NIGHT VENTILATION IN MEDIUM RISE OFFICE BUILDINGS. CASE STUDY: LIMA, PERU

Passive Design Annual Conference PDAC 2014

SUMMER PERIOD EVALUATION OF NIGHT VENTILATION PERFORMANCE IN MID-RISE OFFICE BUILDINGS IN A HOT ARID CLIMATE WITH HIGH RELATIVE HUMIDITY INDEX. CASE STUDY: LIMA, PERU C1469285 1Welsh

School of Architecture, Cardiff University, Cardiff, UK

ABSTRACT: Nocturnal ventilation benefits has been evaluated and demonstrated in different climates; however, given the particular mild temperatures, high humidity index but no appreciable precipitations, and low daily temperature swings, further research in this context was required. This study contributes with nocturnal ventilation evaluation and assessment as a passive cooling technique in a typical summer week, in order to encourage its application and further investigation to maximize its potential. Findings show, due to the typical office related interior incidental gains, indoor temperatures were found higher than the outdoor ones along a summer day. Therefore, mass exposure to external conditions by ventilation was the most effective cooling strategy. Night ventilation, among the other passive strategies by wind air flow, is the least effective for a free running office building, while daytime and full day ventilation take us closer to the comfort band. Furthermore, potential coupling for night ventilation and mechanical cooling is identified, as it can reduce indoor temperatures by 2°C, cut back cooling loads and energy saving, suggestions for further research fall in this area.

Keywords: nocturnal ventilation, hot arid desert climate, high relative humidity, low daily temperature swing, midrise office building.

1. INTRODUCTION Nowadays, the mild temperatures that used to characterize Lima’s meteorological condition has been changing over the past 10 years, as temperature increase can be easily perceived as consequence of global warming. Accordingly, passive cooling strategies application have not been significant in architecture practice up until the moment as environmental awareness has arisen in the recent years. The current Peruvian building code does not contemplate an environmental aspect, yet there are some voluntary sustainable related building parameters (National building regulation Annex EM110. Energy efficient thermic and light fitting). In order to neglect the use of mechanical cooling services so as to overcome the climatic change challenges and cut down greenhouse gasses emissions, an environmental approach in architecture is necessary, along with supportive research that encourages its application. This conference paper aims to evaluate nocturnal ventilation potential in mid-rise free running office buildings during the summer cooling period and the factors that affect its performance in Lima context.

2. LITERATURE Ventilation can improve internal comfort by two means, a direct and indirect mode. The first is related to a direct physiological effect due to air flow, hence, convective heat transfer among occupancy can take place. Nocturnal ventilation refers to the indirect mode, ventilating a space and thermal mass during

night and during the following day, the cooling stored in the thermal mass will reduce the internal temperatures experienced indoor (Givoni, p. 37, 1994).

Extensive research has been conducted in a variety of climates (hot arid, hot humid and temperate climate) evaluating nocturnal ventilation performance in residential and commercial buildings; accordingly, cooling potential has been acknowledged and put in practice.

Three significant factors have been identified in order to maximize its cooling effect. First, climate, best results have been observed in hot arid weather conditions as it presents a high diurnal temperature range (15 °C to 20°C), yet even low daily temperature rages have attained beneficial outcomes. For example, lightweight and heavyweight buildings with an average outdoor temperature amplitude of 8 °C to 10°C can achieve from 1 to 2.5 °C of indoor cooling effect. Second, the technical aspect, which refers to the optimum number of air changes per hour and period of ventilation time required to optimize results, 8 ACH and twelve hours of constant ventilation have been found to be the most appropriate (Blondeau, Spérandio, and Allard, 1997; (Kolokotroni, Webb, and Hayes, 1998). Last but not least, building parameters need to be taken in consideration, fabric density and heat capacity allows a high thermal storage, hence, greater cooling effect.

In addition, cooling efficiency is strongly related to space volume, since the bigger is the ratio of volume to wall surface area, the more effective convective heat exchange is (Blondeau, Spérandio, and Allard, 1997).

