Evaporative Cooling: Alternatives
Introduction |
Status Quo |
Alternatives |
Requirements |
Load Minimisation |
Conclusions |
Bibliography
Having established some idea of the nature and resource demands of the typical Alice Springs summer cooling load, it is now time to survey alternative evaporative cooling solutions.
Air Inlet Devices
Elliot (2007) and Van Meter (1994) discuss residential scale (100m2 to 250m2) passive cooling systems. The most efficient configurations employ a wind-assisted cooling tower of approximately 10m in height with a cross-sectional area of approximately 4m2, further assisted by a complementary wind-escape elsewhere in the building. Consuming from 10W to 150W in operation (presumably the water pump/spray component), the authors claim airflow rates of 1.17 m3/s to 3.77 m3/s, with a daily water consumption of between 190L and 380L (average 227L per day to cool entire house). On a hot dry Arizona day (38°C and 10% humidity), Van Meter claims outlet air temperatures of 18.5°C to 21°C (a 17°C to 19.5°C decrease).
Pearlmutter et al (1996) and Etzion et al (1997) describe a large system located at Sede-Boquer in the Negev Desert in Israel, designed to cool a large one to three-storey building of 1500 m2. The system uses a cooling tower 12m high with a cross-sectional area of 10m2, assisted by a fan of approximately 1.2m diameter at the top of the tower which introduces air into the tower at a measured rate of 36,000 m3/hr (10 m3/s). This system produces a peak cooling output of 100 kW to 120 kW at 36°C and 22% relative humidity, 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. Typical temperature drops through the tower were from 36°C at air inlet to 21.5°C at air outlet (14.5°C decrease).
Perlmutter et al add two important observations regarding the development of passive cooling systems. First, using experimental prototypes they determined that without the assistance of a fan, the intensity of natural down draft due to thermal convection alone is "meagre" and inadequate to achieve substantial temperature reductions in the order of 10°C under summer daytime conditions. However, experiments with a number of wind-catcher configurations proved successful in substantially increasing air flow velocity, to an average maximum of 3.5 m/s with one configuration. Second, the authors completed some experimental work on the size of water droplets created by the spray system, concluding that droplet size is significant and that the best results were obtained using a combination of coarse and fine sprays.
Air Exit Devices
Quantitative data on the performance of wind-escape devices is unavailable at this point, but such devices were integrated into systems described by Elliot (2007).
Khedari et al (2000) found that solar chimneys with a surface area of 6m2 to 9m2 were able to induce air changes of 8 to 15 ACH in a single room of 25m3 volume, although the air velocity was low at a maximum of 0.09m/s. Bansal et al (1993) found that a solar chimney with a surface area of 2.25m2 was able to induce ventilation rates of 100 to 350 m3/hr for solar irradiance levels of 200 to 1000 W/m2.
Traditional Cooling Systems
A variety of sources described the use of highly effective natural cooling systems in traditional Persian, Middle Eastern and North African architecture (Badahori 1978, Fathy & Shearer 1986). These systems employed a variety of windtower configurations in conjunction with means for the evaporative cooling of air, including accessing air from subterranean qanats and drawing incoming air over unglazed terracotta jars and charcoal-filled devices containing water. The traditional Persian systems in particular were highly effective and capable of achieving and maintaining near-frigid temperatures in a variety of building types including public water storage cisterns.
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