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Thermal Storage / Building HVAC

PCM Thermal Energy Storage Simulator

Design latent-heat storage systems with paraffin, salt hydrate, fatty acid or metallic PCMs. Adjust PCM mass, operating temperature range and encapsulation fraction to see the latent/sensible storage balance, energy density, tank volume and charging power update in real time.

Parameters
PCM material
Sets melting point, latent heat, specific heat, density and cost
PCM mass
kg
Operating temperature range
°C
Display only — the actual computation uses cold/hot temperatures
Cold side T_cold
°C
Hot side T_hot
°C
Charging time
h
Encapsulation fraction
Net PCM mass / encapsulated mass (leak control + shape stability)
Results
—
Melting point (°C)
—
Latent capacity (kWh)
—
Sensible capacity (kWh)
—
Total capacity (kWh)
—
Latent fraction (%)
—
Energy density (kWh/m³)
—
PCM tank cross-section — moving melt front

The melt front advances from the heated outer wall toward the centre (Stefan problem). The right-hand bar shows the sensible/latent split of stored energy.

T-Q diagram (sensible → latent → sensible)
Volumetric energy density by PCM type
Theory & Key Formulas

$$Q_{stored} = m \cdot c_p \cdot \Delta T + m \cdot L_f,\quad \rho_E = \frac{Q}{V}$$

m = PCM mass, c_p = specific heat, L_f = latent heat of fusion, Q = total stored energy, V = volume. Total storage is the sum of sensible (cp·ΔT) and latent (L_f) heat; ρ_E is the volumetric storage density.

$$Q_{latent} = m \cdot L_f \cdot \eta_{enc},\quad Q_{sensible} = m\,c_{p,s}(T_m-T_c) + m\,c_{p,l}(T_h-T_m)$$

η_enc: encapsulation fraction, c_{p,s} / c_{p,l}: solid- and liquid-phase specific heats, T_m: melting point, T_c: cold side, T_h: hot side.

Phase Change Material (PCM) Thermal Storage — Latent / Sensible Balance

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I read that "phase change materials (PCMs)" can store heat without changing temperature. Is that actually true?
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Yes, exactly the same principle as ice melting at 0°C. Melting 1 kg of ice takes 334 kJ — equivalent to heating that water from 0 to 80°C. A PCM is just a material engineered so that this huge latent heat happens at a useful temperature window — around 28°C for buildings, several hundred °C for CSP plants.
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Got it. But when I move the "PCM mass" slider on the left, both latent and sensible energy change. Isn't the stored energy supposed to be only the latent part?
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Sharp eye. In practice the tank is heated from the cold side (say 18°C) up to the hot side (say 32°C), so heat goes in and out over the whole range. From 18°C to the melting point (28°C) you store 10°C worth of solid-PCM sensible heat, at 28°C the latent step happens, then from 28°C to 32°C you store another 4°C of liquid sensible heat — three stages. Look at the T-Q diagram on the right: the vertical jump in the middle is the latent plateau.
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Oh I see it! So the higher the "latent fraction", the better the PCM is doing its job?
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Exactly. With the defaults you should see about 87% latent. Once it drops below 40% there is no point using a PCM — you might as well use a water tank. Try widening the operating range by setting the hot side to 80°C — you will see the latent fraction collapse, because most of the stored energy becomes sensible.
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It really does! So in real buildings, the trick is to keep the operating range narrow?
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Yes. A typical PCM ceiling panel might be designed to solidify at 22°C overnight on outside air, then absorb heat when the room exceeds 28°C in the afternoon and release it again as the room cools below 28°C in the evening. That is only an 8°C swing, but it can shave 10–30% off the peak cooling load. The PCM ice banks in data centres use the same trick: charge ice at night on cheap power, discharge during the daytime cooling peak.
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There are so many PCM families — paraffin, salt hydrate, fatty acid, metallic. How do I pick between them?
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Start with the temperature window. Buildings (20–30°C) point to paraffin (28°C) or salt hydrate CaCl₂·6H₂O (29°C); commercial refrigeration points to fatty acids like lauric acid (44°C); CSP plants above 200°C use molten salts or metallics such as Bi-Pb (125°C). Then look at cost and durability. Salt hydrates are cheap but suffer from supercooling and phase segregation, while paraffin is stable over 10,000+ cycles and is the basis of commercial products like BioPCM building materials and Outlast garments. Compare the volumetric density of the four families in the bar chart below.

FAQ

During its melting/solidification phase change, a PCM absorbs or releases a large latent heat at almost constant temperature — about 240 kJ/kg for paraffin and 190 kJ/kg for a salt hydrate. Compared to water sensible heat (4.18 kJ/kg·K × ΔT), even a 5°C swing only gives about 20 kJ/kg, so PCMs deliver an order of magnitude higher storage density. The required tank volume drops to roughly 1/5 to 1/10, which is why PCMs are popular for space-constrained building HVAC and vehicle applications.
The first criterion is that the melting point sits inside the operating temperature range (cold to hot). Winter heating wants 20–25°C, summer cooling 7–10°C, and CSP plants 200–600°C — completely different chemistries for each. After that, rank candidates by latent heat, cycle stability (does it survive 10,000+ cycles?), supercooling (salt hydrates need nucleating agents), leakage control (microencapsulation, shape stabilisation) and cost. Paraffin-based PCMs (BioPCM, Outlast) are the default for buildings.
When the latent fraction (potency factor) drops below 40%, using a PCM stops paying off. If the operating range is much wider than the melting plateau, most of the stored energy is sensible (cp·ΔT) and the system behaves like a plain water or rock store. This tool reports the latent fraction and warns below 40%. Remedies include narrowing the operating range, switching to a PCM whose melting point is closer to the average operating temperature, or stacking several PCMs in a temperature cascade (cascaded PCM storage).
Most practical PCMs are encapsulated or shape-stabilised to prevent liquid-phase leakage and improve heat transfer. Microencapsulation (particle size of a few to several hundred μm) gives a high surface area for fast thermal response and lets the PCM be mixed into concrete or gypsum board. The shell material does not store heat, however, so the effective encapsulation fraction (net PCM mass / total mass) drops to about 0.8–0.9. Salt hydrates also need shape stabilisation to combat supercooling and phase segregation. This tool accounts for that loss via the encapsulationFraction input.

