2.8 The Effect of Urban Geometry on Building Energy Consumption
2.8.1 Cooling Plant Load and Energy Saving
The cooling load in general refers to the load or the energy the space, it reflects the building heat gain
The sensible load calculation depends on the dry bulb temperature of the building, while latent load depends on the wet
high H/W ratio provides more energy saving
SW canyon orientation shows less energy consumption for the all exam
:The daily average energy consumption of the building group with respect to four canyon ratios and orientations E-W, N-S, NE-SW, and NW-SE (Al Znafer
On the other hand, the NW-SE orientation of the canyon shows the highest energy particularly in a low H/W ratio as the obstruction to the sun is less
Moreover, the figure shows a progressive decrease in energy consumption between the 0.5 and 2.0, and SVF between 0.71 and 0.34, this decrease reaches a maximum of
% in the buildings within the deep canyon.
.1 Cooling Plant Load and Energy Saving
The cooling load in general refers to the load or the energy required to release the heat from the space, it reflects the building heat gain. This load covers sensible and latent cooling load.
load calculation depends on the dry bulb temperature of the building, while latent load depends on the wet blub temperature.
72 ratio provides more energy saving at the indoor level.
SW canyon orientation shows less energy consumption for the all examined
:The daily average energy consumption of the building group with respect to four canyon (Al Znafer, 2014)
the highest energy ratio as the obstruction to the sun is less (Figure Moreover, the figure shows a progressive decrease in energy consumption between the
0.5 and 2.0, and SVF between 0.71 and 0.34, this decrease reaches a maximum of
required to release the heat from his load covers sensible and latent cooling load.
load calculation depends on the dry bulb temperature of the building, while
73 Both loads are needed to calculate the design and select the Heating, Ventilation and Air Conditioning (HVAC) system of the building (Cummings, 2012). In a dry climate the calculation of the sensible load is an important factor for estimating the cooling load. In a hot humid climate the latent load also affects the HVAC system selection as it depends on the wet bulb temperatures.
The worst case for both outdoor blub temperature is required to calculate the cooling sensible and latent load, and to design a HVAC system that maintains the desirable indoor air temperature (Sabzi, 2015).
Mahboob et al. (2014) studied the impact of buildings density on cooling load in Dubai, UAE. The researcher explored the impact of a number of internal and external heat gain elements on the cooling load in the hot climate conditions of Dubai, UAE. However, in the UAE, which represents shot, humid climate around 70 % of the energy consumed is for building space conditioning (Mahboob et al., 2014). The researchers compared the cooling load of low-rise and high-rise individual buildings, and they found that the urban density has an inverse impact on cooling load. The cooling load decreases as the buildings density increases. Moreover, the researchers investigated the effect of the insulation material on reducing the heat gained by the building envelope, and consequently reducing the cooling load. Furthermore, the researchers found that building density and proper shading devices can reduce the cooling load by 20 % - 30 %.
Afshari, Nikolopoulou and Martin (2013) investigated the effect of building retrofits on total energy consumption in a study carried out in Abu Dhabi, UAE. The researches illustrated the energy consumption of a case study building in Abu Dhabi and found that the most influenced parameter on cooling load consumption is the outdoor air temperature.
74 Figure 2.33 shows the effect of air temperature compared to the direct solar effect and humidity on cooling load consumption.
Furthermore, the researchers reported that the cooling load contributes to more than 60 % of the total energy consumption of the typical case study building in Abu Dhabi, UAE.
Therefore, reducing the outdoor air temperature is the main target to reduce the load required for cooling and accordingly the total energy consumption.
Figure 2.33:The contribution of air temperature compared to direct solar gain and humidity in cooling load consumption in Abu Dhabi, UAE (Afshari, Nikolopoulou and Martin, 2013)
The air temperature can be expressed by the dry bulb and wet bulb temperatures; both temperatures play a major role in designing the HVAC system of the buildings. The dry bulb temperature is the real temperature of the air and it indicates the real amount of heat in the air and is measured by degrees Celsius (°C), Fahrenheit (°F) or Kelvin (ͦ K), while the wet bulb temperature is the lowest temperature that can be reached by evaporating water into the air.
The wet bulb temperature is a function of the relative humidity, and when the air is saturated the dry and wet temperatures are equal.
Relative humidity is a measure of the actual amount of water carried by the air compared to the maximum amount of water vapor the air
of the air as a percentage. The relative humidity is inversely related to the air’s dry bulb temperature. Figure 2.34 shows the general relation
However, when the humidity is high, the system should from the indoor space and creates a comfortable space.
the desirable wind speed to create natural cooling is in the range of 0.53 The relative humidity has a significant
relative humidity the rage specified for the air temperature is 31 wind velocity of 1.5 m/s, 3m/s, or 6 m/s. At 40
be higher to reach to 33 °C, 35 and Al-Rais, 2011).
Figure 2.34: The inverse relation
Relative humidity is a measure of the actual amount of water carried by the air compared to the maximum amount of water vapor the air can carry. It gives an indication on the saturation . The relative humidity is inversely related to the air’s dry bulb shows the general relationship between both parameters.
However, when the humidity is high, the system should work harder to remove the humidity from the indoor space and creates a comfortable space. For a humid climate such as Dubai, the desirable wind speed to create natural cooling is in the range of 0.53 m/s
The relative humidity has a significant effect on the comfort zone parameters. For 60 relative humidity the rage specified for the air temperature is 31 °C, 32.5
wind velocity of 1.5 m/s, 3m/s, or 6 m/s. At 40 % relative humidity the air temperature could
°C, 35 °C, or 37 °C for the same wind velocity respectively
: The inverse relationship between the air’s dry bulb temperature and relative humidity (Quora, 2016)
75 Relative humidity is a measure of the actual amount of water carried by the air compared to cation on the saturation . The relative humidity is inversely related to the air’s dry bulb
between both parameters.
work harder to remove the humidity For a humid climate such as Dubai,
m/s - 3.04 m/s.
effect on the comfort zone parameters. For 60 %
°C, 32.5 °C, or 34 °C at
% relative humidity the air temperature could
°C for the same wind velocity respectively ( Sallal
dry bulb temperature and relative humidity
76 The importance of the relative humidity or the ambient wet bulb temperature appears in it is effect on the latent heat or latent load. The latent load is the load that affects the design and the size of the cooling tower. It is the load responsible to keep the indoor humidity level at a comfortable level of 55 %. However, the cooling load tower design is affected by the water flow rate, the inlet water temperature and the outlet water temperature plus the ambient wet bulb temperature. Therefore, the wet bulb temperature directly affects the size and the selection of the cooling tower, as the cooling tower provides a water temperature that is higher than the wet bulb temperature by 5 ͦ C -7 ͦ C.
The cooling load has to be increased as the water temperature increases and every location has it is design temperature for the cooling tower. On the other hand, the wet bulb temperature is always between the dry bulb temperature and the dew point. The dew point is the point when the air is 100 % saturated, when the wet bulb is low this means that the air is dry and can hold more water vapor. In the same concept, when the relative humidity is low the dew point is below the dry air temperature, and when the relative humidity is high the dew point temperature is close to the dry air temperature (Figure 2.35).
Figure 2.35:The relation between the relative humidity and the dew point (Quora, 2016)
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