Thermal Energy

Thermal energy is the energy arising from the continuous, random, microscopic motion of atoms and molecules within an object or system. These motions occur through vibration, rotation, and translation of particles and are directly related to kinetic energy. The higher the average kinetic energy of the particles, the greater the total thermal energy of the system. This condition can be measured as temperature. Temperature is the macroscopic measure of the average kinetic energy corresponding to thermal energy and is determined using thermometers in Celsius, Fahrenheit, or Kelvin units. Heat is the transfer of thermal energy from a system at higher temperature to a system at lower temperature when a temperature difference exists. When the temperature difference disappears, the net flow of heat ceases and the systems reach thermal equilibrium.

Thermal Energy Storage (TES) Technology

Thermal energy storage (TES) is an energy management method that stores a specific amount of heat or cold for later use. This technology captures excess energy generated from variable renewable energy sources such as solar and wind and delivers it back when needed. TES systems are employed to reduce temporal mismatches between energy supply and demand, improve energy efficiency, and enhance system reliability.

TES applications are categorized into three main types:

• Sensible Heat Storage: Stores energy by changing the temperature of the storage medium. Materials such as water, molten salts, and rocks are used for this purpose.
• Latent Heat Storage: Stores large amounts of heat at constant temperature through phase transitions such as melting or freezing, using phase change materials.
• Thermochemical Storage: Stores and releases heat through reversible chemical reactions, offering high energy density and long-term storage capability.

TES technologies have a wide range of applications, from building heating and cooling systems to industrial processes, electricity generation, and regional energy management. These systems reduce the need for additional power generation during peak demand periods, lowering energy costs, limiting carbon emissions, and reducing environmental impact while strengthening the resilience of the energy infrastructure.

Advances in materials science, heat transfer, and systems engineering are increasing both the scalability and cost-effectiveness of thermal energy storage. As a result, TES systems are expected to play a strategic role in the global energy transition, particularly in the integration of renewable energy sources into the grid.

Thermal Energy Storage Methods

The storage of thermal energy is examined under three main methods, depending on the mechanism by which heat is retained and subsequently released: sensible heat storage, latent heat storage, and thermochemical heat storage. These methods differ in their application fields, storage densities, costs, and technical challenges.

Sensible Heat Storage

Sensible heat storage is the most commonly used and technically simplest method of thermal energy storage (TES). The fundamental principle is to increase or decrease the temperature of the storage medium without changing its phase. Liquid or solid materials such as water, molten salts, metals, and rocks are preferred for this purpose. The amount of stored energy depends on the mass of the medium, its specific heat capacity, and the temperature difference during the storage process.

Advantages of this method include low capital cost, high reliability, and ease of implementation. However, its limited storage density requires larger volumes to store a given amount of energy. In building heating and cooling applications, water tanks are the most prominent example. For industrial applications requiring higher temperatures, molten salts (up to approximately 600 °C) or metals (up to approximately 1000 °C) are preferred. In systems known as Underground Thermal Energy Storage (UTES), heat is stored underground via boreholes or aquifers, enabling seasonal heating and cooling solutions.

Latent Heat Storage

Latent heat storage is based on the absorption or release of energy during phase changes of the storage medium, such as melting from solid to liquid or freezing from liquid to solid. Materials used in this method are called phase change materials (PCMs). Compared to sensible heat storage, this method offers the significant advantage of storing more energy in the same volume and maintaining it at a constant temperature.

The amount of stored energy is determined by the mass of the phase change material and its latent heat. Common PCM examples include ice, paraffin derivatives, and salt hydrates. For instance, ice is effectively used in cooling and air conditioning systems by undergoing a phase change at 0 °C. However, factors limiting the widespread adoption of this method include the limited availability of suitable phase change materials, the need to improve their typically low thermal conductivity, and integration costs.

Thermochemical Heat Storage

Thermochemical heat storage relies on reversible chemical reactions that store thermal energy in chemical bonds. During an endothermic process, heat energy drives the decomposition of a compound, storing energy as chemical potential. In the exothermic process, when the components recombine, this energy is released as heat.

The greatest advantages of this method are its very high energy storage density and the ability to store energy for long periods with nearly zero heat loss. This feature is of great importance for long-term energy management applications such as seasonal storage. Additionally, the storage duration is theoretically unlimited. However, the technical complexity of the systems, high initial capital costs, and in some cases low conversion efficiency, remain barriers to widespread adoption. Therefore, thermochemical storage is still primarily in the research and pilot application stages.

Geothermal Energy: An Application of Thermal Energy

Geothermal energy is a form of thermal energy derived from heat accumulated at various depths within the Earth’s crust, transferred to the surface via hot water, steam, and various gases. The term “geothermal,” meaning “earth heat,” originates from heat generated by magma layers deep within the Earth and by the decay of radioactive elements. This energy offers significant environmental and technical advantages over fossil fuels due to its renewable and sustainable nature, low greenhouse gas emissions, and ability to generate continuous power independent of weather conditions. Additionally, as it does not require combustible fuel, it is a safe energy source with no risk of fire or explosion.

