PASSIVE AND FREE COOLING OF BUILDINGS

The common feature of both techniques of natural cooling is that they exploit the natural processes of cold generation, such as radiative or evaporative cooling and environment cold. The cold of the environment is a phenomenon that results from the transfer of heat from the surface of the earth through an “atmospheric window” to the space, whose temperature is near to the absolute zero (0 K). This process occurs throughout the day but is more noticeable at night, since the surfaces then do not receive solar irradiation. The “atmospheric window” is radiative property of Earth’s atmosphere that transmits almost all irradiation with wavelengths between 8 and 12 pm, which are exactly the wavelengths at which the environment surface areas and buildings envelope structures emit the largest heat flux. In nature, the effect of environment radiate cooling can be noticed in the cold stored in ambient air as well as in the shallow layer of the soil. Another natural phenomenon that can be utilized by passive as well as by free cooling is the evaporation of water droplets into the outdoor or indoor air. To evaporate, energy needed is transferred from the surrounding air. This has happened on the leaves of greenery and trees and on water surfaces. As a result, air is moisturized but cooled as well.

The passive and free cooling of the buildings will be more efficient if some measures are taken into account when planning buildings. The efficiency of the passive and free cooling will be increased as well, if the outdoor environment will be designed in the urban planning process to mitigate the urban heat island by greened structures and city parks. Figure 15.5 shows the non-mechanical cooling techniques as well as processes that must be implemented to increase the efficiency of passive and free cooling techniques.

Free cooling is sometimes related to the use of ground, sea, or lake water and thermally activated building structures. Free cooling is also one of the operation modes of compressor-driven cold generators where the refrigerant is cooled in by-pass mode in an air heat exchanger at low ambient temperature. Those technologies are not discussed in this chapter, since they are used in combination with mechanical space heating and space cooling systems. Details can be found in Medved et al. (2019).

Adaptation of Urban Environment for Efficient Passive and Free Cooling of Buildings

Replacements of natural ecosystems with the building blocks of the urban environment have significant influence on the thermal and hydrological balance of the urban environment. The phenomenon is known as urban heat island (UHI) and is related to higher air temperatures in cities compared to the suburban or rural areas.

Non-mechanical cooling techniques for cooling of the building as well as measures that increase the efficiency of passive and free cooling techniques

FIGURE 15.5 Non-mechanical cooling techniques for cooling of the building as well as measures that increase the efficiency of passive and free cooling techniques.

Recent research on UHI carried out in Europe indicated different values of the UHI from slight around 0.1°C up to 10°C (Santamouris, 2007) in the cities with 1 million inhabitants. The most effective strategies to mitigate UHI are reducing of solar radiation absorptivity of urban surfaces by using the materials with high reflectance (albedo) of solar radiation, green roofs, and urban vegetation. Santamouris reports that the increase of the albedo of the built environment, which is close to the value 0.35 in a typical city by 0.1, results in decreasing of the average outdoor air temperature by 0.3°C (Santamouris, 2007). An overview study made by Yang et al. (2015) shows that an increase of albedo in cities up to 45% decreased the intensity of UHI up to 3.1°C.

On the smaller scale, the local urban heat island (LUHI) that is in street canyons is defined by the thermal response and the velocity patterns in the settlements. LUHIs could be even more intense than UHIs, and therefore, they have a major impact on both the energy demand for space cooling and the effectiveness of the natural cooling techniques. The most effective way to mitigate the LUHI with thoughtful urban planning of greening the built-up areas and incorporation meadows, water areas, and parks into the settlements. Figure 15.6 shows an example of a LUHI that occurs in the street canyons of row building settlements. The results were obtained by numerical modeling using the CFD technique and are presented for a selected typical summer sunny day in the city with a central European climate (Medved and Vidrih, 2015).

