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Automation in Construction 6 (1997) 417-425

 

Adaptive Architecture:
Integrating Low-Energy Technologies
for Climate Control in the Desert

 

Y. Etzion*, D.Pearlmutter, E. Erell. I. A. Meir

 

The Center for Desert Architecture and Urban Planning
The J. Blaustein Institute for Desert Research
Ben-Gurion University of the Negev
Sede-Boqer Campus 84990 Israel

 

Abstract

The article describes a 'climatically adaptive' approach to intelligent building in which a variety of technologies are integrated in the architectural design to provide thermal comfort with a minimal expenditure of energy. This concept is illustrated by the design of the Blaustein International Center for Desert Studies, a multi-use complex completed recently at the Sede-Boqer Campus of Ben Gurion University of the Negev. In response to the local climate of this desert region, a number of strategies were developed by the authors to exploit natural energy for heating and cooling: earth berming of major parts of the building, 'selective glazing' for seasonal shading and energy collection, a down-draft 'cool tower' for evaporative cooling and a hybrid mechanism for hot-air supply are several of the unique systems whose performance and feasibility are analyzed within the context of the overall building design.

Keywords: Bio-climatic architecture; Selective glazing; Evaporative cooling; Passive heating; Earth berming

 

1. Introduction

Le Corbusier considered buildings 'machines for living in'. Modern buildings have indeed become increasingly complex, involving technologically advanced building materials, and mechanical systems for controlling interior air quality, thermal comfort, lighting and acoustics. These systems, which rely exclusively on the utilization of non-renewable energy, are often expensive to install and energy-intensive in operation. This is particularly true of buildings constructed in locations with extreme climatic conditions, such as deserts, where the difference between ambient conditions and the desired interior conditions is large.

This paper illustrates a radically different approach to the provision of thermal comfort in a building. Rather than invest non-renewable energy to counteract the natural conditions, it is often possible to harness natural energies and exploit the local climate to great advantage - by adapting the architectural design of the building. Climate conscious design requires a thorough understanding of the local climate, and the employment of several strategies and systems for the creation of an agreeable micro-climate with a minimal investment of energy. The success of such a design depends as much on the integration of these strategies and on the proper operation of the building by its users as it does on the individual performance of each technological system.

 

2. The problem: local climatic conditions

The building described below was built at the Sede-Boqer Campus of the Ben-Gurion University of the Negev, located at 30.8 latitude, about 480 m above sea level. The Negev Highlands - (300 m and above) - are characterized by cold and mostly sunny winters, and by summers that are hot during the day but usually pleasant at night. Average annual rainfall is 80 mm, but there is a considerable deviation from year to year. The following analysis was made of the effect of the local climate on the design:

Summers are hot and dry, with a mean daily maximum temperature of 32C [1]. However, nights are cool - the mean daily minimum is 17C - so that the mean daily temperature lies within the thermal comfort range. The problem for the designer is thus limited to overcoming the overheated conditions prevailing around mid-day, while convective cooling is particularly effective at night due to the low ambient temperature. Solar radiation is very intense: global radiation on a horizontal surface averages 7.7 kWh/m per day (during June and July). Thermal comfort in outdoor or semi-exposed areas depends not only on lowering the air temperature, but to a great extent on reducing the exposure to this intense radiation. Ambient relative humidity is very low, between 20% and 40% for most of the day, but may rise to 90% or more at night, when the air temperature drops sharply. On extremely dry days the lack of moisture in the air may cause some discomfort. However, for most of the time, the low relative humidity extends the thermal comfort range, so that temperatures as high as 28C may be tolerated quite comfortably.

Winters in Sede-Boqer are sunny but cool during the day, and cold at night. The mean daily temperature in January is only 9.3C [1]. While the mean daily maximum is 14.9C, night time minimum temperatures average 3.8C, due to the intense radiative cooling characteristic of clear desert skies. Thus, considerable energy is required to heat buildings to comfortable levels. The same clear sky conditions result in high levels of insolation during the daytime: Global radiation averages 3.3 kWh/m per day on a horizontal surface, and about 4.6 kWh/m per day on a south-facing vertical surface. The abundance of solar radiation and the large number of clear days are ideal conditions for the provision of passive heating in buildings, relying on the utilization of solar energy.

