Acronyms
COP	Coefficient of performance
HE	Heat engine
HP	Heat pump
PTES	Pumped Thermal Energy Storage
RTE	Round-trip efficiency
TES	Thermal Energy Storage
Symbols
a	Temperature ratio
C	Heat capacity (J/Ks)
$\dot{Q}$	Heat flow (J/s)
W	Work (J)
P	Power (J/s)
${\overline{P}}_{out}$	Specific Power $P/C_w$ (K)
$\kappa$	Dimensionless factor $\left(\frac{\gamma -1}{\gamma }\right)$
$\dot{m}$	Mass flow (kg/s)
p	Pressure (atm)
r	Pressure ratio
T	Temperature (K)
Greek Letters
$\gamma$	Adiabatic coefficient of ideal gas
$ϵ$	Efficiency parameter
$\eta$	Discharge efficiency
$\nu$	Coefficient of performance
$\rho$	Pressure drops coefficient
$\Phi$	Round-trip efficiency
Subscript
w	Working fluid
c	Compressor
H	Hot storage
L	Cold storage
t	Expander/turbine