Cryocooling for Heat Transfer and Heat Exchanger

Micro technology can be employed to produce miniaturized refrigeration and cryocooling systems. Although process intensification is typically the route used for miniaturization, micro technology can also be used for reducing the size of mechanical components that are necessary for operation. For example, to create a small-scale vapor compression refrigerator, the heat transfer components comprising the condenser and evaporator can be made in a micro-channel configuration for enhanced heat transfer rates. However, the mechanical compressor will often be the determining factor for overall size and weight.

Advanced compressor designs can be developed by replacing existing components with micro- machined ones, or entirely new types of compressors can be developed. In the former area, valve heads and drive mechanisms can be redesigned to take advantage of layered manufacturing techniques so that integrated components result. In the latter area, work has been under way focusing on electrostatically operated compressors micro machined out of silicon. Other approaches using thermopneumatic operation, electro active polymers, and magnetic shape memory alloys have been pursued.

Cryocoolers can benefit from miniature components as well, but fundamental heat transfer issues must be considered. For example, counterflow heat exchangers or regenerators are necessary for cryocooler operation. This is due to thermal isolation requirements of the cold space from ambient temperature. The working fluid must pass relatively unimpeded (i.e., low pressure drop) through the heat exchanger while thermal energy is exchanged between the incoming and outgoing flows. To reduce the size of the overall cooler, each component (including the heat exchanger or regenerator) must be reduced in scale. However, this presents a heat transfer problem in the form of heat leakage into the cold space.

As the size, and hence the length, is reduced, the temperature gradient along the heat exchanger increases, leading to enhanced heat transfer rates to the cold section. On further size reduction, a point is reached where the cooler load is entirely from this leakage and no useful heat lift takes place. Thus, there exists a limit, based on fundamental heat transfer principles, to which coolers can be reduced in size.

With these considerations, several of the techniques used at the macroscale for cryocooling have been investigated for miniaturization. For instance, both pulse tube and Stirling cycle devices have been studied for miniaturization. Conduction through the regenerators and heat exchangers limits the ultimate size of the system, but cryocoolers that are approximately a centimeter in length appear to be practical. Miniaturization of the cyclic compressors needed for cryocoolers is also an important need in this area. Typically, piston-based compressors are used; reducing their size involves all of the challenges as does the micro engine area discussed earlier.

Another possible approach to cryocooling uses a reverse Brayton cycle. Miniaturized turbo machinery could be effectively employed for this application. However, the performance of any Brayton cycle, whether power producing or in cooling applications, is strongly influenced by the efficiency of the individual components. Thus, compressor and turbine elements in a reverse Brayton cycle machine must be relatively efficient for the cycle to be practical. Although this section has not covered solid-state coolers or refrigerators, much work has been under way for producing efficient small-scale devices (e.g., thermo- electric coolers). If high ‘‘figure-of-merit’’ materials become available for thermoelectric cooling, they will play an important role in miniaturized cooling applications.