Engineers at the University of California, Davis, have invented a device that can generate mechanical power at night by linking the natural warmth around us to the cold depths of space. The invention could be used, for example, to ventilate greenhouses or other buildings. The work is described Nov. 12 in Science Advances.
The invention is a type of machine called a Stirling engine. Other machines such as internal combustion engines generate power from a large heat gradient, said Jeremy Munday, professor of electrical and computer engineering at UC Davis and co-author on the paper. In contrast, a Stirling engine can work based on a small difference in temperature, such as that between a hot cup of coffee and its surroundings.
"These engines are very efficient when only small temperature differences exist, whereas other types of engines work better with larger temperature differences and can produce more power," Munday said.
Typically, a Stirling engine is directly connected to a heat source on one side and a cooler environment on the other side.
"If you just set it on the table, it's not going to produce any power on its own because all sides are the same temperature," Munday said.
One way to generate a temperature difference, of course, is to heat up one side by burning fuel. Munday and graduate student researcher Tristan Deppe wondered if instead they could connect the cold side to something very, very cold but also very far away: deep space.
"It doesn't actually have to touch space physically, it can just interact radiatively with space," Munday said. It's like standing outside on a cold, clear night: Your head will quickly start to feel cold as heat radiates away.
Deppe and Munday's idea was to take a simple Stirling engine (essentially a piston driving a flywheel), put it on a panel that acts as a heat-radiating antenna and sit the whole thing on the ground outdoors at night. The ground acts as the warm side of the engine and the antenna channels the cold of space.
A year of night time experiments shows that the small device can indeed generate at least 400 milliwatts of mechanical power per square meter. The researchers used the device experimentally to directly power a fan and also coupled it to a small electrical motor to generate current.
The experiments show that it is possible to generate useful amounts of power from the night sky. The potential is greatest in areas where humidity is low and night skies are usually clear, Munday said. The principle could be used, for example, to ventilate greenhouses or residential buildings.
UC Davis has filed a provisional patent related to the work.
