In this paper, we investigate sensible and latent heat transfer through heat exchanger matrix structures containing phase change material (PCM) in the interstitial spacing. The heat transfer is driven by a temperature difference between fluid flow passages and the phase change material matrix which experiences sensible heat transfer until it reaches the phase change material fusion point; then it undergoes melting or solidification in order to store, or reject, energy. In prior work, a dimensionless framework was established to model heat transfer in a thermal energy storage (TES) device much like effectiveness-NTU analysis methods for compact heat exchangers. A key difference, however, is that in TES units, the overall heat transfer coefficient, U, within the phase change material matrix varies spatially in the unit and with time during storage or extraction. Determination of a mean U for these processes is a key challenge to applying the effectiveness-NTU analysis to design of a TES unit. This paper assesses and identifies strategies for determining the matrix overall heat transfer coefficient in a TES unit from model predictions or experiments. The sensitivity of the TES energy efficiency to the matrix overall heat transfer coefficient is also explored, and the implications for some typical applications are discussed.

This paper establishes a multiscale design evaluation framework that integrates performance models for a thermal energy storage (TES) unit and a subsystem heat exchanger (HX). The modeling facilitates the analysis of transient input and extraction processes for the TES device which uses solid-liquid phase change to store thermal energy. We investigate sensible and latent heat transfer through the unit's matrix structure which contains phase change material (PCM) in the interstitial spacing. The heat transfer is driven by a temperature difference between fluid flow passages and the PCM matrix which experiences sensible heat transfer until it reaches the PCM fusion point; then it undergoes melting or solidification in order to receive, or reject, energy. To capture these physics, we establish a dimensionless framework to model heat transfer in the storage device much like effectiveness-number of transfer units (NTU) analysis methods for compact HX. Solution of the nondimensional governing equations is subsequently used to predict the effectiveness of the transient energy input and extraction processes. The TES is examined within the context of a larger subsystem to illustrate how a high efficiency design target can be established for specified operating conditions that correspond to a variety of applications. The general applicability of the model framework is discussed and example performance calculations are presented for the enhancement of a Rankine power plant via asynchronous cooling.

This paper presents a design analysis framework for a transient cold storage unit that uses solid-liquid phase change for thermal storage. The analytical framework developed in this study establishes non-dimensional parameters that dictate the energy efficiency of the transient energy input and extraction processes, and specifies the links between physical parameters for the system and dimensionless parameters. The resulting governing equations in non-dimensional form are partial differential equations that can be solved numerically. Solutions of the equations predict the thermodynamic efficiency (effectiveness) of the energy storage and retrieval processes, and the time required to input or extract energy from storage for specified values of the dimensionless parameters. The paper illustrates how a high efficiency design target can be established for specified operating conditions using this framework. Application of this framework to a typical example application involving cold thermal storage is described, and the usefulness of this methodology is demonstrated. The use of this methodology for predicting the performance of cold thermal storage for a broad range of potential applications is also discussed.

An abundance of research has been performed to understand the physics of latent thermal energy storage with phase change material. Some analytical and numerical findings have been validated by experiments, but there are few free and open-source models available to the general public for use in systems simulation and analysis. The Modelica programming language is a good avenue to make such models available, because it is object-oriented, equation-based, declarative, and acausal. These characteristics have enabling the creation of component model libraries that can be used to build larger system simulations for design analysis. The authors have previously developed a numerical framework to model phase change thermal storage and have validated model predictions with experiments. The objectives of this paper are to describe the transfer of the numerical framework to an implementation in a Modelica component model and to validate the Modelica model with data from the experiment and the original numerical framework.

Although model predictions of thermal energy storage (TES) performance have been explored in previous investigations, relevant test data that enables experimental validation of performance models has been limited. This is particularly true for high-performance TES designs that facilitate fast input and extraction of energy. In this paper, we present a summary of experimental tests of a high-performance TES unit using lithium nitrate trihydrate phase change material (PCM) as a storage medium. Performance data is presented for complete dual-mode cycles consisting of extraction (melting) followed by charging (freezing). These tests simulate the cyclic operation of a TES unit for asynchronous cooling in a variety of applications. The model analysis is found to agree reasonably well, within 10%, with the experimental data except for conditions very near the initiation of freezing, a consequence of subcooling that is required to initiate solidification.