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Perspectives for short-term thermal energy storage using salt hydrates for building heating
Energy 189 (2019) 116139
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Perspectives for short-term thermal energy storage using salt hydrates for building heating B.C. Zhao, R.Z. Wang* Institute of Refrigeration and Cryogenics, MOE Engineering Research Center of Solar Energy, Shanghai Jiao Tong University, Shanghai, 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2019 Received in revised form 10 September 2019 Accepted 14 September 2019 Available online 17 September 2019
In this forward-looking perspective, the current research status of latent heat storage using salt hydrates for building heating are firstly analyzed from aspects of material development, performance evaluation, heat transfer enhancement and application feasibility. Based on the analysis, barriers for the further promotion of this technology, including narrow application range, imperfect performance evaluation, inefficient heat transfer enhancement and vague market prospect are outlined. To address these issues, perspectives on four aspects are provided. First, further explorations on salt hydrates with high melting points meeting the heat dispatching demand of centralized building heating are strongly recommended. Second, effects of supercooling and phase separation of salt hydrates in practical applications should be considered differently from lab-scale experiments. Third, the combination of multiple heat transfer enhancement approaches can further improve the overall performance of heat storage. Fourth, this technology has a certain prospect in high-temperature heat dispatching in densely populated areas and short-distance mobile heat supply for emergency. The above perspectives provide guidelines for the future material development, device design, system optimization, and application scenario selection of latent heat storage using salt hydrates for building heating. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Thermal energy storage Building heating Salt hydrate Performance evaluation Heat transfer enhancement Application feasibility
1. Introduction The global building heating demand grows rapidly with the promotion of people's living level during past decades. It is reported that the contribution of residential coal burning has exceeded the combination of transportation and power generation on the production of PM2.5 in northern China [1]. Comparing with fossil fuels, industrial waste-heat, renewable energy and electric energy are cleaner and more efficient. However, these heating sources are naturally mismatched in time or space with the demand of building heating. In this case, thermal energy storage (TES) is often used to regulate the supply-demand gap [2]. TES is classified into a long-term and short-term type, considering storage duration. The long-term TES aims to provide thermal energy dispatch over months or even seasons (for example, based on seasonal variations of solar radiation), so it is usually of large scale and thus regionally restricted, especially in densely populated areas. In comparison, the short-term TES performing diurnal heat
* Corresponding author. E-mail address: [email protected] (R.Z. Wang). https://doi.org/10.1016/j.energy.2019.116139 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
regulation (for example, based on peak-valley electricity rates) has a higher operation applicability and deployment flexibility, and therefore received increasing attention in recent years. Among a variety of short-term TES patterns, latent heat storage (LHS) using salt hydrates [3e6] presents a better comprehensive performance in heat storage density, operation controllability, structural compactness and capital cost, which are all highly valued by residential building heating. Investigations and reviews on salt hydrates for the short-term TES have been reported in different aspects of property characterization of developed materials [3,7], development and modification of new materials [8e12], design and optimization of latent heat storage devices [13,14] and economic evaluation of commercial applications [7,15]. Nevertheless, rare study focuses on forwardlooking issues of this technology from a macro perspective of practical building heating application. For instance, there still lacks of mature salt hydrates (or relevant composites) for various building heating scenarios. Besides, common issues like supercooling and phase separation still have potential negative effects on the performance and cycling life of the TES. Moreover, current heat transfer enhancement approaches are still required to be improved in aspects of scalability and versatility. In addition to the above-
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mentioned internal issues, LHS using salt hydrates also faces with external challenges such as poor cost competitiveness against water tank heat storage (WTHS) and uncertain economic effectiveness under current cost levels of thermal resources. All these issues and challenges lead to difficulties in the widespread application of LHS using salt hydrates for building heating. To address these issues, the paper firstly briefly reviews the current study status of the short-term LHS using salt hydrates. Then the main barriers that prevent this TES technology from building heating application are proposed. Finally, several corresponding perspectives are presented from aspects of material development, performance evaluation, heat transfer enhancement, and application feasibility. 2. Current research statues and barriers Salt hydrates make use of the reversible separation and combination of water molecules with inorganic salts to achieve a repeatable storage and release of latent heat. Among various hydrated salts, sodium sulfate decahydrate [16] and sodium acetate trihydrate [17] (SAT) are the most representative ones used for passive building thermal management and distributed building heating, respectively. However, salt hydrates with higher melting temperatures for the heat dispatch of centralized residential building heating are still rare. Natural salt hydrates often suffers from problems of supercooling and phase separation that have negative effects on the discharging stability and cycling life of TES. Although these issues have been suppressed to a great extent by means of adding specific nucleating [18] and thickening agents [19] in lab-scale experiments, there is still a lack of corresponding studies on industrial applications characterized by large-scale (counted based on the heating demand of an entire building) and long-period (counted based on years). Heat transfer enhancement is also a key technology of LHS using salt hydrates. A variety of approaches such as adding additives with high thermal conductivities [20], building form-stable PCM composites using high thermally conductive frameworks [21], and micro/macro PCM encapsulations [22] have been reported. However, further improvements on energy storage density and cost effectiveness are still required in terms of application. Compared with water tank storage that has been widely used for building heating, LHS using salt hydrates seems to have no advantage except a remarkable energy storage density. The cost competitiveness of this TES technology against WTHS may still be weak due to the extra cost of PCMs. Besides, the cost effectiveness of this TES technology is uncertain under current cost levels of thermal resources [23]. Both of the capital cost and application scenario of this TES technology are still lack of investigations. According to the present literature review, four aspects of barriers preventing the short-term LHS using salt hydrates from largescale application in residential building heating are highlighted as below: (1) The development of high-temperature salt hydrates for centralized heat dispatch are under-researched. (2) There exist gaps between scientific research and industrialization in evaluating the thermal performance of salt hydrates. (3) The current heat transfer enhancement methods still limits the storage density and cost-effectiveness of this TES technology. (4) Poor cost competitiveness against WTHS and uncertain cost effectiveness result in an uncertain market positioning of this TES technology.
Detailed descriptions of these barriers and the corresponding perspectives will be presented in the next section. 3. Future perspectives 3.1. Development of high-temperature salt hydrates for centralized heat dispatch A TES integrated into a building heating system is expected to be capable of satisfying heating demands independently when routine heating sources are unavailable, which indicates that it should be able to provide a thermal output meeting the inlet temperature requirement of terminal heaters (for instance, 50e60 C for radiator heating, 40e55 C for underfloor heating, and 35e45 C for fan-coil heating). Accordingly, the required exothermic temperature (i.e. the melting temperature in ideal conditions) of salt hydrates used for different building heating systems can be generally determined, considering the loss of heat transfer (5e10 C for different heat transfer structures) and transportation (0e5 C for distributed heating and 15e25 C for centralized heating). Fig. 1 draws estimated PCM melting temperature ranges (represented by bars with different colors) suitable for different heating terminals including distributed fan-coil heating (D-F), distributed underground heating (D-U), distributed radiator heating (D-R), centralized fan-coil heating (CeF), centralized underground heating (CeU) and centralized radiator heating (C-R). One can easily draw a preliminary concusion on whether a PCM is appropriate for the heat dispatch of these heating terminals by means of checking whether the line representing the melting temperature of the PCM has an intersection with these bars. It is clearly observed that SAT with a melting point of 58 C is only suitable for the distributed heating application scenarios. We also screened out several PCM candidates, including barium hydroxide octahydrate (BHO), magnesium nitrate hexahydrate (MNH) and ammonium aluminum sulfate dodecahydrate (ASSD) for centralized building heating, referring to the selection principles [6,24] of salt hydrates as heat storage media. Although these PCMs are well matched with the application scenarios with relatively high heating temperatures, unfortunately, they also have some fatal drawbacks that limit their large-scale application potentials at the present stage. For instance, BHO is highly toxic and relatively costly [25], MNH is explosion-risky and its latent heat is relatively low [26], and ASSD is highly corrosive and has a large
Fig. 1. Potential salt hydrates for building heating in various application scenarios.
