As electronic circuits become more densely packaged and higher in power, maintaining them within permissible temperature ranges becomes a major issue. This is especially true in the case of such portable devices as laptop computers, cellular telephones, and other hand-held units. Cooling by a conventional approach (for example, by use of a fan and a heat sink) ultimately reduces the useful battery power and the useful operating time of a portable device. Using its own funds, Thermacore Inc. employed knowledge gained from the Small Business Innovation Research (SBIR) program to develop a miniature high-performance heat pipe for use in cooling portable electronic devices.
A heat pipe is a sealed heat-transfer element. It makes use of two-phase heat transfer to carry heat at a small temperature drop from an input area, where a working fluid is evaporated, to an output area where the vapor is condensed, giving up its heat of vaporization. Capillary pumping in a porous wick structure returns the condensed liquid to be re-evaporated, thus making the heat pipe passive in the sense that no external power is needed for its operation.
In a laptop computer, a 3-mm-diameter heat pipe (see Figure 1) is used to spread the heat over a large area that is effectively cooled by natural convection. The ability of the 3-mm heat pipe to adequately transport the excess heat is due to a high-performance sintered powder metal wick structure that lines the interior wall. This wick structure also enables the heat pipe to work in any orientation. The heat-pipe pressure boundary and wick are made from copper, and water is the working fluid.
Given the high volume of computer manufacturing, the cost of heat pipes has been reduced significantly, quite often providing economical solutions to problems of cooling in many applications. Thermacore has used company funds to develop the fabrication process needed and to build the factory required for mass production of miniature heat pipes. Production rates have already exceeded 7,500 units per day.
The commercial success of the miniature heat pipes is spawning other advanced heat-pipe-based products. One such product entering the marketplace is the loop heat pipe (LHP) shown in Figure 2. The LHP includes a pair of narrow tubes (typically 3 mm in diameter), several meters long, along which heat can be transported while the tubes are in any orientation. The key to the performance of the LHP is the a small-pore (≤1 μm) powder metal wick structure in the evaporator section. This wick structure is typically made from nickel or stainless steel. The exterior pressure boundary material of an LHP is made of stainless steel or aluminum.
LHPs were invented in the former Soviet Union in the early 1980s. Expertise in the design and fabrication of LHPs was brought to the United States by Thermacore Inc. in 1990. During the past several years, the ability to fabricate all aspects of LHPs was transferred to Thermacore Inc. and its sister company, Dynatherm Corp. This transfer created a United States supplier of LHPs and of expertise pertaining to LHPs.
A family of these devices has been produced to begin to address several applications, such as cooling of avionics in aircraft and missiles, aircraft anti-icing, regulation of temperatures in spacecraft, and solar heating to produce domestic hot water. Development of an LHP anti-icing system is being funded through the NASA SBIR program and monitored by NASA Lewis Research Center. LHPs will be used to passively transport engine waste heat forward to supply heat to critical surfaces to prevent ice formation. Because waste heat is used here, there is no power penalty and engine efficiency remains high. It is anticipated that an operational system will be demonstrated on an unmanned aircraft in late 1998.
Loop heat pipes are also finding their way into applications on spacecraft. Dynatherm Corp. manufactures loop heat pipes for use on satellites. An American-made loop heat pipe was shown to operate successfully during a microgravitational flight experiment aboard the space shuttle (flight STS-87). Further advancements in LHPs are being funded by NASA Goddard Space Flight Center through the SBIR program.
LHPs are expected to be incorporated into rooftop heating units for producing domestic hot water. An LHP can accept solar energy on a roof and transport it passively to a water heater in the basement. Since the LHP subsystem of the hot-water system operates completely passively, an increase in system efficiency over that achieved without solar/LHP augmentation is anticipated.