A panel has been developed that when exposed to the sky, will cool itself below the ambient air temperature by a mechanism known as radiative sky cooling. In this mechanism, heat is emitted from the panel’s surface to the atmosphere as long-wave infrared radiation. Since the atmosphere is transparent to long-wave infrared radiation, the panel surface is able to do heat exchange with the upper atmosphere, which is typically much colder than the ambient air temperature. Remarkably, this is an entirely passive and renewable mechanism that can be used to reject heat to the environment, even at temperatures below the ambient air temperature. Historically, this mechanism has only been accessed at night. However, we recently demonstrated that a properly designed surface can achieve the same effect during the day, making radiative sky cooling possible even under direct sunlight.
In this paper, a panel with a surface designed for radiative sky cooling is used to demonstrate the passive cooling of water below the dry-bulb temperature with no evaporative water losses, where the only energy input is to pump water. For a surface area of 0.74 m2 (8 ft2), we demonstrate water cooling of 3°C (5.4°F) below the dry-bulb temperature at a water flow-rate between 6-9 L/hr (1.6-2.4 gal/hr). This corresponds to an effective heat rejection rate between 40 and 100 W/m2 (13 and 32 Btu/hr-ft2).
One possible application of these panels is to serve as a modular cooling tower, replacing a traditional cooling tower in a water chiller system. This might be desired under conditions when water resources are constrained, and high efficiency cooling is required. To demonstrate the benefit of the cooling panels on a water chiller system, a thermodynamic analysis using the TMY3 dataset (typical meteorological data) from Las Vegas, NV is presented and the benefit on a typical office building’s cooling system is assessed.