Real-world applications

Building HVAC and peak shaving: Representative products include BioPCM-Q21 (melting point 21°C) from Phase Change Energy Solutions, Outlast temperature-regulating textiles, and German Glauber-salt building boards such as GR27. Microencapsulated PCM mixed into ceiling tiles, floor panels and gypsum board stores heat during the day and releases it at night, cutting peak HVAC power by 10–30%. US DOE studies suggest that PCM walls can reduce residential cooling loads by up to 25%.

Concentrated solar power (CSP): Commercial CSP plants such as Andasol in Spain use NaNO₃/KNO₃ molten salt (melting point around 220°C) as thermal storage so that the turbine keeps running after sunset. Tanks of 500–1,500 MWh capacity providing 6–15 h of dispatchable generation are standard. High-temperature PCMs such as Bi-Pb (125°C) and SiC-matrix cascade PCMs are in active research and pilot deployment.

Data-centre and industrial cooling: Large data centres charge a PCM ice bank on cheap overnight power and discharge it during the daytime cooling peak, cutting electricity cost and grid demand. PCMs also stabilise temperature in cold-chain pharmaceutical containers (vaccine transport), provide backup cooling for cold storage warehouses, and manage satellite thermal loads as spacecraft cycle between sunlight and eclipse.

Industrial waste-heat recovery: Medium-temperature waste heat (150–400°C) from steel mills and cement plants is being captured into sugar-alcohol PCMs (e.g. erythritol) or metallic PCMs and transported by truck (Mobile Thermal Energy Storage) to nearby heat customers. The high energy density makes this kind of inter-site heat matching economically viable.

Common misconceptions and pitfalls

The single biggest trap is assuming the catalog latent-heat value applies directly. Encapsulated or shape-stabilised PCMs only deliver about 80–90% of the pure-material latent heat because the shell stores no latent energy. The encapsulationFraction input in this tool models that loss, so do not confuse it with raw bulk PCM. Salt hydrates additionally lose performance over time because of phase segregation (the water and CaCl₂ separate on melting), so for long-term operation many designers derate the latent heat to about 70% of the manufacturer figure.

Next, thinking of the melting point as a single sharp temperature. Real PCMs melt over a window of ±2–5°C around the nominal value (peak temperature ≠ completion temperature). Nominal 29°C CaCl₂·6H₂O actually distributes its phase change between 27 and 32°C. This tool assumes an ideal isothermal melt, so production designs should re-evaluate the effective latent capacity using a DSC (differential scanning calorimetry) enthalpy curve over the operating range.

Finally, ignoring thermal conductivity and sizing only on mass. Most PCMs have a thermal conductivity of only 0.2–0.5 W/m·K, which makes charge and discharge slow. The charging power in this tool comes from an energy balance — it tells you the average power, not whether the heat can actually reach the centre of the tank at that rate. Practical designs therefore rely on expanded graphite, metal fins or carbon nanomaterials to push conductivity up to 1–10 W/m·K. Securing enough heat-transfer area is just as important as sizing capacity.

How to Use

  1. Select PCM material type (paraffin wax, salt hydrate, fatty acid, or metallic compound) from the dropdown—each has distinct melting points and latent heat values.
  2. Enter PCM mass in kilograms and your operating temperature range (cold and hot setpoints in °C) to define the thermal cycle.
  3. The simulator calculates latent capacity during phase-change, sensible capacity for solid/liquid temperature swing, and total energy density in kWh/m³ to validate system sizing.

Worked Example

Design a thermal buffer for a concentrated solar power plant using 500 kg of paraffin wax (melting point 58°C, latent heat 200 kJ/kg). Set cold temperature 40°C and hot temperature 75°C. Latent capacity = 500 kg × 200 kJ/kg ÷ 3,600 = 27.8 kWh during melt. Sensible capacity (solid/liquid, specific heat ~2.5 kJ/kg·K) = 500 × 2.5 × (75−40) ÷ 3,600 = 12.2 kWh. Total capacity ≈ 40 kWh; energy density ≈ 45 kWh/m³ assuming bulk density 0.9 kg/L. Latent fraction = 69% indicates efficient phase-change utilization.

Practical Notes

  1. Salt hydrates (e.g., Na₂SO₁₀H₂O, 32°C melt, 254 kJ/kg) offer higher energy density than paraffin but require corrosion-resistant encapsulation and supercooling mitigation.
  2. Fatty acids (e.g., stearic acid, 69°C melt) suit building HVAC and district heating; verify thermal cycling stability over 10,000+ cycles to detect degradation.
  3. Metallic PCMs (gallium, 30°C, 80 kJ/kg) achieve extreme density (~6,300 kWh/m³) for spacecraft thermal management but demand hermetic containment and cost >USD 50/kg.
  4. Always include 10–15% design margin for latent capacity to account for incomplete melting and supercooling losses in real encapsulation systems.