Thermal energy storage is a system used for temporarily storing excess heat and releasing it when needed. This allows the use of solar heat also when the sun is not shining. Every solar thermal installation comes by default with thermal energy storage, be it in small installations like individual uses on private buildings or in large-scale applications like district heating and industry.
Thanks to thermal energy storage, no surplus energy is lost, and a continuous energy supply can be guaranteed independently from weather conditions and volatile energy markets. Solar thermal provides the largest energy storage capacity from all renewable energy sources, making it a crucial player for energy security and efficiency.
Did you know?
Thermal energy storage, included per default in every solar thermal installation, is to date by far the cheapest source of energy storage!
How does it work?
There are times when more energy is harnessed from the sun than needed, for example on sunny days and during the summer. Since the produced renewable heat is often not used completely, thermal energy storage is an efficient solution that ensures an energy supply also at night, on cloudy days, and during the winter.
Thermal energy storage optimizes energy costs by providing heat directly to the end user without the need for conversion and thus avoiding energy waste, making solar thermal technology even more cost-efficient. In certain circumstances, thermal energy storage can also be used with other energy sources, such as geothermal or waste heat.
There are two different types of thermal energy storage:
Short-term thermal energystorage
Short-term thermal energy storage is used when the energy demand should be satisfied, for example, on cloudy days or at night. The most common short-term storage method is the displacement of hot/cold water via accumulator tank storage. Usually, these tanks have a vertical cylindrical form made of steel. There are two main types of tanks, pressurized and atmospheric (non-pressurized) tanks.The volume of the tanks varies from a few hundred cubic meters to tens of thousands, based on the application. Across Europe, there are millions of small short-term thermal energy storage units in households. For example, a storage unit with a volume of 300 litres usually comes with an energy storage capacity of approximately 22.5 kWhth.
Large-scale and seasonal thermal energy storage
Seasonal thermal energy storage is indicated for long-term applications such as storing summer heat for winter heating or winter cold for summer air conditioning. Seasonal thermal storage has traditionally been tied to solar heat, allowing the surplus energy in summer to be displaced in the winter, when demand is high and supply low. These can reach capacities above 142 MWth. Four types of large-scale or seasonal thermal energy storage are commonly used worldwide. The four storage concepts include tank and pit thermal energy storage (TTES and PTES), borehole thermal energy storage (BTES), and aquifer thermal energy storage (ATES).
Ensuring flexibility in our energy system
Storage capacity is essential for the flexibility of the energy system. The heat demand in Europe is more than double of the power demand and on top of that with much more extreme peaks. The fact that every solar thermal system integrates by default storage capacity is a major added value to the energy system.