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Passive Design Annual Conference PDAC 2014

Furthermore, there is a variety of sources of information and application of hybrid systems, building-servicing strategies combining natural night and mechanical ventilation, which demonstrate its cooling energy. Mixed mode ventilation in heavyweight building can achieve from 12% to 15% of energy saving, in a hot arid and moderate climate respectively (Blondeau, Spérandio, and Allard, 1997; Kolokotroni and Aronis, 1999).

Accordingly, literature suggests nocturnal ventilative cooling is effective in different climatic contexts, yet there are no studies that confirm nor discard its passive application in the composite climate of Lima, nor its feasible contribution with a mixed mode system.

3. METHODOLOGY Instead of an experimental model, research was based on a computational modelling and simulations were carried out. A typical open plan office model with night ventilation was examined using Design builder, which is able to assess night ventilation parameters.

3.1. Model characteristics An office building with the current Peruvian building specifications and under climatic conditions in Lima was modelled. The office space is assumed to be part of a five stories office building, located on the second floor. A typical office building zoning floor plan has been developed for the office model location (See figure 1).

The model is 9.8 m. - 8.6 m. - 3.0 m. (length width - height) , 30% of window to wall ratio oriented to north, 8mm. single temperate glazing of 5.6 W/m2k W-value solar protected by 0.6 m. hangover, lastly a wood door of 2 W/m2k, 0.90 m. - 2.1 m. (width height). Working hours are 8:00 am. to 20:00 pm, the building have two fronts to north and west. Internal gains such as installed lighting and equipment are 10 W/ m2 each, and occupancy density 0.11 people/ m2.

Figure 1: Typical office building floor plan and office model location. Source: Own

Simulations will be carried out in two stages. The first one involves a series of simulations, assessing night ventilation performance by investigating variables such as, ventilation heat transfer, air flow, fabric heat transfer and indoor thermal conditions.

Second part involves three cases study in order to compare the thermal conditions upon each ventilation condition and nocturnal ventilation. Case 1, between day time and night ventilation; Case 2,

full day and night ventilation; Case 3, no ventilation and night ventilation. Day time ventilation will be arranged from 8:00 am. to 18:00 pm. and nocturnal ventilation from 8:00 pm. to 8 am. Parameters to be used in this stage are the results of the previous evaluation. See Table 1.

Table 1: Ventilation modes in evaluation. Day time, full day, night ventilation and no ventilation.

Case

Model 1

Model 2

1

Day time ventilation

Night Ventilation

2 3

Full day ventilation No ventilation

Night Ventilation Night Ventilation

Data will be collected in terms of air temperature, operative temperature and relative humidity. The cooling potential of the strategy will be evaluated by comparing these results.

3.2. Climate Lima is located at 12°02’ latitude south and 77° 02’ longitude west, it presents a hot arid desert climate according to the Koppen Geiger weather classification (Peel, Finlayson, and McMahon, 2007), mean temperature ranges from 17°C to 24°C, high humidity 80% and low precipitations 1.1 mm. which result in low daily temperature swings from 5°C to 6°C. See table 2 for detail information on the subject climate, regarding temperature, relative humidity and precipitation.

Table 2: Average, minimum and maximum temperature, relative humidity and precipitation rate in Lima. Source: SENAHMI.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Temp. (°C)

Min. Temp. (°C)

Max. Temp. (°C)

RH (%)

PPT. (mm)

23 24 24 22 21 19 19 18 17 18 19 21

20 21 21 19 18 18 17 16 15 16 17 19

26 27 27 25 23 21 21 20 19 20 21 23

23 24 24 22 21 19 19 18 17 18 19 21

0.9 0.3 4.9 0 0 0.3 0.3 0.3 5.4 0.2 0 0.3

4. NOCTURNAL VENTILATION VARIABLES EVALUATION

AND

It is necessary to assess if the potential of the winds in Lima meet the minimum 8 ACH per hour required in the model so as to assure an effective night ventilation performance. As seen in Figure 2 an average of 6 ACH and 9 ACH per hour can be achieved with a 45% and 75% window aperture respectively. Making the assumption of a window type with a 75% aperture, a minimum of 2 ACH and

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Passive Design Annual Conference PDAC 2014

a maximum of 11 ACH per hour was observed. It can be concluded that nocturnal ventilation can be incorporated passively during the summer period and among the schedule of analysis from 20:00 pm. to 8:00 am.