 

3. The response: project overview

The Blaustein International Center for Desert Studies (BIC) building is a multi-functional complex which was designed to house the international activity of the J. Blaustein Institute for Desert Research (Figure 1). The 1100 m building is home to the Institute's library, teaching facilities, a cafeteria and lounge, and administration offices, as well as two apartments and six smaller dormitory rooms for accommodating visiting scientists and scholars. The various building elements are organized around a 500 m central atrium, which straddles a main pedestrian artery linking the existing campus with its future expansion. The protection it provides from the extremes of the outdoor climate has resulted in a flourishing semi-tropical garden, which stands in contrast to the barren landscape outside. The provision of thermal comfort in this relatively large space was one of the main challenges facing the design team, but also provided an opportunity for a comprehensive approach to the climatic conditions in the building complex as a whole.

In winter, the provision of thermal comfort by passive means can be achieved by two strategies - maximizing solar heat gains (and the means of storing the incoming energy), and minimizing heat loss through the envelope of the building. In the case of the BIC building, the atrium was designed to function in winter as a greenhouse. Warm air from the atrium is drawn during the daytime to heat the adjacent spaces. Heat losses are minimized by reducing the area of exposed exterior surfaces, and by providing sufficient thermal insulation. The exterior walls have 5cm of expanded polystyrene insulation, for a total thermal resistance of about 2 mC/W , while the roof has 10cm of insulation, for a total resistance of about 3.6 mC/W. (These values exceed considerably the minimum requirements for thermal insulation in residential buildings set out in Israel Standard 1045[2].)

 

Figure 1: The Building of the International Center for Desert Studies - General layout and sections.

 

In summer, thermal comfort is achieved through a combination of three strategies:

1.Reduction of unwanted heat gains through careful treatment of the building exterior. Insulation of exterior walls is of primary importance, in summer as well as in winter. However, several other strategies were also adopted:

1.1. The exterior surface of the walls is a smooth stucco painted white, so that 70-80% of incident solar radiation is reflected, compared with about 50% for most commonly used finishes, such as limestone or textured stucco [3,4]. The smooth finish was selected to reduce the adherence of airborne dust particles, thus preventing the discoloration of the external walls which occurs commonly in desert conditions.

1.2. The orientation of window openings and glazed areas, (other than the atrium), was designed to allow ventilation, yet reduce heat gains to a minimum. There are no openings on the east or west elevations of the building, where the intensity of solar radiation in summer is higher than on any other surface except the roof (see Table 1). Since these elevations also enjoy less solar radiation in winter than a south facing wall, they are the least desirable orientations. Rooms in the main residential wing open onto the atrium, and have only small windows facing north, the direction of the prevailing winds in Sede-Boqer during the summer, to allow cross ventilation.

1.3. Neutralization of the heat gains from the atrium, by reducing the transmission of solar radiation through the roof surface, by means of a unique selective glazing, and by the addition of internal shading beneath the glazed surface.

 

Table 1: Daily Incidence of Solar Radiation on Building Surfaces by Orientation Sede-Boqer (kWh/m)

  North East South West Roof
July 2.48 5.07 2.62 5.07 7.67
January 1.08 2.54 4.63 2.54 3.33

 

2. Use of high thermal capacity materials to maintain the interior conditions close to the daily average, which, in Sede-Boqer, lies within the comfort range throughout the summer. The building's significant thermal capacity should contribute to stabilizing the large daily fluctuations typical of the desert, and should also increase the building's thermal lag time, which is the time that passes, for instance, between the peak external temperature and the peak internal temperature [5]. The external walls of the building combine insulation with thermal mass. The internal layer of the wall is built of concrete blocks, which constitute an integral part of the storage mass of the building. On the external side of the wall an insulating layer is attached, consisting of 5 cm thick expanded polystyrene, protected on the outside by a special acrylic stucco. The order of the layers in the wall is of great importance: while the thermal resistance of the wall (R-value) is simply the sum of the thermal resistances of all the layers, the thermal storage capacity depends on the degree of thermal contact between the interior air and the wall surface. Should the insulation be placed on the interior surface of the wall, the effective thermal capacity of the envelope would be greatly reduced, as would its thermal time constant.

