|
Evaporative cooling has been utilized in contemporarary
architecture almost exclusively for the conditioning of interior space,
through the use of mechanical equipment. In order to study the extension
of evaporative cooling to include exterior or semi-enclosed spaces, a down-draft
evaporative cool tower was designed, developed and integrated in the design
of a 500 m² glazed courtyard located at the heart of a multi-use building complex
at the Sede-Boqer Campus. The prominent innovation in this type of
system is the potential for passive generation of downward air flow by
free convection, due to the thermal processes of evaporation through the
10 meter height of the tower.
Analysis of the system's performance showed dry bulb
temperature reductions of up to 14°C, a peak cooling output of over
100 kW with a wet bulb depression ratio (cooling efficiency) of 85-90%
during all hours of operation, and a water consumption rate of 1-2 m³/day.
Given the high efficiency of the evaporative process, a subsequent phase
of the research focused on analysis of a wind capture mechanism which could
be employed to increase the volume of air supply and at the same time reduce
reliance on mechanical circulation.
|
For more details see:
Experiments have been carried out over nearly ten years
in a project aimed at applying nocturnal long wave radiation to cooling
buildings. The project began with an experimental study of the basic processes
involved, in particular the effects of wind screens and the location of
thermal mass. This preliminary work made it clear that the key to improving
radiative cooling systems lay in the recognition that sustaining a high
cooling rate was possible only if the radiating surface remained relatively
warm. This required a means of extracting the energy absorbed in the thermal
mass of the building during the daytime to a radiator, where it might be
dissipated to the environment at night.
A cooling system was developed which consisted of a shallow roof
pond insulated from the environment, and flat plate collectors exposed
to the sky, through which the water was circulated at night to be cooled
by long wave radiation and convection. Since the temperature of the radiator
was close to that of the water in the roof pond, and warmer than that of
the ambient DBT for at least part of the time, fairly high cooling rates
could be maintained throughout the night.
The cooling system was installed and monitored in a demonstration
building of approx. 75m2 area. It provided a mean nightly cooling
rate of 80-100 watts per square meter of radiator area, and performed well
regardless of occupant behavior. It was also capable of providing limited
backup heating in a winter heating mode.
Preliminary examinations indicate that in
large areas of the Negev and other hot-arid regions of the world, earth-sheltered
buildings may perform very well, providing thermal comfort with a minimum
utilization of external energy. Earth-sheltered structures are becoming
increasingly popular in the US., both in cold and in warm regions. In Israel,
though, no underground housing has yet been built.
The project investigates and studies earth integrated
structures of the past, evaluating the potential of similar modern structures
and deriving conclusions relevant to the design and construction of such
structures in the future. Emphasis is placed on thermal comfort conditions
and energy conservation.
The Center has already built and monitored an experimental earth covered building
and has integrated this concept in the design of the multi-functional International
Center for Desert Studies building.
The built environment, consisting of buildings
and the open spaces between them, has a great effect on human thermal comfort.
An investigation has been conducted into the thermal behavior of such open
spaces, and into the effect that various urban design pattern and parameters,
have on the thermal behavior of the external spaces.
Within this project, special attention has been focused
on the "legendary" internal court-yard, which is repeatedly mentioned in
the literature as a thermal modifier in traditional arid lands structures.
Conclusions of the study indicated that this is not always the case, and
that the performance of such courtyards depends to a great extent on their
specific design.
Problems and issues of solar design, which have previously required complex calculations and massive use of computers have been analytically solved, to provide designers with simpler design tools that can provide accurate information needed for the design process. These analytic tools enable an insight into solar design and can significantly reduce computation time. In many cases these tools will also replace graphic tools used by architects and designers for many years.
The effect of buildings on the dry deposition of dust was investigated
in Be’er-Sheva, a desert city in southern Israel, and at two reference
points in the surrounding countryside. The mineral and chemical composition
of dust sampled at all sites was similar, reflecting the composition of
the local loess soil, its likely origin. However, dust deposited in the
traps set up in near buildings in the city was significantly coarser than
the dust which accumulated in similar traps at exposed sites in the countryside.
The amount of dust in the urban dust traps was on average more than twice
the amount deposited in the rural area. The differences in grain-size distribution
and quantity of dust are accounted for by the disturbances to the natural
environment caused by the presence of buildings and by human activity in
the city.
The study suggests that strategies commonly employed in the design
of buildings and urban space to reduce exposure to dust, such as the construction
of walled courtyards, are not effective. A significant reduction in the
concentration of dust in the vicinity of buildings in desert cities may
require a comprehensive approach which deals with the entire urban area
and its immediate surroundings, designed to reduce the availability of
erodible particles by means of planting or paving all exposed land surfaces.
