The central research interest of the architectural staff lies in the adaptation of the built environment to desert climatic conditions, with the dual aim of enhancing thermal comfort and conserving non-renewable energy. Investigations are focused on issues which relate to the design of the urban fabric, of the individual building, and of the associated systems for heating and cooling.
URBAN MICROCLIMATE AND OPEN SPACES
The influence of urban design on outdoor thermal comfort
The built environment, consisting of buildings and the open spaces between them, has a great effect on human thermal comfort. A number of investigations have been conducted into the thermal behavior of such open spaces, and into the effect that various urban design patterns and parameters have on the microclimate and comfort conditions within them.
Special attention has been focused on internal courtyards, which are often referred to as thermal modifiers in the built environment of hot regions. However, the conclusions of a number of studies have indicated that this is not necessarily the case, and that the success of such courtyards in creating a favorable microclimate depends to a great extent on their detailed design, requiring careful attention to a range of factors including geometry, finish materials, and the use of vegetation.
In one recent in-depth study (Pearlmutter, 1998), the complex and contradictory microclimatic phenomena operating in an urban "street canyon" were investigated by developing a numerical model to estimate the rate of thermal energy exchange between a pedestrian and his urban environment. 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 overall rate of heat loss or gain in different seasons. Results of the study, while confirming the existence of a canopy-layer "heat island" effect even in small-scale canyons, showed that the overall level of discomfort encountered by a pedestrian is mitigated in such compact street canyons during the most crucial periods of thermal stress.
For more details see:
Pearlmutter, D., Berliner P. and Shaviv E., “Evaluation of urban surface energy fluxes using an open-air scale model,” Journal of Applied Meteorology Vol. 44 No. 4, pp. 532-545, 2005.
Pearlmutter, D., Berliner P. and Shaviv E., “Physical modeling of pedestrian energy exchange within the urban canopy,” Building and Environment, 2005 (in press).
Pearlmutter D., Berliner P. and Shaviv E., "Development of a scale-modeling technique for urban microclimatic analysis." In: Wibig J. and Gajda-Pijanowska I. (eds.) Proceedings of the Fifth International Conference on Urban Climate - Lodz, Poland, September 2003.
Pearlmutter D., Berliner P. and Shaviv E., "Analyzing the microclimatic influence of urban canyon geometry with an open-air scale model." In: Bustamante, W.G. and Collados, B. (eds.) PLEA 2003 - Proceedings of Passive and Low Energy Architecture 20th International Conference - Santiago de Chile, November 2003.
Pearlmutter D., Bitan A. and Berliner P., "Microclimatic analysis of 'compact' urban canyons in an arid zone," Atmospheric Environment Vol. 33, pp. 4143-4150, 1999.
Pearlmutter D., "Street canyon geometry and microclimate: Designing for urban comfort under arid conditions." In: Maldonado E. and Yannas S. (eds.) Environmentally Friendly Cities, Proceedings of PLEA '98, Lisbon, Portugal, June 1998, pp. 163-166, 1998.
Meir I.A., Pearlmutter D., Etzion Y., "On the microclimatic behavior of two semi-enclosed attached courtyards in a hot dry region," Building and Environment, Vol. 30, No. 4, pp. 563-572, 1995.
Meir I.A., "Urban space evolution in the desert - the case of Beer-Sheva," Building and Environment, Vol.27, No.1, pp.1-11, 1992.
Etzion Y., "The thermal behavior of non-shaded closed courtyards in hot arid zones," Architectural Science Review, Vol. 33 pp. 76-80, 1989.
Modeling the urban street canyon
Meteorological measurements required for the development of a design scheme that responds to the local environment are generally recorded by the weather service in stations that are assumed to be representative of the surrounding region. However, no account is taken of the changes in conditions caused by urban development, even though differences between meteorological conditions within cities compared with adjacent rural areas (the heat island effect) have been noted as early as Roman times, and may be substantial. A new computer model (CAT) provides data representing realistic site-specific air temperature in a city street based only on data from standard weather stations such as those operated at local airports and descriptors of the two sites. The CAT model has been tested on field data measured in a monitoring program carried out in Adelaide in 2000-2001. After calibrating the model, predicted air temperature correlated well with measured data in all weather conditions over extended periods. The model may also be used to analyze the effects of urban geometry and morphology on air temperature in the canopy layer.
For further details see:
Erell E. and Williamson T. “Simulating air temperature in an urban street canyon in all weather conditions using measured data at a reference meteorological station”. In press, International Journal of Climatology.
