Three ways to hold energy
All thermal storage does the same thing: it moves energy into a material, keeps it there as well as it can, and takes it out again. What differs is how the material holds the energy. There are three broad mechanisms, and each suits different temperatures, sizes and budgets.
Sensible storage
Sensible storage raises or lowers the temperature of a material without changing its phase. Water in an insulated tank is the familiar example. The energy stored depends on three things: the mass of the material, how much energy it takes to change one kilogram by one degree (the specific heat), and the temperature change.
As a formula: energy = mass × specific heat × temperature change.
A hypothetical example, with rounded numbers: water has a specific heat of roughly 4.18 kJ per kilogram per kelvin. If a tank holds 1,000 kg of water, and it is heated by 30 K, the stored energy is about 4.18 × 1,000 × 30 = 125,400 kJ. Since 1 kWh equals 3,600 kJ, that is about 34.8 kWh. The figures are illustrative only and ignore heat losses.
The strengths of sensible storage are simplicity and low-cost materials. Its limits are size, because a lot of mass is needed for a lot of energy, and the fact that the delivery temperature falls as the store is drawn down. Water works across a limited temperature range at ordinary pressure, so higher-temperature uses may switch to oil, rock, concrete, metal or ceramic. Solid media are often used where temperatures are high and the medium is cheap, with a fluid circulating through to charge and discharge it.
Latent storage
Latent storage uses the energy absorbed or released when a material changes phase, such as melting or freezing. During the change the temperature stays nearly constant, which makes for a steady delivery temperature and compact storage.
Continuing the hypothetical example: melting ice absorbs roughly 334 kJ per kilogram. So melting 1,000 kg of ice absorbs about 334,000 kJ, or about 92.8 kWh (334,000 divided by 3,600). By comparison, warming that same kilogram of water by 10 K takes only about 41.8 kJ, so melting a kilogram of ice absorbs roughly eight times as much energy as that modest warming. This is why ice storage can hold a lot of cooling in a small space.
Other phase change materials melt and freeze at different temperatures, such as certain salt hydrates, paraffin-type waxes and other compounds. The right material is one whose melting point matches the process. Points that need attention include:
- Repeated cycling can degrade some materials.
- Many have low thermal conductivity, so heat moves slowly in and out unless the design adds fins, encapsulation or other measures.
- Some are corrosive, flammable or change volume when they melt, which affects containers and safety.
- Some may separate into layers or supercool, meaning they stay liquid below their normal freezing point.
Because of these details, suppliers’ data and engineering design are needed for any specific material.
Chemical storage
Chemical, or thermochemical, storage uses reversible reactions or sorption processes. Heat drives a reaction in one direction, storing energy in the chemical bonds or in a separated state; later, the reverse reaction releases the heat. Examples include certain salt hydration reactions and sorption of water vapor onto solid materials.
The attractions are potentially high energy density and the ability to keep the energy for long periods with little loss, because the energy is not held as sensible heat that leaks away. The challenges are complexity: reactors, heat and mass transfer, durability over many cycles and cost. Many thermochemical approaches are still at pilot or demonstration scale or are used in specialized settings, so for a typical plant, sensible and latent options are much more common. Treat claims about chemical storage cautiously and look for evidence from comparable installations.
Comparing the three
| Feature | Sensible | Latent | Chemical |
|---|---|---|---|
| How energy is held | Temperature change | Phase change | Reversible reaction |
| Typical complexity | Low | Moderate | High |
| Storage density | Lower | Higher | Potentially highest |
| Delivery temperature | Falls as store is drawn | Nearly constant | Depends on reaction |
| Maturity in industry | Widespread | Used, especially ice | Mostly developing |
This is a general comparison, and real systems vary.
Where stored heat or cold is used to shift load
Plants use these ideas for a few recurring purposes:
- Cooling peaks. Chilled water or ice produced off-peak serves daytime loads, which can reduce the size of the equipment needed at the peak.
- Heat buffering. Hot water or steam accumulators smooth out batch or intermittent heating demand.
- Heat recovery timing. Heat from one process may be stored until another process needs it, which matters when the two do not run at the same time.
- High-temperature processes. Solid media or thermal oil systems can hold heat at temperatures that suit drying, curing or other heating steps.
- Protection. A modest reserve of cold can give a process time to stay in range if cooling equipment trips.
Storage is only useful if the heat can be moved in and out at an adequate rate. Heat exchangers handle that transfer, so their size and cleanliness influence how quickly a store can charge or discharge. At the supply side, equipment such as process heating sources or large scale refrigeration plants must be able to charge the store in the time available.
Design considerations
Several factors tend to shape the choice:
- Temperature range. Which supply and return temperatures does the process need, and how much of the store’s capacity falls in that range?
- Power versus capacity. A store can hold plenty of energy but release it too slowly. Both the total energy and the rate matter.
- Losses. Insulation, surface area and how long energy is held affect how much is lost.
- Space and containment. Storage tanks must be sized, supported and, if pressurized, built and inspected as pressure equipment.
- Controls. Strategies for charging and discharging rely on accurate temperature measurement, so sensor calibration deserves attention.
- Safety. Hot media, high pressure, refrigerants and reactive chemicals each bring hazards that call for manufacturer instructions, local rules, lockout and tagout style procedures and trained, certified personnel.
Outcomes depend heavily on the specific site, and any estimate of savings should be checked by a qualified engineer. This article is general information, not engineering advice.