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supercooling degree [27]. Further investigations on these PCMs are required. In addition to directly looking for appropriate natural salt hydrates, adjusting the melting point of salt hydrates by means of eutectics [28] is also a practicable option to obtain desired PCMs. An eutectic salt hydrate has a lower melting temperature than its principal material. This can be utilized to achieve an artificial downregulation of the melting point of hydrated salts. As recently reported in a literature [29], adding a certain amount of boric acid (10e12%) into oxalic acid dihydrate can obtain a binary eutectic material that has a melting point of 87.3 C, which can be considered as a potential PCM for the heat dispatch of centralized building heating. 3.2. Performance evaluation of salt hydrates in view of TES application The essence of supercooling phenomenon of a solid-liquid PCM is the non-crystallization of the material as its temperature drops below the theoretical crystallization temperature [30] (i.e. the onset temperature of melting), which means that the latent heat would not be released at the melting point of the PCM. Generally, supercooling is considered to have adverse effects on both thermal grade and stability of the discharge of a LHS. However, the degree of this effect varies with different working conditions. Test-tube experiments are often used in laboratory investigations to evaluate the transient discharge performance of hydrated salts, for high experimental efficiency and repeatability. In such experiments, temperature variations of salt hydrates with space are negligible. A typical state evolution of a hydrated salt with supercooling during its exothermic process is drawn in Fig. 2a, where the melted, supercooled and crystallized state of the PCM are represented by red, blue and orange, respectively. Many researchers take the temperature difference between phase II and III as an apparent supercooling degree [24,31,32], and make efforts in mitigating or eliminating this temperature difference. Actually, the apparent supercooling observed in test-tube
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experiments may not have a noticeable negative effect on the output of a storage device with large spatial dimensions, due to the asynchronous solidification of PCMs. Taking a LHS employing fincoil heat exchanger to achieve heat transfer between PCM and heat transfer fluid (HTF) as an example, as shown in Fig. 2b, when the PCM (Point C) near the HTF entrance begin to crystallize at some instant of discharge (corresponding to Phase III in Fig. 2a), the PCM (Point B) located in the middle zone may be going through a supercooling process (corresponding to Phase II in Fig. 2a), and the PCM (Point A) near the HTF exit may still be in a melted state (corresponding to Phase I in Fig. 2a), due to a long flow path. The HTF extracts thermal energy from the PCM with multiple states along the flow path. As a result, the outlet HTF temperature may present a smooth decline during the discharge process, since temperature fluctuations of HTF caused by PCM supercooling can be tempered along the flow path to some extent as shown in the bottom subplot of Fig. 2b. This indicates that a certain supercooling degree is acceptable for hydrate salts used in large-scale LHS devices. In the future, more studies are required to quantify the influence of the supercooling degree of salt hydrates on the thermal performance of LHS devices. Another concern about the industrialization of the short-term LHS using salt hydrates is its practical cycling life, which is related to the phase separation control of salt hydrates. It is known that adding thickening agents, for example, carboxyl methyl cellulose (CMC) into salt hydrates to form a suspension liquid is an effective approach to suppress the phase separation of hydrated salts with low water solubility [31]. However, the degradation of thickening effect is inevitable during a long-period and largespatial-scale operation, due to the aggregation and stratification of thickening agents caused by convection. As a result, the industrial feasibility of salt hydrates as short-term storage media cannot be simply proved by stable performances observed in test-tube experiments. In the future, more investigations on the effectiveness of thickening agents in practical working conditions are required to quantify the performance degradation of LHS devices.
Fig. 2. Comparison of the discharge process of LHS between lab-scale experiments and industrial applications.
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3.3. Efficient heat transfer intensification for LHS using salt hydrates Although salt hydrates have a relatively higher thermal conductivity than organic PCMs, effective heat transfer enhancement on a LHS device using such PCMs is still highly required in practical applications. One method is to directly improve the thermal conductivity of PCMs by means of adding high thermally conductive additives such as graphite nanofibers [33], carbon nanotubes [34] and graphene nano-plates [35]), or introducing high thermally conductive frameworks such as foam metal [21] and compressed expanded graphite [10] to build form-stable composite phase change materials (FS-CPCMs). While the added additives often suffers from particle aggregation and precipitation that deteriorate the effectiveness of heat transfer enhancement, FS-CPCMs are of more stable structures and can provide much greater enhancing effect with the aid of heat conduction channels. However, the introduction of framework materials is always accompanied by a significant decrease in energy storage density either in mass or volume. Besides, the fixed shape of FS-CPCMs also increases the difficulty in assembling a storage device. An alternative approach is to enlarge the heat transfer area between PCMs and HTFs. To achieve this goal, fin-tube heat exchanger [36] (as shown in Fig. 2b) is conventionally used for its high technical maturity and low cost. However, it has drawbacks of low extendability, poor corrosion resistance and high leakage risk. In comparison, the packed-bed LHS [37] based on the macroencapsulation of PCMs is of more flexible deployment, easier leakage control and higher HTF compatibility (allows both gas and liquid HTFs), though it has some advantages including low volume ratio of PCMs, costly encapsulation and poor discharge performance in both thermal power and capacity factor, at the current stage. The combined utilization of the above-described heat transfer enhancement approaches, for instance, metal foams with nanoparticle additives [38] or micro-encapsulation of PCMs [39], can further improve the overall performance of a LHS. Herein, we also propose a new LHS concept, namely multi-granularity packed-bed LHS using high thermally conductive FS-CPCM, for residential building heating. As illustrated in Fig. 3, the LHS consists of a packed-bed randomly stacked by a great number of spherical expanded graphite/salt hydrate FS-CPCM capsules with several specific sizes [40]. By taking advantages of a high thermally conductive compressed-graphite framework and a large heat transfer area provided by the porous region of the packed-bed, heat transfer between the PCM and HTF can be enhanced to a great
Fig. 3. Conceptual design of a multi-granularity packed-bed LHS based on high thermally conductive FS-CPCM.
