Cardy formula

Energy storage systems are crucial for the successful implementation of renewable energy technologies so as to ensure sufficient energy when the renewable energy is not available. Conventional storage of heat/cold is done in water tanks/towers, or electrically using PV cells, and has applications extending from building air-conditioning to nuclear reactors. Although water has been the main medium used in heat/cold stores, PCM can also be considered as a strong alternative due to its high energy density (Sharma et al, 2004).

Cold stores with PCMs work in a similar manner to PCM panels, i.e. they are charged by cool air/water or electricity at night, and discharged when needed. Ice has been commonly employed in cold stores, however, because of its fixed melting point at 0 °C, ice has limited applications. Other PCMs such as inorganic salt hydrates, organic paraffin waxes and eutectics have thus been investigated, and considered suitable for cold stores. For instance, RT5® (peak melting point 7 °C and latent heat of fusion 158 kJ/kg) was proposed to be a suitable candidate for cooling applications, because of its congruent melting, no subcooling, its stability after several heating and cooling cycles, and its self-nucleating properties (Sharma et al, 2004).

Additionally, Bo et al (1999) suggested the use of hexadecane and tetradecane, as well as mixtures of both for cold stores. The phase change temperatures of the different mixtures ranged from 1.7 °C to 17.9 °C, and latent heat from 146 kJ/kg to 227 kJ/kg.

The thermal properties were measured using DSC. The paraffins showed stable chemical and thermal properties, and the volume change during phase changes was below 10%. They concluded that the studied materials qualify for cool storage.

Tay et al (2012) experimentally investigated a tube-in-tank design filled with a salt hydrate with phase change temperature of -27 °C. They based the performance of the PCM on the compact factor and heat exchanger effectiveness of the tank design, and proposed an equation for the average effectiveness of such tanks up to a tube spacing of 70 mm. They concluded that this tank design can deliver a high energy storage density with compactness factors above 90%.

Cristopia Ltd. developed an encapsulated nodule storage, or mini plastic PCM spheres, ranging from diameters of 77mm to 98mm, and phase change temperature of -33 °C to

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0 °C. An example of such PCM nodule is shown in Fig. 2.10 (www.cristopia.com, 2010).

Fig. 2.10. PCM nodule by Cristopia (Cristopia Co. Ltd, 2010)

These PCM nodules are put directly into the storage tank and the secondary heat transfer medium exchanges thermal energy by direct contact with the nodule. This nodular storage system has proven to be very effective, due to its low operating temperatures and high mechanical strength (Cristopia Co Ltd, 2010).

Heat stores are mainly used to capture solar energy for use in water and air heating systems, and sometimes in cooling applications (through absorption chillers) (Godarzi et al, 2013). The Solar Heating and Cooling (SHC) programme (Citherlet et al, 2007) investigated the use of PCM together with water as heat stores. One of the advantages of using water as the heat storage medium is that it can store heat at different temperatures due to stratification. Thus Citherlet et al (2007) designed a water tank with PCM at the top, operating at a relatively high melting temperature. When the water temperature at the top of the tank reaches the PCM melting point, the latter starts charging, therefore maintaining temperature stratification in the water as well as increasing the overall heat capacity of the tank. The PCM used was 90% sodium acetate and 10% graphite, with a melting point of 58°C, melting enthalpy of 180-200 kJ/kg and enhanced conductivity (due to graphite) of 2-5 W/m⋅K.

Other types of configurations may include entire heat stores filled with PCM, with the secondary fluid (water) passing through heat exchangers. Such studies include Mettawee and Assassa (2006) who experimentally investigated a paraffin wax with melting temperature of 54°C and melting enthalpy of 266 kJ/kg in a solar collector.

Their study showed that the low thermal conductivity of the PCM is a limitation at the start of the melting process, which lowers the heat transfer coefficient. However, as the PCM melts and convection becomes more dominant, the heat transfer properties improve. During the discharging process, increasing the water flow rate improves heat transfer. Glauber's salt, calcium chloride hexahydrate, sodium thiosulfate penthydrate,

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sodium carbonate decahydrate, fatty acids, and paraffin waxes were found to be important PCMs for solar heating applications (Koca et al, 2008).

Due to the popularity of PCM heat stores, their application has been extended from the air conditioning of buildings to district heating and cooling, waste heat recovery and heat transportation. The TransHeat® system combines the advantages of district heating with those of latent heat storage. This allows industrial waste heat to be used for other applications such as water heating or cooling (through absorption chillers) at other places. The PCM used in TransHeat® systems is sodium acetate trihydrate with melting point of 58°C but most PCMs in the melting temperature range of 58°C – 180°C have been deemed satisfactory (Mehling et al, 2002), with higher importance placed on the melting enthalpy.

A common drawback for both heat and cold stores is the low conductivity of PCMs.

Direct contact improves the heat transfer properties, but cannot be used for all PCMs.

As a result, appropriate heat exchangers must be used. Furthermore, the use of heat transfer enhancements such as fins, high conductivity metal matrices, etc. were also found to be effective (Sharma et al, 2004). For heat exchangers, the heat transfer coefficient can also be enhanced by using a rough surface between air and the encapsulation (Dolado et al, 2011), promoting turbulence. Medrano et al (2009) experimentally investigated the thermal performance of five conventional heat exchangers working as latent energy storage devices, with RT35® and water as the heat transfer fluid. The authors concluded that a double pipe heat exchanger and a plate heat exchanger are not effective as heat stores. A compact heat exchanger, with PCM between the coils and fins provided the best performance, with 1kW heat transfer at a temperature difference of 25°C.

A report by the International Energy Agengy (IEA) Task 32 (Steicher, 2008) showed that the performance of storage tanks is strongly dependent on the temperature variations in the tank. For temperature variations from 50 °C to 70 °C, a PCM tank with a heat exchanger can be sized to 1/3 of a conventional water tank. However, for macro-encapsulated PCM and a temperature variation of 25-85 °C or 20-70 °C, the PCM store will have the same size as water stores. Hence, hot water designs requiring large variations in temperature will have limited benefits with respect to store size when PCMs are employed.

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