Comparative thermodynamic performance analysis of a cascade system for cooling and heating applications

ABSTRACT

In this study, a comparative thermodynamic performance analysis of cascade system (CCS) for cooling and heating applications is presented and compared for different refrigerant couples. The CCS consists of the low-temperature cycle (LTC) and high-temperature cycle (HTC). The CO₂ was used as working fluid in LTC, whereas the HFE 7000, R134a, R152a, R32, R1234yf, and R365mfc refrigerants were used in HTC. The heating and cooling coefficients of performance (COP_ht, COP_cl) and exergy efficiency of CCS are investigated parametrically according to various factors such as the evaporator, condenser, and reference temperatures. After thermodynamic analyses are completed, the COP_cl of CCS is obtained as 1.802, 1.806, 1.826, 1.769, 1.777, and 1.835 for CO₂-HFE7000, CO₂-R134a, CO₂-R152a, CO₂-R32, CO₂-R1234yf, and CO₂-365mfc refrigerant couples, respectively. Furthermore, the heat exchanger has the highest exergy destruction rate, whereas the expansion valves have the lowest of exergy destruction rate.

KEYWORDS:

• Cascade system
• cooling
• environmentally refrigerants
• energy
• exergy
• heating

Nomenclature

CCS	=	cascade system
CFC	=	chlorofluorocarbon
COP	=	coefficient of performance
e	=	specific energy (kJ/kg)
ex	=	specific exergy (kJ/kg)
$\stackrel{.}{\mathrm{E}}$	=	energy rate (kW)
$\stackrel{.}{\mathrm{E}}x$	=	exergy rate (kW)
$\stackrel{.}{\mathrm{E}}{\mathrm{x}}_{\mathrm{D}}$	=	exergy destruction rate (kW)
GWP	=	global warming potential
h	=	enthalpy (kJ/kg)
HCFC	=	hydro-chlorofluorocarbon
HTC	=	high temperature cycle
LTC	=	low temperature cycle
ODP	=	ozone depletion potential
s	=	entropy (kJ/kg K)
T	=	temperature (°C)
$\dot{m}$	=	mass flow rate (kg/s)
$\dot{Q}$	=	heat transfer rate (kW)
$\dot{W}$	=	work rate (kW)

Subscript

0	=	reference state
c	=	compressor, condenser
ch	=	chemical energy
cl	=	cooling
D	=	destruction
ev	=	evaporator
exp	=	expansion valve
gen	=	generation
ht	=	heating
in	=	input
kn	=	kinetic energy
out	=	output
ph	=	physical energy
pt	=	potential energy

Greek letters

$\psi$

=

Exergy efficiency