Cooling towers evaporate water to dissipate heat in commercial and industrial cooling systems. While water is currently inexpensive, the shear amount of water use is staggering – the Lawrence Berkeley National Laboratory (LBNL) estimates that data centers in the United States will use 660 billion gallons of water by 2020. When including water used in cooling towers for thermoelectric power plants, industrial refrigeration systems, and other large buildings, totals easily surpass one trillion gallons per year. Long term, this amount of water usage is not sustainable and new technologies need to be developed that enable the lower heat rejection temperatures of evaporative systems, but also the simplicity of closed loop, (dry) air cooled systems.
THE SOLUTION
Radiative sky cooling is a passive cooling technique that exploits a natural feature of Earth’s atmosphere: it is partially transparent to electromagnetic radiation in the 8-13 micron wavelength range. This wavelength range overlaps with the thermal radiation emitted by objects at typical terrestrial temperatures (0-50 °C). Thus, sky-facing surfaces at these temperatures emit more energy as thermal radiation to the sky than they receive back, and thereby cool themselves below air temperature. Everyday observations of this effect include the condensation of water on sky-facing surfaces of a car in the morning and the formation of frost on a roof before the air temperature drops below 0 °C.
These surfaces have been applied to panels, which have a similar form factor to solar hot water heaters. Each panel is 39 inches x 77 inches and has a dry weight of 22 lbs (1.1 lb/ft2). The panels are piped together in series and parallel, in a closed system, and used to cool fluids. The panels have no moving parts and only require electricity to run a circulating water pump.
HOW IT WORKS
Radiative sky cooling occurs all the time; however, it is not easily observed during the day with common building materials. This is simply because any outdoor surface facing the sky is also receiving energy and heat from the sun. As a result, this effect has had limited impact on building efficiency and cooling systems.
In 2014, it was shown that specialized nanophotonic surfaces could passively cool up to 5 ˚C below air temperature, or more, even under direct sunlight (Raman et al., 2014; Rephaeli et al., 2013). These specialized surfaces were designed using optical and photonic principles to have a selective spectral emissivity, ε(λ), that was high over infrared wavelengths and low over solar wavelengths (Raman et al., 2014). The surfaces reflected 97 percent of incident sunlight and were strongly emissive of thermal radiation in the 8-13 micron range, where the atmosphere is transparent. These optical properties allow the surfaces to achieve the observed cooling effect, and to radiate and reject heat loads from other sources to below the air temperature. The results also highlighted that heat rejection capacities in excess of 100 W/m2 were possible 24 hours a day. More recently, fluid cooling panels that used these specialized surfaces were developed and used to demonstrate fluid cooling up to 5 °C (9 °F) below ambient air temperature, 24 hours a day at varying flow rates (Goldstein et al., 2017).
The core technology in SkyCool panels was developed with years of research at Stanford University, supported by the US Department of Energy, and National Science Foundation. There have been two third party studies of the technology, by the Pacic Northwest National Lab (PNNL-24904) and Oak Ridge National Lab (on going).
WHERE IT CAN BE USED
The panels can be used in conjunction with traditional air conditioning or refrigeration systems, or even replace more traditional cooling systems in a building. When used with the traditional air conditioner or refrigeration system (compression-based), the panels can be used to cool refrigerant out of the condenser or even replace a standard condenser. By doing so, the condenser pressure can be lowered, and less work is required by the compressor. By doing this, we anticipate saving between 10 to 40 percent electricity, depending on the number of panels used in the system.
The panels can also be combined with thermal storage to replace an air conditioning system in a small one to three story office building. In this scenario, panels are used to cool thermal batteries at night, in the early morning, and late evening. Then, the stored cold can be distributed through a buildings cooling system. When used in this way, the anticipated energy savings will be between 40 to 60 percent relative to traditional compressor-based cooling systems.
The panels can also be used to directly reject the heat from a building. In this mode, the panels are used in a closed fluid loop and provide a high efficiency way to reject heat from buildings. The cooling capacity of the panels relative to the electricity required to run the pump is 10 to 50 times larger than a cooling tower or even a fan. Assuming the panels supplement heat rejection in the typical cooling tower, an array of 500 SkyCool panels would save millions of gallons per year.
