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The increase of the capacity factor of thermal processes which use renewable energies is closely linked to the implementation of thermal energy storage (TES) systems. Currently, TES systems can be classified depending on the technology for storing thermal: sensible heat, latent heat, and sorption and chemical reactions (usually known as thermochemical energy storage). However, there is no standardized procedure for the evaluation of such technologies, and therefore the development of performance indicators which suit the requisites of the final users becomes an important goal. In the present paper, the authors identified the energy density as an important performance indicator for TES, and evaluated it at both material and system levels. This approach is afterwards applied to prototypes covering the three TES technologies: a two-tank molten salts sensible storage system, a shell-and-tube latent heat storage system, and a magnesium oxide and water chemical storage system. The evaluation of the energy density highlighted the difference of its value at the material value, which presents a theoretical maximum, and the results at system level, which considers all the parts required for operating the TES, and thus presents a significantly lower value. Moreover, the proposed approach captured the effect of the complexity and overall size of the system, showing the relevance of this performance indicator for evaluating technologies for applications in which volume is a limiting parameter. ; The work was partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER)). The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. Jaume Gasia would like to thank the Departament d'Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya for his research fellowship (2018 FI_B2 00100). Aran Solé would like to thank Ministerio de Economía y Competitividad de España for Grant Juan de la Cierva, FJCI2015-25741. The authors would also like to thank the participants of IEA ECES Annex 30 for their critical view and feedback during the development of the methodology.

The present document is a manuscript-based dissertation covering Kyle Bassett's PhD research from January, 2015 to January 2017. The research was particularly focused on studying and developing an emerging energy storage technique known as Buoyancy Battery Energy Storage (BBES). The buoyancy energy storage technique is presented and primary components are described and discussed. An idealized system was analyzed to determine governing equations of operation as well as ideal energy storage density. Experimental analysis was conducted to confirm properties of constant discharge force with respect to both float position and storage duration. Discharge testing was conducted with a developed scale system installed in the offshore testing tank and the University of Windsor. To evaluate the scalability of the technique, a utility scale BBES system was designed with power output capacity of 1 MW and energy storage capacity of 1MWh. Several commercially available marine lift bags were considered and evaluated for volume requirements and drag effects at various float speeds. Theoretical roundtrip efficiency for this designed system was found to be 83% based on results from drag calculations, pulley losses and electrical efficiency losses. Numerical simulations of system performance were completed to determine the revenue generation of the designed system based on 2015 Ontario market energy prices. To validate system operation in a marine environment, open water testing was conducted in Lake Huron. Testing validated surface deploy ability and steady state float motion was achieved. To further investigate the market opportunities and challenges facing the grid scale integration of energy storage, an analysis of market conditions was performed using Ontario, Canada as a case study. Ten years of Hourly Ontario Energy Price was analyzed using Fourier transform to reveal periodic trends within the data. It was found that the introduction of Time-of-use billing for electricity was effective in changing energy consumption behavior, improving balance for the electricity grid. Revenue generation simulations were completed for utility scale energy storage systems of various technologies (and thus various roundtrip efficiencies) using historic 2015 energy price data. Simulations included single and multi-cycle storage programs. It was determined that energy storage facilities are not currently financially viable, due to the minimal revenue produced through energy arbitrage transactions. The development of energy storage in Ontario will depend greatly on governmental subsidies and additional revenue-generating ancillary services such as regulation and black start capability. Additional experimental analysis was performed using a modified BBES system designed to convert input energy into mechanical work such that each quantity could be controlled and measured. Three float shapes of interest were tested including a horizontally configured cylinder, a vertically configured cylinder as well as a sphere. Discharge efficiencies greater than 90% were achieved. Roundtrip efficiencies of 78% were recorded. Results suggest that with improved conversion pulleys and component scaling, experimental roundtrip efficiencies should approach the theoretical efficiency used in the 1 MW BBES system designed.

