CAPITULO 2: Caracterización del estado del arte en la enseñanza del diseño de sistemas
2.2 Principales valoraciones acerca de la enseñanza de la Electrónica Digital en las
2.1.1 Phase change Material
Phase change materials are classified as organic or inorganic materials. PCMs are available for a range of different phase change temperatures. Organic PCMs are further divided into paraffins and non-paraffins. Figure 2:1 shows the classification of PCMs.
Figure 2:1: Classification of PCM. Sharma et al. (2009)
2.1.2 Classification of PCM.
Organic PCM such as fat and paraffin have been shown to be thermally stable after several cycles using the DSC(differential scanning calorimeter), showing little or no degradation of the latent heat and phase change temperature range Zalba B (2003). Sharma et al. (2009) mentioned that organic PCMs are regarded as PCMs with “poor thermal conductivity” and not ideal for effective thermal energy storage. However the advantage they provide with thier unique ability to undergo congruent melting, non- toxic nature, self-nucleating (ability to form structures or crystallize on its own) properties has made this class of PCM useful or relevant in latent heat thermal energy storage. Pure paraffin is known to belong to the family of saturated hydrocarbons, which means they contain only carbon and hydrogen of a general formula
C H
no 2no2 .The number of carbon atoms in the chain determines the melting point of the paraffin;paraffin with values of n between 5 and 15 are regarded as liquid paraffin at room temperature. Also the longer the carbon chain, the higher the melting temperature and latent heat of fusion of the PCM Sharma et al. (2005). Lane (1989) mentioned that paraffin is known to be the most utilized viable heat storage PCM and in terms of cost, paraffins are regarded to be cheaper when compared to inorganic PCM.
Inorganic PCMs, include salt hydrates which possess good thermal conductivity when compared to organic PCM. The composition of elements that make up inorganic PCMs are different from organic PCMs. A Eutectic is a homogeneous mixture of substances that freezes at a temperature lower than that of the individual components. The configuration of the eutectic mix is a ratio or percentage of the constituent substances that has a lower melting point than any other composition of those substances Kotzé (2014). An example of an eutectic is calcium chloride hexahydrate and Magnesium chloride hexahydrate, having an individual melting point of 29.54°C and 117°C respectively, but at a percentage or ratio of 66.6% and 33.3%, they melt at a temperature of 25°C. The melting temperature is called the eutectic temperature. A eutectic system may comprise of a number of substances, most often binary or ternary. They have higher volumetric thermal storage density. However, Abhat (1983) observed that they are corrosive and possess poor nucleating properties which results in subcooling of the liquid PCM before solidifying. Subcooling is a phenomenon whereby a PCM only starts to crystallize at a temperature lower than the phase change temperature (melting point). Sub cooling delays the commencement of solidification in phase change material and this inhibits good thermal storage. It is significant in most inorganic phase change material Mehling and Cabeza (2008).In some application, Farid et al. (2004) stated that a small amount of subcooling might be insignificant, however a large amount of subcooling would hamper the performance of the thermal energy storage. Ataer (2006) mentioned that it is important to consider the following when designing a latent heat thermal energy system; a PCM with a desired phase change temperature, storage medium for the PCM and an effective heat transfer fluid which can transfer heat effectively. The selection of a PCM was based on the PCM melting temperature, chosen at a temperature interval below the operating temperature of the application (heat pump). Lane (1989) proposed an interval less than 5°C to prevent excessive degradation of heat within the system and reduction of efficiency. The latent heat, thermal conductivity and phase change temperature of the
PCM play a vital role in determining the size of the heat exchanger. The choice of paraffin for this research was based on cost, non-toxicity and its stability over a number of cycles of charging and discharging.
2.1.3 PCM Selection
Oró et al. (2013) stated that the selection of the suitable PCM required in any application is vital, as it plays an important role with regards to cost, thermal efficiency and utility of the thermal energy storage. PCM undergo several cycles of melting and freezing as they are charged and discharged respectively, hence it is important to take into cognisance, thermal cycling stability, based on the PCMs properties. It is also important for the PCM not to exhibit phase segregation, sub cooling and their thermal properties should be constant over a long period. Requirements of properties of desired PCM has been debated by several researchers; Mehling and Cabeza (2008) mentioned that two key requirements for a PCM to be used in any TES are a high latent heat of fusion and suitable phase change temperature, while Zhou et al. (2012) stated that three key properties are essential in determining the PCM to use for any system: suitable melting temperature, high heat of fusion and thermal conductivity Some researchers have grouped requirements for selecting PCM as; Physical, Technical and Economical. Table 2.1 describes the way the requirements are grouped .The properties of PCM required were explicitly discussed by various researchers.(Sharma et al. (2009), Zalba et al. (2003), Lane (1989)). Differential scanning calorimeters (DSC) has been used by various researchers to determine their thermo physical properties.
Commercial PCM is available from Rubitherm, Cristopia, Climator, TEAP and some can be prepared using reagents from chemical companies. From the list of commercially available PCM and those identified by other researchers, Agyenim and Hewitt (2012) stated that a selection of PCM in the temperature range of 50-60°C are regarded suitable for hot side of a vapour compression heat pump. The choice of the PCM is determined by the application or operating temperature.
Organic PCMs have low conductivity, which require heat enhancement to be used in thermal storage. The choice of selection of a PCM is based on the PCM melting temperature. It should be chosen at a temperature interval below the operating temperature of the application (heat pump). Klimes et al. (2012) stated that the thermal properties of PCM are vital for numerical modelling because they affect the numerical simulation. Esen et al. (1998) mentioned that the thermo physical properties and
geometry design of the store are to be considered together in designing a thermal energy storage system. This was evident in Esen et al. (1998) experiments using four different phase change materials (Calcium chloride hexahydrate (CCHH), Paraffin, Sodium sulphate decahydrate (SSDH) and Paraffin wax). CCHH stored heat energy faster than the other three PCM, because it has the highest conductivity. Also, the PCM melting time for smaller radii geometry was better than larger radii. This is because the thicker the PCM mass the lower the temperature gradient, the slower the heat transfer and the longer it takes to melt.
Table 2:1: Required properties of PCM. Dincer and Rosen (2011)
Thermodynamic Kinetic Chemical Economic
Melting temperature in desired range High nucleation rate Completely reversible Low cost
High latent heat of fusion per unit mass
High rate of crystal growth
Chemical stability Availability
High thermal conductivity Non corrosive High specific heat and
high density
High freeze/melt stability
Small volume changes on phase transition Non-toxic, non- flammable and non-explosive material. Complete melting
Small vapour pressure at operating temperatures
Based on the aforementioned, the research commenced with determining the thermal and physical properties of the PCM available.