## Abstract

Any energy system includes at least two essential entities, namely, energy generators and energy consumers. Each of these elements has its associated characteristics, and it is not necessary that at all times the energy generated is the same as the energy consumed. Moreover, it is not necessary that the energy is generated at the same location where it is consumed. The *transmission grid* is a medium that interconnects the energy generators with the energy consumers.

## Keywords

Storage Device Energy Storage System Thermal Energy Storage Heat Transfer Fluid Thermal Storage## Nomenclature

*A*Area, m

^{2}*C*Electric capacity, F

*c*_{p}Specific heat, J/kg K

*d*Distance, m

*D*Diameter, m

*E*Energy, J

*Ex*Exergy, J

- \( {{\dot{\it E}x}} \)
Exergy rate, W

- d
Distance, m

*f*Friction factor

*H*Enthalpy, kJ or height, m

*h*Specific enthalpy, J/kg, or heat transfer coefficient, W/m

^{2}K*i*Electric current intensity, A

*I*Maximum current intensity, A or moment of inertia, kg⋅m

^{2}*k*Thermal conductivity, W/mK

*K*Flywheel constant

*L*Length, m

- LHV
Lower heating value, MJ/kg

*m*Mass, kg

- \( \dot{m} \)
Mass flow rate, kg/s

*N*Number of heat transfer units

*P*Pressure, bar

*C*Electric capacity, F

*q*Heat, kJ

- \( \dot{Q} \)
Heat rate, W

*R*Electric resistance, Ω

*R, r*Radius, m

*t*Time, m

*T*Temperature, K

*V*Volume, m

^{3}, or velocity m/s or electric potential, V*W*Width, m or work, kJ

- \( \dot{W} \)
Work rate, W

*y*Coordinate, m

## Greek Letters

- \( \delta \)
Thickness, m

- \( \varepsilon \)
Electric permittivity, F/m

- \( \xi \)
Friction factor

- \( \eta \)
Energy efficiency

- \( \psi \)
Exergy efficiency

- \( \rho \)
Density, kg/m

^{3}- \( \sigma \)
Tensile stress, N/m

^{2}- \( \tau \)
Time constant, s

- \( \theta \)
Dimensionless temperature

- \( \omega \)
Angular velocity, rad/s

## Subscripts

- \( \infty \)
Surroundings

- 0
Initial

- c
Charging or capacitor or coolant

- ch
Charging

- d
Destroyed

- dsch
Discharging

- e
Exterior

- i
Interior

- ice
Ice

- in
Inner

- inp
Input

- L
Lateral

- loss
Losses

- LS
Melting

- m
Per unit of mass

- max
Maximum

- opt
Optimal

- out
Outer

- R
Retrieved

- ref
Reference

- ret
Retrieved

- s
System or storage or source

- tot
Total

- T
Total

- w
Wall

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