Erell E. and Williamson T. (2006) “Comments on the correct specification of the analytical CTTC model for predicting the urban canopy layer temperature”. Energy and Buildings, 38:1015-1021.
Erell E. and Williamson T. (2006) “The estimation of air temperature in an urban street canyon on the basis of measured data from a meteorological station in the region”. In Grimmond S. and Lindqvist S. (Eds.) Proceedings of ICUC6 – 6th International Conference on Urban Climate, Goteborg, Sweden, June 12-16 2006.
Erell E. and Williamson T. (2005) “Experimental validation of a model to adapt measured data at a standard weather station to represent site-specific air temperature in an urban street canyon”. In Bloem, J. and Sutherland, G. (Eds.) Proceedings of DYNASTEE 2005 – Dynamic Analysis, Simulation and Testing applied to the Energy and Environmental performance of buildings, Athens, Greece, October 12-14 2005.
Erell E. and Williamson T. (2004) “The CAT model: Predicting air temperature in city streets on the basis of measured reference data”, In: Bromberek, Z. (Ed.) Contexts of Architecture, Proceedings of the 38th Annual Conference of the Architectural Science Association ANZAScA and the International Building Performance Association, Australasia, Launceston, Australia, November 10-12, 2004, pp. 210-215.
Erell E. and Williamson T.J. (2002) “Predicting air temperatures in the urban canopy layer from measured reference data”, in GRECO and ACAD (Eds.) Design with the Environment, Proceedings of the 19th PLEA International Conference, Toulouse, France, July 22-24, 2002, pp. 145-152.
Williamson, T.J. and Erell, E. (2001) “Thermal performance simulations and the urban micro-climate: measurement and prediction”, In Building Simulation 2001, Proceedings of the 7th International IBPSA Conference, Rio de Janeiro, Brazil, August 13-15, 2001, pp. 159-165.
The Effect of Buildings on the Deposition of Dust in Desert Cities
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.
For more details see:
Erell E. and Tsoar H., "Spatial variations in the aeolian deposition of dust - the effect of a city: A case study in Be’er Sheva, Israel," Atmospheric Environment, Vol. 33, pp. 4049-4055, 1999.
Erell E. and Tsoar H., "An experimental evaluation of strategies for reducing airborne dust in desert cities," Building and Environment, Vol. 32, No. 3, pp. 225-236, 1997.
Tsoar H., Erell E., "The effect of a desert city on aeolian dust deposition," Journal of Arid Land Studies, Vol. 5S, pp. 115-118, 1995.
ENERGY ASPECTS OF BUILDING DESIGN AND CONSTRUCTION
Building Technology in the Negev in the Byzantine Period (4-7 c.CE) and its Adaptation to the Desert Environment
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 the attempts which may have been made during that period to create a favorable microclimate for urban dwellers, conditions within the buildings and open spaces of these ancient cities are monitored and simulated, and the results are verified using contemporary building climatology research. A number of CADD tools are employed together with in-situ data collection, and parallel examples of concurrent construction in similar regions and modern construction of similar technology are used as references.
Such information is vital in reconstructing and understanding everyday life, including aspects of environmentally responsive settlement patterns and construction, water storage and use, thermal comfort, dress adaptation, biomass use for heating, and possibly even anthropogenically induced or enhanced desertification - a process currently reaching a worrying magnitude.
Monitoring the Thermal Performance of Existing Buildings
Researchers at the Center have been commissioned by the Israel Ministry of Energy to monitor the thermal performance of various types of buildings constructed in different parts of the Negev desert. Results of the various studies have provided post-occupancy evaluation of the effectiveness of different design approaches to the harsh climatic conditions.
- "Floating Roof" Retrofits - One project monitored involved the retrofit of existing flat-roofed housing units with a ventilated "floating roof." Located at Kibbutz Yotvata in the hot-arid Arava, these uninsulated prefab buildings were found to benefit greatly from the broad white-tiled roofs which were built above each house in a way that effectively shades the actual roof slab and allows a free flow of ventilation air in the interim space.
- Lightweight Immigrant Housing - An in-depth monitoring study of lighweight housing units, which were imported to Israel in massive quantities as a response to the immigration wave of the early 1990s, showed that planning policies at the level of central government may have adverse practical implications when implemented in peripheral regions such as the Negev desert. While such lightweight buildings may have been an acceptable compromise for housing immigrants in the more temperate coastal area where most of Israel's population is concentrated, the introduction of lightweight housing in the Negev desert created severe discomfort conditions and made the installation of air conditioning imperative even in cases where such mechanical means were avoidable with conventional construction.
