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10 H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 2. Sensibleheatstorage Sensible heat storage materials undergo no phase change within the temperature range required for the storage application [14]. Table 4 summarizes the main characteristics of the most common solid and liquid sensible heat storage materials. Within the indicated solids, concrete and cast ceramics have been extensively studied due to their low costs and good thermal con- ductivities, despite their moderate specific heats. In terms of liquids, molten salts [17] and mineral oils [38] are widely used in solar towers and parabolic trough collectors, respectively, being liquid at ambient pressure, providing an efficient and low cost medium, and having their operating temperatures compatible with current high- pressure and high-temperature turbines (temperature range over 120–600 °C). From Table 4, it is clear that the main candidates for liquid high temperature sensible heat storage are either a solar salt, i.e. a binary salt consisting of 60% of NaNO3 and 40% of KNO3, that melts at 221 °C and is kept liquid at 288 °C in an insulated storage tank; or HitecXL, a ternary salt consisting of 48% Ca(NO3)2, 7% NaNO3, and 45% of KNO3 operating beyond its melting point of 130 °C. Common mineral particles, such as silicon carbide, crystobalite silica, exhibit high values of Cp, and are hence subject of increas- ing research as SHS media, as described in Section 6 of this paper. Sensible liquid heat materials have been widely studied and are currently applied in solar thermal plant applications, despite im- portant disadvantages that can affect the storage system design and stability. The low solidification point may be a problem for solar power plants because of the required heat tracing during non- functioning periods. Furthermore, the sensible fraction of thermal energy is seldom fully recovered due to the required temperature difference for the heat transfer driving force. Another important dis- advantage consists in the low energy storage density of sensible heat materials, leading to the requirement of large volumes or quanti- ties in order to deliver the amount of energy storage necessary for high temperature thermal energy storage applications. The above mentioned problems can imply significant increments in the costs of sensible heat storage systems. For solid SHS media, the number of available materials is extensive and current handbooks and da- tabases give no comparison or possible relationships between properties of materials that enable their selection [39]. Materials with the highest Cp are natural and polymeric materials, such as natural rubber or the thermoplastic copolymers (acrylonitrile bu- tadiene styrene) with a Cp value of around 2 kJ/kg K. Other composite materials such as glass fibre reinforced epoxy concretes have Cp values close to 1 kJ/kg K. The specific heat capacity should be con- Table 4 Main characteristics of sensible heat storage solid and liquid materials (adapted from Refs. 34, 36, and 37). Storage medium Sand-rock-mineral oil Reinforced concrete NaCl (solid) Cast iron Silica fire bricks Magnesia fire bricks HITEC solar salt Mineral oil Synthetic oil Silicon oil Nitrite salts Nitrate salts Carbonate salts Liquid sodium Silicon carbide SiO2 (crystobalite) Temperature Cold (°C) Hot (°C) 200 300 200 400 200 500 200 400 200 700 200 1200 120 133 200 300 250 350 300 400 250 450 265 565 450 850 270 53 200 1400 200 1200 Average density (kg/m3) 1700 2200 2160 7200 1820 3000 1990 770 900 900 1825 1870 2100 850 3210 2350 Average heat conductivity (W/mK) 1 1.5 7 37 1.5 1 0.60 0.12 0.11 0.1 0.57 0.52 2 71 3.6 0.92 Average heat capacity (kJ/kg K) 1.3 0.85 0.85 0.56 1 1.15 – 2.6 2.3 2.1 1.5 1.6 1.8 1.3 1.06 1.13 sidered in conjunction with the price of the SHS material. Materials for sensible thermal energy storage in the range of 15–200 °C were considered and presented by Fernandez et al. [39]. Commercial prices vary from 0.02 to 0.08 €/kg for standard concrete, from 0.15 to 0.25 €/ kg for high density concrete, from 0.1 to 0.3 €/kg for common mineral powders, and from 0.5 to 3 €/kg for steel and alloys. 3. Latentheatstorage 3.1. Maincharacteristics Latent storage systems based on PCMs with solid–liquid tran- sition are considered to be very efficient in comparison to liquid– vapour and solid–solid transitions [40]. Liquid–gas transition requires a large volume recipient for the PCM and the solid–solid transi- tion presents a low value of latent heat, making both alternatives not considered as appropriate choices. A large number of materi- als are known to melt with a moderate to high heat of fusion within different ranges of temperature. No material yet studied has all the optimum characteristics required for a PCM, and the selection of a PCM for a given application requires careful consideration of the properties of the various substances and/or mixtures [34,41]. The main disadvantage of a PCM is its low range of thermal conductiv- ity between 0.2 and 0.8 W/mK, whereas sensible heat materials score better. Therefore, improving the PCM thermal conductivity will enhance the TES efficiency by improving its charging/discharging processes as dealt with below. 3.2. PCMsclassification PCMs are generally classified in different categories consider- ing their melting temperature and their material composition. The melting temperature can be divided in two main groups: low tem- perature (<200 °C) and high temperature (>200 °C). The categories for material composition are mainly threefold, being organic, in- organic and eutectic compounds [34], as illustrated in Fig. 10. Organic materials can repeatedly freeze and melt without phase segregations and are usually non-corrosive. Both “paraffin” and “non- paraffin” compounds are limited in use to low and moderate temperatures, hence outside the scope of the present review. Some of the features of this group are their high heat of fusion, low thermal conductivity and instability at high temperatures. Properties are re- viewed in literature by e.g. Zalba et al. [36], Kuravi et al. [34], Jegadheeswaran et al. [37] and other authors.PDF Image | Thermal energy storage: Recent developments
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