To specify a thermal energy storage system well, do not begin with a storage material or a headline number in kilowatt-hours. Begin with the service the customer needs: how much heat or cooling must be delivered, at what temperature and power, for how long, and under which operating conditions.
As explained in our practical introduction to thermal energy storage , TES separates the time when thermal energy is available from the time when it is needed. Specifying the product means translating that principle into measurable requirements that a proposed system can be designed and tested against.
Define the service before the storage
The first question is not, “How large should the store be?” It is, “What must reach the customer process?” A building heating loop, a drying process, a hot-water system and an industrial steam user require different temperatures, flow conditions, response times and interfaces, even when their daily energy consumption is similar.
A clear service definition should state:
- The required thermal output, such as hot water, hot air, steam, cooling or process heat
- The required supply and return temperatures, including acceptable variation
- The peak and normal thermal power required by the load
- The duration and timing of each delivery period
- The available charging source, power and time window
- The operating schedule, expected cycles and seasonal variation
- The site connection, space, mass and control constraints
- The measurements that will demonstrate useful delivery
Hourly or higher-resolution demand data is normally more useful than an annual total. Annual consumption may indicate overall scale, but it does not reveal the power peaks, delivery windows or temperature conditions that determine the design.
Define the load, delivery temperature, power, duration, timing and operating constraints.
Define the energy source, available input power, charging temperature and charging window.
Define usable capacity, permitted temperature range, losses, materials and operating limits.
Define discharge power, outlet conditions, heat transfer, part-load behaviour and verification.
The complete energy pathway must be specified. Storage capacity alone cannot confirm that a system will serve the load.
1. Required delivery temperature
Temperature determines whether the stored energy is useful. A store may contain substantial thermal energy but still be unable to serve a process if its outlet temperature falls below the customer requirement.
A useful temperature specification should include:
- the minimum and normal supply temperature required by the customer;
- the expected return temperature from the thermal loop;
- the maximum permitted temperatures for the process and equipment;
- temperature changes at full and partial discharge power;
- temperature differences across heat exchangers, pipes and other interfaces;
- ambient conditions and any seasonal variation.
Quality matters as well as quantity. One kilowatt-hour of heat available near ambient temperature is not equivalent, for many applications, to one kilowatt-hour available at a much higher temperature.
2. Nominal and usable thermal capacity
Thermal capacity is the quantity of energy retained by the storage system, normally expressed in kilowatt-hours thermal (kWhth) or megawatt-hours thermal (MWhth). The critical word in a specification is usable.
Nominal capacity
A calculated or rated energy quantity associated with the storage material and a defined temperature range. It may not account for all practical temperature limits, heat-transfer constraints or operational reserves.
Usable capacity
The energy that can be delivered to the defined system boundary while maintaining the required outlet conditions and respecting control and safety limits.
For a simple sensible-heat store, the theoretical energy retained in the storage medium can be estimated from its mass, specific heat capacity and temperature change:
Here, m is mass in kilograms, cp is average specific heat capacity in kJ/(kg·K), and ΔT is temperature change in kelvin.
This material-level calculation is only a starting point. A product specification should state the temperature limits, measurement locations and delivery boundary used to determine usable capacity. The stored material may remain warm after the useful discharge limit has been reached.
3. Charge power and charging window
Charge power describes how quickly energy can enter the storage system. It should be stated separately from discharge power because the two duties may be very different.
The specification should identify:
- the charging source, such as electricity, recovered heat, solar thermal energy or a heat pump;
- the maximum and normal input power;
- the available charging time window;
- the source temperature and flow conditions for thermal charging;
- electrical connection requirements for electric charging;
- the permitted ramp rate and any partial-load limitations;
- the energy needed by pumps, fans, controls and other auxiliaries.
Losses, auxiliary consumption, charging limits and the changing state of charge must be considered in detailed design.
A large store with insufficient charging power may never reach its intended state of charge within the available low-price, renewable or waste-heat window.
4. Discharge power and load profile
Discharge power describes the rate at which useful thermal energy reaches the customer. It is not automatically determined by the amount of storage material. Heat exchangers, air or fluid flow, temperature differences, pressure drop, control valves and thermal conductivity can all limit output.
Peak power
The highest thermal output required for a defined period and at defined temperatures.
Continuous power
The output the system can maintain through the required discharge window.
Part-load operation
How outlet temperature, efficiency, flow and control behave below rated power.
The required load profile should be compared with the proposed TES discharge curve. A single maximum-power value does not demonstrate that the unit can sustain the required temperature and power throughout the delivery period.
5. Storage and discharge duration
The word duration is used in two different ways. Discharge duration is the time the system can serve a defined load. Retention duration is the time energy remains stored before use. Both should be stated.
The result is only valid while the system maintains the required delivery temperature and power.
A system with 240 kWhth of genuinely usable capacity could ideally deliver 20 kWth for 12 hours. If 240 kWhth is only a nominal material capacity, the useful duration cannot be confirmed without additional performance information.
6. Thermal losses and retention
A thermal store exchanges heat with its surroundings. The loss rate depends on insulation, thermal bridges, surface area, storage temperature, ambient temperature and time. Therefore, a standing-loss figure is meaningful only when its test conditions are stated.
Useful reporting formats may include:
- kilowatt-hours lost over a stated period;
- average heat loss in kilowatts at a defined storage and ambient temperature;
- percentage of stored energy lost per hour or day at a defined state of charge;
- temperature decay measured at defined locations and conditions.
7. Efficiency and the system boundary
Efficiency can be reported for the storage material, charging equipment, storage period, discharge equipment or the entire energy pathway. These values answer different questions and should not be mixed.