Buildings are responsible for one-third of the world's energy consumption, of which 60% is due to heating and cooling. To accomplish the low-carbon energy goal in the building sector, thermal energy storage offers a number of benefits by reducing energy consumption and promoting the use of renewable energy sources. This manuscript reviews recent advances in the development of thermal energy storage materials for building applications oriented towards zero energy buildings. Volumetric heat capacity of sensible, latent and thermochemical energy storage materials developed for low-to-moderate temperature applications are reviewed and assessed with a special focus on their technical characteristics and development stage. This encompasses most recent publications, international programmes and projects, and commercially available solutions. Physical, thermodynamic, kinetic and chemical properties are addressed, as well as costs. Advantages, drawbacks and challenges of the diverse alternatives are discussed. The analysis shows that solutions with the highest potential for competitive energy efficiency measures are based on latent and sensible energy storage systems, which present a volumetric thermal energy storage density up to 430 and 250 MJ/m3 respectively. Their applications in free-cooling ventilation systems, solar energy storage solutions for short and long-term storage periods, and demand-side management strategies towards the road to zero energy buildings are highlighted as promising, leading to a reduction of energy consumption of more than 30%. On the other hand, thermochemical energy storage does not yet show clear advantages for building applications, despite the potentially high energy density (up to 1510 MJ/m3) and heat availability for long-term storage periods. Currently, there is no available material for thermochemical energy storage that satisfies all the requirements for building operations. Besides, thermochemical solutions require different tanks and heat exchangers that should be carefully addressed for small-scale applications. Additional research efforts are needed to optimise operation conditions, efficiency, costs and system designs. ; European Union SOE1/P3/P0429EU ; Ministerio de Economía y Competitivdad CTQ2014-52763 -C2 -2-R

AbstractThermochemical sorption energy storage (TSES) is the most recent thermal energy storage technology and has been proposed as a promising solution to reduce the mismatch between the energy supply and demand by storing energy for months in form of chemical bonds and restore it in form of synthesis chemical reaction. Compared with sensible/latent thermal energy processes, TSES system has major advantages, including a high energy storage capacity/density and the possibility of long-term energy retention with negligible heat loss. Therefore, a solid–gas thermochemical sorption battery is established and investigated utilizing a composite working pair of MgSO4–H2O based on room temperature expanded graphite (RTEG), treated with sulfuric acid (H2SO4) and ammonium persulfate ((NH4)2S2O8) as a porous additive. The experimental results showed that energy storage density and sorption efficiency increase with the increment of charging temperature or decreasing of discharging temperature at a certain ambient temperature. Under experimental conditions, energy density ranged from 31.7 to 908.8 kJ/kg (corresponding to volume energy density from 11.7 to 335.8 MJ/m3), while sorption energy efficiency ranged from 28.3 to 79.1%. The highest values were obtained when charging, condensation, and discharging temperatures were 95, 20, and 15 °C, respectively. The maximum thermal efficiency was 21.1% at charging/discharging temperature of 95/15 °C with sensible to sorption heat ratio of 3:1.
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1st International Conference on Solar Heating and Coolingfor Buildings and Industry (SHC 2012) ; Thermal energy storage by chemical reactions is one of the most suitable energy storage systems for buildings and industrial applications due to the wide range of working temperatures as well as the high energy storage density provided. Developing and characterizing chemical reactions is now one of the most promising areas of research. Chemical heat pumps are one of the systems used when chemical reactions are utilized as thermochemical energy storage technology. Nevertheless, the main problem to implement this technology is found in the material behaviour within the reactors used. Therefore, material and reaction characterization have been identified as key issues to design a proper system. The main goal of this paper is to highlight the parameters to take into account previous to the design. ; The work is partially funded by the Spanish government (ENE2011-28269-C03-02 and ENE2011- 22722). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2009 SGR 534) and research group DIOPMA (2009 SGR 645).