MODELING OF PASSIVE AND FREE COOLING TECHNIQUES

Buoyancy and Wind-Driven Passive Cooling by Ventilation

Passive cooling is a technique of cooling that can be achieved by high air exchange rates ACH (h') during the daily period when outdoor temperatures are at least several К below the indoor temperature by natural means. Therefore, no auxiliary energy is needed. Air flow between environment and building’s interior is a consequence of the air pressure difference, which results from the difference in the outdoor and indoor air temperature or because the building envelope is exposed to the wind. The higher the air pressure difference, the higher will be the ACH. Regardless of the reason of the pressure difference, adequate ventilation openings in the building envelope must be provided for efficient passive cooling.

Temperature or buoyancy-driven passive cooling occur because colder air is more dense than hotter air. Pressure difference is equal:

where H (m) is the vertical deference in the envelope opening, g (m/s2) is the acceleration due to gravity, and p0i and pte are densities of the air at indoor 0, and outdoor

Building Research Establishment office building is passively cooled by stack ventilation. (From bre.co.uk)

FIGURE 15.18 Building Research Establishment office building is passively cooled by stack ventilation. (From bre.co.uk)

9e temperature. Pressure difference that occurs is normally less than 2 Pa in case of 2-m-high ventilation opening but can be significantly higher in stack buoyancy ventilation (Figure 15.18).

When wind at speed vwin (m/s) is stopped at the envelope of the building, kinetic energy of the control volume of the air is transformed into the pressure energy, and pressure difference is equal:

At a wind speed up to 1.5 m/s, the pressure difference is similar to that of buoyancy- driven ventilation, but at a wind speed (in front of a building) of 5 m/s it rises to 16 Pa, and at a wind speed of 10 m/s to a difference of 65 Pa, compared to the steady indoor air.

Ventilation openings can be large or small, and buildings can be passive cooled by a single side (through ventilation opening(s) installed in the same wall) or cross (through ventilation opening in opposite wall(s)) ventilation. In passive cooling through large ventilation openings, air enters and leaves the building through same opening and pressure-neutral level established somewhere in the area of the opening. Meanwhile, in passive cooling through small openings, air enters from surroundings through openings that are below the neutral pressure level and exit the building through openings above the neutral pressure level (Figure 15.19).

For the purpose of the engineering practice, the volume air flow rate V(m3/s) due to the buoyancy and wind-driven single side airing can be determined by the empirical expressions (EN 15242:2007; Allard, 2012).

For buoyancy (temperature)-driven airing through large opening, the volume air flow rate Vb (m3/s) is:

(a) Passive cooling by ventilation through single-side large ventilation opening, (b) single side, and (c) cross-ventilated building through small opening

FIGURE 15.19 (a) Passive cooling by ventilation through single-side large ventilation opening, (b) single side, and (c) cross-ventilated building through small opening.

where Cd is the discharge coefficient of opening (-0.6); A is the window opening area (m2), 9j, Т; (°C, K) and 0e, Te (°C, K) are the indoor and outdoor air temperatures; g is the acceleration due to the gravitation (m/s2); H is the free area height of the opening (m); and Ca is the coefficient of effective open area of the window determined for the bottom hung window with expression:

where a is the tilt angle of bottom hung window wing with value 0.9 at tilt angle 90° and 1° at tilt angle of 180°.

Case study: At indoor air temperature 0, 20°C and outdoor air temperature 0e 0°C, the volume air flow rate Vb buoyancy-driven airing through a single side window with an area A of 0.35 m2 and height H 1.4 m large with bottom hung wing at the tilt angle 10° (Figure 15.20) will be -44 m3/h. Meanwhile, at outdoor air temperature 0e 15°C, the Vb will decrease to the half (-21.5 mVh).

(■Continued)

Window with bottom hung window wing from case study. In the presented case, tilt coefficient C„ is equal 0.176

FIGURE 15.20 Window with bottom hung window wing from case study. In the presented case, tilt coefficient C„ is equal 0.176.

For multiple single-side ventilation openings:

where Cd is the discharge coefficient of opening (-0.6), A is the total are of ventilation openings (Aj + A0) (m2), and e is the ration between outlet and inlet ventilation openings (Figure 15.19).

For wind-driven airing through a large opening, the volume air flow rate Vw (m-Vs) is

where A (m2) is the window opening (corrected with tilt coefficient Ca in case of bottom hung window), and vw is the wind speed (m/s). At wind speed 5 m/s in a direction perpendicular to the window volume air flow rate Vw through window shown in Figure 15.20, it will be ~28 m-Vh.

It is common that a larger value max (Vb, Vw) is in the calculation of passive cooling effectivity.

 
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