IMPLICACIONES EN LA PRÁCTICA DE LA FISIOTERAPIA

An important consideration is the impregnation of high conductivity porous matrices in PCMs. The combination has gained increasing attention, particularly because of the matrix light weight and high specific surface area for heat transfer.

 Metal foams

Metal foam is a cellular structure consisting of a solid metal, containing a large volume fraction of gas-filled pores. These pores can be sealed i.e. a close-cell foam, or form an interconnected network i.e. open-cell foam (Figure 2-15a). It is an interesting heat exchanger device due to its characteristic of mixing air flow during forced convection (discussed further in Section 2.6). The core advantages of impregnating PCM in metal foam i.e. composite (Figure 2-15b) include: 1) the capability of the foam ligament network to conduct heat throughout the storage media – unlike other techniques, the distribution of the foam ligaments within the PCM phase should make the melting and solidification processes more uniform, and 2) its high specific surface area (per volume or mass) for heat transfer, which results in increasing the ‘effective thermal conductivity’ of the PCM/foam composite1_{. An important parameter to consider is porosity, ε, defined}

as the ratio between the voidage volume and the total volume:

1 _{Note that the effective thermal conductivity is commonly used to describe structures such as sintered}

heat pipe wicks containing a liquid – not unlike a foam containing a PCM in liquid (or solid) form. See for example Maxwell, J.C. [ ] A treatise on electricity and magnetism, Third Edition, Vol. 1, OUP, 1054. Reprinted by Dover, New York, NY, 1981, and Reay et al [ ]. Kew, P.A. and McGlen, R. Heat Pipes, 6th

28 | P a g e 𝑃𝑜𝑟𝑜𝑠𝑖𝑡𝑦, 𝜀 =𝑣𝑜𝑖𝑑𝑎𝑔𝑒 𝑣𝑜𝑙𝑢𝑚𝑒

𝑡𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 𝑥100% Equation 2-5

The porosities of common metal foam/PCM composite are generally >90%, in order to maximize the latent storage capacity [52]. The common foam materials are copper, aluminium, stainless steel, and nickel [53]. The choice of material is a balance between the material thermal conductivity (e.g. copper ~400 W/m.K, stainless steel ~17 W/m.K) and its compatibility with the PCM to avoid chemical degradation or corrosion complications. The cost of the foam is also an important factor. Most works found in the literature present the enhancement due to foams by presenting: 1) the ‘effective

thermal conductivity’ increase e.g. Siahpush et al. [54] measured a 95% porosity copper

foam/eicosane composite conductivity to be 3.06 W/m.K, a significant increase from the baseline eicosane 0.423 W/m.K value, and 2) the reduction in melting/solidification time relative to the baseline PCM e.g. in the previous example, the melting was reduced from 500 min to 250 min (i.e. -50%).

Figure 2-15 (a) The structure of an open-cell aluminium foam, (b) the foam filled (impregnated) with paraffin wax PCM [55].

 Metal meshes

Inspired by the metal foam performance, a novel use of the common metal mesh (which is used for a variety of applications as shown in Figure 2-16 and referred to by some manufacturers as expanded metal – Expamet being one trade name) is to enhance the heat transfer performance of PCM. Advantages of the mesh against metal foam are in its competitive cost and ready availability. Mustaffar et al. [8] reported that the metal mesh is generally significantly cheaper than metal foam per unit volume, up to 96%, for certain types. Shuja et al. [56] pioneered this research by investigating the melt

29 | P a g e behaviour of n-octadecane impregnated with aluminium mesh of different geometries: triangular, square, and hexagonal – see Figure 2-17. The results showed a preference toward the triangular geometry, which resulted in the shortest time taken to melt the PCM. The experimental results conformed to direct 2D numerical simulations [8].

Figure 2-16 Metal mesh from Expamet Building Products (a) commercial variance, (b) common applications, (c) the range of mesh size (in mm) for the raised aluminium

mesh type.

Figure 2-17 (a) Aluminium square mesh (b) the PCM melt progress. An investigation by Shuja et al. [56].

The link between the metal foam, the subject of the research in this thesis, and the metal mesh (expanded metal) is that the latter was found to be a lower cost alternative as a result of ongoing literature studies throughout the research programme. It was therefore decided to highlight it as a potential alternative for applications in heat exchangers and in PCM storage.

30 | P a g e As introduced below, the use of graphite cloth (see Figure 2-14(C)) in Section 2.4.2.1 above) is another alternative good thermal conductor for use in PCM mixes – however it lacks the rigidity of the metallic mesh and the foam. In ‘Advances in Thermal Energy Storage Systems – Methods and Applications’, by Luisa F. Cabeza [57], it is pointed out that the poor mechanical strength is a drawback, although some graphites have been mixed with metal additives (aluminium nitride is mentioned) to enhance thermal conductivity and, the reference implies, rigidity. ‘Expanded graphite’ is a form of graphite (not a mesh) that can be bound with PCM under compression to enhance conductivity, but the structure is essentially random – see Xiao et al. (2015).

 Graphite composites

Graphite composite can be made by dispersing graphite flakes in the PCM. This can be achieved above the melting temperature by mechanical dispersion within the molten PCM or at room temperature by mixing the two components. Pincemin et al. [58] investigated the composite of graphite flakes with several high temperature PCMs (nitrate salts). The thermal conductivity of the composites with 20 wt% graphite effectively increased to 3.5-9.0 W/m.K from the baseline of ~0.5 W/m.K (Figure 2-18).

Figure 2-18 (a) Natural graphite flakes, (b) expanded natural graphite, (c) grounded expanded natural graphite, (d) graphite/PCM composite formed after being poured

into a stainless steel mould [58].

2.4.2.3 Discussion and analysis

Table 2-5 on page 33 analyses the pros and cons of the enhancement techniques. It is not easy to pinpoint the best enhancement technique for any application as each depends upon circumstances. For example, although the metal foam technique is arguably the most superior as attested by the vigorous research upon it compared to the other techniques [59], in the Thermac® project [60], on which the author worked,

31 | P a g e aluminium fins were carefully designed to time 7 hour PCM charging and 4 hour energy discharging periods instead of the metal foam solution – see Figure 2.19. In essence, when a PCM storage application has been selected, the heat transfer solution must take into account economic and technical considerations. In this PhD project, the transient heat sink employed the metal foam solution because rapid energy charge and discharge from the salt hydrate PCM was a requirement. In addition, a high degree of fluid retention was important as it was expected that the viscosity of the salt hydrate to decrease greatly as it changed phase from solid to liquid, which could be made possible by using metal foam due to its tortuous paths [61].

Figure 2-19 The internal and external fins were carefully designed to provide overnight charging and daytime discharging of PCM in the Thermac® project [60].