Thermal energy storage for environmental energy supply
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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.