- Underground and Earth-Sheltered Buildings - In the Negev desert, as in many other regions of the world where the climate is characterized by extreme thermal fluctuations, earth-sheltered buildings may provide thermal comfort with a minimum utilization of external energy.
Temperatures in an experimental earth-covered building constructed at the Institute were monitored over the course of several seasons, and the building's thermal behavior was indeed found to be extremely stable. Since average daily temperatures in the Negev Highlands are within the comfort zone for much of the year, this thermal stability proved to be distinctly advantageous in terms of energy efficiency.
Measurements of temperature within the soil of surrounding earth berms aided in clarifying the mechanisms by which energy is transferred and stored in this type of building, and allowed the formulation of recommendations for improvement in earth-sheltered construction methods. The concept of earth-sheltering has since been applied in new designs, for the multi-functional International Center for Desert Studies building and Samar Community Library.
For more details see:
Pearlmutter D. and Meir I.A., "Assessing the climatic implications of lightweight housing in a peripheral arid region," Building and Environment. Vol. 30, No. 3, pp. 441-451, 1995.
Pearlmutter D., Erell E., Etzion Y., "Monitoring the thermal performance of an insulated earth sheltered structure: A hot-arid zone case study," Building and Environment, Vol. 36, No.1, pp. 3-12, 1993.
Optimal Geometries for Buildings
Manipulating the geometry of the building envelope is the most fundamental way in which an architect adapts a building's design to the constraints of energy and climate. Through specialized geometry, shading of the envelope may be facilitated in summer while solar exposure is enhanced in winter, often with little or no added construction costs.
Aside from window openings, the most energetically sensitive element in a building is the roof. Research has focused on traditional roof forms in desert regions, comparing the thermal performance of vaulted roofs with conventional flat ones, and an optimization study of roof geometry led to the design of a prismatic-section building for use as student dormitories at the Sede-Boqer Campus.
For more details see:
Pearlmutter D., "Roof geometry as a determinant of thermal behavior: A comparative study of vaulted and flat roof surfaces in an arid zone," Architectural Science Review, Vol. 36, No. 2, pp. 75-86, 1993.
Pearlmutter D. and Etzion Y., "Student housing at sede-boker: A geometric response to desert conditions," Journal of Architectural and Planning Research, Vol.10, No. 3, pp. 242-260, 1993.
Etzion Y., Erell E., "The thermal performance of a concrete "finned" wall in hot-arid zone," Energy and Buildings, Vol.17 No. 4, pp. 331-336, 1991.
Controlling the transmission of radiant energy through windows:
A reversible ventilated glazing system
VIEW PROJECT VIDEO
Preliminary experiments with a novel glazing system indicate that it may provide improved visual and thermal performance in buildings with large glazed areas. The glazing system is based on the concept of converting short-wave solar radiation to convective heat and long wave radiation, which in winter is used for heating the interior space behind the opening, and in summer is exhausted outwards. The glazing system consists of two panes of glass: one is a conventional clear glass, providing a weather-proof seal; the other is a highly absorptive glass. The gap between the glass panes is ventilated by openings at the bottom and at the top of the absorptive pane. The two glass panes are assembled in a frame capable of revolving 180 degrees, so that the absorptive glass may face the interior in winter and the exterior in summer.
In winter, the glazing system allows solar space heating but eliminates glare, local over-heating and damage to furnishings caused by exposure to direct solar radiation. In summer, it reduces the penetration of unwanted radiation, thus reducing over-heating and visual glare, without obstructing the view through the window, to an extent that may render external shading devices unnecessary.
The project aims at completing the development of the glazing system and the necessary hardware needed for its utilization. The glazing system will be modeled and evaluated experimentally; a suitable frame will be developed for it; and a design tool required for its application will be developed. The expected outcome is a tested product ready for demonstration and commercial exploitation.
Several institutions co-operate in this project:
- The Desert Architecture and Urban Planing Unit (Coordinator).
- The Center of Built Environment, the Kungliga Tekniska Hogskolan, Sweden.
- The Asociacion de Investigacion y Cooperacion Industrial de Andalucia, Spain.
- The Instituto de Engenharia Mecanica, Portugal.
- The Brandenburgische Technische Universitaet Cottbus, Germany.
- AB Overums Nya Fonsterfabrik, Sweden (where the new glazing system will be manufactured).
