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Annual Review of Heat Transfer, Vol. 15, p.131-177 https://doi.org/10.1615/AnnualRevHeatTransfer.2012004651 Copper 350 8800 35000 The finned tube design is advantageous for applications with the following boundary conditions: high temperatures, corrosive media and pressurized fluids. By using finned tubes, the effective surface of the tube is extended. The aim is the replacement of expensive pressure pipes by less expensive non-pressurized thermally conductive structures. Finned tubes used in a PCM storage system may differ from finned tubes applied in heat exchangers. The distance between parallel tubes and the fin design depends on the specific power requirement. Designs can aim for parallel tubes with a larger spacing compared to conventional heat exchangers. The feasibility of the finned tube concept embedded in PCM (Sandwich concept) has been demonstrated in various DLR research projects using fins made of either graphite foil or aluminum (Steinmann 2008, Laing 2010). Heat transfer enhancement can be also achieved by conductive composites. These composites combine the properties of a high latent heat of a PCM and a good thermal conductivity of an additive. An aspect of interest is the stability of the composites without phase separation after several melting and solidification cycles. A highly conductive and porous graphite matrix can be prepared by the compression of expanded graphite without a binder system. For low- temperature applications (<100 °C), paraffin/graphite composites based on such matrix have been successfully demonstrated. The composites are prepared by an infiltration process (Py 2001, Marín 2005). Conductive composites may also be prepared by other process such as compression. Also of interest are form-stable composite with direct contact to the heat transfer fluid (HTF). If the composites are sufficiently small or provide channels for HTF, high heat transfer rates can be achieved. There are examples at low and high temperatures. At low temperature, form-stable high density polyethylene (HDPE) with a phase change temperature of about 125 °C has been examined in direct contact to ethylene glycol and silicon oil (Takahashi 1981). At high temperatures, form-stable salt-ceramic composites have been developed. Here, anhydrous salts are utilized as a PCM and a ceramic matrix provides form stability (Tamme 1991). Salt-ceramics with PCMs with phase change temperatures in the range 700 to 900 °C were considered. The HTF was air in direct contact to the salt-ceramic composites. Finally, some other methods exit to enhance heat transfer. An intermediate heat transfer fluid can transfer thermal energy between the primary HTF and the PCM. An intermediate “heat pipe” system based on the evaporation and condensation of a suitable intermediate HTF would be very suitable. In this concept, both the intermediate HTF and the PCM undergo a phase change. In this way large temperature differences do not occur and the energy efficiency of the charging and discharging process can be high. If the PCM and the intermediate HTF can operate in direct contact, the primary heat exchanger of this concept can be much smaller compared to a heat exchanger directly embedded in the PCM. There are several requirements with regard to intermediate HTF. The temperatures for boiling and condensation of intermediate HTF must be adapted to the melting temperature of the PCM. If the primary HTF also undergoes a phase change (e.g. water/steam), all three media, namely primary HTF, intermediate HTF and PCM, should undergo a phase change at about the same temperature. Adinberg discusses further details of the so called reflux heat transfer storage concept (Adinberg 2007). The transportation of solid PCM is another concept that can be used to address the heat transfer limitations caused by frozen PCM around the heat transfer structure. Transportation of solid PCM allows a storage system design with two tanks. One tank contains liquid PCM and the other tank stores the solid PCM. In this way the heat exchanger and the storage volume is separated. The heat exchanger of such design can be smaller compared to anPDF Image | Annual Review of Heat Transfer
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