The Desert Architecture Unit of the J. Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, was commissioned by the Israel Ministry of Energy and Infrastructure to design and supervise the construction of an "Energy-Efficient Technology Testing" apartment building in Yaffo, near Tel Aviv. The proposed building VAII utilize various architectural techniques to provide thermal comfort to the dwellers with a minimal use of purchased energies. The project is being carried out within the framework of a bi-national agreement for cooperation with the Ministry of Energy in Catalunya, where an apartment building of similar size will also be constructed. The thermal performance of the buildings will be compared and the application of various methods of climate control evaluated.

The Yaffo site is located on the coastal region of Israel, about 5 kms from the Mediterranean Sea. The climate difficulties it raises are due mostly to summer discomfort, caused by a combination of fairly high temperatures and high relative humidity.

The temperature range in the summer (August) ranges from a mean daily minimum of 22 deg C to a mean maximum of 29 deg C. while the relative humidity at mid-day is usually 65%-75%. The relative modest daily amplitude does not permit significant structural cooling during the night, hence the widespread use of air conditioning. However, the wind pattern in summer is fairly constant, and a sea breeze blowing from the west-northwest may provide some relief from the heat from mid-day onwards.

The design brief calls for the construction of twenty four apartments. four on each floor of the six story building. Apartment buildings of this type are usually very compact. The apartments are served by a single elevator. resulting in an H-form plan which does not allow even exposure of all apartments to the prevailing wind or to direct solar radiation.

Simulations run at the Desert Architecture Unit indicate that comfort ventilation may be a major contribution towards reducing the use of air conditioning during the summer. A special emphasis was put, during the design of the building, on allowing natural ventilation. The design incorporates an extremely large envelope-to floor area ratio, achieved by breaking up the building into four basically separate blocks, served by a single vertical shaft housing the stairs and elevator and linked by open "bridges". Two of the four blocks are stepped, increasing the width of the opening on the lower floors, where wind speed is lower. The superstructure of the building, which is to house some of the solar collectors for the domestic hot water system, will also serve as a wind scoop designed to deflect the airstream at roof height into the open core of the building. creating a vertical flow to augment the horizontal one produced by the natural wind, (This scoop will be sealed by a roll-down metal shutter during winter. to reduce exposure to rainstorms).

Windows on the western facade are to be constructed as cantilevered triangular prisms. One face of the glass-and metal frame will be oriented to the northwest, allowing ventilation through louvered blinds. The second face will be oriented south, creating a "south facing" window on what is basically a west-facing wall, thus allowing some winter heating by direct gain. North facing windows will also protrude from the exterior plane of the masonry walls, and are a variation on the "double fin" design which improves airflow where rooms do not have through ventilation. A high pressure area is created on the windward side of the fin. and a low pressure area on its lee side. A pair of windows equipped with such fins on opposite sides thus serve as an entry and exit for the airflow, even though they face the same orientation.

Thermal comfort in winter will be achieved to a large degree by utilizing passive solar heating techniques. Solar radiation on a south-facing vertical wall totals (on average) over 3 kwh/m2/day, even in December. An attempt was made to maximize exposure to direct solar radiation on the south facing elevation. However, the design brief (four units per floor), coupled with a very open plan designed to maximize ventilation, resulted in insufficient south-facing glazing: not all apartments will be exposed to direct solar radiation for the desired six hours a day. Some supplementary heating will thus have to be provided by purchased energy.

Several mechanical systems are being considered as a backup for passive climate control. The dependency on air conditioning will be reduced by the reintroduction of ceiling fans into large living areas, which will improve human thermal comfort during the warmer hours of the day without charging the structure with extra heat. An option which :is being considered is the installation of mechanical dehydrators, to reduce humidity in closed interior spaces. Also being considered are absorption chillers which will be powered by hot water produced by solar collectors placed on the roof. Several types of systems are being considered for domestic hot water. so that the losses associated with central systems in high rise buildings may be reduced.

The envelope type and internal wall section most appropriate for this combination of climatic conditions was investigated using computer simulations. The effect of thermal mass and resistance to heat transfer were evaluated with two aims:

– Determining the effect of each of the wall materials on the temperature range likely to be attained in a typical apartment using passive climate control methods only, such as cooling nocturnal ventilation or heating by direct solar gain.

– Determining the difference in energy required to maintain comfort conditions in a typical apartment, due to the effect of different wall materials.

The effect of occupant behaviour on energy consumption simulated in order to avoid solutions sensitive to a particular climate control strategy or to the introduction and operation of HVAC systems.

Results ruled out the option of a very lightweight structure, which was shown to be prone to extreme thermal fluctuation even tinder the relatively mild conditions of the coastal plain. Beyond the requirement for a reasonably massive structure with sufficient insulation, however, a wide variety of construction techniques were found to provide suitable thermal performance with only minor differences between them. The best results, in both summer and winter, were achieved by using a wall section which combines conventional concrete and masonry with a 5 cm layer of rigid polystyrene insulation, the latter placed either on the exterior of the wall or sandwiched between two layers of 10 cm block. The option of Ytong as a wall material was not ruled out, as models using such blocks in either 22 or 25 cm thickness were only slightly less efficient.

Regardless of which of these construction details is adopted, the thermal simulations highlighted the importance of the overall design strategies employed in the design of the building. The availability of both ventilation in the summer and direct solar radiation in the winter were shown to increase thermal comfort conditions dramatically in the absence of mechanical cooling or heating, and to provide energy savings of up to 60% in summer and up to 40% in winter if such conditioning is employed to maintain optimum conditions.

The building is now at an advanced stage of design and performance simulation. Construction is expected to begin towards the end of 1994. Upon completion, the thermal performance of the building will be monitored, and the results analyzed and published.