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New rooftop panels could improve air-conditioning efficiency

Abbey Bigler, Special to the Quad

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Autumn is creeping up on us, bringing cooler temperatures. Many of us will enjoy a break from the heat and also from the strain running air conditioners puts on our wallets. Haven’t you ever wished for a way to keep your living space cool over the summer without sending your electricity bill through the roof? Three researchers, Eli A. Goldstein, Aaswath P. Raman and Shanhui Fan from Stanford University, may have found a solution: A rooftop panel which significantly reduces the amount of energy required by cooling systems.

According to the researchers’ paper in Nature Energy, cooling systems currently consume 15 percent of global electricity and create 10 percent of global greenhouse gas emissions. This invention could benefit not just bank accounts but the environment as well.

The published paper describes how the researchers tested their panels during warm, somewhat humid weather in Stanford, Calif. and used the data gathered to predict the efficiency of the panels operating under optimal conditions: the hot, dry climate of Las Vegas, Nev. Their models predict that a typical two-story office building in Las Vegas outfitted with the panels could reduce the amount of energy needed for cooling during the summer months by 21 percent.

To understand how the panels work, you first have to understand the beginning of the process; a vapor-compression air conditioner goes through to cool a room. The first step in the process is putting a liquid called a refrigerant under high pressure, which results in a superheated vapor that is then piped through ambient-temperature air or water. The heat from the refrigerant flows into the air or water, condenses and goes through several other steps to cool the room. One of the largest problems with air conditioners is how the waste heat in the air or water is dealt with because heat building up in the air conditioner makes it less efficient. This is where the fluid cooling panels come in.

According to the researchers’ paper, the panels can take the water heated by a vapor-compression air conditioning system, reduce its temperature to about five degrees Celsius, or nine degrees Fahrenheit, below ambient air temperature and feed it back into the system. What makes this technology so important is that it cools the water and allows it to be reused without consuming electricity or losing water to evaporation.

The mechanism through which the panels lower water temperature is called radiative sky cooling. Essentially, radiative sky cooling enables the temperature of the panels to be lower than the temperature of the surrounding air by reflecting sunlight and releasing heat from the water as mid-infrared spectrum energy. This enables the water to be cooled below ambient air temperature, which means the water can more readily receive heat from the refrigerant and the air conditioner does not have to work as hard, reducing the amount of electricity it consumes.

Each panel has a surface area of 0.37 square meters and is constructed of three relatively simple components: A reflective radiative cooling surface, a plate heat exchanger and an insulating enclosure. The radiative cooling surface consists of a mirror-like silver coating over a piece of plastic and reflects sunlight. The plate heat exchanger directly below it consists of a copper pipe which winds through an aluminum plate. Heat from water in the pipe passes through the radiative cooling surface as mid-infrared spectrum energy and from there radiates through the Earth’s atmosphere and into outer space. The doublewalled acrylic insulating enclosure surrounds both of the other components to protect them from the environment.

According to an article by Robert F. Service, published on Science magazine website on Monday Sept. 4, since their original experiments, the researchers have formed a startup called “SkyCool Systems” to commercialize their invention. They are currently conducting a field trial with new, larger, 1.67 square meter panels in Davis, Calif.

Abbey Bigler is a fourth year English major with minors in buisness and technical writing, communication studies and biology. She can be reached at [email protected]

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The Student News Service of West Chester University
New rooftop panels could improve air-conditioning efficiency