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The focus of this project is the storage of thermal energy in packed beds for bulk electricity storage applications. Packed beds are composed of pebbles through which a heat transfer fluid passes, and a thermodynamic model of the heat transfer processes within the store is described. The packed beds are investigated using second law analysis which reveals trade-offs between several heat transfer processes and the importance of various design parameters. Parametric studies of the reservoir behaviour informs the design process and leads to a set of design guidelines. Two innovative design features are proposed and investigated. These features are segmented packed beds and radial-flow packed beds respectively. Thermal reservoirs are an integral component in a storage system known as Pumped Thermal Energy Storage (PTES). To charge, PTES uses a heat pump to create a difference in internal energy between two thermal stores; one hot and one cold. The cycle reverses during discharge with PTES operating as a heat engine. The heat pumps/engines require compression and expansion devices, for which simple models are described and are integrated with the packed bed models. The PTES system behaviour is investigated with parametric studies, and alternative design configurations are explored. A multi-objective genetic algorithm is used to undertake thermo-economic optimisations of packed-bed thermal reservoirs and PTES systems. The algorithm generates a set of optimal designs that illustrate the trade-off between capital cost and round-trip efficiency. Segmentation is found to be particularly beneficial in cold stores, and can add up to 1% to the round-trip efficiency of a PTES system. On the basis of the assumptions made, PTES can achieve efficiencies and energy densities comparable with other bulk electricity storage systems. However, the round-trip efficiency is very sensitive to the efficiency of the compression–expansion system. For designs that utilised bespoke reciprocating compressors and expanders, PTES might be expected to achieve electricity-to-electricity efficiencies of 64%. However, using compression and expansion efficiencies typical of off-theshelf devices the round-trip efficiency is around 45%.

abstract: For decades, load shifting control, one of the most effective peak demand management methods, has attracted attention from both researchers and engineers. Various load shifting controls have been developed and introduced in mainly commercial buildings. Utility companies typically penalize consumers with “demand rates”. This along with increased population and increased customer energy demand will only increase the need for load shifting. There have been many white papers, thesis papers and case studies written on the different types of Thermal Energy Storage and their uses. Previous papers have been written by Engineers, Manufacturers and Researchers. This thesis paper is unique because it will be presented from the application and applied perspective of the Facilities Manager. There is a need in the field of Facilities Management for relevant applications. This paper will present and discuss the methodology, process applications and challenges of load shifting using (TES) Thermal Energy Storage, mainly ice storage. / Dissertation/Thesis / Masters Thesis Construction 2016

Sweden is only utilizing half of the available excess heat. To utilize more of the excess heat a seasonal thermal energy storage could be implemented to store excessed heat from the summer when the demand is lower to the winter when the demand is higher. This can be achieved by an integration of a seasonal thermal energy storage to the district heating system. A seasonal thermal energy storage may also reduce the need of the system’s peak load, which often is economically costly and adversely affect the environment. The purpose of the paper is to investigate the possibility for Skövde Värmeverk to implement a seasonal thermal storage. The paper is performed by a literature collection and calculations are made by software programs. The result shows that it is technically possible to implement a pit thermal energy storage and a borhole thermal energy storage, but no outcome shows a profitability within 20 years. A pit thermal energy storage can replace the system’s peak load up to 79 percent and a borhole thermal energy storage up to 2,8 percent. The most suitable case for Skövde Värmeverk is to install a pit thermal energy storage with a storage capacity of 4 GWh.

