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[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] This thesis, which consists of eleven papers and reports, deals with nuclear waste repositories in solid rock and with aquifer thermal energy storage systems. All these storage systems induce multidimensional, time-variable thermo-hydro-elastic processes in the ground in and around the storage region. The partial differential equations that govern the physical processes are solved analytically in some cases, and in other cases numerical models are developed. Many methods of classical mathematical physics are employed for the solution. The analytical approach provides a deeper physical understanding of the processes and their interactions. At large depths, the salinity of groundwater, and hence its density, often increases downwards. In the first study, the upward buoyancy flow of groundwater in fracture planes due to heat release from the nuclear waste is studied considering the added effect of a salt gradient. The aim of the study is to determine the natural barrier effect caused by the salt. A simple formula for the largest upward displacement from the repository is derived. There may be a strong natural barrier, which is independent of fracture permeabilities. In two papers, the temperature field in rock due to a large rectangular grid of heat-releasing canisters containing nuclear waste is studied. The solution is by superposition divided into different parts. There is a global temperature field due to the large rectangular canister area, while a local field accounts for the remaining heat source problem. A complete analytical solution is presented. In the next set of papers, the thermoelastic response from the rectangular field of nuclear waste is analysed. Another study concerns the use of heat as a tracer to investigate flow in a fracture plane. Two papers deal with the thermohydraulic evaluations of two aquifer thermal energy storage projects in southern Sweden. Both plants have been successfully simulated using models based on conformal flow and entropy conservation techniques. In the last paper, the conformal flow technique is presented together with the very useful thermal front tracking model
[en] The PULSAR design study is evaluating the pulsed, inductively driven tokamak power plant to assess whether economies can be attained which-are more favorable than those of the steady-state, non-inductively driven tokamak. Considerations of market acceptance and component fatigue lead to the requirement of thermal energy storage (TES) to maintain steady-state power during the cyclic interruptions of fusion power production (open-quotes dwell phaseclose quotes). A major focus of the Study has been to identify and design technically viable TES systems for helium-cooled and liquid lithium self-cooled plants which are economically attractive, safe and environmentally benign. Several basic constraints impact the selection of the TES system. The system must be capable of discharging 2.5 GW during a dwell phase of approximately two minutes (determined by systems code analysis), thus have a capacity of at least 300 GJ. Coolant must be discharged from the TES at the static, burn phase temperature to minimize thermal stress in the steam generator. Several TES options were evaluated: Storage of heat transport working fluid, phase change media and sensible heat storage. Sensible heat storage in the outer shield was selected for PULSAR
[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] In the present study, to determine the flow rate of droplets supplied to heat transfer surface after (j-1) th rebound, Dx[j≥2]*, it was assumed that the rebound droplets are distributed according to the Gaussian distribution from 0 to L, in which the flight distance L is determined by maximum flight distance Lmax. We also assumed that Lmax is dependent on the air flow velocity and mean size of droplets. The local heat flux of a dilute spray in high temperature region was predicted using the newly evaluated Dx[j≥2]*. In addition, the predicted results by the present model were compared with the existing experimental data
[en] The model presented here analyzes solar district heating systems on the basis of the power supplied at the grid feeding point. Consumption patterns are taken into account only in the form of different preset load curves. Processes are selected in consideration of the following aspects: (1) The design of a solar district heating system (collector surface, storage volume) depends on the expected contribution of solar power to electricity supply. For each of the key years 1989, 2005 and 2020, a low, average and high contribution were investigated, from which design concepts for other supply rates can be derived. (2) Yields and economic efficiency of solar systems also depend on collector sites and consumption patterns. 10 variants each with low and very high contributions of solar power were calculated for the key year 2020. (orig.)
