Thermal Energy Storage Technologies Comparison
Thermal energy storage (TES) is the process of collecting thermal energy for future use. Thermal energy storage operates like a battery, using a combination of cooling equipment and energy storage tank to transfer cooling production to off-peak hours, usually nighttime.
Ice or chilled water that is formed / chilled during the night is used to supply the cooling energy during the on-peak hours. The storage cycle for a TES system can be daily, weekly or even seasonal.
After that simple introduction, let us check out some options for thermal energy storage.
Thermal Energy Storage Technologies
Thermal energy storage comes in various technologies that store energy in the form of chemical energy, latent heat or sensible heat. For sensible heat systems, the thermal capacity and sensible heat are the main parameters of the medium in focus. A more specific categorization of thermal energy storage technologies is as shown below.
1. Ice Harvesting
For this technology, the evaporators are placed over the TES Tank. The water return flow goes to the evaporators and ice blocks are formed. Afterwards there is a defrosting process in which the ice blocks are dropped into the tank where there is a mix of water and ice. This system may also be referred to as an ice harvester chiller because it is technically a water chiller and an icemaker.
Ice is formed during the charging cycle. Ice thickness can be as big as 10 mm, but this depends on cycle length.
2. External Melt Ice
This system builds ice and stores it on the exterior surface of a coil. The heat exchange coil used in this case is submerged in a tank full of water. A secondary coolant or liquid refrigerant passes through the heat exchanger coils and causes ice formation on the outside surface. During the discharge phase, the water is circulated from the top of the tank to the bottom of the tank. This water gets cold by melting the ice formation from their external surface.
Tanks used in this technology can be of several shapes. The most common shapes are vertical cylinders and rectangular parallelepiped cylinders. Ice annular thickness can vary a lot, but most ratings average at 36 mm.
Even formation is very important for the external ice melt process. That is why air is passed through the water both at the beginning of the charging cycle and at discharging cycle.
Benefits of this system include:
- No ice water flow restrictions, so ice melt rate can vary immensely.
- The system configuration is simpler than with other types of ice tanks.
Owing to these characteristics, internal melt ice systems are common in industrial and commercial applications.
3. Internal Melt Ice
Like in the external melt ice system, internal melt ice systems form ice on submerged tubes or pipes. This system needs a secondary coolant, mostly 75% water and 25% ethylene glycol, as the heat transfer fluid.
To discharge cooling, warm coolant circulates through the pipes and melts the ice. During the charging cycle, a standard chiller cools the glycol solution. As the cool solution (about -3°C) flows through the tubes within the tank, it causes ice to form on the outer surface.
Some advantages of this system include:
- Water does not leave storage container
- Simple controls-closed loop system
4. Chilled Water
A chilled water thermal energy storage technology relies on the sensible heat characteristic of water. Temperature differential is very critical for this system because it determines tank volume.
TES comprises of two operational phases namely charging and discharging. During the charging phase, chilled water is entering at the bottom of the tank while return water is exiting at the top of the tank. During charging, the warm water is floating on the chilled water. Withdrawal of the water is at low velocity and in horizontal motion to maintain buoyancy as much as possible.
The separation between warm return water and stored cold water affects tank volume. Considering performance and cost factors, natural stratification emerges as the best alternative. Special diffusers maintain the movement of water into and out of the storage tank.
5. Multi Tank
A multi-tank design is essentially in the chilled water technology category, but it comprises of several tanks. Instead of a decoupler or bypass pipe in the single tank scenario, this system comprises of a number of tanks set in parallel from the chiller supply water temperature (for example 5.56° C) and water coming from the cooling consumers (for example at 14.44° C)
We have identified various thermal energy storage technologies. Application of any of these options is largely dependent on heat source type. When you are selecting a thermal energy storage technology, you will need to consider a few attributes including cost of the unit, charging and discharging rate, temperature range and optimum capacity.
If you are looking at overall efficiency, both chilled water and multi-tank technologies score very highly. However, they tend to occupy larger space than the other three technologies. Interestingly, the two technologies are more affordable than the others are. If you liked this post you may be interesting in thermal energy storage tank in district energy systems.