In recent decades, thermal storage has been a topic of great interest because it enables the use of waste energy, increases thermal inertia in buildings, and makes some renewable energies manageable. In any storage system, the most critical factor is always the medium in which the heat is stored. This heat can be stored through three main mechanisms: through a temperature change (sensible heat), through a phase change (latent heat) or through a chemical reaction (thermochemical). For a given material to be considered as a storage medium, it must not only have adequate thermophysical properties in the temperature range of the application, but these properties should not vary significantly over the service life of the system. Therefore, the long-term stability of thermal storage media is a priority issue that should be analyzed in depth when developing any type of storage system, since the proper functioning of the system throughout its service life depends on it.
Since the 1980s, many studies have been carried out on the long-term stability of storage media, especially in the field of latent heat storage materials, also known as phase change materials (PCMs). This is due to the large number of PCMs that are in principle suitable for applications in a very wide temperature range (between 0 °C and 800 °C). These materials range from hydrated salts and organic compounds – such as sugar alcohols, kerosenes, fatty acids, polymers, etc. – for applications below 200 °C; to anhydrous salts and metals for storage at higher temperatures. Most of these PCMs undergo solid-liquid transitions although some of them can store latent heat through solid-solid or liquid-liquid transitions.
Despite the large number of potential applications, commercial deployment of latent heat storage systems is difficult due to the lack of validated PCMs. Most of the studies found in the literature aimed at evaluating the reliability of PCMs as latent heat storage media focus their attention on thermal cycling. When these studies are analyzed, what is observed is a great dispersion in the test conditions, not only in relation to the experimental device used but also to the temperature intervals and, above all, to the number of cycles performed. On the other hand, in many of these studies, thermal cycles are considered as “accelerated” tests, so that the authors establish correlations between the number of cycles and the actual operating time without any prior justification. However, this type of testing does not comply with the accelerated testing requirements clearly established in other branches of science and technology for the validation of materials used in different types of applications. Therefore the results of thermal cycling cannot be extrapolated to predict the long-term performance of a PCM under real operating conditions.
At CIEMAT’s Thermal Storage and Solar Fuels Unit (ATYCOS), we believe that this type of confusion occurs because to date there is still no specific test protocol for validating thermal storage materials, in general, or for PCMs, in particular. Therefore, each author applies his own criteria, not only for the test procedures but also for predicting the long-term behavior of the materials under real conditions based on the number of cycles. Thus, from the ATYCOS Unit of CIEMAT arises the initiative to develop a methodology that allows the validation of storage media, focusing on phase change materials or PCMs.
The validation methodology developed is shown in Figure 1.
This methodology consists of a series of steps that should lead to the validation of a given PCM for a given application. It includes the different steps that should be followed for such validation, such as PCM characterization, preliminary stability testing and accelerated testing. The article also discusses the tests that could be performed in the different steps as well as key concepts of the methodology: control properties, service conditions, degradation factors and service life models.
The long-term stability and long-term performance of thermal storage media is a key issue that needs to be thoroughly analyzed when developing storage systems. However, so far there is no test protocol or guide to validate storage media, so authors apply their own criteria, not only to design test procedures, but also to predict the behavior of the material in long-term operation. Service life relationship models should be obtained to predict the long-term behavior of PCM in service conditions from shorter tests performed under stress conditions.