8.3 THERMAL ENERGY STORAGE
The quantity of heat stored by a sensible heat storage material can be estimated by
Q=mc TΔ =rVc TΔ (8.4)
where
m is the mass of storage material (kg)
c is the specifi c heat of storage material (kJ/kg°C) ΔT is the temperature rise (°C)
r is the density of storage material (kg/m3) V is the volume of storage material (m3)
Iron, which has a high heat capacity and thermal conductivity, is an excellent thermal storage medium. Iron can be used as a high-temperature storage medium. Water, as a thermal energy storage material, has an excellent specifi c heat. It can be pumped. How-ever, if water is used to store heat above 100°C, it should be pressurized. Some oils can be used at a temperature higher than 100°C without the requirement of pressurization. The specifi c heat of oils is only about 2.3 kJ/kg°C compared to 4.18 kJ/kg°C for water.
The latent heat storage system, which provides a much higher storage energy density with a smaller temperature difference between storing and releasing energy, is one of the most effi cient ways to store thermal energy. Latent heat storage systems must have three key components: (1) a substance, also called phase change material, that undergoes a solid-to-liquid phase transition in the required operating temperature range, (2) a container for the storage substance, and (3) a heat exchanging surface to transfer heat between the hot or cold sources and the storage substance.
TABLE 8.5
Thermal Capacities of Some Common Sensible Thermal Energy Storage Materials at 20°C
Materials Density (kg/m3)
Specifi c Heat (kJ/kg°C)
Volumetric Thermal Capacity (MJ/m3°C)
Clay 1458 0.879 1.28
Brick 1800 0.837 1.51
Sandstone 2200 0.712 1.57
Wood 700 2.39 1.67
Concrete 2000 0.880 1.76
Glass 2710 0.837 2.27
Aluminum 2710 0.896 2.43
Iron 7900 0.452 3.57
Steel 7840 0.465 3.68
Gravelly earth 2050 1.81 3.77
Magnetite 5177 0.752 3.89
Water 988 4.182 4.17
Source: Reproduced from Dincer, I., Int. J. Energy Res., 26, 567, 2002. Copyright John Wiley & Son, Ltd. With permission.
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The quantity of heat stored by a latent heat storage material can be estimated by
Q=mh= r (8.5)Vh
where
m is the mass of storage material (kg)
h is the fusion heat of storage material (kJ/kg) V is volume of storage material (m3)
r is the density of storage material (kg/m3)
The properties of some latent heat storage materials are given in Table 8.6. Latent heat storage materials typically include water/ice, inorganic salt hydrates, and organics such as fatty acids (Mazman, et al., 2008). The most popular method for thermal energy storage via latent heat is the conversion of water to ice. Low-volatile, anhydrous organic substances such as glycerol (Bakan et al., 2008), fatty acids (Mazman et al., 2008), and paraffi ns (Demirel and Ozturk, 2006) are also used as thermal energy storage materials.
The thermal conductivity of most phase change materials is too low to achieve a high heat transfer rate between the phase change material and the energy source.
Therefore, the heat transfer has to be increased to effi ciently use the phase change material as an energy storage medium. There are several methods used to enhance the overall heat transfer in a thermal energy storage system: the use of fi nned tubes, the combination of phase change materials with materials with high thermal conduc-tivities such as metals, and the micro-encapsulation of the phase change materials to increase the heat exchanging surface (Mazman et al., 2008).
8.3.4 HOT THERMAL ENERGY STORAGE
Hot thermal energy storage systems are used to store the recovered waste heat from processing facilities and solar heat collected during daytime (Andersen et al., 2008).
Hot thermal energy storage systems can be used to store surplus recovered waste TABLE 8.6
Properties of Phase Change Materials
Storage Materials
Melting Temperature
(°C)
Heat of Fusion (kJ/kg)
Liquid Density (kg/m3)
Solid Density (kg/m3)
Volume Expansion
(%) References
Ice at 0°C 0 340 1000 916 −8 Singh and
Heldman, 2001 80% steric–20%
myristic acid
61–65 191 870 940 10 Mazman et al.,
2008 80% palmitic–20%
lauric acid
55–58 183 850 950 11 Mazman et al.,
2008
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heat to adapt the temporary mismatch between heat loads and waste-heat sources.
Since the availability of solar energy depends on the time of the day and varies in different seasons, a hot thermal energy storage system can be used to store excess solar energy and release it when the energy availability is inadequate or not available (Devahastin and Pitaksuriyarat, 2006).
8.3.5 COOLING ENERGY STORAGE
Cooling energy storage systems can be used to store cooling energy generated at off-peak demand times and supply cooling energy at off-peak demand times in food pro-cessing facilities. They can reduce the size of a refrigeration system and reduce the high demand charge of electricity. In cooling and freezing of foods, refrigeration loads vary signifi cantly with the cooling/freezing time. Cooling storage systems can provide cooling effect at a constant temperature to meet the requirement of dynamic cooling/freezing loads. Cooling storage systems can be recharged using a small refrigeration system during off-peak demand time.
Cooling energy storage systems are a popular demand management tool for utilities and refrigeration. They can help avoid costly plant expansions to meet the requirement of increased cooling/freezing capacity and reduce peak electricity demand. The systems provide cooling energy that is produced using inexpensive electricity during off-peak hours and stored for utilization during on-peak hours when the demand is high and the electricity is more expensive. Cooling energy storage is an economically viable energy conserving technology for a food processing facility. Furthermore, cooling energy storage system is one of the most appropriate methods for correcting the mismatch between the supply and demand of cooling requirement (Cheralathan et al., 2007).
A cooling energy storage system with a glycol–water solution as its storage medium is shown in Figure 8.2. This system uses the temperature change of glycol to
FIGURE 8.2 Glycol cooling energy storage system. (Reproduced from Bakan, K., Dincer, I., and Rosen, M.A., Int. J. Energy Res., 32, 215, 2008. Copyright John Wiley & Son, Ltd.
With permission.)
Storage rank 8 9 7 1 4
5 6
Pump
Chiller
Heat exchanger
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store cooling energy, usually generated from low-cost electricity (Bakan et al., 2008).
It was found that the average energy and exergy effi ciencies of the cooling energy storage system with a capacity of 350,000 kg of 45% glycol–water solution are 80%
and 35%, respectively, at a storage temperature of 2.4°C –5.8°C and ambient tempera-ture of 25°C–45°C (Bakan et al., 2008). Ice on coil, ice harvester or ice slurry, encap-sulated ice, or other phase change materials are popular latent heat cooling storage media (Egolf et al., 2008; Yamaha et al., 2008). Integration of a cooling energy stor-age unit with a chiller can reduce the specifi c energy consumption by charging the system at lower condenser and optimal evaporator temperatures. Cheralathan, et al.
(2007) found that a 1°C increase in evaporator temperature and decrease in condenser temperature can decrease the specifi c energy consumption by 3%–4% and 2.25%–
3.25%, respectively. Cooling energy storage is used mainly in cooling of buildings.
It uses chillers during off-peak hours at nights to produce a low-temperature medium, which can be stored for use during daytime (Rosen et al., 1999)