The evolution of magnetocaloric heat-pump devices
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Introduction Magnetic heat pumping (MHP) is an emerging cooling technology using the magnetocaloric effect (MCE), a property of certain magnetocaloric materials (MCMs) that heat or cool when an applied magnetic field is changed. When the MCM is thermally isolated, the MCE shows up as the magnetocaloric temperature change of the MCM, ΔT. When the MCM is in contact with a thermal reservoir, the MCE shows up as a flow of heat to the reservoir proportional to the magnetocaloric entropy change, ΔS. MHP offers potential advantages over vapor compression (VC), which is currently the most widely used cooling method above 100 K. First, MHP can be done using liquid heat-transfer fluids (HTF) since no phase change of the fluid is required. Thus, there is no need for hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), or other gaseous refrigerant materials. The refrigerant in the MHP system can be a porous solid. The absence of gases reduces the potential for leaks, which is a problem in VC systems. As a result, MHP systems can have greater reliability with reduced maintenance and downtime. The elimination of hydrofluorocarbons and chlorofluorocarbons has environmental benefits, as these gases are ozone-depleting and contribute to global warming.
The response of an MCM to a magnetic field is similar to the response of a gas to compression. Below 4 K, the MCE is large compared to the temperature span, and thus a simple MHP cycle called adiabatic demagnetization refrigeration (ADR) has been used for many years.1 The ADR uses a fixed MCM, a slowly charged and discharged superconducting magnet, and direct thermal contact via solid thermal conduction in the high-field (warm) and low-field (cold) states of the MCM to carry small (less than 1 W) thermal loads. Above 4 K and for cooling loads above a few watts, the MCE is not large enough to directly attain the thermal span and the thermal loads are too large for direct thermal contact.
The active magnetic regenerator cycle Successful MHP systems operating above 10 K have implemented the active magnetic regenerator (AMR) cycle to perform cooling.2,3 This four-stage cycle was described in a 1982 US patent.4 A major advantage of the AMR cycle is that the span (heat-rejection temperature minus heat-absorption temperature) can be much larger than the magnetocaloric ΔT.5,6 Figure 1 shows an implementation5 of the AMR cycle4 with two AMR beds containing MCMs’ porous to fluid flow. This figure assumes MCM temperatures near a paramagnetic to ferromagnetic transition, thus the MCM warms when a
Carl Zimm, Astronautics Technology Center, Astronautics Corporation of America, USA; [email protected] Andre Boeder, Astronautics Technology Center, Astronautics Corporation of America, USA; [email protected] Bryant Mueller, Astronautics Technology Center, Astronautics Corporation of America, USA; [email protected] Kyle Rule, Astronautics Technology Center, Astronautics Corporation of America, USA Steven L. Russek, Astronautics Technology Center, Astronautics Corporation of A
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