PCM Packed-Bed Thermal Storage: Thermocline Front

Bed Operation

HTF inlet temperature 85 °C

Flow rate 1.00 kg/s

PCM capsule material

Simulation speed 4×

Live Readouts

Thermocline depth

Outlet HTF temp

20.0 °C

Energy stored

Bed melt fraction

How it works

A packed bed of encapsulated phase-change-material (PCM) spheres is a real thermal-battery design: hot heat-transfer fluid (HTF) is pumped through the voids between capsules to melt the PCM and store latent heat (charging); later, cold HTF is pumped through to freeze it back and recover that heat (discharging). This is the classic Schumann two-phase packed-bed model.

The bed is discretised into axial layers. Within each layer the fluid reaches local equilibrium quickly (fluid thermal mass ≪ capsule thermal mass), so the fluid temperature leaving a layer follows an effectiveness relation:

T_f,out = T_p + (T_f,in − T_p) · exp(−NTU),   NTU = hA / (ṁ c_f)

Each capsule's bulk temperature then evolves from convective exchange with the fluid passing through its layer:

m_p · c_eff(T_p) · dT_p/dt = hA · (T_f,avg − T_p)

The latent heat of fusion L is absorbed over a narrow melting window ΔT around the melting point T_m using the effective-heat-capacity method — a standard way to fold a phase transition into a sensible-heat ODE:

c_eff(T) = c_solid or c_liquid,  plus  L/ΔT   when |T − T_m| < ΔT/2

Charge / Discharge — sets whether the inlet HTF is hotter or colder than the bed, i.e. whether the bed is melting (storing) or freezing (releasing) the PCM.
Inlet temperature — the HTF temperature entering the top of the bed.
Flow rate — raises ṁ and, through a Nusselt-type correlation h ∝ ṁ^0.6, the convective coefficient — a faster flow moves the thermocline through the bed faster but with a shallower temperature gradient.
Thermocline depth — the fraction of the bed's height that has crossed the midpoint between the initial and inlet temperatures; it is the moving boundary between "charged" and "uncharged" capsules.

Real-world relevance: this exact scheme (macro-encapsulated PCM spheres in a packed column) is used in solar-thermal plants, district-heating buffer tanks and building HVAC storage to store several hours of heat in a fraction of the volume a sensible-heat-only tank would need.

Cold / solid PCM Hot / liquid PCM

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