Thermal energy storage – solutions and trends

Through the eye of an expert

Emilia Basta, Engineer of eco-energy systems

Thermal energy, also known as heat energy, arises from the movement of molecules or atoms within a given physical system, leading to the generation of heat. This heat can be produced through processes like fuel combustion, as well as through the utilization of various energy sources for the same purpose.

How to store thermal energy?

Waste heat can be generated from various technological processes, geothermal sources, and renewable energy sources. This thermal energy can be captured and stored using devices known as Thermal Energy Storage (TES) systems.

These storage tanks hold thermal energy and can be categorized into three types based on the temperature range of the stored medium. Low-temperature systems store heat up to 120°C, medium-temperature systems handle the range between 120 to 500°C, and high-temperature systems manage temperatures above 500°C.

Furthermore, heat can be stored through the following methods:

1. Specific heat: This is a straightforward and cost-effective storage method. Thermal energy can be stored in liquids and solids alike.
2. Heat of phase transitions using Phase Change Materials (PCM): PCM, both organic (paraffins, fatty acids, ionic liquids) and inorganic (hydrated salts), are substances designed to absorb, accumulate, and release energy during phase transition temperatures. Substantial amounts of heat can be absorbed or released during phase transitions at nearly constant temperatures.
3. Heat of chemical transformations: Thermal energy can be stored in chemical form through ongoing chemical reactions. Heat may be released (exothermic reactions) or absorbed (endothermic reactions) during these reactions. The energy is released through exothermic reactions. An example is obtaining heat using hydrogen.

TES systems offer the ability to store both heat and cold for later utilization. An illustrative instance of this is accumulating solar radiation heat during the daytime and using it in colder seasons, or storing cold generated during winter for use in warmer months.

Energy storage operation

Unfortunately, I’m unable to view or interpret images or diagrams. However, if you can provide a description or explanation of the diagram’s contents, I’d be more than happy to help you understand or expand upon the concepts presented in it.

Solar energy harnessed through panels is channeled into an energy warehouse for storage after conversion. Subsequently, this stored energy is dispatched to meet varying demands, catering to external recipients like households and public institutions. Heat storage plays a pivotal role in enabling the retention of thermal energy or cold, which can then be utilized for practical applications. A case in point is the EC-Karolin Veolia Combined Heat and Power (CHP) plant, which has implemented a heat accumulator with a capacity to store more than 1.2 thousand MWh of energy. This capacity showcases the effectiveness of heat storage solutions in efficiently managing and delivering thermal energy to meet utility requirements.

Which heat storage to choose?

The general division of heat storage is as follows:

Heat storage systems can be categorized into two primary divisions, each offering distinct advantages and applications:

1. Underground Thermal Energy Storage (UTES): This category encompasses various types of underground heat accumulators, each tailored to specific requirements:
   • BETS (Borehole Thermal Energy Storage): Utilizes boreholes to create a ground battery for thermal energy storage.
   • ATES (Aquifer Thermal Energy Storage): Involves an accumulator within an aquifer, utilizing the subsurface water layers for storage.
   • PTES (Pit Thermal Energy Storage): Utilizes gravel-filled pits buried in the ground as water accumulators.
   • CTES (Cavity Thermal Energy Storage): Utilizes natural or engineered rock caverns as thermal energy accumulators.
2. Tank Thermal Energy Storage (TTES): Involves the use of water tanks to store thermal energy.

Selecting the appropriate storage solution depends on specific needs and capacity requirements. Factors such as the intended purpose of the thermal energy storage and the desired capacity should guide the choice. Each type of storage has its unique characteristics and suitability for different applications.

By carefully considering these factors, stakeholders can make informed decisions about which heat storage system aligns best with their objectives and offers optimal efficiency for their thermal energy needs.

TTES warehouses

TTES warehouses are the most popular and available on the market thermal energy warehouses. They work in three phases. phase one charging, phase two thermal energy storage, and the third phase discharging.

These types of warehouses offer versatility, as they can operate effectively both in the short term and long term. Short-term TTES systems can reliably fulfill up to 90% of heat demand, while long-term (seasonal) ones can achieve a range of 50 to 70% coverage. This adaptability makes them suitable for various applications, including residential households and public institutions.

