Thermal energy storage systems break down into two types

• Diurnal thermal storage

• Seasonal thermal storage

 

Diurnal thermal storage seeks to capture heat when excess heat is available and release the heat when it is needed. For example, heat can be captured from Solar Thermal panels during the day and released at night. These systems are designed to capture and release heat on a daily basis. As such, they do not require large energy storage systems.

 

Diurnal thermal storage systems are more beneficial in situations where the heat need varies from day to night. For example in desert or prairie climates, the outdoor temperatures can vary from 5C at night to 30C during the day. Diurnal batteries are not effective for more moderate climates such as costal or tropical regions.

 

Most diurnal thermal batteries use water as the medium for energy storage. The energy water-based systems can store is simply a function of the size of the water storage tank and the insulation properties. Depending on the amount of thermal energy required, these tanks can get very large.

 

To address the thermal store size issue, phase change thermal batteries use a material such as a wax that changes from a solid when cold to a liquid when sufficient heat is introduced.  This change in phase stores a much higher amount of energy.  As a result, thermal energy stores that use phase change materials are much smaller.

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Seasonal thermal storage systems seek to capture excess heat during the summer months and then use that heat in the winter months.  As such, these batteries are better suited to northern latitudes where there is more significant variation in temperature from summer to winter.

 

The challenge for seasonal thermal storage is the volume of thermal energy required. The most common is using the earth as the energy store with a large number of vertical boreholes. The efficiency of these systems depends on the local geology and thermal properties of the energy store (see Case Study: Drake Landing).