Thermal Storage (HVAC)

Thermal storage refers to the application of storing thermal energy in materials for later utilisation . Figure below depicts the charge and discharge cycle for thermal storage systems, i.e. the storage of energy (charging) and the use of energy at a later time that benefits the user (discharging).

Charging and discharging cycles for thermal storage systems

Thermal storage can result in the reduction of operating costs by producing and storing the energy during periods of low energy supply cost (off-peak/night time) and utilising the stored energy during periods of high energy supply cost (peak/day time).

Thermal energy may be stored in three main ways:

  1. Sensible Storage
  2. Latent Storage
  3. Thermo-Chemical Storage.

In addition, the two common thermal storage strategies employed are:

  1. Load Levelling Strategy
  2. Load Shifting Strategy

Sensible Heat Storage

Sensible heat storage refers to the heat storage within a medium that does not result in a change of state (e.g. liquid remains liquid or solid remains solid). The two main sources of sensible heat storage applicable to commercial buildings are:

  1. Water Storage – Due to the high heat capacity of water, tanks are commonly used as the thermal storage medium within chilled water and hot water systems
  2. Building Mass – By increasing the thermal mass of the building using dense materials (bricks, concrete slabs, etc.) peak loads can be minimised. These dense materials are able to store heat throughout the day, radiating heat back into the space when ambient temperatures have dropped, or alternatively, cool down over night and remove heat from the space during the day.

Latent Heat Storage

Unlike sensible heat storage, latent heat storage utilises a medium that transfers heat by changing state (e.g. liquid to solid). Given this additional phase change capability, latent heat systems have greater capacity to store energy than those of sensible heat storage systems, when the same physical size. The main methods of latent heat storage within the HVAC industry are:

  1. Ice Storage – Ice is generated and used either directly or indirectly to cool the chilled water system
  2. Phase Change Material (PCM) – PCM typically use specific salt formulations to increase the freezing point of the material above the chilled water supply temperature, so the material can be frozen with chilled water, to store the energy for later use
  3. PCM Building Fabric – Phase change materials can be used in the building fabric to increase the thermal storage in the building mass.

Given the increased capital cost associated with latent heat storage, they are not as prevalent as the sensible heat storage solutions. However, since latent solutions weigh less and are physically smaller than equivalent sensible solutions, the capital costs can potentially be offset by structural savings.

Thermo-Chemical Storage

Thermo-chemical storage is a thermal storage solution that utilises a reversible chemical reaction in the medium for heat transfer. Similar to the other thermal storage solutions discussed above, the thermo-chemical storage consists of three main stages, shown in Figure below.

Charging, storing and discharging stages for a thermo-chemical storage solution

During the charging cycle a thermo-chemical material absorbs heat through an endothermic reaction (heat in) and produces two product chemicals (chemicals A + B). Chemicals A + B are then separated and stored. During the discharge cycle, these chemicals are recombined through an exothermic reaction (heat out) reforming the original thermo-chemical material and liberating heat.

The main advantage of thermo-chemical storage is that it offers approximately six times the storage capacity of the leading latent solutions. However, due to the high capital costs, industry adoption of thermo-chemical storage is low.

Load Levelling Strategy

A load levelling strategy aims to equalise the building loads throughout the day. Such a strategy can be implemented by the use of thermal storage, where the system discharges when the building load is greater than the chiller output and charges when the building load is less than the chiller output. This is illustrated in Figure below where the chiller load is a constant 50% of the building oad. The main advantage of the load levelling system is that the required chiller size can be reduced.

Load levelling load profiles for the building and chiller

Load Shifting Strategy

Load shifting strategy is where thermal storage is charged during off-peak times such that the stored energy can be used during times of peak load. It aims to shift the entire on-peak load to off-peak hours, as shown in Figure below. By reducing or eliminating the chiller operation, the electricity consumed during the day is reduced. This strategy is commonly used to take advantage of the lower off-peak energy costs.

Load shifting load profiles for the building and chiller
Department of the Environment and Energy (Australia)