Tech Insights

Cool Solar: Radiant Cooling Sends Heat Energy Into Space

May 31, 2024 by Kevin Clemens

Researchers have found a way to provide a cooling surface while enhancing solar cell power output.

In 2014, Stanford University engineers invented a coating material to cool buildings by radiating heat and sending it directly into space. The Stanford team developed an ultrathin, multilayered material that deals with light in the visible and invisible spectrum in a novel way. 

Now, a team at Penn State has used Stanford’s radiant cooling concept to develop a cooling and power strategy that can simultaneously collect solar energy using a photovoltaic (PV) solar cell while directing heat away from the Earth and into space without warming the surrounding air. The Penn State team found that combining a solar cell’s electrical generation with radiant cooling could increase energy savings up to 30 percent. 

 

Testing the radiant cooling system.

Testing the radiant cooling system. Image used courtesy of the authors 

 

Heat Transfer

Heat can be transferred in three ways. 

  • Conduction—by direct contact

  • Convection—through the movement of fluids, often air, pulling heat away from an object

  • Radiation—through infrared (IR) light emitted from all objects in proportion to their temperature

Unless blocked, an object’s radiant infrared energy, blackbody radiation, will pass through the atmosphere and end up in outer space. In a radiant cooler, the radiant heat transfer process cools the surface, which can be used as a refrigerator or to cool a building. 

 

Radiant Cooling

The radiant cooling approach typically requires using paints or semisolid materials to reflect nearly all sunlight. This approach is not compatible with harvesting solar energy

Penn State’s system uses visually transparent but IR opaque, low-iron glass as its radiant cooler. During daylight hours, sunlight passing through the transparent radiant cooler hits a solar cell and generates electricity. The surface of the radiant cooler reflects incoming infrared radiation and sends it back into space

At the same time, the radiant cooler emits infrared radiation that is also sent away from the Earth and into space. As a result, the radiant cooler is cool to the touch, even on a hot day in bright sunlight. The radiant cooler emits infrared radiation even on a cloudy day or at night, providing continuous cooling. 

 

Radiant cooling on a solar cell.

Radiant cooling on a solar cell. Image used courtesy of the authors

 

Penn State’s outdoor experiments found the radiant cooler could be as much as 5.1°C (9.2°F) below the ambient air temperature while being hit by ∼1,000 watts per square meter (W/m2) sunlight. The PV solar cell in this system produced 159.9 W/m2 from the same area at the same time. The team also confirmed that two radiant cooler harvesters could be stacked vertically, reducing the space on a building’s rooftop. 

Although the Penn State work is only in its proof-of-concept stage, the solar electricity generated could power a building or be stored for use at night. The temperature drop created by the radiant cooler could cool air or a liquid in place of traditional building air conditioning. The energy savings for a building with such a system would be significant.