Regenerators

Regenerators or regenerative heat exchangers are a key component of cryocoolers such as pulse tube cryocoolers (Cold Facts, August 2014). Regenerator performance greatly affects the coefficient of performance of cryocoolers. Improvements in regenerator design and, in particular, regenerator materials have been an important factor in the improvement of the performance of cryocoolers below 10K.

A regenerator is typically an arrangement or matrix of solid material that transfers heat to and from the working fluid of the refrigeration cycle. Regenerators may be thought of as a type of “heat sponge” for the oscillatory flows found in cryocooler cycles. During one stage of the cycle, the cold working fluid passes through the regenerator material, absorbing heat and cooling the regenerator. During the next stage, the now warmer working fluid transfers heat to the cold regenerator and is cooled back down. Thus, regenerators come into contact with both the hot and cold streams of the cycle. By contrast, recuperative heat exchangers (such as plate-fin or tube-in-shell) transfer heat while keeping the cold and warm streams of the cycle physically separate.

There are a number of functional requirements for regenerators that drive their design and choice of materials. Important requirements include a large surface area to optimize heat transfer between the regenerator and the working fluid, minimization of pressure drop in the working fluid flowing through the regenerator, and a specific heat of the regenerator material that is higher than the working fluid at the operating temperature. Additionally, since the regenerator typically separates the warm and cold sections of the cryocooler, axial heat transfer through the regenerator between these sections should be minimized. The extent to which these requirements are not met has the effect of adding irreversibilities into the regenerator performance that result in a reduction of the coefficient of performance of the cryocooler. The practical effect of this is to require more power at room temperature to provide cooling at cryogenic temperatures. Notice that some of the requirements are intrinsically in conflict (such as the high surface area and the low pressure drop). The conflicting requirements and their impact on cryocooler performance has resulted in significant research and development effort being expended on regenerators.

The physical design of regenerators attempts to balance the need for large surface area and effective heat transfer with the desire to minimize the pressure drop of the working fluid as it passes through the regenerator. Strategies include the use of metal meshes, wires, spheres and rods. The frequency of the oscillatory flow can affect the design of the regenerator. Efforts have been made to model the oscillatory flow’s pressure drop and heat transfer through the regenerator by treating the regenerator as a porous media.

Regenerators perform best if the specific heat of the solid regenerator is as high as possible and preferably higher than that of the working fluid. Since the specific heat of solids decreases rapidly at cryogenic temperatures, this can be a challenging requirement. Different materials are used depending on the operating temperature of the cryocooler (and thus of the regenerator). Examples of regenerator materials and their typical range of temperatures are stainless steel (down to about 50K) and lead (from 50K to 20K). A significant advance in cryocooler performance has resulted from the development of rare earth magnetic materials such as erbium 3 nickel (Er3Ni), erbium nitride (ErN) and holmium nitride (HoN), which have high specific heats below 20K. Figure 1 shows the specific heat as a function of temperature for a number of regenerator materials.

Examples of research on regenerators and regenerator material may be found in: “Performance of 260 MHz pulse tube cooler with metal fiber as the regenerator material,” X. Wang, A. Zhang et al., Adv. Cryo. Engr. Vol. 59a (2014); “Magnetic regenerator material economizing method for 4K Gifford-McMahon cryocoolers using bakelite rod,” S. Masuyama, T. Nagao, et al., Adv. Cryo. Engr. Vol. 59b (2014); “Performance of a new regenerator material in a pulse tube cooler,” A. Kashani, B. P. M. Helvensteijn et al., Adv. Cryo. Engr. Vol. 47a (2002); and “Low Temperature Cryocooler Regenerator Materials,” K. A. Gschneidner, Jr., A. O. Percharsky et al., Cryocoolers 12 (2003). Two recent examples of modeling regenerators as porous materials may be found in “Hydrodynamic and thermal effects of drag and heat transfer coefficients under laminar unsteady flow conditions in porous media,” M. G. Pathak, T. L. Mulcahey et al. and “Numerical simulations of oscillating flow and heat transfer in porous media by lattice boltzmann method,” Q. T. Dai, H. L. Chen et al., both in Adv. Cryo. Engr. Vol. 57b (2012).