Thermal energy storage

Résumé

Thermal energy storage (TES) is achieved with widely different technologies. Depending on the specific technology, it allows excess thermal energy to be stored and used hours, days, months later, at scales ranging from the individual process, building, multiuser-building, district, town, or region. Usage examples are the balancing of energy demand between daytime and nighttime, storing summer heat for winter heating, or winter cold for summer air conditioning (Seasonal thermal energy storage). Storage media include water or ice-slush tanks, masses of native earth or bedrock accessed with heat exchangers by means of boreholes, deep aquifers contained between impermeable strata; shallow, lined pits filled with gravel and water and insulated at the top, as well as eutectic solutions and phase-change materials. Other sources of thermal energy for storage include heat or cold produced with heat pumps from off-peak, lower cost electric power, a practice called peak shaving; heat from comb

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https://en.wikipedia.org/wiki/Thermal_energy_storage

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Peak load levelling through peak shaving and valley ﬁlling is need of the hour in the electricity grid network. A reserve plant or a large scale electric energy storage plant could be the way out to handle the variable load situations. Though the re- serve plants have its limitations and often increase the load on the transmission and grid system, rather than reducing the load during off peak hours. A novel technique of site-independent energy storage is studied in this thesis work, which do have more positives about balancing the electrical grid. While, proﬁtability of the energy stor- age is lie on very thin margins of peak and off peak hour spot electricity market price. The considered thermal storage operates in a heat-pump mode during the charg- ing period and in a thermal-engine mode during discharging period. The idea is to utilise the design concept to integrate with the district energy services, while com- peting with the conventional district services. The design modiﬁcations has been evaluated based on the efficiency performance and the a proﬁtability studyis performed.

2012

Chargement

Energy Integration of Industrial Batch Processes (PinchBATCH) - Phase 2

Pierre Krummenacher

A comparative study has demonstrated that the formerly developed Pinch Analysis (PA) based targeting and design methods outperform (in both heat recovery and cost-effectiveness) the simplified combinatorial design methods proposed by other authors (namely the Omnium Verfahren and the Permutation Method). The PA based methods provide valuable insight in the problem, but aren’t suitable for automated design and optimization. PinchBATCH results therefore in the following advances: 1) a strategy (including guidelines and quantitative criterias) is proposed to restrict the analysis of batch processes to meaningful time slices. Likely variations of the schedule are taken into account; 2) practical constrains limiting the repipe and resequence of heat exchangers are identified and formalized under a matrix format. The formalization of all these constraints is needed for automated design methods to actually focus on feasible solutions; 3) a strategy based on genetic algorithms (GAs) is proposed for the synthesis of direct batch heat exchangers networks (i.e. without heat storage). The strategy features a decomposition into an upper level, which optimizes the overall network, and a lower level which search for the best use of the heat exchangers across time slices; 4) for indirect heat recovery using heat storage, a heuristic, PA inspired approach for targeting and simplified design is proposed and demonstrated. It uses TAM composites and includes a method to identify the minimum number of heat storage units (HSUs) as function of the heat recovery. Most generally, local minimums are to be found just before a storage pinch appears, and the search strategy systematically removes one bottleneck at a time, taking advantage of the available degrees of freedom. A simple graphical tool to assess opportunities to decrease the required capacity of HSUs by rescheduling of streams is proposed. A model suitable for a GA based optimization is also proposed, featuring a decomposition into two levels; 5) detailed specifications of the organization and of the features to be implemented in a software tool to address the heat integration of batch processes have been produced. The very next steps will be concerned with the implementation and the validation of the GA based synthesis approaches.

1999

Chargement

Thermal energy storage using the effects of the latent heat in the solid-liquid phase transformation has found its way into every day applications. Reusable, organic phase change media stabilize low temperatures of parcels, and salt/water tanks act as buffer storage in residential heating/cooling and industrial processes. Emerging, renewable technologies for energy production and conversion profit from high operation temperatures. Alloys from abundant metals such as aluminum, silicon, and copper have suitable melting characteristics to preheat chemical conversion catalysts and process gas streams for heat-to-power applications. They typically absorb and release a large heat of fusion, facilitate better heat transport compared to salts, and are inert to thermal decomposition compared to organic compounds. Fundamental challenges emerge from the high reactivity of molten metals towards the preferred metal encapsulation materials. Stable encapsulations are necessary to protect the storage medium, and to preserve a constant heat exchange surface during the phase transition. This thesis explores the fabrication of stainless steel capsules for use with pure aluminum, as well as Al-12%Si and Al-26%Cu-5%Si alloys. Experimental capsules of cylindrical shape were prepared with or without interface modification, and tested by isothermal exposure up to 800 °C and in a newly developed test-bench with 100+ melting cycles and in-situ performance measurement. Iron aluminide formation on the interior capsule surface reduced performance. Interface modifications and corrosion layers were characterized by scanning electron microscopy, energy dispersive x-ray spectrometry, differential scanning calorimetry, and Vickers hardness micro-indentation. Results were integrated into numerical diffusion and heat transfer modelling, allowing predictions about the effects of atomic diffusivity, solubility, and thermal conductivity on heat storage performance. Ceramic diffusion barrier coatings for stabilized performance were deposited via modified reverse-dip-coating on steel encapsulation and silicon substrates, and characterized using the same methodology. Ceramic coatings and guidelines for conservative operation conditions were effective in stabilizing heat storage density and power. Boron nitride coating was demonstrated to prevent 90 % of the capacity loss occurring in the control group, in a scaled up pilot experiment with 200 MJ latent heat capacity in encapsulated Al-26%Cu-5%Si. Temperature scanning calorimetric data from the cycling test-bench validated numerical heat transfer simulations and performance predictions from diffusion modelling. Results from validated heat and diffusion modelling were used in the design of a 1.4 MJ lab-scale prototype storage, with automated charging and discharging of stacked encapsulated samples, using air as heat transfer fluid. Herein presented guidelines allow design and operation of robust steel encapsulations, which conserve high performance for 10¿000s of full-load-hours. Our experimental data and interpretative modelling focuses on a relevant material class, that combines low material cost with high conductivity, large capacity, and melting temperatures relevant to advanced power cycles. Experiments on lab-, prototype- and pilot-scale demonstrate how our findings contribute to practical implementation of novel highly performant metal latent heat thermal energy storage systems.

EPFL2019

Chargement

Energy Integration of Industrial Batch Processes (PinchBATCH) - Phase 2

Pierre Krummenacher

1999

EPFL2019