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Night Cooling - A new invention exploits the radiant heat losses from earth during night hours to cool buildings

Released on May 28 2015

Source: Canadian Consulting Engineer ( May 2015, Page 37 )

Solar cooling a building at night? At first this idea seems an inherent contradiction of conventional science. But have you ever enjoyed an evening outdoors on a deck watching the sun set and felt the air get cooler? It is not the air temperature that changed. What you are feeling is your body radiating heat to the night sky at a rate of 75 watts/m2. This cooling effect applies to the entire surface of the earth as it loses heat to the cold sky. During the day the incoming radiation from the sun is 10 times greater, which offsets the heat loss. But from sunset to sunrise, the radiation heat loss is one way - from earth to sky.

The nocturnal radiation cooling effect is documented in ASHRAE Handbook HVAC Applications 2011 and is well known in other circles, including agriculture and weather forecasting. However, the renewable energy sector has not fully grasped the opportunity.

John Hollick of Conserval is the inventor of the SolarWall transpired collector, a solar air heating system that has been used on thousands of buildings around the world since the 1990s.

The company's recently patented "NightSolar" unit incorporates the same solar collector as a SolarWall system, but in a reverse way to harness the properties of nocturnal radiation cooling. As the roof cools by as much as 10°C (18°F) below ambient, it can cool air passing through it by as much as 5°C from before sunset to almost an hour after sunrise on clear nights. This cooling has the ability to reduce or even displace conventional air conditioning at night.

As warm night air touches the cooler surface of the NightSolar transpired collector, it transfers its heat to the surface, which cools the air. The chilled air is then drawn in through perforations in the collector and enters the HVAC unit via an economizer cycle.

The system works day and night in the summer, with the daytime solar heat collected in the same way as a conventional SolarWall system. The system simultaneously provides shading and ventilation to the roof. The heat can either be dissipated or used to heat water or industrial processes.

From a structural perspective, the NightSolar system consists of a single sheet of corrugated metal, mounted 100 to 200 mm above an existing roof. The weight is approximately 10 kg/m2 and it has the same appearance as a SolarWall system or a conventional metal roof.

The technology is commercially available and recent field monitored installations are reporting impressive cooling savings on buildings, using existing fans and economizers. Test systems at the National Solar Test Facility in Mississauga have confirmed the ability to cool air below ambient in accordance with ASHRAE Handbook HVAC Applications 2011. A demonstration project on telecom shelters in Turkey showed cooling savings of up to 50%, and closer to home the U.S. military has installed pilot projects that are to be monitored.

Victoria Hollick is vice-president of operations, Conserval Engineering, Toronto. In 2014 the SolarWall and John Hollick, were honoured by the American Society of Mechanical Engineers (ASME) in New York City in an exhibit of 80 of the best inventions of the past two centuries. Other honorees were Thomas Edison, Henry Ford, Willis Carrier, Westinghouse and the Panama Canal.

Last changed:Nov 26 2015