One of the main challenges in the implementation of renewable energy is the mismatch between supply and demand. Energy storage has been identified as one of the solutions to the mismatch problem. Among various storage technologies, thermal energy storage (TES) is foreseen to have a significant role to achieve a low carbon energy systems because of the large share of thermal energy demand and its relatively low cost. However, integrating TES into energy systems requires careful design and implementation since otherwise potential financial and environmental savings may not be achieved. Computational-based design tools are ubiquitous in the design process of modern energy systems and can be broadly categorised into two methodologies: optimisation and simulation. In both cases, designing an energy system with storage technology is significantly more complicated than those without, mainly due to the coupling of variables between time steps. This thesis is concerned with two facets of the application of TES in energy systems. First, the role of TES in improving the performance of renewable-based domestic heating systems. Second, the implementation of optimisation and simulation tools in the design of energy systems with integrated TES. They are addressed by examining two case studies that illustrate the spatial and temporal variance of energy systems: a single dwelling heat pump system with a hot water tank, and a solar district heating system with a borehole thermal energy storage. In the single dwelling case study, the technical and financial benefits of TES installation in a heat pump system are illustrated by the optimisation model. A simulation model which utilises the optimisation results is developed to assess the accuracy of the optimisation results and the potential interaction between the two methodologies. The solar district heating case study is utilised to highlight the potential of a time decomposition technique, the multiple time grids method, in reducing the computational time in the operational optimisation of the system. Furthermore, the case study is also employed to illustrate the potential of installing a similar system in the UK. The latter study was performed by developing a validated simulation model of the solar district heating system. The findings of the analyses reported in this thesis exemplify the potential of TES in a domestic and community-level heating system in the UK. They also provide a basis for recommendations on the improved use of optimisation and simulation tools in the design process of energy systems.

Thermal energy storage systems, precisely latent thermal energy storage (LTES), are systems capable of recovering and storing thermal energy from waste processes, including hot exhaust gases out of combustion engines, or even renewable sources of energy like solar energy. LTES rely on phase change materials (PCMs) to store a significant amount of thermal energy in a relatively small volume. With limited volume and at almost constant temperature, they are capable of storing a large amount of thermal energy, mainly latent energy. Studies of LTES systems have focused primarily on system and process optimization including transient behavior as well as field performance. A major drawback in the development of the use of PCM in LTES has been the low thermal conductivity characteristic of most PCMs. Thus, there is a need to enhance heat transfer using reliable techniques, with the goal of reducing the charging and discharging times of PCM in LTES systems. Some approaches that have been studied in the past include use of finned tubes, insertion of metal matrix into PCM, and microencapsulation of PCM. The performance of TES configurations in forced convection have been characterized using Reynolds numbers (Re), and Stefan numbers (Ste) of the heat transfer fluid (HTF) for different enhancement techniques. The goal of this study is to experimentally investigate the effectiveness of corrugated PCM panels with high surface-to-volume ratio in forced convection as a function of HTF mass flow rate, charging temperature, and flow direction through a corrugated TES unit. The PCM (octadecane) has been segmented using sealed corrugated panels containing several channels immersed in the HTF stream. With this approach, the author expects that the charging and discharging times will be substantially reduced due to the high surface-to-volume ratio of the PCM panel for heat transfer. Of the three conditions examined, the HTF direction influenced the charging and discharging times the most with significant reductions in these times observed when the HTF flow direction through the TES was upwards. Buoyancy effects, observed at high Stefan numbers, were important during the charging (melting) process and greatly influenced the temperature profiles along each channel. Results indicate that the devised TES is more effective than some other TES systems in the literature.

<p>  </p> <p>Thermal energy storage (TES) is used to reduce the operating costs of heating, ventilation, and air conditioning (HVAC) systems by shifting loads away from on-peak periods, to reduce the maximum heating or cooling capacity needed from the HVAC system, and to store excess energy generated by on-site solar power. The most commonly-used form of TES is ice storage with air conditioning (A/C) systems in commercial buildings. There has been extensive research into many other forms of TES for use with HVAC systems, both in commercial and residential buildings. However, this research is often limited to use with either heating or cooling systems.</p> <p>Year-round, high-density storage for both heating and cooling would yield significantly larger cost savings than existing TES systems, particularly for residential buildings, where heating loads are often larger than cooling loads. This dissertation examines the feasibility of using metal hydrides for year-round storage, as well as analyzing the potential of variable-temperature energy storage for optimizing system performance beyond allowing for year-round use.</p> <p>Metal hydrides are metals that exothermically absorb and endothermically desorb hydrogen. Since the temperature this reaction occurs at depends on the hydrogen pressure, hydrides can be used for energy storage at varying temperatures. System architecture for using metal hydrides with an HVAC system is developed. A thermodynamic model which combines a dynamic model of the hydride reactors with a static model of the HVAC system is used to calculate operating costs, compared to a conventional HVAC system, for different utility rates and locations. The payback period of the system is unacceptably high, due to the high initial cost of metal hydrides and the operating costs of compressing hydrogen to move it between hydride reactors.</p> <p>In addition to the metal hydride system model, a generalized model of a variable-temperature TES system is used to determine the potential cost savings from dynamically altering the storage temperature to achieve optimal cost savings. Dynamic tuning does result in cost savings but is most effective for storage tank sizes significantly smaller than the optimal tank size. An alternate system design where the storage tank is charged with the outlet flow from the house achieves larger cost savings even for the optimally-sized tanks. Payback periods calculated for optimal sizing show that year-round storage has a lower payback period than separate cold and heat storage if the year-round storage system is not more expensive than two separate storage tanks. </p>

