The impact of shading on thermal comfort conditions in ... · The impact of shading on thermal...

1072 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

The impact of shading on thermal comfort conditions in perimeter zones with glass facades

A. Tzempelikos, M. Bessoudo, A. Athienitis, R. ZmeureanuConcordia University, Canada

ant and convective perimeter heating systems are usually needed beneath windows to counteract cold interior sur-faces and the related downdraft (Carmody, et al., 2004).High-performance windows (low U-factor, low-e coat-ings, low SHGC) are able to reduce heat losses and gains significantly, and can therefore improve comfort since the interior window surface temperature can be maintained closer to room air temperature. One of the main objectives of this paper is to study whether thermal comfort can be further improved with the use of shading devices. It can be said that the “success” of a building depends on whether a comfortable indoor environment is achieved. Occupants often list thermal comfort as one of the most important requirements of any building. Creating a comfortable indoor environment is also im-portant because occupants will react to any discomfort by taking actions to restore their comfort. Usually these reactions will have an energy cost, such as an occupant opening a window in the winter, when the heating is on, due to overheating (Nicol, Santamouris, 2003).

2. THERMAL COMFORT

Thermal comfort is defined as “that condition of mind which expresses satisfaction with the thermal envi-ronment and is assessed by subjective evaluation” (ASHRAE 55, 2004). It can be complex to evaluate since there exists large psychological and physiological variations from person to person. There are six primary factors which affect thermal comfort:1. Metabolic rate2. Clothing insulation3. Air temperature4. Radiant temperature5. Air speed6. HumidityThe metabolic rate and clothing insulation are consid-ered personal factors while others are considered envi-ronmental factors. The radiant temperature is usually expressed as the mean radiant temperature (MRT), defined as “the uni-form surface temperature of an imaginary black encol-sure in which an occupant would exchange the same amount of radiant heat as in the actual nonuniform

ABSTRACT

This paper describes an experimental study of the ther-mal environment in a highly-glazed perimeter zone of an office building under varying climatic conditions, as well as in a controlled test chamber at the Hydro-Que-bec research facilities in Shawinigan (LTE). Glazing and shading temperatures were measured at several points and the thermal environment was measured using an in-door climate analyzer and a thermal comfort meter. The aim of this study is to present measured thermal comfort conditions in perimeter zones and also to develop rec-ommendations for high-performance façades that will permit the decreased use -or elimination- of perimeter heating as a secondary heating system in office buildings.

1. INTRODUCTION

Occupants situated near windows often experience ther-mal discomfort. In the winter, window surface tempera-tures usually fall below indoor air temperature, caus-ing discomfort due to radiant temperature asymmetry and low operative temperature. In the summer, glass temperatures are usually higher than indoor air tem-perature, causing discomfort due to radiant temperature asymmetry and increased operative temperature. In ad-dition, solar radiation falling directly on the occupant can exacerbate discomfort. Shading devices, which are typically used for visual comfort purposes (glare con-trol), can also be used to improve thermal comfort con-ditions in perimeter zones. The building façade is one of the most important ele-ments in the design of any building. Not only does the façade have a significant influence on a building’s ap-pearance, but it also plays a critical role in the build-ing’s energy consumption, comfort conditions, and en-vironmental impact. As the trend towards highly-glazed façades for commercial buildings continues to grow, it is becoming increasingly important to be able to design façades taking into account its influence on all these con-siderations. Perimeter zones experience the largest fluc-tuations in temperature and human comfort. While the heating and cooling needs of commercial buildings are provided by forced air HVAC systems, additional radi-

PALENC 2007 - Vol 2.indd 1072 7/9/2007 1:25:59 µµ

10732nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

space” (ASHRAE, 2004). It can also be expressed in terms of radiant temperature asymmetry (RTA), defined as “the difference between the plane radiant tempera-ture of the two opposite sides of a small plane element”. The operative temperature is also an important measure in determining thermal comfort conditions since it in-corporates the MRT and air temperature, and is defined as “the uniform temperature of an imaginary black en-closure in which an occupant would exchange the same amount of heat by radiation plus convection as in the actual nonuniform environment”; in moderate thermal environments, it can be estimated to be the average of the MRT and air temperature (ASHRAE, 2004). Thermal comfort can be predicted from physiological and environmental variables. The most widely-used model for prediction of thermal comfort is the PMV-PPD model (Fanger, 1972). The predicted mean vote (PMV) index is used to predict the mean response of thermal sensation of a large group of people, ranging from -3 (cold) to +3 (hot) with 0 being thermal neutral-ity. The predicted people dissatisfied (PPD) index is a function of the PMV, with a minimum value of 5% and maximum value of 80%. Figure 1 shows the summer and winter comfort zones based on acceptable ranges of operative temperature and humidity. The winter zone is based on a clothing insulation value of 1.0 clo and the summer zone is based on a clothing insulation value of 0.5 clo. This chart is valid for a sedentary activity (met-abolic rate between 1.0 – 1.3 met), air speed less than or equal to 0.2 m/s. Conditions held within their respective zone will assure a PMV value between -0.5 and +0.5 and a PPD value less than 20%. The ASHRAE Standard (2004) states that the maximum allowable RTA due to a warm wall (including direct solar radiation) is 23ºC; for a cold wall it is 10 ºC.