Thermal energy recovery
Compare useful thermal energy delivered at the agreed boundary with the thermal energy transferred into the storage system. State whether the storage begins and ends at equivalent conditions.
Whole-pathway performance
Compare useful delivered energy with the primary charging input and include relevant heaters, heat pumps, heat exchangers, pumps, fans, controls and standby consumption.
The numerator, denominator, test period, initial and final states, and included auxiliaries must all be defined.
For electrically charged TES, both thermal efficiency and electrical auxiliary consumption may be relevant. If a heat pump provides the charging energy, its coefficient of performance should be reported separately from the storage system’s energy recovery.
8. Cycling, response and availability
The expected operating pattern affects material selection, mechanical design, control strategy and maintenance. A system charged once per week is subject to a different duty from one cycling every day or following a variable industrial process.
The operating specification should include:
- expected cycles per day, week and year;
- minimum and maximum state of charge;
- response time and permitted ramp rates;
- full and partial charge or discharge operation;
- planned downtime, inspection and maintenance;
- expected performance degradation and how it will be measured.
9. Physical and site-integration requirements
A technically suitable storage concept must also fit the site. Space, mass, access, foundations, ventilation, pipework, electrical capacity, noise, fire strategy and maintenance clearances can determine whether an installation is practical.
| Specification area | Information to define | Why it matters |
|---|---|---|
| Thermal connection | Heat-transfer fluid, supply and return temperatures, flow, pressure and pressure drop | Determines heat exchanger, pipework and pumping requirements |
| Electrical connection | Voltage, phases, available power, protection and metering | Determines whether the site can supply the intended charging duty |
| Controls | Local control, operating modes, alarms, sensors and communication interface | Enables safe operation and exchange with an external BMS or EMS when required |
| Physical installation | Dimensions, mass, lifting, transport route, foundation and service clearances | Confirms that the unit can be delivered, installed and maintained |
| Environment and safety | Indoor or outdoor conditions, enclosure rating, ventilation, temperatures and hazard controls | Aligns the product design with site risk management and applicable requirements |
| Measurement boundary | Meter locations, sampling interval, calibration and accepted performance method | Allows capacity, power, efficiency and customer value to be verified |
A simplified worked example
Twenty kilowatts of heat for ten hours
Consider an illustrative application that requires 20 kWth continuously for 10 hours. The thermal interface must deliver water at no less than 70 °C during that period, and the storage can be charged over a six-hour window.
The minimum useful energy requirement is 20 kWth × 10 h = 200 kWhth. A candidate offering 240 kWhth of usable capacity at the required outlet condition would have an ideal full-power duration of 12 hours.
Recharging 240 kWhth within six hours requires an average storage-side charging rate of at least 40 kWth, before allowing for losses, changing charge power and auxiliary consumption. The design must also confirm that 20 kWth can be delivered while the outlet remains at or above 70 °C.
This example is an initial energy balance, not a complete system design. Detailed engineering would also evaluate the load profile, return temperature, heat exchanger, flow and pressure drop, control margins, heat losses, site conditions and safety requirements.
A practical specification checklist
When comparing concepts or requesting a proposal, ask each supplier or development partner to define the same parameters and measurement boundaries.
| Parameter | Minimum information to request |
|---|---|
| Application duty | Thermal service, load profile, required hours and seasonal operating pattern |
| Temperature | Storage range, required supply and return temperatures, and minimum outlet temperature through discharge |
| Capacity | Nominal and usable kWhth, including the temperature range and delivery boundary |
| Power | Maximum and continuous charge and discharge power, including part-load behaviour |
| Duration | Discharge duration at specified power and temperature, plus intended retention period |
| Losses | Standing losses with time, storage temperature, ambient conditions and test boundary |
| Efficiency | Charging, storage, discharge and whole-pathway definitions, with included auxiliaries |
| Interfaces | Thermal, electrical, mechanical, communication and metering requirements |
| Operation | Cycling, response, controls, alarms, maintenance, availability and degradation assumptions |
| Verification | Test procedure, sensor locations, calibration, acceptance criteria and reporting method |
Common specification mistakes
Starting with capacity alone
A large energy number does not confirm sufficient output power, temperature or integration capability.
Using nominal as usable
Part of the theoretical energy may remain below the required delivery temperature or outside operating limits.
Combining charge and discharge power
The system may charge slowly and discharge quickly, or the reverse. Both duties must be specified independently.
Quoting efficiency without boundaries
A percentage has limited meaning unless the included equipment, initial and final states, and test period are stated.
Ignoring the customer interface
The storage core cannot create value unless useful energy reaches the load through a suitable thermal connection.
Sizing from annual totals
Annual data can hide peaks, short charging windows, seasonal variation and incompatible temperature requirements.
The ThermoReserve perspective
ThermoReserve approaches thermal storage as an engineered product and customer interface, not only as a storage material. Our initial focus is high-temperature modular thermal energy storage, where charging, containment, insulation, heat transfer, measurement, control and safety must work as one system.
For each application, the relevant question is whether the product can accept energy within the available charging window, retain it with acceptable losses, and deliver useful heat at the required temperature and power. An external building-management or energy-management system may exchange signals with the unit, while ThermoReserve controls and monitors the thermal reserve itself.
This service-led specification approach also supports modular development. Energy capacity and thermal power can be considered as related but distinct design requirements, allowing a product configuration to be built around the application rather than around one fixed headline size.
Further reading
Define the thermal requirement before selecting the product
An initial application discussion should establish the load profile, delivery temperature, usable capacity, charge and discharge power, operating windows and site interfaces.