How to measure thermal energy storage
Temitayo Oketola | August 10, 2023In today’s world, the urgent need for sustainable energy has propelled the development and adoption of technologies that harness renewable energy sources. These innovative technologies, such as solar thermal and geothermal systems, are helping to meet a portion of the world’s energy demand while minimizing the detrimental greenhouse gas emissions associated with traditional fossil fuel-based systems.
However, a significant challenge that accompanies these renewable energy sources is their inherent intermittency. For instance, the thermal energy delivered by solar thermal systems fluctuates depending on the time of day, weather conditions and seasons. To address this challenge, the field of thermal energy storage (TES) has emerged as a crucial component of renewable energy systems, particularly for thermal applications.
Figure 1: The field of thermal energy storage (TES) has emerged as a crucial component of renewable energy systems, particularly for thermal applications. Source: Eli/Adobe Stock
Thermal energy storage systems are technologies that allow the capture and storage of excess energy produced during periods of high renewable energy production, which can then be utilized during periods of low energy production. However, as simple as their operation might seem, there are several essential thermodynamic concepts guiding their operation and several kinds of TES technologies available today. Therefore, understanding how these systems work is essential to choosing the right type for a project.
Methods of thermal energy storage
There are commonly three methods of thermal energy storage:
- Sensible heat storage
- Latent heat storage
- Thermochemical storage
Sensible heat storage
Sensible heat storage is a relatively simple and commonly used method of storing thermal energy. The process involves storing thermal energy in a material by increasing its temperature. The stored thermal energy is then extracted by transferring it to a working fluid during periods of low renewable energy production.
The amount of stored thermal energy in the material can be calculated using:
Where:
m = mass of the material (kg)
Cp is the specific heat capacity (j/kgK)
Eq2 is the change in temperature (K)
As can be seen in the equation above, materials with high specific heat have high thermal storage capability. But in addition to choosing a material with high specific heat, engineers must also consider the density of the material. Ideally, materials with high density are recommended because they can store more energy per unit volume (or mass). This is particularly important in applications where space is limited or where high energy storage density is desired.
For instance, water is one of the most commonly used materials for sensible heat storage because of its high specific heat capacity (4,186 J/kgK) and high density (1,000 kg/m3). When compared to air (which has a specific heat capacity of 1,005 J/kgK and a density of 1.2 kg/m3), it will be observed that water can store up to four times more thermal energy per unit mass than air for the same temperature change.
Other common sensible heat storage materials include rocks and concrete.
[Learn more about thermal energy storage at GlobalSpec.com]
Latent heat storage
Latent heat storage is a method of storing thermal energy using the latent heat of a phase change material (PCM). Latent heat is the energy absorbed or released by a material when it undergoes a phase transition, such as liquid to vapor or from solid to liquid. This phase change typically occurs at a constant temperature and is associated with the change in the internal energy of the material.
During the phase change process, the PCM absorbs (or releases) a large amount of energy as it changes its state. This energy is stored within the material until it undergoes a reverse process, at which point the stored energy is released. The amount of thermal energy stored or released through latent heat storage can be calculated using:
Eq3
where:
m = mass of material (kg)
L = latent heat of fusion (kJ/kg)
For instance, consider 10 kg of paraffin wax with a latent heat of fusion of 200 kJ/kg. During a melting phase change process, the paraffin wax would release about 2,000 kJ of thermal energy, assuming there is no change in temperature.
The key advantage of latent heat storage is its high energy storage density. PCMs typically have a higher thermal energy storage capacity than sensible heat storage materials because the latent heat of fusion (or vaporization) involved in the phase change is typically several times greater than the specific heat capacity associated with temperature change. Various types of PCMs are used for latent heat storage, such as organic compounds and eutectic mixtures.
[Learn more about phase change materials at GlobalSpec.com]
Thermochemical energy storage
In thermochemical energy storage systems, reversible chemical reactions are used to store and release thermal energy. The process involves the conversion of thermal energy into chemical energy during the charging phase and the reverse process during the discharging phase.
Thermochemical energy storage offers several advantages, such as the ability to store energy with high energy density and stability. In addition, they can be designed to operate at high temperatures, making them ideal for applications that require high-temperature heat.
Conclusion
Thermal energy storage materials play a significant role in renewable energy systems, especially in applications where the energy supply is intermittent. For instance, thermal energy storage in concentrated solar power systems allows for the storage of excess heat during the day. This stored energy can then be utilized later to produce steam and generate electricity. While seemingly simple in operation, there are several factors that must be considered when designing a thermal energy storage system. It is recommended to reach out to energy storage device suppliers to discuss application requirements.