Energy Density and Thermal Storage: Optimizing the Plant Room Beyond the Traditional Hot Water Cylinder

For an HVAC designer or installer, the plant room is often the place of the “impossible challenge”: fitting generators, pumping groups, and storage systems into spaces that architectural requirements tend to progressively shrink, making the correct sizing of thermal storage a critical design variable.

In this context, the volume occupied by components is not just a logistical matter, but a true design constraint. If the generator size is dictated by the thermal load, the storage size is often the “bottleneck” that prevents the adoption of high-efficiency solutions in limited spaces.

Thermal storage and energy density: the limit of the traditional cylinder

The opportunity cost of technical space

Traditionally, to meet DHW demand peaks calculated according to UNI EN 806 or UNI 9182, the standard solution is sensible (buffer) storage, typically implemented with large-volume domestic hot water (DHW) cylinders, as discussed in the comparison between buffer storage and thermal batteries.

However, the traditional cylinder has an insurmountable physical limit: its low energy density per unit volume.

To store the energy needed to cover the peaks of a hotel or a condominium, water volumes are required that translate into:

• Large footprint, often requiring very spacious dedicated plant rooms.
• Positioning difficulties: A 1,000-litre cylinder has height and diameter constraints that can make it impossible to install in rooms with low ceilings or narrow doors.
• Heat losses proportional to the external surface area, harder to manage in large water storage tanks.

Changing paradigm: latent storage (PCM)

The technical answer to this limitation is moving from the concept of volume to the concept of energy density. i-TES thermal batteries, PCM thermal storage systems (phase change materials), allow the same amount of thermal energy to be stored in a significantly smaller volume than a water tank.

From a technician’s perspective, this approach offers decisive advantages during installation:

1. Volume optimisation (but pay attention to weight): Although the PCM battery takes up much less space, it is essential that the structural designer considers the material’s density. For the same stored energy, the system is more compact, but the specific weight of PCM is higher than that of water, requiring careful assessment of the point load on the slab.
2. Modularity and logistics: The compact and often modular form of i-TES batteries makes it possible to place them in niches or tight plant rooms where a traditional cylinder would not fit.
3. Integration with Heat Pumps: Using PCM makes it possible to keep the generator (HP) correctly sized for the average load, avoiding oversizing of the unit which—while not drastically changing external dimensions—would be more expensive in terms of initial investment.

During the design phase, evaluating energy density therefore becomes an integral part of sizing thermal storage, especially in retrofit projects where technical spaces are already defined.

Conclusions for the professional

Optimising the plant room today means choosing technologies that maximise output for every cubic metre occupied. Energy density thus becomes a true design driver: reducing the storage footprint means freeing up valuable space and, above all, making high-efficiency interventions feasible even where a traditional water storage tank would be physically impossible to install.
i-TES thermal batteries are not just storage, but a spatial optimisation tool for designers who must integrate high performance in constrained contexts.