2.1. The building's exterior is covered with earth berms up to the height of the second floor windows. Previous research on earth-sheltered buildings carried out by the Desert Architecture Unit [6] and others [7,8] has shown that earth berms may reduce significantly the heating and cooling loads on buildings in climatic conditions similar to those of the Negev highlands. The effect of the earth cover is two-fold: first, it reduces the effect of extreme thermal conditions on the building's external surfaces, thus reducing the rates of both energy gain (summer) and energy loss (winter). Second, earth berming increases the thermal inertia of the building by increasing its heat storage capacity, thus reducing its internal temperature fluctuations.

3. A passive cooling system, the evaporative cool tower, was introduced to improve thermal comfort in a selected, high use area in the atrium, where due to the size of the space and to its exposure, the effect of other measures adopted to provide thermal comfort was deemed insufficient.

 

4. Experimental evaluation of the building's thermal performance

4.1. The sunken atrium

All building spaces are arranged around the 500 m sunken and enclosed atrium (Figure 1). The atrium is not only the visual and functional focus of the building, but also a thermal buffer and modifier, creating a microclimatic "oasis'" within the harsh desert surroundings. The atrium is partly shielded from the exterior environment by elements of the building itself, such as the main residential wing on the north side of the building and the library on the south. It also benefits from the effects of earth berming: The lowest level of the courtyard is 2.5 meters below grade, and the north and west wings of the building have been earth bermed against the exterior walls up to a height of 4.5 meters above grade, or 7 meters above the atrium floor.

The roof of the atrium is glazed with a unique double-skinned polycarbonate sheet manufactured in Israel, which is a selective transmitter of solar radiation. The material has small, triangular prisms along the length of the interior surface of its outer skin (Figure 2).

 

Figure 2: Section through a 'selective' polycarbonate sheet, showing internal reflections and selective transmissivity.

 

Figure 3: Calculated and measured transmissivity of selective glazing
in summer (August-September) and winter (January-February).

 

These prisms create internal reflection within the sheet, resulting in variations in the transmissivity of the material which are a function of the angle of incidence of the solar radiation. A large proportion of radiation impinging upon the surface at an angle close to the normal (±9) is reflected, while reflection of radiation at oblique angles is lower. The roof geometry, i.e. its tilt angle (20 facing south) and the direction of the prisms (E-W), was determined so that during the hot hours of the summer most of the incident solar radiation would be reflected, while in winter most would be admitted into the atrium space.

In winter, the atrium functions as a greenhouse. All its openings are closed, and the air within it is heated by the incoming solar radiation. The roof glazing transmits approximately 60% of incident solar radiation (Figure 3), so that interior temperatures at floor level are 5-15C higher than ambient air temperature. The temperature elevation is greatest during clear sunny days, but is evident even at night or during overcast weather. Under normal winter weather conditions at Sede-Boqer, the daytime temperature in the atrium is typically 20-25C, falling to 12-15C at night (Figure 4). The vertical temperature profile displays the effects of thermal stratification, and a temperature difference of up to 5C between the floor and apex of the atrium was recorded on clear, sunny days (Figure 5a).

 

Figure 4: Average temperature at the atrium floor during a typical 3-day period in winter.

 

In summer, thermal comfort in the atrium greatly depends on the extent to which solar heat gain can be minimized, and excess heat removed. The original design called for the reduction of solar radiation by two means:

(a) Selective glazing used in the roof. The glazing panel was intended to act as a shading device in summer, but was found to transmit considerably more radiation than that calculated from the manufacturer's data (Figure 3). Thus some 40-50% of incident solar radiation penetrated the glazing and increased the heat load on the building interior. The high proportion of indirect diffuse radiation, as opposed to direct beam radiation, as well as the accumulation of dust on the glazed roof, may be responsible for the reduction in the performance of the selective glazing, but this remains the subject of a separate study.
(b) Light-reflecting canopy. The original design called for the installation of such a canopy parallel to the interior plane of the roof, to further reduce the penetration of solar radiation and to cut glare. Due to budget constraints, this curtain has been installed in only one section of the roof, and is now being used only as a demonstration of the original intentions.
The removal of excess heat is achieved by two means:
(a) Operable windows installed along the length of the south side of the atrium and along the apex of the atrium are opened during the summer. The effect of these windows is to allow the removal by cross ventilation of excess heat trapped in the upper parts of the atrium.
(b) A large down draft evaporative cool tower. The performance of this cool tower is described at length in a separate section of this article. In the configuration studied, daytime temperatures at the atrium floor level were similar to exterior air temperature, during all but afternoon hours. Figure 5b shows that for this mid-day period of highest heat stress, air temperatures at floor level were up to 4C lower than the ambient, while those measured at the atrium's apex were slightly higher than the ambient. Thus, while the planned cooling effect is not evident in the entire atrium, its benefit is realized in the occupied area adjacent to the cafeteria, where thermal comfort is further enhanced by the airflow generated by the cool tower.

