Active Magnetic Regeneration - Magnetocaloric Energy Conversion: From Theory to Applications - page 94

Chapter 4
Active Magnetic Regeneration

It is well known that the magnetocaloric effect of most magnetocaloric materials at
moderate magnetic elds (up to 1.5 T) is limited to a maximum adiabatic tem-

perature change of 5 K [ 1 , 2 ]. This value is not sufcient for such materials to be
directly implemented into a practical cooling or heating device where temperature
span over 30 K is required. Therefore, in order to increase the temperature span, one
and so far the best option is for a heat regenerator to be included in the magnetic
thermodynamic cycle.
A heat regenerator is a type of indirect heat exchanger where the heat is peri-
odically stored and transferred from/to a thermal storage medium (regenerative
material) by a working (heat-transfer)

fl uid. The regenerative material usually has a

porous structure, through which a working fl uid is pumped in an oscillatory,

counter- fl ow manner (which is more ef cient than a parallel fl ow system). During

the ows through the regenerative material, which

cools down, while the material heats up. As a result, heat is stored in the material.
During the

' hot period ' , a warmer fl uid fl

' cold period ' , a cooler fl uid fl ows through previously heated regener-

ative material, so the uid heats up, while the material cools down. The heat is

therefore transferred back to the

fl

uid or a different one) at a different
phase of the thermodynamic cycle. After a certain number of such steps, a periodic
steady state is reached and, as a result, a temperature pro

fl uid (the same fl

le can be established

along the length of the regenerator [ 3 ].
The need to apply heat regenerators in a magnetic refrigerator was already realized
by Brown [ 4 ] in the rst prototype of a magnetic refrigerator, built in 1976. He applied

a regenerative Stirling-like thermodynamic cycle (very similar to AMR), which
signi

cantly increased the temperature span of the device [ 4 , 5 ]. A few years later
Steyert [ 6 ] and Barclay and Steyert [ 7 ] presented and explained the active magnetic
regenerator, which remains the most applied method for the exploitation of the
magnetocaloric effect at room temperature. Furthermore, all prototypes of magnetic
refrigerators built since then have been based on the AMR process [ 5 ]. An AMR,
unlike a passive (regular) heat regenerator, contains a magnetocaloric material as the
regenerative material. It has a double function in a magnetic regenerator, i.e. it works
as a regenerator and enables an increase in the temperature span as well as working as
a coolant and generating/absorbing heat between the particular phases of the