Technical Description

Seasonal heat storage plays a crucial role in ensuring efficient and sustainable district heating systems, particularly in regions with fluctuating energy demands throughout the year. Here's a breakdown of different options for long-term heat storage for district heating systems:

1. Pit Thermal Energy Storage (PTES):

• PTES involves using a large water reservoir to store thermal energy. Water is preferred as a storage medium due to its non-toxic nature, ability to stratify according to temperature, high capacity for charging and discharging, good heat transfer characteristics, and relatively low cost.
• The design typically involves excavating a pit in the ground, lined with a waterproof membrane, and filled with water. The excavated soil is used to create embankments around the pit, increasing water depth and reducing costs..

2. Borehole Thermal Energy Storage (BTES):

• BTES systems consist of tubes installed in boreholes, which are then operated in conjunction with heat pumps. These systems are suitable for storing thermal energy at low temperatures (0 to 30°C), although they can also handle higher temperatures (up to 90°C).
• The efficiency of BTES can be high, particularly when operating around the average natural temperature of the ground and in areas with minimal groundwater flow.
• BTES requires relatively small land areas, making them suitable for various applications. Pilot projects and larger implementations have been carried out in several countries, including Canada and Germany.

3. Aquifer Thermal Energy Storage (ATES):

• ATES involves storing hot or cold water in natural underground aquifers through vertical wells. Heat exchange occurs directly between the water and the surrounding aquifer.
• This technique is commonly used in countries like the Netherlands and Sweden, often in conjunction with heat pumps for space heating and cooling.
• Aquifer storage can be cost-effective for large-scale storage needs, particularly for low-temperature volumes, if managed properly. Challenges include managing the chemical composition of the aquifer and natural groundwater flow.

Charging and discharging these storage systems involve pumping water through heat exchangers and wells, either to store heat in the ground or extract it for use in the district heating system. Each storage option offers unique advantages and considerations, allowing for flexibility in designing efficient and reliable district heating systems tailored to specific environmental and operational requirements.