Category Physics of Solar Energy
To mitigate the problem of low thermal conductivity of PCMs, the material is often encapsulated in various forms. Figure 12.6 shows an example of a PCM encapsulated in flat or tubular parcels. A heat transfer fluid is required to make it operational.
Figure 12.6 Encapsulation of PCM. An example of PCM encapsulated in flat or tubular parcels, to mitigate the low thermal conductivity of PCM.
Many inorganic salts crystallize with a well-defined number of water molecules to become salt hydrates. Heating a salt hydrate can change its hydrate state. For example, hydrated sodium sulfate (Glauber’s salt) undergoes the transition at 32.4°C
Na2SO410H2O + AQ —> Na2SO4 + 10H2O. (12.16)
In general, the transition is
Salt mH2O + AQ —> Salt raH2O + (m — n)H2O. (12.17)
Thus, at the melting point the hydrate crystals break up into anhydrous salt and water or into a lower hydrate and water. The latent heat could be quite large, thus the storage density could be very high. If the water released is sufficient, a water solution of the (partially) dehydrated salt is formed.
These salt hydrates can be used in solar-operated space-heating or hot-water systems to provide uniform temperat...Read More
Paraffin wax is a byproduct of petroleum refining. The melting point of paraffin wax ranges from 50 to 90° C. Currently, paraffin wax only has a few commercially valuable applications, such are candles and floor wax. For such applications, only these with melting temperature between 58 and 60° are usable. But the supply is abundant. The melting temperature of paraffin matches the range needed for space heating and domestic hot water. It is also nontoxic and noncorrosive. One problem is its low thermal conductivity. This can be mitigated with encapsulation; see Section 12.2.4.
Other organic materials have similar properties as paraffin wax. An example is animal fat. Lard and chicken fat are considered harmful to human health because they can increase blood triglyceride and cause obesity...Read More
As shown in Table 12.3, the latent heat of the freezing of water or melting of ice is one of the highest. The water-ice system has already used in industry to save energy in air-conditioning systems. Figure 12.3 is a photo of a water-ice energy storage system, named Ice Bear, designed and manufactured by Ice Energy, Inc. The system has a large insulated tank filled with water and a lot of copper heat exchange coils. During the night, the refrigerator uses inexpensive electricity and cool air to make ice from the water. As we shown in Chapter 6, the lower the ambient temperature, the higher the
coefficient of performance (COP). Therefore, to make a well-defined mass of ice during the night, the electricity cost is much lower than in the hot daytime...
In sensible heat thermal energy storage systems, the process of charging or discharging of energy is related to a change of temperature, and the temperature is related to the amount of heat energy content. The storage density is limited by the heat capacity of the material. Using phase-change materials (PCMs), a considerably higher thermal energy storage density can be achieved that is able to absorb or release large quantities of energy (“latent heat”) at a constant temperature by undergoing a change of phase. Theoretically, three types of phase changes can be applied: solid-gas, liquid-gas and solid-liquid. The first two phase changes are generally not employed for energy storage in spite of their high latent heats, since gases occupy large volumes...Read More
Because the temperature range of water is limited, in order to store sensible heat at higher temperature, for example, in solar power generation systems, synthetic oil should be used. However, synthetic oil is expensive. A compromised solution is to use a mixture of synthesized oil and inexpensive solid materials, such as pebbles. Figure 12.2 shows such a thermal energy storage system schematically. A thermal energy storage system at high temperature (e. g. 400°C) can be built with limited cost. The heat conduction is mainly through convection of oil, and the pebbles provide heat capacity.Read More
In contrast to water and other liquids, solid materials can provide a larger temperature range and can be installed without a container. However, thermal conductivity becomes a significant parameter. Table 12.2 shows the thermal properties of typical solid materials. For many items, such as soil and rock, the values are only approximate or an average, because those materials vary widely. For example, the thermal parameters of soil could vary by one order of magnitude depending on the water content.
As shown in Table 12.2, materials with high thermal conductivities usually have low heat capacity. To use solid materials with high heat capacities, a long temperature equalizing time is expected.
Table 12.2: Thermal Properties of Solid Materials
As shown in Table 12.1, water has the largest heat capacity both per unit volume and per unit weight. And it is free. Therefore, it is logical to use water as the material for sensible heat storage. A typical case is the hot-water tank used in most homes. The tank is typically insulated by foam polyurethane, which has a thermal conductivity к = 0.02 W/mK and density p = 30 kg/m3.
Table 12.1: Thermal Properties of Some Commonly Used Materials