Water serves as the storage medium within TTES tanks. The transfer of heat energy, either for charging or discharging, involves pumping hot water directly or utilizing heat exchangers. TTES tanks are frequently integrated into the hydraulic circuits of combined heat and power (CHP) plants.

An interesting feature of TTES systems is the utilization of temperature stratification. This phenomenon involves the separation of lighter hot water at the upper part of the tank and denser cold water at the bottom. Between these layers exists a thermocline, acting as a boundary between the distinct water layers. Over time, the thermocline expands and the layers start to mix, potentially affecting the system’s performance. This phenomenon underscores the need for careful design and management to maintain optimal efficiency and effectiveness in TTES systems.

BITES warehouses

Considering the broader European context, Borehole Thermal Energy Storage (BTES) systems have garnered attention. These heat storage facilities directly store thermal energy in the ground, utilizing a network of vertical U-type heat exchangers for the charging and discharging processes.

Situated between 30 to 200 meters below the surface, BTES systems rely on a heat pump to extract heat from the ground. A layer of bentonite, known for its efficient thermal conductivity, fills the space between the borehole and the heat exchanger. This arrangement allows the heated medium to flow through the exchangers, transferring heat to the ground and facilitating heating or cooling.

A notable advantage of BTES systems is their scalability. As the demand for stored heat grows, additional sections of exchangers can be seamlessly integrated into the existing installation, enabling expansion.

However, it’s worth noting that achieving similar thermal capacities as Tank Thermal Energy Storage (TTES) systems requires BTES systems to possess 3 to 5 times the capacity. Moreover, BTES systems come with certain drawbacks, including high drilling costs associated with their installation. Despite these challenges, BTES remains a viable option for regions where ground storage is feasible and offers the potential for long-term, sustainable heat storage solutions.

ATES warehouses

Aquifer Thermal Energy Storage (ATES) systems provide another approach to heat storage by utilizing natural underground aquifers. These systems make use of both water and the surrounding ground to store heat.

During the summer, one set of wells collects and heats water, often using solar collectors and heat exchangers. This heated water is then pumped into another set of wells, creating a thermal front that moves as temperatures change.

In colder periods, the water from the heated layer is drawn and directed through heat exchangers, allowing it to cool down. The cooled water is then pumped back into the second set of wells without any impurities.

When aiming to store water at higher temperatures, the heat reservoir needs to be located at greater depths and insulated with layers of earth. However, the feasibility of constructing an ATES storage facility hinges on the availability of suitable aquifers in the area.

ATES systems present a promising avenue for efficient heat storage, particularly in regions with appropriate geological conditions that can support such installations. Their reliance on natural underground aquifers makes them an environmentally friendly option for long-term thermal energy storage.

CTES warehouses

Heat storage in caverns, known as Cavity Thermal Energy Storage (CTES), offers another method of storing thermal energy. This approach involves pumping hot water into underground chambers or natural hollow spaces for storage.

The chamber, when filled with water, becomes a stratified accumulator, where the differences in density lead to the arrangement of cold water in the lower layers and hot water in the upper layers. Over time, especially with longer storage periods, the layers of water mix, resulting in an average temperature.

To load a CTES storage, hot water is initially introduced into the upper part of the chamber, while colder water is extracted from the lower layers to maintain the temperature stratification. However, this type of storage system can be quite costly due to its construction and maintenance requirements. On the other hand, CTES systems offer high thermal power capabilities.

An example of innovation in this field is Brenmiller Energy, which is developing a thermal energy storage solution using crushed rocks. This approach employs crushed stone as the storage medium for thermal energy, leading to high efficiency and relatively low maintenance costs over the system’s lifespan. The stored heat can subsequently be converted into steam to generate electricity or provide hot water for various industrial processes. This technology has the potential to contribute to more sustainable energy systems by efficiently storing and utilizing excess thermal energy.