Figure 2: Air flow rate per hour achieved by 75% and 45% of window aperture in subject model. Software: Excel

A variety of airflow rates were tested so as to confirm that 8 ACH is the most appropriate one. In table 4, the internal operative temperature result of the air flow rate can be perceived, where the greatest difference is achieved by the variation from 5 to 8 air changes (ACH), while from 8 to 10 and 10 to 13 ACH, difference is negligible. Indeed, as Blondeau, Spérandio, and Allard demonstrated in 1997, there is not major improvement achieved by air flow rates higher than 8 – 10 ACH. Table 4: Night ventilation impact on internal temperature on a typical summer day..

Hour/Air flow

5 ACH

8 ACH

10 ACH

13 ACH

08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00

26.2 29.2 30.4 31.1 31.7 32.2 32.8 33.2 33.6 33.8 34.0 34.1 34.3 30.2 29.5 29.0 28.5

25.9 28.9 30.1 30.8 31.4 32.0 32.5 32.9 33.3 33.6 33.7 33.9 34.0 30.0 29.3 28.8 28.3

25.9 28.8 30.0 30.7 31.3 31.9 32.4 32.8 33.2 33.5 33.6 33.8 33.9 30.0 29.2 28.7 28.2

25.8 28.7 29.9 30.6 31.2 31.7 32.3 32.7 33.1 33.4 33.5 33.7 33.8 29.9 29.2 28.7 28.2

Then, fabric minimum requirements dictated by the Peruvian building regulation is what will be defined from now on as lightweight fabric, for the purpose of this study. On the other hand, a heavyweight construction in the Peruvian construction industry is the one where concrete reinforced predominates. Construction information for both lightweight and heavyweight fabric is shown in Table 4 and 5.

Table 5: Lightweight fabric specification for office building according to the minimum structural requirements (National Building Regulation)

Wall Plaster ext.

Brick Plaster int.

Roof Reinforced concrete Brick Floor Plain concrete

d (m)

λ (W/mk)

ρ (kg/m3)

c (J/kg K)

0.015 0.12 0.015

1.4 0.72 1.4

1860 1920 1860

840 840 840

0.05

2.3

2300

1000

0.2

0.72

1920

840

0.1

0.72

1650

920

Table 6: Heavyweight building.

External Wall Reinforced concrete Partitions Plaster ext.

Brick Plaster int.

Roof Reinforced concrete Floor Plain concrete

fabric

specification

for

office

d (m)

λ (W/mk)

ρ (kg/m3)

c (J/kg K)

0.15

2.3

2300

1000

0.015 0.12 0.015

1.4 0.44 1.4

1860 1920 1860

840 840 840

0.25

2.5

2500

1000

0.1

0.72

1650

920

Night ventilation effect on internal temperatures, taking in consideration the variables mentioned above in terms of fabric, and controlled parameters like twelve hours period and airflow rate (8 ACH) is now assessed. In figure 2, thermal mass effect on internal temperatures and night ventilation performance are presented, heavyweight fabric can achieve a daily average of 30 °C, while a lightweight construction can achieve 31 °C

Figure 2: Thermal mass effect on internal temperature.

Software: Excel

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Passive Design Annual Conference PDAC 2014

5. RESULTS AND DISCUSSION Night ventilation effectivity was assessed by analysing Case 1, Case 2 and Case 3 and simultaneously, internal temperatures proximity to the comfort zone. The fixed parameters in this stage are a heavyweight fabric as specified before, night ventilation with 8 air changes per hour along a period of twelve hours from 20:00 pm. to 8:00 am. An adaptive thermal comfort model is used in this study, given the fact that the subject model is assumed to be part of a free-running office building and clothing adaptation is feasible.