extent. Moreover, the low porosity of the packed-bed also ensures a high volumetric filling ratio of the PCM. However, the efficient preparation of such FS-CPCMs is still a major technical challenge, since expanded graphite is hydrophobic. 3.4. Promising application scenarios for LHS using salt hydrates LHS using salt hydrates has been considered as a substitution for traditional WTHS in building heating for years, due to its much larger energy storage density. Taking a developed SAT-based composite PCM [31] as an example. The ideal volumetric energy storage density of this PCM within a conventional operation temperature range for residential building heating (i.e. 40e80 C) is around 550 MJ m 3, which is about 3.3 times of water (167 MJ m 3). In fact, the advantage of such a LHS in volumetric energy storage density increases with the decrease in the operation temperature range. It can be calculated that the volumetric energy storage density of the above-mentioned PCM is even around 5.6 times of water as the operation temperature range drops from 40 C to 20 C. Based on this, LHS using hydrated salts can perform a much greater volume superiority over normal pressure WTHS in conditions that a relatively high-grade (over 80 C, for example) heat storage is required, because normal pressure WTHS has a ceiling operation temperature of 95 C. Accordingly, more attention should be paid to develop high-temperature salt hydrates for centralized space heating and hot water supply. Benefiting from a higher volumetric energy storage density, LHS using salt hydrates has a higher installation area saving potential than traditional WTHS, which is particularly crucial for densely populated regions where land is short and expensive. With the support of ‘coal to electricity’ policy [41] announced by the Chinese government in 2016, electric-boiler heating integrating with a LHS is of a certain economic feasibility, since a considerable reduction in daily operation costs can be achieved by utilizing time-of-use electricity rates. For example, electricity rates for general industry and commerce in suburban areas of Beijing in heating seasons are 1.32 (0.19), 0.80 (0.11), and 0.31 (0.04) RMB (USD dollar) kWhe 1 for peak, flat and valley periods, respectively. Thus, it can be roughly estimated that the average daily operation cost of such a heating system is slightly lower than that of an equivalent air-source heat pump heating system without heat storage. In addition, electricboiler heating systems can be completely installed indoors, for example, in basement, indicating a more convenient management of operation and maintenance, compared with heat pump heating systems. As a result, LHS using salt hydrates now has a certain market share in residential building heating. While the on-site heat dispatch are used for balancing the gap between heat supply and demand in time, off-site heat dispatchings focus more on solving the mismatch in space. Mobile thermal energy storage (M-TES), as an crucial part of the off-site heat dispatch, has been widely proposed in recent years [5,23,42,43]. Since the economic feasibility of M-TES is heavily sensitive to transportation efficiency per unit mass [44], the superiority in energy storage density of LHS over WTHS is compromised to some extent. Therefore, LHS using salt hydrates could only be economically viable for application scenarios (as shown in Fig. 4) with inexpensive heat resources (including discarded electricity, lowgrade industrial wast-heat and surplus solar thermal energy) and emergency heat demands regardless of costs (including heat demands in communal facilities and disaster areas, and during the first-aid repair of heat supply networks), at the present stage. Short-distance (for instance, within 20 km) automobile transportation in urban areas is strongly recommended for M-TES considering heat preservation and heating cost. Fig. 4 also draws a conceptual design of a plug-and-play container modular-type LHS
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We hope that the above perspectives can provide useful guidelines for the future material development, device design, system optimization and application scenario selection of latent heat storage using salt hydrates for building heating. Acknowledgement This work was supported by the National Key R&D Program of China (Grant No. 2016YFB0601204). This work was also a part supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51521004). References
Fig. 4. Potential application scenarios for mobile latent heat storage.
unit consisting of several well-insulated LHS storage modules, a electric-powered heating system and a pumping system. The storage unit is loaded on a electric truck so as to be thermally charged as the vehicle is electrically charged. 4. Conclusion The short-term latent heat storage using salt hydrates plays an important role in the heat dispatch of residential building heating, especially in urban areas At the current stage, this TES technology faces four aspects of barriers: (1) mature high-temperature salt hydrates for centralized building heating are lack of development, (2) thermal performance evaluation of salt hydrates is still imperfect for industrialization; (3) stronger heat transfer enhancement approaches are required for the improvement of overall performance; (4) poor cost competitiveness and uncertain costeffectiveness result in a vague market prospect. Perspectives targeting at these barriers are proposed as follows: (1) Explorations on salt hydrates with melting points ranging from 75 to 95 C are required to meet the heat dispatching demand of centralized building heating. Barium hydroxide octahydrate, magnesium nitrate hexahydrate and ammonium aluminum sulfate dodecahydrate, as well as some eutectics are potential storage media. (2) Issues like supercooling and phase separation of salt hydrates present different effects on the performance of a latent heat storage between lab-scale experiments and practical applications. Quantitative studies are further required to clear and definite the thermal performance evaluation of salt hydrates for practical applications. (3) The combination of multiple heat transfer enhancement approaches can further improve the overall performance of a latent heat storage using salt-hydrates. The multi-granularity packed-bed latent heat storage using high thermally conductive form-stable composite salt hydrates is one of promising latent heat storage concepts. (4) Latent heat storage using hydrated salts has a volume superiority over traditional normal pressure water tank storage in centralized heating and hot water supply, especially in densely populated regions with large peak-valley electricity rate differences for building heating. Short-distance automobile transportation in urban areas is recommended for MTES considering heat preservation and heating cost.
