Yair Etzion

Radiative cooling of buildings with flat-plate solar collectors

Abstract Roof ponds cooled by nocturnal long wave radiation have often been proposed as a cheap and effective means of providing thermal comfort in buildings in hot-arid locations. Many of the schemes incorporate flat-plate radiators through which the water is circulated at night to be cooled. An analytical model originally developed for heating applications was adapted to the description of such a nocturnal, long wave radiative cooling apparatus. The accuracy of the model was verified on the basis of experimental data from three types of radiators tested at the experimental facilities of the Center for Desert Architecture and Urban Planning at Sede-Boqer, Israel, under a variety of operating regimes. The model allows accurate prediction of outlet temperatures, taking into account the design characteristics of the specific radiator, the environmental conditions and the pattern of operation of the system. Analysis of flat-plate solar collectors converted to radiative cooling of buildings suggests that under typical environmental conditions, they may be less efficient for this type of application than radiators having no fins.

Refining the use of evaporation in an experimental down-draft cool tower

Direct evaporative cooling has long been recognized as an energy-efficient and cost-effective means for space conditioning in hot dry areas. In order to extend the use of evaporative cooling to include exterior or semi-enclosed spaces, a down-draft evaporative ‘cool tower’ was integrated in the project of a 500 m2 glazed courtyard located at the heart of a building complex in the arid Negev Highlands of southern Israel, designed by the authors. The present article describes the development of the cooling tower system, undertaken in three phases: (i) Prototype analysis. Performance of a small-scale tower was monitored, and comparisons were drawn between varying rates and mechanisms of water and air supply. The results indicated a potential for substantial temperature reduction in the order of 10 °C under summer daytime conditions, but a meager cooling output when using a natural draft system. Mechanical-forced air flow was thus utilized in the actual tower. (ii) Field monitoring. The cool tower, approximately 10 m in height and 10 m2 in cross-sectional area, was operated and monitored during a summer season; its performance was analyzed using a series of water supply mechanisms and operating modes. The system produced a peak cooling output of just over 100 kW, with a wet bulb temperature depression of close to 85–95% during all hours of operation, and a water consumption rate of approximately 1–2 m3/day. (iii) Refinement. Potential improvement in the systems operation was investigated through the development of a wind capture mechanism for increasing inlet pressure and air flow to the space. Both fixed and dynamic capture units were investigated, with wind speed and direction as well as internal air speeds measured in the small-scale prototype tower. The wind capture unit with the simplest configuration and best performance is recommended for future integration in the full-scale tower.

On the microclimatic behavior of two semi-enclosed attached courtyards in a hot dry region

Abstract Enclosed and attached courtyards are common architectural patterns—throughout many periods of history and in many regions. They are often referred to in the professional and scientific literature as microclimate modifiers, which may improve thermal comfort conditions in the enclosed as well as the attached built volume. This statement may be correct only under certain conditions, and is subject to a number of specific requirements: the relative dimensions of open space and built volume, the treatment of exposed surfaces, and the orientation of the open space. The results of monitoring undertaken in the Negev Highlands are presented and compared to commonly accepted attitudes on the subject. Measurements were taken in two identically shaped and similarly treated, but differently oriented, courtyards. The results show a definite influence of orientation on the microclimatic conditions of the open space.

Controlling the transmission of radiant energy through windows: a novel ventilated reversible glazing system

State-of-the-art glazing systems can provide very good solutions for cold climate conditions, and fairly effective ones for warm climates. However, there is still no window system on the market that can offer the flexibility required to provide a comfortable visual environment and an efficient energy response in climates where heating is required in winter, and cooling is required in summer. This paper describes an experimental investigation of a novel glazing system, designed to overcome glare and radiation damage to interior furnishings, yet which causes no reduction in the energy efficiency of the glazed opening compared with a conventional window used in direct gain systems. The proposed glazing system (patent pending) incorporates a rotatable frame holding two glazing components: transparent glazing providing a weatherproof seal, and absorptive glazing with a low shading coefficient1. The absorptive glazing is fixed at a small distance from the clear glazing, forming an airspace between them which is sealed at the sides but open at the bottom and top, so that air flows freely through it. In summer, the absorptive glass faces the exterior of the building, absorbing excessive solar radiation and dissipating the heat to the ambient air. In winter, the glazing assembly is rotated so that the absorbing glass faces the interior, reducing glare but allowing effective convective and radiative heating of the adjacent space.

