Energy Savings Potential of Radiative Cooling Technologies

Abstract

EnergyPlus does not support the specification of rooftop radiative heat exchangers, so custom heat transfer modeling was applied to simulate the flows of heat between the heat exchanger, building, and sky, and the anticipated hydronic loop conditions were passed back into the EnergyPlus model. This custom modeling was performed in EnergyPlus's energy management system (EMS) framework, which allows the user to build equations that overwrite certain predetermined points of intervention in the EnergyPlus model. For the nighttime radiative cooler, first-principle thermodynamic equations were used with some simplifying assumptions to model heat flows for conventional heat exchanger surfaces. PNNL partnered with researchers at Stanford who developed the photonic surfaces being investigated for radiative cooling. The researchers provided PNNL with detailed spectral characterizations of the thermodynamic properties of their material. Calculation of radiative heat transfer from photonic materials, however, required mathematical integration functions that are not supported by the EMS. To get around this problem, PNNL used a regression equation for radiative heat exchange based on an integration performed in MATLAB, developed by the Stanford researchers. Results Relative to the VAV system, the proposed photonic radiative cooling system saves 103 MWh electricity in Miami, 55 MWh in Las Vegas, 50 MWh in Los Angeles, 24 MWh in San Francisco and 43 MWh in Chicago, per year. The saved electricity represents 50%, 45%, 65%, 68%, and 55% of the VAV system cooling electricity, respectively in the above five cities. Relative to the high-end nighttime radiative cooling products available in the market, the photonic radiative cooler saves 10 MWh electricity in Miami, 13 MWh in Las Vegas, 8 MWh in Los Angeles, 3 MWh in San Francisco and 6 MWh in Chicago, per year, which represents 9%, 16%, 23%, 22%, and 14% of cooling electricity savings, respectively in the above five cities. Market Assessment and Conclusions Radiative cooling in buildings is best harnessed with hydronic distribution systems. Because achievable chilled water temperatures from radiative cooling are typically well above chilled water temperatures required for forced-air-based delivery systems, this necessitates the simultaneous specification of radiant zone cooling. Both radiative cooling and radiant zone cooling are investigated for market benefits and barriers. There are a wide variety of mechanisms by which radiant cooling and its required set of technologies can produce benefits to building owners and occupants. These include energy savings (and associated energy cost savings), other cost savings from elimination of alternative heating, ventilation and air conditioning (HVAC) infrastructure and downsizing of equipment as well as improved comfort. Besides providing additional electric energy savings, a system that integrates the radiative cooling heat exchanger to a building cooling loop via a cold water storage tank may be a very favorable participant in demand respo...

Colored, Daytime Radiative Coolers with Thin‐Film Resonators for Aesthetic Purposes

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“…Several studies demonstrated that photonic-based radiative coolers offer an important potential to reduce the cooling demand of buildings. In [33], it is estimated that a hybrid photonic cooling structure can save about 118 MWh per year in Phoenix AZ, while in [97], it is estimated that a similar photonic radiative cooling system can save up to 103 MWh electricity in Miami, 55 MWh in Las Vegas, 50 MWh in Los Angeles, 24 MWh in San Francisco and 43 MWh in Chicago, per year. This corresponds to about 50%, 45%, 65%, 68%, and 55% of the electricity consumed by a Variable Air Volume system in the above five cities respectively.…”

Hygrothermal Performance of Cool Roofs Subjected to Saudi Climates

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