[1] Shan YL, Guan DB, Zheng HR, Ou JM, Li Y, Meng J, Mi ZF, Liu Z, Zhang Q. China CO2 emission accounts 1997-2015. Sci Data 2018;5:170201. [2] Dahash A, Ochs F, Janetti MB, Streicher W. Advances in seasonal thermal energy storage for solar district heating applications: a critical review on large-scale hot-water tank and pit thermal energy storage systems. Appl Energy 2019;239:296e315. [3] Amin M, Putra N, Kosasih EA, Prawiro E, Luanto RA, Mahlia TMI. Thermal properties of beeswax/graphene phase change material as energy storage for building applications. Appl Therm Eng 2017;112:273e80. [4] Vasu A, Hagos FY, Noor MM, Mamat R, Azmi WH, Abdullah AA, Ibrahim TK. Corrosion effect of phase change materials in solar thermal energy storage application. Renew Sustain Energy Rev 2017;76:19e33. [5] Lin Y, Alva G, Fang G. Review on thermal performances and applications of thermal energy storage systems with inorganic phase change materials. Energy 2018;165:685e708. [6] Nazir H, Batool M, Osorio FJB, Isaza-Ruiz M, Xu X, Vignarooban K, Kannan AM. Recent developments in phase change materials for energy storage applications: a review. Int J Heat Mass Transf 2019;129:491e523. [7] Kenisarin M, Mahkamov K. Salt hydrates as latent heat storage materials: Thermophysical properties and costs. Sol Energy Mater Sol Cells 2016;145: 255e86. [8] Liu Y, Xie M, Gao X, Yang Y, Sang Y. Experimental exploration of incorporating form-stable hydrate salt phase change materials into cement mortar for thermal energy storage. Appl Therm Eng 2018;140:112e9. [9] Xiao Q, Fan J, Li L, Xu T, Yuan W. Solar thermal energy storage based on sodium acetate trihydrate phase change hydrogels with excellent light-tothermal conversion performance. Energy 2018;165:1240e7. [10] Fang YT, Ding YF, Tang YF, Liang XH, Jin C, Wang SF, Gao XN, Zhang ZG. Thermal properties enhancement and application of a novel sodium acetate trihydrate-formamide/expanded graphite shape-stabilized composite phase change material for electric radiant floor heating. Appl Therm Eng 2019;150: 1177e85. [11] Mao JF, Dong X, Hou PM, Lian HL. Preparation research of novel composite phase change materials based on sodium acetate trihydrate. Appl Therm Eng 2017;118:817e25. [12] Wang Y, Yu KX, Peng H, Ling X. Preparation and thermal properties of sodium acetate trihydrate as a novel phase change material for energy storage. Energy 2019;167:269e74. [13] Liu CZ, Hu PB, Xu Z, Ma XT, Rao ZH. Experimental investigation on thermal properties of sodium acetate trihydrate based phase change materials for thermal energy storage. Thermochim Acta 2019;674:28e35. [14] Li TX, Xu JX, Wu DL, He F, Wang RZ. High energy-density and power-density thermal storage prototype with hydrated salt for hot water and space heating. Appl Energy 2019;248:406e14. [15] Hirschey J, Gluesenkamp KR, Mallow A, Graham S. Review of inorganic salt hydrates with phase change temperature in range of 5 to 60 C and material cost comparison with common waxes. In: International high performance buildings conference. Purdue university; July, 2018. [16] Brown PW, Grimes II JW, Kaetzel L. Evaluation of the variation in thermal performance in a Na2SO4$ 10H2O phase change system. Sol energ mat 1986;13:453e61. €nel T, Emons HH. Thermoanalytical investigation of so[17] Naumann R, Fangha dium acetate trihydrate for application as a latent heat thermal energy storage material. J Therm Anal 1988;33(3):685e90. [18] Al-Shannaq R, Kurdi J, Al-Muhtaseb S, Dickinson M, Farid M. Supercooling elimination of phase change materials (PCMs) microcapsules. Energy 2015;87:654e62. [19] Cabeza LF, Svensson G, Hiebler S, Mehling H. Thermal performance of sodium acetate trihydrate thickened with different materials as phase change energy storage material. Appl Therm Eng 2003;23:1697e704. [20] Singh RP, Kaushik SC, Rakshit D. Solidification behavior of binary eutectic phase change material in a vertical finned thermal storage system dispersed with graphene nano-plates. Energ Convers Manage 2018;171:825e38. [21] Xiao QQ, Zhang MD, Fan JX, Li L, Xu T, Yuan WH. Thermal conductivity enhancement of hydrated salt phase change materials employing copper
6
B.C. Zhao, R.Z. Wang / Energy 189 (2019) 116139
foam as the supporting material. Sol Energy Mater Sol Cells 2019;199:91e8. [22] Milian YE, Gutierrez A, Grageda M, Ushak S. A review on encapsulation techniques for inorganic phase change materials and the influence on their thermophysical properties. Renew Sustain Energy Rev 2017;73:983e99. [23] Xu ZY, Wang RZ, Yang C. Perspectives for low-temperature waste heat recovery. Energy 2019;176:1037e43. [24] Alawadhi EM. The design, properties, and performance of concrete masonry blocks with phase change materials. In: Eco-efficient masonry bricks and blocks. Woodhead Publishing; 2015. p. 231e48. [25] Wang Q, Wang JT, Chen YY, Zhao CY. Experimental investigation of barium hydroxide octahydrate as latent heat storage materials. Sol Energy 2019;177: 99e107. F, C [26] Fernandes D, Pitie aceres G, Baeyens J. Thermal energy storage: “How previous findings determine current research priorities”. Energy 2012;39: 246e57. [27] Zhang SL, Wu W, Wang SF. Experimental investigations of Alum/expanded graphite composite phase change material for thermal energy storage and its compatibility with metals. Energy 2018;161:508e16. [28] Fu WW, Zou T, Liang XH, Wang SF, Gao XN, Zhang ZG, Fang YT. Thermal properties and thermal conductivity enhancement of composite phase change material using sodium acetate trihydrateeurea/expanded graphite for radiant floor heating system. Appl Therm Eng 2018;138:618e26. [29] Xie SL, Sun JH, Wang Z, Liu S, Han LP, Ma GX, Jing Y, Jia YZ. A thermally stable phase change material with high latent heat based on an oxalic acid dihydrate/boric acid binary eutectic system. Sol Energy Mater Sol Cells 2017;168: 38e44. [30] Jones DRH, Ashby MF. Engineering materials 2: an introduction to microstructures, processing and design. Elsevier; 2005. [31] Li TX, Wu DL, He F, Wang RZ. Experimental investigation on copper foam/ hydrated salt composite phase change material for thermal energy storage. Int J Heat Mass Transf 2017:115148e57. [32] Zhou GB, Xiang YT. Experimental investigations on stable supercooling performance of sodium acetate trihydrate PCM for thermal storage. Sol Energy 2017;155:1261e72.
[33] Weinstein RD, Kopec TC, Fleischer AS, D'Addio E, Bessel CA. The experimental exploration of embedding phase change materials with graphite nanofibers for the thermal management of electronics. J Heat Transf 2008;130:042405. [34] Li TX, Lee JH, Wang RZ, Kang YT. Enhancement of heat transfer for thermal energy storage application using stearic acid nanocomposite with multiwalled carbon nanotubes. Energy 2013;55:75261. [35] Singh RP, Kaushik SC, Rakshit D. Solidification behavior of binary eutectic phase change material in a vertical finned thermal storage system dispersed with graphene nano-plates. Energ Convers Manage 2018;171:825e38. [36] Li D, Yang CX, Yang H. Experimental and numerical study of a tube-fin cool storage heat exchanger. Appl Therm Eng 2019;149:712e22. [37] Zhao BC, Cheng MS, Liu C, Dai ZM. Thermal performance and cost analysis of a multi-layered solid-PCM thermocline thermal energy storage for CSP tower plants. Appl Energy 2016;178:784e99. [38] Mahdi JM, Nsofor EC. Solidification enhancement in a triplex-tube latent heat energy storage system using nanoparticles-metal foam combination. Energy 2017;126:501e12. [39] Li WQ, Wan H, Lou HJ, Fu YL, Qin F, He GQ. Enhanced thermal management with microencapsulated phase change material particles infiltrated in cellular metal foam. Energy 2017;127:671e9. [40] Khalili A, Matyka M, Mohammadi RM, Weise J, Kuypers MM. Porosity variation within a porous bed composed of multisized grains. Powder Technol 2018;338:830e5. [41] Barrington-Leigh C, Baumgartner J, Carter E, Robinson BE, Tao S, Zhang YX. An evaluation of air quality, home heating and well-being under Beijing's programme to eliminate household coal use. Nat Energy 2019;4(5):416. [42] Tay NHS, Liu M, Belusko M, Bruno F. Review on transportable phase change material in thermal energy storage systems. Renew Sustain Energy Rev 2017;75:264e77. L, Gasia J, Cabeza LF. Thermal energy storage (TES) for industrial waste [43] Miro heat (IWH) recovery: a review. Appl Energy 2016;179:284e301. [44] Guo SP, Zhao J, Wang WL, Yan J, Jin GY, Wang XT. Techno-economic assessment of mobilized thermal energy storage for distributed users: a case study in China. Appl Energy 2017;194:481e6.
