Optically Triggered Synchronous Heat Release of Phase‐Change Enthalpy and Photo‐Thermal Energy in Phase‐Change Materials at Low Temperatures
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Abstract Phase‐change materials (PCMs) are used in several energy recycling utilization systems due to their latent‐heat‐storage and ‐release ability. However, the inability of PCMs to release heat at temperatures below their freezing point limits their application in distributed energy utilization systems. This paper reports optically‐triggered low‐temperature heat release in PCMs based on a solid–liquid phase change (PC) controlled by the trans – cis ( E – Z ) photo‐isomerization of azobenzene. To achieve this, a photo‐responsive alkyl‐grafted Azo is incorporated into tetradecane (Ted) to create a photo‐sensitive energy barrier for the PC. The Azo/Ted composite exhibits controllable supercooling (4.04–8.80 °C) for heat storage and achieves synchronous heat release of PC enthalpy and photo‐thermal energy. In addition, the Azo reduces the crystallization of Ted by intercalating into its molecular alignment. Furthermore, under light illumination, the Azo/Ted composite releases considerable heat (207.5 J g −1 ) at relatively low temperatures (−1.96 to −6.71 °C). The temperature of the annular device fabricated for energy utilization increases by 4 °C in a low‐temperature environment (−5 °C). This study will pave the way for the design of advanced distributed energy systems that operate by controlling the energy storage/release of PCMs over a wide range of temperatures.Keywords:
Supercooling
Azobenzene
Thermal energy
Atmospheric temperature range
The increasing energy consumption in the building sector enforces development in the field. One aims to cut the link between energy production and consumption, in order for environmental energy supply more of the demand and produce the energy as efficient as possible. In TEK 10 it is stated that minimum 60 % of the total energy demand must be supplied by other energy sources than electricity for buildings of more than 500 m2 floor area. The main goal of this thesis was to assess different possibilities for thermal energy storage in buildings. Different storage technologies and materials apply. Water is the most common substance used for sensible thermal energy storage. Water is cheap, easy accessible and has excellent thermal properties for thermal storage. Rock and heavy building fabric is other materials that could be applied for sensible energy storage. When a material freeze it liberates heat to the surroundings, therefore possibilities for thermal energy storage in Phase Change Materials are of interest. These materials have good latent properties and desirable melting point temperatures. The most common phase change materials are organic, inorganic and eutectics. Around one hundred commercially available PCMs with a melting temperature in the range of 0 ℃ to 100 ℃ were identified. The latent heat of fusion of these materials was found to vary from 100 to 300 kJ/kg. An assessment of these materials showed that the salt hydrates had the highest latent heat of fusion. On the other hand, water has been known to diffuse through the capsule leading to incongruent melting of the PCM. This could lead to degradation of the system performance after numerous cycles. The most common technologies for thermal storage are the one utilizing water stored in a tank supplying the heating or cooling system in the building. Similar systems could be applied using PCMs, however a heat transfer fluid must supply the energy to the load in the building. A model for thermal energy storage was developed in this thesis. Both a model for both thermal stratification of water in a tank and for storage of energy in Phase Change Materials was developed. A given storage capacity was used to simulate a varying thermal load profile in a building. The overall goal of this program was to level the load profile. The model was applied to three buildings, one school, one office buildings and a building used for hotel, residential units and stores. Heating storage was assessed for two buildings while one was assessed for cooling purposes. Actual load profiles at the design outdoor temperature was assessed. The simulations presented showed that the power demand in the school building could be reduced by 38.8% and 36.4% for the office building. The reduction was achieved using water storage of respectively 30 and 25 m3. For the building where cooling storage were assessed, the reduction in the maximum power demand was reduced by 56.7 % from 300 kW to 130 kW when utilizing a chilled water storage of 30 m3.Chilled water storage are a challenge as the maximum density of water occurs at 4 ℃ and the difference in density for temperature close to this are small. This applies for high demand for proper diffusor design in the tank to prevent mixing of water at different temperature level. Proper stratification in the tank is important to maintain the highest possible supply temperature for the longest possible time period. The effect of proper stratification was assessed for a tank of 20,000 L with a fixed volume flow discharged. The result presented showed that the energy supplied from the tank increased with 42.2 % when applying 16 temperature zones compared to a fully mixed tank. The improvement of the tank was decreasing for the increasing number of stratification zones. Different solutions utilizing PCM was assessed. It was presented that the volume of the storage were decreasing when PCM was installed instead of water. One proposed solution indicated that the when Phase Change Materials were applied for a space heating purposes assuming a temperature difference of 10 ℃, an organic PCM was reducing the storage volume by 2.3 times while a salt hydrate reduced the storage volume by 5.3 times. This assessment indicated that salt hydrates are the most energy intensive of the PCM possibilities. Different geometries for the encapsulated PCMs determine the heat transfer to the heat transfer fluid (in many cases water). Three different geometries were assessed: cubic, cylindrical and spherical encapsulations. Applying the same tank volume, the same volume of the PCMs, same volume flow of the heat transfer fluid and the same heat transfer coefficient the spherical capsule would generate 40 % more heat for the heat transfer fluid than the cubic geometry and 24 % more than the cylindrical capsule. There are some weaknesses in the PCM model. It was assumed that the temperature in the tank was uniform. This will not apply for the real case where the heat transfer fluid temperature will increase while transferring through the tank. For a realistic case, the temperature of the first elements will decrease rapidly because of large temperature difference between the heat transfer fluid and the PCMs in the tank.