Master of Science / Department of Mechanical Engineering / Donald Fenton / The need to store and access electronic information is growing on a daily basis as more and more people conduct business and personal affairs through email and the internet. To meet these demands, high energy density data centers have sprung up across the United States and around world. To ensure that vital data centers run constantly, proper cooling must be maintained to prevent overheating and possible server damage from occurring. Emergency cooling systems for such systems typically utilize traditional batteries, backup generator, or a combination thereof. The electrical backup provides enough power to support cooling for essential components within the data centers. While this method has shown to be reliable and effective, there are several other methods that provide reliable emergency cooling at a fraction of the cost. This paper address the lack of information regarding the initial, operation, and maintenance costs of using Thermal Energy Storage (TES) tanks for emergency cooling. From research and various field examples, five emergency cooling system layouts were designed for various peak cooling loads. Looking at the different cooling loads, components, and system operations an economic evaluation of the system over a 20 year period was conducted. The economic analysis included the initial and maintenance costs of each system. In an effort to better understand power consumption of such systems and to help designer’s better estimate the long term costs of TES tanks systems, five layouts were simulated through a program called TRNSYS developed for thermal systems. To compare against current systems in place, a benefit to cost ratio was done to analyze TES versus a comparable UPS. The five simulated systems were one parallel pressurized tank, one parallel and one series atmospheric tank, one parallel low temperature chilled water, and one series ice storage tank. From the analysis, the ice storage and pressurized systems were the most cost effective for 1 MW peak cooling loads. For 5 MW peak cooling loads the ice storage and chilled water systems were the most cost effective. For 15 MW peak loads the chilled water atmospheric TES tanks were the most cost effective. From the simulations we concluded that the pressurized and atmospheric systems consumed the least amount of power over a 24 hour period during a discharge and recharge cycle of the TES tank. From the TRNSYS simulations, the ice storage system consumed 22 – 25% more energy than a comparable chilled water system, while the low temperature storage system consumed 6 – 8% more energy than the chilled water system. From the benefit-cost-ratio analysis, it was observed that all systems were more cost effective than a traditional battery UPS system of comparable size. For the smaller systems at 1 MW the benefit-cost-ratio ranged between 0.25 to 0.55, while for larger systems (15 MW) the ratio was between 1.0 to 3.5 making TES tanks a feasible option for providing emergency cooling for large and small systems.

Thermal energy storage (TES) systems play a crucial part in the success of concentrated solar power as a reliable thermal energy source. The economics and operational effectiveness of TES systems are the subjects of continuous research for improvement, in order to lower the localized cost of energy (LCOE). This study investigates the use of spherical tanks and their role in sensible heat storage in liquids. In the two tank system, typical cylindrical tanks were replaced by spherical tanks of the same volume and subjected to heat loss, stress analysis, and complete tank cost evaluation. The comparison revealed that replacing cylindrical tanks by spherical tanks in two tank molten salt storage systems could result in a 30% reduction in heat loss from the wall, with a comparable reduction in total cost. For a one tank system (or thermocline system), a parametric computational fluid dynamic (CFD) study was performed in order to obtain fluid flow parameters that govern the formation and maintenance of a thermocline in a spherical tank. The parametric study involved the following dimensionless numbers: Re (500-7500), Ar (0.5-10), Fr (0.5-3), and Ri (1-100). The results showed that within the examined range of flow characteristics, the inlet Fr number is the most influential parameter in spherical tank thermocline formation and maintenance, and the largest tank thermal efficiency in a spherical tank is achieved at Fr = 0.5. Experimental results were obtained to validate the CFD model used in the parametric study. For the flow parameters within the current model, the use of an eddy viscosity turbulence model with variable turbulence intensity delivered the best agreement with experimental results. Overall, the experimental study using a spherical one tank setup validated the results of the CFD model with acceptable accuracy.