Figure 1 - ASHRAE comfort chart (ASHRAE Standard 55-2004)

A window influences thermal comfort in three ways:• by long-wave radiation between glass surface and in-terior surfaces in zone;• by transmitting solar radiation to the interior;

• by creating induced air motion (convective drafts) caused by temperature differences. Absorbed solar radiation influences the temperature of the glass, which affects the MRT, while transmitted so-lar radiation falling directly on the occupant can cause discomfort due to radiant temperature asymmetry if it falls directly on the occupant. There have been a limited number of studies into the effect of solar radiation on mean radiant temperature and thermal comfort in the built environment. These studies, however, have not included the effects of shading devices (Le Gennusa et al., 2005).

3. MEASUREMENTS WITH AN EXPERIMENTAL FAÇADE

An experimental setup was established in a perimeter zone with a large glass façade at the Solar & Lighting Laboratory at Concordia University in downtown Mon-treal. The lab is located on the 16th floor of the unob-structed building and its façade is facing 20º west of south. In the lab, the length of the façade is divided into six different sections (Figure 2). Since each section is equipped with its own individual shading device, the section being studied was isolated with partitions to eliminate the effects of the adjacent sections. Each sec-tion is 1.5 m wide, 3.4 m high, and 2.3 m deep. The height of the façade is comprised of a 0.8 m high span-drel, and two sections of 1.3 m high glazing. Perimeter heating under the windows was turned off so as to make it a passive perimeter zone and study free-floating tem-peratures and resulting comfort conditions. Climatic data were collected every minute from exterior sensors placed outside next to the lab. Incident solar ra-diation and daylight levels were recorded using a Li-cor LI-200 pyranometer and a LI-210 photometric sensor. The pyranometer has a spectral response from 280-2800 nm and it is pre-calibrated against an Eppley precision spectral pyranometer under natural conditions. It has a linear response up to 3000 W/m2, a cosine correction for up to 800 angle of incidence and the absolute error is 3%; the response time is 0.01 ms. Several T-type thermocouples were used to record the exterior air tem-perature as well as the temperatures of all the interior surfaces (glazing, shading device, floor), in the air gap between the glazing and the shade, and the indoor air temperature (Figure 3). Indoor environmental conditions were measured using a Br�el & Kjaer Indoor Climate Analyzer Type 1213, a collection of instruments which provides a way to measure individual basic environmental parameters (air velocity, humidity, air temperature, plane radiant temperature in two directions). In addition, a Br�el & Kjaer Thermal Comfort Meter Type 1212 was used for measurement of the operative temperature. This trans-



ducer is able to evaluate the thermal effect that the sur-rounding objects and surfaces would have on the human body. The transducer’s size has been designed such as to closely match the ratio of radiative heat loss to con-vective heat loss of a human body. Its ellipsoid shape is determined by the need to obtain the same angle factors to the individual room surfaces as for a human body. The transducer can be set at different angles depending on the posture of the human body in question. For the case of a seated person it is set at a 45º angle. The in-door climate analyzer and thermal comfort meter were placed 1.3 m from the façade at a height of 1.1 m (Fig-ure 3). The air speed remained at approximately 0.1 m/s and the relative humidity remained around 30%. Two different shading devices were used: a roller shade and a venetian blind. The roller shade is an off-white colour with an average reflectance of 55%, absorptance of 40%, and transmittance of 5%. The venetian blind is aluminum grey, with an average specular reflectance of 5% and average diffuse reflectance equal to 75%. The windows of the façade are double glazed (6mm/12mm air space/6mm) with a low-e coating outside the inte-rior pane and argon filling. The glazing has a total solar transmittance of 36% and a visibile transmittance of 69%; the U-value is 1.59 W/m2 0K and the SHGC is 0.37.

Figure 2 - A view of the façade with different shading devices in each section.

Figure 3 - Experimental setup for the roller shade section with the climate analyzer and the thermal comfort meter.

4. RESULTS AND DISCUSSION

The thermal comfort experiments took place between January and March 2007. The objective was to exam-ine thermal comfort conditions near the façade during both sunny and cloudy days, with and without shading. Each graph presented in the following sections shows the variation of weather parameters (outside tempera-ture and solar radiation), surface temperatures (glazing, shade/blind), room air temperature, operative tempera-ture and radiant temperature assymmetry (between front –facing the window- and back –facing the room). Table 1 presents a summary of the experimental findings.

