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).
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:
- Sensible Storage
- Latent Storage
- Thermo-Chemical Storage.
In addition, the two common thermal storage strategies employed are:
- Load Levelling Strategy
- 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:
- 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
- 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:
- Ice Storage – Ice is generated and used either directly or indirectly to cool the chilled water system
- 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
- 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.
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 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.
Department of the Environment and Energy (Australia)
FREQUENTLY ASKED QUESTIONS
Sensible storage involves storing thermal energy by raising or lowering the temperature of a material, typically water or a phase-change material (PCM), within a specific temperature range. Latent storage, on the other hand, involves storing thermal energy by melting or freezing a PCM, which absorbs or releases energy as it changes phase. Latent storage typically offers higher energy density and more consistent temperature output than sensible storage.
Thermo-chemical storage involves the use of chemical reactions to store thermal energy. This method is still in the early stages of development but has the potential to offer high energy density and long-term storage capabilities. In HVAC systems, thermo-chemical storage could be used to store energy generated from renewable sources, such as solar power, and release it as needed to provide heating or cooling.
The load levelling strategy involves storing energy during off-peak hours and releasing it during peak hours to reduce peak demand and flatten the load profile. The load shifting strategy, on the other hand, involves shifting the entire load from one time period to another, typically from peak to off-peak hours. Load shifting can be used to take advantage of time-of-use (TOU) pricing structures, where energy costs vary depending on the time of day.
Water is a common material used for sensible thermal energy storage due to its high specific heat capacity and low cost. Other materials used for sensible storage include phase-change materials (PCMs), such as paraffin wax or salt hydrates, which can store energy by melting or freezing within a specific temperature range. Some PCMs can also be designed to operate within specific temperature ranges, making them suitable for HVAC applications.
Thermal storage systems can be integrated with BMS through the use of sensors, actuators, and control algorithms. The BMS can monitor energy demand, storage capacity, and ambient conditions to optimize charging and discharging cycles. This integration enables the thermal storage system to respond to changes in building load, weather, and energy prices, ensuring maximum efficiency and cost savings.
Thermal storage is commonly used in HVAC systems for air conditioning, heating, and refrigeration applications. It is particularly useful in buildings with high cooling or heating demands, such as data centers, hospitals, and commercial office buildings. Thermal storage can also be used in district cooling systems, where it can help reduce peak demand and improve overall system efficiency.
