Greg F. Nellis

Review Article: Recent Developments in Room Temperature Active Magnetic Regenerative Refrigeration

Active magnetic regenerative refrigeration (AMRR) systems represent an environmentally attractive alternative to vapor compression systems that do not use a fluorocarbon working fluid. The AMRR concept has previously been demonstrated using superconducting solenoid magnets that are not practical for small-scale commercial applications. However, recent AMRR prototypes that use more practical permanent magnets have proved that AMRR systems can produce cooling over a useful temperature range with a relatively low magnetic field. In addition, families of materials with large magnetocaloric effects and adjustable Curie temperatures have been developed; these materials may be used to construct layered regenerator beds that may have lower cost and provide higher performance than current materials. This paper reviews recent developments in the field of room temperature magnetic refrigeration and discusses some design issues that may affect practical systems.

Predicting the Performance of an Active Magnetic Regenerator Refrigerator Used for Space Cooling and Refrigeration

Active magnetic regenerative refrigeration (AMRR) systems represent an environmentally attractive refrigeration alternative that does not use a fluorocarbon working fluid. Recently, families of magnetocaloric material alloys with adjustable Curie temperatures have been developed. Using these materials, it is possible to construct a layered regenerator bed that can achieve a high magnetocaloric effect across its entire operating temperature range. This paper describes a numerical model capable of predicting the practical limits of performance of this technology applied to space-conditioning and refrigeration applications. The model treats the regenerator bed as a one-dimensional matrix of magnetic material with a spatial variation in Curie temperature and, therefore, magnetic properties. The matrix is subjected to a spatially and temporally varying magnetic field and fluid mass flow rate. The numerical model is solved using a fully implicit discretization of the governing energy equations. The nonlinear aspects of the governing equations (e.g., fluid and magnetic property variations) are handled using a relaxation technique. Modeling results are presented that illustrate how an AMRR system can be optimized for a particular operating condition. The performance of layered and nonlayered AMRRs are compared to current vapor compression technology for space-conditioning and refrigeration applications.

Preliminary microfluidic simulations for immersion lithography

The premise behind immersion lithography is to improve the resolution for optical lithography technology by increasing the index of refraction in the space between the final projection lens of an exposure system and the device wafer. This is accomplished through the insertion of a high index liquid in place of the low index air that currently fills the gap. The fluid management system must reliably fill the lens-wafer gap with liquid, maintain the fill under the lens throughout the entire wafer exposure process, and ensure that no bubbles are entrained during filling or scanning. This paper presents a preliminary analysis of the fluid flow characteristics of a liquid between the lens and the wafer in immersion lithography. The objective of this feasibility study was to identify liquid candidates that meet both optical and specific fluid mechanical requirements. The mechanics of the filling process was analyzed to simplify the problem and identify those fluid properties and system parameters that affect the process. Two-dimensional computational fluid dynamics (CFD) models of the fluid between the lens and the wafer were developed for simulating the process. The CFD simulations were used to investigate two methods of liquid deposition. In the first, a liquid is dispensed onto the wafer as a “puddle” and then the wafer and liquid move under the lens. This is referred to as passive filling. The second method involves the use of liquid jets in close proximity to the edge of the lens and is referred to as active filling. Numerical simulations of passive filling included a parametric study of the key dimensionless group influencing the filling process and an investigation of the effects of the fluid/wafer and fluid/lens contact angles and wafer direction. The model results are compared with experimental measurements. For active filling, preliminary simulation results characterized the influence of the jets on fluid flow.

Experimental validation of a ground heat exchanger model in a hybrid ground source heat pump

This article provides an overview of short time-scale validation of the duct storage model (Hellström 1989) for the simulation of ground source heat pump performance using experimental data acquired from two operational systems. The error in the temperature change across the ground heat exchanger in the first system was within measurement error, but it was larger than the measurement error in the second sytem due to uncertainty in the thermal properties of the ground. A sensitivity study determined that the thermal conductivity and heat capacity have the greatest impact on the model accuracy. An assessment of the error in the model using ASHRAE handbook (ASHRAE 2007) values for thermal conductivity and heat capacity provided a measure of acceptable error. The difficulties of using real-world operational systems for model validation are discussed.