4.1 Measurements under clear skyFigure 4 shows the experimental results for the case with no shading during a cold, sunny day. Although the outside temperature is very low, the interior surface of the glass reaches 30ºC around noon. The operative temperature of the space exceeds the upper limit of 25ºC during a large part of the day (approx. 11:00am – 3:00pm solar time), reaching a maximum of 31ºC (total incident solar radiation is higher than 800 W/m2 during that time). The plane radiant temperature in the direc-tion of the façade is greatly influenced by the presence of direct solar radiation, causing radiant temperature asymmetry to exceed 15ºC during the same time period (the plane radiant temperature transducer cannot meas-ure temperatures higher than 50ºC in any one direction). Room air temperature remained near 25ºC from solar noon until 3:00 pm.

Figure 4 - Results without shading devices- sunny day.

When using a roller shade on a sunny day with slighty higher outside temperature, both the shade and the in-terior surface of the window reached temperatures near 40 ºC – the shade surface is slightly hotter than the glazing surface (Figure 5). However, because the shade does not allow direct sunlight to be transmitted inside, conditions remained within the comfort zone for most



of the day (9:30am – 6:00pm) although the operative temperature reached a maximum of 26 ºC in the after-noon –which is on the upper limit of the comfort zone. Radiant temperature asymmetry remained at or below about 5 ºC, while the room air temperature was between 22-250C during the day.With the venetian blind set at a horizontal position on a cold, sunny day (Figure 6), the glazing and blind reached temperatures of 32ºC. More importantly, how-ever, is the fact that direct solar radiation is allowed into the space, thereby increasing the radiant temperature asymmetry to a maximum of over 20 ºC. The spikes in the radiant temperature asymmetry readings shown in Figure 6 are due to the intermittant shading effect of the individual slats on the radiant temperature asymmetry transducer. The operative temperature remained above the comfort zone from about 11:00am – 5:00pm.

Figure 5 - Results with roller shade- sunny day.

Figure 6 - Results with venetian blind at 0º (horizontal) -sunny day.

Figure 7 - Results with venetian blind at 45º - sunny day.

Tilting the venetian blind to 45º (tilt from horizontal in order to block direct sunlight) has a significant effect on the thermal environment (Figure 7). The venetian blind reaches a maximum temperature of 34ºC. From 10:30 am to 6:00 pm, the operative temperature is maintained between 21-27 ºC. Radiant temperature asymmetry is kept below 10 ºC for almost the entire day, with only a few instances when the direct solar radiation enters the space in the afternoon (low sun angles). Finally, with the venetian blind closed (tilted to 90º), a vertical barrier between the indoor space and the glazing is created. The blind reaches a maximum temperature of 34 ºC and the operative temperature is kept between 20-26 ºC during the day. The RTA never exceeds 3 ºC (Figure 8).

Figure 8 - Results with venetian blind at 90º (closed) –sunny day.

4.2. Measurements under overcast skyThe same experiments were conducted during cold days under cloudy sky, in order to estimate the impact of shading on thermal comfort conditions and temperatures in the absence of direct solar radiation. Nevertheless, because of the high quality glazing, there is no signifi-cant radiant temperature asymmetry due to the window surface temperature for any case. Figure 9 presents the results during a cold cloudy day without shading.



Figure 9 – Results without shading –cloudy day

For the venetian blind at horizontal position on a cloudy day (Figure 10), the indoor air and blind temperature re-mained between 17 - 19 ºC for the entire day. The blind surface temperature is 70C higher than the glass tem-perature on average; the glazing temperature remained between 10 - 13 ºC and RTA does not exceed 4 0C. On a cold cloudy day with the venetian blind tilted at 45º (Figure 11), operative and room air temperatures remain at similar levels as with the horizontal blind case. With the blinds fully closed (Figure 12), operative temperature nev-er falls below 20ºC –outside temperature was a little higher in this case. The blind temperature was 50C higher than the glass temperature, which reached 230C around noon. It is interesting to note, as well, that the perimeter base-board heating use in the room directly beneath the solar laboratory was monitored during the measurement period. This room’s façade has the same orientation and is simi-larly designed, making use of the same roller shades used in this study. It was shown that on cold, sunny days, when the perimeter zone in the laboratory was maintained in the comfort zone for most of the day, the room directly beneath had the perimeter heating on for over 50% of the day. On cold, cloudy days when the perimeter zone was barely in the lower end of the comfort zone, perimeter heating was shown to have been on for up to 90% of the day.

Figure 10 - Results with venetian blind at 0º (horizontal) –cloudy day.

Figure 11 - Results with venetian blind at 45º -cloudy day.

