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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 32°C [1]. However, nights are cool - the mean daily minimum is 17°C - 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/m² 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 28°C 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 9.3°C [1]. While the mean daily maximum is 14.9°C, night time minimum temperatures average 3.8°C, 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/m² per day on a horizontal surface, and about 4.6 kWh/m² 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 m² 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 m² 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 m²°C/W , while the roof has 10cm of insulation, for a total resistance of about 3.6 m²°C/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.
| 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 m² 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 5-15°C 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 20-25°C, falling to 12-15°C at night (Figure 4). The vertical temperature profile displays the effects of thermal stratification, and a temperature difference of up to 5°C 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 4°C 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
1-1.5m³ 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 35-36°C, cooled by evaporation and exhausted at 21-22°C. 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 16°C, while ambient air temperature on a typical day fluctuated between 8°C and 18°C (Figure 8). By introducing warm air, at temperatures of up to 32°C, internal air temperatures inside the heated apartment were maintained at well over 20°C in the daytime, falling no lower than 18°C at night.
Based on the turbine's measured output of 260 m³ per hour and observed temperature differentials of up to 8°C 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
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[10] D. Pearlmutter, E. Erell, Y. Etzion, I. A. Meir, H. Di,
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+972-7-6596881; e-mail: etzion@bgumail.bgu.ac.il
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