The Thermodynamics of Magnetocaloric Energy Conversion - Magnetocaloric Energy Conversion: From Theory to Applications - page 9

I d I M dH I H dM

ð MH Þ¼ þ ¼ 0 ð 1 : 24 Þ

then the work of the closed thermodynamic system equals the work of an open
thermodynamic system.
I

¼l 0 I H dM ¼ l 0 I M dH

I

dw ¼ dw t ð 1 : 25 Þ

By using the character w t we keep in mind the technical work.

1.3.1 The Coef cient of Performance (COP) and Exergy

Ef ciency

The ratio between the cooling capacity q R of the refrigerator and the work w is
de ned as the coef cient of performance (COP). This can also be expressed in

terms of power:

Q R
P

q R
w

COP ¼ COP ¼ ð 1 : 26 Þ

jj

The COP of the ideal Carnot cycle operating at the refrigeration temperature T R
and the heat rejection at ambient temperature can be de ned as follows:

T R
T R

COP c ¼ ð 1 : 27 Þ

j T amb j

in order to
compare such cycles with the COP of the Carnot cycle. Equation ( 1.27 )canbe
assumed for the

Sometimes, the thermodynamic cycles are theoretically “ Carnotized ”

of the Ericsson, Stirling or Rankine cycles. However
in other cycles, where the cooling temperature of the refrigerant varies (as is the case
in, e.g. the Brayton cycle), the

“ Carnotization ”

“ Carnotization ” requires an average value of T R . In this

particular case, we de ne the average refrigeration temperature as [ 41 ]:

q R
D

q R
R b
a

T R ¼

s R ¼

ð 1 : 28 Þ

d q
T R

and the COP of the theoretically Carnotized cycle as:

T R
T R

COP c ¼ ð 1 : 29 Þ

T amb