New water-cooled solar panels could cut costs of air conditioning

Scientists at Stanford University in California, have devised a cooling system capable of reducing the inside temperature of a two-storey building in a desert climate by over 20 per cent without significantly boosting electricity consumption or water evaporation.

The system uses a technique called radiative infrared disruption which involves dispersing infrared thermal energy from rooftop panels directly into the atmosphere and outer space, in order to reduce temperatures to lower than those in the surrounding air.

The physics of radiative sky-cooling is known for many years, but the energy exchange produced by it can be used only on clear, dry nights. The mechanism is not efficient during the day – when the demand for commercial cooling is at its highest.

Now Stanford's Eli Goldstein and his team report in the journal Nature Energy on the creation of ''fluid cooling panels that harness radiative sky cooling to cool fluids below the air temperature with zero evaporative losses, and use almost no electricity''.

Goldstein and his colleagues built prototypes and found that the panels were able to cool water flowing through them to 5 degrees below the external ambient temperature.

On the basis of modelling the researchers concluded that installing the panels downstream of the condenser of a large commercial airconditioning system operating in climate conditions typical of Las Vegas could deliver an internal temperature drop of 21 per cent.

The panels sit atop a roof, and comprise three components, the first a plastic layer topped with a silver coating that reflects nearly all incoming sunlight, keeping the panel from heating up in the summer sun. The second component a snaking copper tube runs under the plastic. Water which is piped through the tube, sheds heat to the plastic.

That heat is then radiated out by the plastic at a wavelength in the middle region of the infrared (IR) spectrum, which is not absorbed by the atmosphere and instead travels all the way to outer space.

The panel is encased in a thermally insulating plastic housing that ensures nearly all the heat radiated away comes from the circulating water and not the surrounding air.