An example of CFD modeled temperatures in

FIGURE 15.6 An example of CFD modeled temperatures in (a) street canyons of the rowbuilding settlement in case of typical albedo-built surfaces and (b) grassy street ground. (From Medved, S. and Vidrih, B., Evaluating the potential of local urban heat island mitigation by the whitening and greening of the settlement surfaces, World Renewable Energy Congress XlV-Clean Energy for a Sustainable Development, 2015.)

LUHI in the most critical street canyon in the row-house settlement with texture of the street canyon ground surface

FIGURE 15.7 LUHI in the most critical street canyon in the row-house settlement with texture of the street canyon ground surface.

On the basis of the CFD simulations, LUHI was determined for the most critical street canyon in a row-building settlement, depending on the street height-to-width ratio and reference wind speed out of the settlement (Figure 15.7). It can be seen that at low wind speed (vw ref < 1.5 m/s, which is quite common) the LUHI is up to 3 К lower in case of grassy street canyons ground compared to a settlement without greened surfaces, and the highest daily temperatures are even lower than air temperatures outside the settlement. At higher wind speed, the mitigation of LUHI is smaller, due to the swirling stream of air in the street canyon.

A regression model for the prediction of the LUHI (°C) was developed (Medved and Vidrih, 2015) in form of the expression:

where regression coefficients for the case of built-up surface with average albedo (0.35) and grassy street canyon ground are equal:

a

b,

b2

bj

b4

c,

c4

Albedo 0.35

2.89

-1.47

0.99

-0.25

0.02

-0.02

-0.28

-0.04

0.08

Grassy street canyons

-0.82

1.22

-0.38

0.06

-0.01

-0.17

0.02

0.04

0.02

Urban parks have an important role in UHI mitigation. Potential for mitigation of the UHI by urban parks, which results from evaporative cooling of the air and shading of the ground surface, are determined by the age (size) of the trees and their planting density. The evaporative cooling by trees depends on the evapotranspiration—the amount of the water that evaporates from ground beneath tree canopy and on the leafs. The area of the leaves is determined by the leaf area index (LAI). The meadow has LAI 1 m2 per m2 of the ground, the dense forest up to 8 m2/m2. The reference amount of the water that evaporates beneath and on the plant is defined by evapotranspiration rate ET. It depends on meteorological parameters, and the reference value (for the plant having LAI 1 m2/m2) is in the range between 1 to 8 kg of water per m2 per day, with reference value ET„ 5 kg/m2day.

To combine the age (size) and density of the trees in the park, the specific dimensionless LAIsp was introduced (Vidrih and Medved, 2013) as shown in Figure 15.8.

Using the CFD modeling technique, the temperature at different lengths was determined for parks with different specific LAI, reference ET„ and different reference wind speeds. As an example, air temperatures in the city pars with LAIsp 3.16 m2/m2 are shown in Figure 15.9 (Vidrih and Medved, 2013). It can be concluded that old, dense parks have a very big impact on the urban microclimate. Because the value of the LUHI is negative, the park cooling island (PCI) can be defined instead of LUHI. A regression model for PCI was developed in the form:

The model is limited to the length L of the park 120 m, but it can be seen from Figure 15.9 that larger parks have a minor impact on urban climate mitigation, since the PCI increases only slightly with L > 100 m.

Specific LAI determined according to the tree age (size) and the density of the tress in the city park. (From Vidrih, B. and Medved, S., Urban For. Urban Green., 12, 220-229,2013.)

FIGURE 15.8 Specific LAIsp determined according to the tree age (size) and the density of the tress in the city park. (From Vidrih, B. and Medved, S., Urban For. Urban Green., 12, 220-229,2013.)

Air temperature in the square-shaped park at different lengths; air temperatures shown are average temperatures in the pedestrian zone

FIGURE 15.9 Air temperature in the square-shaped park at different lengths; air temperatures shown are average temperatures in the pedestrian zone (0.1 to 1.8 m) in plane at distance L from the edge of the part. (From Vidrih, B. and Medved, S., Urban For. Urban Green., 12, 220-229, 2013.)

 
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