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Perspectives for short-term thermal energy storage using salt hydrates for building heating B.C. Zhao, R.Z. Wang* Institute of Refrigeration and Cryogenics, MOE Engineering Research Center of Solar Energy, Shanghai Jiao Tong University, Shanghai, 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2019 Received in revised form 10 September 2019 Accepted 14 September 2019 Available online 17 September 2019
In this forward-looking perspective, the current research status of latent heat storage using salt hydrates for building heating are firstly analyzed from aspects of material development, performance evaluation, heat transfer enhancement and application feasibility. Based on the analysis, barriers for the further promotion of this technology, including narrow application range, imperfect performance evaluation, inefficient heat transfer enhancement and vague market prospect are outlined. To address these issues, perspectives on four aspects are provided. First, further explorations on salt hydrates with high melting points meeting the heat dispatching demand of centralized building heating are strongly recommended. Second, effects of supercooling and phase separation of salt hydrates in practical applications should be considered differently from lab-scale experiments. Third, the combination of multiple heat transfer enhancement approaches can further improve the overall performance of heat storage. Fourth, this technology has a certain prospect in high-temperature heat dispatching in densely populated areas and short-distance mobile heat supply for emergency. The above perspectives provide guidelines for the future material development, device design, system optimization, and application scenario selection of latent heat storage using salt hydrates for building heating. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Thermal energy storage Building heating Salt hydrate Performance evaluation Heat transfer enhancement Application feasibility
1. Introduction The global building heating demand grows rapidly with the promotion of people's living level during past decades. It is reported that the contribution of residential coal burning has exceeded the combination of transportation and power generation on the production of PM2.5 in northern China [1]. Comparing with fossil fuels, industrial waste-heat, renewable energy and electric energy are cleaner and more efficient. However, these heating sources are naturally mismatched in time or space with the demand of building heating. In this case, thermal energy storage (TES) is often used to regulate the supply-demand gap [2]. TES is classified into a long-term and short-term type, considering storage duration. The long-term TES aims to provide thermal energy dispatch over months or even seasons (for example, based on seasonal variations of solar radiation), so it is usually of large scale and thus regionally restricted, especially in densely populated areas. In comparison, the short-term TES performing diurnal heat
* Corresponding author. E-mail address: [email protected] (R.Z. Wang). https://doi.org/10.1016/j.energy.2019.116139 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
regulation (for example, based on peak-valley electricity rates) has a higher operation applicability and deployment flexibility, and therefore received increasing attention in recent years. Among a variety of short-term TES patterns, latent heat storage (LHS) using salt hydrates [3e6] presents a better comprehensive performance in heat storage density, operation controllability, structural compactness and capital cost, which are all highly valued by residential building heating. Investigations and reviews on salt hydrates for the short-term TES have been reported in different aspects of property characterization of developed materials [3,7], development and modification of new materials [8e12], design and optimization of latent heat storage devices [13,14] and economic evaluation of commercial applications [7,15]. Nevertheless, rare study focuses on forwardlooking issues of this technology from a macro perspective of practical building heating application. For instance, there still lacks of mature salt hydrates (or relevant composites) for various building heating scenarios. Besides, common issues like supercooling and phase separation still have potential negative effects on the performance and cycling life of the TES. Moreover, current heat transfer enhancement approaches are still required to be improved in aspects of scalability and versatility. In addition to the above-
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mentioned internal issues, LHS using salt hydrates also faces with external challenges such as poor cost competitiveness against water tank heat storage (WTHS) and uncertain economic effectiveness under current cost levels of thermal resources. All these issues and challenges lead to difficulties in the widespread application of LHS using salt hydrates for building heating. To address these issues, the paper firstly briefly reviews the current study status of the short-term LHS using salt hydrates. Then the main barriers that prevent this TES technology from building heating application are proposed. Finally, several corresponding perspectives are presented from aspects of material development, performance evaluation, heat transfer enhancement, and application feasibility. 2. Current research statues and barriers Salt hydrates make use of the reversible separation and combination of water molecules with inorganic salts to achieve a repeatable storage and release of latent heat. Among various hydrated salts, sodium sulfate decahydrate [16] and sodium acetate trihydrate [17] (SAT) are the most representative ones used for passive building thermal management and distributed building heating, respectively. However, salt hydrates with higher melting temperatures for the heat dispatch of centralized residential building heating are still rare. Natural salt hydrates often suffers from problems of supercooling and phase separation that have negative effects on the discharging stability and cycling life of TES. Although these issues have been suppressed to a great extent by means of adding specific nucleating [18] and thickening agents [19] in lab-scale experiments, there is still a lack of corresponding studies on industrial applications characterized by large-scale (counted based on the heating demand of an entire building) and long-period (counted based on years). Heat transfer enhancement is also a key technology of LHS using salt hydrates. A variety of approaches such as adding additives with high thermal conductivities [20], building form-stable PCM composites using high thermally conductive frameworks [21], and micro/macro PCM encapsulations [22] have been reported. However, further improvements on energy storage density and cost effectiveness are still required in terms of application. Compared with water tank storage that has been widely used for building heating, LHS using salt hydrates seems to have no advantage except a remarkable energy storage density. The cost competitiveness of this TES technology against WTHS may still be weak due to the extra cost of PCMs. Besides, the cost effectiveness of this TES technology is uncertain under current cost levels of thermal resources [23]. Both of the capital cost and application scenario of this TES technology are still lack of investigations. According to the present literature review, four aspects of barriers preventing the short-term LHS using salt hydrates from largescale application in residential building heating are highlighted as below: (1) The development of high-temperature salt hydrates for centralized heat dispatch are under-researched. (2) There exist gaps between scientific research and industrialization in evaluating the thermal performance of salt hydrates. (3) The current heat transfer enhancement methods still limits the storage density and cost-effectiveness of this TES technology. (4) Poor cost competitiveness against WTHS and uncertain cost effectiveness result in an uncertain market positioning of this TES technology.
Detailed descriptions of these barriers and the corresponding perspectives will be presented in the next section. 3. Future perspectives 3.1. Development of high-temperature salt hydrates for centralized heat dispatch A TES integrated into a building heating system is expected to be capable of satisfying heating demands independently when routine heating sources are unavailable, which indicates that it should be able to provide a thermal output meeting the inlet temperature requirement of terminal heaters (for instance, 50e60 C for radiator heating, 40e55 C for underfloor heating, and 35e45 C for fan-coil heating). Accordingly, the required exothermic temperature (i.e. the melting temperature in ideal conditions) of salt hydrates used for different building heating systems can be generally determined, considering the loss of heat transfer (5e10 C for different heat transfer structures) and transportation (0e5 C for distributed heating and 15e25 C for centralized heating). Fig. 1 draws estimated PCM melting temperature ranges (represented by bars with different colors) suitable for different heating terminals including distributed fan-coil heating (D-F), distributed underground heating (D-U), distributed radiator heating (D-R), centralized fan-coil heating (CeF), centralized underground heating (CeU) and centralized radiator heating (C-R). One can easily draw a preliminary concusion on whether a PCM is appropriate for the heat dispatch of these heating terminals by means of checking whether the line representing the melting temperature of the PCM has an intersection with these bars. It is clearly observed that SAT with a melting point of 58 C is only suitable for the distributed heating application scenarios. We also screened out several PCM candidates, including barium hydroxide octahydrate (BHO), magnesium nitrate hexahydrate (MNH) and ammonium aluminum sulfate dodecahydrate (ASSD) for centralized building heating, referring to the selection principles [6,24] of salt hydrates as heat storage media. Although these PCMs are well matched with the application scenarios with relatively high heating temperatures, unfortunately, they also have some fatal drawbacks that limit their large-scale application potentials at the present stage. For instance, BHO is highly toxic and relatively costly [25], MNH is explosion-risky and its latent heat is relatively low [26], and ASSD is highly corrosive and has a large
Fig. 1. Potential salt hydrates for building heating in various application scenarios.
