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Annual Review of Heat Transfer, Vol. 15, p.131-177 https://doi.org/10.1615/AnnualRevHeatTransfer.2012004651 Figure 15: Heat transfer enhancement concepts for latent heat storage systems There are two general types of extended heat transfer surface concepts. One approach is the encapsulation of the PCM. The terms micro-encapsulation and macro- encapsulation refer to the physical size of the capsules. Macro- and micro-capsules prevent the PCM from leaking in the liquid phase. Micro-capsules may be embedded in a solid matrix (e.g. wall board) or suspended in a heat transfer fluid. The latter concept is also known as micro-encapsulated PCM slurry. The large heat transfer surface area of the micro-capsules results in high heat transfer rates. Similarly, a heat transfer fluid, such as water or air, can surround macro- capsules in order to achieve a high transfer rate. Design criteria of the capsule include the following: Wall thickness (flexible or stiff design) Inside and outside pressure levels Compatibility of capsule material with the PCM and heat carrier fluid Void volume for the expansion of the PCM during melting Thermo-mechanical stress and long-term stability of the capsules Encapsulation is commercially employed at low temperatures (<120 °C) where polymers can be used. For CSP applications at higher temperatures, capsules with metallic walls must be used (Figure 16). Basically, containers for PCM can be either thin-walled (flexible) with equal pressure inside and outside or thick-walled (stiff) with different pressures. For systems using molten salts, the walls must have a minimum thickness to ensure a sufficient life expectancy regarding corrosion aspects. Consequently, a design using flexible containers is not possible for nitrates as the PCM. A significant drawback of stiff capsules is the necessity to include a gas volume to compensate the expansion of the PCM during melting (Steinmann 2008). It has been found that only about 60% of the volume inside the pressure vessel can be filled by PCM capsules. A lab scale test unit was designed and manufactured by DLR. Cylindrical capsules containing a total mass of 4.5 kg of NaNO3-KNO3 (eutectic) were stored in a pressure vessel (Figure 16). Although the feasibility of the concept was proven by experiments, this approach was not pursued any further due to economic aspects. Regarding costs, the macro- encapsulation of PCM is not very attractive due to the limited effective volume share of the storage material and the significant amount of steel needed for the capsules and the pressure vessel. An additional problem is the necessity to ensure a high quality sealing of the capsules, since contamination of the steam due to leakages must be avoided.PDF Image | Annual Review of Heat Transfer
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