 

 

Figure 5: The vertical temperature profile in the atrium on typical (a) summer and (b) winter days.

 

Figure 6: Schematic section of cool tower, showing installation in atrium and typical temperature profile on a summer day.

 

The degree of mixing of outside air with interior air in the lower parts of the atrium is a critical factor in determining the overall thermal comfort. The balance between evaporative cooling provided by the cool tower and comfort cooling provided by the natural airflow has still not been resolved, and the extent to which the atrium should be opened to natural ventilation during the summer is a matter for further study.

4.2. The evaporative down-draft cool tower

As previously mentioned, in summer cool air is provided to the atrium by a large evaporative cool tower. Evaporative cooling is a familiar and energy-efficient tool for space conditioning in arid regions, where daytime temperatures are high and relative humidity is low [9]. The primary innovation of its use here is the exploitation of convective forces for the conditioning of a relatively open public space.

Figure 6 shows a vertical cross section of the tower: its height is approximately 12 meters, and its horizontal section is octagonal, with a 4 meter width from side to side. Water injectors and sprayers - which were selected empirically - saturate the air in the tower with water, causing fast and intensive evaporation and significantly lowering the air temperature. More water is injected into the air than can be completely evaporated, in order to ensure the highest possible evaporation rate and temperature reduction. The excess water falls into a small collection pond at the bottom of the tower, to be recycled by a small pump to the top of the tower and to be re-sprayed into the air. The result is that no water is lost except that volume which is evaporated and which cools the air. The quantity of water which is evaporated by the air moving through the tower is about 1-1.5m per day, depending on the ambient external conditions. At the top of the tower, a low-rpm fan was installed to supplement the natural convective down-draft that is caused by the temperature difference between air in the upper and lower parts of the tower. Calculated as a function of volumetric air flow and temperature depression, up to 120 kW of cooling power is provided by the tower in the hottest hours of the summer [10]. Cool air is supplied at the lowest point of the atrium, so that it accumulates in the sitting area of the cafeteria and ascends only as it begins to warm up. As the warm air rises, it is replaced by cooler air supplied by the tower, so that the lower layer of air, which is closer to the atrium floor, remains relatively cool.

 

Figure 7: Temperature depression by evaporative cooling in the down-draft cooling tower.

 

Figure 7 shows the performance of the tower on a typical summer day. At mid-day, outside air is drawn into the tower at 35-36C, cooled by evaporation and exhausted at 21-22C. Although the air leaving the tower is close to saturation, upon mixing with the internal air of the atrium its humidity drops and the resulting relative humidity in the occupied sitting area is less than 65%. Calculated as a proportion of the maximum possible temperature depression obtainable by evaporation alone, the system's efficiency was found to exceed 85% during all hours of operation.

Upon observation of the cool tower's performance, a number of possible improvements were identified. The most important of these concerns the up-draft of air through the tower, due to wind-generated suction at the inlet above roof level. This phenomenon, which is the result of the particular geometry of the roof and the height of the air inlet above it, counteracts the thermal down-draft and reduces the efficiency of the fan at the top of the tower. The reduction in the air flow rate through the tower results in a lower overall cooling output. Further experiments carried out at the Desert Architecture Unit [10] have resulted in the development of a wind capture unit that is to be installed at the head of the tower. The dual intent of this measure is to deflect the natural airflow above the roof into the tower, and to prevent the reverse flow observed under some conditions, thus increasing airflow and possibly reducing the system's dependence on mechanical means.

An unforeseen aspect of the cool tower's operation was the accumulation of sediment in the pool beneath it. Dust found in suspension in the ambient air is washed out by the water droplets in the tower, and deposited in the pool. Since the volume of air flowing through the tower is quite large, the amount of dust washed out is enough to require frequent cleaning of the pool. This "rinsing" effect does, however, have a decidely positive side benefit, since it allows cleaner air to be introduced into the atrium.

