Night Sky Radiant Cooling

What is “Solar Cooling”?

Cedar Mountain Solar is a pioneer in the field of “solar cooling”, led by our own Bristol Stickney. “Solar cooling” is so named only because the technology we widely use for solar heating can also be appropriate for cooling. This type of cooling is better named “Night-sky radiation” or “night-sky radiant cooling (NSRC).”

Cedar Mountain Solar partner Bristol Stickney collaborated with Mark Chalom of Solar Design and Analysis on a New Mexico state grant to develop night-sky radiation as a viable cooling system. The grant work included installing and data-logging several test systems and development of a design guide for NSRC implementation. Bristol and Mark, with the National Renewable Energy Labs, have also designed a combined heating and cooling system for the entirely off-grid Sleeping Rainbow Ranch in Utah.

The theory of operation is as follows: Solar heating collectors can also be effective heat radiators. In a heating system, the ability to transfer heat from collectors to storage is dictated in part by the difference in temperature between the collectors and tank, or delta T. Heat is transferred primarily by conduction – this means the heat is conducted from the hotter collectors to the cooler tank. The larger the temperature differential, or delta T, the more efficiently heat is conducted from collectors to tank.

In a night-sky radiation cooling system, heat is transferred from the warmer collectors to the cooler night sky. However, radiation functions independently of ambient temperatures. Electrons are emitted from the warm collectors and radiated at the speed of light to outer space. Therefore, the surface temperature of a radiating plate can be well below ambient temperature (this is why your car windshield can be frosty even when the ambient temperature is not below freezing.)

Cooling is accomplished by basically operating a solar heating system “backwards”, drawing heat from a radiant floor or water storage tank and radiating it at night to the night sky. Unlike evaporative cooling, which is very common in the southwestern US, this type of cooling system uses no water once filled. It also uses very little electrical energy for pumping. Cooling the temperature of a large mass such as a concrete radiant floor only several degrees can have a major impact on comfort level.

Selective surface flat plate collectors, painted flat plate collectors, unglazed metal collectors, and plastic pool heating collectors can all be used for night-sky radiation. A rule of thumb is that the better a collector is at heating, the worse it will be at radiating, because high-quality heating collectors are designed to hold heat and are well-insulated.

Night-sky radiation can allow a user of a solar heating to gain additional benefit during the cooling season by further reducing utility costs, water consumption, and greenhouse gas emissions.

Cedar Mountain Solar has built several prototype “solar cooling” systems in northern New Mexico. Please check back to this page to learn more about our experiences as we develop night-sky radiation technology for mainstream applications.

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