[en] Short communication

[en] Highlights: • The application of PCM in building was experimentally studied. • Weather forecast data were used in order to improve the energy saving. • The importance of an accurate weather forecast was investigated. • An electrical saving up to 90% per day was achieved. • Successful peak load shifting was achieved. - Abstract: This study experimentally investigated the application of weather forecasting in combination with the price-based control method for solar passive buildings. Two identical lightweight test huts were used for the experimental study, one finished with ordinary gypsum board and the other finished with PCM-impregnated gypsum boards. Based on the experimental results, the application of weather forecast data showed significant energy saving when PCM is used. In some days, an electrical energy saving up to 90% per day was achieved using the proposed method. The results also showed that the application of inaccurate weather forecasts can significantly deteriorate performance of the control system and even lead to more energy consumption in the PCM hut.

[en] 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 reviews previous 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. Three aspects have been the focus of this review: PCM materials, encapsulation and applications. 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. Paraffin waxes are cheap and have moderate thermal energy storage density but low thermal conductivity and, hence, require large surface area. Hydrated salts have larger energy storage density and higher thermal conductivity but experience supercooling and phase segregation, and hence, their application requires the use of some nucleating and thickening agents. The main advantages of PCM encapsulation are providing large heat transfer area, reduction of the PCMs reactivity towards the outside environment and controlling the changes in volume of the storage materials as phase change occurs. The different applications in which the phase change method of heat storage can be applied are also reviewed in this paper. The problems associated with the application of PCMs with regards to the material and the methods used to contain them are also discussed

[en] Thermal energy storage is a technology under investigation since the early 1970s. Since then, numerous new applications have been found and much work has been done to bring this technology to the market. Nevertheless, the materials used either for latent or for sensible storage were mostly investigated 30 years ago, and the research has lead to improvement in their performance under different conditions of applications. In those years a significant number of new materials were developed in many fields other than storage and energy, but a great effort to characterize and classify these materials was done. Taking into account the fact that thousands of materials are known and a large number of new materials are developed every year, the authors use the methodology for materials selection developed by Prof. Ashby to give an overview of other materials suitable to be used in thermal energy storage. Sensible heat storage at temperatures between 150 and 200 C is defined as a case study and two different scenarios were considered: long term sensible heat storage and short term sensible heat storage. (author)

[en] A model has been solved in order to determine the thermal losses of a storage tank, where thermal energy is stored as sensible heat of a diathermic fluid at medium temperatures. A parametric analysis has been performed in order to evaluate the influence of various design parameters on the thermal discharge of the accumulator

[en] Short communication

[en] Highlights: • Optimum melting temperature match in air pre-heater with multi-stage LHS unit is derived with entransy analysis. • Entransy dissipation rate in air pre-heater with LHS unit is discussed and compared with that in the case of SHS unit. • It is concluded that air pre-heater with multi-stage LHS unit is favored, from the viewpoint of entransy analysis. • Entransy dissipation is always reduced in the case of multi-stage LHS unit, constraining heat storage amount the same. - Abstract: Boiler air pre-heater with multi-stage LHS unit is investigated with entransy analysis. Optimum melting temperature match is derived. It is concluded that optimum melting temperature decreases alongside flow direction of flue gas. Optimum melting temperature at inlet increases with stage number, while that at outlet decreases. Temperature difference of PCMs in two nearby portions is the same but decreases with stage number. Entransy dissipation in LHS unit is compared with that in LHS unit. It is concluded that normalized entransy dissipation is not always reduced in LHS unit in the whole period, due to the decrease of heat transfer rate in SHS unit. However, entransy dissipation is always reduced in LHS unit, except single-stage unit, when heat storage amount is constrained. Reduction degree increases with stage number. Therefore, air pre-heater with multi-stage LHS unit is favored for recovery of waste heat in flue gas and improvement of boiler efficiency.

[en] Thermal energy storage is a key technology for reduced cost solar thermal power generation. This high-temperature application requires storage operation above 100 °C. Possible options are sensible, latent and thermochemical heat storages. A combination of sensible and latent heat storage seems a promising option for thermal energy storage with an increased specific heat capacity. Salt mixtures with a melting range as opposed to a melting point combine the effects of both latent and sensible heat storage. These provide the possibility of utilizing not only latent but in addition sensible heat during the melting and solidification process. The present paper focuses on a binary mixture of 30 wt.% potassium nitrate (KNO₃) and 70 wt.% sodium nitrate (NaNO₃). The measurement systems include a differential scanning calorimeter, a melting point apparatus, a custom-built adiabatic calorimeter and a lab-scale storage unit. The sample masses ranged from about 20 mg to 156 kg. Tests with the lab-scale storage unit indicate that salt mixtures with a melting range may be successfully utilized in large-scale applications. -- Highlights: • Enhanced capacities are possible by using anhydrous salt mixtures in the melting range. • As a suitable binary salt mixture was sodium nitrate and potassium nitrate identified. • A distribution of the enthalpy of fusion within the melting range was determined. • Cyclic stabilities of the selected salt mixture were measured. • The effect of dimension on the cyclic stability was examined in various test rigs

[en] Seasonal storage for solar energy have been the object of intensive theoretical studies and practical experiments. Within the frame of the research programme of the International Energy Agency 11 objects were set up and tested in practice. It was the aim of this research and demonstration projects to examine thermal long term storage for the use of solar energy and waste heat from industrial processes. In the following the actual state of the art of long term storage of solar energy for district heating in buildings will be described and possibilities of its use in Austria will be assessed. (author)

[en] Highlights: ► Carbon nanofiber (CNF)/paraffin wax composite is found to be a promising electric thermal storage material. ► The thermal storage capacity of CNF/paraffin wax composite is five times of traditional electric thermal storage material. ► CNF is shown to be an effective conductive filler for the composite. - Abstract: The research of electric thermal storage (ETS) has attracted a lot of attention recently, which converts off-peak electrical energy into thermal energy and release it later at peak hours. In this study, new electric thermal storage composites are developed by employing paraffin wax as thermal storage media and carbon nanofiber (CNF) as conductive fillers. Electric heating and thermal energy release performances of the CNF/paraffin wax composites are experimentally investigated. Experimental results show that, when the composites are heated to about 70 °C, the developed electrically conductive CNF/paraffin wax composites present a thermal storage capacity of about 280 kJ/kg, which is five times of that of traditional thermal storage medium such as ceramic bricks (54 kJ/kg). The CNF/paraffin wax composites can also effectively store the thermal energy and release the thermal energy in later hours.