[de]Die Behandlung solarer Nahwaermesysteme beschraenkt sich hier auf die Energiebereitstellung am Netzeinspeisepunkt. Die Verbraucher werden nur durch die Vorgabe unterschiedlicher Lastverlaeufe beruecksichtigt. Die Auswahl der Prozesse erfolgte unter der Beruecksichtigung folgender Gesichtspunkte: Die Auslegung des solaren Nahwaermesystems (Kollektorflaeche, Speichervolumen) ist stark vom angestrebten solaren Deckungsanteil abhaengig. Fuer die Stuetzjahre 1989, 2005 und 2020 wurden je ein geringer, ein mittlerer und ein hoher solarer Deckungsanteil untersucht. Auslegungen fuer andere Deckungsanteile koennen dann naeherungsweise interpoliert werden. Der Aufstellungsort der Kollektoren und das Bedarfsprofil der Verbraucher beeinflussen ebenfalls Ausbeute und Wirtschaftlichkeit der Solarsysteme. Fuer das Stuetzjahr 2020 wurden je 10 Varianten mit geringem und sehr hohem solaren Deckungsanteil gerechnet. (orig./VA)
[en] The feasibility of using solar energy instead of electricity for air conditioning and refrigeration has been investigated as part of a trend to explore sustainable and low-carbon technologies that minimize the use of fossil fuels. Solar desiccant cooling systems provide direct air conditioning for indoor spaces using solar energy, with the added benefit of full outdoor air provision. The system can handle the space sensible and latent loads while achieving good indoor air quality. This paper proposed an effective optimization method for a solar desiccant cooling system developed by a simulation model. The parameters for suitable operation and control were determined. The objective of optimization is to maximize the solar fraction against the involvement of auxiliary electric heating. This is a constrained optimization problem since the room temperature should be maintained within a comfortable range. As such, traditional numerical optimization methods cannot be used. However, the evolutionary algorithm (EA) can effectively handle the constrained, nonlinear and multidimensional engineering problems such as those encountered in this solar desiccant cooling system. This study included a comparative evaluation of the 3 major paradigms of the EA in an effort to evaluate a suitable approach for the current engineering application with demanding computational function calls. A model of the solar desiccant cooling system was built and its year-round performance was maximized. 17 refs., 2 tabs., 10 figs
[en] Highlights: • A new index, Savings per energy unit, is defined to assess the effectiveness of CTES. • CTES systems were used to perform demand management strategies, removing partial load operations and shaving peak loads. • CTES was used to perform price arbitrage, exploiting the difference between peak and off peak electricity rates in Singapore. • Results showed that it is possible to enhance the efficiency of the whole system, achieving both energy and economic savings. • Depending on the sizing scenario, the pay back periods ranged from a minimum of 8.9 years to a maximum of 16 years. - Abstract: This paper investigates the feasibility of Cold Thermal Energy Storage (CTES) for building demand management applications in hot climate characterized by a cooling season lasting all year long. An existing office building, located in Singapore, serves as case study. The CTES is coupled to the existing cooling systems in order to address the opportunity of improving overall energy efficiency and to perform price arbitrage, exploiting the spread between peak and off-peak energy tariffs. Six different sizes for the CTES are analyzed, addressing different percentage of the daily cooling energy demand. A new index, Savings per energy unit, is defined to assess the effectiveness of CTES. Results indicate that it is possible to enhance the efficiency of the whole cooling system, achieving both energy and economic savings. The payback periods of the different solutions range from a minimum of 8.9 years to a maximum of 16 years. All these aspects make CTES applications a viable option. However, a large amount of space in direct proximity to the building is necessary and, especially in largely urban environment, this is not always available.
[en] Highlights: • Paraffin based PCM’s have been noted to undergo irreversible physical change with time changing their thermal performance. • The melting point and latent heat of RT21 mixture change while kept at elevated temperatures. • Ester based PCM have shown to have superior thermo-physical characteristics with reduced fire hazardous. - Abstract: Thermal energy storage using phase change materials (PCMs) have been a focal point in the efficient energy utilisation in buildings for over 30 years. The possible use of PCM in buildings is becoming more and more attractive due to the large energy storage density and nearly isothermal nature of the PCM storage when compared to sensible heat storage. Although there is large amount of information available on this topic, literatures show very little information about the long term thermal performance of phase change materials. In this research, changes in thermal characteristics of two commercial organic PCMs when exposed to a constant temperature above their melting point were examined. The thermal characteristic (i.e. melting range and latent heat of fusion) of Rubitherm 21 (RT21, a paraffin mixture) and propyl stearic and palmitate mixture with a melting point in the range of 18–25 °C were tested after an exposure to storage temperatures of 30 and 55 °C. The results obtained indicated that the paraffin based mixture such as RT21 experienced a significant irreversible physical change with time. The data collected and analysis indicated a shift in the peak melting point from 21 to 28 °C and increase in latent heat of fusion from 134 to 170 J/g over a period of 120 days when kept at a temperature of 55 °C. On the other hand, the mixed esters experienced almost no change in mass due to their lower vapour pressure
[en] This article presents experimental analysis on performance augmentation of a single hole cored brick regenerator using turbulence inducers. Experiments were carried out for different velocities with air as the working fluid for both charging and discharging processes of a 455 mm long aluminum regenerator with inner and outer diameters of 26 mm and 40 mm, respectively. Two numbers of turbulence inducers of 1.5 mm diameter and 13 mm long were placed in ten different combinations and the results were compared with the trials wherein no inducers were used. The mean temperature of the cored brick, exit temperature during discharge, ratio of heat transfer rate to pressure drop, and exergetic efficiencies are the characteristics that were used to study the performance of the regenerator. Placement of inducers increased the mean temperature of the regenerator and the ratio of heat transfer rate to pressure drop by about 15% and a maximum of 40%, respectively, during charging. The exit air temperature during discharge exhibited maximum improvement of 18%. Increased exergetic efficiencies of more than 10% and 5% were estimated for charging and discharging, respectively. It was also observed that the addition of inducers does not necessarily result in an increased performance, and some of the combinations in fact deteriorated the performance of the regenerator.