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Characterization of Alkanes and Paraffin Waxes for Application as Phase Change Energy Storage Medium
Abstract Latent thermal energy storage is one of the favorable kinds of thermal energy storage methods considered for renewable energy source utilization, as in solar photothermal systems. Heat is stored mostly by means of the latent heat of phase change of the medium. The temperature of the medium remains more or less constant during the phase transition. A large number of materials have been identified for low, intermediate, and high operating temperatures for application as latent thermal energy storage media. In the present paper a method for characterization of alkanes (C1,-C100) and paraffin waxes for application as the low-temperature (298-323 K) phase change energy storage medium is introduced. A computational technique is introduced by which the alkanes and paraffin waxes could be evaluated, and possibly upgraded, as the phase change energy storage media. It is demonstrated that the family of n-alkanes has a large spectrum of latent heats, melting points, densities, and specific heats so that the heat storage designer has a good choice of n-alkanes as storage materials for any particular low-temperature thermal energy storage application. As an example of the proposed method, a particular paraffin wax for which appropriate experimental data are available is analyzed and the results of the analysis are presented.
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Salt hydrate phase change materials used for thermal storage in space heating and cooling applications have low material costs, but high packaging costs. A more economic installed storage may be possible with medium priced, high latent heat. Latent heat storage is one of the most efficient ways of storing thermal energy. Unlike the sensible heat storage method, the latent heat storage method provides much higher storage density, with a smaller temperature difference between storing and releasing heat. This paper work on latent heat storage and provides an insight to recent efforts to develop new classes of phase change materials (PCMs) for
use in energy storage. There are large numbers of phase change materials that melt and solidify at a wide range of temperatures, making them attractive in a number of applications. Hydrated salts have larger energy storage density and higher thermal conductivity but experience super cooling and phase segregation, and hence, their application requires the use of some nucleating and thickening agents. Sodium carb-onate, sodium phosphate and sodium sulfate tested as phase change material by crystallization in this work.
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Exergy efficiency
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Because of the unstable and intermittent nature of solar energy availability, a thermal energy storage system is required to integrate with the collectors to store thermal energy and retrieve it whenever it is required. Thermal energy storage not only eliminates the discrepancy between energy supply and demand but also increases the performance and reliability of energy systems and plays a crucial role in energy conservation. Under this paper, different thermal energy storage methods, heat transfer enhancement techniques, storage materials, heat transfer fluids, and geometrical configurations are discussed. A comparative assessment of various thermal energy storage methods is also presented. Sensible heat storage involves storing thermal energy within the storage medium by increasing temperature without undergoing any phase transformation, whereas latent heat storage involves storing thermal energy within the material during the transition phase. Combined thermal energy storage is the novel approach to store thermal energy by combining both sensible and latent storage. Based on the literature review, it was found that most of the researchers carried out their work on sensible and latent storage systems with the different storage media and heat transfer fluids. Limited work on a combined sensible-latent heat thermal energy storage system with different storage materials and heat transfer fluids was carried out so far. Further, combined sensible and latent heat storage systems are reported to have a promising approach, as it reduces the cost and increases the energy storage with a stabilized outflow of temperature from the system. The studies discussed and presented in this paper may be helpful to carry out further research in this area.
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The latent heat thermal energy storage system using phase change material (PCM) is quite attractive, mainly due to their high energy storage density and their ability to provide heat in a narrow temperature range. A latent heat thermal energy storage unit is designed, which is installed conveniently and have a fast period of charge and discharge. The latent heat thermal energy storage unit is applied to a demonstration project in conjunction with solar collectors and tank. Results showed that the designed unit in this paper eases the large fluctuations of water temperature in tank and make the system work more stably. The latent heat thermal energy storage unit plays a significant role in project.
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