An analysis of absorbed radiation by domed and vaulted roofs as compared with flat roofs

Abstract Domed or vaulted roofs have been extensively used in buildings throughout the Middle East and other hot arid regions. Many studies have focused on climate-related considerations, but very little quantitative research exists in the literature. In this study, an attempt was made to calculate the insolation absorbed by these roofs, as compared with flat roofs, based on angular dependence of absorptance and solar geometry. Results showed that a domed or vaulted roof will absorb more solar radiation than its corresponding flat roof. The ratios K b , K tot increase with the increase of the half dome or vault angle but are insignificantly affected by climate characteristics and latitude of the location. It was also seen that a south–north facing vaulted roof both reduces the solar heat gain of buildings in summer months and increases solar heat gain in winter months compared to one which faces east–west; the greater the proportion of area exposed to the sun is, the smaller the amount of beam radiation that will be absorbed by a curved roof. Furthermore, results showed that even if absorptance is assumed to be constant this would affect total solar heat gain of the roofs studied by less than 4%.

Heating experiments with a radiative cooling system

A hybrid space-cooling system for hot arid zones previously investigated by the authors 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 W/m2 of collector under the sunny but cool conditions typical of Sde-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.

An improved solar shading design tool

Abstract A method which extends the use of the well established chart for the design of solar shading devices was developed. The new method provides a quantitative proportion of the openings area which is exposed at any given time to direct solar radiation, in addition to the binary answer returned by the old chart whether any part of the opening is exposed or not exposed to such radiation at all. The paper explains the development of the new tool and demonstrates its use.

A Radiative Cooling System Using Water as a Heat Exchange Medium

A passive cooling system, integrating radiative and convective cooling, was tested for a test cell in Sde-Boqer, Israel. The system was based on circulating water from a pond on the roof of the cell through a system of radiators, which removed energy from the system by nocturnal long wave radiation and convective losses. The water flowing through the system kept the radiators relatively warm, thus increasing radiative losses and eliminating convective gains. Wind screens were not necessary in this case. The radiators also served as a shading device over the roof during the day.

Thermal behavior of buildings with curved roofs as compared with flat roofs

In this paper, a detailed finite element model dealing with heat transfer through a domed or vaulted roof is suggested based on a three-dimensional heat transfer equation and solar geometry. This model allows a comparison of the thermal behavior of curved and flat roofs in terms of heat flux and daily heat flow through them into an air-conditioned building under different climatic conditions. The results of numerical calculations show that the ratio of daily heat flow through curved roofs to that through flat ones is not affected by the curve radius, thickness and construction material of the roof, but is significantly influenced by the half rim angle θ0 of the roofs and the ambient temperature. Compared to a flat roof, under typical hot dry climatic conditions, the daily heat flow through a domed roof of θ0=90° is about 40% higher, whereas the daily heat flow through a south–north oriented and an east–west oriented vault of θ0=90° is about 20 and 27% higher, respectively. The reason for this is mainly attributed to the convective heat transfer between the enlarged curved roof and ambient air. However, when θ0<50°, heat flux and daily heat flow through a curved roof is close to that through a flat roof. The results also confirm that curved roofs are not suitable for areas with higher air temperature and intense sky diffuse radiation typical of hot humid areas.

Mapping the potential for climate-conscious design of buildings

Abstract Where climatic conditions vary substantially across a country, it may be of benefit to map the potential of different areas for the application of various techniques for energy-conscious architecture. A zoning approach is proposed that is based on careful selection of climatic variables from meteorological data commonly available, and which uses the statistical technique of cluster analysis for regional grouping. The proposed approach is illustrated for the case of Israel, whose climatic conditions are extremely diverse despite the countrys small size. The proposed approach may help to determine the spatial limits to which certain performance standards may apply, or in which particular climate-conscious design techniques may be beneficial.