Impact of weather variation on ground-source heat pump design

This article presents an investigation on the impact of year-to-year weather variability on the optimal design of a system for heating and cooling a building using a ground-source heat pump. The designs of a boiler–ground-source heat pump hybrid and a cooling tower–ground-source heat pump hybrid were optimized using a typical meteorological year (TMY2) weather file and also using 15 years of actual weather data. The results indicate that a design based on a TMY2 weather file may be undersized for a severe weather year; this is particularly true if the severe weather year is encountered during the first year of system operation. A cooling tower–ground-source heat pump hybrid model was developed, which includes the use of a backup boiler placed on the building side (rather than the loop side) of the system. It was found that the use of a boiler backup mitigated much of the negative impact of a severe weather year. The boiler supplied the heating during periods of particularly severe weather so that the ground loop length could be maintained at a reasonable value.

Simulation of the coupled thermal optical effects for liquid immersion micro-/nano-lithography

Immersion lithography has been proposed as a method for improving optical microlithography resolution to 45 nm and below via the insertion of a high refractive index liquid between the final lens surface and the wafer. Because the liquid will act as a lens component during the imaging process, it must maintain a high, uniform optical quality. One potential source of optical degradation involves changes in the liquid’s index of refraction caused by changing temperatures during the exposure process. Two-dimensional computational fluid dynamics models from previous studies have investigated the thermal and fluid effects of the exposure process on the liquid temperature associated with a single die exposure. Here, the global heating of the wafer from multiple die exposures has been included to better represent the “worst case” liquid heating that will occur as an entire wafer is processed. The temperature distributions predicted by these simulations were used as the basis for rigorous optical models to predict effects on imaging. This paper presents the results for the fluid flow, thermal distribution, and imaging simulations. Both aligned and opposing flow directions were investigated for a range of inlet pressures that are consistent with either passive systems or active systems using filling jets.

Technologies for Cooling of Large Distributed Loads

In future space applications, large and distributed loads will require active thermal control if their lifetimes are to be extended beyond one or two years. Examples include Zero Boil-Off (ZBO) cryogenic systems for exploration missions, and cooling of widely distributed sensor arrays and large deployable structures, such as mirrors and sunshades, for space science missions. These applications will require efficient means of heat transfer from extended structures or from several discrete elements to one or more remotely located heat rejection packages consisting of active and passive components. More or less stringent temperature control will also be required. We have recently undertaken a program to develop a number of technologies relevant to the issues associated with distributed cooling. These include circulation networks that transfer heat via steady flows of cold pressurized gas; gas rectifiers for use with linear pressure wave generators; and MEMS-based throttling valves for precise temperature control. This paper describes the studies that are underway to establish the performance potential of each.

Controlling template response during imprint lithography

Step-and-Flash Imprint Lithography (S-FILTM) is a principal candidate for the next-generation lithography at the 45-nm node (and below). In imprint lithography, a monomer solution is dispensed onto the wafer. The monomer fills small features in a template that is lowered onto the wafer. The monomer is cured, causing it to solidify so that a three-dimensional replica of the template features is produced and remains on the wafer after the template is removed. Because this is a one-to-one process, any distortions of the template during the squeezing process will be manifested directly as errors in the features that are imprinted on the substrate. A finite element (FE) structural model of the S-FIL template has been created to predict the distortions due to mounting, gravity, and the fluid pressure distribution that arises from the viscous flow of the polymer liquid during the imprint process. Distortions take the form of both in-plane and out-of-plane displacements. An axisymmetric, finite difference (FD) model is used to predict the pressure distribution over the template due to viscous flow and surface tension effects. The FE and FD models are coupled using an iterative process in which the pressure distribution and template distortions are calculated at progressing time intervals until the final, desired gap height is achieved, nominally 200 nm. The coupled models are capable of characterizing the fluid-structure interaction that occurs during the imprint process. The results of the model will facilitate the design of system components that are capable of meeting the stringent error budgets associated with the sub-45-nm nodes.

Editorial: Cryosurgery: An Emerging Application for Low-Temperature Refrigeration

Cryosurgery refers to the application of very low temperatures to undesirable tissue, typically cancerous tumors, in order to destroy it. Cryosurgery is a medical technique that has existed for more than a century. However, recent advances in cryosurgical devices, medical imaging, and understanding of the physical mechanisms that lead to tissue destruction have expanded the number of conditions that can be treated using some form of cryosurgery. The challenges faced by those working on cryosurgical systems are largely engineering challenges that are familiar to the refrigeration engineer, and therefore this is an exciting technology area where the HVAC&R and medical device development industries overlap.