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supercooling degree [27]. Further investigations on these PCMs are required. In addition to directly looking for appropriate natural salt hydrates, adjusting the melting point of salt hydrates by means of eutectics [28] is also a practicable option to obtain desired PCMs. An eutectic salt hydrate has a lower melting temperature than its principal material. This can be utilized to achieve an artificial downregulation of the melting point of hydrated salts. As recently reported in a literature [29], adding a certain amount of boric acid (10e12%) into oxalic acid dihydrate can obtain a binary eutectic material that has a melting point of 87.3 C, which can be considered as a potential PCM for the heat dispatch of centralized building heating. 3.2. Performance evaluation of salt hydrates in view of TES application The essence of supercooling phenomenon of a solid-liquid PCM is the non-crystallization of the material as its temperature drops below the theoretical crystallization temperature [30] (i.e. the onset temperature of melting), which means that the latent heat would not be released at the melting point of the PCM. Generally, supercooling is considered to have adverse effects on both thermal grade and stability of the discharge of a LHS. However, the degree of this effect varies with different working conditions. Test-tube experiments are often used in laboratory investigations to evaluate the transient discharge performance of hydrated salts, for high experimental efficiency and repeatability. In such experiments, temperature variations of salt hydrates with space are negligible. A typical state evolution of a hydrated salt with supercooling during its exothermic process is drawn in Fig. 2a, where the melted, supercooled and crystallized state of the PCM are represented by red, blue and orange, respectively. Many researchers take the temperature difference between phase II and III as an apparent supercooling degree [24,31,32], and make efforts in mitigating or eliminating this temperature difference. Actually, the apparent supercooling observed in test-tube
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experiments may not have a noticeable negative effect on the output of a storage device with large spatial dimensions, due to the asynchronous solidification of PCMs. Taking a LHS employing fincoil heat exchanger to achieve heat transfer between PCM and heat transfer fluid (HTF) as an example, as shown in Fig. 2b, when the PCM (Point C) near the HTF entrance begin to crystallize at some instant of discharge (corresponding to Phase III in Fig. 2a), the PCM (Point B) located in the middle zone may be going through a supercooling process (corresponding to Phase II in Fig. 2a), and the PCM (Point A) near the HTF exit may still be in a melted state (corresponding to Phase I in Fig. 2a), due to a long flow path. The HTF extracts thermal energy from the PCM with multiple states along the flow path. As a result, the outlet HTF temperature may present a smooth decline during the discharge process, since temperature fluctuations of HTF caused by PCM supercooling can be tempered along the flow path to some extent as shown in the bottom subplot of Fig. 2b. This indicates that a certain supercooling degree is acceptable for hydrate salts used in large-scale LHS devices. In the future, more studies are required to quantify the influence of the supercooling degree of salt hydrates on the thermal performance of LHS devices. Another concern about the industrialization of the short-term LHS using salt hydrates is its practical cycling life, which is related to the phase separation control of salt hydrates. It is known that adding thickening agents, for example, carboxyl methyl cellulose (CMC) into salt hydrates to form a suspension liquid is an effective approach to suppress the phase separation of hydrated salts with low water solubility [31]. However, the degradation of thickening effect is inevitable during a long-period and largespatial-scale operation, due to the aggregation and stratification of thickening agents caused by convection. As a result, the industrial feasibility of salt hydrates as short-term storage media cannot be simply proved by stable performances observed in test-tube experiments. In the future, more investigations on the effectiveness of thickening agents in practical working conditions are required to quantify the performance degradation of LHS devices.
Fig. 2. Comparison of the discharge process of LHS between lab-scale experiments and industrial applications.
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3.3. Efficient heat transfer intensification for LHS using salt hydrates Although salt hydrates have a relatively higher thermal conductivity than organic PCMs, effective heat transfer enhancement on a LHS device using such PCMs is still highly required in practical applications. One method is to directly improve the thermal conductivity of PCMs by means of adding high thermally conductive additives such as graphite nanofibers [33], carbon nanotubes [34] and graphene nano-plates [35]), or introducing high thermally conductive frameworks such as foam metal [21] and compressed expanded graphite [10] to build form-stable composite phase change materials (FS-CPCMs). While the added additives often suffers from particle aggregation and precipitation that deteriorate the effectiveness of heat transfer enhancement, FS-CPCMs are of more stable structures and can provide much greater enhancing effect with the aid of heat conduction channels. However, the introduction of framework materials is always accompanied by a significant decrease in energy storage density either in mass or volume. Besides, the fixed shape of FS-CPCMs also increases the difficulty in assembling a storage device. An alternative approach is to enlarge the heat transfer area between PCMs and HTFs. To achieve this goal, fin-tube heat exchanger [36] (as shown in Fig. 2b) is conventionally used for its high technical maturity and low cost. However, it has drawbacks of low extendability, poor corrosion resistance and high leakage risk. In comparison, the packed-bed LHS [37] based on the macroencapsulation of PCMs is of more flexible deployment, easier leakage control and higher HTF compatibility (allows both gas and liquid HTFs), though it has some advantages including low volume ratio of PCMs, costly encapsulation and poor discharge performance in both thermal power and capacity factor, at the current stage. The combined utilization of the above-described heat transfer enhancement approaches, for instance, metal foams with nanoparticle additives [38] or micro-encapsulation of PCMs [39], can further improve the overall performance of a LHS. Herein, we also propose a new LHS concept, namely multi-granularity packed-bed LHS using high thermally conductive FS-CPCM, for residential building heating. As illustrated in Fig. 3, the LHS consists of a packed-bed randomly stacked by a great number of spherical expanded graphite/salt hydrate FS-CPCM capsules with several specific sizes [40]. By taking advantages of a high thermally conductive compressed-graphite framework and a large heat transfer area provided by the porous region of the packed-bed, heat transfer between the PCM and HTF can be enhanced to a great
Fig. 3. Conceptual design of a multi-granularity packed-bed LHS based on high thermally conductive FS-CPCM.
