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Annual Review of Heat Transfer, Vol. 15, p.131-177 https://doi.org/10.1615/AnnualRevHeatTransfer.2012004651 2.1. Sensible heat storage in liquids Liquids offer the advantage of a possible use as both storage medium and heat transfer fluid. The most widely applied media, in this respect, are water and thermal oil. The storage approach using liquids can be realized as a single tank or a two tank concept. The two tank concept consists of two individual tanks at different temperature and fluid level. In a single heat storage vessel, thermal stratification is desired, because the value of high temperature heat is maintained in one part of the vessel, while low-temperature fluid (as backflow from a heat consumer) can still be stored in another part of the vessel. The solid filler materials can suppress free convection within the liquid and hence improves thermal stratification. Due to parasitic transient internal heat conduction, the stratification is destroyed in the course of time, even in very well insulated vessels. For media other than water, the liquid storage medium often creates the predominate cost of the entire storage system. In these cases low-cost solid filler materials can replace expensive liquid storage materials (e.g. cast iron in oil, molten salt thermocline designs). Due to the direct contact of the liquid and solid filler their compatibility must be ensured. Table 2 lists some characteristic liquids together with their thermo-physical properties at atmospheric pressure. For high-temperature storage systems, molten alkali metals, such as sodium (Tm = 98 oC) and sodium-potassium, could be used. Experience with these metals exists from nuclear reactor designs. Major advantages are the high thermal stability and the high thermal conductivity of these metals. However, at elevated temperatures the reactivity of alkali metals with air and water is high and require a carefully designed containment. Hence, molten alkali metals are not considered further here. The following subsections discuss systems with water, thermal oil and molten salt as liquid heat storage media. Table 2: Thermophysical properties of liquids for sensible heat storage T, oC Water 20 ρ, cp, λ, 106×a, [kg/m3] [kJ/(kg·K)] [W/(m·K)] [m2/s] 10−3×b, [J/(m2Ks1/2)] Material Silicone oil (AK250) Transformer oil Molten salt (K-NaNO3) Paraffin (liquid) Sodium 998 4.183 0.598 0.142 1.58 25 970 1.465 0.168 0.118 0.49 60 842 2.09 0.122 0.069 0.46 230 1950 1.57 20 900 2.13 100 927 1.385 0.50 0.16 1.24 0,26 0.14 0.71 85.84 66.85 10.50 2.1.1. Low-temperature water systems (< 100 °C) The most widely used liquid for thermal storage is water (Tamme 2009). Water has the following advantages: It is abundant and inexpensive It has a relatively high thermal effusivity (b = 1.58 × 103 Jm−2 K−1s−1/2) A relatively low thermal (temperature) diffusivity (a = 0.142 × 10−6 m2/s); an advantage for thermal stratification within a hot-water storage tank It can be easily stored in all kinds of containers The control of water flow systems is highly flexible and is often state-of-the-art Water can be used without heat exchangers It is easy to handle, nontoxic, noncombustible, experience with water is common It is easily mixable with additives (antifreeze, anticorrosive) Water has also some disadvantages, which can be summarized as follows:PDF Image | Annual Review of Heat Transfer
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