Technology: PCM Latent Thermal-Storage Media - Heating and Cooling without Energy Consumption?
Natural sources of energy exist in adequate quantities on earth, but they are not always available when one needs them. How can daytime warmth be preserved till deep in the night or cool night temperatures maintained far into the day? One traditional means of doing this is to exploit the thermal storage capacity of solid materials, such as timber, stone, bricks or concrete. In lightweight forms of construction, however, building elements of great mass are not used. In such situations, latent thermal-storage media offer new scope for storing heat. Around the melting point, the thermal-storage capacity of the aggregate leaps to several times the normal level (ills. 5, 6). This enormous energy gain is not perceptible in the form of higher temperatures, which is why it is referred to as "latent". When the reverse process takes place and the melted substance cools, it reverts to its crystalline state, yielding the stored energy (the melting enthalpy) in the form of heat of solidification. Since the major part of the thermal-storage process takes place when the aggregate changes its state (from liquid to solid or vice versa), latent thermal-storage media are sometimes referred to as "phase-change materials" (PCM). For the heating and cooling of buildings, materials with a melting point close to the desired room temperature should be chosen. Two main groups of materials have asserted themselves in practice with melting points in the temperature range between roughly 20 °C and 35 °C: organic PCMs such as paraffin; and hydrates of salt with a melting point around 27 °C or sodium sulphate deca-hydrates with their melting point at 32 °C.