PTES warehouses

Another approach to thermal energy storage involves the use of water and gravel, known as Pit Thermal Energy Storage (PTES) or water-gravel accumulators. These storage systems are buried below ground level and typically insulated with a protective layer of foil. The PTES system operates by pumping water into and out of the storage unit, or alternatively, through a network of tubular heat exchangers.

One of the significant advantages of PTES systems is that they can be discreetly integrated into the environment, particularly in urban areas. Since the storage units are placed underground, they don’t disrupt the landscape. Additionally, these systems offer design flexibility, as they can take various shapes such as cuboids, cylinders, truncated pyramids, or even combinations of these forms.

The combination of water and gravel in PTES systems allows for effective heat storage and exchange. As the water flows through the gravel-filled reservoir, it can absorb and release heat efficiently, providing a reliable method for storing and distributing thermal energy as needed. This type of system holds promise for contributing to more sustainable and efficient energy use, especially in urban environments where space is limited and aesthetics are important.

Use of solar energy

The technology for harnessing thermal energy from solar radiation has reached an advanced stage and is increasingly competing with traditional heating methods.

Utilizing solar energy for thermal production offers several advantages, including:

1. Universal Accessibility: Solar energy is available virtually everywhere on the planet, making it accessible to a wide range of regions and communities.
2. Low Operating Costs: Once the initial investment is made, solar thermal systems have minimal ongoing operational expenses. The energy source is free and abundant.
3. Environmental Friendliness: Solar thermal systems produce heat without emitting greenhouse gases or other harmful pollutants, contributing to a cleaner environment and helping combat climate change.
4. No Impact on Earth’s Energy Balance: Unlike fossil fuel combustion, solar energy utilization does not contribute to depleting Earth’s finite resources, and it does not disturb the natural energy balance of the planet.

One of the key advantages of solar energy is its sustainability – the sun is an inexhaustible source of energy. However, there are certain limitations and challenges associated with solar thermal systems, such as daily and seasonal variations in sunlight availability, varying levels of solar intensity, and relatively higher upfront investment costs compared to conventional heating systems. Nonetheless, ongoing advancements in technology and increasing adoption are steadily reducing these limitations, making solar thermal energy an increasingly viable and attractive option for sustainable heating solutions.

Innovative ways of storing thermal energy

Indeed, solar ponds offer a unique approach to storing and utilizing solar thermal energy. These ponds are classified as low-temperature active solar energy systems, and they provide a means of accumulating and storing solar energy for various heating applications.

Solar ponds work by preventing the dissipation of heat energy into the environment, allowing it to accumulate within the pond. There are two main types of solar ponds:

1. Convection Pond: In this type of solar pond, heat losses due to evaporation are minimized by covering the surface of the pond, often with a transparent insulating material such as a foil. The covering helps retain the accumulated heat during nighttime and periods without direct solar radiation.
2. Non-Convection Pond: This type of solar pond reduces heat loss by inhibiting convection. It utilizes a high concentration of salt in the water. The saltwater, due to its high density, remains at the bottom of the pond even when heated. This prevents the transport of heat through convection from the bottom of the pond to the surface and the surrounding environment.

Solar ponds have been used for various heating purposes, including:

• Heating Facilities: Solar ponds are used to heat swimming pools, sports halls, greenhouses, and even for central heating in residential buildings and housing estates.
• Power Generation: Larger solar ponds have been employed to generate electricity. An example is the 5 MW power plant in Israel that uses a 21-hectare solar pond to produce power.

Solar ponds offer a way to store thermal energy in a natural and efficient manner, contributing to sustainable energy solutions. They are especially useful in regions with abundant sunlight and can serve as a valuable addition to the renewable energy landscape.

Exploitation of solar ponds

In solar ponds, the natural convection process is manipulated through salinity variations to accumulate and store thermal energy. The design of solar ponds prevents the usual circulation where warm water rises to the surface and then cools upon contact with the air. Instead, solar ponds are engineered to maintain heat in the lower layers, which are heavily salted to store and concentrate the energy.

