar effect on energy consumption. On the other hand, the subcases configurations are varied according to tree size, where its height and width differs. A range of height (H: 7, 10, 13m) and width (W: 2, 4, 6m) are chosen based on the used tree type (the Conocarpus Erectus), also with regard to the houses’ height, so a reasonable proportion is created. Trees geometric presentation is simplified by two bi-dimensional surfaces formed by oval and rectangle surfaces duplicated and intersected at 90°,

Fig 4: Simulated typical Saudi house within typical neighbourhood model/base-case.

4. Results and discussion

The performed simulation considers only the limitation of solar access on the building envelope due to vegetation, which corresponds to a decrease of internal load. The simulated three reference cases were found to have different electricity consumption due to the change in orientation. Since the typical house has a rectangular layout (W/L = 1.1), when the street is oriented toward east it consumed 1.7% more energy than being south oriented, while when oriented towards west it consumed, even more, about 3.4% compared to the south oriented one as seen in Table 4. Thus, having three different reference cases was essential to measure the exact effect of orientation of trees placement according to street orientation. It should be noted that all reference cases have no street trees in them to have a base of comparison when adding trees in the later tested cases.

While controlling the parameters, and designing the reference cases, it was noticed that the effect of shading by trees did not only occur by the trees in front of the plot of the tested housing unit, but the effect extended by the tress in front of the neighboring housing units on the left and right of the test one. Where each house plot can have three street trees in front of it, because the distance between trees is limited to 6m; the number of trees studied was decided according that.

Results showed that there is defendant shading impact of street trees on the energy consumption. In all the subcases configuration, there was reduction in the electrical consumption of the tested housing unit. Of course, it ranged from minor amount of 0.1% to a significant one of 7.2%. Table 4 lists the results of reduction in consumption where each subcase is compared to its reference case. The larger the tree the greater the impact; however, there should be a limitation to the growth of trees as it might cause some serious damage in site as mentioned earlier in literature. Also, large trees have one more disadvantage as it can block the direct solar radiation from reaching the building. The received incident solar radiation on the shadowed building’s wall is illustrated in Figure 4 (a, b, and c) for the same tree size (H:10, W:4) but in different orientation (East, South, and West). The amount of incident solar radiation is less in all shaded walls, and the hours of received direct radiation is cut to half in some cases. Moreover, sunshine duration analysis shows the progressive reduction of sunshine hours on the surfaces of the building with the increasing size of trees, as shown monthly in watts for the whole run-period in Figure 6. Despite that, large street trees can provide privacy in a better environmental way than other unattractive solutions sometimes made by local residents to rise up their property wall for more privacy.

The CO₂ equivalent produced by consumption of electricity was reduced almost 5% for the highest electricity reduction subcase where the street is oriented towards west and trees of size (H13,W6). The monthly produced amount of CO₂ equivalent is illustrated in kilograms per unit together with the its reference case amount (Figure 7).

Figure 5: The shading impact of street trees on the incident solar radiation received by building affected wal

Figure 6: The Reduction in incident solar radiation per tree size, East orientation

Figure 7: The Reduction in CO₂ Equivalent amount due to the shading of street trees

5. Conclusion

The investigation was conducted on typical Saudi house in a typical neighborhood in Riyadh city to measure the shading impact of street trees on the building’s energy performance, through thiry simulated models. The tested housing unit front setback distance from the street side is the minimum prerequisites by the city municipality, also, the vegetation characteristics and regulation. Results showed a noticeable reduction in some of the cases; there was found an apparent positive relationship between the tree size and the amount of reduced electricity. In general, the west street orientation although was found to increase the energy consumption in the cases studied here, yet it has the highest potential of reducing electrical consumption to almost 7.2%. Findings could be more or less, that depends as well on the setback distance which was a fixed parameter in this study. The municipal guidelines for street vegetation were found beneficial in the matter of reducing energy. However, the shading of trees should be considered in the development of residential neighborhood and not just as an element to beautify the city. Furthermore, results can be more relevant if evapotranspiration was calculated. A thoughtful consideration is needed if wanting to benefit from street trees regarding energy consumption. On the other hand, the CO₂ equivalent produced due to electrical consumption was reduced by 5% in some tested cases, and this is just one bright side of the many environmental benefits of urban vegetation.

This study is a modest attempt to shed light on the importance of vegetation within a residential neighborhood, as it discussed the issue of energy performance of buildings in relation the shading impact of street trees. However, there are many more aspects that can be investigated to identify the related problems and find their solutions. The Saudi residential sector with its high rate of energy consumptions in need of green trends such as the one studied here. The kingdom’s need to get practicing its strategy of sustainable future, for its cities to be more breathable, more sustainable, in this age of global warming.

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