Phase Change Material Thermal Storage Calculator

How this phase change material (PCM) storage calculator works

This calculator estimates the thermal energy that can be stored in a phase change material (PCM) as it is heated from an initial temperature, passes through its melting point, and reaches a higher final temperature. It combines sensible heat (temperature change in solid and liquid phases) with latent heat (the energy absorbed during melting) and then applies an optional efficiency factor to account for real-world losses.

Use it to compare different PCMs, size storage for building or equipment applications, or sanity-check manufacturer datasheet values. The tool assumes that the material starts fully solid below its melting temperature and ends fully liquid at or above its melting temperature.

Energy storage formula

The total ideal thermal energy stored in a PCM that is heated from an initial temperature below its melting point to a final temperature at or above its melting point is:

E_ideal = m [ c_s(T_m − T_i) + L + c_l(T_f − T_m) ]

where:

• m = mass of PCM (kg)
• c_s = specific heat in solid phase (kJ/kg·°C)
• c_l = specific heat in liquid phase (kJ/kg·°C)
• L = latent heat of fusion (kJ/kg)
• T_i = initial temperature (°C)
• T_m = melting temperature (°C)
• T_f = final temperature (°C)

The usable stored energy accounts for losses such as imperfect heat transfer, tank insulation, and control strategy. It is approximated by:

E_usable = η · E_ideal

where η is the storage efficiency (between 0 and 1).

Formula in MathML

The same relationship in accessible MathML form is:

The calculator reports energy in kilojoules (kJ) and converts to kilowatt-hours (kWh) using:

1 kWh = 3,600 kJ

Interpretation of the results

The output gives you an estimate of the total thermal energy that can be stored during one full charge cycle under the stated temperature range.

• Energy from solid heating shows how much sensible heat is stored while the PCM warms up from the initial temperature to just before it starts to melt.
• Latent heat is the energy used to change phase at the melting temperature without a temperature increase. For PCMs, this portion often dominates.
• Energy from liquid heating is the additional sensible heat stored as the already-molten PCM is heated from the melting temperature up to the final temperature.
• Total ideal energy is the sum of the three contributions assuming no losses.
• Usable energy applies your chosen efficiency factor to approximate what you can realistically extract.

In many applications, the latent term is the main reason for using a PCM. A high latent heat value allows significant storage over a narrow temperature range, which is useful when you need stable operating temperatures (for example, around room temperature in buildings or around specific setpoints in electronics).

Worked example

Consider 100 kg of a paraffin wax PCM used for building thermal storage. Assume:

• Mass, m = 100 kg
• Solid specific heat, c_s = 2.1 kJ/kg·°C
• Liquid specific heat, c_l = 2.4 kJ/kg·°C
• Latent heat, L = 200 kJ/kg
• Initial temperature, T_i = 20 °C
• Melting temperature, T_m = 60 °C
• Final temperature, T_f = 70 °C
• Storage efficiency, η = 0.8

Step 1: sensible heating in solid phase:

ΔT_solid = T_m − T_i = 60 − 20 = 40 °C

E_solid = m · c_s · ΔT_solid = 100 · 2.1 · 40 = 8,400 kJ

Step 2: latent heat at melting:

E_latent = m · L = 100 · 200 = 20,000 kJ

Step 3: sensible heating in liquid phase:

ΔT_liq = T_f − T_m = 70 − 60 = 10 °C

E_liq = m · c_l · ΔT_liq = 100 · 2.4 · 10 = 2,400 kJ

Step 4: total ideal stored energy:

E_ideal = 8,400 + 20,000 + 2,400 = 30,800 kJ

Step 5: usable stored energy with efficiency:

E_usable = η · E_ideal = 0.8 · 30,800 = 24,640 kJ

Convert to kWh:

E_usable,kWh = 24,640 / 3,600 ≈ 6.84 kWh

This means that, under these conditions, the PCM bank can store roughly the same energy as a 1 kW electric heater running for almost 7 hours. You can adjust mass, temperature range, or material properties in the calculator to explore different designs.

Typical phase change material properties

The table below shows indicative properties for a few common PCMs. Actual values depend on grade, additives, and supplier data, so always consult product datasheets before final design.

Organic PCMs like paraffin waxes are chemically stable and non-corrosive but have low thermal conductivity. Salt hydrates provide higher volumetric storage but may need additives or encapsulation to control issues such as phase separation or supercooling. The efficiency factor in the calculator lets you experiment with how such practical limitations might reduce usable energy.

Comparison: PCMs vs sensible heat storage

PCMs are often compared with conventional sensible heat storage (for example, water tanks or concrete slabs). A simplified comparison is shown below.

The calculator focuses on the PCM case, but you can conceptually compare the kWh results with a sensible-heat-only system using E = m·c·ΔT for a reference material such as water.

Assumptions and limitations

The model behind this tool is intentionally simple and has important assumptions:

• Temperature ordering: The formula assumes T_i < T_m ≤ T_f. If the initial temperature is already above the melting point or the final temperature is below it, the energy distribution between sensible and latent parts will differ from what this calculator assumes.
• Complete melting: It assumes the entire PCM mass fully melts at T_m. Partial melting or non-uniform temperatures within the storage medium are not modeled.
• Uniform properties: Specific heats (c_s, c_l) and latent heat (L) are treated as constants, even though they can vary with temperature, composition, and cycling.
• No subcooling or superheating: Effects such as delayed solidification (supercooling) or the need to exceed the nominal melting point to initiate melting are ignored.
• Uniform temperature field: The PCM is assumed to be well mixed, with no thermal gradients, stratification, or local hot/cold spots.
• Losses simplified into efficiency: All practical losses (heat leaks, imperfect heat exchangers, control limitations) are combined into the single user-entered efficiency factor η.

Because of these simplifications, the results should be treated as a first-order estimate. For critical systems, use detailed thermal modeling and experimental validation.

Frequently asked questions

How should I choose the melting temperature of a PCM?

Pick a melting temperature close to the desired operating temperature range of your system. For building applications, that may be between 20 °C and 30 °C. For process or solar thermal storage, higher melting points may be appropriate. The closer the melting point is to your target temperature, the more useful the latent heat will be.

What units does this calculator use?

Inputs for mass are in kilograms (kg), temperatures in degrees Celsius (°C), specific heats in kJ/kg·°C, and latent heat in kJ/kg. The outputs are given in kilojoules (kJ) and kilowatt-hours (kWh).

How accurate is this estimate for real systems?

For many conceptual designs, the estimate will be within the right order of magnitude. Real systems often store less usable energy than the ideal value because of heat losses, incomplete phase change, or property variations. Adjusting the efficiency factor downward (for example, 0.6–0.9) can approximate these effects, but detailed design should use more advanced models.

Can I use this for cooling as well as heating?

Yes. The underlying energy balance is the same whether you charge the PCM with heat or cold. As long as the material moves through its phase change over your temperature range and the inputs are consistent, the calculated kWh represent the magnitude of energy stored or released.