Figure 12 - Results with venetian blind at 90º (closed) –cloudy day.

5. CONCLUSION

This paper summarizes the experimental results of ther-mal comfort conditions in a highly-glazed perimeter zoneswith shading devices. In cold and sunny conditions, a roller shade is shown to improve the thermal environment by decreasing RTA due to direct solar radiation and maintaining the opera-tive temperature of the space within the comfort zone for most of the day. Venetian blinds are also able to im-prove the thermal environment, but this depends on the tilt angle of the slats and solar altitude. With the slats in a horizontal position, for example, discomfort will be experienced due to high operative temperature and RTA. Moving the slats to a tilt of 45º improves thermal comfort by decreasing RTA to below 10ºC for most of the day, although allowing a small time with RTA great-er than 23ºC in the late aftenoon. Closing the venetian blinds completely will keep the operative temperature in the comfort zone for most of the day while decreas-ing RTA for the entire day. These results show that the installed perimeter heating is not needed for almost the entire day on cold and sunny days; the main VAV system in the zone can supply nearly all the required heating.



For cloudy days, the differences in thermal comfort con-ditions between using a shading device and not using one are less apparent. This is due to the high-perform-ance, double-glazed, low-e glazing used on the façade. Even on very cold, cloudy days, the operative tempera-ture of the perimeter zone never falls below 18ºC. Using shading devices on cold, cloudy days does not provide any significant improvement in the thermal environ-ment when high-performance glazing is used. This is important, since most occupants generally choose not to use any shading device on cloudy days in order to maximize view and daylighting without any detrimen-tal effects to visual comfort. In this experiment, an insulated glazing unit with a low-e coating and argon filling was used. Usually, double-glazed windows with standard low-e coatings are used on glass facades. For similar perimeter zones with stand-ard double-glazed windows, the impact of shading on thermal comfort would be even higher. With the results of these measurements, in contrast with the perimeter heating data collected from a similar space directly be-neath the solar laboratory, it can be seen how important it is to take the façade design into consideration, includ-ing glazing, shading, and orientation, when devising the heating strategy for perimeter zones and selecting pe-

rimeter heating capacity and control options.

ACKNOWLEDGEMENTS

The authors would like to thank Dr. Michael Collins from the University of Waterloo for measuring the properties of the shading devices and SOMFY Canada for domating the motorized shades and blinds. Financial support of this work was provided by NSERC through the Solar Buildings Research Network.

REFERENCES

ASHRAE 55-2004. Thermal comfort conditions for human occu-pancy. Atlanta: American Society of Heating, Refirigerating and Air-Conditioning Engineers, Inc., 2004Carmody, John, Stephen Selkowitz, Eleanor S. Lee, Dariush Ar-asteh, and Todd Willmert. Window Systems for High-Perform-ance Buildings. New York: W.W. Norton & Company, 2004. Fanger PO. Thermal comfort. New York: McGraw-Hill, 1972La Gennusa, Maria, Antonio Nucara, Gianfranco Rizzo, and Gi-anluca Scaccianoce. “The Calculation of the Mean Radiant Tem-perature of a Subject Exposed to the Solar Radiation - a General-ised Algorithm.” Building and Environment 40 (2005): 367-375.Nicol, F. “Thermal Comfort.” Solar Thermal Technologies for Build-ings. Ed. M Santamouris. London: James & James Ltd, 2003. 164.

Sunny daysRange of glazing

surface temp (ºC)

Range of shade/blind temp (ºC)

Average temp dif-ference between blind and glazing

(Tshade - Tglass)

Range of op-erative temp

(ºC)

Maximum RTA (ºC)

Range of room air temp (ºC)

No shading 11 – 31 - - 19 – 32 > 21 18 - 26Roller shade 13 - 41 18-41 2 18 - 26 6 18 - 25

Venetian blind (horizontal) 14 – 33 20 – 33 2 19 - 31 > 23 19 - 26

Venetian blind (45º) 11 – 32 19 – 34 4 17 – 27 >23 19 - 24

Venetian blind (closed) 12 – 36 19 – 34 3 18 – 25 3 18 - 24

Overcast daysRange of glazing

surface temp (ºC)

Range of shade/blind temp (ºC)

Average temp dif-ference between blind and glazing

(Tshade - Tglass)

Range of op-erative temp

(ºC)

Maximum RTA (ºC)

Range of room air temp (ºC)

No shading 13 – 18 - - 18 – 21 4 18 - 21Venetian blind

(horizontal) 11 – 13 18 – 19 6 18 – 19 4 17 - 18

Venetian blind (45º) 12 – 16 19 – 23 6 18 – 20 3 17 - 19

Venetian blind (closed) 15 – 22 20 – 27 4 20 – 23 2 18 - 22


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