**PCM-Air Heat Exchangers: Slab Geometry**

Pablo Dolado, Ana Lázaro, José María Marín and Belén Zalba *University of Zaragoza / I3A - GITSE Spain* 

## **1. Introduction**

Energy efficiency and the search for new energy sources and uses are becoming main objectives for the scientific community as well as for society in general. This search is due to various environmental issues and shortages of conventional and non-sustainable energy resources, for example fossil fuels, that are essential to industrial development and to daily life. Free-cooling in buildings, bioclimatic architecture applications, demand and production coupling in renewable energy sources, as solar energy, are examples of thermal energy storage contributions to achieve these objectives. The application of Phase Change Materials (hereafter PCM) in Thermal Energy Storage (hereafter TES) is an expanding field due to the variety of materials being developed. There are four critical considerations for the technical viability of these applications: 1) The features of both the PCM and the encapsulation material must be stable during the system lifetime; 2) A reliable numerical model of the system to simulate different operational conditions; 3) The thermophysical properties of the PCM; 4) The cost of the system.

Specifically, the solid-liquid phase change phenomenon of the PCM is being widely studied in the field of TES, both experimentally and numerically, because this technology is of great interest among different fields: from applications in electronics, textile, transport... to applications in aerospace or thermo-solar power plants. The incorporation of these materials on the market, as stated before, is conditioned partly by its price. To cope with this situation, manufacturers often sell PCM as non-pure substances or mixtures which, on the one hand, lower their costs but, on the other hand, condition its thermophysical properties so that they are not as well established as in pure substances. Generally, this determining factor leads to a nonlinearity of the temperature dependence of the thermophysical properties of the PCM. This issue is another aspect to consider when simulating the thermal behaviour of these substances. Therefore, it is essential a good determination of these properties as they are input values to the theoretical models that simulate the thermal performance of devices based on these materials, some of which may strongly condition the results of the simulations.

When working at ambient temperatures, there are different situations where TES with PCM can be applied. Zalba et al., 2003, presented a comprehensive review on latent heat TES and its applications. The authors remarked that low values of λPCM can lead to real problems in the systems since there could be insufficient capacity to dispose of the stored energy quickly enough. Later, Sharma et al., 2009, presented another review highlighting that there was

PCM-Air Heat Exchangers: Slab Geometry 429

property regarding heat transfer. As a result of the application of the existing methods to analyze the liquid and solid phases, the most suitable method is chosen and the setup was started up. Besides the energy storage capacity and the thermal conductivity as a function of temperature, other properties are also important to be known, such as the compatibility of

In order to obtain the most suitable method to determine enthalpy as a function of temperature during the solid-liquid phase change, two main thermal analysis methods were studied: differential scanning calorimetry (DSC) and adiabatic calorimetry. In addition, a customized method was studied: the T-history method. A complex review of the work on thermophysical properties was carried out with some conclusions being (Lazaro, 2009):

DSC is the most used method for determining the storage capacity because it is the

There are several problems with using DSC for non pure and low thermal conductivity

The number of authors that use the enthalpy vs. temperature curves to express the

DSC, adiabatic calorimetry and T-history method were studied and compared. Factors considered in the method selection are: sample size, heating and cooling rate, obtainability of the h-T curve, introduction to the market, easiness to build, cost, use, maintenance. The Thistory method was selected as it provides the enthalpy vs. temperature curves and also uses sample sizes and heating/cooling rates similar to those used in real applications.

Zhang et al., 1999, developed a method to analyze PCM enthalpy. The T-history method is based on an air enclosure where the temperature is constant and two samples are introduced at a different temperature from the temperature in the air enclosure. During cooling processes, three temperatures are registered: the ambient (air enclosure) and those of the two samples. The two samples are one reference substance whose thermal properties are known (frequently water) and one PCM whose thermal properties will be determined

with the results of the test. Figure 1 shows the basic scheme of the T-history method.

the PCM with the encapsulation material.

substances (Arkar & Medved, 2005).

**2.1.1 The T-history method** 

Fig. 1. Scheme of T-history installation.

**2.1 Determination of enthalpy as a function of temperature** 

most common commercial device (Zhang D. et al., 2007).

storage capacity of PCM is increasing (Zalba et al., 2003).

scarce literature on the melt fraction studies of PCM used in the various applications for storage systems. Many of these applications have been studied widely in the last years; most are related to buildings and several to heat exchange between PCM and air as the heat transfer fluid:


In any case, it is crucial to achieve efficient heat exchange between the heat transfer fluid and the PCM. This point is strongly affected by the heat exchanger geometry, as the TES unit has limited periods of time to solidify. Lazaro, 2009, compared the PCM-air heat exchange geometries studied by different researchers (Arkar et al., 2007; Turnpenny et al., 2000; Zalba et al., 2004; Zukowsky 2007). The author pointed out the difficulty of comparing between the different results provided by the authors, since each one show the results in its own way. Therefore, Lazaro concluded the need to standardize for proper comparison. Lazaro et al., 2009b, also presented experimental results for melting stage of real PCM-air heat exchangers pointing out the importance of the geometry. Geometry issues also affect the pressure drop of the TES unit and the air pumping requirements of the system, i.e., the electrical energy consumption. Regarding experimental studies, the evaluation of the thermal behaviour of the TES unit under statistical approaches or mathematical fitting leads to expressions that are very useful tools when designing such units. Among others, Butala & Stritih, 2009, and Lazaro et al., 2009b, followed this methodology when they evaluated their results.

In this chapter, a specific case study on slab geometry of a PCM-air heat exchanger is presented for temperature maintenance in rooms. However, the methodology posed here can be extrapolated to other different PCM geometries and system setups.