4.3. Indirect space heating from solar heated air

The upper stories of the north wing of the building, which serve as guest accommodations, are heated by drawing in warm air from the apex of the atrium. Air is channeled during the warm daytime hours through 6" ducts stretching from the top of the atrium into each room, drawn in by small, individually operated air turbines positioned at each of the duct outlets.

 

Figure 8: The effect of space heating by forced convection of atrium air in winter.

 

A comparison of the temperatures in two similar rooms on the same floor illustrates the effect of the heating system. A reference room, well insulated and enjoying the benefits of its exposure to the mild conditions in the atrium, but having no other source of heat, remained stable at about 16C, while ambient air temperature on a typical day fluctuated between 8C and 18C (Figure 8). By introducing warm air, at temperatures of up to 32C, internal air temperatures inside the heated apartment were maintained at well over 20C in the daytime, falling no lower than 18C at night.

Based on the turbine's measured output of 260 m per hour and observed temperature differentials of up to 8C on a typical sunny day, the system provided a peak heating power of about 600 W and a daily heat output of 3.7 kWh, requiring only 40 W of electric power for operation. Calculation of the net heat output was based on the temperature difference between room air and the warmer air at the duct inlet, and reflects the low amount of energy required to maintain the well-insulated room at a comfortable level. Performing the same calculation for a colder room would result in a significantly higher net heat output, since the temperature of the air supplied by the duct depends only on the conditions at the apex of the atrium.

 

Conclusions

The design of the International Center for Desert Studies incorporates several innovative energy saving strategies in concert. The evaluation of these measures indicates that some, such as the evaporative cool tower and the solar heated air system appear to be very successful and cost-effective means for the provision of thermal comfort in desert climates. The performance of others, such as the selective glazing, while below the manufacturer's claims, are worthy of consideration in other projects. Most of the innovative features described in this paper, such as the down-draft cool tower, may benefit from further research and optimization. The effect of the earth cover in this building remains a subject for future investigation. It is anticipated that the completion of the building, involving the installation of the interior shading canopy underneath the atrium roof, and of a wind capture mechanism in the cool tower, should further enhance the thermal conditions inside the atrium in summer.

 

References

[1]A. Bitan, S. Rubin, Climatic Atlas of Israel for Physical and Environmental Planning and Design, Ramot Press, Tel-Aviv University, Tel-Aviv, 1991.
[2] The Standards Institution of Israel, Thermal Insulation of Residential Buildings - Israel Standard 1045, The Standards Institution of Israel, Tel-Aviv, 1989.
[3] B. Anderson, Solar Energy, McGraw Hill, New York, 1977.
[4] G.G. Gubareff et al., Thermal Radiation Properties Survey, 2nd edn., Honeywell, Mineapolis, 1960.
[5] B. Givoni, Man, Climate and Architecture, Van Nostrand Reinhold, New York, etc., 1969, pp. 120-144.
[6] D. Pearlmutter, E. Erell, Y. Etzion, Monitoring the thermalperformance of an insulated earth-sheltered structure: a hot-arid zone case study, Architectural Sci. Rev. 36 (1)(1993) 3-12.
[7] W.B. Davis, Earth temperature: its effect on undergroung residences, in: Moreland, Higgs, Shih (Eds.), Earth Covered Buildings: Technical Notes, U.S. Dept. of Energy and University of Texas at Arlington, 1979, pp. 205-209.
[8] A. Rahamimoff, S. Rahamimoff, A. Silberstein, D. Faiman, A. Zemel, D. Govaer, Design considerations for an earth-integrated education centre in the Israeli desert, Tunnelling and Underground Space Technol.2 (1) (1987) 69-71.
[9] B. Givoni, Passive and Low Energy Cooling of Buildings, Van Nostrand Reinhold, New York etc., 1994, pp. 131-147.
[10] D. Pearlmutter, E. Erell, Y. Etzion, I. A. Meir, H. Di, Refining the use of evaporation in an experimental down-draft cool tower, Energy and Buildings 23 (1996) 191-197.

 

* Corresponing author. Tel.: +972-7-6596875; fax:
+972-7-6596881; e-mail: etzion@bgumail.bgu.ac.il

 

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