extent. Moreover, the low porosity of the packed-bed also ensures a high volumetric filling ratio of the PCM. However, the efficient preparation of such FS-CPCMs is still a major technical challenge, since expanded graphite is hydrophobic. 3.4. Promising application scenarios for LHS using salt hydrates LHS using salt hydrates has been considered as a substitution for traditional WTHS in building heating for years, due to its much larger energy storage density. Taking a developed SAT-based composite PCM [31] as an example. The ideal volumetric energy storage density of this PCM within a conventional operation temperature range for residential building heating (i.e. 40e80 C) is around 550 MJ m 3, which is about 3.3 times of water (167 MJ m 3). In fact, the advantage of such a LHS in volumetric energy storage density increases with the decrease in the operation temperature range. It can be calculated that the volumetric energy storage density of the above-mentioned PCM is even around 5.6 times of water as the operation temperature range drops from 40 C to 20 C. Based on this, LHS using hydrated salts can perform a much greater volume superiority over normal pressure WTHS in conditions that a relatively high-grade (over 80 C, for example) heat storage is required, because normal pressure WTHS has a ceiling operation temperature of 95 C. Accordingly, more attention should be paid to develop high-temperature salt hydrates for centralized space heating and hot water supply. Benefiting from a higher volumetric energy storage density, LHS using salt hydrates has a higher installation area saving potential than traditional WTHS, which is particularly crucial for densely populated regions where land is short and expensive. With the support of ‘coal to electricity’ policy [41] announced by the Chinese government in 2016, electric-boiler heating integrating with a LHS is of a certain economic feasibility, since a considerable reduction in daily operation costs can be achieved by utilizing time-of-use electricity rates. For example, electricity rates for general industry and commerce in suburban areas of Beijing in heating seasons are 1.32 (0.19), 0.80 (0.11), and 0.31 (0.04) RMB (USD dollar) kWhe 1 for peak, flat and valley periods, respectively. Thus, it can be roughly estimated that the average daily operation cost of such a heating system is slightly lower than that of an equivalent air-source heat pump heating system without heat storage. In addition, electricboiler heating systems can be completely installed indoors, for example, in basement, indicating a more convenient management of operation and maintenance, compared with heat pump heating systems. As a result, LHS using salt hydrates now has a certain market share in residential building heating. While the on-site heat dispatch are used for balancing the gap between heat supply and demand in time, off-site heat dispatchings focus more on solving the mismatch in space. Mobile thermal energy storage (M-TES), as an crucial part of the off-site heat dispatch, has been widely proposed in recent years [5,23,42,43]. Since the economic feasibility of M-TES is heavily sensitive to transportation efficiency per unit mass [44], the superiority in energy storage density of LHS over WTHS is compromised to some extent. Therefore, LHS using salt hydrates could only be economically viable for application scenarios (as shown in Fig. 4) with inexpensive heat resources (including discarded electricity, lowgrade industrial wast-heat and surplus solar thermal energy) and emergency heat demands regardless of costs (including heat demands in communal facilities and disaster areas, and during the first-aid repair of heat supply networks), at the present stage. Short-distance (for instance, within 20 km) automobile transportation in urban areas is strongly recommended for M-TES considering heat preservation and heating cost. Fig. 4 also draws a conceptual design of a plug-and-play container modular-type LHS
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We hope that the above perspectives can provide useful guidelines for the future material development, device design, system optimization and application scenario selection of latent heat storage using salt hydrates for building heating. Acknowledgement This work was supported by the National Key R&D Program of China (Grant No. 2016YFB0601204). This work was also a part supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51521004). References
Fig. 4. Potential application scenarios for mobile latent heat storage.
unit consisting of several well-insulated LHS storage modules, a electric-powered heating system and a pumping system. The storage unit is loaded on a electric truck so as to be thermally charged as the vehicle is electrically charged. 4. Conclusion The short-term latent heat storage using salt hydrates plays an important role in the heat dispatch of residential building heating, especially in urban areas At the current stage, this TES technology faces four aspects of barriers: (1) mature high-temperature salt hydrates for centralized building heating are lack of development, (2) thermal performance evaluation of salt hydrates is still imperfect for industrialization; (3) stronger heat transfer enhancement approaches are required for the improvement of overall performance; (4) poor cost competitiveness and uncertain costeffectiveness result in a vague market prospect. Perspectives targeting at these barriers are proposed as follows: (1) Explorations on salt hydrates with melting points ranging from 75 to 95 C are required to meet the heat dispatching demand of centralized building heating. Barium hydroxide octahydrate, magnesium nitrate hexahydrate and ammonium aluminum sulfate dodecahydrate, as well as some eutectics are potential storage media. (2) Issues like supercooling and phase separation of salt hydrates present different effects on the performance of a latent heat storage between lab-scale experiments and practical applications. Quantitative studies are further required to clear and definite the thermal performance evaluation of salt hydrates for practical applications. (3) The combination of multiple heat transfer enhancement approaches can further improve the overall performance of a latent heat storage using salt-hydrates. The multi-granularity packed-bed latent heat storage using high thermally conductive form-stable composite salt hydrates is one of promising latent heat storage concepts. (4) Latent heat storage using hydrated salts has a volume superiority over traditional normal pressure water tank storage in centralized heating and hot water supply, especially in densely populated regions with large peak-valley electricity rate differences for building heating. Short-distance automobile transportation in urban areas is recommended for MTES considering heat preservation and heating cost.
