Advanced exergy analysis of transcritical CO₂ heat pump system based on experimental data

Abstract

Conventional exergy analysis identifies component exergy destruction, however, does not give clear picture whether occurs because of component itself or due to remaining components and then the advanced exergy analysis is emphasized. The main purpose of this study is to apply advanced exergy analysis to the transcritical CO₂ vapor compression heat pump system for simultaneous water cooling and heating, along with the conventional exergy analysis. The novelty of the present study is the advanced exergy analysis (with modified methodology) of transcritical superheat cycle based on experimental data. Endogenous, exogenous, avoidable and unavoidable exergy destructions are determined for each component of this system. Results show that compressor is having highest exergy destruction among the CO₂ system components and also its major part is avoidable endogenous exergy destruction. According to conventional exergy analysis, the sequence for improvement seems to be compressor, gas cooler, expansion valve and evaporator; however, the in-depth analysis (advanced exergy analysis) shows compressor is the first component followed by evaporator, gas cooler and expansion valve to be considered for improvisation. Thus it is concluded that compressor provides highest opportunity for improvement and new design of the same will improve overall transcritical CO₂ system performance.

Appendices

Nomenclatures

p:

pressure (MPa)

T:

temperature (K)

${\dot{\text{E}}}$ :

exergy rate (kW)

e:

specific exergy (kJ/kg)

${\dot{\text{m}}}$ :

mass flow rate (kg/s)

h:

specific enthalpy (kJ/kg)

s:

specific entropy (kJ/kg K)

s_gen :

specific entropy generation (kJ/kg K)

ε:

exergetic efficiency

${\dot{\text{W}}}$ :

power (kW)

$\Delta T_{\hbox{min} }$ :

pinch point temperature difference (K)

Subscripts

CM:

compressor

GC:

gas cooler

TV:

throttling valve

EV:

evaporator

k:

kth component

0:

property at dead state

f:

fuel

d:

destruction

p:

product

tot:

overall system

l:

losses

x:

property at (T₀, p)

Superscripts

AV:

avoidable

EN:

endogenous

EX:

exogenous

UN:

unavoidable