For more details see:
Erell E., Etzion Y., Carlstrom N., Sandberg M., Molina J., Maestre I., Maldonado E., Leal V. and Gutschker O., "SOLVENT: Development of a reversible solar-screen glazing system," Energy and Buildings, Vol. 36, pp. 468-480, 2004.
Leal V., Erell E., Maldonado E. and Etzion E., "Modelling the SOLVENT ventilated window for whole building simulation," Building Services Engineering Research & Technology, Vol. 25, No. 3, pp.183-195, 2004.
Erell E. and Etzion Y., "SOLVENT Development of a Ventilated, Solar-screen Glazing System," Final research report to the European Commission, ENERGIE Program, 2004.
Etzion Y. and Erell E., "Controlling the transmission of radiant energy through windows: A novel ventilated reversible glazing system", Building and Environment, Vol. 35, No.5, pp.433-444, 2000.
Erell E. and Etzion Y., "A novel ventilated reversible glazing system," Proceedings of the 1999 ISES Solar World Conference, July 1999, Jerusalem.
INNOVATIONS IN PASSIVE HEATING AND COOLING
SEE IT WORK! Evaporative Cooling of Public Spaces
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:
Etzion Y., Pearlmutter D. Erell E., Meir I.A., "Adaptive architecture: Integrating low-energy technologies for climate control in the desert," Automation in Construction, Special issue: Intelligent Buildings, Vol. 6, pp. 417-425, 1997.
Pearlmutter D., Etzion Y., Erell E., Meir I.A., Di H., "Refining the use of evaporation in an experimental down-draft cool tower," Energy and Buildings, Vol. 23, No. 3, pp. 191-197, 1996.
Roof-mounted systems for passive climatization of buildings
Roofs are a primary source of undesirable energy absorption, leading to overheating buildings in hot climates. Current techniques try to minimize the problem by insulation. However, the roof is also the best place for installing various cooling systems.
Experiments have been carried out over nearly ten years into a project aimed at applying nocturnal long wave radiation to cooling buildings. A system was developed which consists of a shallow roof pond which is insulated from the environment, with flat plate collectors exposed to the sky through which the water 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 he roof pond and warmer than that of the ambient dry bulb temperature for at least part of the time, fairly high cooling rates could be maintained throughout the night. The system was installed and monitored in a building of approximately 75 square meters. It provided a mean nightly cooling rate of 80-100 Watts per square meter of radiator.
Preliminary investigations indicate that the radiative cooling system discussed above using roof ponds, with no physical modifications, could supply a significant portion of winter heating requirements in areas where summers are hot yet winters are cold enough to require heating systems. The system's heat output averaged 370 watts per square meter of collector under cool, sunny conditions, though on windy, overcast days the system was inoperative. Heat output is determined primarily by the intensity of solar radiation on the collectors, wind speed and temperature difference between the water in the roof pond and the ambient air.
Further modifications to the radiative cooling system, as well as an innovative evaporative cooling system, were tested within the framework of the ROOFSOL (Roof Solutions for Natural Cooling) project, funded by the EU and carried out in collaboration with researchers from six other research institutions in Europe. This project was aimed at developing and testing roof systems suitable for use in Mediterranean or desert climates which use natural cooling techniques to extract unwanted heat from buildings.
For more details see:
Yannas, S., Erell, E. and Molina, J.L., Roof Cooling Techniques A Design Handbook, James & James Science Publishers, London, 2005, 332p.
Erell E. and Etzion Y., "Radiative cooling of buildings with flat-plate solar collectors," Building & Environment, Vol. 35, No. 4, pp. 297-305, 2000.
Etzion Y. and Erell, E., "Low-cost long-wave radiators for passive cooling of buildings," Architectural Science Review, Vol. 42, pp. 79-86, 1999.
Erell E. and Etzion Y., "Analysis and experimental verification of an improved cooling radiator," Renewable Energy Vol. 16, pp. 700-703, 1998.
Erell E. and Etzion Y., "Heating experiments with a radiative cooling system," Building and Environment, Vol. 31, No. 6, pp. 509-517, 1996.
Erell E. and Etzion Y., "A radiative cooling system using water as a heat exchange medium," Architectural Science Review, Vol. 35, No. 2, pp.39-49, 1992.
Etzion Y., Erell E., "Thermal storage mass in radiative cooling systems," Building and Environment, Vol., 26, No. 4, pp. 389-394, 1991.
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