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PH ASE CH A NGING M ATERIA LS
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Ther m al stor age is a n efficient way of energy conservation possible by the incor por ation of l atent heat (concealed heat) stor age in building products. Energy storage in the walls, ceiling a nd floors of buildings m ay be enh a nced by a pplying suita ble ph ase ch a nge m aterials (PCMs) within these surfaces to ca ptur e sol a r energy dir ectly a nd incr ease hu m a n co mfort by m aintaining the te m per atur e in the desir ed interval for a longer period of tim e. PCMs a bsor b a nd r elease heat when the m aterial ch a nges fro m one ph ase to a nother. Solid-liquid ph ase ch a nge is the m ain ph ase ch a nge of inter est since other types, such as liquid-gas ph ase ch a nge m aterials, a r e gener ally not pr actica ble for m ost energy stor age a pplications. As a m atter of fact, liquid-gas ph ase ch a nges in volv e l a rge ch a nges in volu m e or pr essur e when going fro m the liquid to the gas ph ase, which pr ev ent effectiv e im ple m entation. So m e m aterials exhibit solid/solid ph ase ch a nges, in which the crystalline structur e is ch a nged at a certain te m per atur e. These a r e avail a ble in limited te m peratur e r a nges
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PH ASE CH A NGING M ATERIA LS
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How does it wor k? Initially, u nlike con v entional energy stor age m aterials, when solid-liquid PCMs r each the te m per atur e at which they ch a nge ph ase (their m elting point), they a bsor b l a rge a m ou nts of heat without a significa nt rise in te m per atur e. Despite the heat input, the te m per atur e of the m aterial stays r el ativ ely consta nt, while the ph ase ch a nge is ta king pl ace (Figur e 1). When the te m per atur e a rou nd a liquid m aterial falls, the PCM solidifies a nd r eleases its stor ed l atent heat. The sim plest a nd chea pest ph ase ch a nge m aterial is water. The fr eezing te m per atur e of water is fixed at 0°C. But wh at if you r equir e this heat at a te m per atur e other th a n zero? Within the hu m a n co mfort r a nge of 20° C to 30°C, so m e PCMs a r e v ery effectiv e. They stor e 5 to 14 tim es m or e heat per u nit volu m e th a n con v entional storage m aterials such as water, m asonry, or rock.[1] PCMs ca n be used in a nu m ber of ways, such as ther m al energy stor age wher eby heat or coolness ca n be stor ed fro m one process or period in tim e, a nd used at a l ater date or differ ent location for space heating, hot water, air conditioning syste ms or gener ating electricity. For ex a m ple, a r evolutiona ry new drywall, incor por ating one third ph ase ch a nging m aterial, h as the sa m e heat stor age ca pacity as a 23 c m thick brick wall in the critical te m per atur e r a nge for living co mfort of 22 to 26 °C. It wor ks by e m bedding “ph ase ch a nging microca psules” fro m BASF called Micronal into drywall, a nd is sold in Europe as KNAUF PCM Sm a rtboa rd. The ph ase ch a nge m aterials inside the BASF ca psules keep a roo m cool in m uch the sa m e way th at ice cubes chill a drink, by a bsor bing heat as they m elt. Each polym er ca psule contains pa r affin wa xes th at m elt
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PH ASE CH A NGING M ATERIA LS
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Each polym er ca psule contains pa r affin wa xes th at m elt at a rou nd roo m te m per atur e, ena bling the m to keep the te m per atur e of a roo m consta nt throughout the day (Figur e 2).
All in all, useful PCMs should r elease a nd a bsor b l a rge a m ou nts of energy, to do this they need to h av e a l a rge l atent heat a nd to be as dense as possible. H aving a fixed a nd clea rly deter mined ph ase-ch a nge te m per atur e is also of gr eat im porta nce which m ea ns the PCM needs to fr eeze a nd m elt as in a sm all te m per atur e r a nge as possible. In addition, a PCM should r e m ain sta ble a nd u nch a nged ov er m a ny fr eeze/m elt cycles a nd should keep its energy stor age ca pa bility for quite a long tim e. R efer ences
1.http://w w w.pc m products.net/ 2.http://w w w.energyefficiency.basf.co m/ecp1/micronal/ study _ effect _ of _ micronal 3.http://w w w.micronal.de/portal/str ea m er?fid =443847 4.http://w w w.delta m e m br a nes.co m/pdf/DELTA-COOL24 _ 2007. pdf 5.http://w w w.ther m alcor e.info/Ther m alCor e.pdf 6.http://w w w.rubither m.co m/english/index.htm