R. Teyber

Magnetic heat pumps: An overview of design principles and challenges

Active magnetic regeneration is one of the most promising alternative technologies for the development of heat pumps and cooling systems for applications around room temperature. In the open literature, numerous works can be found in which much effort has been put on the development of magnetocaloric materials, magnetic circuits and prototypes. In this article, the authors discuss some of the main challenges encountered in the literature and how design choices impact cooling power and work requirements from a system engineering perspective. First, based on a generic schematic representation of a magnetocaloric heat pump, or refrigerator, various problems and challenges found in the current state of the art are pointed out and discussed. Second, different design principles for magnetic heat pumps are examined. As a means to improving performance, an extended design/optimization methodology is proposed based on entropy generation minimization with performance criteria. Finally, some initial optimization results are presented and discussed.

Material screening metrics and optimal performance of an active magnetic regenerator

A variety of metrics to rank the magnetocaloric materials can be found in the literature, but a quantitative assessment showing their efficacy has not been reported. A numerical model of an active magnetic regenerator cycle is used to assess the predictive ability of a set of material metrics. The performance of eight cases of known magnetocaloric material (including first order MnFeP1-xAsx and second order materials Gd, GdDy, Tb), and 15 cases of hypothetical materials are considered. Using a fixed regenerator matrix geometry, magnetic field, and flow waveforms, the maximum exergetic cooling power of each material is identified. Several material screening metrics such as relative cooling power (RCP) are tested and a linear correlation is found between maximum RCP and the maximum exergetic cooling power. The sensitivity of performance to variations in the hot side and cold side temperatures from the conditions giving maximum exergetic power are determined.

Multiple points of equilibrium for active magnetic regenerators using first order magnetocaloric material

First order transition material (FOM) usually exhibits magnetocaloric effects in a narrow temperature range which complicates their use in an active magnetic regenerator (AMR) refrigerator. In addition, the magnetocaloric effect in first order materials can vary with field and temperature history of the material. This study examines the behavior of a MnFe(P,Si) FOM sample in an AMR cycle using a numerical model and experimental measurements. For certain operating conditions, multiple points of equilibrium (MPE) exist for a fixed hot rejection temperature. Stable and unstable points of equilibriums (PEs) are identified and the impacts of heat loads, operating conditions, and configuration losses on the number of PEs are discussed. It is shown that the existence of multiple PEs can affect the performance of an AMR significantly for certain operating conditions. In addition, the points where MPEs exist appear to be linked to the device itself, not just the material, suggesting the need to layer a regenerator...

Topology optimization of reduced rare-earth permanent magnet arrays with finite coercivity

The supply chain risk of rare-earth permanent magnets has yielded research efforts to improve both materials and magnetic circuits. While a number of magnet optimization techniques exist, literature has not incorporated the permanent magnet failure process stemming from finite coercivity. To address this, a mixed-integer topology optimization is formulated to maximize the flux density of a segmented Halbach cylinder while avoiding permanent demagnetization. The numerical framework is used to assess the efficacy of low-cost (rare-earth-free ferrite C9), medium-cost (rare-earth-free MnBi), and higher-cost (Dy-free NdFeB) permanent magnet materials. Novel magnet designs are generated that produce flux densities 70% greater than the segmented Halbach array, albeit with increased magnet mass. Three optimization formulations are then explored using ferrite C9 that demonstrates the trade-off between manufacturability and design sophistication, generating flux densities in the range of 0.366–0.483u2009T.

Experimental characterization of multilayer active magnetic regenerators using first order materials: Multiple points of equilibrium

Multiplepoints of equilibrium (MPE) have recently been observed in single layer active magnetic regenerators (AMRs) using first order magnetic materials (FOMs). Here, we describe experiments using three multilayer MnFeP1-xAsx FOM regenerator beds characterized under a range of applied loads and rejection temperatures. Thermal performance and the impacts of MPE are evaluated via heating and cooling experiments where the rejection (hot side) temperature is varied in a range from 283u2009K to 300u2009K. With fixed operating conditions, we find multiple points of equilibrium for steady-state spans as a function of warm rejection temperature. The results indicate a significant impact of MPE on the heating and cooling temperature span for a multilayer MnFeP1-xAsx FOM regenerator. Unlike single material FOM tests where MPEs tend to disappear as load is increased (or span reduced), with the layered AMRs, MPEs can be significant even with small temperature span conditions.Multiplepoints of equilibrium (MPE) have recently been observed in single layer active magnetic regenerators (AMRs) using first order magnetic materials (FOMs). Here, we describe experiments using three multilayer MnFeP1-xAsx FOM regenerator beds characterized under a range of applied loads and rejection temperatures. Thermal performance and the impacts of MPE are evaluated via heating and cooling experiments where the rejection (hot side) temperature is varied in a range from 283u2009K to 300u2009K. With fixed operating conditions, we find multiple points of equilibrium for steady-state spans as a function of warm rejection temperature. The results indicate a significant impact of MPE on the heating and cooling temperature span for a multilayer MnFeP1-xAsx FOM regenerator. Unlike single material FOM tests where MPEs tend to disappear as load is increased (or span reduced), with the layered AMRs, MPEs can be significant even with small temperature span conditions.