Photo above: Two large day-to-day storage tanks at the Avedøre Plant near Copenhagen optimize the plant’s economy by allowing electricity production when prices are high and storing surplus heat for later use.
Thermal Energy Storage (TES) is a pivotal technology in advancing sustainable district heating systems. By storing excess thermal energy generated from various sources, TES helps balance energy supply and demand, enhances system efficiency, and contributes to the reduction of greenhouse gas emissions.
Benefits of Thermal Energy Storage
Grid Balancing and Renewable Integration
TES systems play a crucial role in balancing the electrical grid, especially with the increasing integration of renewable energy sources like wind and solar. By storing excess heat during periods of low electricity prices and discharging it when prices are high, TES helps stabilize the grid and optimize the use of renewable energy. This is akin to functioning as a “virtual battery” for the energy system.
Read what some of our most experienced experts write in this HOT|COOL article about the virtual battery.
Operational Efficiency and Cost Savings
TES enables district heating systems to operate more efficiently by reducing peak load production and optimizing the use of Combined Heat and Power (CHP) plants. This not only leads to cost savings but also extends the operational lifespan of production facilities. For instance, due to these efficiencies, the PTES system in Copenhagen is projected to provide an operational benefit of DKK 6.1 million annually by 2025.
Read an article about preparations for the pit thermal energy storage in Greater Copenhagen.
Environmental Impact
By enabling the storage and use of excess heat from renewable sources, TES significantly reduces the reliance on fossil fuels and lowers carbon emissions. This aligns with Denmark’s goal of achieving a 70% reduction in CO2 emissions by 2030. Furthermore, TES supports the integration of surplus heat from industries and data centers into district heating networks, further enhancing environmental sustainability.
The Value of Thermal Energy Storage
“Thermal energy storage offers numerous benefits, including improved economic performance and increased flexibility in district heating systems. Decoupling production from consumption enables the integration of more sustainable heat sources.”
A unique feature of district heating is the ability to store hot water both on a daily and seasonal basis. Similar to storing any other product, thermal energy storage separates the time of production from the time of consumption. This means that heat can be stored when it is generated from sources such as Combined Heat and Power (CHP) plants, solar collectors, surplus wind electricity, and industrial processes. The stored heat can then be used when needed, providing significant flexibility and efficiency to the system. In Denmark, both central and decentralized district heating systems incorporate thermal energy storage.
Types and Applications of Thermal Energy Storage
Accumulation Tank for Heat Storage
An accumulation tank is a flexible and proven technology that stores heat from plants that produce heat for later use. The tank is primarily used to smooth out peak loads in heat demand or production, making it suitable for short-term storage in peak/reserve load situations. Accumulation tanks are used in conjunction with other heat-producing units such as CHP (Combined Heat and Power) and renewable energy (RE) plants with fossil-free fuel sources like solar heat, biomass, and heat pumps. Read more.
Pit Thermal Energy Storage (PTES)
The figure above: Seasonal storage systems, comparable in size to several swimming pools, are filled with water and covered with insulation to store heat for extended periods, ensuring availability during high-demand seasons.
PTES involves storing heat in large, insulated underground pits filled with water. This method has been refined over the past 30 years and is now a mature technology in Denmark. For example, a PTES in Greater Copenhagen has a storage capacity of 70,000 m³, providing a charging and discharging capacity of 30 MW and an energy storage capacity of 3,300 MWh. It functions as a weekly storage solution, expected to be charged and discharged 25-30 times a year.
Read the article “New Pit Thermal Energy Storage in Greater Copenhagen.”
High-Temperature Storage
High-temperature storage solutions, such as molten salt storage, can store heat at temperatures up to 700°C. This method is suitable for generating high-temperature process steam, which can be converted back to electricity using steam turbines. High-temperature storage is particularly useful for industrial applications and can help decarbonize sectors that traditionally rely on fossil fuels.
How Thermal Energy Storage Works: Daily and Seasonal Solutions
Day-to-Day Storage
Short-term thermal energy storage is a critical component of Danish district heating networks. Its primary purpose is to decouple power production at CHP plants, allowing these plants to optimize their cogeneration of electricity and heat according to fluctuating electricity market prices without compromising the heating supply.
The CHP plants produce electricity and heat when prices are high (typically in the morning and afternoon) and store the hot district heating water until it is needed throughout the day. This flexibility is crucial for optimizing the entire energy system, both economically and environmentally.
Short-term storage is also beneficial when integrating external heat production that cannot be controlled by the district heating company, such as industrial surplus heat. For example, industries like fish processing plants, which sporadically produce surplus heat, can store this heat in smaller daily storage units.
Seasonal Storage
Seasonal thermal energy storage is used primarily for large-scale solar collectors that generate more heat during the summer than is immediately needed. These storages are typically large pits lined with water and covered with a floating insulation layer.
Another solution involves groundwater storage systems, where surplus heat is stored at lower temperatures and later used at higher temperatures through heat pumps. Groundwater systems are particularly suitable for urban areas with limited space for large pit storage. Heat stored in these systems can be used during autumn and winter when demand increases.
Seasonal storage will become increasingly important for Danish district heating systems as part of the green transition, utilizing energy that would otherwise be wasted.
Thermal Energy Storage for Solar Heating and Cooling
Integrating thermal energy storage with solar heating systems allows for the efficient use of solar energy, which is abundant in the summer but scarce in the winter. By storing excess heat generated during sunny periods, it can be used for heating during colder months, ensuring a consistent and sustainable heat supply.
Thermal Energy Storage for Data Centers
Data centers generate significant amounts of heat, which can be captured and stored using thermal energy storage systems. This not only helps in managing cooling demands but also provides a sustainable way to repurpose excess heat, enhancing overall energy efficiency.
Utilizing Surplus Wind Energy for District Heating
In 2019, wind energy accounted for 46.9% of Denmark’s electricity consumption. Due to the intermittent nature of renewable energy sources, Denmark sometimes experiences surplus electricity sold at very low or even negative prices. While no full-scale technology is available for directly storing renewable energy, surplus electricity can be used to produce district heating. District heating companies can use this electricity to generate hot water either directly through boilers or via heat pumps, leveraging both short-term and long-term storage solutions.
Challenges and Innovations
Technical Challenges
One of the primary challenges in PTES technology is the design and maintenance of the insulating lid. Issues such as water accumulation, air relief, and thermal expansion have historically posed problems. However, recent advancements, like the new lid design by Aalborg CSP, have addressed these issues, making PTES systems more reliable and scalable.
Collaborative Efforts
The successful implementation of TES technologies requires collaboration among various stakeholders, including energy producers, industry partners, and researchers. Denmark’s approach, which involves extensive knowledge-sharing and collaboration across sectors, serves as a model for other countries looking to develop and integrate TES solutions.
See the video with Astrid Birnbaum presenting the PTES in Høje Taastrup.
Conclusion
Thermal energy storage is a transformative technology that enhances the efficiency, sustainability, and resilience of district heating systems. As Denmark continues to innovate and expand its TES infrastructure, the benefits of these systems will become increasingly apparent, providing a blueprint for global adoption in the quest for greener, more efficient energy solutions.