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Night Radiation Cooling with Roof Mounted SolarWall® Panels

Released on Jun 18 2007

SolarWall used for night coolingToronto, Ontario; June 2007 - The use of a building’s roof to take advantage of long-wave radiation to the night sky has been long identified as a potentially productive means to reduce space cooling in buildings. The ASHRAE Handbook (2007) devotes a separate section to cooling by nocturnal radiation and states “Radiative building cooling has not been fully developed.” Their most recent references are from 1984 which is before the SolarWall solar collector had been invented.

Radiation cooling to the night sky is based on the principle of heat loss by long-wave radiation from one surface to another body at a lower temperature. On a clear desert night, a typical sky-facing surface will cool at a rate of about 75 W/m2. This means that a metal roof facing the sky can be significantly colder than the surrounding air temperature. The daily average cooling potential for the month of July amounts to 63 - 110 Wh/m2 of roof surface in U.S. depending on the climate locations. Over a 10 hour night, this amounts to about 250 - 450 W/m2 if all of the energy could be effectively utilized.

D. S. Parker of Florida Solar Energy Center (FSEC) (April 2005) reports that the night-time temperature depression of a white metal roof surface in Florida averages 3-4°F below the aspirated air temperature from 9 PM to 7 AM. The minimum roof surface temperature is limited by the dew point temperature. Researchers at FSEC are currently monitoring a metal roof in Florida but the conversion efficiency appears to be much lower than the efficiencies that should be possible with a perforated metal roof or SolarWall collector panel.

The unglazed perforated metal solar collector, SolarWall, developed by Conserval Engineering, has become the most popular method of solar heating air for commercial and industrial buildings. The transpired solar collector is unique in that ambient air passes through a large surface in a controlled manner with the same volume of air per unit area of surface. Its collection efficiency can be over 80%. Given that this level of efficiency has been established for heating, it is expected that high efficiency levels can also be obtained in the cooling arena. By mounting the transpired solar panels on the roofs of buildings, it should be possible to capture the cool night air and deliver it into buildings at higher efficiencies than have been previously documented to date.

A perforated solar panel allows the external heat boundary layer to be drawn into the air cavity, essentially doubling the heat transfer efficiency. The same principle is expected to apply to the cold boundary layer on roofs. Without perforations, the external heat or cold boundary layer is not transferred to the air.

The Cool Roofs program of which California has subscribed to with Title 24, requires a roof to have a total reflectance of 0.7 or better and a high emissivity of 0.75 or better. The Cool Roofs program assumes that the heat is not required or heating is not as important as cooling energy and thus the heat must be reflected away. Clearly there are many buildings where the solar heat can be utilized. Any night time cooling system that can perform double or triple duty such as day time solar energy capture for summer cooling or hot water heating as well as winter heating needs is likely to improve the economics of the installed system.

Energy Savings:
The main benefit to a night cooling system will be the ability to utilize the SolarWall collectors over more months each year. At present, solar heating collectors generally require five to six months of minimum use to achieve a reasonable payback. The longer the usage the shorter the payback. A nine month heating season offers a 50% longer usage which can lower the payback to under five years. By adding a cooling component, the combined heating cooling system can operate twelve months a year if necessary. Realistically, a combined system should be used at least nine to ten months a year making it a much more viable system for moderate climates such as California where both heating and cooling or mainly cooling are required.

For a typical roof of 225 square meters, the cooling potential is between 6 to 14 kW or 1.5 - 4 tons of cooling each summer night. This is normally sufficient to cool the building during the 10 to 12 hours of night time. This translates into between 5000 kWh and 10,000 kWh per cooling season depending on location. With electrical rates varying between 12 cents to 25 cents/ kWh, the annual dollar savings are $600 to $2500.

The ventilation heating supplied by these panels (225 m2) on a typical commercial building can displace between 1000 to 4000 therms (30,000 to 120,000 kWh) a year depending on location and building heating requirements. This provides a dollar savings of $1000 to $4000 a year when displacing natural gas.

Total energy savings from combining summer night cooling and winter space heating, for this example, is expected to range between $2000 and $6000 a year. Northern cities will benefit more from space heating and southern cities will have higher savings from the cooling benefit.

Night radiation cooling has been known for decades but no one has been able to successfully and economically capture the cooling benefit to cool buildings. When the unglazed perforated metal panel or SolarWall system was developed by Conserval’s engineers, it was considered a breakthrough in solar technology since glazing was eliminated and the method of passing a precise amount of air through each perforation was perfected. This same technology can be adapted to night time cooling.

Conserval Engineering is evaluating night cooling and soon expects to be involved in pilot projects and demonstration projects. Conserval is looking for suitable locations to conduct these demonstrations and welcomes enquiries to host night time cooling projects.

Last changed:Nov 12 2008