[1] Shan YL, Guan DB, Zheng HR, Ou JM, Li Y, Meng J, Mi ZF, Liu Z, Zhang Q. China CO2 emission accounts 1997-2015. Sci Data 2018;5:170201. [2] Dahash A, Ochs F, Janetti MB, Streicher W. Advances in seasonal thermal energy storage for solar district heating applications: a critical review on large-scale hot-water tank and pit thermal energy storage systems. Appl Energy 2019;239:296e315. [3] Amin M, Putra N, Kosasih EA, Prawiro E, Luanto RA, Mahlia TMI. Thermal properties of beeswax/graphene phase change material as energy storage for building applications. Appl Therm Eng 2017;112:273e80. [4] Vasu A, Hagos FY, Noor MM, Mamat R, Azmi WH, Abdullah AA, Ibrahim TK. Corrosion effect of phase change materials in solar thermal energy storage application. Renew Sustain Energy Rev 2017;76:19e33. [5] Lin Y, Alva G, Fang G. Review on thermal performances and applications of thermal energy storage systems with inorganic phase change materials. Energy 2018;165:685e708. [6] Nazir H, Batool M, Osorio FJB, Isaza-Ruiz M, Xu X, Vignarooban K, Kannan AM. Recent developments in phase change materials for energy storage applications: a review. Int J Heat Mass Transf 2019;129:491e523. [7] Kenisarin M, Mahkamov K. Salt hydrates as latent heat storage materials: Thermophysical properties and costs. Sol Energy Mater Sol Cells 2016;145: 255e86. [8] Liu Y, Xie M, Gao X, Yang Y, Sang Y. Experimental exploration of incorporating form-stable hydrate salt phase change materials into cement mortar for thermal energy storage. Appl Therm Eng 2018;140:112e9. [9] Xiao Q, Fan J, Li L, Xu T, Yuan W. Solar thermal energy storage based on sodium acetate trihydrate phase change hydrogels with excellent light-tothermal conversion performance. Energy 2018;165:1240e7. [10] Fang YT, Ding YF, Tang YF, Liang XH, Jin C, Wang SF, Gao XN, Zhang ZG. Thermal properties enhancement and application of a novel sodium acetate trihydrate-formamide/expanded graphite shape-stabilized composite phase change material for electric radiant floor heating. Appl Therm Eng 2019;150: 1177e85. [11] Mao JF, Dong X, Hou PM, Lian HL. Preparation research of novel composite phase change materials based on sodium acetate trihydrate. Appl Therm Eng 2017;118:817e25. [12] Wang Y, Yu KX, Peng H, Ling X. Preparation and thermal properties of sodium acetate trihydrate as a novel phase change material for energy storage. Energy 2019;167:269e74. [13] Liu CZ, Hu PB, Xu Z, Ma XT, Rao ZH. Experimental investigation on thermal properties of sodium acetate trihydrate based phase change materials for thermal energy storage. Thermochim Acta 2019;674:28e35. [14] Li TX, Xu JX, Wu DL, He F, Wang RZ. High energy-density and power-density thermal storage prototype with hydrated salt for hot water and space heating. Appl Energy 2019;248:406e14. [15] Hirschey J, Gluesenkamp KR, Mallow A, Graham S. Review of inorganic salt hydrates with phase change temperature in range of 5 to 60 C and material cost comparison with common waxes. In: International high performance buildings conference. Purdue university; July, 2018. [16] Brown PW, Grimes II JW, Kaetzel L. Evaluation of the variation in thermal performance in a Na2SO4$ 10H2O phase change system. Sol energ mat 1986;13:453e61. €nel T, Emons HH. Thermoanalytical investigation of so[17] Naumann R, Fangha dium acetate trihydrate for application as a latent heat thermal energy storage material. J Therm Anal 1988;33(3):685e90. [18] Al-Shannaq R, Kurdi J, Al-Muhtaseb S, Dickinson M, Farid M. Supercooling elimination of phase change materials (PCMs) microcapsules. Energy 2015;87:654e62. [19] Cabeza LF, Svensson G, Hiebler S, Mehling H. Thermal performance of sodium acetate trihydrate thickened with different materials as phase change energy storage material. Appl Therm Eng 2003;23:1697e704. [20] Singh RP, Kaushik SC, Rakshit D. Solidification behavior of binary eutectic phase change material in a vertical finned thermal storage system dispersed with graphene nano-plates. Energ Convers Manage 2018;171:825e38. [21] Xiao QQ, Zhang MD, Fan JX, Li L, Xu T, Yuan WH. Thermal conductivity enhancement of hydrated salt phase change materials employing copper
6
B.C. Zhao, R.Z. Wang / Energy 189 (2019) 116139
foam as the supporting material. Sol Energy Mater Sol Cells 2019;199:91e8. [22] Milian YE, Gutierrez A, Grageda M, Ushak S. A review on encapsulation techniques for inorganic phase change materials and the influence on their thermophysical properties. Renew Sustain Energy Rev 2017;73:983e99. [23] Xu ZY, Wang RZ, Yang C. Perspectives for low-temperature waste heat recovery. Energy 2019;176:1037e43. [24] Alawadhi EM. The design, properties, and performance of concrete masonry blocks with phase change materials. In: Eco-efficient masonry bricks and blocks. Woodhead Publishing; 2015. p. 231e48. [25] Wang Q, Wang JT, Chen YY, Zhao CY. Experimental investigation of barium hydroxide octahydrate as latent heat storage materials. Sol Energy 2019;177: 99e107. F, C [26] Fernandes D, Pitie aceres G, Baeyens J. Thermal energy storage: “How previous findings determine current research priorities”. Energy 2012;39: 246e57. [27] Zhang SL, Wu W, Wang SF. Experimental investigations of Alum/expanded graphite composite phase change material for thermal energy storage and its compatibility with metals. Energy 2018;161:508e16. [28] Fu WW, Zou T, Liang XH, Wang SF, Gao XN, Zhang ZG, Fang YT. Thermal properties and thermal conductivity enhancement of composite phase change material using sodium acetate trihydrateeurea/expanded graphite for radiant floor heating system. Appl Therm Eng 2018;138:618e26. [29] Xie SL, Sun JH, Wang Z, Liu S, Han LP, Ma GX, Jing Y, Jia YZ. A thermally stable phase change material with high latent heat based on an oxalic acid dihydrate/boric acid binary eutectic system. Sol Energy Mater Sol Cells 2017;168: 38e44. [30] Jones DRH, Ashby MF. Engineering materials 2: an introduction to microstructures, processing and design. Elsevier; 2005. [31] Li TX, Wu DL, He F, Wang RZ. Experimental investigation on copper foam/ hydrated salt composite phase change material for thermal energy storage. Int J Heat Mass Transf 2017:115148e57. [32] Zhou GB, Xiang YT. Experimental investigations on stable supercooling performance of sodium acetate trihydrate PCM for thermal storage. Sol Energy 2017;155:1261e72.
[33] Weinstein RD, Kopec TC, Fleischer AS, D'Addio E, Bessel CA. The experimental exploration of embedding phase change materials with graphite nanofibers for the thermal management of electronics. J Heat Transf 2008;130:042405. [34] Li TX, Lee JH, Wang RZ, Kang YT. Enhancement of heat transfer for thermal energy storage application using stearic acid nanocomposite with multiwalled carbon nanotubes. Energy 2013;55:75261. [35] Singh RP, Kaushik SC, Rakshit D. Solidification behavior of binary eutectic phase change material in a vertical finned thermal storage system dispersed with graphene nano-plates. Energ Convers Manage 2018;171:825e38. [36] Li D, Yang CX, Yang H. Experimental and numerical study of a tube-fin cool storage heat exchanger. Appl Therm Eng 2019;149:712e22. [37] Zhao BC, Cheng MS, Liu C, Dai ZM. Thermal performance and cost analysis of a multi-layered solid-PCM thermocline thermal energy storage for CSP tower plants. Appl Energy 2016;178:784e99. [38] Mahdi JM, Nsofor EC. Solidification enhancement in a triplex-tube latent heat energy storage system using nanoparticles-metal foam combination. Energy 2017;126:501e12. [39] Li WQ, Wan H, Lou HJ, Fu YL, Qin F, He GQ. Enhanced thermal management with microencapsulated phase change material particles infiltrated in cellular metal foam. Energy 2017;127:671e9. [40] Khalili A, Matyka M, Mohammadi RM, Weise J, Kuypers MM. Porosity variation within a porous bed composed of multisized grains. Powder Technol 2018;338:830e5. [41] Barrington-Leigh C, Baumgartner J, Carter E, Robinson BE, Tao S, Zhang YX. An evaluation of air quality, home heating and well-being under Beijing's programme to eliminate household coal use. Nat Energy 2019;4(5):416. [42] Tay NHS, Liu M, Belusko M, Bruno F. Review on transportable phase change material in thermal energy storage systems. Renew Sustain Energy Rev 2017;75:264e77. L, Gasia J, Cabeza LF. Thermal energy storage (TES) for industrial waste [43] Miro heat (IWH) recovery: a review. Appl Energy 2016;179:284e301. [44] Guo SP, Zhao J, Wang WL, Yan J, Jin GY, Wang XT. Techno-economic assessment of mobilized thermal energy storage for distributed users: a case study in China. Appl Energy 2017;194:481e6.