Geothermal Energy Resources

Geothermal energy resources are classified into different categories based on their geological environment, temperature levels, and composition:

• Hot Water Resources: Formed when underground water naturally reaches high temperatures. These resources, located near the surface, can be used directly for heating systems or electricity generation.
• Steam Resources: Sources of dry steam generated under high temperature and pressure, which are directed directly to turbines for electricity production.
• Hot Dry Rocks: Deep rock formations containing no water but possessing high temperatures. Water is injected under pressure to heat the rocks, and the resulting hot water or steam is extracted.
• Geothermal Basins: Areas located in volcanic or tectonically active regions with intense geothermal activity and generally high energy potential.

Geothermal Energy Production Methods

Different technologies are used to extract electricity or heat energy from geothermal resources:

• Dry Steam Plants: Dry steam extracted directly from underground is transported via pipelines to turbines for electricity generation.
• Flash (Flashing) Steam Plants: High-pressure hot water is released into a low-pressure tank. The sudden pressure drop causes part of the water to vaporize, and the resulting steam drives the turbines.
• Binary Cycle Plants: Medium-temperature geothermal water heats an organic fluid with a lower boiling point, causing it to vaporize. The vapor drives the turbines. Since the system operates in a closed loop, the geothermal water is reinjected into the ground.
• Geothermal Heat Pumps: Systems that utilize the stable temperature difference a few meters below the Earth’s surface to meet building heating and cooling needs.

Applications of Geothermal Energy

Geothermal energy can be utilized across a broad spectrum, from direct heating and electricity generation to industrial and agricultural activities:

• Electricity generation
• Regional heating and cooling systems
• Providing process heat for industries such as food, paper, and textiles
• Heating greenhouses and agricultural fields, drying products
• Thermal tourism and health applications (spas, therapeutic pools)
• Production of minerals and gases from geothermal fluids (e.g., lithium, boron, CO₂)
• Control of water temperature in aquaculture

Geothermal Energy in Türkiye and Worldwide

Geothermal potential is generally high along tectonic plate boundaries and regions with intense volcanic activity. Globally, the Pacific Ring of Fire and the Alpine-Himalayan belt are significant areas. Leading production regions include the Geysers Complex in the United States, the Larderello area in Italy, Iceland, the Philippines, Indonesia, and Mexico.

Türkiye possesses significant geothermal energy potential due to its young tectonic structure and dense fault systems located along the Alpine-Himalayan seismic belt. The highest concentration of geothermal resources in the country is found in the Aegean Region and Southern Marmara. Provinces such as Aydın, Denizli, Manisa, İzmir, and Çanakkale are among the main areas with concentrated geothermal power plants. In Türkiye, geothermal energy is effectively used not only for electricity generation but also for residential and greenhouse heating and thermal tourism. The Denizli-Kızıldere Plant, Türkiye’s first geothermal electricity generation facility, marks a pivotal milestone in the development of this sector.

Advantages of Geothermal Energy

Geothermal energy is a resource that offers significant environmental and economic benefits:

• Renewable and sustainable: It relies on natural heat continuously supplied by the Earth’s crust, resulting in a low risk of depletion.
• Low greenhouse gas emissions: Compared to fossil fuels, it emits significantly less harmful gases such as carbon dioxide (CO₂) and sulfur dioxide (SO₂), contributing to reduced air pollution.
• Continuous and reliable production: Unlike wind and solar energy, it provides uninterrupted power 24 hours a day, 7 days a week, independent of weather conditions.
• Small land footprint: Geothermal plants require less land area relative to their energy output capacity, reducing interference with natural habitats.
• Energy security: As a domestic and national resource, it reduces dependence on imported fuels and insulates the country from price fluctuations of imported energy.
• Multifunctional use: In addition to electricity generation, it can be used for direct heating, greenhouse farming, industrial processes, tourism, and healthcare applications.

Disadvantages of Geothermal Energy

Despite its many advantages, geothermal energy has certain technical, environmental, and economic limitations:

• Location-dependent resource: Geothermal potential is concentrated in specific geological regions; therefore, it is not viable for every country or region.
• High initial cost: Exploration, drilling, and infrastructure investments require substantial upfront capital.
• Reservoir depletion risk: If not operated sustainably, the temperature and pressure of underground reservoirs may decline over time.
• Environmental impacts: Some geothermal fluids may contain harmful minerals and gases such as arsenic, boron, or ammonia; uncontrolled release of these substances can cause environmental contamination.
• Seismic risks: Especially in hot dry rock technology, water injection can induce small-scale seismic activity.
• Waste fluid management: Discharging used geothermal fluids to the surface poses risks of water and soil contamination; therefore, reinjection is required.