A study was made of design grids and three-dimensional envelopes that optimize the use of energy and other building resources, while offering adequate living and working conditions. The research involved a study of various sections and layouts of buildings which were developed and tested. A recent study compared the thermal performance of flat and vaulted roofs, and another involved the design of a prismatic-section building to be used as a dormitory for students at the Sede-Boqer Campus.
A simple numerical model was developed to estimate the total radiative and convective energy exchange between a pedestrian and a given urban canyon environment, based on empirical micro-climatic data. Long-term measurements in a number of low-rise street canyons were integrated with the model in order to analyze the effect of canyon orientation and sectional geometry on a pedestrian's rate of heat loss or gain in different seasons.
The Center was commissioned by the Ministry of Energy to monitor the thermal performance of various types of buildings constructed all over the Negev desert. The study provided a post-occupancy evaluation of the effectiveness of different design approaches to the harsh climatic conditions.
Roofs are a primary source of undesirable energy gains leading to the overheating of buildings in hot climates. Current building practice seeks to minimize energy transfer through the roof by means of thermal insulation. However, the roof is also the part of the building envelope most suited to the installation and operation of various cooling systems. The research undertaken within the framework of a multinational project promoted by the EU is aimed at developing and testing roof systems or components suitable for use in Mediterranean or desert climates, which utilize natural cooling techniques in order to extract unwanted heat from buildings.
The cooling systems under investigation include various types of roof ponds; a nocturnal radiative cooling system involving the use of flat plate collectors as radiators; and various types of roof tiles as radiators for cooling air circulated from building interiors.
The project is currently at a preliminary stage, involving the selection of prototypes and the establishment of experimental facilities. Detailed testing and improved design are expected to last about two years.
Protected membrane roofs (sometimes known
as "inverted roofs") are roof systems in which a flat, concrete roof slab
is water-proofed by means of an external sealing membrane which is in turn
covered by thermal insulation laid on top of it. Such systems are recommended
where extremes of temperature or radiation damage the sealing membrane
and reduce its service life unless it is protected. The section of the
Protected Membrane Roof is also consistent with the conventional desert
construction technique in which it is recommended to install the insulation
on the outside surfaces of the building envelope in order to improve its
thermal performance. The thermal insulation commonly used consists of expanded
polystyrene boards, which are grooved on their lower side to allow drainage
of rainwater, and kept in place by a 5cm layer of gravel ballast.
The gravel ballast, which is used for mechanical reasons
only, was found to have detrimental side effects in both winter and summer
conditions. This is attributed to the creation of pockets of still air
trapped between the pebbles. In winter, air cooled by the evaporation of
rain water is removed very slowly, thus exposing the outer surface of the
insulation, for a period of up to 24 hours after the rain stopped, to lower
temperatures than would otherwise be measured. Also, the drainage system
of the roof, based on letting the water flow underneath the insulation
layer, exposes the roof slab to the temperature of the rain water which
is in direct contact with the slab. The temperature of this water is at
most the cool ambient temperate, or even a lower temperature. In summer,
solar radiation is absorbed by the gravel, and excess heat, otherwise removed
by convection, is trapped. The outer surface of the roof (above the insulation)
is thus exposed to temperatures up to 15°C higher than would be experienced
on a flat concrete surface.
A hybrid space-cooling system for hot-arid
zones previously investigated was based on the nocturnal radiative cooling
of water circulated through flat plate radiators. Preliminary investigations
also indicated that the same system, with no modifications to the physical
set-up, could provide a significant proportion of the winter heating requirements
of buildings exposed to these climatic conditions, where summers are hot
yet winters are frequently cold enough to justify the installation of heating
systems.
The heat output of the system averaged 370 Watts
per square meter of collector under the sunny but cool conditions typical
of Sede-Boker winters; However, on windy and overcast days the thermostat
control prevented water circulation and the system was inoperative. The
primary factors determining the heat output were the intensity of global
solar radiation incident on the collectors, wind speed and the temperature
difference between the water in the roof pond and the ambient air. An expression
was derived linking these parameters, which may be used to predict the
heat output of the specific system with a high degree of accuracy, and
thus define the climatic conditions where such a system may be of value.
Design and construction have undergone a series of changes and adaptations through history. The role environmental constraints have played in this process has been the subject of academic discourse over the years. Of special interest in this discourse is the evolution of urban settlements in the Negev desert during the Nabatean, Roman, and especially during the Byzantine period. To gain a better understanding of micro-climatic conditions during that period a number of CADD tools and in situ data collection are employed. Parallel examples of contemporary construction in similar regions and modern construction of similar technology are used as reference.
|
|
|