The equation for Humphrey’s adaptation comfort model is: đ?‘‡đ?‘?=11.9+0.534 đ?‘‡đ?‘œ where Tc is comfort temperature (°C) To is monthly mean outdoor temperature (°C).

In figure 3, comfort zone boundary has been calculated in relation with the external weather conditions in Lima. Clearly, during the summer period, external temperatures are in great percentage within the comfort band; however, the conditions experimented inside the subject building will be higher due to occupancy, equipment and solar internal gains; hence, overheating.

Case 2 shows night ventilation remains to be the least cooling effective compared with full day ventilation, as this mode can achieve 28 °C average daily temperature and only 1 °C far from the comfort band at peak temperatures.

Figure 5: Case 2, full day and night ventilation effect in internal temperatures and thermal comfort. Software: Excel

Clearly, because the external temperatures are higher than the internal ones along the day in summer, ventilation period increase have the best outcomes for free running office buildings. Case 3 shows that no ventilation is by far the worst scenario as temperatures would reach easily 36 °C at maximum, 7 °C higher than the comfortable band, and an average daily temperature of 33 °C. In this case, 2 °C reduction on internal gains can be attained by night ventilation with heavy weight fabric, and around 1 °C if the fabric complies only with the minimum building requirements in Peru (See figure 6).

Figure 3: Humphrey’s adaptability comfort model in Lima.

Case 1 shows operative temperatures obtained from daytime ventilation and night ventilation.

Nocturnal ventilative cooling has the highest internal temperatures, around 34 °C, 5°C above the comfort zone during the peak temperatures and an average of 30°C, on the contrary, daytime ventilation reaches a maximum of 31 °C and an average of 29.5°C.

Figure 4: Case 1, daytime and night ventilation effect in internal temperatures and thermal comfort. Software: Excel

Figure 6: Case 3, no ventilation and night ventilation effect in internal temperatures and thermal comfort. Software: Excel

Overall, in an unconditioned office building, full day ventilation is the most beneficial, followed by daytime and night ventilation. Results can be pushed further by investigating other variables like external colour, fabric (E.g. phase change materials).

Moreover, thermal stratification is a fact, hence, total building evaluation is necessary to have a complete performance overlook.

Nonetheless, nocturnal ventilative cooling application can take place in a mixed mode system, coupling such passive strategy with a mechanical cooling system. Further research is required in order to quantify the cooling loads reduction, energy savings and so as to evaluate its performance as part of a hybrid mode system.

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Passive Design Annual Conference PDAC 2014

6. CONCLUSION Given the mild meteorological conditions presented in Lima, nocturnal ventilation application is not suitable as a passive strategy only, since external temperatures are not unfavourable for cooling objectives in the subject building use. Due to the climatic benefits, day time and full day comfort ventilation can achieve better outcomes than night ventilation, thermal wise. However, if internal temperatures cannot reach comfort by passive means, there is cooling energy potential in night ventilation for a mix mode system, as it assists to the building cooling during night.

7. REFERENCES Blondeau, P., Spérandio, M. and Allard, F. (1997) ‘Night ventilation for building cooling in summer’, Solar Energy, 61(5), pp. 327–335.

Givoni, B. (1994) Passive low energy cooling of buildings (architecture). United States: John Wiley & Sons.

Kolokotroni, M., Webb, B. C. and Hayes, S. D. (1998) ‘Summer cooling with night ventilation for office buildings in moderate climates’, Energy and Buildings, 27(3), pp. 231–237.

Kolokotroni, M. and Aronis, A. (1999). Coolingenergy reduction in air-conditioned offices by using night ventilation. Applied Energy, 63(4), pp.241-253.

Peel, M. C., Finlayson, B. L. and McMahon, T. A. (2007) ‘Updated world map of the Köppen-Geiger climate classification’, Hydrology and Earth System Sciences Discussions, 4(2), pp. 439–473.

SENAMHI (no date) Available at: http://www.senamhi.gob.pe/main_mapa.php?t=dHi (Accessed: 18 December 2015).

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