Determining the minimum mass and cost of a magnetic refrigerator

An expression is determined for the mass of the magnet and magnetocaloric material needed for a magnetic refrigerator and these are determined using numerical modeling for both parallel plate and packed sphere bed regenerators as function of temperature span and cooling power. As magnetocaloric material Gd or a model material with a constant adiabatic temperature change, representing a infinitely linearly graded refrigeration device, is used. For the magnet a maximum figure of merit magnet or a Halbach cylinder is used. For a cost of \$40 and \$20 per kg for the magnet and magnetocaloric material, respectively, the cheapest 100 W parallel plate refrigerator with a temperature span of 20 K using Gd and a Halbach magnet has 0.8 kg of magnet, 0.3 kg of Gd and a cost of \$35. Using the constant material reduces this cost to \$25. A packed sphere bed refrigerator with the constant material costs \$7. It is also shown that increasing the operation frequency reduces the cost. Finally, the lowest cost is also found as a function of the cost of the magnet and magnetocaloric material.Comment: 12 pages, 10 figure

Efficient modulation of the magnetocaloric refrigerator capacity

Magnetocaloric energy conversion devices (e.g., room air conditioners and household refrigerators) have the potential to significantly reduce the emissions associated with refrigerant leakage into the atmosphere but still have lower efficiencies compared to mature vapor compression systems. The efficiency of a magnetocaloric cooling device derives not only from its design characteristics (e.g., solid refrigerant, hydraulic system, and magnet system) and its operating temperature span but also from its modulating capability. Owing to the lack of experimental data regarding this topic, the advantage of modulating the cooling capacity (i.e., the part-load performance) of an active magnetic regenerator prototype is demonstrated experimentally for the first time. The capacity modulation is carried out by means of regulating both the cycle frequency of the device and the volumetric flow rate of the heat transfer fluid. At a 14 K temperature span and a 1.4 Hz frequency, the magnetocaloric refrigerator prototype using 3.8 kg of gadolinium provided a maximum cooling capacity of 452 W with an appreciable coefficient of performance of 3.2, which corresponds to a second-law efficiency of 15.5 %. At part-load operating conditions, the device can produce a cooling capacity of 245 W with an increased second-law efficiency of 29.7 %, or a coefficient of performance of 6.2, making it more competitive with traditional vapor compression systems. In future studies, the experimental data obtained may be implemented in a dynamic building energy model to quantify the energy-saving benefits of part-load operation by estimating the overall system efficiency during a typical cooling season.This work was in part financed by the RES4Build project, which received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 814865

Performance assessment of a rotary active magnetic regenerator prototype using gadolinium

We present the experimental results for a rotary magnetocaloric prototype that uses the concept of active magnetic regeneration, presenting an alternative to conventional vapor compression cooling systems. Thirteen packed-bed regenerators subjected to a rotating two-pole permanent magnet with a maximum magnetic field of 1.44 T are implemented. It is the first performance assessment of the prototype with gadolinium spheres as the magnetocaloric refrigerant and water mixed with commercial ethylene glycol as the heat transfer fluid. The importance of various operating parameters, such as fluid flow rate, cycle frequency, cold and hot reservoir temperatures, and blow fraction on the system performance is reported. The cycle frequency and utilization factor ranged from 0.5 to 1.7 Hz and 0.25 to 0.50, respectively. Operating near room temperature and employing 3.83 kg of gadolinium, the device produced cooling powers exceeding 800 W at a coefficient of performance of 4 or higher over a temperature span of above 10 K at 1.4 Hz. It was also shown that variations in the flow resistance between the beds could significantly limit the system performance, and a method to correct those is presented. The performance metrics presented here compare well with those of currently existing magnetocaloric devices. Such a prototype could achieve efficiencies as high as conventional vapor compression systems without the use of refrigerants that have high global warming potential