Thermal Energy Storage Capacity Calculator

The Thermal Energy Storage Capacity Calculator sizes an ideal thermal store, or evaluates the capacity of one you already have. It handles storage by temperature change alone, by phase change alone, or by both together, and it reports the sensible and latent components separately as well as the total.

Around that capacity it works out the source-side energy needed to charge the store at the charge efficiency you enter, the useful energy delivered to the load after discharge losses, the round-trip efficiency, the stored energy per kilogram of medium, and how long a charge and a discharge take at the constant powers you give it.

It runs in two directions. In evaluate mode you enter a mass and it returns the capacity. In sizing mode you enter the useful delivered energy you need and it solves for the mass of medium required.

It works in SI units: kilograms, degrees Celsius, kilojoules per kilogram per kelvin, kilojoules per kilogram, and kilowatts and kilowatt hours thermal.

The capacity it reports is ideal. There are no standby losses, no stratification and no containment, so treat the result as an upper bound for preliminary sizing rather than as the capacity a built store will deliver.

Purpose and model boundary

This model evaluates an entered thermal-storage mass or sizes the mass required to deliver an entered energy target. It supports ideal sensible, latent, and combined sensible-plus-latent mechanisms, then applies charge/discharge efficiencies and constant powers to report ideal cycle energies and durations. It is not equipment, vessel, fire, materials-compatibility, or economic design.

Inputs and units

Inputs use kilograms, degrees Celsius, kilojoules per kilogram-kelvin, kilojoules per kilogram, kilowatts, kilowatt-hours, and fractional efficiencies. The calculation mode selects entered-mass evaluation or target-delivered-energy sizing. The storage-mechanism selector controls which specific-heat and latent-heat properties are active. Combined mode also uses a phase-change temperature between the initial and final temperatures.

Governing relationships

For sensible storage with effective specific heat c_p:

E_sensible = m c_p |T_f - T_i| / 3600

where division by 3600 converts kilojoules to kilowatt-hours. For latent storage:

E_latent = m L / 3600

For combined storage, the workbook adds the low-phase sensible segment, a complete latent transition, and the high-phase sensible segment:

E_total = m [c_p,low |T_pc - T_i| + L + c_p,high |T_f - T_pc|] / 3600

The exact segment directions follow the entered heating or cooling path. In evaluation mode m is the entered mass. In sizing mode the workbook solves:

m_required = E_target / (η_discharge × energy-per-unit-mass)

The energy boundaries are:

E_delivered = η_discharge × E_total

E_charge,input = E_total / η_charge

round-trip efficiency = η_charge × η_discharge

charge duration = E_charge,input / P_charge

discharge duration = E_delivered / P_discharge

Calculation sequence

1. Validate mode/mechanism selections, active properties, temperatures, mass or target, powers, and efficiencies.
2. Determine heating/cooling direction and the active sensible/latent segments.
3. Calculate stored energy per unit mass.
4. Use entered mass or solve required mass from the delivered-energy target.
5. Calculate sensible, latent, total stored, delivered, and source-input energies plus both ideal durations.
6. Evaluate workbook status and plot the sensible/latent contribution bars.

Outputs and interpretation

Headline outputs are total stored thermal energy and usable delivered energy. Supporting results expose mass used, energy density, sensible and latent components, temperature direction, charge input, round-trip efficiency, and ideal charge/discharge durations. The contribution chart is a decomposition of the workbook result, not a time-dependent state-of-charge curve.

Validation and status logic

This is the complete workbook TES_Model_Status logic. Invalid states return protected zero numeric outputs, which are placeholders rather than valid zero-capacity results.

Assumptions and limitations

• Specific heats and latent heat are constant user-entered effective properties.
• Combined mode represents a complete phase transition with the phase-change temperature strictly between the two endpoint temperatures.
• Charge power is source-side thermal power before charge losses; discharge power is useful load-side power after discharge losses.
• Power and efficiency remain constant through each idealized full cycle.
• Partial melt fraction, hysteresis, supercooling, stratification, standby loss, parasitic power, degradation, cycling limits, and rate-dependent capacity are not modeled.
• The workbook contains no material or vendor database and does not establish heat-exchanger approach temperatures or usable operating limits.

Restrictions and non-computing states

Active specific heat, latent heat, mass or target energy, and charge/discharge power must satisfy their allowed ranges; efficiencies must be positive and no greater than one. Combined mode requires a complete endpoint crossing of the phase-change temperature. Inputs that violate a bound or supply an undeclared mode are rejected before calculation.

Errors and warnings

NOT VALID means the selected storage path cannot be evaluated or sized. The workbook has no separate caution state: OK confirms only that the ideal energy arithmetic completed. A calculation-service or connection failure is not zero capacity and must be handled as an operational error.

The energy boundaries, mechanism visibility, sizing equation, duration relationships, named output formulas, and status logic were verified against the delivered workbook, its published input rules, tests, and reviewer packet. The workbook identifies these technical sources:

• NBSIR 79-1737: Testing of Pebble-Bed and Phase-Change Thermal Energy Storage Devices — Sensible-plus-latent capacity and ideal fill-time energy balance.
• NREL Methodology for Phase-Change Thermal Energy Storage — Sensible and latent capacity integrals and the phase-change enthalpy contribution.
• NIST Special Publication 811 — SI temperature conventions and the exact kilowatt-hour conversion.