Phase Change Materials and Solar Water Heaters

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1. Introduction
2. Types of Phase Change Materials
3. Selection criteria
4. Principle of solar water heater with phase change materials 5. Heat transfer analysis between water and PCM’s
6. Results
7. Conclusion

PHASE CHANGE MATERIALS: The materials which undergo either boiling or condensing by absorbing heat from a medium or liberating heat to a medium are called as phase change materials. Characteristics

PCMs latent heat storage can be achieved through solid–solid, solid–liquid, solid–gas and liquid–gas phase change. However, the only phase change used for PCMs is the solid–liquid change. Liquid-gas phase changes are not practical for use as thermal storage due to the large volumes or high pressures required to store the materials when in their gas phase. Liquid–gas transitions do have a higher heat of transformation than solid–liquid transitions. Solid–solid phase changes are typically very slow and have a rather low heat of transformation. Initially, the solid–liquid PCMs behave like sensible heat storage (SHS) materials; their temperature rises as they absorb heat. Unlike conventional SHS, however, when PCMs reach the temperature at which they change phase (their melting temperature) they absorb large amounts of heat at an almost constant temperature. The PCM continues to absorb heat without a significant rise in temperature until all the material is transformed to the liquid phase. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its stored latent heat. A large number of PCMs are available in any required temperature range from −5 up to 190 oC. Within the human comfort range of 20° to 30°C, some PCMs are very effective. They store 5 to 14 times more heat per unit volume than conventional storage materials such as water, masonry or rock.

The PCM’s can be classified as follow
Organic PCMs
Paraffin (CnH2n+2) and fatty acids (CH3(CH2)2nCOOH)
* Advantages
1. Freeze without much super cooling
2. Ability to melt congruently
3. Self-nucleating properties
4. Compatibility with conventional material of construction 5. No segregation
6. Chemically stable
7. High heat of fusion
8. Safe and non-reactive
9. Recyclable
* Disadvantages
10. Low thermal conductivity in their solid state. High heat transfer rates are required during the freezing cycle 11. Volumetric latent heat storage capacity is low
12. Flammable. This can be easily alleviated by a proper container 13. To obtain reliable phase change points, most manufacturers use technical grade paraffins which are essentially paraffin mixture(s) and are completely refined of oil, resulting in high costs Inorganic

Salt hydrates (MnH2O)[4]
* Advantages
1. High volumetric latent heat storage capacity
2. Availability and low cost
3. Sharp melting point
4. High thermal conductivity
5. High heat of fusion
6. Non-flammable
* Disadvantages
7. Change of volume is very high
8. Super cooling is major problem in solid–liquid transition 9. Nucleating agents are needed and they often become inoperative after repeated cycling Eutectics
Organic-organic, organic-inorganic, inorganic-inorganic compounds * Advantages
1. Eutectics have sharp melting point similar to pure substance 2. Volumetric storage density is slightly above organic compounds * Disadvantages
3. Only limited data is available on thermo-physical properties as the use of these materials are very new to thermal storage application Hygroscopic materials
Many natural building materials are hygroscopic, that is they can absorb (water condenses) and release water (water evaporates). The...
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