Concentrated solar thermal power generation is becoming a very attractive renewable energy production system among all the different renewable options, as it has have a better potential for dispatchability. This dispatchability is inevitably linked with an efficient and cost-effective thermal storage system. Thus, of all components, thermal storage is a key one. However, it is also one of the less developed. Only a few plants in the world have tested high temperature thermal energy storage systems. In this paper, the different storage concepts are reviewed and classified. All materials considered in literature or plants are listed. And finally, modellization of such systems is reviewed.

State of the art on high temperature thermal energy storage for power generation. Part 1—Concepts, materials and modellization

Phase change materials for thermal energy storage

Various types of solid–liquid phase change materials (PCMs) have been reviewed for thermal energy storage applications. The review has shown that organic solid–liquid PCMs have much more advantages and capabilities than inorganic PCMs but do possess low thermal conductivity and density as well as being flammable. Inorganic PCMs possess higher heat storage capacities and conductivities, cheaper and readily available as well as being non-flammable, but do experience supercooling and phase segregation problems during phase change process. The review has also shown that eutectic PCMs have unique advantage since their melting points can be adjusted. In addition, they have relatively high thermal conductivity and density but they possess low latent and specific heat capacities. Encapsulation technologies and shell materials have also been examined and limitations established. The morphology of particles was identified as a key influencing factor on the thermal and chemical stability and the mechanical strength of encapsulated PCMs. In general, in-situ polymerization method appears to offer the best technological approach in terms of encapsulation efficiency and structural integrity of core material. There is however the need for the development of enhancement methods and standardization of testing procedures for microencapsulated PCMs.

/pdf/review-of-solid-liquid-phase-change-materials-and-their-4qrepox763.pdf

Review of solid–liquid phase change materials and their encapsulation technologies

Phase change material (PCM) based latent heat thermal storage (LHTS) systems offer a challenging option to be employed as an effective energy storage and retrieval device. The performance of LHTS systems is limited by the poor thermal conductivity of PCMs employed. Successful large-scale utilization of LHTS systems thus depends on the extent to which the performance can be improved. A great deal of work both experimental and theoretical on different performance enhancement techniques has been reported in the literature. This paper reviews the implementation of those techniques in different configurations of LHTS systems. The influence of enhancement techniques on the thermal response of the PCM in terms of phase change rate and amount of latent heat stored/retrieved has been addressed as a main aspect. Issues related to mathematical modeling of LHTS systems employing enhancement techniques are also discussed.

Performance enhancement in latent heat thermal storage system: A review

Organic phase change materials and their textile applications: An overview

Phase change materials (PCMs) are thermal storage materials with a high storage density for small temperature range applications. In the design of latent heat storage systems, the enthalpy change of the PCM has to be known as a function of temperature with high precision. During dynamic measurements, the sample is not in thermal equilibrium, and therefore the measured value is not the equilibrium value. The influence of non-equilibrium on the measurement results can be quantified by doing measurements during heating and cooling with any measurement instrument. Measurements carried out by differential scanning calorimetry (DSC) and by the T-history method are presented and discussed. To characterize encapsulated PCM objects, measurements on the whole objects should be carried out. A measurement setup for this purpose is also presented. The obtained precision meets typical application requirements, and good agreement between results obtained with the different methods is demonstrated.

Enthalpy of Phase Change Materials as a Function of Temperature: Required Accuracy and Suitable Measurement Methods

Thermal energy storage by latent heat allows storing high amounts of energy working in narrow margins of
temperature. The use of phase change material (PCM) for the latent heat storage has been studied in different
applications and it has been commercialized in containers to transport blood, products sensible to temperature, to
decrease their energy demand. The use of PCM in cooling and refrigeration has been attracting a lot of interest lately,
but for all applications, the properties of these materials need to be known with sufficient accuracy. Regarding heat
storage, it is necessary to know the enthalpy as a function of temperature. The most widely used calorimeter is the heatflux
differential scanning calorimetry (hf-DSC). The objective of this study is to investigate different methods for hf-
DSC analysis, namely the dynamic method and the step method, and to test their accuracy in the determination of
enthalpy–temperature relationship of PCM. For the dynamic method, a strong influence of heating/cooling rate was
observed. For the step method, the resulting enthalpy–temperature relationship is independent of heating/cooling rate.
Commercial PCM RT27 was chosen as sample material to avoid subcooling and kinetic effects in the test
measurements. The approach introduced in this study can be used to carry out similar investigations for other classes of
PCM and/or other DSC instruments.

Determination of the enthalpy of PCM as a function of temperature using a heat-flux DSC—A study of different measurement procedures and their accuracy

Phase change materials (PCM) are able to store thermal energy in small temperature intervals very efficiently due to their high latent heat. Accurate knowledge of the enthalpy as a function of temperature, or the storage capacity at each temperature, is the key to design any application. Conventional methods for thermal analysis however often lack sufficient accuracy or sample size to be applied to PCM. The T-history method is a simple method to determine the storage capacity of PCM and allows the use of large sample sizes. The experimental setup and methods of data analysis have been significantly improved in recent years. In this paper, a proper methodology to verify the correct setup and data analysis method of a T-history installation using standard materials with known properties is described and tested. The implementation of the T-history method has been done at the ZAE-Bayern. Three standard materials, gallium, water and hexadecane, were measured, as well as two commercial PCM, RT27 and sodium acetate trihydrate graphite compound (SAT+G). The obtained results confirm that the T-history installation can be used to analyse different PCM.

http://iopscience.iop.org/article/10.1088/0957-0233/17/8/016/pdf

Verification of a T-history installation to measure enthalpy versus temperature curves of phase change materials

Polyurethanes as solid–solid phase change materials for thermal energy storage

Phase change materials (PCM) are able to store thermal energy in small temperature intervals very efficiently due to their high latent heat. Particularly high storage capacity is found in salt hydrates. Salt hydrates however often show subcooling, thus inhibiting the release of the stored heat. In the state of the art simulations of PCM, the effect of subcooling is almost always neglected. This is a practicable approach for small subcooling, but it is problematic for subcooling in the order of the driving temperature gradient on unloading the storage. In this paper, we first present a new algorithm to simulate subcooling in a physically proper way. Then, we present a parametric study to demonstrate the main features of the algorithm and a comparison of computed and experimentally obtained data. The new algorithm should be particularly useful in simulating applications with low cooling rates, for example building applications.

Eva Günther

Papers

Enthalpy of Phase Change Materials as a Function of Temperature: Required Accuracy and Suitable Measurement Methods

Determination of the enthalpy of PCM as a function of temperature using a heat-flux DSC—A study of different measurement procedures and their accuracy

Verification of a T-history installation to measure enthalpy versus temperature curves of phase change materials

Polyurethanes as solid–solid phase change materials for thermal energy storage

Modeling of subcooling and solidification of phase change materials