Here are the key features of solar ponds:

1. Salt Concentration: The water in a solar pond is intentionally salted, often using solutions like NaCl (table salt) and MgCl2 (magnesium chloride). This creates a layer of high-salinity water at the bottom, which is dense and remains there.
2. Temperature Zones: A solar pond typically consists of three main layers:
   • Surface (Insulating) Layer: This upper layer, with a temperature around 30°C, is influenced by air temperature and acts as an insulating barrier.
   • Indirect (Buffer) Layer: Below the surface layer, there is an intermediate zone where the temperature gradually increases with depth.
   • Bottom (Accumulation) Layer: The bottom layer has the highest temperature, reaching up to 100°C. It also has the highest salt concentration, enabling efficient heat storage.
3. Prevention of Convection: The stratification of the layers prevents convection currents, allowing heat to be trapped in the accumulation layer.
4. Freeze Protection and Algae Control: The high salinity of the bottom layer prevents freezing and inhibits the growth of algae, maintaining the desired thermal stratification.

Solar ponds are usually shallow with a large surface area (1-2 meters deep) to maximize solar absorption. They operate within a temperature range of 50-95°C, making them suitable for various heating applications.

The process of obtaining thermal energy in a solar pond involves the following steps, as illustrated in the diagram below:

1. Absorption: Solar radiation penetrates the surface layer and is absorbed by the accumulation layer.
2. Heat Concentration: The concentrated brine in the bottom layer absorbs and stores the heat energy.
3. Thermal Storage: The accumulated heat remains trapped in the bottom layer, ready for use in heating applications.
4. Extraction: When heat is needed, the hot brine is pumped from the bottom layer and used for heating purposes.
5. Recharging: Cold brine is returned to the bottom layer to continue the accumulation process.

Solar ponds are an innovative way to capture and store solar thermal energy, offering a sustainable solution for various heating needs while leveraging the unique properties of saline water stratification.

Scheme of obtaining thermal energy in a solar pond; Source: www.oxyfuel.eu

Solar ponds utilize the unique properties of stratified water layers to capture and store solar energy effectively. As solar radiation permeates the water, it is absorbed by the darkened bottom, leading to a higher temperature in the bottom layer compared to the rest of the liquid. The surface layer, shallow at 0.3 to 0.5 meters, maintains lower brine concentration and temperature, allowing for heat exchange with the environment through convection and water evaporation.

Within the intermediate layer, spanning 0.8 to 1.2 meters, concentration and temperature gradually rise with depth. This layer plays a dual role by transmitting solar radiation to the bottom layer while acting as an insulating barrier, preventing mixing between the extreme layers.

The bottom layer, extending 1 to 7 meters deep, is characterized by high salinity and density, resulting in the accumulation of heat from the upper layers. Heated brine, with greater density than water, sinks to the bottom, where heat can be extracted through bottom-mounted heat exchangers or external systems.

Optimal functioning of this setup requires maintaining high transparency in the upper layer to enable unhindered penetration of solar rays. The upper layers also act as thermal insulation for the warmer regions beneath. The accumulated energy is subsequently recovered using heat exchangers or heat pumps. Notably, the salt concentration in the water increases as depth increases, contributing to the distinct thermal properties of each layer within the solar pond.

Profitability of innovative accumulation of thermal energy.

A solar pond proves to be economically viable when constructed on a flat terrain with consistent water access and affordable salt availability. Its successful operation hinges on continuous water supply, the maintenance of a concentrated brine solution, and proper separation from the ground to prevent brine contamination of groundwater.

Desert areas and coastal regions, particularly close to the sea, offer the optimal conditions for solar pond installation. Although this technology is not as well-established in Poland compared to other countries, potential sites along the Baltic coast hold promise.

Solar ponds offer distinct advantages, including lower heat loss to the surroundings compared to solar collectors and enhanced power and efficiency. Moreover, these ponds can store heat over prolonged periods, even spanning several months.

The applications of solar ponds encompass heating buildings, supplying tap water, and generating electricity through the Rankine cycle. Nevertheless, a challenge lies in maintaining the appropriate concentration ratio. Additionally, it’s important to note that while solar ponds offer greater efficiency, their initial investment costs tend to be higher than alternatives like solar collectors.

/