Inspired Heat-Pipe Technology
In 1963, Los Alamos physicist George Grover successfully demonstrated his invention of the heat pipe. Grover's inspiration for the heat pipe came from rudimentary heat-conducting pipes used by British bakers more than 170 years ago. The development of such pipes began in 1839, when American inventor Jacob Perkins patented the hermetic tube boiler. Angier March Perkins (Jacob's son) modified the tube boiler, and in 1936 he patented what he called the Perkins Tube, which saw widespread use in locomotive boilers and working ovens (including a mobile oven for the British Army). The Perkins Tube served as a "jumping off point" for Grover's development of modern heat pipes, which depending on their application can be as short as a hypodermic needle, or up to 24 feet long.
A heat pipe is a heat-transfer device that consists of a sealed metal tube with an inner lining of a wick-like capillary material and a small amount of fluid in a partial vacuum. One end of the pipe absorbs heat through fluid vaporization while releasing at the other end, through vapor condensation. There are a variety of liquids and wicks used in heat pipes, but the principle is the same: A liquid evaporates into a gas that travels to the cooler end of the pipe, where it condenses back into a liquid and returns via the wick.
Grover originally developed the heat pipe to conduct heat from a nuclear reactor's core to a thermoelectric element or a heat engine. A thermoelectric element generates electricity from the temperature difference between a heat source (the reactor core) and a heat sink. A heat engine produces mechanical motion from this temperature difference.
From Water or Sodium to Lithium
Early Los Alamos heat pipes contained water or sodium. In the mid-1980s, Los Alamos developed a lithium heat pipe that transferred heat energy at a power density of 23 kilowatts per square centimeter—to understand the intensity of that amount of heat energy, consider that the heat emitted from the sun's surface is only 6 kilowatts per square centimeter. Lithium is placed inside a molybdenum pipe, which can operate at white-hot temperatures approaching 1,477 K (2,200°F). Once heated inside the pipe, the lithium vaporizes and carries heat down the pipe's length.
In this photo taken in the early 1960s, physicist George Grover tests a heat pipe.
Although heat pipes were not used to conduct heat from a nuclear reactor's core, they have been successfully used to manage temperatures inside spacecraft, where the heat generated by electronic equipment can build up and damage equipment. Because spacecraft travel in an excellent thermal insulator—the vacuum of space—the only way a spacecraft can dissipate heat is to radiate it to space. Heat pipes efficiently conduct the heat generated by electronics inside the spacecraft to heat radiators on the spacecraft's exterior.
In 1996, the space shuttle Endeavor carried into space three Los Alamos heat-pipe prototypes. The designs of these liquid-metal prototypes were for use in advanced spacecraft. The pipes operated at temperatures in excess of 900°F, and performed flawlessly in all tests. In 2000, Los Alamos worked with NASA's Marshall Space Flight Center in developing heat pipes to generate electricity and propulsion in spacecraft designed to journey to the solar system's outer limits.
Because heat pipes work efficiently in zero gravity environments, routine applications for them are to cool electronic elements aboard geostationary communication satellites.
Currently, miniature heat pipes cool central- and graphics-processing units in mainframe computers, as well as microprocessors found in laptop computers.
In addition, heat pipes increase the efficiency of solar water heaters. In such an application, heat pipes are sealed within a copper tube with distilled water inside, thus enabling efficient conduction of heat along the tube's length. Once placed in an evacuated glass tube, sunlight heats the copper tube. This "evacuated-tube" solar collector is up to 40% more efficient than the more-traditional "flat-plate" solar collectors used to heat water. Evacuated-tube collectors also do not need the antifreeze additives that flat-plate collectors require.
Heat pipes are also used to dissipate heat at the Trans-Alaska Pipeline. Without such pipes, heat picked up by the oil from its underground sources (120°F) and through friction and turbulence (as the oil moves through the pipeline) would go down the pipeline's supports anchored to the ground and would likely melt the permafrost. If the permafrost melted, the pipeline would sink. To prevent such a disaster, more than 124,000 heat pipes mounted on top of the pipeline's vertical supports keep the permafrost frozen and intact by conducting heat from the supports to the ambient air.
Today's modern electronics generate heat that can cause damage. In space, such damage is irreparable. With its ability to transfer and dissipate heat, the heat pipe will continue to play a crucial role in cooling electronics such as computers, pipes along the Trans-Alaska Pipeline, and heat-sensitive electronics aboard satellites and spacecraft.
-Octavio Ramos Jr.
Heat pipes on the Trans-Alaska Pipeline supports prevent hot oil moving through the pipeline from melting the permafrost. Photo courtesy of the United States Geological Survey.